What is Cold weather concreting and effects, defects and precaution during concreting

What is Cold weather concreting

Any concrete operation performed at a temperature below 5°C is known as cold weather concreting. As per code provisions, the concreting is not recommended to be placed at a temperature below 5°C without special precautions. 

The normal procedure adopted for concreting in fair weather will not be valid for concreting under special conditions. The performance of concrete is adversely affected unless appropriate measures are taken to control it. The concreting under special conditions include: 

• Cold Weather concerning
• Hot Weather Concreting
• Under Water Concreting.

The production of concrete in cold weather may generate special and peculiar problem may affect the overall performance of concrete. The effects of cold weather on concerting summarized below.

Effects of low Temperatures on Concrete

(i) Delayed Setting and hardening

Rate of hydration is dependent upon the temperature low temperatures the Development of concrete strength is retarded as compared to the strength Development at normal temperature. If the temperature is low, concrete takes a long time to set and even longer time to harden (i.e., to attain the desired strength). 

The delay in setting and hardening of concrete results in increased stripping time of formworks, thus, affecting the progress of work and economy. The delay in setting time makes the concrete vulnerable to frost attack and other related disturbances.

(ii) Early freezing of concrete

When the temperature goes below freezing point, the free water available in plastic concrete tends to freeze (as ice crystals). Freezing of water, not only prevents the hydration of cement but also make the concrete to expand. 

If the concrete is allowed to freeze before a certain prehardening period, it may suffer irreparable loss in its properties. The prehardening period depends upon the type of cement and environmental conditions. 

(iii) Stresses due to differential temperature

The concrete subjected to cold weather may experience a large temperature differential within the concrete member that may promote cracking. This may also have a harmful effect on durability. Such conditions may arise when the formwork is removed. 

Proportioning of Concrete Ingredients during cold weather concreting.

The following are the properties of concrete ingredients that are given below.

1. As the quantity of cement in the mix affects the rate of increase in temperature therefore an additional quantity of cement may be used.
2. It is advisable to use high alumina cement for concreting during frost conditions. The main advantage of using high alumina cement is that it generates higher heat of hydration in the first 24 hours.
3. No accelerators should be used if high alumina cement is used.
4. Air-entraining agents are recommended for use in cold weather as they offer better resistance to freezing and thawing. They also improve the workability of fresh concrete.

Precautions before concreting to be taken into account are the following below.

1. All the ingredients of concrete (i.e., cement and aggregates) should be stacked on wooden platforms. These should be covered with tarpaulins (plastic sheets), gunny bags, etc. Temporary shelters should be constructed around the construction site.
2. Mixing and batching plants should be covered from direct exposure of winds. Windbreakers should be erected with the help of plastic sheets, tarpaulin to avoid adverse effects of cold weather. 
3. The surface prepared for concreting should be covered with insulating material to prevent The surface from freezing until the concrete is placed. Concrete should not be placed on freezing ground.
4. The temperature of ingredients should be so decided that the resulting concrete sets at a temperature of 10° to 20°C. The water should be heated (below 60°C) and the aggregates should be heated if the temperature is below freezing temperature. The ager heated by passing steam through pipes embedded in aggregate storage bins.
5. It may be noted that cement should not be heated under any circumstances.
6. The temperature of the water should not exceed 65°C as the flash set of cement will when the hot water and cement come in contact in the mixer. Therefore, the aggregate and water should be mixed first in the mixer and then should be added.
7. The use of accelerators is recommended but under strict supervision and controlled to quantity. 

Precautions after placing concrete are the following below.

1. A large amount of heat is generated during the hydration of cement. This heat can be conserved by having proper insulation of formwork. The cover to formwork can be of timber, clean, straw, blankets, tarpaulins, plastic covering sheets, etc. and are used in conjunction with an air gap as insulation. 
2. The concrete surface should be covered until it hardens.
3. During the period of freezing or in near-freezing conditions, water curing is not applicable.
4. Because of a slower rate of gain of strength during the cold weather, the formwork and props have to be kept for a longer time as compared to normal concreting practice.