For safety purposes, please wear work gloves and safety goggles when carry out the repair work.
Filling in a minor crack
- To inspect the damage, first sweep away any dirt and debris with a brush. To ensure a good repair, also brush away any new debris created by following step 2.
- Use a club hammer and cold chisel to “undercut” (open up) along the crack line by forming an inverted V-shape – with the point of the V at the top surface of the crack line. This provides a better gripping surface for the mortar.
- Prime the crack with a coat of PVA adhesive. Follow the manufacturer’s instructions for diluting the adhesive and use an old paint brush to apply it. The adhesive will also help the mortar to grip the surfaces of the crack line.
- Use a trowel to mix one part of cement with two or three parts of sand mixed with diluted PVA adhesive (again, follow the manufacturer’s instructions). The mix should be buttery, not weak – it is better to err on the dry side rather than having a runny mixture.
- Using the trowel, press the mortar press the mortar into the crack. Level it off with the surface of the floor, using the edge of the trowel. Skim off any excess before it dries. Leave the mortar to dry overnight, then brush away any loose dust.
- To fill a wide crack, instead of using the trowel to level the repair as in step 5, use a wood batten – move it from side to side as you pull it toward you.
Sealing in dust
To bind the dust layer and seal it in, simply roll or brush diluted PVA adhesive or a proprietary concrete floor sealer from a builder’s merchant over the floor. Follow the manufacturer’s recommendations for the drying time necessary before laying a floor covering.
If the damp-proof membrane has been breached, you can seal the floor by brushing on a waterproofing emulsion, which is available at builder’s merchants. Because dampness can spread beyond the treated area, it is best to treat the whole floor.