What must happen to the temperature of a material for thermal expansion to occur?
in most cases the temperature must increase for thermal expansion to occur. Most substances expand as temperature increases because the atoms or molecules vibrate faster as temperature increases and experience greater separation.
There are a few substances that contract when the temperature increases, but this is only within a particular temperature range. For example, water contracts as its temperature increases within a few degrees of its freezing point. It occupies more volume frozen than it does in the liquid phase.
Thermal expansion is a physical property of substances. Some undergo more thermal expansion than others. Metals, for example, tend to expand more than other materials when heated. The coefficient of thermal expansion of a substance describes mathematically its amount of expansion per degree of temperature change.
Expansion joints are built into expanses of concrete such as sidewalks and driveways to prevent cracking of the concrete when the temperature increases.