How To Pour Concrete In Cold Weather | Recommended Practices And Precautions | LCETED

How To Pour Concrete In Cold Weather | Recommended Practices And Precautions | Concrete Under Extreme Environmental Conditions

 

Any concreting operation done at a temperature below 5°C is termed cold weather concreting. Most codes do not advocate concreting to be done at an atmospheric temperature below 5°C without special precautions.

Due to low temperature, the problems are mainly due to the slower development of concrete strength; the concrete in the plastic stage can be damaged if it is exposed to low temperatures which cause ice lenses to form and expansion to occur within the pore structure, and subsequent damage may occur due to alternate freezing and thawing when the concrete has hardened.

 

EFFECTS OF POURING CONCRETE IN HOT WEATHER

The effects of cold weather concreting may be summarized as follows.

1. Delayed setting At low temperatures,  the development of concrete strength is retarded as compared with the strength development at normal temperatures. The setting period necessary before the removal of formwork is thus increased. Although the initial strength of concrete is lower, the ultimate strength will not be severely affected provided the concrete has been prevented from freezing during its early life.

 

2. Early freezing of concrete When plastic concrete is exposed to freezing temperature, it may suffer permanent damage. If the concrete is allowed to freeze before a certain prehardening period, it may suffer irreparable loss in its properties so much so that even one cycle of freezing and thawing during the prehardening period may reduce compressive strength to 50 per cent of what would be expected for normal temperature concrete. The prehardening period depends upon the type of cement and environmental conditions. It may be specified in terms of time required to attain a compressive strength of the order of 3.5 to 7.0 MPa. Alternatively, it can be specified in terms of period varying from 24 hours to even three days depending upon the degree of saturation and water-cement ratio.

 

3. Stresses due to temperature differential A large temperature differential within the concrete member may promote cracking and has a harmful effect on durability. Such situations are likely to occur in cold weather at the time of removal of formwork.

 

SETTING TIME OF CONCRETE AT VARIOUS TEMPERATURE

 

RECOMMENDED PRACTICE WHILE POURING CONCRETE IN COLD WEATHER

As per IS: 7861 (Part-II)–1981, the following measures should be taken:

Temperature Control of Ingredients

The temperature at the time of setting of concrete can be raised by heating the ingredients of the concrete mix. It would be easier to heat the mixing water. The temperature of the water should not exceed 65°C as the flash set of cement will occur when the hot water and cement come in contact with the mixers. Therefore, the heated water should come in direct contact with the aggregate first, and not the cement. The aggregates are heated by passing steam through pipes embedded in aggregate storage bins as shown in Fig. Another precaution taken along with the heating of ingredients is to construct a temporary shelter around the construction site. The air inside is heated by electric or steam heating or central heating with circulating water. The temperature of ingredients should be so decided that the resulting concrete sets at a temperature of 10 to 20°C.

 

Use of Insulating Formwork and Blanket Covers

A fair amount of heat is generated during the hydration of cement. Such heat can be gainfully conserved by having insulating formwork covers capable of maintaining a concrete temperature above the desirable limit up to the first three days (or even seven days) even though the ambient temperatures are lower. The formwork covers can be of timber, clean straw, blankets, tarpaulins, plastic sheeting, etc., and are used in conjunction with an air gap as insulation. The efficiency of the covers depends upon the thermal conductivity of the medium as well as ambient temperature conditions. For moderately cold weather, timber formwork alone is sufficient.

 

 

Fig. Cold-weather concreting

 

Proportioning of Concrete Ingredients

The important factor for cold-weather concreting is the attainment of suitable temperatures for fresh concrete. Since the quantity of cement in the mix affects the rate of increase in temperature, an additional quantity of cement may be used. It would be preferable to use high alumina cement for concreting during frost conditions, the main advantage is that a higher heat of hydration is generated during the first 24 hours. During this period, sufficient strength (approximately 10 to 15 MPa) is developed to make the concrete safe against frost action. No accelerator should be used if high alumina cement is used. Alternatively, the rapid hardening of Portland cement or accelerating admixtures used with proper precautions can help in getting the required strength in a shorter period. Air-entraining agents are generally recommended for use in cold weather. Air-entrainment increases the resistance of the hardened concrete to freezing and thawing and normally, at the same time, improves the workability of fresh concrete. The calcium chloride used as an accelerating admixture may cause corrosion of reinforcing steel. In any case, calcium chloride should not be used in prestressed concrete construction.

 

Placement and Curing

Before placing the concrete, all ice, snow and frost should be completely removed. Care should be taken to see that the surface on which the concrete is to be placed and eminent parts are sufficiently warm. During periods of freezing or in near-freezing conditions, water curing is not applicable.

 

Delayed Removal of Formwork

Because of a slower rate of gain of strength during the cold weather, the formwork and props have to be kept in place for a longer time than in usual concreting practice.

 

The problem of concreting in cold weather can be minimized by adopting the precast construction of structures. Precast members are manufactured in factories where adequate precautions can be taken and concreting can be done in controlled conditions.

TIPS FOR POURING CONCRETE IN HOT WEATHER

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