Pouring Concrete in Hot Weather

Concreting in Hot Weather – What You Should Know

Changing your concreting methods to hot-weather concreting methods is important when the freshly mixed concrete is in danger of becoming compromised due to the accelerated rate of moisture loss and the accelerated rate of the cement hydration.
Some factors which come into play during hot-weather concreting include a high ambient temperature, a high concrete temperature, a low relative humidity, the wind speed and solar radiation.
A good rule of thumb is to start thinking about hot-weather concreting whenever the air or concrete temperature exceeds 80 degrees.

The Use of Aggregate
Your ready mix supplier will aim to get the concrete as cool as possible before it makes its trip to the job site. Since aggregate has a big effect on the concrete’s temperature, the supplier will likely shade the aggregate piles. Cool water will also likely be used to further reduce the concrete’s temperature. Some concrete suppliers in hot climates also use chilled water or ice to lower the concrete’s temperature.

The Use of Retarding Admixtures
Using retarding admixtures is another way to control the temperature of the concrete during the warmer summer months. The retarder is generally added at the job site, as it greatly increases the concrete’s setting time. It is important to remember, however, that curing is critical when using retarders.
Retarders may also be of the water-reducing variety. Mid-range water reducers increase the air content of the concrete. Be aware that if too much retarder is added to the concrete, the slab may begin to crust.
The Use of Hydration Controlling Admixtures
Hydration controlling admixtures may be used during extremely hot weather conditions, or when the concrete has to travel a long distance to get from the supplier to the job site. Ready mix suppliers can add this material to delay set-up by as much as five hours.

The Use of Fly Ash
Fly ash may also be used to slow the set time of the concrete, although this may change the color of the concrete. In other words, make sure you don’t alter the percentage of fly ash in all of your shipments, or you could end up with discolored concrete. In addition, fly ash is not very effective in higher temperatures.

Ideally, you should place your concrete in the early evening, or perhaps in the early evening, during the warmer summer months.  Ensure that your crew and their equipment is prepared to start working as soon as the concrete is brought on site, as concrete’s temperature can increase considerably simply by waiting in the truck before it is placed.
If possible, it is best to protect the concrete from the sun.  The use of sun shades may be useful.  Ideally, you should also keep your tools and equipment shaded from the sun, specifically ones that will touch the concrete directly.
If you are laying exterior concrete down on a subgrade, make sure you spray down all of the elements with cold water – including the subgrade and the forms -before you begin installing the concrete. This prevents the subgrade and the forms from absorbing water from the concrete.

Once you have set the concrete and it is bull floated, then you should utilize an evaporation retarder or a monomolecular film to slow down the evaporation process, retaining surface water.