One of the most common questions asked when it comes to decorative concrete finishes is “Will it crack?”. The simple answer is yes, hairline cracking is normal and should be expected. Having said this it is not always the case. There are different types of cracks and to varying degrees and due to varying factors. Understanding these factors and taking certain measures or precautions in both the planning and execution phases, the risk of cracking can be reduced. Because concrete is a natural material affected by external factors such as weather, drying and cure times, no one can ever guarantee a crack free surface. However taking care to ensure best practices are followed when placing concrete, the degree or severity of cracking should be greatly reduced.
Cement is also not a flexible material, so movement and temperatures causing expansion and contraction may cause fine hairline cracks which is part of the natural element.
In the case of decorative overlays and coatings that are usually around 1.5-2mm in thickness, considerations of the substrate beneath must be taken into account as any issues, cracks or weaknesses in the base are sure to transition through to the top layer. In some cases, remedial work can be carried out prior to the application of the overlay.
There are many factors that can influence the formation of cracks, and here is a summary of the most common…
Incorrect amount of water added to the cement mix
Adding too much water at the time of mixing, with the idea that this will make it easier to work with, causes excessive evaporation of the excess water, leading to excessive shrinkage, the result of which is cracking. This also affects the overall strength of the concrete.
During the curing phase, concrete requires water. This hydration is a long process and takes up to 28 days. By adequately curing the concrete (usually by wetting daily) you can assist this process to take place at the correct rate and not too dry out too quickly which could lead to excessive evaporation of the moisture present in the concrete,
resulting in cracking. Rapid drying is also more of a problem for external applications where the concrete is exposed to the elements. This can be controlled by protecting the site with shade cloths, polythene etc
Incorrect cement strength in the mix
Cement is available in a range of strengths, and the correct strength for the specific job is very important. It is important to specify the intended application so that the concrete mix meets the performance expectations. Concrete is a mixture o f water, cement, aggregate, and admixtures and the ratio of water to cement plays an important role in determining the strength of the concrete.
Incorrect expansion and control joints – Expansion joints are placed or cut so that the concrete has sufficient space to move during the hydration or curing phase (expansion and contraction will also take place with temperature changes). The placement of the expansion joints is vitally important to minimise the visibility of possible structural movement cracking which usually occurs on corners of walls, in doorways, etc. In larger areas, it is a good idea to place expansion joints at certain intervals (say every 10-12m²) to allow the correct expansion and contraction of the concrete as it cures. Control joints are placed in order for the contractor to be able to complete each section timeously (working on large areas without control joints creates difficulty and time pressures for contractors trying to place and finish timeously and correctly and may lead to one or a number of the factors mentioned above and ultimately cracking). Quite simply expansion and control joints are “planned cracks” – in other words, if and when a structural crack does occur, it should take place where you’ve planned it to be, and in a straight line, thereby minimising the visual impact of such cracks. These joints should be cut deep enough and soon enough and can later be filled with a flexible/expandable material like grout or joint filler (these are usually available in a range of colours, adding another possible decorative element to your project) to improve the aesthetic as well as to prevent dirt and debris becoming lodged in these cuts.