Like other chemical reactions, hydration of cement depends on temperature and time, as well as the presence of moisture. As concrete hardens, it becomes stronger, resists damage, and increases in durability. Hydration of cement stops when concrete dries. This means concrete must be kept from drying for as long as possible to reach its greatest strength. Warm, moist conditions are nearly ideal for concrete placement and curing. However, when the weather is hot, dry & windy, a number of potential problems arise.
Some of the more critical problems include:
- More water is needed to make concrete workable
- Warm concrete dries faster
- Concrete may set too fast
- Handling, finishing, and curing may be more difficult and may require more effort
- Concrete contracts more as it cools if its temperature is high
- Plastic shrinkage cracking is more likely to occur because the surface dries before curing begins
- Concrete will be more porous and srink more because more water must be used in the mix
The statement, “start curing concrete immediately” cannot be overemphasized. Curing must begin as soon as placement and finishing is complete. If concrete surfaces dry before the concrete hardens, or is alternately wet and dried, cracks may appear on the surface. To prevent cracks, steps can be taken to reduce evaporation during finishing and thereafter.
All materials needed for protection from early drying and curing should be available and ready for use before the concrete arrives, for simplicity & ease of application during finishing and thereafter.
- Sprinkling or fog spraying – Sprinklers must be setup to ensure complete slab or flatwork coverage to keep the surface wet. Keep water pressure and flow long enough to avoid surface damage.
- Straw or hay – Wet loose materials such as straw or hay can be used, however they can discolor the surface. Keep material moist & prevent from blowing away in windy conditions.
- Poly sheeting – Plastic film must be at least 4 mils in thickness. For hot weather, white colored poly is preferred for its heat-reflecting quality. Remember, any poly is better than none at all. Wrinkles in the poly will result in discoloration of the concrete. Ensure it is secure at all edges to prevent it from blowing off/away.
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The water is absorbed through the chemical reaction. It does not evaporate and the slab or whatever you are pouring will cure evenly. If you are pouring in extreme heat you can use type 4 cement. It has a low heat of hydration to resist premature curing from outside heat.