Concrete Basics & The need for Joints
What Causes Concrete To Crack?
Many surface cracks in concrete slabs are not a result of structural failure of the slab but are formed by uncontrolled
shrinkage of the concrete. These types of cracks can be avoided.
Shrinkage cracks are the result of the concrete volume decreasing as water is lost from within the concrete mixture. These cracks can still occur some months after the slab has been poured. However, the more rapid the drying process the more likely it is that shrinkage cracks will develop. The same type of cracking occurs in many other situations where moisture is lost from its structure. (eg. Clay soils will crack in times of low rainfall).
How To Control Cracking
In concrete slabs it is possible to reduce the width of these cracks by placing steel bars (reinforcement) in the concrete, which effectively holds the cracks together. Although shrinkage still occurs the cracks formed are closer together and are held together so tightly that they are usually too small to be visible.
It is generally not economically feasible to place enough reinforcement in large concrete slabs to eliminate all cracking. The aim of reinforcement is to eliminate large
visible cracks, and in conjunction with joint placement, minimise those that remain.