As the summer approaches, the need to maintain cool concrete temperatures increases. Hot weather can decrease workability and finishability and can reduce long-term strength. One reason this occurs is because temperature can influence the reaction rate of portland cement and water. When temperature speeds cement hydration, the bonding that results between the cement grains is weaker. Because cement hydration releases heat, it is difficult to keep concrete cool enough in hot weather so that long-term strength does not suffer. High temperatures near the center of large placements also cause that zone to expand while cooler concrete contracts. Thermal cracking can become a serious problem if the temperature disparity becomes too great.
Concrete temperatures can be reduced through several methods. Cooling the mix ingredients can help, or using chilled water, crushed ice, or liquid nitrogen can cool the entire mix. Using other cementitious materials to supplement portland cement can reduce the temperature rise in the concrete while meeting strength and durability requirements.
Reducing aggregate temperature provides the greatest reduction in concrete temperature. Putting cold water on aggregate piles quickly drops the aggregate temperatures to about 5 to 10 degrees above the water temperature. Cold water aggregate cooling is often the least expensive way to reduce concrete temperature. Jobs involving 500 cubic yards or less of concrete probably do not warrant the capital expense of aggregate cooling. Producers for these jobs might be better served by adding ice to aggregate piles. However, it can be difficult to transport and store ice, and to determine how much ice should be added to the aggregate. Liquid nitrogen can be used to cool batch water, produce an ice and water mixture, or cool the cement. A heat pump can provide adequate cooling by circulating the batch water through a refrigerated system, and at a lower cost than liquid nitrogen.