Admixtures can be natural or manufactured chemicals that are added to the concrete before or during mixing. Admixtures are used in difficult construction situations such as hot or cold weather placements, pumping requirements, or early-age strength requirements. This can enhance workability of fresh concrete and the durability of hardened concrete. There are a few chemical admixtures that are most common:
- Air-Entraining Admixtures-Used for exterior pours that will be exposed to freezing/thawing cycles
- Water Reducer Admixtures-Used to lower the water content in fresh concrete to increase its strength or to obtain higher slump without adding additional water.
- High Range Water Reducers (HRWR)-Used for high strength, high performance concretes.
- Retarders-Used in large pours or hot days to delay the initial setting of concrete by more than an hour.
- Accelerators-Used in cold weather situation to reduce the initial setting time of concrete and produce higher strength at early stages.