EXTREME WEATHER CONCRETING






In countries which experience extreme weather conditions special problems are encountered in preparation, placement and curing of concrete. India has regions of extreme hot weather (hot-humid and hot-arid) as well as cold weather. The Indian Standards dealing with extreme weather concreting are: IS: 7861 (Part 1-1975 Reaff. 2007)-Hot weather concreting and IS: 7861 (Part 2-1981 Reaff. 2007) -Cold weather concreting.

 HOT WEATHER CONCRETING:

Special problems are encountered in the preparation, placement and curing of concrete in hot weather. High temperature result in :

Ø  Rapid hydration of cement
Ø  Increased evaporation of mixing water
Ø  Greater mixing water demand
Ø  Large volume changes in concrete resulting in cracks.
Ø  Reduction in strength.

The climatic factors affecting concrete in hot weather are:

Ø  High ambient temperature
Ø  Reduced relative humidity
Ø  Increased wind velocity

Problems associated with hot weather concreting shall be addressed as follows:

Ø  Controlling the temperature of concrete ingredients
Ø  Suitable proportioning of concrete mixes.
Ø  Controlling the temperature of concrete as placed.
Ø  Controlling the processes such as concrete production and delivery
Ø  Carrying out effective protection and curing of placed concrete.

Controlling the temperature of concrete ingredients:

The most direct approach to keep concrete temperature down is by controlling the temperature of its ingredients. The contribution of each ingredient to the temperature of concrete is a function of the temperature, specific heat and quantity used of that ingredient. The aggregates and mixing water exert the most pronounced effect on temperature of concrete. Thus, in hot weather all available means shall be used for maintaining these materials at as low temperatures as practicable.

Aggregates 

Any one of the procedures or a combination of the procedures given below may be used for lowering the temperature or at least for preventing excessive heating of aggregates.

 Shading stockpiles from direct rays of the sun.

Sprinkling the stockpiles of coarse aggregate with water and keeping them moist.
This results in cooling by evaporation, and this procedure is specially effective when relative humidity is low. Such sprinkling should not be done haphazardly because it leads to excessive variation in surface moisture and thereby impairs uniformity of workability. When coarse aggregates are stockpiled during hot weather, successive layers should be sprinkled as the stockpile is-built up. If cold water is available, heavy spraying of coarse aggregate immediately before use may also be done to have a direct cooling action. Coarse aggregates may also be cooled by methods, such as inundating them in cold water or by circulating refrigerated air through pipes or by other suitable methods.

Water

The mixing water has the greatest effect on temperature of concrete, since it has a specific heat of about 4.5 to 5 times that of cement or aggregate. The temperature of water is easier to control than that of other ingredients and, even though water is used in smaller quantities than the other ingredients, the use of cold mixing water will effect a moderate reduction in concrete placing temperatures. For a nominal concrete mixture containing 336 kg of cement, 170 kg water, 1850 kg of aggregate per ma, a change in 2°C water temperature will effect a 0.5 º C change in the concrete temperature.

Efforts shall be made to obtain cold water, and to keep it cold by protecting pipes, water storage tanks, etc. Tanks or trucks used for transporting water shall be insulated and/or coloured and maintained white or yellow. Under certain circumstances, reduction in water temperature may be most economically accomplished by mechanical’ refrigerator or mixing with crushed ice. Use of ice as a part of the mixing water is highly effective in reducing concrete temperature since, on melting alone, it takes up heat at the rate of 80 kcal/kg. To take advantage of heat of fusion, the ice shall be incorporated directly into the concrete as part of the mixing water. Conditions shall be such that the ice is completely melted by the time mixing is completed.

NOTE :- If the ice is not melted completely by the time mixing is completed, there can be a possibility of Ice melting after consolidation of concrete and thus leaving hollow pockets in concrete, with detrimental effects.

Recommended procedure for concreting during hot weather conditions is given below:

Ambient temperature shall be below 40° C at the time of placement of concrete.  Concreting may be planned during morning and evening hours.
The period between mixing and delivery (placing) shall be kept an absolute minimum.

Keep aggregates under shade and cool aggregates by sprinkling water.
Formwork, reinforcement shall be sprinkled with cool water just prior to placement of concrete.



COLD WEATHER CONCRETING:

The production of concrete in cold weather introduces special and peculiar problems which do not arise while concreting at normal temperatures. Quite apart from the problems associated with setting and hardening of cement concrete, severe damage may occur if concrete which is still in the plastic state is exposed to low temperature, thus causing ice lenses to form and expansion to occur within the pore structure. Hence it is essential to keep the temperature of the concrete above a minimum value before it is placed in the formwork. After placing, concrete may be kept above a certain temperature with the help of proper insulating methods before the protection is removed. During periods of low ambient temperature, special techniques are to be adopted to cure the concrete while it is in the formwork or after its removal.


The Precautions to be taken and methods adopted for concreting in sub-zero temperature is listed below.

  1. Utilization of the heat developed by the hydration of cement and practical methods of insulation.

  1. Selection of suitable type of cement

  1. Economical heating of materials of concrete
(Heating of water is the easiest to be adopted)

  1. Admixtures of anti-freezing materials

  1. Electrical heating of concrete mass

  1. Use of air-entraining agents.
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