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

WHAT IS COLD WEATHER CONCRETING?

Any operation of concreting, whether mixing, placing, casting, etc., done at about 5 deg.C atmospheric temperature or below is termed as Cold 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 COLD WEATHER ON CONCRETE

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

The major effects which are generally noted are as following:

• Forming of Ice-Lenses: Severe damage may occur if concrete, which is still in the plastic state, is exposed to low temperature; thereby causing the ice lenses to form and expansion to occur within the pores of the mixed and poured concrete.
• Delayed Setting: When the temperature falls to about 5 deg.C or below, the development of concrete strength is retarded compared with the strength that is developed at normal temperatures. The hardening period necessary before the removal of forms is thus increased and the forms cannot be used rapidly.
• Freezing of Concrete at Early Ages: When concrete is exposed to freezing temperatures, there is a risk of concrete suffering irrevocable loss of strength and other qualities, that is, permeability may increase and the durability may be impaired.
• Repeated Freezing and Thawing of Concrete: If concrete is exposed to repeated freezing and thawing immediately after final setting and/or during the hardening period, the final qualities of the concrete are also affected.
• Stress Due to Temperature Differentials: Large temperature differentials within the concrete members may promote cracking and have harmful effects on the durability. Such differentials are likely to happen in cold weather conditions at the time of removal of forms, which more or less act as an insulation barrier

PRECAUTIONS & MEASURES TO BE TAKEN

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

• The forms should be kept in position longer
• Rapid hardening cement should be used
• Covering the top of concrete surface as well as the forms with insulating material
• Ensuring that the concrete temperature is not less than 5 deg.C at the time of placing it.
• Making sure that the concrete is not placed against a frozen sub-grade or against reinforcement or forms covered with snow or ice.
• Heating the aggregates and water by means of steam or hot blowers.
• Making sure that the concrete is delivered to the point of placing at not less than 10 deg.C, followed by placing quickly and insulating; or making sure that concrete is delivered to the point of placing at not less than 5 deg.C, placing quickly and providing continuous heating to the concrete externally.

OTHER SECONDARY MEASURES

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

• While heating the aggregates, the temperature of any particular individual batch should not exceed 65 deg.C. The heating of aggregates higher than 15 deg.C is rarely necessary while using mixing water at 60 deg.C.
• If coarse aggregates are free of frozen lumps, ice and/or snow, only sand should be heated up to a temperature of about 40 deg.C. The heating of aggregates can be done by embedded steam pipes for larger works, or with the help of steel drums embedded in heaped aggregates and filled with firewood in case of small jobs.
• To avoid possibility of flash set when either aggregate or water is heated to a temperature in excess of 40 deg.C, water and aggregate should be mixed together in the mixer before cement is added to it. The heated water should come into contact with the aggregate first and not the cement.
• The Mix design should separately be done for the cold weather conditions and additives if any should be used only on results of the extensive study of the trial mixes made on the mix design.

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Post Tags: Construction, Tutorials, Technical-Jargon

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