Concrete Cracking Part 3: How can it be Reduced?

This is the final blog of our three-part series on concrete cracking, if you would like to read our previous parts, you can find them here:
Part 1: Settlement & How much is too much?
Part 2: Shrinkage & Thermal Behaviour

Concrete is going to crack, however, there are methods that can help reduce the amount and ensure the structural integrity of the slab. This includes the use of forming concrete joints, which force cracks to occur at specific locations, giving the concrete a cleaner finish and allow these cracks to be easily monitored. These joints are also sometimes required to improve the performance of the material and allow the concrete to expand and/or contract without damaging the rest of the structure.

Soil Compaction & Tree Removal

To avoid concrete slabs being affected by settlement as described in Part 1 of this three-part series, ensure that all soil is properly compacted before pouring the slab. It is also strongly suggested that you avoid planting trees near slabs for two reasons. Firstly, you don’t want the roots growing underneath the slab, and secondly, trees will draw moisture from the soil, causing high plasticity soil to shrink even more than it normally would and can cause soil movement that will further cause concrete to crack.

Concrete Curing

Concrete curing can help prevent cracks caused by plastic shrinkage by stopping the slab’s surface from drying too quickly. Curing involves maintaining the concrete moisture content and temperature so that it can develop and achieve the designed properties of the mixture. The process begins immediately after the concrete is poured and is complete once the concrete has developed the desired strength and durability. This is very important for reducing concrete cracking as mentioned previously in Part 2. For further information on concrete curing techniques be sure to check out this blog.

Contraction Joints

Concrete poured during hot weather will contract as the weather cools down, therefore contraction joints are placed in the concrete to help minimise the impact. Contraction joints are formed, sawed or tooled into a concrete structure to weaken a section of the concrete, causing a crack to result at that specific location when the concrete shrinks. This is usually in a straight line and is placed so they produce panels that are as square as possible, never exceeding a length to width ratio of 1.5 to 1. These joints are generally placed approximately 30 times the slab thickness apart (3-4.5m max) and should be a minimum depth of ¼ of the thickness of the slab when using a groove for the contraction joint.

Expansion Joints

During the cooler seasons, it is important to place expansion joints within concrete slabs. Expansion joints minimise the cracking when temperatures rise by allowing the concrete to expand within the joints, rather than cracking elsewhere. This allows for thermal expansion and contraction without inducing stress into the slab. It should be noted that in these joints, some, but not necessarily all, bonded reinforcement is interrupted. When placed in pavement slabs directly on the ground, a compressible filler material is used to fill the separation.

Notes on Joints

When installing any type of joint to reduce cracking in concrete it’s important to note that they generally need to be formed within the first 12 hours, or even sooner during hot weather. If the joints are formed after this it may be too late as the concrete will have already began to shrink or swell, causing cracks to already begin occurring. During this time the concrete is still soft and special equipment is necessary to be used to perform what is known as the ‘soft cut’.

While using tooling equipment is fine for 100mm slabs, larger slabs will require a deeper groove than most tooling equipment is capable of (approximately 25mm max). For example, if the slab is 150mm thick, it will require a 37.5mm minimum groove cut, which normal tooling equipment isn’t capable of. Therefore, care should be taken as to ensure not only are joints added quickly, but the correct equipment for the job be used to ensure it is done right.