Hot Weather Concreting

When temperatures rise, adverse site conditions can impact the quality of concrete and how it should be poured. Hot weather concreting, as defined by ACI 305R, is any combination of the following conditions that tends to impair the quality of freshly mix or hardened concrete by accelerating the rate of moisture loss and rate of cement hydration or otherwise causing detrimental results: high ambient temperature, high concrete temperature, low relative humidity, high wind speed and solar radiation.

If the temperature of freshly mixed concrete approaches approximately 77 degrees Fahrenheit adverse site conditions can impact the quality of concrete. Ambient temperatures above 90 degrees Fahrenheit and the lack of a protected environment for concrete placement and finishing (enclosed building) can contribute to difficulty in producing quality concrete.

“Concrete must be protected from the adverse conditions created due to weather,” says Gene Hightower, technical services director for ACIA.  “Suppliers should be sprinkling their coarse aggregate piles and promoting the use of chilled water or ice to lower the batch temperature. They can also promote the use of set retarders or hydration stabilizers. What we are ultimately trying to do is buy time.”

Heat will accelerate the hydration process and shorten the initial set time, which will reduce the time for placement. The concrete should be placed at as low of a temperature as possible using conventional methods that are low in cost, placing concrete at cooler times of the day or at night, and protecting the concrete from rapid moisture loss during placement and after the initial set.

Curing in Hot Weather

The purpose of curing is to eliminate or greatly reduce the moisture loss from the concrete. By doing so, the shrinkage cracking will be eliminated or greatly reduced. Curing in hot weather should be done as soon as possible as should the sawing of control joints. As a rule of thumb, the slower the hydration the better the end strength of the concrete.

Set-controlling admixtures are chemicals used to slow down the hydration process and reduce the heat of hydration produced in the concrete. Some are designed to slow down this process over a long period of time and some are designed to hold off the hydration for a controlled period and then allow the hydration to occur in a normal manner.

When pouring concrete in hot weather, you must be prepared for some possible problems. Due to the temperatures during the pour, shrinkage cracks can be quite deep. This deep cracking is because the concrete has little capacity to resist shrinkage stresses. The cracks can continue to widen and propagate until the shrinkage stresses are relieved.

“When its all said and done, concrete is noting but time and temperature,” Hightower says.

Hot weather concreting can be a challenge, but with proper planning and execution concrete can be successfully placed and finished to produce high quality durable concrete at temperatures of 95 degrees Fahrenheit or more.

*To minimize the disadvantages and make your next hot weather concreting project a success, use ACI’s resources, including ACI University online courses, free online education presentations, publications, and journal articles.

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