Specifications for dams and power plants routinely require the use of materials and methods that reduce peak concrete temperatures. This controls thermal cracking in mass concrete. Mass concrete includes more than dams and power plants, however. It's any concrete that expands or contracts too much because of differences between the air temperature and the peak internal temperature. WHY MASS CONCRETE CRACKS When cement and water react in concrete, heat is given off. The size and location of most concrete structures allow this heat to escape. But in some footings, piers, abutments, and walls, the heat builds up internally. It can't escape fast enough because the heat is generated too fast, the concrete section is too thick, or the outside temperature is too high. If the peak temperature is too high, cracking may occur soon after concrete placement or months later. HOW TO CONTROL THERMAL CRACKING There are many ways to reduce thermal cracking in concrete: 1. Use Type II cement 2. Reduce the cement content 3. Cool the freshly mixed concrete 4. Control the cooling rate of hardened concrete 5. Cure properly RECOGNIZE MASS CONCRETE Mass concrete is any concrete that requires extra care to cope with volume changes caused by heat generated during cement hydration. Be sure to recognize mass concrete situations and use mass concreting methods during design and construction.