The success of many hot-weather concreting operations depends on the steps taken to slow the cement hydration reactions within the concrete and to minimize the rate of evaporation of moisture from the freshly mixed concrete.
ACI 305 “Hot Weather Concreting” defines hot weathers as any combination of the following conditions that tends to impair the quality of the freshly mixed or hardened concrete:
- High ambient temperature
- High concrete temperature
- Low relative humidity
- Wind speed
- Solar radiation
Potential concrete problems in hot weather are likely to include:
- Increased water demand
- Increased rate of slump loss
- Increased rate of setting
- Increased tendency for plastic-shrinkage cracking
- Increased difficulty in controlling entrained air content
- Decreased 28-day and later strengths
- Increased tendency for differential thermal cracking
- Greater variability in surface appearance
- Increased permeability
The temperature of freshly mixed concrete depends upon the temperature of each ingredient. Concrete can be cooled to a moderate extent by using chilled mixing water. The quantity of cooled water cannot exceed the mixing water requirement. The maximum reduction in concrete temperature that can be obtained is approximately 6°C (10°F).
Rule of Thumb #1
For every 0.5°C (1°F) drop in concrete temperature desired, the mix water temperature must be reduced by 2 degrees C (3.6 degrees F)
On a weight basis the mix water has the greatest effect on the final concrete temperature. This stems from its higher specific heat, which is about five (5) times as high as for the other concrete making materials.
Using the above rule, a concrete producer would chill his 21°C (70°F) mix water down to 9°C (52°F) to lower his concrete temperatures from 27°C to 24°C (80°F to 75°F).
If a water chiller is not available, or the desired reductions in mix temperature involve chilling the water beyond its freezing point (0°C or 32°F), ice is the obvious answer. The amount of cooling is limited by the amount of mixing water available for ice substitution. For most concrete, the maximum temperature reduction is approximately 11°C (20°F). For correct proportioning the ice must be weighed.
Ice has a two-way cooling effect. First it draws heat from the concrete for melting of ice, then the resulting water at 0°C (32°F) provides continuing cooling capacity.
Rule of Thumb #2
For every 0.5°C (1°F) drop in concrete temperature desired, replace 2% of the total mix.
To cool a concrete mix containing 166 kg/m3 (280 lbs/yd3) of water from 27°C (80°F) down to 16°C (60°F), replace 40% (66 kg/m3 or 112 lbs/yd3) of the mix water with ice.
The ice must be added directly into the concrete as part of the mixing water.
The moment cement contacts water, hydration begins and heat is liberated. Admixtures meeting the requirements of ASTM C494, Type D (water-reducing retarders such as Daratard® and Recover®) are beneficial in hot weather concreting due to their ability to slow the chemical reaction between water and cement.
Daratard and Recover, when used at their recommended dosage rates, decrease the water required for a given workability, slow the rate of hydration and extend the set time to allow for proper finishing.
Chilled water, ice and admixtures, used either in combination or separately, are all helpful tools for successful hot weather concreting. For more information on this topic refer to ACI 305, “Hot Weather Concreting.”
gcpat.com | North America Customer Service: 1 877-4AD-MIX1 (1 877-423-6491)
We hope the information here will be helpful. It is based on data and knowledge considered to be true and accurate and is offered for consideration, investigation and verification by the user, but we do not warrant the results to be obtained. Please read all statements, recommendations and suggestions in conjunction with our conditions of sale, which apply to all goods supplied by us. No statement, recommendation, or suggestion is intended for any use that would infringe any patent, copyright, or other third party right.
RECOVER is a trademark, which may be registered in the United States and/or other countries, of GCP Applied Technologies Inc. This trademark list has been compiled using available published information as of the publication date and may not accurately reflect current trademark ownership or status.
© Copyright 2018 GCP Applied Technologies Inc. All rights reserved.
GCP Applied Technologies Inc., 62 Whittemore Avenue, Cambridge, MA 02140 USA.
In Canada, 294 Clements Road, West, Ajax, Ontario, Canada L1S 3C6.
This document is only current as of the last updated date stated below and is valid only for use in the United States. It is important that you always refer to the currently available information at the URL below to provide the most current product information at the time of use. Additional literature such as Contractor Manuals, Technical Bulletins, Detail Drawings and detailing recommendations and other relevant documents are also available on www.gcpat.com. Information found on other websites must not be relied upon, as they may not be up-to-date or applicable to the conditions in your location and we do not accept any responsibility for their content. If there are any conflicts or if you need more information, please contact GCP Customer Service.
Last Updated: 2018-11-29