Cement
The effects of different types of cement have already been
described in the section on Cements. Within one type the properties will vary, but if the supply is derived from one works only, this variation will be small.
Aggregates
The overall grading of the aggregate affects the amount of water that must be added because, in simplified terms, 'fine' grading require more water than 'coarse' grading to obtain the same degree of consistence. The aim is to combine the different sizes of aggregate in such a way as to achieve the optimum packing of the particles and so reduce voids to a minimum. The special considerations applicable to air-entrained concrete are discussed on page 16, under Air-entraining admixtures.
Aggregate particles that have an angular shape or a rough texture, such as crushed stone, give greater strength for a given free water/cement ratio but need more water than smooth and rounded particles to produce concrete of the same consistence. With smaller sized aggregates, the amount of sand needed to fill the voids increases with a corresponding increase in water demand. To maintain the free water/cement ratio necessary for strength and durability, at the specified consistence, more cement and/or admixture is necessary.
The sand and coarse aggregates need to be proportioned to produce a stable, cohesive mix at the required consistence with the minimum amount of water. Badly proportioned constituents require an
excessive amount of water to achieve the required slump, and this will result in concrete of lower strength and durability, as well as resulting in a mix prone to segregation.
Water
Water quality is the most consistent of the constituents of concrete but water quantity, as it affects the free water/cement ratio, is most important for control of consistence, strength and durability. The amount of water used should be the minimum necessary to ensure thorough compaction of the concrete. When deciding how much water is required, allowance must be made for absorption by dry or porous aggregates and for the free surface moisture of wet aggregates, as explained under Storage of aggregates on page 1 3 and Water on page 14.
Admixtures
Admixtures have been described in the section on Admixtures. All admixtures are batched in small quantities and need great care in dispensing and mixing to ensure dispersion through the mix.
Trail mixes
It may be necessary to establish that the proposed mix proportions, including cement content, will produce concrete of the required fresh and hardened properties, or satisfy a requirement to meet a maximum free water/cement ratio. This can be achieved either from examination of previous data or by the use of trial mixes.
It should be noted that, when ready-mixed concrete is supplied with third-party certification, trial mixes by the producer are not needed. Purchasers normally receive certificates for the intended mix designs.