Hot Weather Concreting Guidelines

During hot summer months all projects benefit by implementing hot weather concrete practices. Appropriate hot weather concreting practices are described in ACI 305R. Most project specifications that address hot weather concreting practices only mention that concrete should be placed in accordance with ACI 305, or mention a maximum concrete placement temperature of 90 degrees Fahrenheit. The reality is that most architects, engineers, and contractors aren't familiar with ACI 305R, and only specifying a maximum concrete temperature may not be enough to avoid concrete problems.

Hot weather is defined by ACI 305R as any combination of the following conditions that tends to impair the quality of freshly mixed or hardened concrete by accelerating the rate of moisture loss and rate of cement hydration:

• High ambient temperature
• High concrete temperature
• Low relative humidity
• Wind speed
• Solar radiation

Concrete problems in hot weather include increased water demand, increased rate of setting, increased tendency for drying shrinkage, decreased durability, and greater variability of surface appearance. Any damage to concrete caused by hot weather cannot be fully corrected. During the warm summer months it is important to take preemptive measures so that concrete performs properly.

The following measures will reduce or avoid potential problems of hot weather concreting:

• Address hot weather concreting within the project specifications
• Discuss hot weather concreting requirements during preconstruction meetings
• Select a contractor that has experience with hot weather concreting
• Discuss hot weather concreting requirements during preplacement meetings
• Proportion mixes for hot weather concreting
• Utilize chemical admixtures, particularly high-range water reducing admixtures, to maintain workability
• Use cool water or ice during concrete batching
• Store aggregate sources in the shade
• Sprinkle and fog spray coarse aggregates
• Use transport mixers that are free of buildup of hardened concrete and excessive wear of mixer blades
• Minimize time to transport, place, consolidate, and finish concrete
• Minimize the amount of concrete mixing
• Do not add water in excess of the proportioned maximum water-cementations materials ratio (w/cm)
• Place concrete during times of the day or night when weather conditions are favorable
• Place concrete on a moist subgrade
• Protect the concrete from moisture loss during placement and curing
• Perform field testing of concrete to determine that the concrete is within design parameters for temperature, slump, and air content

Basically, concrete can be placed with favorable results during hot weather periods as long as all members of the construction team understand what is involved with hot weather concreting and all members of the construction team know what their role is to produce, place, test, and cure concrete appropriately.

Philip R. Barber, P.E.
Director of CMT