What is the fundamental mechanism causing thermal cracking in massive concrete structures?

The fundamental mechanism causing thermal cracking in massive concrete structures is the development of tensile stresses due to differential temperature changes within the concrete mass, primarily caused by the heat of hydration. Massive concrete structures, such as dams, bridge piers, and large foundations, generate a significant amount of heat during cement hydration, the chemical reaction between cement and water. This heat causes the interior of the concrete mass to become much hotter than the exterior surfaces. As the interior heats up and expands, the cooler exterior restrains this expansion, creating compressive stresses in the interior and tensile stresses near the surface. These tensile stresses can exceed the tensile strength of the concrete, leading to cracking. The thermal gradient, the difference in temperature between the interior and exterior, is the driving force behind this cracking. The magnitude of the thermal gradient depends on factors such as the size of the concrete member, the type of cement used, the ambient temperature, and the rate of heat dissipation. Cracks typically form at the surface and propagate inward. Strategies to mitigate thermal cracking include using low-heat cements, employing cooling pipes within the concrete mass, placing concrete in lifts to reduce the thermal gradient, and providing insulation to slow the rate of heat loss from the surface. For instance, in dam construction, cooling pipes are embedded in the concrete to circulate chilled water, reducing the peak temperature and minimizing thermal stresses.