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Hot Weather Concreting
Cold Weather Concreting

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ACI 305 “Hot Weather Concreting” defines hot weathers as any combination of the following conditions that tends to impair the quality of the freshly mixed or hardened concrete: High ambient temperature High concrete temperature Low relative humidity Wind speed Solar radiation The success of many hot-weather concreting operations depends on the steps taken to slow the cement hydration reactions within the concrete and to minimize the rate of evaporation of moisture from the freshly mixed concrete.

Potential concrete problems in hot weather are likely to include: Increased water demand Increased rate of slump loss Increased rate of setting Increased tendency for plastic-shrinkage cracking Increased difficulty in controlling entrained air content Decreased 28-day and later strengths Increased tendency for differential thermal cracking Greater variability in surface appearance Increased permeability

 

High temperatures accelerate the hardening of concrete and more water is generally required to maintain workable consistencies. If the water-cement ratio is not maintained by adding additional cement, strength and durability will be reduced. For example, if the temperature of concrete is increased from 50 degrees F (10ºC) to 100 degrees F (38ºC), about 33 pounds (roughly 15 liters) of additional water is needed per cubic yard of concrete to maintain a three-inch slump. If the water content of concrete is increased without increasing the cement content, the strength and other properties of concrete are adversely affected.

High water contents also mean greater drying shrinkage. Furthermore, setting is accelerated which shortens the time within which the concrete can be handled and finished. Curing concrete requires that it be kept damp so that hydration can be completed, therefore, it cannot be allowed to dry too rapidly.

 

 

Keeping equipment such as mixers, chutes, wheelbarrows, etc. in the shade or covered with wet burlap will reduce the effect of the sun's heat. Forms, reinforcing steel, and subgrade should be sprinkled with cool water just before the concrete is placed. Wetting down the area cools the surrounding air and increases the relative humidity. This not only reduces the temperature but also minimizes the evaporation of water from the concrete.

Aggregates have a pronounced effect on fresh concrete temperature because they represent 60 to 80 percent of the total weight of concrete. Stockpiles should be shaded from the sun and kept moist by sprinkling. Since evaporation is a cooling process, sprinkling provides effective cooling, especially when the relative humidity is low. Cement temperature has only a minor effect on the temperature of freshly mixed concrete because it represents a relatively small percentage of the total mixture.

Concrete Curing is a hydration process and requires water to be present. Forms should be sprayed with water to keep them damp, or they will absorb part of the mixing water. As soon as the concrete has set sufficiently to prevent marring the surface, it should be covered with straw, burlap, or other materials such as sand and kept moist for several days. Covering with polyethylene plastic sheets or using curing compounds helps in preventing evaporation by air or wind. Anything to prevent rapid dehydration will be helpful.

High ambient temperature cause low relative humidity due to which loss of water from concrete is very high which result in high water requirement for mix preparing.

Precautions in hot weather concreting:

To control the high concrete temperature following steps may be taken:

1. Schedule concreting. The concrete can be scheduled to time when the temperature is not high e.g. in summer it can be schedule to night or early morning

2. Material and mix proportions: use material and mix design proportion having good weather resistance e.g. select sand having low specific heat.

3. Covered environment: The concrete can be done in a covered environment.

4. Chilling: The chilling of aggregate can be done by watering or keeping them covered.

5. Efficient work force: employ efficient workforce and machinery to improve the handling.

6. Use of low heat cement: Use low heat cement.
   

Temperature of Concrete:

The formula to calculate temperature of concrete is given as:

OR

Where Ta, Tc,Tw & Twa and Ma, Mc, Mw, Mwa are temperature and mass of aggregate, cement, water and water content in aggregate respectively. Mi is the mass of ice.

To decrease the temperature of concrete by 1 ºC, decrease the temperature of any one of the following by:

Cement OR

Water OR

Aggregate

Sometimes ice is used to decrease the temperature of concrete

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