Technical Bullentins & Memos

Hot Weather

Those properties of concrete that make it an excellent construction material can be affected adversely by hot weather. Hot weather being defined as any combination of the following conditions that tend to impair the quality of freshly mixed or hardened concrete by accelerating the rate of moisture loss and rate of cement hydration , or otherwise resulting in detrimental results: a) high ambient temperature, b) high concrete temperature, c) low relative humidity, d) wind velocity, e) solar radiation. Strength, impermeability, dementional stability, and resistance of the concrete to weathering, wear, and chemical attack all depend on the following factors: selection and proper control of materials and mixture proportioning, initial concrete temperatures, wind velocity, solar radiation, ambient temperatures, and humidity conditions during placement and curing periods.

Concrete mixed, placed, and cured at elevated temperatures normally develop higher early strengths than concretes produced and cured at lower temperatures, but strengths are generally lower at 28 days and later ages.

Laboratory tests have demonstrated the adverse effects of high temperatures and lack of curing on concrete strength. Specimens molded and cured in air at 73 deg. F, 60 % relative humidity; and at 100 deg. F, 25 % relative humidity, produced strengths of only 73% and 62% , respectively, of that obtained for standard specimens moist cured at 73 deg. F for 28 days. The data illustrate that inadequate curing in combination with high placement temperatures impairs the hydration process and reduces strength.

Potential problems in hot weather for concrete in the freshly mixed state are likely to include:
a) increased water demand
b) increased rate of slump loss and corresponding tendency to add water at the jobsite
c) increased rate of setting resulting in greater difficulty with handling, compacting, finishing, and a greater risk of cold joints
d) increased tendency for plastic shrinkage cracking
e) increased difficulty in controlling entrained air content.

It is important to recognize that any damage to concrete caused by hot weather can never be fully alleviated. Therefore, good judgment is necessary to select the most appropriate compromise of quality, economy, and practicability. The following is a list of practices and measures to reduce or avoid the potential problems of hot weather concreting.

a) Modify mixtures as appropriate; retarders, pozzolanic admixtures(grancem) or other proven local solutions.

b) adequate manpower to quickly place, finish and cure the concrete.

c) limit the addition of water at the jobsite- add water only on arrival at the job site to adjust slump. Later additions should be avoided; in no instance should they exceed 2 to 2 gallons per cubic yard. Never add water to concrete that is more than 1 hours old.d) d) d) slabs on grade should not be placed on polyethylene sheeting-if a vapor barrier is required, then a bed of damp sand should be placed over it.

e) finish as soon as the sheen has left the surface; start curing as soon as finishing is completed. Continue curing for at least 3 days; cover to prevent evaporation or use a liquid membrane curing compound, or cure slabs with water.

f) moisten the subgrade, forms and reinforcing prior to placement. However aviod standing water.

g) do not use accelerators.