Why concrete cracks

One of the most common questions we can hear is, “will the decorative concrete overlay develop cracks?” and, “what can we do about it.”

If a job develops cracks, for whatever reason, we sometimes get feedback that the homeowner is questioning why their beautiful decorative concrete is cracking and they wonder if the applicator did a shoddy job.

This is a valid comment as they see the evidence of cracking, however, to answer the question, we need to understand a few key points about concrete and why concrete cracks in the first place.

When concrete is installed properly, it is one of the most durable and long-lasting products that can be used on any property. Therefore, the very first thing a property owner needs to do is to ensure that they have a good concrete contractor that follows well-established guidelines with respect to concrete placement.

One fact is sure, that good quality, durable, high strength, and crack resistant concrete does not happen by accident.

So let’s step back a little further and discover why concrete would crack in the first place. By this stage, you should be realising that a decorative concrete overlay will naturally perform in a similar way to the concrete it is resurfacing, based on the quality of the concrete.

So the problem may not necessarily be with the overlay, but be a consequence of the condition the concrete is in.

The first reason that concrete cracks are from excess water in the concrete mix.

Concrete does not require much water to achieve maximum strength. However, a wide majority of concrete used in residential work has too much water added to the concrete on the job site.

This water is added to make the concrete easier to install. There is a downside to this as the excess water in the concrete mix also greatly reduces the strength of the concrete.

Shrinkage is also the main cause of cracking. As concrete hardens and dries, it shrinks. This is due to the evaporation of excess mixing water. The wetter or soupier the concrete mix, the greater the shrinkage will be. Concrete slabs can shrink as much as 1-2 centimetres per 60 meters. This shrinkage causes forces in the concrete that literally pull the slab apart. Therefore, cracks are the end result of these forces.

The bottom line is a low water to cement ratio is the number one issue effecting concrete quality – and excess water reduces this ratio.

There is not much you as the property owner can do about it, but the concrete contractor can, that’s why you need to get a good contractor.

A good contractor should know the allowable water for the mix that they are pouring. It is more expensive to do it right, it simply takes more manpower to pour stiffer mixes.

The second reason that concrete cracks are from the concrete drying too quickly.

Also, rapid drying of the slab will significantly increase the possibility of cracking. The chemical reaction, which causes concrete to go from the liquid or plastic state to a solid-state, requires water. This chemical reaction, or hydration, continues to occur for days and weeks after you pour the concrete.

Therefore, the concrete contractor needs to make sure that the necessary water is available for this reaction by adequately curing the slab.

The third reason concrete cracks are using a lower strength of concrete than what should have been used.

Concrete is available in many different strengths, therefore, the concrete contractor should verify what strength the concrete you are pouring for the job should be poured at.

The fourth reason concrete cracks is a lack of control or expansion joints.

Control joints help concrete crack where you want it to. The joints should be of the depth of the slab and no more than 2-3 times (in meters) of the thickness of the concrete (in centimetres). For example, a 100mm slab, which is generally standard, should have joints 3 meters apart.

Most plans don’t have detailed joint spacing marked on them or they have been left out altogether! So don’t leave this important part of concrete construction to chance.

Placing expansion joints is often not taken seriously enough and then when the “saw cutter” comes to your job to put in expansion joints, they can put the cuts where he feels they belong or where it is convenient for him.

As you can imagine, if the expansion joints are not put at the correct place the concrete will try to expand or contract, have nowhere to go, because concrete is brittle, and an unsightly crack will form.

Another reason for concrete cracks is that concrete should never be poured on frozen ground.

While this is not a problem in most parts of Australia, it is significant enough to mention as the ground where concrete will be placed must be compacted and prepared according to the soil conditions.

Understand what your contractor is doing about each of the above-listed items and you will get a good concrete job and prevent the most common causes that produce cracks.

Therefore, if a decorative concrete solution is subscribed over concrete that has not been properly laid, then the risk rises that the overlay may crack too. A property owner cannot always point the blame to the concrete resurfacing company, as although the preparation is done as best as possible, it may not be enough to stop the effects of imperfect concrete and therefore even a perfect resurfacing job can crack, even when the best products are used.

Barefoot Concrete takes all these factors into consideration and although we offer a flexible concrete polymer in our overlay system, and are fastidious with our preparation, use high-quality products that are noted for their strength, flexibility, adhesion and weatherability, the bottom line is that if the substrate cracks the overlay can crack.