COLD WEATHER CONCRETE GUIDELINES
Cold Weather presents many problems for the concrete industry. The most common problem is with having the concrete freeze and/or go through freeze thaw cycles before acquiring adequate strength. This and other problems can be avoided with precautionary steps. Protecting exposed concrete in these conditions can prevent costly issues down the road..
WHAT IS COLD WEATHER?
ACI 306 defines cold weather when the following conditions exist for three consecutive days:
- The average daily temperature is below 40°F
- Air temperature does not rise above 50° for more than half a day in any 24-hour period.
WHAT’S THE PROBLEM?
- Water begins to freeze in voids of concrete at 28°F
- Water expands when it freezes causing cracks in the concrete.
- Up to 50% strength reduction can occur if concrete freezes before reaching 500-PSI, which typically takes about two days after concrete placement.
WHAT’S THE SOLUTION?
The following facts and guidelines should be followed to assure quality of the concrete in cold weather:
1. Use air-entrained concrete when exposure to moisture and freezing and thawing conditions are expected.
2. Keep surfaces in contact with concrete free of ice and snow and at a temperature above freezing prior to placement.
3. Place and maintain concrete at the recommended temperature.
4. Place concrete at the lowest practical slump.
5. Protect plastic concrete from freezing or drying.
6. Protect concrete from early-age freezing and thawing cycles until it has attained adequate strength.
7. Limit rapid temperature changes when protective measures are removed.
HOT WEATHER CONCRETE GUIDELINES
Hot weather problems are most frequently encountered in the summer, but the associated climatic factors of high winds and dry air can occur at any time, especially in arid or dry climates.
High temperatures alone cause increased water demand, which in turn will raise the water-cement ratio and yield lower potential strength. Higher temperatures tend to accelerate loss of slump and entrained air. Hot weather conditions can produce a rapid rate of moisture evaporation from the surface as well as the accelerated setting time, among other problems. Concrete that is cure at high temperatures early will not be as strong at 28 days as the same concrete cured at more moderate temperatures (70°F). Generally, high relative humidity tends to reduce the effects of high temperature.
High temperatures, high wind velocity, and low relative humidity can affect fresh concrete in two ways: the high rate of evaporation may induce early plastic shrinkage or drying shrinkage crackage, and the evaporation rate can remove surface water necessary for hydration unless proper curing methods are employed. Thermal cracking may result from rapid drops in the temperatures of the concrete, such as when slabs or walls are placed on a hot day followed by a cool night. High temperature also accelerates cement hydration and contributes to potential cracking in massive concrete structures.
The keys to successful hot weather concreting are: (1) recognition of the factors that affect concrete and (2) advanced planning to minimize their effects. Use proven recommendations for adjusting concrete proportions, such as use of water reducing admixtures. Perhaps a moderate heat of hydration cement (ASTM Type II-moderate heat), pozzolanic admixture (fly ash) or ground granulated furnace blasted slag can reduce the effects of high temperatures.
Advance timing and scheduling to avoid delays in delivery, placing, and finishing is a must: trucks should be able to discharge immediately and adequate personnel should be available to place and handle the concrete. When possible, deliveries should be scheduled to avoid the hottest part of the day. In case of extreme temperature conditions or with mass concrete, the concrete temperature can be lowered by using chilled water or ice as part of the mixing water. Other measures, such as sprinkling and shading the aggregate can be used to help lower the temperature of the concrete. If low humidity and high winds are predicted, then windbreaks, surface evaporator retarder, sunscreens, or mist fogging may be needed to avoid plastic shrinkage cracking in slabs.
Plastic Shrinkage Cracking
Download this informative pdf to find out what plastic shrinkage cracking is and how to prevent it.