Hot weather concreting: effects, precautions

Effects of hot weather (above about 30 to 35 °C, or low humidity and wind) on concrete

• Faster setting and loss of slump, so more water tends to be added (higher W/C).
• Rapid evaporation, causing plastic shrinkage cracks.
• Higher early strength but lower 28-day and long-term strength.
• Difficulty in placing and finishing, and a risk of cold joints.
• More thermal cracking and drying shrinkage, higher permeability, and difficulty in controlling air entrainment.

Precautions (IS 456 Cl. 14.1, which refers to IS 7861 Part 1; ACI 305)

1. Keep the concrete temperature at placing below 40 °C (IS 7861 Part 1 limit). Shade aggregate stockpiles, paint tanks and pipes white, sprinkle water on aggregates, and use chilled water or crushed ice as part of the mixing water.
2. Use low-heat cement or fly ash blends; use retarding or water-reducing admixtures (not extra water).
3. Reduce transit time; mix, transport and place quickly; schedule casting in the early morning, evening or night.
4. Dampen the subgrade and formwork before casting; keep forms shaded.
5. Use windbreaks, sunshades, fog sprays or evaporation retardants to prevent plastic shrinkage.
6. Start curing immediately: wet hessian, ponding, continuous spraying, or a membrane curing compound; cure for at least 10 days (IS 456, Cl. 13.5.1).
7. Place smaller lifts and provide enough joints.