MC Magazine

By Alex Morales
Alex Morales is NPCA’s Director of Technical Education and a member of the NPCA TechTeam.

What is “hot weather” as far as precast concrete is concerned? According to ACI 305, “Hot Weather Concreting,” hot weather is any combination of the following weather conditions: high ambient temperature; low relative humidity; solar radiation; and wind.

Although wind is not traditionally associated with hot weather, it is important to note that the effects of the first three conditions listed become more pronounced with increases in wind speed. This is why efforts to preserve concrete quality on a windy, sunny day are more critical than those required on a calm, humid day – even if ambient air temperatures are the same.

It is important to know when hot weather conditions will strike because you may need to adjust your mix beforehand. It is unlikely that you will need to take all of the following precautions – you should analyze your particular hot-weather situation and plan accordingly.

Decrease concrete temperature and reduce moisture loss. The hotter the ambient temperature, the more difficult it is to maintain a constant concrete temperature – especially because hydration inherently produces heat internally within the concrete. Nevertheless, maintaining the temperature of fresh concrete at approximately 55 F (13 C) will prevent many hot-weather problems. Since moisture loss can result from increased concrete temperatures, these precautions can also help maintain concrete water content.

Controlling water temperature is typically the easiest way to lower concrete temperature. Water can be cooled to as low as 33 degrees F, but substituting ice for water is also a good option. However, do not substitute ice for all of the water. Specifications usually limit the amount of ice to 75 percent of the required mixing water. Typically, the temperature of the mix can be decreased by 1 degree F for every 4-degree F reduction in water temperature – but specifications usually limit how much you can reduce concrete temperatures by water cooling.

While water has the greatest impact per unit weight on concrete temperature, aggregates have the most overall significant impact. A 1-degree decrease in concrete temperature can be realized with a 2-degree aggregate temperature decrease. Consequently, you should make every effort to keep aggregates cool during hot weather.

After concrete placement, prevent moisture loss by immediately covering with any moisture-retaining material such as burlap or a curing compound. Retention of moisture will optimize the cement hydration process.

Increase initial set time. Substituting your current cement type with ASTM C 150 Type II cement, or ASTM C 595 Type IP or Type IS blended cements can help increase initial set time and help with concrete handling. Remember, however, that slower-setting cements can increase the potential for plastic shrinkage cracking. So consider this option carefully. You can also affect initial set with the use of ASTM C 494 set-retarding admixtures. Work closely with your admixture supplier to determine the ideal dosing rate for your particular hot-weather condition, cement content and cement type.

Decrease slump loss. ASTM C 494 water-reducing admixtures can help curb slump loss without affecting the water demand of the mix. Since the efficacy of chemical admixtures is conditional upon cement type, you should work with your admixture supplier for proper admixture selection and dosage rate.

Prevent cracking and loss of air entrainment. Admixtures that increase the bleeding rate may also require additional consolidation after the majority of bleeding has subsided. Work with your supplier to optimize the dosage of any admixture you choose, including air-entraining admixtures, which may need to be increased depending on the combination of hot-weather precautions you take. The use of fiber reinforcement can help prevent drying shrinkage cracks and can be added to the mix per the manufacturer’s recommendations.

Hot weather conditions can challenge the way you think about concrete but should not impact the quality of your final product. As with any mix design, your hot-weather mix will likely be a result of trial-and-error – be patient.

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