Concrete Basics and 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.

Combined with the use of reinforcement, one of the most effective means of reducing the amount of cracking in slabs is by using the correct type and placement of joints in the slab. This effectively breaks the pour into several smaller sections which reduces the resistance to contraction forces within each slab section. This was traditionally done by the old system of pouring adjacent slabs alternatively in a chequerboard pattern. However, in today's environment where larger pours are necessary in order to keep jobs on a tight schedule, this out dated system is often not practical. To enable a continuous pour, yet still break the slab into several smaller slabs, it is common practice to install contraction or control joints in the wet concrete. The aim of this type of joint is to allow the shrinking concrete to move freely away from the joint and thus prevent random cracking elsewhere in the slab.

The correct type of joint and the appropriate placement of the joint in a slab is considered by many contractors to be a good insurance policy against customer complaints. It is far easier to explain why a joint was placed in the slab than it is to explain why there is a random crack in a slab with no joints.

(NOTE) This brochure is in Adobe Acrobat pdf format. To read pdf files you need the freely available Acrobat Reader on your system. If you do not have Acrobat Reader you can download Acrobat Reader here.