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Mortar is generally used for relatively small repairs (typically less than 1 m2) that are placed by hand. The mortars themselves are usually proprietary products containing sand, cement, polymers and, in some cases, fibres. The polymer acts as a plasticiser, reducing the amount of water required to produce a workable mix, decreasing the permeability of the hardened mortar and improving the bond with the substrate. Fibres are used to give the mortar more cohesion so that it can be placed in thicker layers in vertical and soffit repairs. Fibres also help to reduce shrinkage cracking. The products should be mixed in accordance with the manufacturer’s instructions.
Mortars are applied by trowel or by gloved hand onto either a soaked substrate or, if a bonding aid has been used, while it is still tacky. The mortar has to be compacted against the substrate and around the reinforcement so that all crevices are filled and there are no voids. In deeper repairs the mortar has to be built up in layers; subsequent layers should be applied as soon as the previous layer is strong enough to sustain the combined load of the two layers. The layer thickness should be in accordance with the manufacturer’s literature. If the mortar starts to sag it should be removed completely and started again with thinner layers. The final layer is finished with a steel or plastic float.
Good curing is essential and needs to start as soon as finishing is completed. Thin patch repairs are particularly vulnerable as they can dry out very quickly.
Acknowledgement: The Concrete Society
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