Concrete admixtures are used to improve the performance of concrete in construction applications. They are available in a wide range of liquid formulations for use during mixing at the concrete batching plant or on construction sites. They are injected into the concrete to enhance workability and improve water retention. They can be useful in situations where a project needs to be completed quickly, or when weather conditions prevent normal curing of concrete. However, they must be used by experienced construction professionals who understand the limits and benefits of this type of concrete.
The most common types of concrete admixtures are accelerating, retarding, and shrinkage-reducing admixtures. Accelerating admixtures are added to the concrete mixture to increase the rate of cement hydration, which speeds up setting time and early strength development. They are often used in cold-weather concreting, but can also be effective in warm weather. They are commonly used along with retarding admixtures to achieve high early-strength concrete.
Retarding admixtures slow down the rate of hydration and delay the set of the concrete. They are useful when concrete must be placed in a restricted time period, such as when a bridge deck is required to be opened up for traffic or when the structure must be closed up before a new building is occupied. They are often used in conjunction with accelerating admixtures to produce high-early-strength concrete.
Shrinkage-reducing admixtures reduce the amount of cracking caused by drying shrinkage in the concrete. They are typically added to the concrete during mixing, and may be used in combination with accelerating and retarding admixtures to create high-early-strength or structural concrete.