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Thermal cracking occurs when there is excessive temperature difference within a concrete structure and the ambient temperature. This temperature difference causes the external part of the concrete with a lower temperature to contract more. But this contraction is restrained by the interior of the concrete with a higher temperature.
This causes tensile stresses in the concrete. When such stresses exceed the tensile strength of the concrete, thermal cracking occurs. Thermal cracking can also occur when the temperature of concrete is very high compared to ambient temperature.
Many problems of concrete can be attributed to thermal cracking.
Fly ash in concrete can reduce thermal cracking. The temperature rise in concrete can be lowered through the use of fly ash as an additive or replacement of Portland cement. Research has shown that fly ash significantly reduced the total temperature rise in concrete. Test results obtained demonstrates that the high volume replacement of cement by fly ash is advantageous, particularly for mass concrete where thermal cracking due to excessive heat rise is of great concern.
Heat of hydration is important not only in mass concrete but also in structures which are more than 600 mm thick. These structures are massive enough to accumulate enough heat of hydration to cause thermal cracking. This is a very significant advantage for the durability of concrete because thermal cracking would reduce the water tightness.