Construction World January 2019

Restrained Drying Shrinkage The correct way to prevent restrained drying shrinkage cracking and ensure a hard-wearing concrete surface is effective and timeous application of the curing process. Curing ensures that the concrete remains moist enough for cementing reactions to take place. This is best achieved by using a suitable atomising spray to apply an approved white-pigmented, resin-based membrane forming compound. To ensure uniform consistence and dispersion of the pigments in the curing materials, it should be agitated in the supply container immediately before transfer to the distributor and kept agitated during distribution. The vertical sides of the pavement should be coated within an hour after the removal of the side formwork. Any areas of the coating that show signs of damage within a week should immediately be repaired. Other curing methods, such as placing white or light coloured plastic sheeting over the concrete, may be considered where membrane curing is not possible. However, such coverings may mark the surface. The plastic sheeting should be placed while the surface is still moist. If the surface looks too dry, it should be wetted with a fine spray before the sheeting is placed. Adjacent sheets should be lapped 500 mm and weighted so that they remain in contact with the concrete surface. The sheeting should extend beyond the pavement edges to completely cover the sides of the slab after the side forms have been removed. The sheeting should remain place throughout the curing period. Damp hessian covering or damp sand as a means of curing are not recommended. All methods of curing should be in place for at least seven days. 

• Erecting windbreaks to reduce wind velocity over the concrete surface; • Providing sunshades to control surface temperature of the concrete; • Avoiding excessive temperature differences between concrete and the air; • Lowering of concrete temperatures in hot weather; • Avoiding overheating of the concrete in cold weather; • Protecting the concrete with temporary coverings during any appreciable delay between placing and finishing; • Reducing the time between placing and the start of curing by improved construction procedures; • Protecting the concrete during the first few hours of placing and finishing to minimise evaporation. The application of moisture to the with a fog spray nozzle will help prevent evaporation of the concrete until effective curing procedures can be applied to the surface; and • Using mono-molecular films to reduce evaporation between placing and finishing operations. This is particularly applicable to concrete road construction. The films protect fresh concrete from the effects of excessive moisture loss in rapid drying conditions, without increasing the bleed. The product is sprayed over fresh concrete surfaces to form a continuous barrier film over the entire surface. Damage To prevent damage to the concrete surface in rainy and hail conditions, frame-mounted waterproof plastic sheeting covers or other suitable materials should be used to protect unhardened concrete surfaces. In fact, when rain appears imminent, all concrete paving operations should stop.

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CONSTRUCTION WORLD JANUARY 2019