COLD WEATHER CONCRETING:
Any concreting operation carried out at a temperature below 5 ° C is called cold weather concreting. Most codes do not recommend that concreting be carried out at an atmospheric temperature below 5 ° C without special precautions.
Due to the low temperature, the problems are mainly due to the slower development of concrete strength; Concrete in the plastic stage can be damaged if exposed to low temperatures that cause ice lens formation and expansion within the porous structure, and subsequent damage can occur due to alternating freezing and thawing when the concrete has been hard.
The effects of cold weather concreting may be summarized as follows.
1. Delayed setting
At low temperatures, the strength development of concrete is delayed compared to the strength development at normal temperatures.
This increases the setting time required before stripping. Although the initial strength of concrete is lower, the ultimate strength will not be seriously affected as long as the concrete has been prevented from freezing during its initial life.
2. Early freezing of concrete
When plastic concrete is exposed to freezing temperatures, it can suffer permanent damage. If concrete is allowed to freeze before a certain pre-hardening period, it can suffer an irreparable loss in its properties, so much so that even a freeze-thaw cycle during the pre-hardening period can reduce compressive strength by 50 percent. then would be expected for normal conditions. concrete temperature.
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The pre-hardening period depends on the type of cement and the environmental conditions. It can be specified in terms of the time required to achieve a compressive strength of the order of 3.5 to 7.0 MPa. Alternatively, it can be specified in terms of a period ranging from 24 hours to even three days, depending on the degree of saturation and the water-cement ratio.
3. Stresses due to temperature differential
A large temperature differential within the concrete member can promote cracking and have a detrimental effect on durability. Such situations are likely to occur in cold weather when removing formwork.
Recommended practice
As per IS: 7861 (Part-II)–1981, the following measures should be taken:
Temperature control of ingredients:
Temperature can be increased during concrete setting by heating the ingredients of the concrete mixture.
Mixed water makes it easier to heat. When the water temperature does not exceed 65 ° C, the lightning setting of the cement occurs when it comes into contact with hot water and cement mixers. Therefore, hot water must first come into direct contact with the aggregate and not with the cement.
The aggregates are heated by passing steam through the pipes embedded in the total storage bins as shown in Fig.12.2. Another precaution taken in addition to materials heating is to build temporary shelters around the construction site.
The air inside is heated by central heating with an electric or steam heating or circulating water. The temperature of the materials must be determined, resulting in the concrete being set at a temperature of 10 to 20 ° C.
Use of insulating formwork and blanket covers:
During the hydration of the cement, a considerable amount of heat is generated. Even at room temperature, that heat can be conserved by having insulating form covers capable of maintaining concrete temperatures in excess of the desirable limit for the first three days (or seven days).
The formwork covers can be made of wood, clean straw, blankets, tarps, plastic sheets, etc., and used as an air gap insulation.
The efficiency of the covers depends on the thermal conductivity of the medium and the ambient temperature conditions. For moderately cold climates, only wooden formwork is sufficient.
Proportioning of concrete ingredients:
An important factor for cold weather is reaching the right temperatures for fresh concrete. As the amount of cement in the mixture affects the rate of temperature rise, an additional amount of cement can be used.
It is desirable to use cement with a high content of alumina for concrete in ice conditions, and the main advantage is that it creates more heat of hydration in the first 24 hours. During this period, sufficient strength (about 10-15 MPa) is developed for the concrete to be safe from frost.
The accelerator should not be used when using high alumina cement. Alternatively, fast hardening of Portland cement or acceleration
Mixtures used with proper precautions can help to gain the required strength in less time.
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Air training equipment is generally recommended for use in cold climates. The introduction of air increases the resistance of hardened concrete to freezing and boiling and usually improves the processing of fresh concrete at the same time.
Calcium chloride, used as an accelerator mixture, can corrode reinforcing steel. In any case, calcium chloride must not be used in prestressed concrete.
Placement and curing:
Before placing the concrete, all ice, snow, and ice should be completely removed. Care should be taken to see the surface on which the concrete is to be placed and the eminent parts are warm enough. During periods of freezing or near freezing, water treatment is not applicable.
Delayed removal of formwork:
Due to the slower reinforcement rate during cold weather, formwork and props should be stored for a longer time than in the usual practice of concreting.
The problem of concreting in cold weather can be minimized by adopting ready-made concrete structures. Prefabricated members are manufactured in factories where appropriate precautions can be taken and can be concreted in controlled conditions.