Temperature variations in concrete
Changes in the temperature of concrete can cause cracking. As the concrete gets hotter, it expands; as concrete gets cooler, it contracts. This is similar for many other materials you have probably learned about.
If the concrete wants to expand or contract but can't, it will most likely crack. For example, the concrete near the foundation of a dam is restricted by the foundation -- there, the concrete can't expand or contract if it needs to and this might result in cracking.
But what would cause a change in temperature?
Well, there are a number of reasons:
First, there will be some serious temperature goings-on during construction. As you learned on the What is concrete? page, heat is a major result of the reaction between cement, water, and aggregate to form concrete. The bigger the structure, the more heat that's going to be generated. As the concrete cools, it may experience "shrinkage" -- some water evaporates and the concrete gets a little smaller. If shrinkage is not controlled, cracks can form.
There are a couple ways to try to deal with high heats of hydration during construction:
precooling: use ice to make the concrete or chill the aggregate
postcooling: circulate cold water through coils around the structure
use less cement: the heat of hydration is directly proportional to the amount of cement used to make concrete
add a retarding agent to the concrete which reduces the rate of heat hydration
Too much cooling to combat the heat of hydration could lead to problems though. If the concrete is cooled too much, it will have to warm back up to reach an equilibrium, resulting in expansion and possibly cracking. This was a difficulty in Fontana dam.
There may be seasonal difficulties during construction: the foundation of a dam may be poured in the fall and then pouring is stopped until after the winter. The top of the concrete block may be exposed to cold temperatures and want to contract but the bottom of the concrete block is restrained by the foundation -- cracks!
There may be differences in the temperatures of the concrete and the surrounding air and water.
Finally, direct solar heat on the downstream face of the dam can increase the temperature of the dam dramatically -- this is another of the problems with the Fontana dam.
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