Placing Concrete in Hot Weather
Caution needs to be applied when placing concrete in hot weather. Without the proper care, concrete may have reduced strength and will be very prone to cracking due to rapid drying. It also may stiffen quickly making finishing quite difficult.
At some point, usually between 75ºF. and 100ºF., hot weather problems for concrete may begin. The combination usually causing the most problems is low relative humidity and high wind velocity. These conditions, when added to sun and high temperature, create a very high potential for problems.
There are several methods of cooling concrete. The most efficient way is to cool the aggregates, which may be done as simply as sprinkling them with water and allowing the evaporation process to cool them. Other methods of cooling the concrete include using ice or injecting liquid nitrogen into the mixer. However, both methods add cost to the concrete. The contractor should also be prepared with sunshades, windbreaks and other means to prevent rapid drying.
Basic Guides for Hot Weather Concreting
1. Plan in advance. Have equipment and materials ready before the hot weather arrives.
2. Keep the subgrade and forms moist so they will not absorb water from the mix.
3. Keep sunshades and windbreaks available and use them whenever possible.
4. Have everything prepared before the ready mix truck arrives. Don’t make the truck wait for you.
5. Keep in constant communication with the ready mixed concrete provider. Coordination between contractor and producer is key.
6. Concrete should be placed, struck off and darbyed immediately.
7. Use evaporation retardants, fogging or misting with water, or cover with a vapor-proof sheet after screeding. This will help prevent rapid drying, crusting, plastic shrinkage and rubber sets.
8. Temporary covers, like continuously moistened burlap, may be placed over the fresh concrete and removed in small sections immediately ahead of the finishers.
9. Substituting a burlap drag or broom finish will eliminate other high risk finishing practices, such as smooth trowel finish.
10. Curing should take place when the surfaces are hard enough to resist marring.
11. Seal with a good, quality sealer a minimum of 30 days after placing and curing.
12. Remember to protect the crew in high temperatures. Drink plenty of fluids and be careful with long exposure to the sun.