CEMENT & CONCRETE
PREPPING FOR
non-structural repairs
There are essential steps to follow before and during the repair of relatively minor yet unsightly concrete defects. Bryan Perrie, managing director of The Concrete Institute, outlines what to do – and avoid – when tackling these projects.
R
epairs 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 original concrete and the repair material. Before doing the repair, all dust and detritus must be removed from the sur face to be treated by, for example, washing vertical and near-vertical sur faces 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.
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IMIESA February 2021
Bryan Perrie, MD, The Concrete Institute
Good adhesion between the original concrete and the repair material is essential and, to ensure this, the sur face 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. But it is important to remove the unwanted concrete in such a way that the remaining concrete is not badly damaged. Sandblasting, which can remove small volumes of concrete, is an excellent means of achieving a rough sur face free of loosely adhering material.
When to use mechanical fixing In cases where the repair areas are large (say over 0.1 m2), 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 sur face preparation is complete, but before the sur face 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 film at the inter face but should not desiccate the repair material. This limited suction can
