Hot Weather Concreting Practices for Contractors
Most concrete contractors are well aware of the difficulty of handling, placing, finishing and curing concrete during the hot summer months. While producers work hard to keep concrete temperatures down and use set retarding admixtures to minimize slump loss, there are still many things that can go wrong at the job site.
There are special precautions that concrete contractors need to take during hot weather. The American Concrete Institute (ACI) defines conditions such as high ambient temperature, high concrete temperature, low relative humidity and high wind speed as conditions, in which precautions should be considered. Such conditions tend to impair the quality of freshly mixed or hardened concrete by accelerating the rate of moisture loss and the rate of cement hydration, or otherwise causing detrimental results. These conditions can cause surface water to evaporate too quickly, leading to plastic shrinkage cracking. However, there are several ways to ensure problem-free projects, despite hot weather.
- Consult with ready mix producer. They may recommend mixes optimized for hot weather in your area, including mixes containing synthetic microfibers to minimize drying shrinkage cracking.
- Prepare the site. Dampen the subgrade, forms and reinforcement prior to placing the concrete. Be sure to avoid ponding of excess water. Have windbreaks and sunscreens available to protect the concrete.
- Plan staff accordingly. Trucks should be able to discharge concrete immediately and you should have workers on hand to quickly place, finish and cure the concrete
- Schedule carefully. Schedule the rate of your concrete deliveries to match the placement rate of your crew and equipment. Plan your deliveries to avoid having concrete delivered during the hottest part of the day.
- Limit added water at the jobsite. High heat may cause the concrete to lose slump. Limit any addition of water to not exceed the quantity of mixing water established for the concrete mixer. Your ready mix producer can also supply concrete treated with CONCERA™ admixtures, which can provide highly flowable concrete with excellent slump retention, without adversely affecting the setting of the concrete.
- Speed is essential. You will want to start the final finishing as soon as the water sheen is gone from the concrete surface, and start curing as soon as finishing is done. This can be done by covering the concrete with wet burlap and plastic sheeting or liquid membrane curing compounds to protect the concrete. Alternatively, you can cure the concrete with water using mist foggers.
For more information on hot weather concreting, consult ACI 305R.