Concrete 40. It is not recommended to keep at temperatures above0C and below 50c Without reasonable precaution as prescribed in IS: 7861 (Part-I or Part-II as the case may be).
IS:7861 Part-I deals with hot weather concreting and Part-II deals with cold weather concreting.
hot weather concreting
Special problems arise in preparing, applying and curing concrete in hot weather. Higher temperatures result in rapid hydration of the cement, increased water evaporation of the mixture, greater water demands, and large volume changes resulting in cracking. Hot weather problems on concrete are exacerbated by a number of factors, such as the use of rapid hardening cement, handling large batches of concrete, etc.
40. any operation of concreting at atmospheric temperatures above0C can be placed under hot weather concreting. In the absence of special precautions prescribed under IS: 7861 (Part-1), the effect of hot weather can be as follows:
a) quick setting
The higher temperatures of fresh concrete result in more rapid hydration of the cement and reduced workability/quick setting. This reduces the handling time of concrete.
b) decrease in strength
Concrete mixed, laid and cured at high temperatures typically develops a higher initial strength than concrete produced and cured at normal temperatures but strength is usually lower after 28 days or so.
c) increased tendency to crack
Rapid evaporation can cause the plastic to shrink and crack, and the subsequent cooling of the hardened concrete will create tensile stresses.
d) rapid evaporation of water during the curing period
It is difficult to retain moisture for hydration and maintain proper temperature conditions during the curing period.
e) Difficulty in control of air content in air-entrained concrete
It is more difficult to control the amount of air in air-entrained concrete. This adds to the difficulty of controlling the functionality. For a certain amount of air-penetrating agent, heated concrete will penetrate less air than concrete at normal temperature.
To avoid the harmful effects of hot weather concreting, IS: 7861 (Pt.1) recommends that the temperature of the material be controlled so that the temperature of the concrete produced is low. Adding water has the greatest effect on lowering the temperature of the concrete. The use of cold water/flaked ice in the mix results in a substantial reduction in the temperature of the concrete.
To control the temperature of the concrete and avoid the adverse effects of hot weather, it is desirable to limit the maximum temperature of the concrete to 35.0C to keep margin for temperature rise during transit.
cold weather concreting
Concrete production in cold weather presents special and peculiar problems that do not arise when concreting at normal temperatures. In addition to the problems associated with the setting and hardening of cement concrete, serious damage can occur if the concrete, which is still in a plastic state, is exposed to low temperatures, causing ice lenses to form and pores to expand within the structure. Is. Therefore it is necessary to keep the temperature of the concrete above the minimum value before it is placed in the form-work. After laying, the concrete can be kept above a certain temperature with the help of proper insulating methods before the protection is removed. During periods of low ambient temperature, special techniques must be adopted to cure the concrete while it is in form-work or after it has been removed.
5. any concreting operation performed at a temperature lower than0C is called cold weather concreting.
IS: 7861 (Part-II) recommends special precautions to be taken during cold weather concreting.
In the absence of special precautions, the effect of concreting in cold weather can be as follows:
a) delayed setting
When the temperature is around 5. falling till0C or below, the development of strength of concrete is retarded compared to development at normal temperature. Thus, the time period for deletion of form work has to be increased.
b) solidification of concrete in the initial stage
Permanent damage can occur if concrete in its fresh state is exposed to solidification before some pre-hardening period. Concrete can suffer irreparable damage to its properties to such an extent that compressive strength may be as low as 50% of that required for normal temperature concrete.
c) stress due to temperature difference
Large temperature differences within a concrete member can promote cracking and adversely affect its durability. It is a common experience that large temperature differences within a concrete member can promote cracking and have a detrimental effect on durability. Such differences are likely to occur at the time of removal of form insulation in cold weather.
d) frequent freezing and thawing of concrete
If the concrete is exposed to repeated freezing and -thawing after the final set and during the hardening period, the final properties of the concrete may also deteriorate.
Considering the above, the minimum temperature of concrete should be set at 5. desirable to limit0C