CURING AND PROTECTING NEW CONCRETE
Freshly placed and finished concrete needs to be kept moist. As water is essential to the hydration process taking place within the concrete, keeping it moist helps it retain, or if necessary absorb, the additional moisture it needs to complete the hydration, setting and hardening process. This process, known as ‘curing’, needs to be carried out carefully and properly if concrete is to maximize its hardness and strength.
Although it will achieve its final set in around 10 hours, the concrete is still comparatively weak at this stage. It will gain strength in the days that follow and achieve around 90 per cent of its final strength after approximately three weeks; it may then continue to strengthen for decades.
If concrete loses its moisture content too quickly, tensile stresses build up in it. The concrete is still too weak at this point to resist these stresses and both internal and surface cracks can result. These not only spoil the surface finish but also reduce the concrete’s final strength, making it less durable and more susceptible to frost damage.
New concrete must also be protected from the sun and the wind, which will both speed up the evaporation process, causing shrinkage cracking, particularly at the surface. These problems are especially significant where concrete is being laid in thin sections. Mixing and placing concrete in such weather conditions will have the same injurious effect on the water content during these processes and should be avoided if possible.
It is vital to cure concrete properly in the first three days to increase its early strength development and long-term strength, impermeability and durability. Ideally, it should then be kept in conditions of controlled temperature and humidity. In practical terms, this is achieved by a number of methods, depending on the situation:
• Periodically spraying the concrete surface with water.
• Where possible, flooding or ‘ponding’ the entire surface – sometimes it may be necessary to build earth or clay dams around the area being cured.
• Covering the surface with hessian or coconut matting that is maintained in a wet state.
• Covering the surface with wet sand.
• Covering the surface with waterproof paper or polythene to retain the moisture.
Curing should begin as soon as the concrete surface is sufficiently hard that it will not be marked or damaged by the water spray or covering.