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 ready- mixed 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. the responsibility for selection of mix proportions; 2. the terms in which the mix is specified;

3. 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.

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.