[Tutorials]: Concreting Under Special Conditions >> Hot Weather Concreting

WHAT IS HOT WEATHER CONCRETING?

Any operation of Concreting done at atmospheric temperature above 40 deg.C; or any operation of Concreting (other than steam curing) where the temperature of concrete at time of its placement is expected to be beyond 40 deg.C may be termed as Hot Weather Concreting. Special precautions are required to be taken so as to ascertain the desired qualities of the cast structures in such conditions.

EFFECTS OF HOT WEATHER ON CONCRETE

Before analyzing the remedial measures that one should take while carrying out concreting in extreme hot weather conditions, let us first try to understand what effects the hot weather may impart on the concrete as well as the cast structure.

The major effects which are generally noted are as following:

• Rapid Hydration: Higher temperatures result in rapid hydration of cement, increased evaporation of mixing water, greater demand of mixing water, and large volume changes resulting in cracks. Climatic factors affecting concrete in hot weather are high ambient temperature and reduced relative humidity, the effects of which may be more pronounced with increase in the wind velocity.
• Accelerated Setting: High Temperature increases the initial setting of concrete. The duration of time during which the concrete can be handled is reduced. Quick stiffening may necessitate undesirable re-tempering by addition of water. It may also result in cold joints.
• Reduction in Strength: High temperature results in the increase of the quantity of mixing water to maintain the workability with consequent reduction in strength.
• Increased Tendency to Crack: Either before or after hardening, plastic shrinkage cracks may form in the partially hardened concrete due to rapid evaporation of water. Cracks maybe developed in hardened concrete either by increased drying shrinkage resulting from greater mixing water used or by cooling of the concrete from its elevated initial temperature.
• Affected Curing: It is difficult to retain moisture for hydration and maintain reasonably uniform temperature conditions during the curing period.
• Difficulty in control of Air Content in Air-Entrained Concrete: It is more difficult to control the air content in air-entrained concrete. This adds to the difficulty of controlling workability. For a given amount of air-entraining agent, hot concrete entrains less air than concrete at normal temperatures.

PRECAUTIONS & MEASURES TO BE TAKEN

The major precautions, which are required to be taken for Hot Weather conditions, are listed hereunder:

• Sprinkling of the stockpiles of coarse aggregates with water and keeping them moist. This results in cooling by evaporation, and this procedure is especially 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.
• Circulating refrigerated air through pipes or other suitable means may also cool coarse aggregates.
• Ice flakes may be incorporated directly into concrete as part of mixing water in such a way that ice is completely melted by the time concrete is mixed failing which, there can be possibility of ice melting after consolidation of concrete, thus leaving hollow pockets in concrete resulting in detrimental effects.

OTHER SECONDARY MEASURES

Besides taking the above points into consideration, certain other secondary measures and checks should be adopted:

• Forms, reinforcement, and sub-grade may be sprinkled with cold water prior to placement of concrete.
• Fresh concrete, as soon as laid, should be covered by fog sprays, wet burlap, cotton mats, or other similar means.
• Moist curing should commence once the concrete has attained some degree of hardening, sufficient to withstand any kind of surface damage.
• Continuous curing for a period of not less than 10 days should be done without fail since volume changes due to alternate wetting and drying promote development of surface cracks.

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Filed under: Construction, Tutorials, Technical-Jargon

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