Guidelines on use of Ready Mixed Concrete

GUIDELINES ON USE OF READY MIXED CONCRETE

1. Temperature of concrete:

Concrete is not recommended to be placed at a temperature above 40 oC
and below 5o C without proper precaution as laid down in IS: 7861 (Pt.I or
pt. II as the case may be ). IS:7861 pt. I deals with hot weather concreting
and Pt.II deals with cold weather concreting.

1.1 Hot weather concrete:

Any operation of concreting done at atmospheric temperature above 40 oC may be put under hot weather concreting. In the absence of special precautions as laid down under IS: 7861 (Pt.I), the effect of hot weather may be as follows:

a) Accelerated setting: A higher temperature of fresh concrete results in a more rapid hydration and leads to reduced work ability accelerated setting. This reduces the handling time of concrete.

b) Reduction in strength: Concrete mixed, placed and cured at higher temperature normally develops higher early strength than concrete produced and cured at normal temperature but at 28 days
or later the strength are generally lower.

c) Increased tendency to crack: Rapid evaporation may cause plastic shrinkage and cracking and subsequent cooling of hardened concrete would introduce tensile stresses.

In order to avoid harmful effect of hot weather concreting IS: 7861 (Pt.1) recommends that temperature of ingredients should be controlled so that the temperature of produced concrete is lower. Mixing water has the greatest effect on lowering of temperature of concrete. The use of chilled water/ flaked ice in mixing produces adequate reduction in concrete temperature.

In order to control the temperature of concrete and to avoid adverse effect of hot weather, it is desirable to limit the maximum temperature of concrete as 35 oC to keep margin for increase in temperature during transit.

1.2 Cold weather concreting:

Any concreting operation done at a temperature below 5 oC is termed as cold weather concreting. IS: 7861 (Pt.II) recommends special precautions to be taken during cold weather concreting.

In the absence of special precautions, the effect of cold weather concreting may be as follows:

a) Delayed setting:- When the temperature is falling to about 5oC or below, the development of strength of concrete is retarded compared with development at normal temperature. Thus, the
time period for removal of form work has to be increased.

b) Freezing of concrete at early stage:- The permanent damage may occur when the concrete in fresh stage is exposed to freeze before certain pre-hardening period. Concrete may suffer irreparable loss in its properties to an extant that compressive strength may get reduced to 50% of what could be expected for normal temperature concrete.

c) Stresses due to temperature differentials:- Large temperature differentials within the concrete member may promote cracking and affect its durability adversely.

In view of above, it is desirable to limit the lowest temperature of
concrete as 5o C