One of the main ingredients of concrete and concrete block is water. There is an abundance of water in the mortar as well but only a portion of that water is consumed for chemical reactions leading to the setting of concrete.
After pouring the concrete for footing and foundations walls, the extra water will evaporate gradually in the process of concrete hardening.
Evaporation of water results in a slight shrinkage in the concrete walls and footing. While the wall wants to shrink its connection to other structural elements restricts this shrinkage which results in vertical cracks forming on the wall.
Another reason for vertical cracks in the foundation walls is sudden changes in temperature which causes shrinkage in the walls, thus creating cracks. As an example, a drastic drop in temperature during night and the sudden temperature increase during daytime may cause vertical cracks.
These kinds of vertical cracks usually do not affect the structural integrity or the resistance capacity of the foundation walls. Although it is still advised that owners monitor the crack and record the dimensions as outlined in Article 1.
As explained in previous articles, an expert’s opinion is likely required if the crack is live and spreading.