Hot climate concreting and curing

Hot-weather concreting (ambient above about 30 to 35 °C, with low humidity or wind) is carried out so that the concrete temperature at placing stays below 40 °C (IS 7861 Part 1, referred to by IS 456 Cl. 14.1) and moisture loss is prevented.

Appropriate methods before and during concreting

• Select low-heat cement or Portland pozzolana cement; use a retarding water-reducing admixture so that the W/C does not need to be raised.
• Cool the ingredients: shade and sprinkle aggregate stockpiles, keep water tanks and pipes shaded or white-painted, and use chilled water or crushed ice in the mix.
• Keep the haul short, use covered transit mixers, mix just before placing and avoid delays.
• Cast in the early morning, evening or night.
• Wet the subgrade, forms and reinforcement before casting (with no standing water); use shades and windbreaks.
• Place in thin layers, vibrate properly, and plan joints to avoid cold joints.

Specific measures during finishing and curing

1. Prevent plastic shrinkage: use a fog spray or an evaporation-retardant film, or cover with polythene between finishing operations.
2. Begin curing as soon as the surface can take it without being marred.
3. Cure continuously by ponding, wet hessian or sand, or sprinkling; do not let the surface dry even for a short time (wet-dry cycles cause cracking).
4. Alternatively, apply a curing compound or cover with white polythene sheet to reflect heat.
5. Cure for at least 10 days in hot and dry weather (IS 456, Cl. 13.5.1), and preferably 14 days with blended cement.
6. Delay the stripping of forms, protect the concrete from heat and wind, and keep the forms wet.
7. Control joint spacing and keep the temperature difference between the concrete and the air small, to avoid thermal cracking.