Concrete Floor - Will it Crack?
The most asked question when it comes to concrete floors is..."Will it crack?" The honest answer is "Yes" but with the exception of hairline cracking, cracks can be reduced.
There are several factors that can influence the formation of cracks in a decorative cement floor. By its nature, cement is not a flexible product. Any movement and temperatures causing expansion and contraction may cause fine hairline cracks on the surface, but this is part of the natural element and adds to the look and feel of a decorative cementitious floor. It's also important to bear in mind that decorative cementitious coatings - that are generally between 1.5 to 2mm thick - are affected by the condition of the underlying substrate.
Insufficient or too much water added to the product
Where contractors add too much water to make working with the cement product easier, this causes excessive evaporation of the excess water, leading to excessive shrinkage, which results in cracking and also affects the overall strength of the concrete.
Cement is available in a range of strengths, and the correct strength for the specific job is very important. It is also important to match the strength of the substrate concrete to the strength of the decorative cement coating for best results.
Incorrect expansion / control joints
Expansion / control joints are essential for the substrate concrete to have plenty of space to move during the hydration or curing phase. Positioning of expansion / control joints is vitally important to minimise the visibility of structural movement cracking, which usually occurs on corners of walls, in doorways, etc. In essence, expansion and control joints are 'planned cracks' – making sure that if and when a structural crack does occur, it is where you’ve planned it to be, and in a straight line, rather than randomly on its own path which may become unsightly.
Large areas require expansion joints at certain intervals to allow 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 and therefore correctly – trying to do panels that are too big for the contacting team to handle usually leads to one or a number of the factors mentioned above, leading to cracking.
It is important to cut these expansion / control joints deep enough and soon enough; they can later be filled with an expandable material to lessen visibility and prevent dirt and debris getting trapped in them.
Rapid drying when curing
The process whereby concrete sets or cures requires water. This chemical reaction, called hydration, is a long process and takes anywhere from days to weeks. With daily wetting of the surface you can assist in the process occurring at the correct rate, and not to dry out too quickly, again leading to excessive evaporation and resulting in cracking.
Where cement is used for external applications, rapid drying is also more of a problem since the concrete is exposed to the elements. However, this can be easily controlled by protecting the site with shade cloths and barriers; a planning step that can make all the difference between a successful and unsuccessful project.
As mentioned previously, direct sunlight, wind and rain can all affect the hydration process, as can temperature. Excessive heat (both dry and humid heat) will play a role in the hydration process, as well as freezing conditions. For cement, freezing conditions start at 5°C, not 0°C as one would presume. At 5° C and below, water begins to expand before it actually turns to the solid state of ice. This expansion creates bigger voids in the cement as it cures, and once this ice melts and evaporates, leaves bigger voids than the cement can handle (also breaking the cement bonds), resulting in cracking.
Hairline cracks in cement-based coatings may be the norm, but by hiring an experienced contractor bigger cracks than that are unlikely to appear. Cemcrete have a recommended list of contractors that they deal with to ensure any job is done right the first time. For more information visit www.cemcrete.co.za