Fix cracks in concrete

Cracks in concrete happen all the time as a result of shrinkage or movement. As concrete cures and dries out it has a tendency to shrink, and that's why it is important to lightly spray concrete with water during the curing or drying process.


Another reason why concrete can crack is incorrect preparation of the mix, which is why it is essential to know the right ratios of sand, stone and cement.


The use of too much water used when mixing concrete can cause excessive cracking and is the single most common cause of cracks in concrete.   The evaporation of the excess water is what causes the shrinkage and reduces strength.  The runnier the mix - the more shrinkage will occur.  It is essential to follow the manufacturers recommendations for mixing concrete for specific projects.

After laying concrete, it is also important to prevent the concrete from drying to rapidly by lightly spraying water on the surface during the curing process, or by covering the concrete with a plastic sheet to retain moisture during the drying time. 

Larger areas of concrete require what is called control joints, or cut lines in the concrete that creates a weak area and forces any cracks to this point.

To reduce the risk of cracks, rebar or wire mesh embedded in the concrete add strength and reinforce the installation.

Repairing cracks in concrete

Any cracks in concrete should be repaired as soon as possible, to prevent additional cracking and eventual failure of the slab.

For smaller, hairline cracks you can use a hammer and chisel to enlarge the crack in a 'V' shape. The shape allows you to insert cement-based filler, such as RockSet. Polycell RockSet is recommended for patching floors where rapid setting is required. Repaired concrete floors are ready for normal use (vehicular and foot traffic) after one hour. When fully cured, the applied product has a greater compression strength than conventional concrete.

Mix RockSet to a thick paste with 1 part water to 4 parts RockSet and apply with a putty knife or paint scraper, pushing down into the 'V' shaped gap.

You will find Polycell RockSet at your local Builders Warehouse.

For longer, larger cracks it recommended that you use an angle grinder and masonry disc to cut out a 'V' shaped opening along the length of the crack.


Always clean out cracks with a vacuum cleaner before applying filler.

After application, use a wood float to smooth-finish the surface, or wait until the product has cured and sand smooth with 120-grit sandpaper. Drying time at a depth not exceeding 20mm is about 1 hour at 23 °C