Weather conditions at a jobsite-hot or cold, windy or calm, dry or himid-may be vastly different from the optimum conditions assumed at the time a concrete mix is specified, or selected. Hot weather can create difficulties in fresh concrete, such as:increased water demand
accelerated slump loss
increased tendency for plastic cracking
difficulties in controlling entrained air
critical need for prompt early curing
Adding water to the concrete at the jobsite can adversly affect properties and serviceability of the hardened concrete, resulting in:
decreased durability and watertightness
nonuniform surface appearance
increased tendency for drying shrinkage
Only by taking precautions to alleviate these difficulties in anticipation of hot-weather conditions can concrete work proceed smoothly.
In hot weather, the tendency for cracks to form is increased both before and after hardening. Rapid evaporation of water from freshly placed concrete can cause plastic-shrinkage cracks before the surface has hardened . Cracks may also develop in the hardened concrete because of increased drying shrinkage due to higher water content or thermal volumn changes at the surface due to cooling.
Air entrainment is also affected in hot weather. At elevated temperatures, an increase in the amount of air-entraining admixture is required to produce a given air content.
Because of the detrimental effects of high concrete temperatures, operations in hot weather should be directed toward keeping the concrete as cool as is practicable.
Preparation Before Concreting
Before concrete is placed, certain precautions should be taken during hot weather to maintain or reduce concrete temperatures.
Forms, reinforcing steel, and subgrade should be fogged or sprinkled with cool water just before the concrete is placed. Fogging the area during placing and finishing operations cools the contact surfaces and surrounding air and increases its relative humity. This reduces the temperature rise of the concrete and minimizes the rate of evaporation of water from the concrete after placement. For slabs on ground, it is a good practice to moisten the subgrade the evening before concreting. There should be no standing water or puddles on forms or subgrade at the time concrete is placed.
Transporting, Placing, Finishing
Transporting and placing concrete should be done as quickly as is practical during hot weather. Delays contribute to loss of slump and an increase in concrete temperature. Sufficient labor and equipment must be available at the jobsite to handle and place concrete immediately upon delivery.
Prolonged mixing, even at agitating speed, should be avoided. If delays occur, the heat generated by mixing can be minimized by stopping the mixer and then agitating intermittently. The Standard Specifications for Ready Mixed Concrete requires that discharge of concrete be completed within 1 hour or before the drum has revolved 300 times, whichever occurs first. During hot weather the time limit can reasonable reduced to 1 hour or even 45 minutes.Since concrete hardens more rapidly in hot weather, extra care must be taken with placement techniques to avoid cold joints. For placement of walls, shallower layers can be specified to assure enough time for consolidation with the previous lift. Temporary sunshades and windbreaks help to minimize cold joints.
Floating should be done promptly after the water sheen disappears from the surface or when the concrete can support the weight of a finisher. Finishing on dry and windy days requires extra care. Rapid drying of the concrete at the surface may cause plastic shrinkage cracking.
Curing and Protection
Curing and protection are more critical in hot and cold weather than in temperate periods. Retaining forms in place cannot be considered a satisfactory substitute for curing in hot weather; they should be loosened as soon as practical without damage to the concrete.
The need for moist curing is greatest during the first few hours after finishing.
To prevent the drying of exposed concrete surfaces, moist curing should commence as soon as the surfaces are finished and continue for at least 24 hours. In hot weather, continuous moist curing for the entire curing period is preferred. However, if moist curing can not be continues beyond 24 hours, the concrete surfaces should be protected from drying with curing paper, heat-reflecting plastic sheets, or membrane-forming curing compounds while the surfaces are still damp. Moist-cured surfaces should dry out slowly after the curing period to reduce the possibility of surface crazing and cracking.
For unusual cases in hot weather and where careful inspection is maintained, a retarding admixture may be beneficial in delaying the setting time, despite the somewhat increased rate of slump loss generally resulting from their use.
Heat of Hydration
Heat generated during cement hydration raises the temperature of the concrete to a greater or lesser extent depending on the size of the concrete placement, its surrounding environment, and the amount of cement in the concrete. A general rule is that a 10F to 15F temperature rise per 100 lb of cement can be expected from the heat of hydration. There may be instances in hot-weather-concrete work and massive concrete placements when measures must be taken to cope with the generation of heat and attendant thermal volume changes to control cracking.