Chemical admixtures are materials in the form of powder or fluids that are added to plain concrete mixes at the time of batching or mixing in order to give the finished concrete certain properties. In normal use, admixture content is usually less than 5 per cent by mass of cement. There are a number of common types of admixtures:

• Accelerators speed up the hydration and early hardening and strength gain of concrete, which is important when trying to combat the effects of concreting in cold weather.

• Retarders slow the hydration of concrete, and are used in large or difficult pours where partial setting before the pour is complete is undesirable.

• Air entrainers add and distribute tiny air bubbles within the concrete, which reduces damage during freeze-thaw cycles in winter, thus increasing the concrete’s durability. However, entrained air has a disadvantage in that there is a trade-off with strength – each 1 per cent of air can result in 5 per cent reduction in compressive strength.

• Plasticizers increase the workability of ‘fresh’ concrete, allowing it to be placed more easily and compacted with less effort. Alternatively, plasticizers can be purposely employed to

reduce the water content of a concrete; in these circumstances they are referred to as ‘water reducers’. This improves the strength and durability characteristics of the concrete whilst still maintaining workability during placing.

• Pigments or colourizers can be used to change the colour of concrete, for aesthetic and design purposes. Most pigments come in a liquid form for adding to the mixing water.

• Corrosion inhibitors are used to minimize the corrosion of steel and steel reinforcement bars in concrete.

Подпись: Admixtures As admixtures alter the properties and performance of concrete, they must always be used in accordance with manufacturer’s instructions. They must be gauged very carefully at the mixing stage and from one batch to the next, so that each successive batch contains exactly the same amount as the previous one.

Bonding agents are used to create a bond between old and new concrete.