early strength concrete admixtures are used to accelerate or delay the setting time of concrete. They can also provide other benefits including improved concrete quality, manageability and performance.
The term admixture refers to the ingredients that are added directly to concrete, besides Portland cement, water and aggregates. Admixtures may be added during the batching or during pouring, and they can improve concrete quality, manageability, acceleration or delay setting time, and achieve other specific effects by changing properties such as water-cement ratio, hydration rate, and other properties.
Accelerating admixtures accelerate the hydration reaction of the cement paste and thus increase the early strength of concrete. They shorten the setting time, thereby reducing the amount of plasticity that occurs during the pouring, shipping and placing process.
Non-chloride containing set accelerator admixtures are available that conform to ASTM C 494 Type C and can help to eliminate the corrosion problems associated with calcium chloride. Although calcium chloride is effective for early strength development, it can cause corrosive damage to embedded or stressed reinforcement and other concrete components.
Aggregate gradation uniformity is important for early strength, workability and long-term durability. Intermediate size aggregates fill voids typically occupied by less dense cement paste and optimize concrete density.
Adding air entrainers to concrete can reduce water consumption by up to 10% and improve concrete fluidity. Adding these entrained air bubbles can be beneficial in cold weather concreting because they retain the heat of hydration and prevent freezing, thus increasing strength.