CAUSES OF CRACKING
Effects of Applied Loads
Direct or flexural tension in concrete arising from applied external service loads, from temperature gradients due to solar radiation,or from the containment of liquids at temperatures above ambient,may cause cracking in the concrete.
Temperature andMoisture Effects
Changes in the temperature of the concrete and reinforcement and in the moisture content of the concrete cause dimensional changes which, if resisted internally or externally may crack the concrete. The distribution and width of such cracks may be controlled by reinforcement, together with the provision of the movement joints.
Heat is evolved as cement hydrates,and the temperature will rise for a day or more after casting and then fall towards ambient. Cracking usually occurs at this time, while the concrete is still weak.
Subsequent lower ambient temperature and loss of moisture when the concrete is mature will open these cracks although the loss of moisture at the surface under external drying conditions is usually low.
structure built in the summer but not filled or an external structure standing empty will usually be subjected to greater drops in temperature than the same structure filled.