Swimming Pools: Design and Construction

Swimming Pools

Copyright 2000 Philip H Perkins

Swimming Pools

Fourth edition

Philip H Perkins

London and New York

Copyright 2000 Philip H Perkins

This edition published in 2000 by E & FN Spon 11 New Fetter Lane, London EC4P 4EE Simultaneously published in the USA and Canada by Routledge 29 West 35th Street, New York, NY 10001 This edition published in the Taylor & Francis e-Library, 2003.

E & FN Spon is an imprint of the Taylor & Francis Group First edition 1971 Second edition 1978 Third edition 1988 (Elsevier Applied Science Publishers Ltd) © 2000 Philip H Perkins All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made.

British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging in Publication Data A catalog record for this book has been requested ISBN 0-203-47788-X Master e-book ISBN

ISBN 0-203-78612-2 (Adobe eReader Format) ISBN 0-419-23590-6 (Print Edition)

Copyright 2000 Philip H Perkins

Contents

Preface 1

The planning and layout of swimming pools General considerations 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12 1.13

Introduction Basic requirements for all swimming pools Pools for private houses, clubs, hotels and schools Covered pools for private houses, hotels, clubs and schools Teaching/learner pools Public swimming pools Floor gradients The drainage of walkways and wet areas Hydrotherapy pools Pools used for sub-aqua activities Facilities for the disabled Swimming pools with movable floors Wave-making machines

Recommended procedure for getting a pool built: contracts and dealing with disputes 1.14 Introduction 1.15 Contracts: how to proceed 1.16 Dealing with disputes Further reading 2

Basic characteristics of the materials used in the construction of swimming pools 2.1 Introduction 2.2 Portland cements 2.3 Aggregates from natural sources for concrete and mortar 2.4 Admixtures

Copyright 2000 Philip H Perkins

2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17

Additions Water for mixing concrete, mortar and grout Steel reinforcement Spacers Non-ferrous metals Bimetallic corrosion Curing compounds for concrete and mortar Polymers Reactive resins Joint fillers Joint sealants Ceramic tiles British standards and euro codes

References Further reading 3

Factors affecting the durability of reinforced concrete and cement-based materials used in the construction of swimming pools 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9

Introduction Corrosion of steel reinforcement in concrete Carbonation of concrete Chloride-induced corrosion of reinforcement Deterioration of the concrete Chemical attack on cement-based mortar Swimming pool water and chemicals used in water treatment Moorland water and the Langelier Index Alkali-silica reaction

Further reading 4

Construction of swimming pool shells in insitu reinforced concrete 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11

Introduction Site investigations Under-drainage of site Flotation (uplift) of the pool shell General comments on design and construction Concrete construction in cold weather Concrete construction in hot weather Plastic cracking Thermal contraction cracking Swimming pools with floor slabs supported on the ground Construction of the walls of the pool

Copyright 2000 Philip H Perkins

4.12 Construction of walkway slabs and floors of wet changing areas 4.13 Curing the concrete floor and walls of the pool 4.14 Construction of suspended pool shells 4.15 Thermal insulation of swimming pool shells 4.16 Under-water lighting and under-water windows Further reading 5

Construction of swimming pool shells in reinforced sprayed concrete and other materials Reinforced sprayed concrete (shotcrete) 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8

Introduction Design and specification Methods of application Execution of the work Thermal insulation Pipework Testing for watertightness Under-water lighting

Swimming pools constructed with reinforced hollow concrete block walls and insitu reinforced concrete floor 5.9 5.10 5.11 5.12 5.13 5.14 5.15 5.16 5.17

Introduction Construction of the floor Construction of the walls Pipework Under-water lighting Curing the concrete and protecting the blockwork Testing for watertightness Back-filling around the walls Thermal insulation

Sandwich type construction with insitu reinforced concrete core wall and concrete blocks as permanent form work 5.18 5.19 5.20 5.21 5.22 5.23 5.24 5.25

Introduction Construction of the floor Pipework Construction of the walls Under-water lighting Finishes to floor and walls Testing for watertightness Back-filling around the walls

Copyright 2000 Philip H Perkins

5.26 Thermal insulation Other methods of construction 5.27 5.28 5.29 5.30 5.31 5.32 5.33

General comments Pools constructed with mass (gravity) type walls Curing the concrete Testing for watertightness Pools constructed in very stable ground such as chalk or rock Pools constructed of precast post-tensioned concrete units Pool shells of steel

Further reading 6

External works 6.1 6.2 6.3 6.4

General considerations Paving Surface water drainage Walling

Further reading 7

Finishing the pool shell and associated structures; problems with pool hall roofs Finishing the pool shell and associated structures 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13

Cement-sand rendering to insitu concrete walls Cement-sand rendering to sprayed concrete walls Cement-sand rendering to concrete block walls Cement-sand screeds on insitu concrete floors Cement-sand screeds on sprayed concrete floors Ceramic tiles and mosaic Walkways and wet changing areas Testing the completed tiling Marbelite Coatings and paints Sheet linings to swimming pools Glass-fibre polyester resin linings Finishes to walls of pool halls

The roofs of swimming pool halls 7.14 General considerations 7.15 Pressurised roof voids 7.16 The warm-deck roof Further reading

Copyright 2000 Philip H Perkins

8

Water circulation and water treatment Water circulation 8.1 Flow-through pools 8.2 Pools where the pool water is in continuous circulation 8.3 Ducts for pipework Water treatment 8.4 8.5 8.6 8.7 8.8 8.9 8.10 8.11 8.12 8.13 8.14 8.15

Layout of treatment plant Filtration and filters Chemical dosing of the pool water The disinfection of pool water Chlorination Ozone Bromine Chlorine dioxide Metallic ions (silver and copper) Ultra-violet radiation The base-exchange process for softening pool water Sulphates in swimming pool water

Further reading 9

Notes on heating swimming pools and energy conservation 9.1 9.2 9.3 9.4

Heating open-air swimming pools Heating the water in indoor swimming pools Heating and ventilation of pool halls and adjoining areas Solar heating of swimming pools

Further reading 10 Maintenance and repairs to swimming pools Maintenance of swimming pools 10.1 General considerations 10.2 Routine supervision: smaller pools 10.3 Shut-down periods 10.4 Algal growths: prevention and removal 10.5 Foot infections Repairs to external works: paving 10.6 Remedial work to insitu concrete paving for pedestrians 10.7 Remedial work to insitu concrete paving for light commercial vehicles 10.8 Remedial work for precast concrete flag paving

Copyright 2000 Philip H Perkins

10.9 10.10 10.11 10.12

Remedial work to precast concrete block paving Remedial work to clay pavers Remedial work to slippery paving Preventing trips and falls

Repairs to external works: walling 10.13 Remedial work to free-standing walls 10.14 Remedial work to earth-retaining walls Remedial work to pools under construction 10.15 10.16 10.17 10.18 10.19

General comments Remedial work to thermal contraction cracks Remedial work to drying shrinkage cracks Remedial work to honeycombed concrete Inadequate concrete cover to the reinforcement

Remedial work to existing pools: tracing leaks and investigations 10.20 Introduction 10.21 Tracing leaks 10.22 General investigations Remedial work to existing pools: repairs following leak tracing and investigations 10.23 10.24 10.25 10.26

Remedial work to leakage Improving support to the pool floor Structural lining to the pool shell Remedial work to finishes

Further reading Appendix 1 Conversion factors and coefficients Appendix 2 Testing swimming pools shells, walkway slabs and other wet areas for watertightness. Commissioning swimming pools Introduction Testing new pools Testing existing pools The leakage test procedure General comments on testing Watertightness test for walkway slabs and other wet areas Commissioning swimming pools (filling and emptying) Appendix 3 Investigations, sampling and testing General considerations

Copyright 2000 Philip H Perkins

Sampling and laboratory testing Cover-meter survey Appendix 4 The consultant/designer as an expert witness Introduction The form of the Expert’s Report The expert witness and the Construction Act 1996 Appendix 5 Notes on safety in swimming pools Introduction Water depths for diving Signs for water depths in the pool Other safety signs Outlets for water in the pool floor Water slides and play equipment Slipping and tripping on floors of walkways, changing rooms etc. Chemicals in water treatment Appendix 6 List of organisations relevant to this book

Copyright 2000 Philip H Perkins

Preface

Since the third edition of this book was published in 1988 there have been no startling changes in the materials used for the construction of swimming pools. A similar comment can be made about the design of reinforced concrete swimming pool shells. The number of swimming pools has continued to increase both in the public and private sectors. This is particularly so with private club leisure centres which offer a wide range of activities. There has been significant developments in the field of National Specifications and Code of Practice relating to construction due to the intensive work on the preparation of Euro Standards and Codes and the issue of Directives from the EEC. The latter set out minimum quality standards for a wide range of constructional materials, and establish the responsibility of suppliers and designers. Of particular importance are The Construction (Design and Management) Regulations 1994 which became completely effective in December 1995. These Regulations make people assess risks and take precautions rather than waiting to deal with problems when they occur. They target the health and safety of those who build, maintain, install and demolish buildings and plant. The Construction Products Regulations came into force at the end of 1991 to implement the Construction Products Directive. The potential scope of the Regulations is very wide indeed as they are applicable to all types of product which are intended for permanent incorporation in buildings and civil engineering works. The Regulations provide for the application of the European Community regulatory mark—the CE mark—to construction products. The Building Research Establishment Information Paper IP. 11/93 gives information on Ecolabelling of building materials and building products. The British Standards Institution emphasise that the Kite Mark will continue to ensure that the level of quality is above the minimum legal requirements. Health and Safety Regulations have been extended and tightened up and there is increasing awareness of the need for a more enlightened and professional approach to treatment of swimming pool water. The Committee which produced the publications for the Department of the Environment on the purification of swimming pool water is no longer in existence. It has virtually been replaced by the independent Pool Water Treatment Advisory Group.

Copyright 2000 Philip H Perkins

It is important to observe recognised safety precautions when using certain materials, and also all types of plant and equipment. Concrete itself is not a hazardous material; however, Portland cement when mixed with water is highly alkaline (it has a pH of about 13.5) and is considered a caustic alkali. It can cause burns to the skin, particularly to people who are vulnerable to dermatitis. A safety warning is included as an Appendix in all British Standards for Portland cement. It recommends that precautions be taken to prevent dry cement entering the eyes, nose or mouth, and prevent skin contact with wet cement. Polymer resins are now widely used in construction and there are hazards associated with the use of some of these compounds. Users should obtain information from the manufacturers and be aware of the requirements of the publications of the Health and Safety Executive relating to the use of substances hazardous to health. The corrosion of steel reinforcement continues to be the number one cause of deterioration in reinforced concrete structures. Research Focus, No. 37, May 1999, states that: ‘Corrosion of reinforcing steel in concrete structures…is estimated to be costing the UK £550 million a year. Many of these structures continue to require maintenance or replacement…’ It is therefore surprising that the protection of rebars by properly formulated and applied epoxy resin coatings (see BS 7293 and ASTM Specification A775) is still only used on a comparatively small scale in the UK. The author acknowledges with gratitude the encouragement, and many useful comments, he has received from his wife. He also records the help he has been given by numerous people, organisations and firms, and in particular, David Butler of the Sports Council, Andrew Alphick of the Pool Water Treatment Advisory Group, Ralph Riley of the Institute of Baths and Recreation Management, Geoffrey Roberts and Jim Gordon of Buckingham Swimming Pools Ltd.

Copyright 2000 Philip H Perkins

Chapter 1

The planning and layout of swimming pools

GENERAL CONSIDERATIONS

1.1 Introduction In the United Kingdom, the construction of the shell of a swimming pool (without ancillary buildings such as plant house, changing rooms etc.) is unlikely to require a Building Permit under the Building Regulations, but planning permission may be required. It is therefore advisable for any one wishing to build a swimming pool to consult their Local Authority, and also the water supply company as there may be special requirements, such as metering of the supply, restriction on the amount of water used etc. While there are regulations relating to swimming pools open to the public, the legal control over the purity of water in pools for private houses, clubs and hotels is minimal. Recommendations for the treatment and quality of swimming pools water have been issued by the Pool Water Treatment Advisory Group (PWTAG), namely the Pool Water Treatment and Quality Standards. The PWTAG is an independent body supported by all the organisations involved in the operation of swimming pools. In the United States, the position is different; for example in California regulations are in force which apply to all swimming pools except private pools maintained by an individual for use by his family and friends. The regulations specifically apply to pools belonging to hotels, clubs, schools and health establishments. Important aspects of design, layout, operation and maintenance are detailed and clear directions given. Requirements for the chemical and bacteriological quality of the water are included.

1.2 Basic requirements for all swimming pools The recommendations given below are intended to apply to all swimming pools constructed of what may be termed ‘long-life’ materials such as concrete. 1. 2.

The pool shell (floor and walls) must be structurally sound. The shell must be watertight against loss of water when the pool is full or

Copyright 2000 Philip H Perkins

Figure 1.1 Sketch showing safety step.

3.

4. 5.

6.

partially full, and if constructed below ground level, against infiltration of ground water when the pool is empty or partly empty. The internal surface of the floor and walls must be finished with a smooth, reasonably impervious, easily cleaned, attractive material. The water must be maintained at a proper standard of purity and clarity. A walkway of adequate width (minimum about 1.5 m), with a non-slip, easily cleaned and durable surface should be provided around the pool. A safety step (or ledge) should be provided on all the walls of pools used by young children and non-swimmers. This safety step should be located not more than 900 mm (0.9 m) below top water level (Figure 1.1). A diving board should not be provided unless the dimensions of the diving area and the water depth comply with the recommendations of the Amateur Swimming Association (ASA). For pools used for international diving competitions, the regulations of the Federation Internationale de Natation Amateur (FINA) should be followed.

Copyright 2000 Philip H Perkins

1.3 Pools for private houses, clubs, hotels and schools 1.3.1 Open-air pools: location With pools in this category, there is generally a limited choice of location as they usually have to be built on the same plot as the main building. An exception is school pools as these may form part of sports ground facilities which are likely to be some distance from the school. For open-air pools for private houses, and hotels, the following points should receive consideration. 1. 2.

3.

4. 5. 6.

7. 8.

9.

A position should be selected which receives as much sun as possible, particularly in the afternoon. The vicinity of large trees or potentially large trees should be avoided. Tree roots can cause damage to foundations, and to drains and other pipelines. Leaves can cause discolouration of the pool water and staining of the pool finish which is difficult to remove. It is advantageous to utilise a natural wind-break, such as a thick hedge, garden wall, or part of the main building, and if it does not exist, to provide one as part of the landscaping. The position of existing drainage, water supply, electricity and gas supply lines is important. Depending on the method of construction of the pool (see Chapters 4 and 5), access for materials and plant required for the construction can be critical. A small building (or room in the main building) will be needed for plant and equipment and storage of cleaning materials and the chemicals used for water treatment. It is desirable for the distance from the changing accommodation to the pool to be as short as practical bearing in mind the points mentioned above. For private houses and hotels, landscaping of the area in which the pool is to be located should be given careful thought and professional advice is usually worthwhile. People often find it difficult to envisage from a two-dimensional sketch what the completed three-dimensional project will look like. The cost of a simple model and/or an isometric drawing could be justified.

Figures 1.2 and 1.3 illustrate alternative positions for a private pool.

1.3.2 The shape and dimensions of swimming pools The shape and dimensions of a swimming pool are mutually interdependent. The primary use of the pool will be a major factor in determining both shape and dimensions.

Copyright 2000 Philip H Perkins

Figure 1.2 Pool adjacent to building.

If the primary use is for training and swimming, then a rectangular shape is normally chosen. The length should be a simple fraction of 100 m, and the width a number of swimming lanes which are usually to be 2.0 m wide (ASA for 25 m pools). The materials used in the construction of the pool shell will also influence its shape. Pools constructed in insitu reinforced concrete can be of any shape, but the cost of a free-formed pool would be very high due to the cost of the formwork, compared with a pool constructed in sprayed concrete (shotcrete). But this cost differential is influenced by the size of the pool, it being greater for smaller pools than for larger ones. The smaller domestic and hotel pools, constructed in sprayed reinforced concrete can be any shape, with little difference in cost between rectangular and free-formed.

Copyright 2000 Philip H Perkins

Figure 1.3 Pool near boundary of plot.

Copyright 2000 Philip H Perkins

As these pools are likely to be used by children, non-swimmers and weak swimmers, the provision of a safety step around the pool at a depth not exceeding 900 mm (0.9 m) below top water level is strongly recommended. This is a standard feature of hotel pools in Switzerland (Figure 1.1).

1.3.3 Requirements for swimming Even the smallest pool should be large enough for a swimmer to take several strokes; the minimum size would be about 6.00 m long by about 4.00 m wide with a minimum water depth of 1.00 m. However, a water depth of 1.00 m is not sufficient from a safety point of view for even a very flat dive. For general comfort, there should be an allowance of about 4.5 m2 for each person who wants to swim.

1.3.4 Requirements for diving The depth of water and the dimensions of the diving area for competitive diving are covered in the UK by the requirements of the ASA. For international events these matters are covered by the world governing body, the Federation Internationale de Natation Amateur (FINA). There are minor differences between these two sets of regulations but both provide adequate safety for diving in properly designed pools. The relevant publications of both organisations should be consulted and followed by the designers of any swimming pool which is intended to include a diving board. The designer should check and comply with the latest recommendations. It is emphasised that the dimensions given are essential for safe diving from a position not more than 1.00 m above the water level in the pool. A natural question is ‘What about diving from the sides of the pool?’. The only form of dive recommended into shallow water from the pool sides is what is known as a flat racing dive, which can only be safely executed by experienced swimmers; even then the minimum depth of water is 1.50 m, which must be Table 1.1 Examples of rectangular swimming pools for private houses, hotels, clubs and schools

Copyright 2000 Philip H Perkins

Figure 1.4 Section through 25 m pool with diving pit.

maintained forward for a distance of 7.6 m and the water level in the pool should not be more than 0.38 m below the pool edge. These recommendations are given by the courtesy of the Institute of Baths and Recreation Management. Diving should not be permitted nor attempted into pools which do not meet the above recommendations. Should an accident occur to a person diving into a pool from a diving board which does not meet authoritative safety recommendations, the pool owner/manager may be faced with a claim that would be difficult to contest.

1.4 Covered pools for private houses, hotels, clubs and schools There are obviously many advantages in having a covered swimming pool instead of an open-air one. A covered pool can be used in comfort 365 days a year compared with the ‘season’ for an open-air pool of about 150 days. The conditions under which the pool has to operate are much less onerous; problems arising from freezethaw do not arise, staining of the walls and floor is much reduced, and discolouration of the water from leaves and air-borne dirt will be eliminated. See Figures 1.5–1.6 for views of private house pools, and Figures 1.7–1.10 for views of hotel, club, and school pools. A major problem with covered pools is the occurrence of condensation on the walls, windows and ceiling, and, depending on the method of construction, within the roof space. The environment in the hall of a heated indoor swimming pool can be considered as particularly hostile to many building materials; the air temperature is relatively high—probably about 28 °C to 30 °C, and the relative humidity is also high, say, 70–75%. The surfaces in contact with the air in the pool hall will generally have a lower temperature than the temperature of the air in the hall; if the air is saturated

Copyright 2000 Philip H Perkins

Figure 1.5 Pool with Roman end and steps and fully automatic cover. Courtesy, Buckingham Swimming Pools Ltd.

Figure 1.6 Indoor private house pool. Courtesy, Buckingham Swimming Pools Ltd.

Copyright 2000 Philip H Perkins

Figure 1.7 Indoor hotel deck-level pool with spa pool. Courtesy, Buckingham Swimming Pools Ltd.

Figure 1.8 Open-air pool at private club leisure centre. Courtesy, Buckingham Swimming Pools Ltd.

Copyright 2000 Philip H Perkins

Figure 1.9 Indoor, 25 m school pool. Courtesy, Buckingham Swimming Pools Ltd.

Figure 1.10 Indoor hotel pool, Switzerland.

Copyright 2000 Philip H Perkins

with water vapour, condensation will occur on the contact surfaces. The temperature at which condensation occurs is known as the dew point. The design and detailing of the roof requires special attention and this is discussed briefly in Chapter 7.

1.5 Teaching/learner pools This section deals with general principles relating to layout and dimensions of teaching pools irrespective of whether they belong to a school or form part of a large swimming pool complex (leisure centre) run by a local authority. The first principle is that the pool must be absolutely safe for nonswimmers. The pools are usually rectangular on plan with an almost level bottom. The water depth generally varies from 0.80 m to 1.00 m. A useful size is 12.00 m by about 7.0 m. There are often shallow steps into the pool extending the full length of the short side. There are different opinions as to whether the walkway around the pool should be lower than the deck to enable the teacher to carry out his duties without having to bend down, or whether the pool shell should be elevated similar to the hydrotherapy pool shown in Figure 1.15 and briefly described in Section 1.9. In the UK, it is customary for the teaching pool to be quite separate from the main swimming pool so that the two different types of use do not interfere with each other. If the teaching pool is in a separate enclosed part of the main building, it is usual for the temperature of the water and the air in the pool hall to be a few degrees above that in the main part of the building.

1.6 Public swimming pools 1.6.1 Introduction In the UK and most countries in the temperate zone, all new large swimming pools which are publicly owned are covered to enable them to be used throughout the year. Table 1.2 Examples of dimensions of teaching pools

Copyright 2000 Philip H Perkins

There are a number of large open-air pools in the UK which are owned by local authorities, but these were built many years ago, generally before the Second World War. These are only in operation for four or five months in the year, usually from May to September. A few of these are heated. They are rectangular on plan and some contain sea water which contributes to a high rate of general deterioration. In Europe, mainly in Switzerland and Germany, in spas, there are large openair heated pools, some with wave-making machines. Figures 1.11–1.13 are examples of pools in public leisure centres.

1.6.2 Location It is not possible to lay down detailed rules for the location of public swimming pools, but the following are matters which should receive careful consideration: 1. 2. 3. 4.

provision of adequate public transport; provision for adequate car parking; provision of public sewers (foul and surface water), water supply, electricity, gas and telephone; adequate access for emergency services, fire brigade and ambulance;

Figure 1.11 View of part of pool at Bletchley Leisure Centre.

Copyright 2000 Philip H Perkins

Figure 1.12 View of pool in Rushcliffe Leisure Centre. Courtesy, British Cement Association. Photographer, T.Jones.

Figure 1.13 View of pool in Swansea Leisure Centre with wave machine in operation. Courtesy, British Cement Association. Photographer, T.Jones.

Copyright 2000 Philip H Perkins

5.

sub-soil conditions including water table levels, presence of aggressive chemicals and presence of contaminated ground. This is of particular importance in view of the alleged shortage in the UK of ‘good building land’, which in some cases exerts pressure to build on land-fill sites. The Environmental Protection Act 1990 should be studied. Requirement C2 of Schedule 1 of the Building Regulations 1991 states that ‘precautions shall be taken to avoid danger to health and safety caused by substances found on or in the ground covered by the building…’ Approved Document C, 1992 edition, sets out detailed requirements for dealing with containments. Reference can also be made to Section 4.2, and the list of Further Reading at the end of this chapter.

1.6.3 Types, shapes and dimensions When the first edition of this book was published in 1971, the standard shape of public swimming pools in the UK was rectangular or L-shaped. Some large pools had two shallow ends. In L-shaped pools, the long leg can be used for swimming and the short leg for diving. However, with the advent of the leisure centre, the shape, size and use of pools have changed considerably. Figures 1.11–1.13 show examples of public swimming pools in leisure centres. In these centres, it is usual for the main pool to be freeformed and incorporate a sloping ‘beach’ and the installation of a wave-making machine which is switched on for relatively short periods several times a day. It is emphasised that for competitive swimming, diving and aquatic sports, the requirements of the ASA (for national events) and FINA (for international events) must be fully complied with. The requirements mentioned in this book are only a few of the very detailed requirements laid down by these two organisations. The following are examples of some of these requirements: 1.

2. 3.

4.

For competitive swimming (national events), the water depth in front of the starting blocks must not be less than 1.80 m and this must extend forward for a distance of 6.00 m. Stairs and steps must be accommodated outside the pool dimensions, i.e. they must be recessed. For water polo, the minimum depth of water over the whole playing area must not be less than 1.80 m; the playing area must not exceed 30 m× 20 m and must not be less than 20 m×8 m. For life saving certificates, a water depth of 2.0 m is required and this must extend for a length of 6.00 m over the full width of the pool.

The provision of a diving pit as part of the main pool is deprecated as diving into a pool in which persons are swimming is unpleasant and can be dangerous. There are many advantages in having a separate diving pit which is used only for diving.

Copyright 2000 Philip H Perkins

By having a separate diving pit, swimming and diving events can be held simultaneously, and the same applies to training. Pools intended for competitive swimming are normally 25 m or 50 m long with a width based on a number of swimming lanes, either 2.0 m for national events or 2.5 m for international events. Reference must always be made to the latest edition of the relevant regulations.

1.7 Floor gradients 1.7.1 The pool The floor of the pool must be laid with a fall (slope) towards the outlet with such a gradient that the pool can be effectively emptied. However, the gradient should not be so steep that non-swimmers and learners can lose their balance and/or slip. It is generally considered that the depth of water at which boyancy is likely to affect a person’s balance is about 0.75 of the person’s height. The steeper the gradient, the sooner a person will reach the point of over-balance. As the point of over-balance varies with the height of the person, it is suggested that the maximum gradient for the floor of a pool used by children and non-swimmer/learners should be 1 in 40 (25 mm in 1.00 m). For efficient emptying of the pool, hydraulic considerations require a gradient of about 1 in 80 (25 mm in 2.00 m). The gradient should be uniform between clearly marked locations and depth markers on all walls are essential. A further safety precaution is to provide non-slip tiles on the floor. The above comments on gradient do not apply to the steep slope down to a diving pit as shown in Figure 1.4. Water outlets in the floor at the deep end of the pool should be fitted with small aperture gratings. See Section 8.2.6.

1.7.2 Walkways and wet areas In this context ‘wet areas’ include all those areas, such as changing areas, walkways around the pool, shower cubicals etc., which are made constantly wet by pool users and by cleaners. It is in these areas that injuries resulting from slipping are most likely to occur. There is a conflict between the need for a non-slip (or slip resistant) surface and the need for easy cleaning and efficient drainage (run-off). At the time of writing, there does not appear to be any formal and recognised gradients for floors in these areas. There is also the problem of ponding. To avoid ponding, a gradient of about 1 in 40 (25 mm in 1.00 m) is normally required, but for safety a gradient of 1 in 60 (25 mm in 1.50 m) is probably needed. The frictional characteristics of the finished surface when in contact with the bare feet of pool users are relevant. It should be noted that the gradient suggested here is considerably less than for the floor of the pool in Section 1.7.1.

Copyright 2000 Philip H Perkins

1.8 The drainage of walkways and wet areas Where walkways and wet areas form part of a covered pool complex, the drainage of these areas should not discharge into the water circulation system of the pool but should be connected to the main drainage system of the building.

1.8.1 General comments In some cases the public sewers are designed on the ‘separate’ system in which surface water is carried in surface water sewers, but in other areas, the sewerage system is ‘combined’ and the foul and surface water is carried in the same sewer. It is important that an adequate number of inspection chambers/man-holes should be provided and drainage system laid to gradients which will provide a self-cleansing velocity. With open-air pools, the surrounding paving should slope away from the pool.

1.9 Hydrotherapy pools The advantages of carrying out special exercises under water have been known to the medical profession for many years; the weight of the body is reduced by the weight of water displaced and thus movements are made much easier with less muscular effort. While spas in the UK have declined in popularity, this has not happened on the continent of Europe. There, special health resorts, with names starting with ‘Bad’ in Germany, Switzerland and Austria, continue to flourish and attract large numbers of visitors/patients. The spas are mainly situated in beautiful country and make a very pleasant location for a holiday. Many of the special baths and swimming pools contain naturally heated highly mineralised water from springs, which in some places is slightly radio-active. There are often a number of pools which operate at different temperatures and possess therapeutic properties. Under-water massage by powerful jets located at different depths below the water surface is a common feature of many of these pools. The length of stay in the pool is strictly limited. Figure 1.14 shows a pool at a spa in Switzerland. Figure 1.15 shows a hydrotherapy pool at a school for pupils with severe learning difficulties. The pool is 10 m×5.00 m and the depth varies from 0.90 m to 1.50 m. It is a deck level pool and the water temperature is maintained at 32°C. In the UK there are many therapeutic pools but these are mainly attached to hospitals, recuperation homes and similar institutions and are used for treatment prescribed by a physician. They seldom form part of a holiday resort. Special features to be taken into account in the design of such pools include the following:

Copyright 2000 Philip H Perkins

Figure 1.14 Open-air pool with wave machine in operation Bad Vals, Graubunden, Switzerland.

Figure 1.15 Hydrotherapy pool in school for pupils with severe learning difficulties. Courtesy, Buckingham Swimming Pools Ltd.

Copyright 2000 Philip H Perkins

1. 2. 3.

4.

5.

6. 7.

8.

The floor should have a flat gradient of about 1 in 80 (25 mm in 2.00 m) which should be adequate for emptying. The floor of the pool and all wet areas should be finished with non-slip ceramic tiles or ceramic mosaic. If the pool is not deck-level, then glazed ceramic scum channels should be provided as these are more efficient in providing good water circulation than skimmer outlets. The turn-over period (the time required to completely circulate all the water in the pool) should not exceed 3 hours; some pools operate on a 1½hour cycle. See Section 8.2.1. The water temperature and the air temperature in the pool hall and changing areas should be maintained at a higher temperature than in normal swiming pools. A water temperature of 30–32°C and an air temperature of 33 °C is adopted in many pools. All fittings should be corrosion resistant (austenitic stainless steel or phosphorbronze). As these pools will certainly be used by disabled persons, special arrangements should be included to enable such persons to enter and leave the pool easily; see also Section 1.11. If the pool contains saline water, then a detailed chemical analysis should be obtained, including information on any variations in the concentration and type of dissolved salts. This is essential in order to decide whether special protective measures are needed for the pool shell, finishes and fittings.

1.10 Pools used for sub-aqua activities Sub-aqua activities have become very popular in all parts of the world. Training in the sea, lakes and rivers in the UK and other countries in the temperate zone is often difficult owing to low temperatures, low visibility, currents etc. Thus, there are many advantages in carrying out training in a swimming pool. The British Sub-Aqua Club (BSAC) requires that every beginner should receive basic training in a swimming pool. The use of public swimming pools is not always permitted by Baths Managers owing to interference with public use of the pool and possible damage to the finish of the pool and walkways by the divers equipment. With reasonable care, the equipment used by the Club’s members should not cause damage if the finishes are high quality ceramic tiles or ceramic mosaic. In any event, these finishes require maintenance and repair in the course of time. Small damaged areas can be repaired under-water which eliminates the need to lower the water level or empty the pool. The BSAC have published a booklet giving detailed information on all aspects of aqualung diving—see Further Reading at the end of this chapter.

Copyright 2000 Philip H Perkins

Figure 1.16 Aqualung training in public pool. Courtesy, British Sub-Aqua Club. Photographer, T.Jones.

The BASC Code of Conduct contains some 21 directions, including emphasis on the prohibition of dropping heavy equipment in the pool and anywhere in the pool premises. As far as the pool itself is concerned, the requirements for sub-aqua activities are very modest. The minimum dimensions required for a group lesson are 3.60 m×5.00 m, with a minimum depth of water over this area of 1.50 m. These requirements can be increased with advantage, with special reference to water depth to 3.50 m and if possible 5.50 m. Designers should contact the BSAC for their latest recommendations. Figure 1.16 shows sub-aqua training.

1.11 Facilities for the disabled The absolute need to provide satisfactory arrangements for disabled persons to use public swimming pools is now recognised. The specification and design of the necessary facilities require special study at the design stage as it can be difficult and costly to provide these facilities at a later date. Work in this field is done by a number of organisations and reference should be

Copyright 2000 Philip H Perkins

made to BS 5810 1979 Code of Practice for Access for the Disabled to Buildings and to the publications of the Thistle Foundation.

1.12 Swimming pools with movable floors The desirability of having separate pools for swimming, diving and teaching has been mentioned earlier in this chapter. Such separation entails additional capital investment and increased operating and maintenance costs; also, the teaching and diving facilities are only used from time to time. This led to the development of hydraulically operated movable pool floors and separating walls. The depth of water can be reduced over part of the pool by raising a section of the floor thus forming a teaching/learner area. This feature has proved more popular in Europe than in the UK where the number of public pools with movable floors is small and very few have been constructed in recent years. Figure 1.17 shows a movable floor in the raised position.

Figure 1.17 View of movable floor in public pool. Courtesy, Buckingham Swimming Pools Ltd.

Copyright 2000 Philip H Perkins

1.13 Wave-making machines 1.13.1 Introduction The provision of equipment which generates artificial waves in swimming pools has become increasingly popular in recent years. When the first edition of this book was published in 1971, there was only one pool with wave-making equipment in operation in the UK, namely the large open-air Portobello pool at Edinburgh, which was installed in 1936. During the past 25 years, wave-making equipment has been installed in many new leisure centre pools. The wave-making machines are usually switched on at stated times for about 15–20 minutes. There are several methods of creating artificial waves in swimming pools, the main methods being by (1) swing arm equipment, and (2) compressed air.

1.13.2 Swing arm equipment The makers usually make a model of the pool so that they can assess all important hydraulic features, such as wave height, location of ‘breaking’ point, backwash, cross currents etc. The shape and sloping floor create the effect of a sloping beach with the waves breaking naturally. The shape also provides adequate area of shallow water for non-swimmers. The wave-making equipment consists of two swing arms which operate together but not in complete unison. For example, one arm oscilates at 17.5 oscillations per minute and the other at 18.0 oscillations per minute. A specially designed screen is provided in front of the wings.

1.13.3 Compressed air equipment There are a number of patented systems using compressed air to create artificial waves. The creation of waves of the desired height and distance from crest to crest (wave-length) is not a simple matter and all relevant factors must be taken into account. This usually includes the making of a scale model. Figures 1.13 and 1.14 show pools with a wave machine in operation.

Copyright 2000 Philip H Perkins

RECOMMENDED PROCEDURE FOR GETTING A POOL BUILT: CONTRACTS AND DEALING WITH DISPUTES

1.14 Introduction The recommendations which follow are intended mainly for private persons, club committees and owners of small hotels, although it is hoped that even large hotel groups and local authorities will find some of the points mentioned useful.

1.15 Contracts: how to proceed It is not recommended that a swimming pool should be built on a do-it-yourself basis. Although there is a theoretical saving in capital cost the dividing line between success and failure is a very narrow one and the cost saving does not justify the risk. The two procedures recommended are: 1.

2.

To engage a qualified professional person with proven experience in swimming pool design and construction who will take responsibility for the preparation of the design, drawings, specification and other contract documents, and obtaining all necessary permits. The consultant should recommend a list of, say, three contractors, send out the invitations to tender, recommend to the client the adjudication of the contract, certify the contractor’s accounts, and inspect the work at appropriate stages. The consultant should be a Chartered Civil or Structural Engineer or Chartered Architect. In the case of large contracts for public swimming pools/ leisure centres, there will be several professional firms involved responsible for structural and civil design, heating and ventilating, electrical and mechanical and architectural, and quantity surveyors; the Architect is usually the head of the team. To employ a consultant to advise on the selection of a suitable ‘package deal’ contractor. The names and addresses of swimming pool contractors can be obtained from the Swimming Pool and Allied Trades Association. The consultant, in discussion with the client (referred to as the Employer in the contract), should prepare a clear brief setting out the requirements (see Section 1.2). It is important that the contractors should submit a list of recently completed pools; these should be checked by site visits by the consultant, with the client. The financial standing of the selected contractors should also be checked, and information obtained on the extent the contractors employ sub-contractors. A test for watertightness should be clearly described and included in the

Copyright 2000 Philip H Perkins

contract; see Appendix 2 at the end of this book for details of this test. The consultant’s brief should also include for periodic site inspections to help ensure that the contractor is carrying out the work in accordance with the contract.

1.15.1 Insurance-backed guarantees and warrantees In recent years there have appeared on the market ‘insurance-backed guarantees’. These are offered by contractors/sub-contractors and material suppliers claiming that should the work prove defective, then the insurance company will provide the funds to have remedial work put in hand in the event of the contractor/supplier failing to do so. This suggests that the client will avoid the necessity of legal action to obtain redress. These ‘guarantees’, which are sometimes referred to as ‘warrantees’, are stated to be valid for periods of 10–20 years from the completion of the work. A careful scrutiny of these guarantee/warrantees will usually reveal that they contain many anomalies and uncertainties. Such documents should be examined by a solicitor experienced in that particular field. A consultant would be unwise to recommend reliance on such a guarantee without first taking competent legal advice.

1.16 Dealing with disputes The above may appear to be exaggerated, but experience suggests that caution and attention to detail is the best approach. Irrespective of which procedure is adopted, if things do go wrong, such as work unduly delayed, poor workmanship, the use of sub-standard materials, the failure of the pool to pass the leakage test etc., the building owner will find he is faced with the following limited choice: 1. 2.

He can accept the situation, which he would be most unwilling to do, or He can instruct the contractor to put things right, in accordance with the terms of the contract, and if he fails to do so he can follow the procedure laid down in the Conditions of Contract.

If the faults are serious, it is unlikely that even after completion of the remedial work, the finished job will be as satisfactory as if it had been done properly the first time. One of the worst things that can happen is for the contractor to go into liquidation during the contract. The cost of employing another contractor to complete the project will be very high and the chance of obtaining financial compensation from the original contractor is extremely small. This is why it is important for the consultant not to feel obliged to recommend the acceptance of the lowest tender even though the tenders are from a list which he has drawn up. There can be many reasons why a contractor will submit an

Copyright 2000 Philip H Perkins

exceptionally low price. The client would be unwise to disregard his consultant’s advice in this matter. The value of an experienced consultant, a properly drawn-up contract, and care in the selection of the contractor cannot be over emphasised.

1.16.1 General comments The method of dealing with disputes which may arise during or after completion of a contract will depend mainly on whether the contract comes within the scope of the Housing, Grants, Construction and Regeneration Act 1996 which came into force on 1 May 1998 (known as the Construction Act 1996). If the contract falls outside the scope of the Act then dealing with disputes would follow established procedure. However if the contract falls within the Act, entirely new procedures would have to be followed. Unfortunately, the wording of the Act in a number of important matters is lacking in clarity and at the time of writing this book, there is little reported experience in the operation of the Act. The Act covers a very wide field, but the construction of a private swimming pool is likely to be outside the Act as it would most probably come under the exemption given to residential contracts. The coming into force of the Construction Act has necessitated the revision of the Standard Forms of Building Contract and Sub-Contract, and the ICE Conditions of Contract. The February 1999 issue of Construction Briefing issued by Merricks, Solicitors, London, states: ‘There has been a fundamental review of the legal system in this country over the last three years. A review carried out by Lord Woolf (“Access to Justice”) will culminate this year in major changes to come into effect on 26 April 1999 which will affect the speed and cost of legal proceedings. The changes can be divided into three areas: (a) The restriction of legal aid; (b) The expansion of contingency fee arrangements (‘No win, No fee’) for all proceedings except crime and family; (c) Fundamental changes to court proceedings which should decrease costs and increase speed. In the main, all Personal Injury work will be excluded from the new Legal Aid structure…’ Except in certain cases, ‘the case will be dealt by another route, e.g. a conditional fee arrangement, or via alternative dispute resolution.’ (This change in dealing with personal injury cases is likely to affect claims arising from accidents in swimming pools.) ‘Even before litigation starts there will be changes in the manner in which

Copyright 2000 Philip H Perkins

parties conduct themselves. There will be a series of preaction protocols dealing with various categories of litigation…There is a single objective to the new court rules—to enable the court to deal with the cases justly. Lord Woolf felt that the two evils of modern litigation were delay and disproportionate costs… Expert evidence will be more restricted and may not be adduced without leave of the court. Normally experts will not be allowed to give oral evidence but will provide a written report and answers to written questions put to them by the opposing party…’ There will be considerable control of costs. ‘Prior to taking interlocutory steps details of the costs must be given to the other side…’ All the above changes face the test of practical use and there may well be further changes in the light of experience.

1.16.2 Notes on procedure for contracts outside the construction Act 1996 As stated in Section 1.16.1 above, the situation may arise where the owner is faced with the choice of accepting an unsatisfactory swimming pool or taking legal action against the contractor, and/or the consultant. A Solicitor experienced in construction disputes would be able to advise the client on the appropriate procedure. The action to be taken is generally laid down in the General Conditions of Contract. Action against the consultant could arise if it was considered by the employer’s Solicitor that he had been guilty of professional negligence. The majority of construction contracts (prior to the 1996 Act), contain a provision for referring disputes as a last resort to arbitration. But subject to certain conditions, a party can apply to the Court to have the matter settled by Court action. It is important to remember that Arbitration can be more expensive than Court action as the Arbitrator has to be paid (Arbitrators fees are high), and payment has to be made for the hire of the arbitration room. Costs usually ‘follow the event’ which means that the losing party may have to bear his own legal costs and those of the other party. Court judgments can be quite surprising. This, together with the high cost of litigation, is no doubt why so many disputes are settled out of Court (about 75–80%). In the event of Arbitration or Court action, the employer would be advised by his Solicitor to engage a professional person to act as an Expert Witness. Some information of the duties of an Expert Witness are given in Appendix 4. In Court proceedings, difficult technical considerations can arise if a defence of ‘Limitation’ is put forward. Such a defence is only likely to arise some years after the completion of the pool. A defence of limitation would

Copyright 2000 Philip H Perkins

involve the Limitation Act 1980 and the Latent Damage Act 1984, and the expert witness would be asked for his opinion on the following two issues: 1. 2.

When did significant damage first occur? What was the earliest date on which the Plaintiff had both the knowledge required to bring an action for damages in respect of the relevant damage and the right to bring such action?

Such questions give rise to very complex technical considerations to which there is unlikely to be a clear-cut technical answer. Due to the enormous cost of High Court actions and the very considerable delay which occurs between the time of the issue of the Writ and the handing down of the judgment, proposals have been made in recent years to find alternative methods of settling disputes. This is generally known as ‘Alternative Dispute Resolution (ADR)’. While the majority of cases in the Official Referee’s court and in arbitration settle before trial, few do so early enough to avoid the substantial costs incurred in the preparations leading to trial. The essence of ADR is to create a framework in which the parties involved in a dispute can reach a solution for themselves. This usually requires the assistance of a neutral third party. There are a number of ADR techniques which include: Conciliation; Mediation; Mini-trial; Expert fact finding and adjudication. The success of ADR depends entirely on the willingness of all parties to resolve their dispute in a mutually satisfactory way, and this requires considerable give and take. Some references on ADR are given under Further Reading at the end of this chapter.

1.16.3 Notes on procedure under the construction Act 1996 The comments which follow are intended to supplement those made in Section 1.16.1. The Act gives parties to a construction contract the right to refer a dispute arising under the contract to adjudication in accordance with a clearly defined procedure. The procedure is intended to provide a fast-track method of resolving disputes. A party to a construction contract has a right, but not an obligation, to refer a dispute for adjudication under the procedure laid down in the Act.

Copyright 2000 Philip H Perkins

The Adjudicator must then be appointed within seven days of the issue of a notice by the party wishing for adjudication. The Adjudicator must reach a decision on the dispute within 28 days of referral, unless both parties agree to an extension of time. With the consent of the referring party, the Adjudicator may extend the 28–day period by up to 14 days. The decisions of the Adjudicator are binding until the dispute is finally determined by legal proceedings or by arbitration or by agreement between the parties. The orders of the Adjudicator must be complied with and they are binding until the dispute is finally determined. The Act contains numerous new concepts and conditions and anyone intending to have work carried out by contract after 1 May 1998 should seek legal advice on whether the contract will come within the scope of the Act. Reference can usefully be made to the publication Construction Briefings, issued by Merricks, Solicitors, Chelmsford and London, and Notes of a Seminar on the Construction Act, given by Lawrence Graham, Solicitors, London.

Further reading

General British Sub-Aqua Club. Pools for Sub-aqua Use. Cottam, G. Adjudication under the Scheme for Construction Contracts. Thomas Telford, London, 1998. Department of the Environment. Building Regulations (Amendment) Regulations 1998, S.I. 2561—Revision to Part M, Access and Facilities for Disabled People. Institute of Baths and Recreation Management. Practical Leisure Centre Management, Vol. 2. Institute of Baths and Recreation Management. Diving in Swimming Pools. International Board for Aquatic, Sports and Recreation Facilities. International Standards Swimming Pools: Part B, Construction, Finish and Equipment, 1977. Sports Council. Safety in Swimming Pools, 1998. State of California, Department of Public Health. Laws and Regulations Relating to Swimming Pools, excerpts from the California Health and Safety Code and the California Administrative Code. Swimming Pools and Allied Trades Association. Swimming Pool Guide, 1995.

Construction Act 1996 Merricks, Solicitors. Construction Briefing—The Housing Grants, Construction and Regeneration Act 1996, Merricks, Chelmsford and London, May 1998. Lawrence Graham, Solicitors. Four papers at a seminar on The Housing Grants, Construction and Regeneration Act 1996, Lawrence Graham, London, July 1998.

Copyright 2000 Philip H Perkins

Health and safety and environmental law Fink, S. Health and Safety Law for the Construction Industry, Thomas Telford, London, 1997. Health and Safety Executive. Slips and Trips HS (G) 155. Stubbs, A. Environmental Law in the Construction Industry, Thomas Telford, London, 1998.

Alternative dispute resolution McKenna and Company, Solicitors. Law Letter, Autumn/Winter 1989, McKenna, London, pp. 14–15. McKenna and Company, Solicitors. Alternative dispute resolution, Litigation Update, May 1995, McKenna, London, pp. 6–8. Hollands, D.E. Alternative dispute resolution, Journal CIArb, February 1992, pp. 57–9. Grove, J.B. The role of arbitration in an ADR environment, Journal CIArb, November 1997, pp. 244–5.

The expert witness Newman, P. Professional liability of expert witnesses, Journal CIArb, August 1993, pp. 173–81. Lord Taylor. The Lund lecture—The expert witness, Journal CIArb, May 1995, pp. 113–17. The Times, Law Report: 6 Oct. 1999: Court of Appeal Judgement 27 July 1999; Stevens v Gullis (Pile third party).

EEC construction legislation Department of the Environment. Construction products directive, Euronews, Construction , Special supplement, September 1991. Kay, T. and Wyatt, B. European Standards for protection and repair, J Concrete, September 1997, pp. 11–17. Taylor Joynson Garrett, Solicitors. The Construction (Design and Management) Regulations, 1994, Construction Review, Issue No. 1, 1995.

Copyright 2000 Philip H Perkins

Chapter 2

Basic characteristics of the materials used in the construction of swimming pools

2.1 Introduction The objective of this chapter is to provide information on the principal materials used in the construction of swimming pools, including those used in external works described in Chapter 6. The materials are: Portland cements; Aggregates from natural sources for concrete and mortar; Admixtures; Additions; Water for mixing the concrete/mortar; Steel reinforcement including stainless steel; Spacers; Non-ferrous metals; Curing compounds; Polymers and reactive resins; Joint fillers and joint sealants; Ceramic tiles; Notes on bimetallic corrosion; Notes on British Standards and Euro Codes. The information given in this chapter is intended to be of a general nature and specifiers and users should always refer to the latest edition of the relevant National Standard. Work is continuing at the British Standards Institution on the revision of existing Standards and Codes and the production of new Euro Standards and Codes. It is therefore essential that anyone wishing to incorporate into a contract requirements for compliance with British Standards should ensure that they are still valid and have not been replaced by a Euro Standard. Reference can also be made to BRE Digest 397, September 1994: Standardisation in Support of European Legislation.

Copyright 2000 Philip H Perkins

2.2 Portland cements It is made by burning at high temperature a mixture of chalk and clay in a rotary kiln. The clinker is ground and gypsum (calcium sulphate) is added to control the set. British Standard BS 12 limits the amount of sulphur (expressed as SO ) to 3 3.5%. The fact that Portland cement contains sulphate is important when investigating the possibility of sulphate attack on concrete or mortar which is discussed in Chapter 3. The addition of water to the cement results in a complex reaction accompanied by the evolution of heat. Revised British Standards for cements were published in 1996. The new designations for Portland cements likely to be used for the construction of swimming pools and external works are as follows: Portland cement class 42.5 to BS 12 1996 (CEM 1); Portland cement class 52.5 to BS 12 1996; Portland cement class 42.5R to BS 12 1996; Sulphate-resisting Portland cement class 42.5 to BS 4027 1996; Portland Masonry cement to BS 5224 1995, ENV 413.1. The letter R denotes high early strength. The revisions were mainly concerned with methods of test and terminology and were intended to agree with the European Standard for cement, ENV 197–1. Minor changes in composition were also introduced. In the early 1990s a complete and major revision was carried out to BS 5328 Concrete, and this was issued in four parts:

Part 1 Part 2 Part 3 Part 4

Guide to Specifying Concrete 1995; Methods for Specifying Concrete Mixes 1991; Specification for the Procedures to be Used in Producingand Transporting Concrete 1990; Specification for the Procedures to be Used in Sampling, Testing and Assessing Compliance of Concrete 1990.

In 1993, BSI issued a Published Document PD 6534 1993 Guide to the Use in the UK of DD ENV 206 1992. The principal characteristics of Portland cement are: 1. 2.

A very fine powder, particle size 1–50 microns (1 micron equals 0.0001 mm). The paste (cement and water) is highly alkaline, having a pH of about 13.5. The high alkalinity is relevant to the protection of steel reinforcement, and also to the occurrence of alkali-aggregate reaction. The interaction

Copyright 2000 Philip H Perkins

3. 4. 5.

6.

7.

8.

betweenalkalis in the cement and certain types of silicious aggregates is discussed in Section 3.9. The setting time (initial and final) is in the range of 45 minutes to 10 hours. The compounds which are principally responsible for the cementing action of the cement paste are mainly the calcium silicates (the C2S and the C3S). It is the hydration products of the cement which, other things being equal, determine the strength of the concrete/mortar. The hydration products are very complex chemical compounds. The principal compounds are calcium silicate gel, calcium hydroxide (about 20%) and tricalcium aluminate hydrate. The calcium hydroxide (Ca(OH)2) is liberated by the hydrolysis of the calcium silicates. The various hydration products hydrate at different rates, but the hydration is rapid to start with and then slows down. The three major factors which influence the rate of gain of strength are the chemical composition, the fineness, and the temperature of the hydrating mix. With modern cements, the increase in strength after the first 28 days is likely to be very small and can generally be ignored. The amount of water in the mix (usually referred to as the water/cement ratio, w/c) is a vital factor in determining the strength, permeability, absorption and durability of the concrete/mortar. Generally, other factors being equal, the higher the water/cement ratio the lower the strength and the higher the permeability and absorption. This is why it is often necessary to use a plasticizer in the mix when high quality concrete is required. Due to its high alkalinity, Portland cement is very vulnerable to attack by acids. The reaction between the cement and the acid takes place immediately the two are in contact. See Section 3.5.2.1.

2.2.1 Sulphate-resisting Portland cement Sulphate-resisting Portland cement (SRPC) should be specified as Low Alkali Sulphate-resisting, class 42.5 complying with BS 4027 1996. The cement is similar in its strength and other physical properties to Ordinary Portland cement (OPC), but the tricalcium aluminate content (the C3A) is limited in the relevant British Standard (BS 4027) to a maximum of 3%. It is the C3A which is attacked by solutions of sulphates of various bases. This can have important consequences as the reaction products are expansive in character and is discussed in Section 3.5.2.2. The low alkali content, not exceeding 0.6% equivalent sodium oxide, is useful in minimising the risk of alkali-silica reaction; this is discussed in Section 3.9.

2.2.2 Blended cements Blended cements consisting of mixtures of Portland cement and pulverised fuel ash (pfa) and Portland cement and ground granulated blast furnace slag (ggbs) are used in concrete for special purposes such as reduction of heat of hydration and to

Copyright 2000 Philip H Perkins

improve sulphate resistance. Generally, these additions slow down the rate of gain of strength of the concrete.

2.2.3 Portland masonry cement This should be specified as Portland masonry cement class MC 12.5 complying with BS 5224 1995. It contains an air-entraining agent which increases resistance to freeze-thaw conditions. The inclusion of additives to impart workability and improve water retention are particularly useful. It is used for mortar for brickwork and blockwork and for external rendering.

2.3 Aggregates from natural sources for concrete and mortar The relevant British Standard for concreting aggregates, which is still valid at the time of writing, is BS 882, and covers gravel, crushed rock and sand. The British Standards for fine aggregate (sand) for mortars and external rendering are BS 1199 and BS 1200 1976/1996, and these cover sand for mortar for plain and reinforced brickwork, blockwork and masonry. The Standards should be referred to for their detailed requirements, which include grading limits, flakiness, shell content, and limits on clay, dust and chlorides. Regarding durability, BS 882, Appendix B makes the point that ‘No simple tests for durability and resistance to frost or wear of concrete can be applied; hence, experience of the performance made with the type of aggregate in question and a knowledge of their source are the only reliable means of assessment.’ With sea-dredged aggregates, special attention should be paid to the shell and salt (mainly sodium chloride) contents. In the UK, aggregates from some sources in Scotland and the north of England possess high shrinkage characteristics. When there is any doubt about an aggregate, reference should be made to BS 812 Testing Aggregates; this is in 23 parts published between 1985 to 1995. Part 120 details test methods for determining drying shrinkage of mortar prisms made with the suspect aggregates and recommendations are given for the interpretation of the results. There are different opinions among experienced engineers on the effect of absorption of aggregates on the permeability of concrete used for water-retaining structures. The relevant Code, BS 8007 1987 Code of Practice for the Design of Concrete Structures for Retaining Aqueous Liquids, places a limit of 3% on the absorption of aggregates. However, published information on properly conducted tests which would justify this restriction are conspicuous by their absence. The Standards for sands for mortar are BS 1199 and 1200: Building Sands from Natural Sources.

Copyright 2000 Philip H Perkins

2.4 Admixtures 2.4.1 Introduction An admixture can be defined as a chemical compound that is added in comparatively small quantities to concrete, mortar, or grout at the time of batching or mixing, to produce some desired characteristic in the mix and/or in the mature concrete, mortar or grout. While the use of admixtures in the UK has increased significantly in recent years, this country lags behind continental Europe, the USA and other developed countries. The main types of admixtures in general use are: Water-reducers, plasticizers/workability aids; Superplasticizers; Accelerators; Set retarders; Air-entraining admixtures. The general use of admixtures is covered by various Codes and by BS 5328 Parts 1 and 3 and by ENV 206 (draft European Standard). The ENV puts an upper limit on the use of admixtures in a mix at 5% by mass of the cement and a lower limit of 0.2%. The ENV also requires that when the dosage of admixtures in liquid form exceeds 3 litres/m3 of concrete, this shall be taken into account when calculating the water/cement ratio of the mix. The British Standards are performance specifications. The USA Standard for admixtures for concrete is ASTM C494–86. It should be noted that BS 8110 Structural Use of Concrete refers to pigments as an admixture, but PD 6534 1993 Guide to the Use in the UK of ENV 206 1992 Concrete, clause 4.5 includes pigments under the heading of Additions and this practice has been followed in this book.

2.4.2 Water-reducing admixtures/workability aids/plasticizers For concrete, these admixtures are covered by BS 5075 Part 1. This type of admixture is a compound which increases the workability of a concrete mix with a constant w/c ratio, or permits the w/c ratio to be reduced without reducing the workability of the concrete. It should not increase the air content of the mix. The Standard also covers ‘accelerating water/reducing’ admixtures and ‘retarding water-reducing’ admixtures.

Copyright 2000 Philip H Perkins

2.4.3 Superplasticizing admixtures These admixtures are covered by BS 5075 Part 3, and by the draft European Standard ENV934–2. This type of admixture is a compound which when added to a concrete mix imparts very high workability to the mix, or allows a large decrease in the w/c ratio while maintaining a given workability. The very high workability obtained (150–200 mm) slump ensures that the concrete is virtually self-compacting. However, the super workability only lasts for a limited period, usually in the range of 2–4 hours. In the context of this book, this type of admixture can be very useful for placing concrete in positions where compaction is very difficult, e.g. in members containing congested reinforcement and repairs to honeycombed concrete. The two main basic types of superplasticizers are sulphonated naphthaleneformaldehyde condensates, and sulphonted melamine-formaldehyde condensates.

2.4.4 Accelerators These are covered by BS 5075 Part 1 and draft European Standard ENV 934–2. There is no British Standard for accelerators for mortar and grout. This type of admixture increases the rate of reaction between the cement and water in a concrete mix, and thus accelerates the setting and rate of gain of strength of the concrete. Some accelerators contain chlorides as an active ingredient and the British Standard requires that the chloride content must be stated by the manufacturer. Standards for concrete now strictly limit the chloride ion content of concrete which contains ferrous metals, see BS 5328 Parts 2 and 3.

2.4.5 Set retorders This type of admixture is covered by BS 5075 Part 1. It is a compound that reduces the rate of reaction between the cement and water in a concrete/mortar, thus reduces the rate of setting of the concrete/mortar. The relevant British Standard for set retarders for mortar is BS 4887 Part 2; the draft European Standard is ENV 934–2. The British Standard covers building mortars and rendering, but not mortars for floor screeding. Suppliers of readymixed building mortars make extensive use of this type of admixture.

2.4.6 Air-entraining admixtures This type of admixture is covered by BS 5075 Part 2. It is a compound which when added to a concrete mix incorporates air during the mixing; it should not significantly affect the setting of the concrete. The draft European Standard is ENV 934–2. For mortars, the relevant British Standard is BS 4887 Part 1.

Copyright 2000 Philip H Perkins

The presence of the entrained air increases the resistance of the concrete (and mortar) to frost attack (freeze-thaw conditions). However, there is a reduction in the compressive strength compared with a control mix of the same mix proportions and W/C ratio; this is important in concrete and should be allowed for. The actual reduction in compressive strength depends on a number of factors, but a figure of 4% for each 1% of entrained air is often used as a guide. The size of the bubbles of entrained air is about 50 microns (0.05 mm). It should be noted that with cement contents in excess of about 350 kg/m3 difficulties are likely to arise in entraining the air. This type of admixture is normally specified for all external insitu concrete paving to prevent damage by frost and freeze—thaw conditions.

2.5 Additions 2.5.1 Introduction These are materials which are added to a mix (concrete or mortar or grout) in much larger quantities than admixtures. The reason for the use of Additions is similar to that for the use of admixtures, namely to impart some desirable characteristics to the mix and/or the concrete, mortar or grout. The materials described below are generally considered as Additions.

2.5.2 Pulverized fuel ash Pulverized fuel ash (pfa) is covered by BS 3892 Part 1 1997 Pulverized-fuel Ash for Use in Concrete, and European Standard BS EN 450 1995 Fly Ash for Concrete—Definitions, Requirements and Quality Control. Pulverized fuel ash is also classed as a cement replacement and in fact that is its principal use in the concrete industry. The material is a by-product of pulverized coal-fired electricity generating stations. It is a fine powder, and the approximate composition is: 50% silicon (SiO ); 2 28% alumina (Al O ); 2 3 11% iron oxide (Fe O ); 2 3 11% oxides of calcium, magnesium, sodium and potassium. British Standard BS 3892 Part 1 covers pfa for use as a cementitious compound in structural concrete. Part 2 covers pfa for use in grouts (excluding grout used in prestressing ducts) and for miscellaneous uses in concrete. The two main differences in the BS EN 450 compared with BS 3892 lies in the permitted fineness; the former allows a much coarser material. The increase in fineness results in a decrease in strength (other factors being equal). This is discussed in some detail in Research Focus No. 31, November 1997.

Copyright 2000 Philip H Perkins

The main advantages claimed for the inclusion of pfa in concrete are: 1. 2. 3. 4. 5.

reduction in heat of hydration; improved workability with constant w/c ratio; increased resistance to sulphate attack; reduced permeability to liquids; long-term increase in compressive strength.

There appears to be some reason to believe that the inclusion of pfa in the concrete may render it more resistant to alkali-silica reaction. Reference should also be made to BS 6588 Specification for Portland Pulverized Fuel Ash Cements which lays down requirements for composition, strength, chemical and physical properties for two combinations of Portland cement and pfa. While the presence of pfa in hardened concrete can be determined by microscopic examination of thin sections, it is not possible by chemical analysis to determine the proportion of pfa in concrete, mortar or grout. The pfa content in fresh concrete can be determined by the chemical method described in BS 6610 Specification for Pozzolanic Pulverized Fuel Ash Cement, or by the particle density method described in Annex D in Part 128 of BS 1881 Methods for the Analysis of Fresh Concrete.

2.5.3 Ground granulated blastfurnace slag This material should comply with BS 6699 Specification for Ground Granulated Blastfurnace Slag (ggbs) for use with Portland cement. The slag is a waste product produced in steelworks. It can be used as an aggregate for concrete or as an addition to Portland cement for concrete. When used in combination with OPC, it increases the resistance of the concrete to sulphate attack, and to alkali-silica reaction by limiting the alkali content of the binder (cement plus ggbs). It also reduces the heat of hydration. The proportions used with OPC depends on the required characteristics of the hardened concrete. Generally, mixes containing 40% ggbs and 60% OPC, to 65% ggbs and 35% OPC are used.

2.5.4 Condensed silica fume Condensed silica fume is used as an Addition in concrete, although there is no British Standard for this material at the time this book was written, but a European Standard (an EN) is in course of preparation. Condensed silica fume is a waste product of the ferrosilicon industry. It consists of about 88% silicon dioxide (SiO ) with very small percentages of carbon, ferric 2 oxide, aluminium oxide (alumina) and oxides of magnesium, potassium and sodium. It is a very fine greyish powder with a specific surface about fifty times that of normal Portland cement and is a highly reactive pozzolan.

Copyright 2000 Philip H Perkins

The addition of condensed silica fume to a concrete or mortar mix has a significant effect on the properties of the plastic mix as well as on the hardened concrete or mortar. The dosage is usually in range of 2% to 10% by mass of the cement, and it imparts a number of beneficial characteristics to the concrete/mortar; these are: 1. 2. 3. 4. 5.

increased cohesion; reduced permeability; increase in compressive strength; increase in resistance to sulphate attack, except possibly ammonium sulphate; increase in resistance to a number of aggressive chemicals.

The very small particle size increases the water demand of the mix and can result in premature stiffening if placing and compaction is delayed. It is normally used with a superplasticiser. In the UK and USA, it is mainly marketed as a stabilised slurry which contains a plasticiser or superplasticiser. The Agrement Certificate for a proprietary slurry marketed in the UK states that the pH is 5.5 plus or minus 1.0. It is not possible to determine by chemical tests the percentage of condensed silica fume in a mix.

2.5.5 Pigments The relevant British Standard is BS 1014 Pigments for Portland Cement and Portland Cement Products. The USA Standard is ASTM C979–82. Table 1 of BS 1014 lists seven pigments of which four are oxides of iron, one is carbon black, one is chromic oxide and one is titanium dioxide. The principal pigments used are the oxides of iron and are in the form of very fine powders, having a particle size of about 0.1 microns. This can be compared with Portland cement 1.0–50.0 microns and sand 150–5000 microns (1 micron= 0.001 mm). Some comments on their use are in Section 6.2.

2.6 Water for mixing concrete, mortar and grout The relevant British Standard is BS 3148 Methods of Test for Water for Making Concrete. Water used for mixing concrete, mortar and grout should be free from compounds which adversely affect the setting and hardening of the mix and/or have an adverse effect on the properties of the hardened concrete/mortar/grout. The impurities may be organic or inorganic. Sulphates in solution (as SO ) should 3 not exceed 1000 ppm. Water which is fit for drinking (potable water) is suitable for making concrete/ mortar/grout, but water which is unfit for drinking may be quite suitable.

Copyright 2000 Philip H Perkins

2.7 Steel reinforcement Steel reinforcement for concrete is covered by the following British Standards:

BS 4449 BS 4482 BS 4483 BS 4486 BS 5896 BS 7295 BS 6744

Carbon Steel Bars for the Reinforcement of Concrete; Cold Reduced Steel Wire for the Reinforcement of Concrete; Steel Fabric for the Reinforcement of Concrete; Hot Rolled and Processed, High Alloy Steel Bars for the Prestressing of Concrete; High Steel Wire and Strand for the Prestressing of Concrete; Fusion Bonded Epoxy Coated Carbon Steel Bars for the Reinforcement of Concrete; Austenitic Stainless Steel Bars for the Reinforcement of Concrete.

The coefficient of thermal expansion of plain carbon steel is 12×10-6. All reinforcing steel should meet the tests prescribed by CARES (UK Certification Body for Reinforcing Steels) and purchasers should check that suppliers hold the Quality Assurance Certificate issued by CARES.

2.7.1 Galvanised reinforcement At the time of writing, there is no British Standard specifically for galvanized reinforcement, but there is a Standard BS 729 for hot dipped galvanized coatings for iron and steel articles. The author is indebted to the Galvanisers Association for the following information. Galvanised reinforcement was first used in Bermuda in the 1930s; it became widely used during the war years when sea-dredged aggregates were used for concrete structures. In the UK, it appears to be mainly used in precast concrete units for large building projects. When Portland cement concrete/mortar is placed around galvanised rebars, there is a chemical reaction between the zinc coating and the calcium hydroxide in the hydrating cement paste. The zinc surface is passivated with the evolution of hydrogen. The passivation occurs with the initial formation of a layer of zinc hydroxide; further chemical reactions follow, resulting in the formation of a complex stable zinc compound, zincate. For durability in aggressive conditions, it is essential that this passivated film on the zinc be undamaged. The protection of the steel provided by the zinc coating is mainly dependent on the thickness of the coating and therefore the thickness should be specified to meet the anticipated exposure conditions.

Copyright 2000 Philip H Perkins

The presence of a very small concentration of chromate (about 20 ppm) in the cement will inhibit the reaction between the cement paste and the zinc and thus limit the formation of hydrogen.

2.7.2 Fusion-bonded epoxy coated reinforcement This method of protecting steel reinforcement from corrosion has been in use in the USA since the early 1970s, but in the UK its acceptance has been much slower. The American Standard is ASTM A775 Standard Specification for Epoxy-coated Reinforcing Bars. The UK Specification is BS 7295 Parts 1 and 2. The coating is an epoxy powder specially formulated to resist impact and abrasion, and to possess a sufficient degree of flexibility to accommodate bending stresses in the bars and to possess high bond to the bars. The epoxy resin is defined in BS 7295 as a thermosetting epoxy powder consisting mainly of epoxy resin plus curing agent and pigments. Two conditions are paramount for the coating to protect effectively the steel rebars: 1. 2.

high bond strength to the surface of the rebars; toughness to reduce damage to the coating during transport and fixing and this includes an adequate coating thickness which latter should be in the range 130–300 microns.

2.7.3 Stainless steel reinforcement The relevant British Standard is BS 6744 Specification for Austenitic Stainless Steel for the Reinforcement of Concrete. Of the three basic types of stainless steel, martensitic, ferritic and austenitic, the latter steel types 302, 315 and 316 are by far the most resistant to attack by concentrations of chlorides. Tests by the Building Research Establishment, UK, have shown that austenitic steel embedded in concrete containing 3% chloride by mass of the cement, showed no sign of corrosion after 17 years. Type 316 steel contains 18% chromium, 10% nickel, and 3% molybdenum. However, even with this steel if it is exposed to warm humid conditions and is very highly stressed, corrosion can occur. This was illustrated by the failure of stainless steel hangars supporting a reinforced concrete ceiling slab in a swimming pool in Switzerland in 1985. Stainless steel is much more expensive than ordinary carbon steel and its use for reinforcement is only justified in special cases. The coefficient of thermal expansion of austenitic stainless steel 18×10-6 compared with 12×10-6 for carbon steel.

Copyright 2000 Philip H Perkins

2.8 Spacers The use of correct spacers is an essential part of the construction of reinforced concrete. There is no British Standard for spacers, but in 1991 the Concrete Society published a manual Spacers for Reinforced Concrete. Spacers are used to ensure correct cover to the rebars. Cover less than that required by the relevant Code of Practice can increase considerably the risk of corrosion of the rebars resulting in premature deterioration of the concrete member. Spacers are normally made of plastics, but some are made of fibrereinforced cement based material which has the advantage of bonding to the surrounding concrete.

2.9 Non-ferrous metals Only a limited number of non-ferrous metals are likely to be used in the construction and fitting-out of swimming pools, and these are phosphor-bronze, gunmetal and copper.

2.9.1 Phosphor-bronze Bronze is an alloy of copper and tin, and phosphor-bronze contains phosphorus as copper phosphide. These alloys are corrosion resistant and are used for fittings and fixings. The coefficient of thermal expansion is about 20×10-6. Reference should be made to the comments in Section 2.10.

2.9.2 Copper Copper is resistant to most conditions met in building construction. For many years it was used as water bars in concrete water-retaining structures but has been entirely superseded by PVC. Copper is corroded by solutions of chlorides and by solutions of ammonium salts which latter may be present in some organic adhesives used for floor coverings. BS 2870 1980 covers copper and copper alloys for sheet, strip and foil. Reference should be made to the comments in Section 2.10.

2.10 Bimetallic corrosion When using dissimilar metals, it is important to ensure that either they are not in contact with each other, or that bimetallic corrosion will not take place. Comprehensive recommendations on this subject are contained in a BSI publication, PD 6484 1979 Commentary on Corrosion at Bimetallic Contacts and its Alleviation. For example, mild steel can be seriously corroded if it is in contact with copper, phosphor-bronze or stainless steel (Figure 2.1).

Copyright 2000 Philip H Perkins

Figure 2.1 Corrosion of mild steel rebar caused by direct contact with phosphor-bronze tie.

2.11 Curing compounds for concrete and mortar The efficient curing of concrete and mortar is essential to ensure strength, resistance to shrinkage cracking, and resistance to abrasion, and long-term durability. Materials used for curing are in three forms: Spray-applied membranes; Sheet materials; Wet/water curing.

2.11.1 Spray-applied membranes The relevant British Standard for testing spray-applied membranes is BS 7542 Methods of Test for Curing Compounds for Concrete. The USA Standard is ASTM 309–81 Standard Specification for Liquid Membrane-forming Compounds. These compounds are generally either water-based or resin-based and should be applied as soon as possible after completion of compaction and finishing. When

Copyright 2000 Philip H Perkins

concrete is cast in formwork, curing should commence as soon as the formwork is removed. Many of these compounds gradually weather away in the course of time, but if it is intended to apply a coating or other layer to the concrete (e.g. rendering or bedding for tiles etc.) then the suppliers of the curing compound should be consulted as most curing compounds adversely affect bond at the interface with the base concrete.

2.11.2 Sheet materials The principal sheet material used for curing concrete is polyethylene sheeting (trade name polythene). This is very effective in reducing moisture loss provided it is laid as soon as practical after casting the concrete or mortar, and the sheets are held down around the edges, and kept in position for at least four days. Also, 1000 gauge sheeting should be used; this gauge material is 250 microns (0.25 mm) thick.

2.11.3 Wet/water curing The curing of concrete and mortar by water spray is only carried out in special cases mainly when it is required to keep the temperature of the concrete under control. It is specifically recommended for high alumina cement concrete.

2.12 Polymers The term polymers includes a wide range of materials, but in the context of this book they are materials used in a concrete or mortar mix to provide some desirable characteristics to the mix, such as: 1. 2. 3. 4. 5.

improved workability of the mix with constant w/c ratio or reduced w/c ratio with constant workability; increased bond with the substrate; reduced permeability and absorption; improved resistance to carbonation; some limited increase in resistance to chemical attack.

They are mainly available in liquid or powder form. The liquids are dispersions (also referred to as latexes) and are generally whitish in colour. The solid content and the viscosity vary; the solid content is usually in the range of 40–70%. The polymers in most general use include: Modified polyvinyl acetates (PVAs); Ethylene vinyl acetate (EVAs); Acrylics; Styrene butadiene rubber (SBR); Styrene acrylics.

Copyright 2000 Philip H Perkins

When these polymers are added to a mix, the proportions recommended by the suppliers should generally be followed. In the case of SBRs, the proportions are usually in the range of 15–25% by mass of cement, depending on the reason for the addition. It is assumed that the dispersion weighs 1 kg/litre. All polymers are expensive and so the amount used justifies careful consideration.

2.13 Reactive resins These materials are generally used for protective coatings, and when mixed with selected fine aggregates, for thin bonded repairs. The main resins used for mortars for the repair of concrete are: Epoxies; Polyesters; Polyurethanes.

2.13.1 Epoxy resins These resins are by-products of the petrochemical industry. The basic resin is a liquid with a fairly high viscosity and will remain in this condition almost indefinitely. For use, it must be mixed with a hardener/accelerator. The hardener reacts chemically with the epoxy and changes it from a liquid to a solid. Adequate mixing by mechanical means is essential to ensure effective dispersion of the hardener in the epoxy. The great advantage with epoxies is that they can be formulated to suit particular conditions of application and end use. The principal properties of epoxy resins are: 1. 2. 3. 4. 5. 6. 7.

high bond strength to many materials, with special reference to concrete and steel; very low shrinkage during curing; high resistance to a wide range of chemicals; high resistance to water penetration; high compressive, tensile and flexural strength when used with selected aggregates; coefficient of thermal expansion of sand-filled epoxy is about three to four times that of concrete made with natural aggregates; epoxies suffer loss of compressive strength with increase in temperature; with temperatures above about 75 °C the loss of strength can be considerable.

Copyright 2000 Philip H Perkins

2.13.2 Polyester resins These resins are similar in many respects to epoxies; the main differences are: 1. 2. 3.

coefficient of thermal expansion is about 1.5 times that of epoxies; shrinkage during curing is appreciably higher than that of epoxies; the shelf life of the basic resin is strictly limited. The curing of polyester resins is adversely affected by the presence of moisture.

2.13.3 Polyurethane resins These resins are used mainly for floor sealants and protective coatings. There is a large variety of polyurethane resins, all of which possess specific qualities; it is therefore important when specifying these resins that the performance characteristics be clearly defined. They have a number of excellent characteritics including flexibility, resistance to abrasion and resistance to chemical attack. They bond well to concrete and mortar, but the surface of the substrate must be dry to avoid formation of blisters.

2.14 Joint fillers These materials are used in ‘in-house’ design joints and are sometimes referred to as back-up materials. They provide support to the sealant, but should not bond to it; they also help prevent the entry into the joint of stones and debris during construction as the sealant is usually applied later in the contract. The materials used for joint fillers should fulfil the following requirements: 1. 2. 3. 4. 5.

It must be durable under service conditions; ideally, the service life should be the same as that of the structure in which it is inserted. It must be chemically inert and non-toxic. It should be resilient but should not extrude so as to interfere with the sealant. It should not bond to the sealant; if it is liable to do so, a bond breaker should be used. It must be formed easily and be inserted readily into the joint.

The main materials used for joint fillers include: 1. 2. 3.

cork granules bonded in a resin which is resistant to long-term immersion in water; wood fibre with bitumen (not suitable for use in damp conditions); cork granules bonded in bitumen (may be unsuitable for use in contact with potable water).

Copyright 2000 Philip H Perkins

2.15 Joint sealants These materials can be divided into two basic groups: Insitu compounds; Preformed compounds. Materials in both groups should possess the following characteristics: 1. 2. 3. 4.

For external use and in liquid retaining structures, the material must be virtually impermeable. Ideally, the service life should be the same as that of the structure in which it is used. It must bond well to the side of the joint, but should not bond to the joint filler (a bond-breaker may be required). It should deform in response to the movements in the structure without extruding and without losing its integrity.

For detailed advice on the use of sealants in structures, reference should be made to BS 6213 Guide to the Selection of Constructional Sealants. This document deals mainly with the selection of insitu sealants. Table 4 indicates that the only sealant recommended as suitable for use in swimming pools is a flexible epoxy. Sealants in joints in swimming pools have to function in particularly severe conditions and consequently few materials can be relied upon to give satisfactory service. Another relevant British Standard is BS 6093 Code of Practice for Design of Joints and Jointing in Building Construction.

2.15.1 Insitu compounds The insitu sealants can be divided into two main classes: Hot applied sealants; Cold applied sealants (pouring-grade and gun-grade). As far as swimming pools and ancillary structures are concerned, the sealants in general use are the gun-grade sealants. The relevant British Standards are:

BS BS BS BS

5215 5889 4254 5212

Copyright 2000 Philip H Perkins

One-part Gun-grade Polysulphide-based Sealants; One-part Gun-grade Silicone-based Sealants; Two-part Polysulphide-based Sealants; Cold-applied Joint Sealant Systems for Concrete Pavements.

The first three Standards listed above have been declared obsolete and not replaced at the time of writing this book. In the circumstances, it is recommended that reference be made to International Standards Organisation, Standard ISO 11600 Building Construction Sealants, Classification and Requirements.

2.15.2 Preformed sealants Preformed sealants suffer from one practical disadvantage, namely the sides of the joints have to be smooth and even, as the preformed sealant does not accommodate itself well to out-of-true surfaces. This may require that the sides of the joint have to be trued-up with an epoxy resin mortar; cement/sand mortar is not recommended for this type of repair. Neoprene and EPDM (ethylene-propylene diene monomer) are particularly resistant to a wide range of chemicals and this makes them suitable for use in floors of stores holding chemicals for water treatment and in swimming pools.

2.16 Ceramic tiles Ceramic tiles are covered by BS 6431–EN87 Ceramic Wall and Floor Tiles, which is published in 23 parts. There are two basic categories of tiles used for swimming pools in the UK. The difference arises mainly from the method of manufacture, and they are usually described as pressed tiles and extruded tiles. In each category, there are many divisions and full details can be obtained from the manufacturers. Generally, the pressed tiles are thinner and the body of the tile is relatively more absorbent, but the dimensional tolerances are smaller so that there is very little variation in the declared size of the tiles. This means that the joints can be narrower. Extruded tiles are thicker and the body is wholly or partly vitrified, and dimensional tolerances are larger. This results in the absorption being much lower and the joints being wider. The thicker tiles generally have deeper indentations or ‘frogs’ on the back. Generally, extruded tiles with very low absorption are recommended for swimming pools. For outdoor pools the tiles must be frost resistant. Further information on ceramic tiles is given in Section 7.6.

2.17 British standards and Euro codes Frequent reference is made in this book to British Standards and Codes of Practice, and to Euro Codes. Theoretically, once a Euro Code or Standard has been finally approved and published, the equivalent British Code or Standard will be withdrawn. However, in practice, it is likely that certain sections of some of the British Codes and Standards will be retained on the grounds that they have special application to conditions in the UK.

Copyright 2000 Philip H Perkins

The following terminology is in general use: CEN CPD EN prEN prENV

European Committee for Standardisation; Construction Products Directive; European Standard; Draft European Standard; Draft European PreStandard.

References American Society for Testing Materials (ASTM): Portland Cement, C150. Steel Wire Plain for Reinforcement of Concrete, A82. Steel Welded Wire Fabric for Reinforcement of Concrete, A185. Steel Wire Deformed, for Reinforcement of Concrete, A496. Aggregates for Concrete, C33. Chemical Admixtures for Concrete, C494. Pigments for Concrete, C979. Epoxy Coated Reinforced Bars for Concrete, A775. Silica Fume, C1240.

British Standards Institution. Building Sands From Natural Sources, BS 1199 and 1200. British Standards Institution. Corrosion-resistant Stainless Steel Fastners, BS 6105. British Standards Institution. Rolled Copper and Copper Alloy Sheet, BS 2870.

Further reading American Concrete Institute. Chemical Admixtures for Concrete, ACI Committee 252, 1991. American Concrete Institute. Superplasticisers for Concrete, ACI Committee 252, 1993. American Concrete Institute. Aggregate for Concrete, ACI Committee 211, 1997. Brown, B. Aggregates for concrete, Concrete, May 1998, pp. 12–14. Concrete Society. Polymers in Concrete, Technical Report No. 39, 1994. Concrete Society. Guidance on the Use of Stainless Steel Reinforcement, Technical Report No. 51, November 1998. Dennis, R. Polymer dispersions, in Construction Materials Reference Book, Ed. D.K. Doran, Butterworth Heinemann, 1992. Ryle, R. Technical aspects of aggregates for concrete, Journal of Quarry Management, April 1988, pp. 27–31. Walters, D.G. Comparison of latex-modified Portland cement mortars, ACI Materials Journal, July/August 1990, pp. 372–7.

Copyright 2000 Philip H Perkins

Chapter 3

Factors affecting the durability of reinforced concrete and cement-based materials used in the construction of swimming pools 3.1 Introduction When one considers the need for a structure to be durable the following questions arise: 1. 2. 3.

What maintenance is likely to be required? The time lapse between construction and the need for repairs? What is the likely useful life of the structure before partial or complete replacement is considered appropriate?

The following terms are important when dealing with the above questions: 1.

2. 3. 4.

Durability. A material can be considered durable if it fulfils its intended duty for the whole of its design life with an acceptable amount of maintenance including general repair. Design Life. This is the length of time which the designer estimates the material will remain durable. Service Life. This is the actual length of time the material remains durable. Maintenance. The work and materials which when applied to a structure enables the structure to fulfil its duty during its service life. Maintenance should include cleaning, minor repairs, decorating and replacing parts when required.

This is a very important subject and guidance is given in two British Standards to which reference should be made:

BS 7543 BS 8210

Copyright 2000 Philip H Perkins

Guide to the Durability of Buildings, Building Elements and Components; Guide to Building Maintenance Management.

The suggestions/recommendations put forward are linked to the anticipated level of maintenance (including repair) applicable to the type of structure under consideration. Unfortunately, the two publications referred to above do not include specific reference to swimming pools, but the basic principles are applicable. The main causes of deterioration of reinforced concrete and the commonly used cement-based materials are discussed in this chapter, with particular reference to swimming pools. Consideration should be given to how maintenance and repairs can be carried out. This is particularly important with swimming pools as generally these are constructed in the ground and inspection and repair to the pool shell can be difficult and disruptive to the use of the pool. It is necessary to distinguish between the causes of the deterioration of the concrete and of the steel reinforcement. The corrosion of steel reinforcement is the most serious durability problem affecting concrete structures; in other words, steel reinforcement is the ‘Achilles Heel’ of reinforced concrete. However, in swimming pool shells which are finished on the inside with rendering, screed and tiling/high-quality decorative coating, the chance of reinforcement corrosion is reduced. Unfortunately, concrete and cracking are often closely associated in peoples’ minds. Cracks can result in corrosion of steel reinforcement due to the admission of moisture, air and agressive chemicals in solution. For this to occur the crack must be of a certain minimum width at the interface of the concrete and the rebars. It is usually assumed that if the width of the crack at the surface of the concrete is not more than 0.3 mm wide, corrosion is unlikely to occur; this is valid if the concrete is good quality and the cover to the rebars is adequate taking into account the exposure conditions. Cracks which penetrate right through the wall or floor of a swimming pool are likely to form a source of leakage and need attention. However, even in these circumstances, very fine cracks may be self-healing (known as autoginous healing). The types of cracks which normally occur in the floor and walls of swimming pools are discussed in Chapters 4 and 5. Remedial work to cracks which are found to be a source of leakage is described in Chapter 10.

3.2 Corrosion of steel reinforcement in concrete 3.2.1 Introduction The corrosion products of steel, known generally as rust, consist of oxides of iron. For rusting to occur, moisture and oxygen must be present and for steel embedded in the vast majority of concrete structures, both are present to a greater or lesser degree. Some chemicals used in water treatment are acidic in solution, e.g. sodium bisulphate, and aluminium sulphate; see Section 3.7. Steel does not corrode when it is surrounded with concrete or cement-based mortar, unless special factors are

Copyright 2000 Philip H Perkins

present. The high alkalinity of the cement paste passivates the steel due to the formation of a protective film of oxidation products, such as ferric oxide. As long as this film is maintained, further oxidation of the steel is inhibited, and the steel does not corrode. It therefore follows that subsequent corrosion (oxidation) of the steel must be due to a breakdown of the passivating film on the surface of the steel. This loss of passivation can be caused by a number of factors, of which the principal are set out below: 1. 2.

3. 4.

5.

physical damage to the concrete surrounding the steel, resulting in exposure of the steel; development of cracks in the concrete extending down to the rebars, of sufficient width to allow the ingress of moisture, oxygen, or aggressive chemicals, e.g. chlorides. Such cracks are normally due to shrinkage and/or stress. See Sections 3.2.2.1–3.2.2.3; high permeability/porosity of the concrete surrounding the rebars allowing ingress of moisture etc.; inadequate thickness of the cover coat of concrete or mortar. The recommended thickness/depth of cover will depend on the exposure conditions and the permeability/porosity of the cover coat. See BS 5328 Part 1, with special reference to Table 6. The presence in the concrete of chlorides in excess of the ‘safe’ recommended concentration as laid down in Standards and Codes of Practice. See BS 5328 Part 1 Clause 4.2.2.

Carbon dioxide is present in the air, and while it does not damage the concrete it lowers the pH of the cement paste to about 9.5 which can result in depassivation of the steel; this is known as carbonation; see Section 3.3.

3.2.2. Causes of cracking in concrete All concrete contains micro-cracks and these do not adversely affect the performance of the concrete. However, macro-cracks, if they extend down to the rebars, can result in loss of passivation leading to corrosion of the rebars. The main types of cracks likely to be found in swimming pool shells and associated structural members are: Shrinkage cracks; Flexural cracks; Thermal contraction cracks.

3.2.2.1 Shrinkage cracks Shrinkage cracks can result from a high water/cement ratio, or the use of shrinkable aggregates (see Section 2.3), and/or inadequate curing of the concrete. Comments on water/cement ratio and curing are given in Sections 4.5 and 4.13.

Copyright 2000 Philip H Perkins

3.2.2.2 Flexural cracks The width of this type of crack is limited in BS 8110 to 0.3 mm; the width is measured at the surface of the member and decreases in width with depth. It is considered that cracks of this type and width are unlikely to result in corrosion of the rebars. See also BS 8007 1987 Code of Practice for the Design of Concrete Structures for Retaining Aqueous Liquids, which is the main Code for the design of swimming pool shells.

3.2.2.3 Thermal contraction cracks This type of crack is not uncommon in reinforced concrete swimming pool walls. They are caused by the cooling of the concrete after the removal of the formwork when inadequate distribution steel has been used to control this type of cracking. This is discussed in Section 4.9.

3.3 Carbonation of concrete Carbon dioxide in the air reacts with the calcium hydroxide in the hydrating cement paste to form calcium carbonate: Ca(OH) +CO =CaCO +H O 2

2

3

2

The reaction results in a significant reduction in the pH of the cement paste (from about 12.5 to about 9.5). The surface of concrete exposed to the air carbonates very rapidly, forming a carbonated layer of micron thickness. In good quality concrete, the rate of penetration is very slow and depends on many factors, the principal ones being porosity, permeability and moisture content. The floor and walls of the majority of swimming pools are not exposed directly to the air once the back-filling has been completed. It is therefore reasonable to assume that the carbonation of the concrete shell is unlikely to endanger the reinforcement during the lifetime of the pool.

3.4 Chloride-induced corrosion of reinforcement 3.4.1 General considerations Corrosion of rebars can occur in un-carbonated concrete due to the presence of chlorides. Chlorides are present either because they were added to the concrete mix, or were present in the aggregates, and/or the mixing water, or they had penetrated into the hardened concrete from an outside source. When present, chlorides are in solution in the pore water. When a salt is dissolved in water it is immediately split up into electrically charged particles known as ions:

Copyright 2000 Philip H Perkins

NaCl=Na++ClIt is the negatively charged chloride ions which destroy the passivity (the layer of ferric oxide) on the surface of the rebars. In practice, the chloride ions present in the pore water exist in two forms, free chloride ions and combined chloride ions. The combined chloride ions are combined with the hydration products in the cement paste, mainly the tricalcium aluminate (C3A). It is generally agreed that it is the free chloride ions which damage the passivity of the steel, resulting in the corrosion of the rebars. It can thus be seen that the higher the concentration of C3A in the hydrating cement paste, the higher the percentage of chloride ions which will be ‘locked up’ and not free to attack the steel. From the point of view of dealing with potential chloride attack on reinforcement, the use of a cement with a high C3A content is to be preferred. Ordinary Portland cement has a C3A content in the range 8–12%, compared with sulphate-resisting Portland cement which has a C3A content not exceeding 3.5% (see BS 4027). The formation of rust by chloride attack can cause cracking and spalling of the concrete due to the considerable increase in volume of the steel when it is converted into rust (an increase of three to four times the original thickness of steel). There are two main types of corrosion of rebars, general corrosion and local corrosion (pitting). The general corrosion is more likely to cause cracking and spalling of the concrete, but local corrosion can be more serious due to significant reduction in the diameter of the rebars at the ‘pits’. These pits may penetrate the rebars by more than 50% of the bar diameter; even a careful visual examination of the concrete may not detect localised/pitting corrosion. Localised/pitting corrosion is more likely to be the result of chloride ions in the concrete in contact with the steel than carbonation of the concrete in contact with the rebars.

3.4.2 Chlorides in swimming pool water Concern is sometimes expressed about the durability of reinforced concrete in continuous contact with chlorinated swimming pool water. A careful literature search has not revealed any authorititive research or detailed study of this problem. This suggests that significant corrosion of steel reinforcement in pool shells due to chlorides introduced into the pool water for disinfection of the water has so far not been detected and/or has not caused serious concern. However, a brief discussion on the subject is considered justified. A great deal has been written about the durability of reinforced concrete marine structures, and consideration of the characteristics of sea water is worthwhile as this water contains a high percentage of dissolved salts, mainly chlorides. Comparatively few swimming pools contain sea water; the majority of pools in the UK contain water of drinking quality. For reasons of hygiene, the water in the pool is filtered and treated with various chemicals including a disinfectant. The

Copyright 2000 Philip H Perkins

disinfectant in most general use in public swimming pools in the UK is chlorine which is generally produced by dosing the water with sodium or calcium hypochlorite. Sea water contains a high percentage of dissolved salts. Figures given by Lea, in The Chemistry of Cement and Concrete, 3rd edition, are quoted below:

This Table can be converted to:

The dosage of chlorine into swimming pool water depends on a number of factors as the aim is to maintain the ‘free chlorine residual’ at about 0.5–1.0 ppm. If it is assumed for the purpose of this discussion that the actual dosage of chlorine is as high as 5 ppm, when the bathing load is heavy, then it can be seen that the concentration of chloride ions in swimming pool water is less than 0.01% of that in sea water. On this basis, the chance of chloride-induced corrosion of reinforcement occurring in a properly constructed swimming pool shell can be considered as insignificant, and can be reasonably disregarded until authentic research proves otherwise.

3.5 Deterioration of the concrete 3.5.1 Physical damage The usual causes of physical damage to concrete structures are unlikely to be relevant to swimming pools with the possible exception of open-air pools, which could be in possible danger from frost (freeze-thaw conditions). However, all swimming pools are provided with a finish, i.e. a decorative coating or tiling/mosaic which would protect the concrete from the effects of frost, although the finish itself may be damaged.

Copyright 2000 Philip H Perkins

3.5.2 Chemical attack on the concrete This is a wide field for discussion as concrete is vulnerable to a range of chemical compounds in solution. However, concrete swimming pools are only likely to come into contact with a limited range of aggressive agents and these are discussed below.

3.5.2.1 Acids and industrial chemicals in the subsoil All Portland cement-based materials will be attacked by acids (acidic solutions). Generally speaking, inorganic acids such as the three listed below are more aggressive than organic acids: Sulphuric acid (H SO ); 2 4 Hydrochloric acid (HCl); Nitric acid (HNO ). 3

These are only likely to be found in sub-soil contaminated by use as an industrial tip and should be detected by a proper site investigation, see Section 4.2. Some salt solutions are acidic, e.g. sodium bisulphate, and alum which are used in the treatment of swimming pool water to adjust the pH; this is referred to in Section 3.7. Organic acids are present in moorland water and if this class of water is used in a swimming pool without pretreatment, quite significant attack on exposed concrete and cement-based mortars can take place. Acidic solutions have a pH below the neutral point of 7.0 and alkaline solutions have a pH above 7.0. A solution with a pH of 5.0 has 100 times the hydrogen ion concentration than a solution with a pH of 7.0. The pH of a liquid can be readily measured by the use of indicator papers or by a pH meter. The pH alone does not define the type nor the amount of acid present; it measures the intensity of the acidity. It is important to remember that the same concentration in solution of different acids will give different pH values.

3.5.2.2 Sulphates in solution As far as the reinforced concrete shell of the pool is concerned, the most likely source of sulphate in sufficient concentration to cause deterioration of the concrete is from sub-soil and ground water. However, it must be pointed out that if close control of the addition of chemicals to the pool water is not adhered to, sulphates can build up in the pool water and attack mortar joints between tiles, the tile bedding, and the screed and rendering. See Section 3.6 and Chapter 8. An exception is where the concreting aggregates are contaminated by sulphates, e.g. the Arabian Gulf or the gauging water contains sulphates in solution as would be the case if brackish/saline water had to be used for mixing.

Copyright 2000 Philip H Perkins

Figure 3.1 Concrete surface attacked by sulphates in solution.

Sulphates and acids in solution in the sub-soil and ground water are dealt with in some detail in Building Research Establishment Digest 363 1991 Sulphate and Acid Resistance of Concrete in the Ground. Sulphates in solution react with the hydrates of tricalcium aluminate (C3A) in the cement forming calcium aluminium hydrate (known as ettringite). This reaction is expansive in character and this expansion can cause disruption of the concrete. Figure 3.1 shows a concrete surface attacked by sulphates in solution. If the sulphate in the sub-soil and/or ground water exceeds the concentrations given in BRE Digest 363, it is advisable to use a Portland cement with a low C3A content such as sulphate-resisting Portland cement in which the C3A content is limited to 3.5% (BS 4027). Ordinary Portland cement can contain up to about 12% C3A. Increased sulphate resistance can also be obtained by reducing the w/c ratio and increasing the cement content; also by the inclusion of pfa or ggbs as an addition to the mix. See also Section 2.5. Portland cement contains gypsum (calcium sulphate) expressed as SO , by mass 3 of the cement as this is added during manufacture to control the setting of the

Copyright 2000 Philip H Perkins

cement. There is reason to believe that because gypsum is added during manufacture it does not form ettringite on the hydration of the cement paste. The formation of ettringite may be considered as ‘standard’ sulphate attack. Early in April 1998 the technical press carried reports of severe deterioration of concrete bridge foundations and this was claimed to be due to sulphate attack involving the formation of the mineral thaumasite. Thaumasite is a calcium silicate sulphate carbonate hydrate. When this form of attack occurs the surface of the concrete becomes very soft, rather like lime putty. The reaction needs wet cold conditions and a source of carbonate (which is usually found in calcareous aggregates) and the presence of sulphates or sulphides in the sub-soil in contact with the concrete. Laboratory tests showed that attack on concrete due to the formation of thaumasite could be produced by continuous exposure to high concentrated sulphate solutions using limestone aggregate. The fact that this reaction could take place had been known for many years, but only about four cases had been reported world-wide prior to the report on the UK motorway bridge foundations. The quality of concrete recommended for swimming pool shells is likely to possess considerable resistance to general sulphate attack. See Chapter 4 for recommended concrete mixes.

3.6 Chemical attack on cement-based mortar It can be seen from what has been written so far that it is the cement which is the ingredient in concrete and mortar that is most vulnerable to chemical attack. Hand-applied mortar, rendering, bedding for tiles and mosaic and building mortar have appreciably higher porosity and permeability than concrete and therefore may suffer attack in conditions where concrete would be virtually immune.

3.7 Swimming pool water and chemicals used in water treatment The water used in the vast majority of swimming pools is taken from a public supply. While such water is quite fit for drinking, the chemical characteristics of the raw water can vary considerably. In the UK, such water can be assumed to fall into the following general categories: 1. 2. 3.

soft, slightly acidic water, low in alkalinity and total dissolved solids; pH in the range 5.0 to 6.5; waters with a pH range between 6.5 and 7.5; alkalinity about 200 ppm; waters with higher alkalinity and a pH in the range of about 7.5 to 8.5.

Chapter 8 gives information on the basic principles of water treatment which will not be repeated here. The following compounds are in common use for the treatment of swimming pool water. Which compounds are used and the dosage will depend

Copyright 2000 Philip H Perkins

on a number of factors, including the characteristics of the ‘raw’ water and the bathing load: Sodium hypochlorite (NaOCl); Calcium hypochlorite (Ca(OCl) ); 2 Sodium bisulphate (Na SO ); 2 4 Hydrochloric acid (HCl); Aluminium sulphate (alum) (Al (SO ) ); 2 4 3 Carbon dioxide (CO ). 2

The hypochlorite (sodium and calcium) are both strongly alkaline chemicals and tend to raise the pH of the pool water. They are used to provide chlorine for the disinfection of the pool water. As the pool water should have a pH in the range 7.2 to 7.8, an acidic compound usually has to be added to adjust the pH to the required level. The hypochlorite is not aggressive to cement-based materials in the concentrations used in water treatment. A concentrated solution of sodium hypochlorite will attack concrete slowly but this is only likely to occur from spillage in storage areas. As previously stated, hydrochloric acid is very aggressive to all cement-based materials and protective measures should be adopted in storage areas where spillage is liable to occur. Sodium bisulphate is acidic in reaction (it is often referred to as ‘dry acid’), but is significantly less aggressive to cement-based products than hydrochloric acid. Nevertheless, in areas where spillage of the concentrate may occur, protective measures to concrete floors and cement/sand screeds should be adopted. Its use in the pool water (to correct the pH) can result in the slow build-up of sulphate to undesirable levels. For this reason it is advisable for cement-based bedding and jointing for tiles and mosaic to be formulated to resist sulphate attack. Reference should be made to Chapter 7. Aluminium sulphate (alum) is strongly acidic and is used mainly to provide a ‘floc’ for the efficient filtration of the pool water. It is also used to adjust the pH of the water. The concrete floors of storage areas should be protected by the provision of a suitable coating based on epoxy or polyurethane resins. Carbon dioxide is a gas and when dissolved in water forms carbonic acid which is a weak acid and it is used to adjust the pH of the pool water necessitated by the use of hypochlorite. When not properly controlled, it can cause etching of cementbased mortar joints in tiling. Even though the pH of the pool water may be above the neutral point of 7.0 (i.e. slightly alkaline), it may still be ‘lime dissolving’ and therefore needs special attention. This is discussed in the next paragraph.

3.8 Moorland water and the Langelier Index Soft moorland waters can be particularly difficult to deal with in their raw state as their characteristics can vary considerably through the year. It is not unusual to

Copyright 2000 Philip H Perkins

Figure 3.2 Severe etching of concrete water channel by moorland water.

find that the pH of the raw water drops appreciably after heavy rain, and may even go as low as 3.5. The characteristics of these waters are: Low calcium hardness; Low alkalinity; Some dissolved carbon dioxide and often some organic acids; Low total dissolved solids (tds). These waters are aggressive to cement-based materials and Figure 3.2 shows attack on a concrete water channel in a moorland area. The Langelier Index was developed in the USA in the 1930s by Dr Langelier to assess the characteristics of boiler feed water. A positive Langelier Index indicates that the water is lime depositing, while a negative index indicates that it is lime dissolving. Lime in this context is calcium carbonate. The chemistry behind the calculation of the Index is complicated, but it can be summarised by accepting that if the Index is negative the water will be lime dissolving and consequently potentially aggressive to Portland cement. If the Index is positive the water will be lime depositing and will not be aggressive to Portland cement. There is a scarcity of published guidance on the practical interpretation of

Copyright 2000 Philip H Perkins

the Index. An ISO (International Standards Organisation) document, N18E, of February 1983 indicates a classification of the Langelier Index for water as follows: Highly aggressive Moderately aggressive Non-aggressive

-2.0 and lower 0.0 to -2.0 any positivevalue

If swimming pool water has a negative Langelier Index, it is recommended that measures be adopted to increase the resistance to attack on the bedding mortar and grouted joints between tiles and mosaic. It is unlikely that the concrete shell of the pool will suffer attack. Figure 7.6 shows severe erosion of grouted tile joints by a soft moorland water.

3.9 Alkali-silica reaction Although a library search has not disclosed any reports of alkali-silica reaction (ASR) affecting swimming pools, it is considered that some basic information on this important subject is justified, as ASR was confirmed some years ago in the concrete of a water reservoir in the UK. The environmental conditions in which swimming pools operate (relatively high temperature and high humidity) are favourable to the occurrence of ASR, but much more than this is needed for attack to take place, and this is discussed below. Alkali-silica reaction was first reported in 1940 in the USA. Since then it has been identified as the cause of expansion and cracking in concrete in many countries. There are differences of opinion on the number of confirmed cases of ASR in the UK, and an even greater difference on the extent to which concrete affected by ASR suffers loss of strength, load carrying capacity, frost resistance, and increased risk of rebar corrosion. Agreement is general that the effects of ASR are long term and about five years is the minimum period required for any visible signs to appear. Alkali-silica reaction arises from chemical reaction between alkalis in the concrete and certain types of siliceous aggregates. The alkalis generally originate in the cement and are present in the pore fluid. This reaction results in the formation of an alkali-silica gel. This gel in contact with water expands and causes visible cracking. The situation is complicated by the fact that with aggregates in the UK there is a maximum percentage of reactive silica beyond which expansion decreases. This maximum amount (the ‘pessimum’), varies from one type of aggregate to another. For the reaction to take place, there must also be a high moisture level within the concrete. The main precautions which can be taken to avoid ASR damage is to either use a non-siliceous aggregate or use a low alkali cement, or both.

Copyright 2000 Philip H Perkins

Also, steps should be taken to prevent solutions of alkalis coming into contact with and penetrating the concrete. This latter is most unlikely to occur with swimming pools, except from a highly alkaline ground water in an industrial tip, or external concrete paving treated with de-icing salts. The alkali content of cement is expressed as ‘equivalent sodium oxide’ (Na O). 2 At the time of writing this book, the recommended limit in the UK for equivalent sodium oxide is 3 kg/m3 of concrete. General guidance on ASR is given in: BS 5328 Part 1 1991; BS 8110 Part 1 1985, clause 6.2.5.4; BRE Digest 330 1988; Concrete Society Technical Report No. 30 1987. Further reading American Concrete Institute. A Guide to the Use of Waterproofing, Damp-proofing, Protective and Decorative Barrier Systems for Concrete, ACI 515–1R–79 43, 1979, reviewed 1985. American Concrete Institute. Guide to Durable Concrete, ACI 201–2R–77, reaffirmed 1982. American Concrete Institute. Permeability of Concrete, 11 papers, 1988. American Concrete Institute. Corrosion of Steel in Concrete, 1996. British Cement Association. Minimum Requirements for Durable Concrete, Ed. D.W. Hobbs, 1998. British Standards Institution. Guide to the Durability of Buildings, Building Elements and Components, BS 7543. British Standards Institution. Guide to Building Maintenance, BS 8210. Building Research Establishment. Sulphate and acid resistance of concrete in the ground, Digest 363, 1991. Concrete Society. Alkali-silica Reaction: Minimising the Risk of Damage to Concrete, Technical Report 30, 3rd edition, 1999. Department of the Environment. The Thaumasite Form of Sulphate Attack: Risks, Diagnosis, Remedial Works and Guidance on New Construction, January 1999. Langelier, W.H. The analytical control of anti-corrosion water treatment, Journal AWWA, 28(1), October 1936.

Copyright 2000 Philip H Perkins

Chapter 4

Construction of swimming pool shells in insitu reinforced concrete

4.1 Introduction This chapter sets out to discuss a number of important matters which are generally applicable to concrete water-retaining structures but with special reference to swimming pools. It is not possible to draw a clear dividing line between design, specification and construction. Designers should possess a working knowledge of how the structure will be built and the problems the contractor is likely to face on site. Equally necessary, the contractor should understand the basis of the design and the reasons for the specification requirements. Detailed treatment of the structural design of the pool shell is outside the scope of this book as the design of concrete liquid-retaining structures are covered by a number of well-known publications, some of which are included under Further Reading at the end of this chapter. The three most important factors in the construction of swimming pool shells in insitu reinforced concrete are: structural stability, durability, and watertightness. For durability, the steel reinforcement must be protected against corrosion during the life-time of the structure. This requires that the concrete cover to the rebars should be dense and virtually impermeable. It is difficult to ensure that the concrete cover to the rebars (usually specified as 40 mm) is fully compacted and there is no loss of grout (cement and fines) through joints in the formwork. General (standard) type formwork consists of timber, fibre-glass panels, or steel. The type of formwork used determines the type of finish to the concrete when the formwork is removed. It is important to ensure that the correct type of release agent is used, and also that the joints between the formwork panels are grout tight. The use of what is known as ‘controlled permeability formwork’ can help to achieve the necessary quality of the concrete cover to the rebars. This type of formwork was originally developed in Japan in the 1980s and is now used in Europe and other developed countries. The formwork is designed to be permeable to water and air but not to cement particles. The forms consist of an outer shutter, a filter and a drain.

Copyright 2000 Philip H Perkins

Work by the Cement and Concrete Association (now the British Cement Association) showed that about 90% of the pressure on the formwork by the plastic concrete is caused by the pore water. Therefore, by removing a substantial proportion of the pore water, the pressure on the formwork will be substantially reduced. On removal of the formwork, the concrete surface has relatively few blow holes and a fine textured finish. It is recommended that the use of this type of formwork be considered for large insitu reinforced concrete pools. The pool shell should be tested for watertightness, as detailed in Appendix 2, before any finishes are applied.

4.2 Site investigations 4.2.1 General comments It is essential that site investigations should be carried out in the early stage of the design process. The work should only be entrusted to experienced firms and they should be asked in the brief to give practical interpretation and advice on the results of the investigation. The theory and practice of sub-soil surveys falls within the province of soil mechanics and is outside the scope of this book. Three important publications relevant to this subject, particularly to larger projects are:

BS 5930 1981 BS 8004 1986 DD 175 1988

Code of Practice for Site Investigations; Code of Practice for Foundations; Code of Practice for the identification of contaminated land and its investigation.

4.2.2 Reasons for site investigations There are many reasons for carrying out a site investigation of which the following three are the most important: 1.

2.

3.

to obtain information on the sub-soil, its physical and chemical characteristics, to enable the designer to decide on the type and dimensions of the foundations and other parts of the structure below ground level; to ascertain whether the sub-soil and/or ground water is likely to be aggressive to the concrete in contact with it, including information on whether there are hazardous and/or toxic substances present; to obtain sufficient information for the contractor to appreciate the problems involved in carrying out the work below ground level and to price his contract accordingly, including the time required to carry out the work.

It is essential that there should be an adequate number of trial pits or boreholes to ensure that the information obtained gives a clear picture of sub-soil conditions over the whole site.

Copyright 2000 Philip H Perkins

4.2.3 Interpretation of results In all sub-soil investigations, except rock, the pH of the soil and ground water should be given. If the pH is below 6.5 or above 9.5, a chemical analysis should be carried out to ascertain the composition of the compounds present which give rise to the low (acidic) or high (alkaline) pH. Information should be obtained (if possible), on the likely variations in water table level. The following factors should be taken into account: 1.

There are many practical difficulties in obtaining truly representative samples of sub-soil and ground water, and in carrying out the analysis. 2. All test results should be viewed with caution and carefully considered with all other relevant information. For example, ground water may be practically static, i.e. the velocity of flow may be very slow, as in clay, or appreciable as in gravels. 3. Due to excavation, and to the control of ground water by pumping subsurface conditions can change appreciably and these changes can be significant during the construction period, and the life-time of the structure. Where aggressive chemicals come from a specific deposit, e.g. an industrial tip on adjacent land, their quantity may be limited and perhaps removed altogether if the adjoining site is developed. 4. The degree of exposure to attack from aggressive chemicals should be assessed in the site investigation report. The following conditions are listed in increasing potential for attack on the concrete: (a) conditions, above water table level; (b) conditions below static ground water level; (c) conditions below flowing ground water level. 5. Seasonal fluctuations in ground water level and changes in its chemical composition, as well as its direction and velocity of flow can have an important bearing on the severity of attack. For example, in peaty sub-soils, the pH can fall significantly after heavy rain and then rise again. It may be worthwhile to consider the practicality of permanently lowering the water table by means of a properly designed sub-soil drainage system. Some general recommendations on sub-soil drainage are given in the next paragraph. It may be necessary to lower the water table level to enable the pool to be constructed in the dry. This can be effected by well point dewatering or water collecting pits. However, it is necessary to take into account the effect of lowering the water table on the foundations of adjacent/nearby structures. 4.3 Under-drainage of site Under-drainage of sites are carried out for two main reasons: 1. to permanently lower the water table for the reasons given in the preceding paragraph;

Copyright 2000 Philip H Perkins

2. to prevent the build-up of ground water pressure which may result in uplift (flotation) causing serious damage to the structure. 4.3.1 Materials and layout The pipes generally used for sub-soil drainage are: 1. 2.

unglazed clayware field drainage pipes to BS 1196 1989; plastics pipes for sub-soil field drains to BS 4962 1989.

It is also possible, and in some types of fine-grained soils desirable, to use dense concrete pipes to BS 5911 Part 3 1982 or vitrified clay pipes to BS 65 1991 with open joints, the joints being surrounded with graded aggregate. Dense concrete pipes may be attacked by an aggressive groundwater unless protected by a suitable coating. The layout of the under-drainage system will depend on site conditions. On many sites, the provision of a simple perimeter drain with inspection chambers/ manholes at each change of direction would provide adequate access for periodic inspection and clearing.

4.4 Flotation (uplift) of the pool shell It is unusual for flotation to be a serious problem with the construction of insitu concrete swimming pools because the dead weight of the concrete shell, without taking into account the friction between the ground and the walls, is usually more than adequate to counter the upward pressure of the groundwater even when the water table is high. In any case, the design should be checked for the posibility of flotation.

4.5 General comments on design and construction 4.5.1 Introduction Anyone undertaking the design and/or construction of a reinforced concrete swimming pool is recommended to become acquainted with the relevant provisions in BS 8007 Code of Practice for the Design and Construction of Concrete Structures for Retaining Aqueous Liquids, BS 8110 The Structural Use of Concrete, and BS 5328 Concrete Parts 1–4. The basic publication in the UK is BS 8007, and the equivalent document in the US and Canada is the American Concrete Institute publication ACI 350 R-89. Essentially, BS 8007 sets out recommendations for the quality of the concrete and the structural design of the structure to meet the design life under service conditions, which is stated to be in the range of 40–70 years. However,

Copyright 2000 Philip H Perkins

it is pointed out that some components such as jointing materials have a shorter life than the concrete and are likely to require replacement at intervals. See comments in Sections 2.14 and 2.15 which discuss joint fillers and sealants. British Standard BS 8007 is linked to the principal structural design Code BS 8110. Where the two Codes differ on a specific point, the recommendations of BS 8007 take precedence. British Standard BS 8007 recommends that the minimum conditions of exposure should be taken as ‘severe’ (see Table 3.2 of BS 8110 and Tables 5 and 6 of BS 5328 Part 1). Calculated crack widths are set at 0.2 mm and 0.1 mm depending on conditions of exposure. This is the width of cracks at the surface of the concrete. Cracks in reinforced concrete members are caused by applied loads, by thermal effects (heat of hydration of the hydrating cement and by external temperature changes), and by drying shrinkage which occurs as the concrete matures. The crack width and crack spacing is controlled by reinforcement and the location and type of joints.

4.5.2 Joints The selection of the type of joints and their location is fundamental to the calculation of the distribution steel for crack control. There is some conflict between the wish to keep the amount of distribution steel within ‘reasonable’ limits and the known fact that joints are the major source of leakage in a concrete water retaining structure. The author’s experience is that joints should be kept to a practical minimum. In the case of swimming pools which are finished with tiles/mosaic, it is strongly recommended that there should be close co-operation between the suppliers of the tiles/mosaic and the designer of the pool shell. The reason for this is that joints in the pool shell which may have to accommodate movement should be carried through the tiling/mosaic and the cutting of tiles should be avoided as far as this is practical. The joints considered here are: Full movement joints (also known as expansion joints); Contraction joints; Partial contraction joints; Stress relief joints; Construction joints (also known as day-work joints); Sliding joints.

4.5.2.1 Full movement joints This type of joint is designed to cater for both expansion and contraction of the concrete on each side of the joint. Figures 4.1 and 4.2 show these joints.

Copyright 2000 Philip H Perkins

Figure 4.1 Full movement joint in floor slab.

Figure 4.2 Full movement joint in wall.

It is desirable that a water bar be provided and that the joint should be sealed. It should be noted that this type of joint is about 15 mm to 20 mm wide. Reinforcement does not cross the joint. However, if it is decided that there may be non-uniform settlement across the joint, it may be desirable to insert dowel bars (as in a road slab), but this is unusual. It is important that the wall should be of adequate thickness to enable the concrete to be thoroughly compacted around a centrally located water bar. The thickness is likely to be 300 mm to 400mm.

4.5.2.2 Contraction joints This type is shown in Figure 4.3. There is no initial gap left between the adjacent concrete, but no attempt is made to secure bond at the interface. The reinforcement is stopped off each side of the joint.

Copyright 2000 Philip H Perkins

Figure 4.3 Contraction joint in floor slab.

The joint becomes a Partial Contraction Joint if some of the reinforcement (up to 50%) crosses the joint.

The joint should be sealed, and provided with a water bar which is centrally located in a wall, and located on the underside of a floor slab; see note above on minimum wall thickness for installation of centrally located water bars.

4.5.2.3 Stress relief joints These joints are formed in the freshly placed concrete in floor slabs, and sometimes in walls. A crack inducer is fixed at a predetermined position in the underside of the slab and directly over this a slot is wetformed in the top surface of the slab to a depth of one-third of the overall depth of the slab. The surface slot can be sawn, but the correct time after casting the concrete to carry out the sawing is very difficult to determine. If too early, the concrete is likely to ravel and, if too late, a crack may

Copyright 2000 Philip H Perkins

form which is unlikely to be straight; in either event, sealing will be very difficult. Reinforcement crossing the joint is generally reduced by 30–50%. The principle is that thermal contraction and drying shrinkage will cause a crack to form through the slab in a straight line which can be sealed. The joint should be sealed and provided with a water bar as for a contraction joint. An alternative type of stress relief joint in a wall is shown in Figure 4.4. The reinforcement crossing the joint is reduced by 30–50%. The diameter of the void should be about one-third of the wall thickness.

4.5.2.4 Constructionlday work joints These joint are formed to facilitate the construction of the pool shell; also, they become necessary if there is a serious delay in the supply of concrete. Such joints should be formed with a stop-end and all necessary steps taken to ensure maximum bond between the ‘old’ concrete and the newly placed concrete when placing starts again. The surface of the hardened concrete should be lightly bush hammered or other means used to expose the coarse aggregate, followed by removal of all loose

Figure 4.5 Concrete surface at construction/day-work joint prepared to secure bond.

Copyright 2000 Philip H Perkins

grit and dust. Reinforcement should be carried across the joint. The objective is to make the concrete at the joint as monolithic as possible. Figure 4.5 shows a concrete surface prepared.

4.5.3 The concrete Since the third edition of this book was published in 1988, the recommendations for concrete mixes to meet specific requirements for buildings and civil engineering structures have been greatly extended. These requirements are included in BS 5328 Concrete Parts 1–4. The contractor has the option of deciding on site mixing or using ready-mixed concrete. The use of ready-mixed concrete is recommended. At the time this book was being revised, about 80% of concrete used in the UK was supplied by readymixed concrete firms. It is recommended that ready-mixed concrete should be supplied by a QSRMC Registered Company from a plant holding current QSRM Certification for Product Conformity. The QSRMC Certification Mark should be on all quotations and delivery tickets. For swimming pool shells, the concrete should be either a designed, a prescribed, or designated mix as detailed in Part 2 of BS 5328. The main differences in the selection of the type of mix referred to in BS 5328 relate to: 1. 2. 3.

the responsibility for selection of mix proportions; the terms in which the mix is specified; the main parameters used for judgement of conformity.

For a designated mix, the concrete should have a characteristic strength of 35 N/ mm2, with a maximum w/c ratio of 0.50 (but a lower w/c ratio plus the use of a plasticiser is recommended by the author. Aggregates should be 20 mm maximum size, well graded and complying with BS 882 Aggregates from Natural Sources for Concrete. The workability should be adequate for full compaction (say a nominal slump of 75 mm). It is essential that the supplier of the concrete, whether this is a ready-mixed concrete firm or a contractor who wishes to mix the concrete on site, is given the necessary information so that it is clear exactly the type and standard of concrete required. Tables in Part 2 of BS 5328 provide detailed information for the specification requirements of designed, prescribed and designated mixes. It was emphasised in Section 1.2 that the pool shell should be watertight against loss of water when the pool is full and against ingress of ground water when the pool is empty. This necessitates a practical water test, and the details of a suitable test are given in Appendix 2. Concrete has a pore structure and water can very slowly move through it, but with good quality concrete and proper design, this permeability is unlikely to have an adverse effect on the durability of the structure.

Copyright 2000 Philip H Perkins

British Standard BS 8007 emphasises this and states that the concrete should possess low permeability which is one of the important characteristics required to ensure durability of the structure (see also Chapter 3). Permeability may be defined as the characteristic of a material which allows fluids to pass through it under differential pressure. Low permeability helps to ensure resistance to chemical attack, and protection of the steel reinforcement. To secure low permeability, the mix proportions of the concrete have to be carefully designed and this is emphasised in the Code. Consideration should be given to the effect of the heat of hydration on the maximum temperature likely to be reached by the concrete, which is important when the concrete is cast in timber formwork, particularly in hot weather. This can result in thermal contraction cracking and the Code deals with this in some detail under Temperature and Moisture Effects. Moisture effects in this context refer to the drying out of the concrete after casting, and these ‘effects’ consist of drying shrinkage which can result in cracking. The thermal effects (cracking) will occur in the early age of the concrete, generally within a few days after removal of the formwork, while shrinkage cracks are likely to appear later. This is the reason for the suggestion given above to reduce the w/c ratio and thus reduce the amount of water in the mix when it is placed, and this in turn reduces the risk of drying shrinkage cracking. Thermal contraction cracking can be controlled by specific design of the reinforcement, e.g. by increasing the amount of distribution steel, and/or by reducing the length of wall or floor slab between joints. The reduction of the amount of Portland cement in the mix by replacing, say, 20% with ground granulated blastfurnace slag (ggbs), or pfa, will also help. The type of aggregate used also has a significant effect on the thermal movement (expansion or contraction) of the concrete. The coefficient of thermal expansion of concrete made with a flint gravel is generally taken as 12–14× 10-6 per °C, while for the same mix using a limestone aggregate, it would be about 7–8×10-6 per °C. It has recently been suggested that the use of limestone aggregate concrete in contact with a high concentration of sulphates in the ground water may trigger the occurance of thaumasite attack. It is important that the specified nominal cover to all reinforcement is maintained by the use of spacers and careful fixing of the reinforcement. It is recommended that the cover to reinforcement in walls be checked by means of a cover-meter survey as soon as practical after the removal of the formwork. A similar exercise should be carried out on the floor slab as soon as practical after casting and finishing the concrete. Some information on cover meter surveys is given in Section 10.22.2 and in Appendix 3.

Copyright 2000 Philip H Perkins

4.6 Concrete construction in cold weather 4.6.1 General considerations The information given here is intended to bring out important principles which should be followed when concreting in cold weather. For more details on this subject, readers are referred to Further Reading at the end of this chapter. The important factors involved in using Portland cement concrete when air temperatures are near freezing point are set out below: 1.

2.

3.

When the temperature of the setting and maturing concrete is lowered, the chemical reaction between the cement and the mixing water is slowed down and the rate of gain of strength decreases. As the temperature of the concrete approaches freezing point, the hardening process practically ceases. However, if the concrete is not saturated with water and if it has reached a compressive strength of not less than 3 N/mm2, and has a reasonable cement content (300 kg/m3), then even if the concrete does freeze it is unlikely that permanent damage will result. When the temperature of the concrete rises again the maturing process (hardening) will recommence and will continue at a rate proportional to the temperature of the concrete. It is the temperature of the concrete which is the key factor and the concrete should not be placed unless it has a temperature of at least 10 °C.

The precautions to be taken to prevent frost damage to maturing concrete must be directed towards maintaining the temperature of the concrete as high as practical, by providing thermal insulation and/or using heated concrete. However, the temperature of the heated concrete should not exceed about 30 °C at the time of placing.

4.6.2 Recommended precautions to be taken The following simple rules, if properly applied, will enable concreting to proceed in the severe weather likely to be experienced in the UK. 1. 2.

Frozen aggregates and icy water must not be used. Concrete must not be placed on frozen ground nor in frozen formwork. Wet curing should not be used. The cement content should not be less than 300 kg/ m3. It can be advantageous to increase the cement content by, say, 1½ bags (75 kg) per m3 unless there is a specific reason not to do so. Or to use rapidhardening Portland cement. Sulphate-resisting Portland cement generally has a slower hardening rate than ordinary Portland cement, and if sulphate-resistant Portland cement is required to resist sulphate attack, it may be necessary to suspend concreting until the air temperature rises.

Copyright 2000 Philip H Perkins

3.

4.

The temperature of the concrete at the time of placing should not be lower than 10 °C. This can be achieved by ordering heated concrete from a readymixed concrete plant which has the equipment to produce this type of concrete. The usual way of producing heated concrete is to heat the mixing water, but the maximum temperature should not exceed about 50 °C. The concrete must be well insulated as soon as the finishing processes have been completed. This is particularly important when casting slabs on the ground, as the exposed surface area to volume ratio is high. The degree of exposure of the site should also be given consideration.

4.7 Concrete construction in hot weather 4.7.1 General considerations It may be thought that, in temperate climates (such as in the UK), there would be no problems arising from concreting in the summer months. However, with temperatures in the high twenties and low thirties, difficulties can be experienced, including premature stiffening of the concrete which makes placing and compaction difficult, and an increased risk of plastic cracking and thermal contraction cracking. The adverse consequences of the neglect of proper curing and protection against direct sunshine and strong wind will be increased. It has been mentioned in the preceding paragraph that increase in the temperature of the concrete speeds up the chemical reaction between the mixing water and the cement. In the summer, the increase in ambient temperature provides this additional heat. For example, concrete placed at a temperature of, say, 10 °C may reach a temperature of about 30 °C in 24 hours, while the same mix placed under the same external conditions but with a casting temperature of 20 °C may reach a temperature of about 55 °C after 20 hours. Also, the heat of hydration of the cement type used will affect the temperature rise of the concrete.

4.7.2 Recommended precautions to be taken The following matters should be given careful consideration: 1.

2.

3.

If the specified cement content exceeds 330 kg/m3, it may be advantageous to reduce this, but the need to maintain the specified strength and low permeability must be kept in mind. The possible change in the cement type from OPC to a cement containing pfa or ggbs (BS 6588 or BS 4246), but such a change may increase the striking time for formwork. The aggregate stock piles can be sprayed with water as this will lower the temperature of the aggregate.

Copyright 2000 Philip H Perkins

4.

5. 6.

The mixing water should be as cool as possible. Water drawn from a public supply main will be reasonably cool, but if kept in a black painted steel tank it will become quite hot, and water storage tanks should be painted white. The use of a retarder is likely to be essential when ready-mixed concrete has to be transported over a long distance. Special attention to proper curing is essential and consideration may have to be given to the use of tentage for floor and roof slabs.

4.8 Plastic cracking There are two types of plastic cracking, namely plastic shrinkage cracking and plastic settlement cracking. The former is more common, and the latter is unlikely to be encountered in the construction of a swimming pool shell and associated floor slabs.

4.8.1 Plastic shrinkage cracking This type of cracking may occur on the surface of floor and roof slabs while the concrete is still plastic. Investigations by various authorities have established that the principal cause is the rapid evaporation of moisture from the surface of the concrete while it is still in a plastic or semi-plastic state. When the rate of evaporation exceeds the rate at which water (known as bleed water) rises to the surface of the concrete, plastic shrinkage cracking is very likely to occur.

Figure 4.6 Plastic shrinkage cracks in floor slab. Courtesy, G.F.Blackledge.

Copyright 2000 Philip H Perkins

The cracks are usually very fine and are often not noticed until the next day (Figure 4.6). These fine cracks are usually straight and short and transverse in direction, and rarely extend to the slab edge. They are sometimes parallel to each other and the spacing can vary from about 50 mm to 300 mm; the cracks are shallow and seldom extend to below the top layer of reinforcement. Figure 4.6 shows plastic shrinkage cracking in the floor slab of a swimming pool. While these cracks generally occur in hot weather, they can also appear on cooler days if the concrete is exposed to a strong wind. They should be grouted in with a Portland cement grout, preferably including a styrene butadien (SBR) emulsion, say 10 litres to 50 kg cement. The treated surface should be covered with polythene sheeting, held down around the edges with planks or blocks. The following are recommended precautions to be taken when conditions are likely to be suitable for this type of cracking to occur: 1.

2. 3.

4. 5.

The formwork (if any) should be well damped down prior to placing the concrete. Slabs cast ‘on the ground’ should be separated from the sub-base by 1000 gauge polythene sheeting. The aggregates, particularly if they are dry, should be sprayed with water. It may be advisable to use a slightly finer grading of sand. An air entraining admixture can often be used with advantage. The mean air content of the mix, when using 20 mm maximum size aggregate should be 5.5% by volume of the fresh concrete (BS 5328 Part 1 clause 4.3.3). The allowable tolerance on the 5.5% is given in Part 4 of the Standard, clause 3.6. The placing, compacting and finishing should be proceeded with as quickly as possible without delay between each operation. Curing should be commenced as soon as possible after finishing is complete and the surface of the concrete should be protected from hot sun and strong wind.

4.9 Thermal contraction cracking 4.9.1 General considerations This is a common type of cracking in the walls of reinforced concrete water retaining structures including swimming pools. During the setting and early hardening of concrete, considerable heat is evolved by the chemical action between the mixing water and the cement which results in a rise of temperature of the concrete; this has been referred to in Section 4.7. The actual rise, the peak temperature and the time taken to reach the peak and then to cool down depend on a large number of factors of which the following are the most important: 1. the temperature of the concrete at the time of placing;

Copyright 2000 Philip H Perkins

2. 3. 4. 5. 6. 7. 8.

the type of formwork used (whether timber, steel or plastic) and the time the formwork is kept in position; the ambient air temperature; the volume/exposed surface area of the concrete; the thickness of the section cast; the type of cement and the cement content of the mix; the method of curing; the amount and detailing of crack control reinforcement, see BS 8007.

As the temperature of the concrete rises it expands and when it cools down it contracts. By the time it starts to cool, the concrete has already started to harden so that tensile stress set up by the contraction can only be accommodated without crack formation if the tensile strength of the concrete and/or the bond strength with the distribution reinforcement is not exceeded. It must be kept in mind that the walls are restrained at the base as the joint between the kicker and the wall panel is specially prepared to secure maximum bond and minimum permeability. Figure 4.7 shows a thermal contraction crack in the wall of swimming pool. This type of crack is usually narrow, seldom exceeding 1.0 mm in width, but it can pass

Figure 4.7 Thermal contraction crack in wall.

Copyright 2000 Philip H Perkins

through the wall and is a potential source of leakage. They are often present when the formwork is stripped but are frequently not noticed until some time later when their ‘discovery’ causes consternation to designers and contractors alike.

Once the formwork is removed, drying shrinkage will start to take effect, and unless controlled by careful curing will tend to widen the existing thermal cracks. Cracks of ‘design’ width, i.e. 0.1 mm and 0.2 mm, are unlikely to allow water to reach the rebars provided the specified cover (normally, 40 mm) has been provided. Experience shows that some cracks seal themselves by ‘autogenous healing’, but this cannot be relied upon.

4.10 Swimming pools with floor slabs supported on the ground 4.10.1 Casting the floor slab The floor slab should be cast on a sliding layer consisting of two sheets of 1000 gauge polythene laid on a compacted granular sub-base, not less than 100 mm thick. The concrete should be thoroughly compacted by a vibrating tamper working off the side forms. The type of finish required should be clearly stated in the specification. The tamper can be a single or double beam (twin beam compactor). Poker vibrators should be used around the perimeter (next to the side forms) to help ensure adequate compaction. The concrete should be placed with a surcharge of about 50 mm to allow for reduction due to compaction. The actual surcharge will depend on the slump of the concrete and the thickness of the slab. The surface left by the tamper will be ribbed. To provide a smoother finish, the tamper should be taken back every 1.5 m to 2.0 m and then moved forward slowly over the compacted surface to smooth out the ridges and furrows left by the first pass of the beam. The finish can be further improved by the use of a power float followed by a power trowel. To achieve satisfactory results, the concrete must have reached the right degree of stiffness before the power floating. The use of the power float and power trowel should not be necessary if the floor slab is to receive a screed. However, the power floating and trowelling would be desirable if the finish is to be a proprietary coating or PVC sheeting.

4.10.2 Slab reinforcement The reinforcement for the main floor slab could be either high yield deformed bars or high tensile fabric, and is normally located in the top of the slab with 50 mm cover. There is usually a perimeter bay which forms the heel of the wall. The detailing of reinforcement in the perimeter bay at the four corners of a rectangular

Copyright 2000 Philip H Perkins

pool requires careful thought and experience, as the wall reinforcement has to be securely anchored into this slab.

4.10.3 Joints in the floor slab The selection of the types of joints and their location is an essential part of the design, and has been discussed in some detail in Section 4.5.2.

4.10.4 Pipework through the floor slab The main outlet pipe for the pool is located at the deep end and special care is required to ensure that leakage does not take place at this point. In some special systems of water circulation, the inlets for the treated water are located longitudinally on the centre line of the floor. There are two ways of carrying the pipes through the floor (and in fact the same applies to the walls), namely by boxing-out, or building-in, as the work proceeds. There are differences of opinion as to which method gives the better results from a watertightness point of view, and this is discussed below. It should be noted that the majority of the pipes used for water circulation are now plastic instead of steel or cast iron.

4.10.4.1 Boxing-out This method has to be adopted when for one reason or another it is not practical to insert the pipe at the time the slab is cast, e.g. the pipe may not be available or its exact position has not been finally determined. The boxed-out hole must be of adequate size to accommodate the pipe and allow for compaction of the concrete around it. For small diameter pipes, up to, say, 75 mm diameter, mortar can be used instead of concrete. The mortar should have a low w/c ratio and a styrene butadiene emulsion should be added to the mix (10 litres of SBR to 50 kg cement). Thorough compaction is essential. The pipe should be cleaned and all dirt and coating/paint removed prior to fixing. A flange should be provided on the water face of the floor slab. Figure 4.8 shows a detail of a boxed-out steel pipe with a flange on the water face of the floor slab or wall. If plastic pipe is used it will be difficult to obtain a good bond between the pipe and the concrete/mortar. The surface of the pipe should be roughened and a specially formulated resin bonding coat applied. This should be sprinkled with coarse sand while it is still tacky and then allowed to harden, the object being to assist bond with the concrete/mortar. An alternative is to accommodate the pipe in a galvanized steel sleeve which would overcome the problem of bond.

Copyright 2000 Philip H Perkins

Figure 4.8 Boxing-out for pipe through wall.

4.10.4.2 Building-in This method should be adopted whenever possible and involves casting-in the pipe as the concrete is cast. The same precautions should be taken to ensure good bond between the pipe and the concrete. A flange should be provided on the water face of a floor slab, but can be centrally located in a wall panel which in fact is the usual location for what is known as a ‘puddle’ flange.

4.11 Construction of the walls of the pool 4.11.1 Casting the concrete The walls are cast within formwork firmly secured at the base to the kicker which forms part of the perimeter bay of the floor slab. The thickness of the wall will be determined by the design, but it must be thick enough to allow for thorough compaction of the concrete, bearing in mind that there will be four layers of reinforcement and two lots of cover. Also, the designer may have decided to specify the use of water bars in contraction joints as well as in expansion joints. This means that the minimum thickness of the wall would be 300 mm without a water bar, and probably 400 mm with water bars. Figure 4.2 shows a PVC water bar in an expansion joint in a wall. The PVC water bar must be securely fixed to the reinforcement otherwise if may interfere with the placing and compaction of the concrete. Displaced water bars are a source of leakage which is very difficult to rectify. The top surface of the kicker should be carefully prepared so that maximum bond is obtained at this position where stress is likely to be at a maximum. This preparation can consist of bush hammering to lightly expose

Copyright 2000 Philip H Perkins

Figure 4.9 Top surface of kicker prepared to receive placing of concrete for wall.

the coarse aggre gate, or the use of high-velocity water jets. If the surface is scabbled then all grit and dust must be removed before the concrete is cast on it. The surface should be well damped down but care taken to avoid standing water between the pieces of exposed aggregate. Figure 4.9 shows the top surface of a kicker prepared for the casting of the wall panel. The use of a water bar in the kicker is not recommended as it is very easily displaced during the placing and compaction of the concrete. The wall panels should be cast to their full height in one lift.

4.11.2 Joints in the walls The question of type and location of joints has been discussed in Section 4.5.2. The joint between the formwork and kicker must be grout tight otherwise honeycombing at the base of the wall is likely to occur resulting in leakage when the water test is applied. The same comment applies to joints in the formwork.

4.11.3 Execution of the work The reinforcement should be checked for specified cover and for general condition. It should be free from grease and loose scale rust; light powdered rust does no hard and may in fact improve bond with the concrete.

Copyright 2000 Philip H Perkins

The problem of thermal contraction cracking has been discussed in Section 4.9 and this is relevant to the type of formwork used (whether timber or steel) and the length of time it is kept in position. If there is any fear about the probable occurrence of thermal contraction cracks, it is better to leave the formwork in position for a day or so longer. The time lost and the cost of repairing these cracks can be considerable.

4.11.4 Pipework through walls Pipes for the water circulation system will pass through the walls below top water level and the precautions outlined in the previous paragraph for the floor of the pool also apply to the walls. However, in the case of the walls, if the pipes are cast in as the work proceeds, a puddle flange can be provided in the centre of the wall. A flange on the water face is not required unless the pipes are sleeved. The omission of the surface flange would simplify the formwork.

4.12 Construction of walkway slabs and floors of wet changing areas 4.12.1 Introduction If these slabs are suspended to provide space below for some specific purpose, such as storage, plant rooms etc., then it is essential that they should be designed and constructed so as to be completely watertight. They should be tested for watertightness as described in Appendix 2. Slabs uniformly supported on the ground need not be designed for watertightness. The design and method of construction will depend on whether the pool is open air or enclosed.

4.12.2 Suspended slabs The slabs should be designed to the ‘water retaining’ Code (BS 8007). This may sound obvious, but it is surprising the number of cases where this has not been done. This omission can result in serious leakage through the slab into the utilised area below (Figure 4.10). The leakage is often only reported some time after the completion of the swimming pool complex and can only be rectified at considerable cost and serious dislocation of the use of the pool. The standard of watertightness of the slab itself should be the same as for the roof slab of a building. Account should not be taken of applied finishes such as screeds and non-slip ceramic tiles or mosaic. The slab should be tested for watertightness, as described in Appendix 2. Seepage is likely to occur through cracks and joints, and pipework passing through the slab such as drainage channel, outlets etc. This requires careful detailing

Copyright 2000 Philip H Perkins

Figure 4.10 Seepage through walkway slab into plant room below.

and consultation between the designer and the suppliers of the fittings and pipework is desirable. The provision of a flange on the top and/or bottom surface of the slab is likely to provide a practical solution. The precautions described in the previous paragraph for pipes passing through the pool floor are applicable here. These slabs can be designed as flat slabs and all interior joints are detailed as construction joints so that the whole slab is virtually monolithic. The perimeter of the slab can be tied to the supporting structure, but this restraint may introduce complex stresses and therefore a sliding joint is often preferred (Figure 4.11). If the joint is tied, any movement joints in the supporting walls should be carried through into the suspended slab and this will increase the problem of ensuring complete watertightness.

4.12.3 Slabs supported on the ground If the pool is an open air one, then the walkways can be constructed as described in Chapter 6. If the pool is enclosed, then the walkways and other ‘wet’ areas can be constructed as normal ground floor slabs as required by the Building Regulations and the relevant Approved Documents made thereunder.

Copyright 2000 Philip H Perkins

Figure 4.11 Sliding joint between walkway slab and supporting structure.

4.13 Curing the concrete floor and walls of the pool Some general information on the materials used for curing concrete has been given in Section 2.11. Recommendations for the curing of the concrete of swimming pool shells is given below. Concrete swimming pools are normally provided with a decorative and easily cleaned finish consisting of either a proprietary coating or ceramic tiles/mosaic, or, in the case of private pools, an insitu terrazzo known as marbelite. All these finishes are required to bond strongly either directly to the base concrete or to an applied rendering which also has to bond well to the base concrete. For this reason, the use of a spray-applied curing membrane is not recommended as this is likely to interfere with the bond at the interface of the finish with the concrete. Wet curing with water is also not recommended as it is very difficult to ensure proper control. A practical solution is to use polythene sheeting securely fixed so that wind cannot blow underneath it. The sheeting should be kept in position for a minimum of four days after completion of the casting of the concrete floor. For the walls, special frames are needed to secure the sheeting and these should be kept in position for four days after removal of the formwork. If the weather is ‘favourable’ (mild and damp), it may be practical to keep the formwork in position for a few extra days and omit special curing procedure.

Copyright 2000 Philip H Perkins

4.14 Construction of suspended pool shells 4.14.1 Introduction Very few pools are designed and constructed as completely suspended structures; when this is done it is usually required to utilise the space below the pool itself. Examples are large public pools when it is required to locate plant rooms, stores etc. beneath the pool shell. A pool located on the upper floor of a building has to be designed as a suspended structure and is constructed in a special structural void. The standard of watertightness required is much higher than required for a normal pool located on or in the ground. This creates a number of special problems of which the most important is the need to ensure that there is no moisture penetration through the pool shell into the areas below. While a damp patch may

Figure 4.12 Sketch showing suggested arrangement for reinforced concrete pool on upper floor of a building.

Copyright 2000 Philip H Perkins

be tolerated on the ceiling of a plant room, it would not be accepted on the ceiling of a hotel room or similar. The type of finish required, i.e. a decorative waterproof coating or ceramic tiles/mosaic, will influence the design and method of construction of the pool shell. The precautions outlined below may be considered as rather exaggerated, but thought should be given to how a leak can be located and repaired.

4.14.2 Methods of construction It is assumed that the pool shell is located in a structural void in the building so that the outside of the walls and floor are available for inspection. It is also recommended that the void should be tanked so that should leakage occur it will be contained and will not penetrate to other parts of the building. All pipework should be in accessible ducts (Figure 4.12). Suggested methods of construction are: 1.

Insitu reinforced concrete post-tensioned to ensure that the walls and floor of the pool are in a state of permanent compression (Figure 4.13). It would be logical to post-tension the walkway slab and the floor slab of

Figure 4.13 Reinforcement with nominal prestressing for pool on upper floor of a building in London. Courtesy, S.B.Tietz & Partners, Consulting Engineers and Scarlett, Burkett, Griffiths, Chartered Architects.

Copyright 2000 Philip H Perkins

2. 3.

wet changing areas, but this is a matter of design. If joints, other than daywork joints, can be eliminated, this would be very advantageous from the point of view of watertightness. Insitu reinforced concrete with sandwich type membrane. This increases the dead load considerably. Insitu reinforced concrete finished with either fully bonded PVC or similar sheeting or a decorative waterproof coating applied by spray.

In methods 2 and 3, the walkways around the pool, and wet changing areas if there are any, should be provided with a membrane fully bonded to the structural floor slab. The membrane can be sheet material or a waterproof coating applied by spray; the important point is that the membrane must be fully bonded to the base concrete and be sufficiently tough to withstand the laying of the finishes on top. An alternative is to form the pool shell out of steel. However, there can be problems with the application of finishes, the flexing of the pool shell, and corrosion. See paragraph 5.33. The void in which the pool is located should be large enough for men to enter, carry out a detailed inspection, and if necessary repair any leakage. Permanent lighting and power points should be provided in this working space. There should be a regular routine inspection, say, every six months which should be recorded.

4.14.3 Additional matters for consideration When the pool is located on the upper floor of a residential building special attention should be given to: 1.

2.

The method to be adopted for the disinfection of the pool water. Chlorine, due to its penetrating smell is not suitable and some other method should be selected such as ozone, or metallic ions. See Chapter 8. An efficient system of mechanical ventilation to ensure that the warm humid air in the pool hall does not find its way into other parts of the building. An alternative is complete aerial disconnection between the swimming pool hall and associated rooms, and the rest of the building.

4.15 Thermal insulation of swimming pool shells It is sometimes suggested that thermal insulation should be provided to the pool shell. By far the greatest loss of heat is from the surface of the water with only a small percentage through the floor and walls to the surrounding ground unless the water table is high. With a permanently high water table, the loss of heat from the heated water in the pool may be significantly increased, in which case the installation of

Copyright 2000 Philip H Perkins

thermal insulation is worth considering. To be effective, the insulation must be virtually impermeable to water.

4.16 Under-water lighting and under-water windows The provision of under-water lighting can be very attractive, and it can improve safety for the bathers. Under-water windows require an access corridor or similar for use by viewers, and should be installed between two under-water lights. It is essential that recesses for the light fittings should be formed when the pool shell is being cast, and the same applies if the light fitting is installed in a watertight tube. If it is decided to install the fittings after the pool shell has been constructed, it can prove very difficult indeed to make such openings watertight. The whole installation should be corrosion-proof and water-proof. The window(s) must be made of high-quality safety glass. The equipment and wiring etc. should be installed and tested in accordance with the latest edition of Regulations for Electrical Installations, published by the Institution of Electrical Engineers (IEE). The light fittings should preferably be installed so that bulbs can be changed without having to lower the water level in the pool. The fittings themselves are proprietary items and recommendations for installation should be obtained from the manufacturers. The International Board for Aquatic Sports and Recreational Facilities recommend that, in swimming pools, the lights should be installed at a depth of about 0.90 m below the water surface as this will prevent dazzle to swimmers and spectators. In deep pools, it may be desirable to install a second row of lights. The longitudinal distance between the lights is usually 2.0 m to 3.0 m. In diving pools, lights should only be installed in the side walls. Inspections and tests should be carried out at intervals prescribed by the IEE, and these should be recorded.

Further reading American Concrete Institute. Environmental Engineering Concrete Structures, ACI 350-R-89. American Concrete Institute. Testing Reinforced Concrete Structures for Watertightness, ACI Committee 360/AWWA Committee 400–511. American Concrete Institute. Practitioners Guide to Hot Weather Concreting, 1996. American Concrete Institute. Cold Weather Concreting, ACI Committee 306, 1988. American Concrete Institute. Practitioners Guide to Cold Weather Concreting, 1997. American Concrete Institute. Guide to Sealing Joints in Concrete Structures, ACI 504-R-90. American Concrete Institute. Control of Cracking in Concrete Construction, ACI Committee 224, 1989.

Copyright 2000 Philip H Perkins

Anchor, R.D. Design of Liquid Retaining Concrete Structures, 2nd edition, Edward Arnold, London, 1992. British Cement Association. Concrete on Site No. 11; Winter Working, 1993. British Standards Institution. Code of Practice for Site Investigations, BS 5930. British Standards Institution. Code of Practice for Foundations, BS 8004. British Standards Institution. Code of Practice for Building Research Establishment, Sulphate and Acid Resistance. British Standards Institution, Code of Practice 4: Indentification of Contaminated Land and its Investigation, DD175, 1988. Concrete Society. Formwork—A Guide to Good Practice, CS 030, 1995. Concrete Society. Joints in insitu concrete, Digest No. 10, CS 053, 1988. Concrete Society. Non-structural Cracks in Concrete, Technical Report 22, 1992. Concrete Society. Curing concrete, Digest No. 3, 1985. Deacon, R.C. Watertight concrete construction; Cement and Concrete Association, 46.504 1980. Harrison, T.A. Early-age crack control in concrete. CIRIA Report 91, 1981. International Standards Organisation. Building Construction Sealants, Classification and Requirements, ISO 11600. Pink, A. Winter Concreting, British Cement Association, 1978. Price, W. Recent developments in the use of controlled permeability formwork, Concrete, March 1998, pp. 8–10. Shirley, D. Concreting in Hot Weather, British Cement Association, 1980.

Copyright 2000 Philip H Perkins

Chapter 5

Construction of swimming pool shells in reinforced sprayed concrete and other materials

REINFORCED SPRAYED CONCRETE (SHOTCRETE)

5.1 Introduction The term gunite was originally used in the UK, for pneumatically applied mortar (cement, sand and water); in the US this was known as shotcrete. With the increasing use of a coarse aggregate in addition to the sand, the term now generally used in the UK is sprayed concrete. The American Concrete Institute (ACI), defines shotcrete as ‘mortar or concrete pneumatically projected at high velocity onto a surface’. The Concrete Society (UK), published a specification for sprayed concrete in 1979, and Guidance Notes on the measurement of sprayed concrete in 1981. The UK Code for the design of concrete water-retaining structures (BS 8007) only devotes one short clause (6.7) to pneumatically applied mortar and does not offer any detailed advice or information on the use of this material. The Code comments: ‘It is a specialist operation…’ and ‘the designer should agree a full specification with the contractor for materials, mix proportions, mixing, placing, equipment and curing…’ The implication of this statement is that structures constructed of pneumatically applied mortar are outside the scope of the Code. No reference is made in the Code of pneumatically applied concrete, and this is unfortunate as sprayed concrete has been used satisfactorily for many years for swimming pools, both large and small. The material is also used for repair and strengthening reinforced concrete structures and for lining tunnels. In the US and Canada the relevant Code for concrete water retaining structures is ACI 350 R-89 Environmental Engineering Concrete Structures. This is similar in many respects to the UK Code (BS 8007), and does not refer to the construction of water retaining structures with shotcrete. The use of sprayed concrete has a number of advantages: 1. 2.

Formwork (which is very expensive) is virtually eliminated. The pool can be formed to any desired shape without undue difficulty and significant increase in cost.

Copyright 2000 Philip H Perkins

3. 4.

5.

High speed of construction is possible. The pool can be constructed on a congested site where access for materials and equipment is severely restricted, because the delivery hose to the gun can be at least 100 m long. The only joints normally found in sprayed concrete pools are plain, butt construction joints which are usually not provided with a sealing groove and sealed with sealant. There are a number of disadvantages with shotcrete pools:

1. 2.

3.

The design and construction is not specifically covered by the UK Code (BS 8007). The problem of flotation can be a real one and this should be checked in all cases, and where necessary, suitable precautions should be taken. See Section 4.4 and 5.4.1 The usual design incorporates a wide cove angle at the junction of the wall and floor; this prevents the use of ceramic tiles as a finish. The alternative is to use ceramic mosaic, or to design the pool shell to eliminate the wide cove at the junction with the floor (Figures 5.1 and 5.2). Pools constructed for national and international competitions should

Figure 5.1 Reinforcement prepared for construction of pool shell in sprayed concrete. Courtesy, Cement Gun Co.

Copyright 2000 Philip H Perkins

Figure 5.2 Construction of pool shell in reinforced sprayed concrete. Courtesy, Buckingham Swimming pools Ltd.

4.

5.

have a right angle at the junction of the walls and floor in order to comply with ASA and FINA requirements. Special skill and care is required by the gun operator to ensure that all the rebars are properly embedded. It is particularly difficult to ensure full embedment of the rebars in the floor slab and at wall junctions. Sprayed concrete construction can be more vulnerable to low winter temperatures than insitu concrete due to the virtual absence of formwork. See Section 5.5.

5.2 Design and specification As mentioned above, the design of liquid-retaining structures constructed in sprayed concrete are not specifically referred to in the UK Code of Practice (BS 8007). However, if the design procedure set out in the Code is followed it could reasonably be claimed that the Code had been complied with. It appears that some experienced contractors specialising in swimming pool construction have evolved their own designs with the object of eliminating movement and stress relief joints, and this has generally proved satisfactory in practice.

Copyright 2000 Philip H Perkins

The specification for the concrete mix should be suitable for the type of equipment used, and also the concrete should possess low permeability and low shrinkage and have an adequate cement content. It is usual for shotcreting contractors to use ready-mixed concrete. It is recommended that the ready-mixed concrete should be supplied by a QSRMC Registered Company from a plant holding current QSRMC Certification for Product Conformity. The mix should contain not less than 325 kg of Portland cement per cubic metre, and the aggregates should comply with BS 882 Aggregates from Natural Sources for Concrete.

5.3 Methods of application There are two methods of application, the wet mix and the dry mix. 1.

2.

In the dry mix method, the cement and aggregates are weigh batched without the addition of water and the mix is then conveyed pneumatically to the ‘gun’ which consists of a mixing manifold and nozzle. It is here that the water is admitted by the gun operator. The operator has thus complete control over the amount of water in the mix. The mix is then conveyed at high velocity into place. Volume batching should not be used. In the wet mix method, the cement and aggregates are weigh batched and a predetermined quantity of water is added. It is usual for the concrete to be ready-mixed to a specification prepared by the designer or by the packagedeal contractor. The mix is then pumped to the nozzle where compressed air is admitted which conveys the mix at high velocity into place.

The Building Research Establishment in the UK carried out tests on the compressive strength of cores taken from both dry mix and wet mix sprayed concrete. The dry mix cores gave a compressive strength range of 50–72 N/mm2, and the cores from the wet mix gave a compressive strength range of 37–40 N/mm2. These figures are not necessarily accepted by experienced contractors as being representative of good quality wet mix construction. The properties and performance of sprayed concrete depends largely on the experience and skill of the operators, but the mix proportions, grading of the sand and coarse aggregate and the type and condition of the equipment used are also important. It should be particularly noted that the mix proportions of the sprayed concrete in place are likely to be different to the proportions at the time of batching. This is due principally to what is known as ‘rebound’. The amount of rebound is affected by the w/c ratio, grading of the sand and placement velocity, and the placement factor, i.e. whether the shotcrete is applied to a floor, wall or a ceiling. Experience in the US (see ACI 506-R-90 Guide to Shotcrete) gives the following figures for rebound.

Copyright 2000 Philip H Perkins

5.4 Execution of the work The majority of swimming pools are constructed wholly or partly below ground and the comments given in Chapter 4 on site investigations, under-drainage of site and flotation are applicable for pools constructed in sprayed concrete with special reference to flotation/uplift.

5.4.1 Flotation of the pool shell The thickness of the floor and walls of sprayed concrete is less than a conventionally designed insitu reinforced concrete pool. Also, the density (mass per unit volume) may be rather less than for well compacted insitu reinforced concrete. Consequently, the weight of the shell may be appreciably less than a reinforced concrete pool of the same size. With a high water table, the danger of uplift can be very real and the necessary precautions should be taken. If a calculation shows that flotation may occur, then the mass of the shell should be increased by thickening the floor slab to ensure a reasonable factor of safety, or by installing pressure relief valves in the floor. Many pool contractors install these valves as standard procedure. When a pressure relief valve operates, it admits ground water into the pool when the ground water pressure is higher than the pressure of the water in the pool. These valves, being mechanical devices, can fail to operate, and then the pool shell may be damaged by uplift. An increase in the mass of the pool shell in accordance with flotation calculations has obvious advantages. A further point for consideration is that the ground water may be contaminated, or may become contaminated, and if admitted to the pool through pressure relief valves, this could constitute a health hazard. A third solution to the problem of uplift is to control ground water level by means of under-drainage and level controlled pumps. The effect on the foundations of adjacent structures by the lowering of the water table must be considered and expert advice taken before a decision is made.

5.4.2 Application of the reinforced sprayed concrete One of the most difficult, but at the same time most important, problems is to ensure that the sprayed concrete is of consistent density throughout and that there are no voids or sand pockets behind the reinforcement. Corners and the floor require special care in this respect. Reinforcing bars should be fixed so that they are at

Copyright 2000 Philip H Perkins

least four bar diameters apart or 50 mm which ever is the greater, to help ensure full embedment of the steel. It is important to check for the presence of hollow-sounding areas behind reinforcement during the construction of the pool shell. This can be done by simple tapping with a light hammer or rod. For large projects, ultra-sonic pulse velocity equipment or impulse radar can be used. The nominal cover should be 40 mm. The cover should be checked with a cover meter as soon as practical after completion of the placing of the shotcrete (see Section 10.22.2 and Appendix 3 for information on cover meter surveys). In swimming pools, it is usual for the walls and adjoining bays of the floor to be gunned first, followed by the central portion of the floor. The walls are usually gunned in panels to their full height and thickness, followed by the adjoining floor bay, in one operation. This means that the size of the panel is largely governed by the capacity of the equipment, the organisation of the work and the output of the gun operator. Formwork is not used for the walls but only for a ring beam at the top of the walls, and for any intermediate wall beams which may be included in the design of deep pools. On the outer face of the walls, hessian or hardboard is fixed to a timber frame which forms the background to which the sprayed concrete is applied. If the excavation has been cut accurately and is stable, it is sometimes used as the ‘back shutter’. Prior to the commencement of the gunning, the hessian is sprayed with mortar (cement, sand and water) to stiffen it. Hardboard and plywood can be used instead of the hessian, but plaster-board should not be used as it is composed largely of gypsum (calcium sulphate) and the sulphates may, in the course of time, migrate into the sprayed concrete resulting in sulphate attack. The sub-base is often formed of large shingle (known as ‘rejects’), laid to a depth of 150–200 mm. Compacted granular material of adequate thickness can be used as a sub-base. On this the reinforcement is fixed and the sprayed concrete applied to the thickness required by the design. Figures 5.1 and 5.2 show swimming pools being constructed of sprayed concrete. It can be seen that the amount of reinforcement is considerable. Movement joints are not normally provided in walls and floor and thus the pool shell is virtually monolithic. This is discussed in the next paragraph.

5.4.3 Joints Although movement joints are not normally provided in the shell of sprayed concrete pools, even a small private house pool will not be gunned in one continuous operation. This requires the use of day-work/construction joints. There are differences of opinion about how these joints should be formed. The ACI in their Recommended Practice for Shotcrete recommends featheredging, but in the UK it is considered good practice to form a plain butt joint down to the

Copyright 2000 Philip H Perkins

reinforcement, and below this the joint is tapered. The object is to make the joints monolithic and to assume that movement across the joint does not take place to a degree likely to cause cracks exceeding about 0.2 mm wide at the surface as the weight of reinforcement normally used would ensure adequate crack control. The virtual absence of formwork will make a major contribution to reducing temperature rise in the sprayed concrete. Cracks of this width (0.1–0.2 mm) are most unlikely to be seen on the rough surface of sprayed concrete, unless carefully and specifically looked for. The width of very fine cracks on such a surface are very difficult to measure.

5.4.4 Curing the sprayed concrete Proper curing, as described in Section 4.13 is essential, although there is a tendency to neglect it or even omit it altogether. Lack of careful curing can result in serious drying shrinkage cracks which may penetrate down to below the reinforcement and result in corrosion of the steel.

5.4.5 Finishing the sprayed concrete Sprayed concrete can be left ‘in the rough’ straight from the gun, or it can be worked over with a wood float, depending on what subsequent finishes are required. The time after ‘gunning’ for the application of the wood float must be carefully judged by an experienced operator; if too early, it will disturb the newly gunned material, and if too late it will be ineffective in providing a relatively smooth, even surface. The ‘as gunned’ material provides a good key to rendering, and screed, but the surface must be well brushed down, to remove all loose material (this cleaning can also be carried out by compressed air), prior to the application of the rendering/ screed. For pools which are finished with tiles or mosaic, it is normal good practice to apply rendering in order to ensure a true and even background for the tiling/ mosaic.

5.4.6 Construction in cold weather and hot weather The problems associated with placing concrete in cold weather with air temperature close to or below 0°C, and in hot weather with air temperature above about 28°C, have been discussed in Sections 4.6 and 4.7. The basic principles involved apply to the application of sprayed concrete. Sprayed concrete walls of swimming pools are particularly vulnerable to low temperature as the inside (water) face does not have any formwork and the outer face has only hessian or hardboard or thin plywood. The cement content of sprayed concrete is usually higher than insitu concrete and the resulting increase in the heat of hydration is advantageous. However, in very cold weather, the use of heated concrete and efficient thermal insulation are likely to be essential. If these cannot be provided then it would be prudent to suspend concreting until the weather improves.

Copyright 2000 Philip H Perkins

5.5 Thermal insulation The provision of thermal insulation to the floor and walls of swimming pool shells constructed below ground level in insitu reinforced concrete has been discussed in Section 4.15 to which readers should refer. A sprayed concrete pool is likely to have thinner walls and floor than an insitu concrete pool and therefore thermal insulation would be comparatively more effective. However, the basic fact that the heat loss from the pool surface is substantially greater than through the walls and floor applies and this is why thermal insulation is seldom used.

5.6 Pipework Recommendations for dealing with pipes which pass through the floor, and walls below top water level have been given in Sections 4.10.4 and 4.11.4. The comments and recommendations made can be considered as generally valid for pools constructed in reinforced sprayed concrete. An exception is the use of puddle flanges for pipes passing through the walls. The presence of a flange within the wall thickness would create problems in gunning behind the flange and therefore it is generally better to provide the flange on the inner surface of the wall and floor.

5.7 Testing for watertightness Theoretically, the water test described in Appendix 2 should be carried out before any finishes such as rendering, screed etc. are applied. However, the finish to the sprayed concrete is usually not acceptable to tilers, or suppliers of proprietary coatings and sheet vinyl linings, and this necessitates the application of rendering and screed. This constitutes a reason for carrying out the water test after application of rendering and screed.

5.8 Under-water lighting The provision of under-water lighting has been discussed briefly in Section 4.16 to which readers are referred, as the same principles apply to sprayed concrete pools.

Copyright 2000 Philip H Perkins

SWIMMING POOLS CONSTRUCTED WITH REINFORCED HOLLOW CONCRETE BLOCK WALLS AND INSITU REINFORCED CONCRETE FLOOR

5.9 Introduction This is a popular method of constructing small swimming pools for private houses. British Standard BS 5628 Part 2 1985 The Structural Use of Reinforced Masonry, deals with the design of laterally loaded walls based on limit state principles and it is recommended that these design principles be followed. Figure 5.3 is a sketch showing a suggested section through a reinforced blockwork wall and insitu reinforced concrete floor of a small swimming pool with a maximum depth of 1.50 m, and 8 m×4 m on plan. The blocks should be dense aggregate two core blocks, to BS 6073, having a minimum compressive strength of 10 N/mm2. All batching of concrete should be by weight, but mortar is normally batched by volume. The method of construction described here does not comply with the

Figure 5.3 Sketch through wall and floor of pool constructed with reinforced blockwork walls and insitu reinforced concrete floor.

Copyright 2000 Philip H Perkins

recommendations of the Code of Practice for concrete water retaining structures (BS 8007). The joint between the concrete block walls and the insitu concrete floor is particularly vulnerable to leakage and the provision of a substantial cove in cement/ sand rendering is recommended.

5.10 Construction of the floor The floor is cast first with a 100 mm high kicker on which the wall is built, as shown in Figure 5.3. The top surface of the kicker must be horizontal to accommodate the blockwork. The floor slab for these small pools (dimensions on plan should not exceed about 8 m×4.0 m) can be cast without a transverse joint provided the reinforcement is designed accordingly. If a joint is provided it can be considered as a contraction joint and detailed as shown in Figure 4.3. The reinforcement can consist of high tensile fabric to BS 4483, and the type of mesh should be determined by the dimensions of the slab. The fabric should be fixed so that the cover (to the top surface) is 50 mm. The cover should be checked with a cover meter as soon as practical after casting. See Section 10.22.2 and Appendix 3 for information on cover meter surveys. The mix proportions for site mixed concrete would be 360 kg OPC, 550 kg sand, and 1100 kg coarse aggregate (20 mm maximum size). The w/c ratio should not exceed 0.50 and the nominal slump should be 75 mm. To ensure full compaction of the concrete a plasticiser may be required. If the concrete is ready mixed then the order for the concrete should be: Designated mix to BS 5328, characteristic strength of 35 N/mm2, w/c ratio not exceeding 0.50, and nominal slump of 75 mm. The concrete should be cast on a slip membrane consisting of two sheets of polythene laid on either 50 mm of oversite concrete or 75 mm of compacted granular material blinded with sand. The floor slab should be not less than 150 mm thick. The floor slab should be cured by covering it with polythene sheeting held down around the perimeter and kept in position for at least four days. The floor would normally be finished with a cement/sand screed, having mix proportions of 1:4½ cement to concreting sand, medium grading (BS 882), preferably pre-packed. For additional information on floor screeds see Chapter 7. Any joints in the floor slab should be carried through the screed and any rigid finish such as tiles etc.

5.11 Construction of the walls 5.11.1 Reinforcement The vertical rebars must be located as accurately as practical and securely fixed into the floor slab.

Copyright 2000 Philip H Perkins

Reliance has to be placed on the infill concrete and the rendering to the blockwork walls to protect the rebars from corrosion as the concrete blocks have relatively high permeability, and to ensure watertightness.

5.11.2 The hollow concrete block walls The two-core hollow concrete blocks for the walls should comply with BS 6073 Precast Concrete Masonry Units, having a compressive strength of 10 N/mm2. The blocks should be 440 mm×215 mm×215 mm. The top surface of the kicker should be roughened and all grit and dust removed prior to the commencement of the blockwork so as to provide good bond to the building mortar. Horizontal reinforcement should be provided in each course of blockwork as shown in Figure 5.3 and the diameter of these rebars should be determined by the length of the wall. The mortar mix for the blockwork would be determined by the design of the wall, as required by BS 5628. The joints of the blockwork should be raked out to a depth of 10 mm to improve the key for the rendering. The walls should be finished on the inside with two coats of a cement/sand render containing 10 litres of SBR to 50 kg of OPC; the mix should be 1:3½ cement to sand. The thickness of the first coat should be between 12 and 15 mm, and the thickness for the second coat should be between 5 and 8 mm. The second coat should not be applied sooner than four days after the completion of the first coat to allow the first coat to mature. The final coat can be finished with a wood float if ceramic tiles or mosaic is specified. However, if a proprietary coating is specified, then the finish to the render should comply with the recommendations of the coating supplier. A substantial cove should be formed at the junction of the wall and floor. As a precautionary measure, it is recommended that the outside of the walls be given two coats of a proprietary waterproofing compound. If a bituminous based material such as Liquapruf is used this should be protected by hardboard to prevent damage by the back-filling. An alternative is to render the outside of the walls.

5.11.3 The concrete infill The concrete infill for the blocks should have mix proportions of 1:2:2½ by mass, using 10 mm maximum size aggregate, with a slump of about 150 mm, and a w/c ratio not exceeding 0.5; this would require the use of a plasticiser or superplasticiser.

5.11.4 The ring beam The wall is finished at the top with a reinforced concrete ring beam; the vertical reinforcement in the walls is extended into the ring beam. The mix for the ring beam can be the same as that used for the floor or the infill for the blocks.

Copyright 2000 Philip H Perkins

5.12 Pipework Pipework which has to pass through the floor slab should be carried out as recommended in Section 4.10.4. The passing of pipework through the block walls below top water level presents considerable difficulty and special care is needed. A flange should be provided on the inside (water) face.

5.13 Under-water lighting For under-water lighting, reference should be made to Section 4.16, but it must be emphasised that with the walls constructed in reinforced blockwork, there are serious practical difficulties in inserting lighting fittings.

5.14 Curing the concrete and protecting the blockwork The floor slab should be cured as described in Section 5.10. The rendering and screed should be cured as recommended in Sections 7.1.4.2 and 7.4.5. It is unlikely that the blockwork walls will require curing unless the weather is particularly hot and windy. The mortar joints in the blockwork are vulnerable to freezing temperatures and building the walls should be suspended during very cold weather.

5.15 Testing for watertightness The pool should be required to pass the water test described in Appendix 2 with the following modifications: 1.

2.

3.

The test should be carried out after the rendering and screed have been applied and allowed to mature for at least 14 days, and before the back-filling around the walls is carried out. The ‘initial soakage’ period should be 21 days as the precast concrete blocks have higher absorption than insitu reinforced concrete or sprayed concrete. The maximum permitted water loss over the test period of seven days should theoretically, not exceed 10 mm, but from a practical point of view, a somewhat higher figure may have to be accepted.

5.16 Back-filling around the walls This should be carried out after the pool has passed the watertightness test, and not sooner than 28 days after completion of the walls.

Copyright 2000 Philip H Perkins

The back-fill material should be carefully placed and compacted in 150 mm thick layers; see also the recommendation in Section 5.11.2.

5.17 Thermal insulation Thermal insulation has been discussed in Section 4.15 and in Section 5.5, to which the reader is referred.

SANDWICH TYPE CONSTRUCTION WITH INSITU REINFORCED CONCRETE CORE WALL AND CONCRETE BLOCKS AS PERMANENT FORMWORK

5.18 Introduction This method of construction is favoured by many package-deal contractors. As generally constructed, it does not comply with the Code of Practice, BS 8007 for concrete liquid retaining structures. However, when properly designed and constructed, it can give satisfactory service.

Figure 5.4 Sketch through wall and floor of pool constructed with insitu reinforced concrete core wall with concrete blocks as permanent formwork.

Copyright 2000 Philip H Perkins

The main question that arises concerns the walls. The concrete blocks which act as permanent formwork to the insitu core wall cannot be considered as forming a structural part of the wall because the blocks themselves do not comply with the requirements for concrete quality in the Code. This difficulty can be overcome by designing the insitu core wall to comply with the Code requirements without taking into account the concrete blocks, and then the suggested restrictions on size and depth in Section 5.19 would not apply. However, this is likely to be uneconomic compared with taking account of the blockwork in the design. The recommendations which follow are based on the assumption that the design is an empirical one and does not comply with the BS 8007. Figure 5.4 shows a section through the wall of this type of pool.

5.19 Construction of the floor The floor should be constructed in insitu reinforced concrete and provided with a kicker 100 mm high which forms the base on which the wall is constructed as shown in Figure 5.4. The top surface of the kicker should be horizontal so that the blockwork bed joints are also horizontal. The size of the pool should be limited to about 10.0 m×6.0 m×2.0 m deep. For a pool of this size, the insitu reinforced concrete floor can be cast without movement joints, provided the reinforcement is designed accordingly. Should the contractor decide to cast it in two bays then the transverse central joint can be a contraction joint formed by a stop end, and detailed as shown in Figure 4.3. Starter bars for the wall should be securely fixed in the floor slab. The main reinforcement for the floor is usually a high tensile fabric to BS 4483, located 50 mm from the top surface of the slab which should have a minimum thickness of 150 mm. The cover to the reinforcement should be checked with a cover meter as described in Appendix 3. The concrete mix should be as recommended in Section 5.10; and curing should be carried out as described in Section 5.14. The construction of the floor slab should be as described in Section 5.10.

5.20 Pipework For the walls, the pipes can be either cast-in or boxed-out. For pipework passing through the floor, reference should be made to Section 4.10.4.

5.21 Construction of the walls The reinforcement should be securely fixed so that the minimum cover of the insitu concrete is 40 mm.

Copyright 2000 Philip H Perkins

The concrete blocks should be solid dense aggregate blocks complying with BS 6073, with a minimum strength of 10 N/mm2. The mortar mix should be 1 part OPC, ½ part lime and 4½ parts of sand Type S to BS 1200, and the gauging water should contain 10 litres of SBR to 50 kg cement. As an alternative, masonry cement can be used as this eliminates the need to use lime. The mix would then be 1 part masonry cement to 3 parts sand Type S to BS 1200. As the height of the wall is limited to 2.0 m, the blockwork can be built to the full height and then the concrete for the core wall can be cast in one pour not less than 7 days after completion of the blockwork. Both skins of blockwork should be supported while the core wall is being cast and the supports should be left in place for 48 hours after completion of casting. A suitable mix for the concrete core wall would be: 360 kg cement; 550 kg sand; 1100 kg coarse aggregate (20 mm maximum size); Maximum water cement ratio, 0.50; Nominal slump, 100 mm. The use of a plasticiser is likely to be required in order to obtain the 100 mm slump with the w/c ratio of 0.50. Galvanised wall ties should be inserted in each course at 450 mm centres and staggered vertically.

5.22 Under-water lighting The procedure suggested in Section 5.20 for pipework should be adopted for underwater light fittings. Reference should also be made to Section 4.16.

5.23 Finishes to floor and walls The floor would normally be finished with a cement/sand screed and the inside surface of the walls with two coats of cement/sand rendering. It is recommended that the outside of the walls be given two coats of a proprietary waterproofing compound. If a bituminous-based compound is used, it would be desirable for the coating to be protected by plywood or hardboard to prevent damage by back-filling. A cement/sand rendering could be used instead of a proprietary waterproofing compound. The final finish can be ceramic tiles/mosiac, or chlorinated rubber paint. Package deal contractors often use marbelite, see Section 7.9.

5.24 Testing for watertightness The pool should be tested for watertightness not less than 14 days after completion of floor screed and rendering, as described in Appendix 2, modified as recommended in Section 5.15.

Copyright 2000 Philip H Perkins

5.25 Back-filling around the walls The back-filling should be carried out as recommended in Section 5.16.

5.26 Thermal insulation This has been discussed in Sections 5.5 and 5.17, to which the reader is referred.

OTHER METHODS OF CONSTRUCTION

5.27 General comments Chapter 4 and parts of this chapter are intended to cover the methods of construction adopted for the vast majority of swimming pools. There are however other methods which may be suitable and convenient in special circumstances. For these methods of construction, the pools should be limited in size to about 5 m×4 m with a maximum depth of water of 1.5 m. Also, for these pools, other than the precast post-tensioned type, a higher rate of water loss than the figure of 10 mm drop in level given in Appendix 2 may have to be accepted. Problems arise with joints in the walls. The installation of inlets and outlets and the circulating pipework are likely to create serious problems. When these factors have been taken into account, it is probable that the cost and time for construction exceeds that of a conventional pool. Where reinforced concrete is used for the floor, the nominal cover to the rebars should be 50 mm and this should be checked with a cover meter after casting the concrete; see Appendix 3 for information on cover meter surveys.

5.28 Pools constructed with mass (gravity) type walls 5.28.1 The walls The walls must be structurally stable by their own weight when subjected to water pressure from the inside when the pool is full and when subjected to sub-soil and ground water pressure when the pool is empty. Mass (gravity) type walls should be constructed on independent foundations. The walls can be mass concrete with mix proportions of about 1:3:6 by mass using 40 mm maximum size aggregate. The walls can be finished with cement/ sand rendering on the inside carried out as described in Section 5.11. The walls can often be conveniently constructed by casting against the sides of the excavation which has been covered with polythene sheeting. The insertion in the base/foundation of a steel flat as a waterbar is recommended.

Copyright 2000 Philip H Perkins

An alternative method is to use dense aggregate two-core hollow blocks (to BS 6073), 440×215×215 mm, with a minimum compressive strength of 10 N/mm2. The hollow blocks are filled with insitu concrete, and thus the wall is basically similar to the pool wall described earlier in this chapter, with the vertical reinforcement omitted. A waterbar cannot be used at the junction of the wall and the foundation. The wall must be stable by its mass; it is a gravity type wall and the thickness is determined by calculation. Such a wall should be finished with cement/sand rendering. The mortar for the blockwork should have mix proportions given in Section 5.11.

5.28.2 The floor The floor can be insitu reinforced concrete, 150mm thick, constructed as described in Section 5.10.

5.29 Curing the concrete Curing of the mass concrete, the insitu reinforced concrete floor and the cement/sand rendering should be carried out as recommended in Section 5.14.

5.30 Testing for watertightness The pools described here should be tested for watertightness as recommended in Section 5.15, but a higher water loss may have to be accepted.

5.31 Pools constructed in very stable ground such as chalk or rock Such pools can be satisfactorily constructed by the application of a lining of reinforced sprayed concrete direct to the sides of the excavation. The excavation should be carried out carefully to line and level so as to ensure a reasonably uniform thickness of the sprayed concrete. The principle is similar to that used for the lining of rock tunnels and similar work and therefore would be suitable for a flowthrough pool as briefly described in Section 8.1. The application of the sprayed concrete should be carried out generally as described earlier in this chapter. The thickness of the sprayed concrete and the amount of fabric reinforcement required would have to be decided in the light of site conditions, but the thickness should not be less than 75 mm. The sprayed concrete should be finished with cement/sand rendering and screed as described earlier in this chapter. A final finish with chlorinated rubber paint is recommended.

Copyright 2000 Philip H Perkins

Figure 5.5 Section through pool constructed in chalk or rock and lined with reinforced sprayed concrete.

Figure 5.6 Precast post-tensioned concrete units under erection. Courtesy, Dickerhoff & Widmann in association with IBACO International.

Copyright 2000 Philip H Perkins

The pool should be tested for watertightness as described in Section 5.7 but a somewhat higher water loss would not be unexpected. If the pool is intended to have water treatment, then the problems associated with the installation of the circulating water pipework as previously mentioned would apply. Figure 5.5. shows a section through such a pool.

5.32 Pools constructed of precast post-tensioned concrete units This type of construction has been used on a number of public swimming pools in Germany and Switzerland. Figure 5.6 shows such a pool under construction at Chur, Switzerland. As far as the author is aware, this method has not been used for swimming pools in the UK, and in Europe it has not progressed as originally anticipated due to the popularity for pools which are not rectangular on plan. The precast units are only suitable to rectangular pools.

5.33 Pool Shells of Steel With the object of ensuring complete watertightness and reducing dead load, a small number of pool shells have been constructed of welded steel sheets (carbon steel and stainless steel). Carbon steel shells have given poor performance, mainly due to problems of corrosion and deterioration of the finishes. In recent years stainless steel has been used for a few small pools. A valid assessment can only be made after the pools have been in use for several years.

Further reading American Concrete Institute. Guide to Evaluation of Shotcrete, ACI-506–4R-94. American Concrete Institute. Guide to Shotcreting, ACI-506R-90. American Concrete Institute. Specification for Shotcrete, ACI-506–2–95. Austin, S.A. and Robbins, P.J. (eds). Sprayed Concrete—Properties, Design and Application Whittles Publishing, Scotland, 1995. British Standards Institution. Precast Concrete Masonry Units, BS 6073, Parts 1 and 2. Building Research Establishment. Sprayed Concrete Tunnel Support Requirements and the Wet Mix Process, Current Paper CP18/77. Concrete Society. Specification for Sprayed Concrete, CS 021, 1979. Concrete Society. Guidance Notes on the Measurement of Sprayed Concrete, CS 022, 1981. Tomsett, H.M. The practical use of ultra-sonic pulse velocity measurements in the assessment of concrete quality, Magazine of Concrete Research, Vol. 32, No. 110, March 1980.

Copyright 2000 Philip H Perkins

Chapter 6

External works

6.1 General considerations External works on a large scale are only likely to be required for major swimming pool projects such as leisure and sports centres. Nevertheless, some external works will be needed for even a small private pool, and it is important to remember that good landscaping can transform a rather dull uninteresting site into a most attractive one. The advice of an experienced landscape architect is generally worth the cost. The term external works comprises paving, walling and surface water drainage. As most open-air pools are wholly or partly below ground level, the excavated material can be used to adjust levels and form terraces and embankments, which require proper compaction. On a sloping site, the pool can be constructed into the side of the hill, but care must be taken to ensure that the whole of the pool is built on solid undisturbed ground or is uniformly supported in some other way. The floor can be designed as a suspended slab supported on reinforced concrete columns or load-bearing walls, but this can be very expensive and is seldom adopted except for large projects. Paved areas laid on compacted fill can present problems arising from long-term settlement, and this is discussed in Section 6.2.2. Natural depressions in the ground can be useful in saving excavation for the pool. With unsymmetrical sites, the construction of a free-formed pool instead of a rectangular one can assist in producing an attractive layout. Consideration can be given to the construction of a circular pool for private houses when serious swimming is not contemplated. Large covered pools which form part of a leisure centre are sometimes designed with the plant rooms, stores etc. below the pool shell and/or below walkways and changing rooms. This is discussed in Sections 4.12 and 4.14. For outdoor pools, it is advisable for the edge of the pool and the surrounding paving to be raised slightly, say, 50–75 mm above the level of the adjoining ground. This will help to keep insects from crawling into the pool and will generally help in maintaining cleanliness in the pool. Paved areas adjacent to the pool should be laid to slope away from the pool; a minimum gradient of 20 mm in 1.00 m (1 in 50) is adequate; this is to prevent water used for washing down the paving and rain

Copyright 2000 Philip H Perkins

water, from draining into the pool. A proper system of surface water drainage may be required, depending on the layout and size of the project. See also Section 6.3.

6.2 Paving 6.2.1 Introduction There are a number of materials and methods of construction which can be used satisfactorily: Insitu concrete: plain or reinforced; pigmented; pattern-imprinted; chemically stained; Precast concrete flags—plain or pigmented, or chemically stained; Natural stone flags; Precast concrete paving blocks—plain or pigmented; Clay paving bricks (pavers); Asphalt or coated macadam, plain or pigmented. All the above can be used for paving for pedestrians and for vehicles but the end use must be taken into account when specifying the material and the method of construction. Precast and insitu terrazzo, and natural marble, while very attractive, are not suitable for paving around a swimming pool nor for any form of external paving as the finished surface is very smooth resulting in a real danger of slipping, particularly when wet. Action to provide a non-slip surface to these materials is likely to spoil their natural attractive appearance. It is not possible in this book to give a detailed specification for each of the above types of paving laid on various sub-soil conditions, but it is hoped

Fgure 6.1 Illustration of terms used in concrete paving.

Copyright 2000 Philip H Perkins

that the information which follows will be useful. Figure 6.1 illustrates the terms used. The finished level of paving close to buildings must be at least 150 mm lower than any damp-proof course (dpc) in the walls of the building.

6.2.2 Paving for pedestrians If the paving has to be laid on compacted fill, then the use of insitu concrete can give rise to problems, particularly for small jobs when heavy compacting plant is not available. Even well-compacted fill is liable to settle in the course of time resulting in cracks, and unevenness across the cracks which is very difficult to rectify without complete relaying. Owners can be bitterly disappointed when they find their new paving has developed an uneven surface and a considerable amount of cracking which can only be rectified at high cost. The use of compacted hardcore to make up levels is likely to give appreciably better results, but care must be taken to ensure that the hardcore does not contain material such as gypsum, which can attack concrete paving laid on it. The hardcore should be broken up into pieces not exceeding 50 mm in size and ‘blinded’ with a thin layer of sand.

6.2.3 Insitu concrete For the reasons given in Section 6.2.2, this type of paving is not recommended if it has to be laid on filled ground. It is better to use concrete flags or concrete or clay paving blocks as settlement of the sub-base can be readily corrected. The following recommendations apply when the insitu paving is laid on undisturbed ground. Top soil must be stripped down to the appropriate level and shape, to the finished level of the surface of the paving. On clay/peat, it is advisable to lay 75 mm of compacted gravel or similar material as the sub-base. A separating layer of 1000 gauge polythene sheeting should be laid on the subbase. On natural ground such as gravel or sand, the separating layer can be laid directly on the prepared ground (known as the sub-grade). Plain unreinforced concrete should be 100 mm thick and laid in bays not exceeding 2.5 m×2.0 m. The length should not exceed 1.5 times the width. If ready-mixed concrete is used, this should be specified as GEN 4 in accordance with the relevant clauses in BS 5328: Concrete. The concrete must be thoroughly compacted, and after finishing must be cured for not less than four days by covering with polythene sheeting held down around the edges. For site-mixed concrete, the mix proportions recommended are: 1 bag (50 kg) cement (about 1¼ cubic feet or 0.036 m3);

Copyright 2000 Philip H Perkins

2.5 ft3 (0.072 m3) coarse, clean concreting sand; 3.75 ft3 (0.108 m3 coarse (20 mm) aggregate. This gives mix proportions by volume of 1:2:3. Sufficient water should be added to make a workable mix which can be easily spread, compacted and finished.

6.2.3.1 Pigmented concrete Many building owners do not like the grey colour of concrete and want a coloured (pigmented) concrete. The amount of pigment should be decided by trial and the pigment added to the mix before water is added. The result can be disappointing due to variations in the tone (intensity of colour). It is not possible to obtain the same uniformity of colour as is given by a pigmented coating. These variations in pigmented insitu concrete arise from inevitable variations in mix proportions, standard of mixing, amount of water in the mix, variations in compaction and finishing. The pigments used should comply with BS 1014 Pigments for Portland Cement and Portland Cement Products. However, some colours are adversely affected by ultra-violet light (mainly blues and greens). There is one further problem with pigmented concrete and that is efflorescence or lime-bloom. This is a whitish discolouration which can appear on the surface of the concrete irrespective of whether it is pigmented or plain. It is much more noticeable on pigmented concrete, particularly if dark pigments have been specified. It wears off in time, but can be very disfiguring before it finally disappears. Some information on pigments is given in Section 2.5.5.

6.2.3.2 Pattern-imprinted concrete Pattern-imprinted concrete has been in use for many years but on a small scale. The colour is imparted to the surface of the plastic concrete in the form of a ‘colour-hardener’ which is sprinkled evenly by hand in two applications. The first application uses about 60% of the total dosage. The colour hardeners usually consist of mixtures of Portland cement, pigments, fine hard aggregate, and an admixture to assist the imprinting process. The imprinting tools consist of rubber mats of various shapes and patterns, which are pressed into the surface after the second application of the sprinkled colour-hardener. The final operation consists in the application of a wax or acrylic type sealant. The process should only be entrusted to specialist contracters with a proven record of successful jobs. The basic principles of construction given in Section 6.2.3 should be followed. Figure 6.2 shows a garage drive of patterned concrete.

Copyright 2000 Philip H Perkins

Figure 6.2 Garage drive in imprinted concrete.

6.2.3.3 Chemical staining of concrete Chemical staining of concrete is carried out after it has hardened for at least a month. These stains are water-based solutions of metallic salts and are claimed to react with the free lime in the concrete and form stable coloured deposits in the surface layers of the concrete. Concrete to be treated by this process should not be given a floated finish as this will significantly reduce the penetration of the metallic salt solution into the concrete. There will be variations in the intensity of the surface colour and unless this fact is accepted, the results can be very disappointing. Only specialist contractors should be employed with a proven record of satisfactory work.

6.2.4 Precast concrete paving flags These can be obtained in a range of standard sizes, rectangular and square. The thickness varies from 50 mm to 70 mm. These flags should comply with BS 7263 Precast Concrete Flags, Channels, Kerbs, Edgings and Quadrants, Part 1

Copyright 2000 Philip H Perkins

Specification. They should be laid in accordance with Part 2 Code of Practice for Laying. The Precast Concrete Paving and Kerb Association (Interpave) have issued a number of explanatory leaflets giving detailed information on these products and how they should be laid. The precast flags can be pigmented and, as they are factory made, variations in colour intensity should be much less than with insitu concrete. There is still a potential problem with change in colour intensity (fading) due to ultra-violet light, and efflorescence as referred to in Section 6.2.3.1. Generally, the flags should be laid on a bed of mortar, sand, or crushed rock fines, 25 mm compacted thickness. The bedding should be laid on a properly prepared sub-base. The flags must be uniformly bedded and well punned down onto the sub-base which should consist of Type 1 Granular Material, cement-bound material or wet lean concrete. The Interpave Information Sheets give details of these alternative materials suitable for the sub-base. The joints between the flags can be narrow (2–4 mm) and filled with fine sand, or they can be wide (5–10 mm) and filled with mortar (1:4½ cement to sand). British Standard BS 7263 requires that the difference in level between adjacent flags must not exceed 3 mm. This is important, particulary for paving around a swimming pool where people walk with bare feet. It is not unusual for this 3 mm ‘lip’ between adjacent flags to increase with time due to consolidation of the bedding and sub-base. When this occurs, it is relatively easy to take up the offending flags and rebed them. This is a sound reason for using precast flags instead of insitu concrete.

6.2.5 Natural stone flags These are best specified to have a riven finish to help prevent slipping. They can be laid in the same way as precast concrete flags, but the minimum joint width is 6 mm to allow for slight variations in the slab surface. The joints should be made with cement-sand mortar, having mix proportions of 1 part cement to 4½ parts of fine sand. Difference in level between adjacent flags should not exceed 3 mm. See BS 5385 Part 5 Wall and Floor Tiling.

6.2.6 Precast concrete paving blocks This type of paving has become very popular for both pedestrian areas and car parks, garage drives and access roads for commercial vehicles (Figures 6.3 and 6.4). The blocks should be manufactured to BS 6717 Precast Concrete Paving Blocks Part 1. Specification for Paving Blocks, and laid in accordance with Part 3 Code of Practice for Laying. The blocks are generally rectangular, 200×100 mm, but can be obtained in

Copyright 2000 Philip H Perkins

Figure 6.3 Concrete block paving around garden pool. Courtesy, Redland Precast Ltd.

Figure 6.4 Concrete block paving for light commercial traffic. Courtesy, Marshalls plc.

Copyright 2000 Philip H Perkins

a number of proprietary shapes and sizes. They are made in a range of thicknesses and colours and are frost resistant. Where the block paving is intended for pedestrian use only, then the full design procedure recommended by the Code (BS 7533) and detailed in the Interpave Information Sheet Concrete Block Paving, Structural Design is not required. Informed consideration must be given to the type of sub-grade (California Bearing Ratio, CBR), and the thickness of the Type 1 Granular Material used for the subbase. The level of the water table must also be taken into account. A block thickness of 50 mm would generally be sufficient for pedestrian use only. The recommendations on the Information Sheets issued by Interpave relating to installation and detailing should be followed.

6.2.7 Clay paving bricks (pavers) The relevant Standards and Codes are:

BS 7533 Guide for Structural Design of Pavements Constructed with Clay or Concrete Block Pavers; BS 6677 Clay and Calcium Silicate Pavers for Flexible Pavements Part 1 Specification for Pavers; Part 2 Code of Practice for Lightly Trafficked Pavements; Part 3 Method for Construction of Pavements.

Figure 6.5 Flexible clay paving under construction in herringbone pattern. Courtesy, Brick Development Association.

Copyright 2000 Philip H Perkins

Figure 6.6 Decorative clay pavors in large pedestrian area. Courtesy, Brick Development Association.

The Brick Development Association have issued a Design Note (No. 9) Flexible Paving with Clay Pavers, and this, together with the Standards and Codes referred to above should provide the information required for the design and laying of this type of paving (Figures 6.5 and 6.6).

6.2.8 Paving for light vehicular traffic This is intended to cover parking areas and access roads for private cars and light commercial vehicles. The materials discussed in this section are: insitu reinforced concrete; precast concrete paving blocks; clay paving bricks (pavers); asphalt and coated macadam.

6.2.8.1 Insitu reinforced concrete for use by private cars and light commercial vehicles For this type of use, the following is recommended, based on the use of readymixed concrete laid by a local contractor.

Copyright 2000 Philip H Perkins

The order to the supplier should state the purpose for which the concrete is to be used. The mix should comply with BS 5328 Designated Mix PAV 1 Table 13 Part 1 and Table 6 of Part 2. The concrete should be air-entrained (mean 5.5% entrained air); see Chapter 2 for information on air entraining admixtures. A minimum thickness of 150 mm is recommended, laid on a slip membrane of 1000 gauge polythene sheeting, laid on a compacted sub-base not less than 100 mm thick. The slab should be reinforced unless it is laid in bays not exceeding 3.0 m×2.5 m. The use of fabric reinforcement located 50 mm from the top surface of the slab would allow the bays to be increased in length and width, the dimensions depending on the weight of reinforcement used. The transverse joints can be stop-end joints with the reinforcement stopped back 75 mm each side of the joint. Tie bars are sometimes used in these joints. The use of saw-cut joints in this class of work can result in practical difficulties over timing of the sawing. Tie bars are recommended at longitudinal (warping) joints; they are usually located at 500 mm centres and are located at the neutral axis, and are bonded throughout their length. The concrete should be laid between side forms, well compacted, and properly cured for four days by covering with polythene sheeting or by the application of a resin-based curing compound. OCCASIONAL USE BY HEAVY COMMERCIAL VEHICLES Paving subject to occasional use by heavy commercial vehicles (e.g. petrol bowsers) should be designed, specified and laid on the same principles as those adopted for highways, which are laid down in the publications of the Department of Transport in the UK and to the following authorities in the USA and Canada: American Association of State Highways and Transportation Officials (AASHTO); The American Concrete Institute; The Portland Cement Association (USA); The Ministry of Transportation and Communications (Ontario); The Portland Cement Association (Canada). The work should be carried out by an experienced contractor under reasonable site control.

6.2.8.2 Precast concrete paving blocks This type of paving is suitable for light and heavy commercial vehicles provided the recommendations of BS 7533 Guide for the Structural Design of Pavements Constructed with Clay or Concrete Block Pavers, supplemented by the Information Sheets issued by Interpave, are followed.

Copyright 2000 Philip H Perkins

Essentially, the procedure is to establish the CBR value for the sub-grade, followed by the selection of the sub-base material and determination of its thickness. The paving blocks should be laid in accordance with BS 6717 Precast Concrete Paving Blocks Part 3 Code of Practice for Laying. It should be noted that if the sub-grade material is susceptible to frost attack then the thickness of the sub-base would have to be substantially increased and expert advice should be sought.

6.2.8.3 Clay pavers The paving should comply with BS 6677 Clay and Calcium Silicate Pavers for Flexible Pavements Parts 1, 2 and 3, supplemented by the detailed recommendations given in the Brick Development Association (BDA) Design Note No. 9 Flexible Paving with Clay Pavers. In brief, it is necessary to establish the CBR value, and from this select the material for the sub-base, i.e. Type 1 Granular Material, lean concrete, soil cement, or cement-bound granular material (all in accordance with the DoT Specification for Highway Works) and decide its thickness based on the CBR value. If the sub-grade is frost susceptible, the sub-base should not be less than 450 mm thick. Care should be taken to prevent the water table rising to less than 600 mm from the pavement surface. This may require sub-soil drainage.

6.2.8.4 Asphalt and coated macadam This material is suitable for garage drives, car parks for light commercial, and heavy commercial vehicles. It can also be used for the resurfacing of deteriorated concrete paving; some information on this is given in Section 10.7. It has the advantage of being relatively easy to lay to close tolerances. Relevant British Standards are BS 594 Hot Rolled Asphalt for Roads and Other Paved Areas, and BS 4987 Coated Macadam for Roads and Other Paved Areas. The Quarry Products Association have issued a set of Information Sheets giving details of this type of construction for pavements and roads. Details of these publications are given under Further Reading at the end of this chapter.

6.3 Surface water drainage To help eliminate ponding, cross falls to all types of paving should not be less than 1 in 60 and drainage channels should have an appropriate longitudinal fall; factorymade channels are now available on the market. Surface water drainage for a large paved area normally consists of a channel or channels which collect the run-off from the paving and these channels discharge to road gulleys located at predetermined positions. The gulleys are connected to the main drainage system which is either a surface water sewer, a combined sewer, water course or soakaways. The design and construction of soakaways need careful consideration and reference should be made to BRE Digest 365, September 1991 Soakaways and CIRIA Report No. 156 Infiltration Drainage Manual of Good

Copyright 2000 Philip H Perkins

Practice 1996. The information in these publications is largely based on theoretical considerations. New research by the Hydraulics Research Station at Wallingford, UK, will be based on how soakaway systems actually work, taking into account rainfall, and the run-off from various types of surface. The design of large drainage systems should be based on the recommendations in BS 6367 Code of Practice for Drainage of Roofs and Paved Areas. The details of surface water drainage design and construction are outside the scope of this book. For the small projects, reference to BS 8301 Code of Practice for Building Drainage is recommended.

6.4 Walling 6.4.1 Intoduction For external works associated with swimming pools there are essentially two types of walling, namely free-standing walls and earth-retaining walls.

6.4.2 Free-standing walls These walls are usually constructed in clay bricks and one of the best-known references is the Brick Development Association’s publication Design of Freestanding Walls DG12. Also relevant is BS 5628 Part 3 Code of Practice for the Use of Masonry Materials, Components, Design and Workmanship. The most important factor in the design of such walls is the correct assessment of the exposure conditions and reference should be made to the Driving Rain Index map or the Wind Zone map of the UK which is included in BS 5628, and the BSI publication DD 93 Methods for Assessing Exposure to Wind Driven Rain. Ordinary clay bricks to BS 3921 Clay Bricks and Blocks, are generally suitable for use in sheltered and moderate zones, provided the wall is provided with an overhanging coping and has a damp-proof course of two courses of engineering bricks or two courses of slates half-lapped and bedded in mortar, located 150 mm above adjacent ground level. It is recommended to use sulphate-resisting Portland cement for the mortar for the full height of the wall. A mix of 1:¼:3 up to dpc level. Then a mix of 1:½:4½(cement, lime, sand) to coping level, or masonry cement and sand mix 1:3 by volume. For sites exposed to freeze-thaw cycles, frost resistant bricks should be specified. Copings should be provided with a throat and bedded on a bituminous felt damp-proof coursing and can be precast concrete or stone, and should project not less than 50 mm beyond the face of the wall on both sides.

Copyright 2000 Philip H Perkins

Figure 6.7 Boundary wall (free-standing) in contrasting facing bricks. Courtesy, Brick Development Association.

Movement joints should extend the full height of the wall (from foundation to top of the wall including the coping). The location of these joints depends mainly on the design of the wall (Figure 6.7); see Brick Development Association publication DG 12 for details. The wall should be structurally designed in accordance with the procedure set out in the BDA publication DG 12. As an alternative for small projects, the Building Research Establishment publication Building Brick or Blockwork Free-standing Walls, Good Building Guide 14 gives useful practical advice based on ‘rule of thumb’ procedures for this type of wall.

6.4.3 Earth retaining walls These can be constructed in reinforced concrete, mass concrete, stone, concrete blocks, or clay bricks. The first question which arises is under what circumstances should a garden retaining wall be ‘structurally designed’. There is no clear answer to this, but it is suggested that reference should be made to the Building Research Establishment’s

Copyright 2000 Philip H Perkins

Good Building Guide GBG 27 which provides rule of thumb guidance for the safe construction of brick and block earth retaining walls, up to a maximum retained height of 1.75 m. For masonry, clay bricks or concrete blocks, the relevant Code is BS 5628 Code of Practice for the Use of Masonry. Specific information on the use of clay bricks is given in the BDA publication Design of Brickwork Retaining Walls, DG 2. It is recommended that low sulphate clay bricks be used with a sulphate-resisting Portland cement (SRPC) mortar. ‘Weep holes’ should be provided near the base of the wall at about 2.0 m centres longitudinally. The mortar mix should be by volume, 1 part SRPC to ¼ lime to 3 sand (type S to BS 1200). For concrete block retaining walls, useful practical advice is given in the BRE publication GBG 27 Building Brickwork and Blockwork Retaining Walls. The mortar mix should be the same as for clay bricks. See Figure 6.8 for the view of an attractive clay brick retaining wall. Insitu reinforced concrete would only be used for larger projects and should be designed to BS 8110 Structural Use of Concrete. Useful information is contained in Civil Engineering Code of Practice No.2 1951 Earth Retaining Structures.

Figure 6.8 Reinforced clay brick retaining wall, 3 m high and 337 mm thick. Courtesy, Brick Development Association.

Copyright 2000 Philip H Perkins

Further reading Bennett, D. Chemical stain, concrete terrazzo and exposed aggregate finishes, Concrete, November/December 1992. British Standards Institution. Code of Practice for Assessing Exposure to Wind-driven Rain, BS 8104. Department of Transport. Specification for Highway Works, Parts 3 and 7, HMSO, London, 1986/89. Department of Transport. Structural Design of New Road Pavements, HD 14/87, with amendments, HMSO, London, December 1989. Department of Transport. Joints, New Civil Engineer Concrete Engineering, November 1997, pp. 32–3. Precast Concrete Paving and Kerb Association. Concrete Block Paving—Structural Design of the Pavement. Precast Concrete Paving and Kerb Association. Concrete Block Paving—Detailing. Precast Concrete Paving and Kerb Association. Paving Flags—Techniques for Laying. Precast Concrete Paving and Kerb Association. Kerbs and Footways, Model Specification Clauses. Quarry Products Association—Asphalt Information Service. Construction and Surfacing of Car Parking Areas, Information Sheets 1, 2 and 3. Quarry Products Association—Asphalt Information Service. Decorative and Coloured Finishes for Asphalt Surfacings, Information Sheet 4. Roeder, T. Update on pattern-imprinted paving, Concrete Quarterly, Autumn 1992, pp. 14– 15. Transport Research Laboratory. Design for Road Surface Dressing, Road Note 39, 4th edition. Whitehead, T. Rain relief, New Civil Engineer Concrete Engineering, November 1997, pp. 36–7.

Copyright 2000 Philip H Perkins

Chapter 7

Finishing the pool shell and associated structures; problems with pool hall roofs

FINISHING THE POOL SHELL AND ASSOCIATED STRUCTURES For the purpose of this chapter, the term associated structures means walkway slabs and floors of wet changing areas. The substrates to which the finishes are: applied are: Insitu concrete; Sprayed concrete; Precast concrete blocks. The finishes considered here in some detail are cement/sand rendering and screed followed by ceramic tiles and mosaic. Brief mention is made of marbelite (an insitu white terrazzo), coatings, sheet linings, and glass-fibre polyester resin linings. While ceramic tiles and mosaic can be applied successfully direct to insitu concrete, this should only be attempted by experienced concreting contractors due to the practical difficulties in obtaining the required surface tolerances on the concrete so that it is suitable for the laying of the tiles or mosaic. These tolerances depend on the type of bedding used. The Code is BS 5385 Part 4 and this requires that with thin-bed cement-based adhesives the gap beneath a 2.00 m straightedge must not exceed 3 mm; with thick-bed adhesives, the ‘gap’ must not exceed 6 mm. It will be noted in Sections 7.1.2 and 7.1.3 that Sulphate Resisting Portland cement (SRPC) is recommended instead of Ordinary Portland cement. The reason for this is that the author has investigated a number of cases of deterioration of tile bedding and cement-based rendering and screeds caused by sulphate attack. On ‘the balance of probabilities’, the source of the sulphate has been the pool water. The temperature of the pool water 26 °C to 28 °C assists the chemical reaction between the sulphates in solution and the tricalcium aluminate (C3A) in Ordinary Portland cement. See Sections 2.2.1, 3.5.2.2, 3.6, 3.7, and 8.8.

Copyright 2000 Philip H Perkins

British Standard BS 5385 Part 4 sets great importance on providing an adequate period for drying out of the pool shell when insitu concrete or sprayed concrete is used for walls and floor. A period of six weeks is recommended between the curing of the pool shell and the commencement of rendering or screed. The curing of the pool shell is likely to take about seven days after the removal of the formwork, or completion of the application of the sprayed concrete. It is assumed that pool shells constructed in insitu concrete or sprayed concrete have been successfully tested for watertightness (as described in Appendix 2) before the rendering and screed are applied. Compliance with the recommended periods result in the following time sequence: From casting to test (for structural reasons): Filling for test, 2 days; preliminary soakage, 7 days; leakage test, 7 days: Drying out after completion of test including 2 days for emptying (2+42)

28 days 16 days 44 days Total: 88 Days

This is a total period of almost 3 months from casting the pool shell to the start of the rendering/screed.

7.1 Cement-sand rendering to insitu concrete walls 7.1.1 Preparation of the base concrete It is essential to obtain maximum bond between the base concrete and the rendering as bond failure is probably the most frequent cause of serious trouble (failure) with rendering. The surface of concrete prepared for application of screed and rendering should be checked and accepted by the supervising officer before the screed/rendering is allowed to proceed. The most effective way of ensuring a high standard of bond is to prepare the surface of the concrete so that the coarse aggregate is slightly exposed. This type of surface is shown in Figure 7.1 and can be obtained by the following means: 1. 2. 3.

percussion tools such as bush hammers and Kangos; grit blasting (wet and dry); high-velocity water jets.

Of the above, method 3 is much preferred. A depth of exposure of the coarse aggregate of 3 mm is adequate; excessive use of percussion tools can cause fracture of the coarse aggregate resulting in a weak surface. Methods 1 and 2 can be create considerable dust and fine grit and

Copyright 2000 Philip H Perkins

Figure 7.1 Close-up view of concrete surface prepared for application of rendering or screed.

method 1 considerable noise. All grit and dust must be removed before the first coat of rendering is applied. Percussion tools should not be used earlier than 21 days after completion of the casting of the concrete; in cold weather this may have to be increased, see notes on concreting in cold weather in Section 4.6. Methods 2 and 3 can be started seven days after casting the concrete in ‘normal’ weather conditions. However, there is some advantage in delaying the exposure of the coarse aggregate until after the completion of the water test. The amount of water used with high-velocity water jets is comparatively small, about 50 litres per minute per jet, of which about one-third is dissipated as mist and spray. The pressure at the nozzle is in the range of 25 N/mm2 to 30 N/mm2. The jetting leaves the concrete clean and damp and very suitable for the application of rendering and screed (Figure 7.2). An alternative to exposure of the coarse aggregate, but rather less satisfactory, is to use a spatter-dash coat direct on the concrete, as described in Section 7.1.2. It should be noted that very high-pressure water jets can be used for cutting concrete (Figure 7.3). It is recommended that the preparation of the base concrete to receive screed

Copyright 2000 Philip H Perkins

Figure 7.2 The use of high-velocity water jet to expose the coarse aggregate in preparation for application rendering.

and rendering be carefully checked to ensure that the preparation is satisfactory before the application of the screed/rendering.

7.1.2 Spatter-dash coat A spatter-dash coat is not required if the concrete surface has been prepared by the exposure of the coarse aggregate as described in Section 7.1.1. It is necessary, however, if rendering is to be applied to a smooth dense concrete surface. Before the spatter-dash is applied, it is necessary to brush down the surface to remove all dust, dirt and the remains of the release agent and curing membrane (if the latter has been applied). Then immediately before the spatter-dash is applied the concrete surface should be well damped down. The object of this coat is to provide a firm rough surface with reasonably uniform suction, and this should ensure a good bond with the first coat of rendering. The mix proportions by volume should be: 1 part SRPC (Sulphate Resisting Portland cement) class 42.5 to BS 4027 1991; 2 parts of sharp clean dry sand. If damp sand is used allowance should be made for ‘bulking’ up to 25%. The grading of the sand should comply with

Copyright 2000 Philip H Perkins

Figure 7.3 Concrete wall cut by very high-velocity water jet.

Type A of Table 1 of BS 1199 Building Sands from Natural Sources for External Rendering. The cement and sand should be mixed with sufficient water to give a consistency of a thick slurry. It is applied as a very thin coat not exceeding about 2 mm thick. About an hour after the application, the spatter-dash should be lightly sprayed with water to ensure adequate hydration of the cement. Also, it must be protected from hot sun and/or strong winds by properly secured covers. About 36–48 hours after application, the first coat of rendering can be applied. The timing depends on the weather. Figure 7.4 shows a spatter dash coat being applied.

7.1.3 The first and subsequent coats of rendering The number of coats, and to some extent the thickness of each, will depend on the total thickness of the rendering, and this will depend on the accuracy of the as-cast surface of the base concrete. If, owing to inaccuracies in the

Copyright 2000 Philip H Perkins

Figure 7.4 Application of spatter-dash to concrete wall. Courtesy, British Cement Association.

surface of the concrete, the rendering has to exceed 25 mm in thickness, then it is advis able for a lightweight steel mesh to be provided as reinforcement. The mesh should preferably be stainless (austenitic) steel. The mesh must be pinned into the wall by stainless steel fixings, care being taken to ensure that the stainless steel does come into contact with the reinforcement of the concrete otherwise bimetallic corrosion is likely to occur (see Section 2.10). The thickness of the rendering is critical on the short end walls if the pool is used for competitive swimming. In the UK, the Amateur Swimming Association (ASA) requires a tolerance on the length of the pool of plus 30 mm, but with no minus tolerance. Unfortunately, the ASA does not say how the length should be measured, nor how a zero tolerance can be achieved in practice. The batching of rendering is generally done by volume in spite of the known inaccuracies involved. The UK Code of Practice is BS 5262 1991, and the British Cement Association publication External Rendering, reference number 47.102,

Copyright 2000 Philip H Perkins

1992, give useful and practical advice. The mix proportions recommended here for the first coat are: 1 part SRPC class 42.5 to BS 4027 1991; 3 parts clean dry sharp sand, graded to Type A, Table 1 of BS 1999 1976. The addition of 10 litres of SBR latex to 50 kg cement is recommended; the SBR acts as a plasticiser and thus improves workability; it also reduces permeability and the leaching of lime from the mix if the pool water has a negative Langelier Index (see Section 3.8). For the second coat, a slightly weaker mix is recommended, e.g. 1:3½ by volume, also with the inclusion of SBR. All mixing should be by mechanical pan mixer as an ordinary concrete mixer gives considerable variations in the actual proportions of the mixed material. For rendering onto dense concrete, the overall thickness should ideally not exceed 15 mm, except for small localized areas. The first coat would be about 10 mm thick and the second about 6 mm thick. It is important that the second coat should be appreciably thinner than the first coat. If for any reason a third coat has to be applied, this must be thinner than the second coat. So that for a total thickness of, say, 20 mm the first coat could be 12 mm thick and the second 8 mm. With a total thickness of 25 mm, the third coat could be 5 mm. The first coat should be scraped or scratched to provide a key for the second coat, and the same applies to the second coat if a third coat is required (Figure 7.5). If the final finish is ceramic tiles or mosaic, then the final coat of rendering should be finished with a wood float, and/or lightly scratched to provide a key for cement-based adhesive. If an organic-based adhesive is used, advice should be obtained from the adhesive supplier on the recommended finish to the rendering. Each undercoat should be allowed to mature for several days before a subsequent coat is applied; the exact time between coats has to be determined by weather conditions. In cold wet weather, the period should be increased.

7.1.4 Application of the rendering 7.1.4.1 Panel sizes and joints If the pool has been designed and constructed without full or partial movement joints, then the rendering can be applied in panels for the full height of the walls and with a length not exceeding about 5.0 m. Full movement joints in the pool shell must be carried through the rendering and any rigid applied finish such as ceramic tiles/mosaic. These joints are usually 15–20 mm wide and this width should be carried right through all applied finishes. Partial movement

Copyright 2000 Philip H Perkins

Figure 7.5 Combing freshly applied undercoat to provide key for subsequent coat. Courtesy, British Cement Association.

joints (contraction and stress relief joints) should also be carried through, but these can be of a width to coincide with the joint widths of the tiles. The joints in the rendering can be saw cut or wet formed. A discussion with recommendations for tiling is given in Section 7.6. The part of a full movement joint within the thickness of the rendering should be filled with a suitable backup material such as resin-bonded cork. It is recommended that apart from the desirability of lining-up movement joints in the pool shell with joints in the rendering and tiling, movement joints in the tiling should be carried down through the rendering to the base concrete. The whole question of making provision for movement in the pool shell, rendering and tiling is complicated and it has to be decided by experience, and practical consideration. Joint decisions between all parties concerned should be undertaken as early as possible in the design process.

7.1.4.2 Curing the rendering There is no need to wet-cure rendering apart from the spatter-dash coat as recommended in Section 7.1.2. However it is most important that each coat of rendering should be protected from hot sun and drying winds for a period of not less than 48 hours after application.

Copyright 2000 Philip H Perkins

It is strongly recommended that rendering should not be carried out in very cold weather as the 10–15 mm thickness is very vulnerable to low temperature especially when it is applied to cold concrete. When swimming pools are constructed inside buildings, it is sometimes thought that there is no need to protect the rendering as recommended above. The fact is that in a building under construction it is likely that doors and windows will not be fixed at the time the pool shell is rendered. A funnel effect can be created and the resulting blast of air can have a most adverse effect on the rendering resulting in serious drying shrinkage cracking, and debonding.

7.2 Cement-sand rendering to sprayed concrete walls The ‘as gunned’ surface of sprayed concrete is rough and only requires brushing down to remove all loose material, and damping, to be fit to receive the rendering. In most cases, two coats should be adequate and the mix proportions should be as set out in Section 7.1.3 and curing in Section 7.1.4.2. A limited amount of dubbing-out may be required. Sprayed concrete pools are normally constructed without movement joints, and construction joints are intended to be monolithic. The panel lengths can therefore line up with the panel sizes for the tiling, namely at about 4.5 or 5.0 m centres, and at other locations as recommended in Section 7.6.2.3. The joints can be wet formed or cut with a disc.

7.3 Cement-sand rendering to concrete block walls Dense aggregate concrete blocks provide a good key for cement-sand rendering provided they are first well brushed down to remove all grit and loose particles and then damped down immediately prior to the application of the rendering. The only other preparation needed is for the mortar joints to be recessed about 10 mm as the wall is built. Dense, very smooth faced blocks should be given a spatter-dash coat, but this type of block is unlikely to be used for the walls of swimming pools. The first coat of rendering should be 10–15 mm thick and each subsequent coat should be thinner than the preceding one. Generally, there should be at least two coats. The mix proportions recommended by volume are: First coat:1 part SRPC; 3.5 parts clean sharp sand Class A, to BS 1199 Table 1; 10 litres of SBR to 50 kg cement.

Copyright 2000 Philip H Perkins

The addition of the SBR (styrene butadiene rubber latex) improves workability and reduces water penetration. Second coat:1 part SRPC; 4 parts sand to BS 1199 (as for first coat); 10 litres of SBR to 50 kg cement. The rendering should be finished, protected and cured as recommended for rendering on to insitu concrete. See paragraph 7.1.

7.4 Cement-sand screeds on insitu concrete floors 7.4.1 Introduction Screeds are laid to provide a true and level surface with good suction on which to lay ceramic tiles and mosaic. High-quality dense concrete does not provide adequate suction to secure the standard of bond needed in a swimming pool if ordinary cement-sand mortar is used for bedding the tiles. Also, insitu concrete floors are generally not finished with sufficient accuracy for the laying of tiles and mosaic directly on them if thin or thick bed cement-based adhesives are used for tile bedding. Reference can usefully be made to BS 5385 Part 4 1986 Wall and Floor Tiling; Code of Practice for Ceramic Tiling and Mosaics in Specific Conditions. Section 13 of the Code covers swimming pools.

7.4.2 Preparation of the concrete The surface of the concrete should be prepared by exposure of the coarse aggregate as detailed in Section 7.1.1.

7.4.3 Mix proportions and laying The mix proportions recommended are set out below, and whenever possible the batching should be by weight/mass. 1 part SRPC; 4 parts clean concreting sand to grading limits C or M in Table 3 of BS 882. The w/c ratio should not exceed 0.5. The addition of 10 litres of SBR latex to 50 kg cement is recommended; this will assist bond, reduce permeability and improve workability. The mixing should be by pan mixer as concrete mixers give wide variations in the proportions of the resulting mix. The mortar should be laid between screeding boards and well tamped down either by a mechanical tamper or other suitable

Copyright 2000 Philip H Perkins

means to secure good bond with the base concrete and to ensure good compaction. The screed can be finished with a screed board, but this is not adequate for use as a compacting tool.

7.4.4 Joints Movement joints, both full and partial, should be carried through the screed. Monolithic joints can be ignored. Movement joints in tiling should be carried down through the screed; these joints are usually at 4.5–5.0 m centres.

7.4.5 Curing the screed The screed should be properly cured for a minimum period of seven days by covering with polythene sheeting held down around the perimeter and placed in position as soon as possible after the finishing operations are completed.

7.5 Cement-sand screeds on sprayed concrete floors The recommendations given in Section 7.4 apply here with the exception of the preparation of the surface of the sprayed concrete floor. The ‘as gunned’ surface of the sprayed concrete provides an adequate key for the mortar screed provided all dust, loose particles are first removed by light wet blasting. The laying of the screed, mix proportions, compaction, treatment of joints and curing are all as recommended for screeds laid on insitu concrete.

7.6 Ceramic tiles and mosaic 7.6.1 Introduction It is important that before any tiles/mosaic are fixed that the substrate (rendering and screed) should be carefully checked for bond to the pool shell. This can be detected by tapping with a light hammer or rod; a hollow sound indicates loss of bond (adhesion). The tapping should be done systematically, with particular attention to the edges of the bays. All hollow sounding areas should be clearly marked and the extent of each defined. Areas of defective bond, provided they are not extensive, can be grouted in with a low-viscosity polymer resin. Any cutting out of the rendering or screed should be kept to a minimum and should be done by sawing, percussion tools should not be used as the vibration is liable to reduce the bond in adjacent areas. For all practical purposes, the recommendations relating to the installation of ceramic tiles also apply to the installation of ceramic mosaics. Significant differences are drawn to the readers attention in Section 7.6.3.

Copyright 2000 Philip H Perkins

7.6.2 The tiles 7.6.2.1 General considerations The relevant British Standard is BS 6431 EN87 Ceramic Wall and Floor Tiles, which is a comprehensive document in 23 parts. Also relevant is BS 5385 Wall and Floor Tiling. Part 4 of this Standard is a Code of Practice for Ceramic Tiling and Mosaics in Specific Conditions, Tables 3 and 4 list the suitability of wall and floor tiles in specific conditions. For tiles continuously immersed (e.g. swimming pools), users are advised to ‘refer to manufacturers to confirm suitability’. For swimming pools, the tiles should be vitrified extruded tiles with a water absorption not exceeding 3% by mass when measured in accordance with Part 11 of BS 6431 Ceramic Floor and Wall Tiles. This type of tile is frost-proof and can be used for open-air pools. The floor tiles in the shallow end of the pool should have a slip resistant finish; this recommendation also applies to the walkways and wet changing areas and other areas which are frequently washed down and used by bathers with bare feet. There are various finishes which are slip-resistant and the choice is largely a matter of experience and personal preference. Reference should be made to Section 2.16 for additional information on ceramic tiles.

7.6.2.2 Laying the tiles The tiles should be laid with joints 3–4 mm wide and fixed with a proprietary cement-based adhesive suitable for continuous immersion, such as Ardex Arduflex 5000 or BAL High polymer modified adhesive. For greater resistance to sulphate attack and leaching of lime from the cement as a result of a negative Langelier Index, a proprietary epoxy-based adhesive, such as Ardipox WS or BAL Epoxy LV, could be used. The cost of an epoxy-based adhesive is significantly higher than a cement-based one, but may be worth the extra compared with the cost and great inconvenience of closing down a pool for extensive remedial work to the tiling, should the pool water prove more aggressive than originally anticipated. The adhesive should be mixed as directed by the manufacturers. It is emphasised in Section 8.6.1 that the pH of the pool water should be strictly maintained in the range 7.2 to 7.8 for effective water treatment. If the pH falls below 7.0, there is a risk that acid attack on the grouted joints between the tiles will take place (Figure 7.6). The tiles must be fully bedded, the adhesive being applied to the back of the tiles with a toothed and notched trowel, and the tiles being firmly pressed and tamped into position. The thickness of the bed will depend on the regularity of the substrate to which the tiles are fixed. Thin-bed adhesives should be used when the substrate checked with a 2 m straight edge does not reveal any gaps behind the straightedge which exceed 3 mm in depth. Suitable thick-bed adhesives should be

Copyright 2000 Philip H Perkins

Figure 7.6 View of erosion of grouted joints by pool water of low pH (acidic).

used when the gaps are between 3 mm and 6 mm. It can be seen that the regularity/ trueness of the substrate is of great importance. Grouting the tiles should not be carried out earlier than three days after completion of fixing. The grout should be a proprietary product from the same manufacturer as the adhesive and compatible with the adhesive. Figures 7.7 and 7.8 show pools completed with high-quality ceramic tiles and mosaic.

7.6.2.3 Movement joints Movement joints should be provided in the tiling at 4.5 m to 5.0 m centres, carried down through the rendering/screed to the structural pool shell. Movement joints are also required at all internal angles and changes of direction, and in the floor, at changes of gradient. Movement joints in the tiling are normally 6 mm wide. As movement joints in the pool shell must be carried up through the rendering/ screed and tiling, it follows that there must be close co-operation between the tile suppliers, tile layers and pool designer if these basic requirements are to be properly met. All movement joints must be sealed with a suitable sealant. The relevant British Standard is BS 6213 1982 Selection of Constructional Sealants. According to Table

Copyright 2000 Philip H Perkins

Figure 7.7 View of leisure centre pool finished with high-quality ceramic mosaic. Courtesy, Pilkington’s Tiles Ltd.

Figure 7.8 View of part of leisure centre pool and walkway slab finished with high-quality ceramic mosaic and ceramic tiles. Courtesy, Pilkington’s Tiles Ltd.

Copyright 2000 Philip H Perkins

4, the only material recommended without reservation is a flexible epoxy. Other materials are recommended, subject to reference to the manufacturers for their suitability. This indicates a fairly wide choice, but Table 2 of BS 6213 suggests that the ‘expected service life’ of the silicone, polysulphide and polyurethane type sealants is about 20 years. The estimated ‘life’ of the flexible epoxy sealants is not given, but it would be reasonable to anticipate an appreciably longer life than 20 years. The British Standards for polysulphide and silicone based sealants are listed by BSI as obsolete. Reference should be made to Section 2.15. The provision of movement joints recommended for ceramic tiling is also applicable to ceramic mosaics.

7.6.2.4 Tolerances on finished surface Acceptable tolerances on surface regularity are: 1. 2.

3 mm under a 2 m straightedge; difference in level across joints including movement joints: 1 mm for joints not exceeding 6 mm wide; 2 mm for joints exceeding 6 mm wide.

7.6.2.5 Scum channels and deck level pools Ceramic scum channels provide better circulation of the pool water than skimmer outlets. They should be securely bedded on the shelf formed in the pool wall. The perimeter channel which are an essential feature of deck level pools should be either glazed ceramic or finished with a smooth, impervious and durable coating such as chlorinated rubber paint (see Section 7.10.4), or epoxy resin. The internal surface of the concrete channel should not be left with the bare concrete as this becomes dirty and almost impossible to keep clean. With deck level pools, the perimeter channel usually discharges to a balancing tank and it is strongly recommended that the internal surface should also be finished with a suitable coating. Water circulation is dealt with in Chapter 8.

7.6.3 Mosaic There are two types of mosaic, ceramic mosaic and glass mosaic, the former being the type mostly used. It is particularly suitable for free-formed pools and pools constructed in sprayed concrete when there is a wide cove at the junction of the walls and floor. The ceramic mosaic are fully vitrified and are

Copyright 2000 Philip H Perkins

therefore frost proof. The tesserae (pieces of mosaic) can be obtained in a variety of shapes, sizes and colours; standard sizes are 20 mm×20 mm and 25 mm× 25 mm. The basic recommendations for laying ceramic mosaic are similar to those for ceramic tiles and are covered by BS 5385 Part 4 Code of Practice for Ceramic Tiling and Mosaic in Specific Conditions. The experience of the author is that the use of a nylon or other mesh embedded on the back of the mosaic can reduce the bond between the mosaic and the substrate (screed and rendering). It is better if the sheets of mosaic have paper on the face which is removed after laying. Very attractive patterns can be formed with mosaic as can be seen in Figure 7.8. For the best results, the work should only be entrusted to experienced firms.

7.7 Walkways and wet changing areas The recommendations given for screeds and tiling for insitu reinforced concrete pool shells apply to walkways around the pool and wet changing areas and shower rooms. It is essential that the surface of these areas should be ‘non-slip’, i.e. slip resistant, and when the floor slabs are suspended and use made of the space below, the floors must be completely watertight to the same standard as the roof of a building. See Section 4.12. The non-slip requirement can be readily met by the provision of slip resistant tiles, but the requirement for complete watertightness requires special attention to design, specification and execution. The design should be based on the water retaining Code (BS 8007). In view of the serious trouble which has occurred in a number of public pools where these floor areas have not been watertight, it is recommended that a waterproof membrane be incorporated in the floor. An insitu brush or spray applied coating in two coats is recommended. The second coat can be sprinkled with coarse sand to provide a key for the cement—sand screed. Because the key provided is not as good as that obtained by exposure of the coarse aggregate in the concrete slab, the screed should be not less than 30 mm thick and laid in bays not exceeding 3 m in width. The length of the bay is less important than the width as the fine transverse cracks which occur can be readily grouted in before the tiles/mosaic are laid. The membrane should be carried up walls which are built directly from the slab, and all openings in the slab for pipes and gulleys must be carefully detailed so as to be watertight. If a sheet membrane is used this will completely debond the screed from the base concrete and the screed should be not less than 75 mm thick and should be concrete with 10 mm maximum size coarse aggregate. Such a construction would increase the dead load of the floor. The floors must be laid to falls to drainage channels which discharge to the drainage system and not to the water circulation system of the pool. A gradient of

Copyright 2000 Philip H Perkins

1 in 40 (25 mm in 1.00 m) should be adequate to prevent ponding, but for safety reasons (to prevent slipping) a gradient of 1 in 60 (25 mm in 1.50 m) is recommended.

7.8 Testing the completed tiling The whole of the tiling should be tested for adhesion (detection of hollow sounding tiles by tapping with a rod) in the same way and for the same reason as recommended for rendering and screeds. Difficulty can arise in the interpretation of results and the fixing of a line between acceptance and rejection, particularly as the defects in adhesion may only extend over part of the area of individual tiles. The decision requires considerable experience as the vibration caused by the removal of defective tiles may increase the debonded area.

7.9 Marbelite Marbelite is a white terrazzo applied insitu to the walls and floor of swimming pools in the private sector. Package deal swimming pool contractors appear to favour this material for small pools, especially for private houses (Figure 7.9). It is composed of white Portland cement, white marble chippings, usually graded from 3 mm down and should be free from dust. The mix proportions are generally in the range of 1 part cement to 2–2½ parts marble. There is no Code of Practice for the use of insitu terrazzo as a finish to swimming pools, but the National Federation of Terrazzo, Marble and Mosaic Specialists have issued a specification for the material, mainly for use as flooring. The marbelite should be applied to cement-sand rendering and screed to ensure an even base. The surface of the substrate should be combed or scratched to provide a key for the marbelite which is usually applied in one coat to a finished thickness of 6 mm on the walls and about 10 mm on the floor. A minimum period of 21 days should elapse between the completion of the rendering/screed and the application of the marbelite. It should only be entrusted to contractors who specialise in the application of insitu terrazzo, and with a good ‘track record’ for the use of the material in swimming pools. It is not advisable to have the work carried out by an ordinary plasterer. The following list outlines some of the important points in its application: 1.

The marbelite should be applied in one coat to the required thickness (6–10 mm).

Copyright 2000 Philip H Perkins

2.

3.

4.

Some applicators will try to finish the whole of a small pool in one day, which is not advisable. The marbelite should be applied in panels, square if possible, otherwise the length/height should not exceed 1.5 times the width. Shrinkage can then take place along the line of the panel joints instead of forming random cracks. Even a 1:2.5 mix is very rich in cement and liable to shrinkage cracking and crazing. The marbelite must be protected for at least 48 hours by means of damp sacking or polythene sheets properly secured to prevent sun and wind impinging on the newly applied marbelite. Care must be taken to ensure that the surface of the floor and of steps leading into the pool are not too smooth, as polished merbelite is very slippery particularly to wet feet.

Marbelite stains easily because, in spite of the polishing, the surface is absorbent and stains are very difficult to remove and usually require grinding. This applies particularly to open-air pools; the stains from leaves and other organic matter are virtually impossible to remove without grinding. To combat staining, it is usual to provide a row of ceramic tiles or mosaic about

Figure 7.9 Application of marbelite to wall of private swimming pool.

Copyright 2000 Philip H Perkins

300 mm wide at the water line as these are less liable to become stained, and if this does occur, cleaning is relatively easy. Marbelite is particularly vulnerable to acid attack which can occur if the pH of the pool water is not maintained consistently in the recommended range of 7.2 to 7.8. The marbelite should be subjected to a test to detect defective bond as described in Sections 7.1 and 7.8. Figure 7.9 shows marbelite being applied to the wall of a small pool.

7.10 Coatings and paints 7.10.1 Introduction In cases where shortage of funds prevent the use of ceramic tiles/mosaic, the alternative is to use a proprietary coating (also referred to as paints). There is a fairly wide choice of materials, and for swimming pools the desirable characteristics are set out below:

7.10.2 Desirable characteristics The main features include the following: 1. 2. 3. 4. 5.

6.

The coating must be attractive in colour and appearance. It must have a smooth impermeable surface which can be readily cleaned. The coating must be capable of forming a good bond to the substrate. The normal substrate is cement based (concrete, rendering, screed) and the coating must not be adversely affected by the alkalies in the cement. The coating must be durable under the conditions in which it has to exist, namely warm chlorinated water containing solutions of chemicals used in water treatment. In the case of open-air pools, the coating must be resistant to weathering, ultra-violet light, and the effect of frost. It should possess some degree of elasticity (elastomeric), as the pool shell to which it is applied will move to some extent during filling and emptying, changes in temperature and foundation movement.

Reference should be made to BS 3900 Methods of Tests for Paints. The following Parts are of special interest: Part Part Part Part Part

C3 Through Dry Test for Multi-coat Systems; C5 Determination of Film Thickness; E2 Scratch Test; E6 Cross-cut Test; E10 Pull-off Test for Adhesion.

Copyright 2000 Philip H Perkins

7.10.3 Material types The basic types of available materials may be summarised as follows: 1. 2. 3.

Chlorinated rubber paint; Polymers such as acrylics, epoxies and polyurethanes; Cement-based paints.

Some types of coatings are vulnerable to blistering if there is moisture or vapour trapped behind the coating. A reasonable period must elapse between the completion of the substrate and the application of the coating and also before the admission of water into the pool. Directions for these two periods should be obtained from the manufacturers.

7.10.4 Chlorinated rubber paint This type of paint is very popular for swimming pools, both covered and open air. The paint should be the high-build type and generally consists of chlorinated rubber, a plasticiser, inert pigments and a thickening or thixotropic agent. It should be applied to a dry film thickness of 0.10–0.15 mm. The best results are obtained when the paint is applied to good quality cement/sand rendering/screed which has been finished with a wood float; the wood float gives a dense even surface without laitance. The substrate must be clean and dry and it is advisable to neutralise the surface by means of a dilute acidic wash (1 part hydrochloric acid to 10 parts water). Immediately prior to the application of the acid, the surface should be washed down with water as this will help prevent the acid being absorbed into the substrate. After about five minutes, the surface should be well washed down with water and the damp surface tested by a pH indicator; the pH should be in the range recommended by the paint supplier. The paint should be applied in three coats, the second coat being applied at right angles to the first to eliminate pin holes. A minimum of 24 hours should elapse between coats, and 48 hours is better. A period of 14 days should elapse between the completion of the final coat and the filling of the pool with water. One disadvantage with chlorinated rubber is that the makers usually recommend that it be applied within a fairly narrow range of temperature and humidity; this can be particularly difficult with open-air pools. The above information is intended as a general guide, and the detailed directions of the supplier should be followed. The useful life of chlorinated rubber is very difficult to predict and depends on many factors such as whether the pool is indoors or open-air, the standard of water treatment and cleaning and maintenance, and the amount of back-pressure (if any) arising from moisture and/or vapour trapped behind the coating. However, a useful

Copyright 2000 Philip H Perkins

life of four years would be a reasonable average for open-air pools and seven years for indoor pools. Figure 10.8 shows a pool completed with chlorinated rubber paint.

7.10.5 Epoxy resins and polyurethanes For basic information on epoxies and polyurethanes the reader is referred to Section 2.13. These paints can be obtained in a standard range of colours and when properly applied give very satisfactory results and should have a useful life of 5 to 7 years for open-air pools and 7–10 years for indoor pools. The following factors are important: 1.

2.

The material must be of the highest quality and a statement should be obtained from the manufacturer on the percentage of resin in the paint. Epoxies and polyurethanes bond strongly to concrete, rendering and screeds. The correct preparation of the substrate is of primary importance and the manufacturer’s directions should be carefully followed. However, the following indicates the steps which are generally recommended: (a) Grit blasting is advisable, but this should be very light as it is only required to remove the thin layer weak laitance. With good-quality concrete about 1 mm is all that should be removed. (b) A minimum of two coats is required, but three coats are preferred. Two coats should eliminate ‘holidays’ (pin holes), but three coats will certainly do so. When applied by brush, the second and subsequent coats should be applied at right angles to the preceding coat. It is usual to apply a primer before the first coat. (c) Epoxy resins can now be formulated to bond to damp concrete and it is advisable to select such a resin for external application. (d) Manufacturers supply information on the temperature and ‘shelf life’ of the resin and this information should be noted and adhered to. (e) If the material is two pack (resin plus accelerator), proper mechanical mixing is essential. These resins are normally used with a primer which should be supplied with the resin.

There is a distinct advantage if the supplier of the material is also responsible for its application as this avoids divided responsibility if something goes wrong.

7.10.6 Cement-based paints These paints are based on white or pigmented Portland cement, to which are usually added accelerators, waterproofers, and inert fillers. As normally used, they give a matt finish and are available in a range of colours including white. By the addition of a glaze, a gloss can be imparted to the surface.

Copyright 2000 Philip H Perkins

This type of paint is particularly suitable for cementitious backgrounds such as concrete, dense concrete blocks and rendering and screeds. The paint is mostly used for private and club pools as it has the great merit of low cost and ease of application and it can be applied over many types of existing decorated surfaces provided these are clean and sound. In the case of dusty, friable or absorbent surfaces, it is advisable to apply first a priming coat or stabilising liquid supplied by the paint manufacturer. The manufacturers always give detailed directions for the mixing and application of the paint which should always be followed. Approximate coverage on different surfaces is also given. A minimum of two coats is recommended. A minimum period of 24 hours should elapse between successive coats and in cool weather this may have to be extended to 48 hours. It is not possible to say how long such a coating should last as many factors are involved such as the location of the pool (open-air or indoors), the number of persons using the pool and the standard of maintenance. It is likely that an openair pool would need repainting each year if the owner required a high standard of finish. Reference can be made to BS 4764 Specification for Powder-cement Paints.

7.11 Sheet linings to swimming pools There are various types of flexible sheeting materials which can be used to provide a waterproof lining to small swimming pools. However, the type of material which is used almost exclusively in the private swimming pool market is polyvinyl chloride (PVC). These pools are known as liner pools. The pool shell must be structurally sound and if ground water rises above the level of the underside of the floor of the pool, the pool shell should also be watertight against infiltration of ground water. Unless this is taken into account, then if the pool is emptied, ground water pressure may force the liner out of position and it will prove very difficult, if not impossible, to rectify this. When the liner is originally fitted, it is ‘stretched’ and fits tightly against the inside surface of the pool shell. The PVC liner can be obtained in a variety of attractive colours and patterns. The ‘life’ of a high-quality PVC liner which has been correctly installed and carefully used is likely to be in the range of six to ten years. When a new liner is fitted, a different colour and/or pattern can be selected. For satisfactory service, it is essential that the liner should be supplied and fitted by the same firm as this eliminates divided responsibility. The suppplier of the liner should also provide detailed instructions for cleaning the pool and for advice on the treatment of the pool water. Figure 7.10 shows a pool completed with PVC sheet lining.

Copyright 2000 Philip H Perkins

Figure 7.10 Pool completed with liner of PVC sheeting. Courtesy, P.G.Sales and Services Ltd.

7.12 Glass-fibre polyester resin linings This type of lining was originally introduced into the UK in the 1960s and was first used for the renovation of old swimming pools. The use was extended to new private and club pools and then it was used for for a limited number of new public pools. As far as public pools were concerned, the results were not up to the original expectations in most cases. The pool shell should be watertight against loss of water from the pool and against infiltration of ground water when the pool is empty. This type of lining is intended to be fully bonded to the pool shell (floor and walls) and this normally requires the shell to be constructed of insitu reinforced concrete or sprayed reinforced concrete. This requirement does not apply when the material is used to renovate an old existing pool. The concrete surface should be prepared by grit blasting to lightly expose the coarse aggregate. All loose grit and dust must be removed. The following is a brief description of how the material is generally applied, but the details vary from one specialist applicator to another. The polyester resin, glass fibre and a catalyst are applied by compressed air through a three-nozzle gun, to a first coat thickness of about 1 mm. It is consolidated by metal rollers to ensure that the glass fibres are completely embedded in the

Copyright 2000 Philip H Perkins

resin. The first coat is immediately followed by the application of glass tissue sheet about 0.3 mm thick to seal in completely any glass fibres which may be projecting through the resin. The next stage consists of the application of a gel coat of polyester resin; this coat is allowed to semi-cure (the time taken depending on the formulation of the resin) and can be modified to suit the execution of the work. A second gel coat is then applied and contains pigments. The overall thickness of the laminate is between 3–5 mm. When properly executed this type of lining can be very attractive, but certain facts should be kept in mind when considering its use. These are summarised below: 1.

2. 3.

4.

Polyester resins have high shrinkage characteristics and this can lead to a build-up of stress at angles, corners and points of entry of pipework, resulting in formation of cracks. There is a considerable difference in the coefficient of thermal expansion of the resin and the glass fibres, resulting in internal stresses in the laminate. It can be difficult to secure good bond between the concrete/ rendering/screed and the laminate. If water penetrates through cracks in the lining, debonding can occur and spread over a considerable area. As the gel coats are of different composition to the body of the laminate, blistering and flaking sometimes occurs.

Briefly, while this type of laminate lining has performed well in many cases, there have been some instances of serious trouble. Special care should be taken in drawing up the contract documents to help ensure guarantees of satisfactory performance.

7.13 Finishes to the walls of pool halls 7.13.1 General considerations The selection of attractive and durable finishes to the inside surface of swimming pool halls requires careful consideration due to the conditions under which the finishes have to operate. The notes which follow are intended to highlight the problems and offer practical solutions.

7.13.2 The use of the natural (unprotected) surface of the structural material The increase in cost of both labour and materials in recent years has led to the search for structural materials which can be produced with a finish which is aesthetically satisfying and has a long maintenance-free life. This applies particularly to insitu and precast concrete. Good-quality Portland cement concrete will not suffer deterioration by the atmosphere in swimming pool halls. The high humidity, relatively high temperature and the presence of chlorine compounds will have no corrosive effect on the concrete. Some staining will occur but this will

Copyright 2000 Philip H Perkins

not generally be noticeable if it is above eye level, and the effect can be mitigated by the application of a good-quality silicone water repellent at about five-yearly intervals. However, this type of finish is not recommended in any position where it can come into contact with persons using the pool as serious staining by grease and dirt will result from such contact. The untreated surface of concrete is absorbent and this type of staining cannot be removed without grinding and this changes the light reflecting properties of the surface making the areas clearly visible. Figure 7.11 shows the wall of a pool hall left in its ‘natural state’. For the reasons given above this is not recommended.

7.13.3 Applied finishes The finishes briefly described here are all considered suitable for the walls, columns and similar structures of swimming pool halls and some for the walls of shower rooms:

Figure 7.11 Wall of pool hall left in its ‘natural state’. This is not recommended for reasons given in the text.

Copyright 2000 Philip H Perkins

Ceramic tiles and mosaic; Terrazzo; Marble.

7.13.3.1 Ceramic tiles and mosaic The relevant UK Code of Practice is BS 5385 Part 4 Code of Practice for Ceramic Tiling and Mosaics in Specific Conditions. It is recommended that this Part of the main Code be followed rather than Part 1 which gives recommendations for normal internal tiling. Part 4 contains detailed recommendations for tiling which will have to operate under wet and damp conditions, which covers pool halls, shower rooms etc. The principal points to note are: 1.

2. 3.

The tiling should be fixed to concrete or dense aggregate concrete blocks, or to cement/sand rendering. The structure behind the tiles must be watertight. This ‘tanking’ must be carried up from the floor to an appropriate height. In the case of the walls of pool halls, not less than 2.5 m; in shower rooms, for the full height of the walls. Cement-based and organic-based adhesives can be used. It is advisable, but not essential if the walls are ‘tanked’, for the grout used for the tile joints to be impervious.

Detailed advice for waterproofing/tanking the substrate to which the tiles are fixed can be obtained from the manufacturers of the tiles and adhesives.

7.13.3.2 Terrazzo and marble The same basic principles as described in Section 7.13.3.1 apply to terrazzo and marble. Thin bed adhesives should not be used, and organic-based adhesives are not recommended. Generally, these materials can be used on the walls of the pool hall but not in shower rooms. For detailed recommendations for the fixing of terrazzo and marble, reference should be made to the National Federation of Terrazzo, Marble and Mosaic Specialists. See the note in Section 7.13.3.1 on tanking the walls behind the tiles.

7.13.4 Toppings and coatings for plant rooms and stores for chemicals High-quality insitu or precast concrete is not really suitable as a finish for the floors of plant rooms and chemical stores. It is better from the point of view of long-term durability to provide an applied finish (coating or thin topping) of a polymer resin such as epoxy or polyurethane. Coatings normally do not exceed about a millimetre in thickness are likely to be adequate for private and hotel pools. These are applied in two coats on a primer, and can be obtained in a range of

Copyright 2000 Philip H Perkins

colours. The surface of the concrete should be wire brushed and all grit and dust removed prior to the application of the primer. For larger pools (clubs and public pools), the extra expense of providing a resinbased topping 3–4 mm thick is usually justified. The 3 mm thick toppings are often self-levelling. The material consists of selected aggregates and fillers and the resin and accelerator/activator. The base concrete must be good quality and laitence should be removed and the surface left free of dust and grit. If, for reasons of economy, it is decided to use a coating, then the tests for paints set out in BS 3900 Part E6 Cross-cut Test, and E10 Adhesion, should be included in the Contract Specification.

THE ROOFS OF SWIMMING POOL HALLS

7.14 General considerations As mentioned in Section 1.6, there are serious problems associated with roofs of swimming pool halls arising from the comparatively high air temperature and high humidity in the pool hall. If chlorine is used in the water treatment process, this will aggravate the situation from the point of view of corrosion of any unprotected ferrous metal. The result can be condensation in the roof space, leading to corrosion of ferrous metals, deterioration of electrical equipment and timber used in the roof structure. Cases have been reported where stainless steel has suffered serious corrosion when highly stressed in roof voids. Thermal insulation is required to prevent unacceptable heat loss and a vapour barrier is needed to avoid interstitial condensation. The principle is that the vapour barrier should be on the warm side of the thermal insulation. The details of construction depend on the design of the roof structure. In a swimming pool hall this problem can be overcome in two basic ways: (1) The provision of a pressurised roof void, or (2) The provision of a ‘warm deck’ roof.

7.15 P