chemical admixtures used in concrete play a vital role in the construction process. Their judicious use improves the quality of fresh and hardened concrete, reduces cost, helps in meeting specific project requirements & overcomes emergencies during the concrete operations. They can be classified as water reducing admixtures, retarders, accelerators, air-entraining agents & super plasticizers.
The main function of chemical admixtures in concrete is to improve the workability of the concrete without changing the water-cement ratio or cement content significantly. The admixtures are added during mixing or immediately before pouring and curing to achieve a desired slump & workability. They are further used to control the hydration of cement, increase or decrease the concrete permeability, modify the concrete surface, and meet certain emergency situations.
Water reducing admixtures are used to reduce the water content in concrete by upto 30-40% while maintaining the flowable characteristics of concrete. They also improve the concrete strength and durability and reduce the risk of cracking during the hardening process. Retarders are used to slow the setting rate of the concrete allowing for longer periods of placement & finishing. They also prevent concrete from stiffening too quickly which may hinder the proper placement of materials & increase labor costs.
Air entraining admixtures are used to increase the density of concrete by introducing pockets of air into the concrete mix. This increases the resistance to freeze-thaw cycles and improves the concrete performance in varying temperatures. It can also help in reducing shrinkage cracking & enhance the concrete aesthetics.