The water/cement ratio is the ratio of the weight of water to the weight of cement in the fresh concrete mix. The water/cement ratio controls the strength of the concrete and its permeability. The lower the water/cement ratio the higher the compressive strength and the lower the permeability of the hardened concrete. The lower the permeability the greater the potential durability of the concrete.
The water content affects the amount of drying shrinkage, the water cement ratio, and the workability of the plastic concrete. Water contents can be reduced by using water reducing admixtures. It is preferable to control the water content to a practical minimum.
Gases can be deliberately or accidentally incorporated into concrete leaving voids in the hardened concrete. The compressive strength of concrete tends to decrease with increasing voids content. Concrete should be well compacted to remove entrapped air. All concretes will contain some entrapped air voids, even if very well compacted.
Pigments are mixed into the concrete to produce colours, other than the normal grey colour. If light colours are required, it is normal to use white cement and a pigment. The pigment is usually added during batching as a finely divided coloured inert powder or as a liquid. These may also contain plasticisers to reduce water demand and other constituents that reduce the occurrence of surface efflorescence and minimise colour fade caused by UV light.
Silica fume (sf) is a by-product of the manufacture of silicon metal and ferro-silicon alloys. The process involves the reduction of high-purity quartz (SiO2) in electric arc furnaces at temperatures in excess of 2000°c. It is a very finely divided material, many hundreds of times finer than cement. It reacts with lime and water in a similar way to fly ash, also known as pulverised fuel ash (pfa), to produce a binder.
Ground granulated blastfurnace slag (GGBS) is the slag from iron producing blastfurnaces that is rapidly quenched in water and then ground into a powder. Chemically it is similar to, but less reactive than, Portland cement (PC).
Portland Cement is the most common form of cement used for general purposes. It is produced from firing a mixture of clay or shale, and limestone or chalk. The clinker produced in the kiln is ground to a fine dust. Historically it was known as Ordinary Portland Cement (OPC) but with current standard BS EN 197-1 the closest classification would be CEMI (typically 42,5R or 52,5N).
Safety Note – Most cements when mixed with water are highly alkaline and can produce alkali burns to unprotected skin. Avoid skin contact with fresh wet concrete.
Normal weight aggregates, used for most concretes, are naturally occurring gravels from land or marine sources, or crushed rocks, for example, limestone, granite or basalt.
Fine aggregate is material that will pass through a 4mm sieve. Generally fine aggregate is a naturally occurring sand material, crushed rock material, or a combination of both. The distribution of particle sizes, known as grading, is important because it effects some properties of the plastic concrete, e.g. water demand and bleeding. A uniform distribution of particle sizes is preferable.