| TECHNICAL |
REPAIRING NON-STRUCTURAL CONCRETE DEFECTS There are essential steps to follow before and during the repair of relatively minor, yet unsightly concrete defects. Bryan Perrie, of Concrete and Cement South Africa (CCSA), outlines what to do – and avoid – when tackling such projects Firstly, repairs to new concrete should be done as soon as possible after the formwork has been removed to reduce differential shrinkage and improve the bond between the orig inal concrete and the repair material. Before doing the repair, all dust and detritus must be removed from the surface to be treated by, for example, washing vertical and near-vertical surfaces with clean water. For manual repairs, suitable for small areas, feather edging – trying to seamlessly blend the repair material into the existing concrete – should be avoided. The outline of a repair should be cut with a masonry cutting disc or saw to ensure a square edge. Surface preparation Good adhesion between the original concrete and the repair material is essential and, to ensure this, the surface of the original concrete must be strong, rough and clean. Any loose or weak material must be removed with sharp chisels, driven by relatively light hammers. However, it is important to remove the unwanted concrete in such a way that the remain-
ing concrete is not irreparably damaged. Sandblasting, which can remove small volumes of concrete, is an excellent means of achieving a rough surface free of loosely adhering material. In cases where the repair areas are large (say, over 0,1m²) and especially where persons could be injured by falling fragments, it is strongly recommended not to rely solely on adhesion between repair and background concrete, but to provide mechanical fixing. Such fixing should be done with corrosion‐resistant metal devices such as screws or rods, preferably of stainless steel. Fixing devices should be installed after the surface preparation is complete, but before the surface is cleaned. To ensure strong adhesion of fresh concrete or mortar to a substrate of hardened concrete, the substrate should have enough suction to absorb the water fi lm at the interface, but should not desiccate the repair material. This limited suction can be achieved in different ways, depending on the age and density of the concrete. If the concrete is fairly new – say, within 48 hours of being placed – simply allowing
