Answer: First off, you have the right attitude toward heat reduction. There is no single solution for keeping the temperature of concrete down when the ambient air temperature is high. You have to have a flexible strategy for hot-weather concreting. The most important thing to do is start with the fundamentals of concrete hydration and what causes fresh concrete to have a higher temperature. The hydration reaction is exothermic, meaning the concrete gives off heat. This reaction is generated by the reaction between water and cement. The more cement, the more reaction and consequently the more heat.
Strategy 1: Whenever possible, reduce cement content. This does not mean you should reduce cementitious materials content, just the cement content.
This leads to Strategy 2: Whenever possible, use fly ash. The use of fly ash can significantly decrease the temperature rise in hydrating concrete. In addition, you will generally get improved workability performance and higher compressive strength. Strategies 1 and 2 often are enough to reduce temperature and, in most cases, reduce your production costs, but as with any strategy in ready-mix, you need to be flexible.
Strategy 3: Make use of chemistry. The chemical admixture industry produces numerous chemical retarders that can reduce the temperature of the curing concrete by slowing the hydration process. If none of these strategies help either alone or in combination, it may be time to take other measures.
Strategy 4: Nothing beats the heat like something ice cold. In this case, the use of ice shavings, ice chips, or ice blocksùas either a partial or complete replacement of water by weightùcan have a significant effect on the temperature of the concrete. The proof of this rests with the following equation:
H(TaWa + TcWc) + TaWwa + TwWw
Tconcrete = H(Wa + Wc) + Wwa + Ww
Where H is the specific heat of cement and aggregates (0.22 Btu/lb-F), Wa, Wc, Wwa, and Ww are the weights of water in pounds, and Tc, Ta, and Tw are the temperature of the cement, aggregates, and mixing water, respectively. If you reduce the temperature of the water, you can reduce the temperature of the concrete. However, this does not guarantee positive results, since ambient air and forms temperatures can quickly add heat back into the concrete, regardless of the original water temperature.
The chart compares the overall impact and effectiveness of keeping the temperature low not just for workability but also the quality of hardened concrete and the relative cost.
For this reason, the best producers use a combination of the above strategies so that they can keep concrete cool when the ambient temperature is high.
We invite questions from readers on all aspects of concrete production and use. We also invite comments on our published answers. Send questions and comments to Troubleshooting Editor, THE CONCRETE PRODUCER, 426 S. Westgate St., Addison, IL 60101; fax: 630.543.3112.