Concrete work

In situ concrete 

Concrete is a mixture of cement, fine aggregate, coarse aggregate and water. The strength and durability of concrete are dependent on the proportions of materials in a particular mix. The methods for specifying prescribed and designed mixes of concrete, either site mixed or ready mixed, are covered by BS 5328. 

Designed mixes

Designed mixes are those for which the purchaser is responsible for specifying the performance, and the producer is responsible for selecting mix proportions that will conform with the particular performance requirements. 

Prescribed mixes 

Prescribed mixes are those for which the purchaser specifies the mix proportions that will produce a concrete with the required performance. There are two types of prescribed mix: 

Ordinary prescribed mixes 

These are specified in accordance with the requirements of BS 5328. The grade of concrete is prescribed in Table 1 of the standard, together with permitted types of cement and aggregate, aggregate size, etc. 

Special prescribed mixes

These generally meet the same requirements as ordinary prescribed mixes, but mix proportions in kilograms are given for each constituent material. 

Materials

Cement 

The most widely used cement is Ordinary Portland Cement, which is manufactured to perform to B2 12. In the examples that follow we use ordinary Portland cement. There are of course other types of cements produced to satisfy particular criteria. 

Aggregate 

The term 'aggregate' is used to describe the gravels, crushed stone and other materials which are mixed with cement and water to make concrete. Aggregates defined as fine aggregate are natural sands, crushed rock or gravel or other materials which will pass through a 5 mm BS sieve. Coarse aggregate, such as natural gravel, crushed rock or crushed gravel, is retained in this sieve. 

Water 

Water for use in concrete should be reasonably free from such impurities as suspended solids, organic matter and- dissolved salts, which may adversely affect the properties of the concrete. 

Admixtures 

No admixtures are permitted to be incorporated in ordinary prescribed mixes. Therefore the development of costs in the examples in this chapter does not include costs for admixtures. However, the estimator must be aware of and account for any such requirement in the specification for admixtures in designed or special prescribed mixes. 

Weights and volumes 

The unit of measurement for the main structural items of in situ concrete work is the cubic metre (m3). However, constituent materials that form concrete are sold by weight. Thus we must develop the costs for such items by weight, and then convert the costs by weight to costs by volume for dry materials. Table 6.1 gives average weights per cubic metre for various materials. 

Weights of coarse and fine aggregates may vary for a number of reasons, and the estimator can account for these variations where required. For example, the weight per volume depends on how wet or dry the material is, and on the source of the material. 

The mass of aggregate to be used with 100 kg of cement can be obtained from Tables 1 and 2 of BS 5328. 

Shrinkage in concrete mix

During the process of mixing materials to form concrete, a reduction in bulk takes place. This is due to the finer particles of sand and cement filling the voids or interstices of the