Tools: Circular saw with masonry blade; 1.25kg club or lump hammer; mason’s chisel; wood float; sponge float to match rough concrete texture; corner edging tool; vacuum cleaner and broom; release agent for wood forms (light oil, WD-40); gloves; dust, eye and hearing protection.

Materials: Sand-and-cement, or concrete mix (in 20 or 40kg bags); 1 litre Bond-Crete, Cembond or similar.

Repairing slabs

1. Set a masonry blade at a 5° angle so that the cut-out is slightly wider at the bottom than at the top. This helps to lock the patch in place. Cut a 10–12mm-deep ‘shoulder’ around the edge of the spalled area with a saw and masonry blade. Move the saw slowly as you cut, making certain that you cut back into solid concrete to ensure a strong bonding surface.


 

2. Break out all weak and loose concrete with a club hammer and cold chisel. Wear safety goggles to protect against sharp concrete chips that may fly off the surface.


3. Clean the chips and dust thoroughly from the repair area with a broom, vacuum cleaner or water and sponge. For the patching mix, use either a pre-packaged sand-and-cement or concrete mix, or a specialised patching material selected for the depth of the repair. A 40kg bag of dry mix makes enough concrete for a patch 1m long x 50mm thick x 500mm wide. Some patching materials are also available in 10 and 20kg bags for smaller jobs.


4. Paint the exposed surface with a PVA emulsion, such as Bond-Crete or Cembond, and pack in the patching mix with a wooden float. Leave the mix slightly higher than the surface of the old concrete. The mix should be just wet enough to hold together when it is trowelled into the repair. Do not be tempted to add too much water – that will result in weaker concrete.