Hot Weather
Hot weather can be defined as any period of high temperature in which special precautions need to be taken to ensure proper handling, placing, finishing, and curing of concrete. Other climactic factors such as high wind and dry air can also produce a rapid rate of evaporation of moisture from the surface of the concrete. Any combination of hot, dry, or windy weather can:
• Increase slump loss, through rapid surface evaporation.
• Accelerate set.
• Increase the potential of plastic shrinkage cracking.
• Decrease air content.
• Reduce strength, especially if water is added to offset slump loss.
High humidity can offset some of these problems.
Tips to Minimize the Effects of Hot Weather
• Contact your Prairie salesperson to discuss cooling concrete, reducing heat of hydration, and using a water-reducing, set-retarding admixture.
• Dampen the sub-grade, the forms, and any reinforcing steel to cool them
• Erect temporary windbreaks and sunshades.
• Consider early morning, evening, or nighttime concrete placement.
• Schedule ready mix trucks to minimize waiting time.
• Provide a sufficient work force to eliminate delays during construction.
• Protect the concrete between finishing passes.
• Apply final curing immediately after final finishing to control moisture loss and surface temperature.
• Saw cut the slab as soon as possible.
• Protect the surface; cover slab for at least 3 days.
Protection Between Finishing Passes
• Spraying a "evaporation retardant" on the surface can reduce evaporation by up to 80% for concrete under shade and windy conditions and up to 40% for concrete exposed to direct sunlight.
• Covering with white plastic sheeting or damp burlap.
• Continuous fog spraying to produce a fog blanket, spray should not exceed ½ gallon per minute. Optional Fog Sprayer: Use a pressure washer that delivers 2 to 3 gpm at 2000 to 2500 psi. Use a 40 to 50 degree wide-angle nozzle; in windy situations, use a 10 to 15 degree nozzle.
Cold Weather
Cold weather is defined as any period when the average daily air temperature is less than 40° F or less than 50° F for more than 12 of any 24 hour period on more than 3 consecutive days. Slabs lose moisture and /or heat rapidly in cold weather which affects setting time, strength gain and long term durability of the concrete. Cold weather:
• Can freeze mix water, reducing concrete strength up to 50%.
• Increases concrete set time up to one-third for each 10° F decrease in air temperature.
• Retards concrete stiffening.
Tips for Protecting Concrete in Cold Weather
• Warm the subgrade, the forms, and any reinforcing steel-don't place concrete on frozen subgrade.
• Maintain concrete temperature above 50° F by insulating or heating for three days. Use insulation blankets to retain internal heat generated by the concrete or provide heaters for the scheduled concrete placement.
• Use heated enclosures to block wind, keep out the cold, and conserve heat.
• Use air-entrained concrete.
• Use proper curing methods as soon as possible. Cure to protect against freezing until concrete reaches 500-psi compressive strength (usually about 2 days at 50° F).
• Contact your Prairie salesperson to discuss heating water and aggregates, or adding a non-chloride accelerating admixture, increasing cement content or using Type III Cement.
• Protect concrete against multiple freeze-thaw cycles until it reaches 3500-psi compressive strength.
What is Carbonation?
Carbonation is a chemical reaction in the concrete that forms calcium carbonate (which kills cement) and creates a soft dusty surface on normal concrete. Carbon dioxide gases accelerate this action. Carbonation is most prevalent in cold weather concreting.
To Lessen Carbonation Damage
• Use a water cure or a membrane curing compound and block carbon dioxide gas from the concrete. Membrane film should be uniform and heavy.
• Use an accelerating set admixture in the concrete.
• Use vented heaters.
• Open the building to fresh air during pouring to ventilate exhaust fumes from trucks, troweling machines, etc.
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