Thermal Cracking

Summary

Thermal Cracking - is caused by a temperature difference across the depth of a concrete slab during hydration, resulting in uneven shrinkage.

Technical Information

The hydration of cement generates heat. If the rate of heat produced in hydration is greater than the loss of heat from the concrete surfaces, the concrete will expand. On the other hand, if the rate is lower there will be an overall contraction. From the generation of the difference in temperatures within the slab, tensile stresses can form between the cooler, exterior part of the slab and the interior part of the slab, in which shrinkage occurs slower, causing it to crack.

Some ways to reduce the temperature gradient in very thick slabs:

Using certain cement replacing materials, or different cement types which hydrate more slowly than a straight Portland cement (most common).
Casting deep panels in two passes.
Using insulation to keep the surface temperature closer to the core temperature (less common).
Having reinforcing steel protruding out of the element, conducting heat from the interior of the element to the air outside (less common).

This process can also be referred to as early age thermal cracking.