Civil Engineering April 2022 | Vol 30 No 3

42 April 2022 Civil Engineering I t is not generally realised that all newly cast concrete must be properly cured to ensure that hydration continues until the full potential strength of the hardened concrete is achieved and to minimise the tendency to crack. This means ensuring that the concrete is kept damp and at a temperature that allows cementing reaction to proceed. The more extender in the mix, the slower the strength development, calling for longer curing operations. Curing has the greatest effect on the surface, and near surface zone, of concrete and is especially important in reinforced concrete and concrete subject to abrasion. CURING METHODS For proper concrete curing the following steps should be followed: Q Q Cover the surface with a water- retaining material such as sand, earth, straw or hessian so that it is kept continuously damp. Q Q Sprinkle or spray with water often enough to keep the concrete continu- ously moist. Q Q Pond water on the concrete surface (commonly used for flat surfaces such as slabs or roads). Berms can be used to contain water on sloping slabs. Q Q Cover the concrete with plastic sheeting or waterproof paper. The covering must be held in place at its edges in a way that does not damage the concrete and must be sufficiently overlapped at joints. Q Q A fine water spray can be used to apply water continuously and in a uniform manner. Q Q Apply membrane-forming compounds. Such components are supplied in liquid form and applied by spray, roller, or brush. On horizontal surfaces the membrane should be protected against damage. Curing compounds must be applied as soon as possible without marring horizontal surfaces, or within two hours of formwork removal on vertical surfaces. Q Q Leave formwork in place and covering any exposed concrete surfaces. Q Q Small precast elements (such as decora- tive paving stones) can be demoulded and cured by immersion in water. If freshly placed concrete is exposed to hot sunshine or drying winds, evaporation should be prevented by covering with plastic sheeting immediately after placing and fin- ishing. If the plastic sheeting could damage the surface, water-filled atomiser sprays of the type used for spraying insecticides on fruit trees can provide a mist over the fresh concrete until the surface is hard enough to permit one of the above-mentioned curing methods. It may also be necessary to pro- vide adequate wind breaks for the concrete during cooler parts of the day. In winter, newly placed concrete should be protected from frost by covering it with insulated material such as sacking or straw. The temperature of placed and compacted concrete should not be allowed to fall below 5°C until the concrete has attained a strength of 5 MPa. Curing should be continued for at least five days after placing concrete, and longer – about a week – in cold weather. Adequate curing is absolutely essential for concrete to attain its potential compres- sive strength at 28 days. With adequate curing, the concrete will become stronger and durable, more impermeable, and more resistant to abrasion.  Cement AND Concre te Bryan Perrie Pr Eng CEO Cement & Concrete SA bryan.perrie@cemcon-sa.org.za Careless curing can ruin the best concrete Concrete that has been correctly specified, batched, mixed, placed, and finished can still fail without adequate curing. Curing is usually the last step in a concrete project and, unfortunately, is often neglected – even by professionals. All concrete must be properly cured to ensure that hydration continues until the full strength of the hardened concrete is achieved