Concreting may proceed in cold weather provided that special precautions are taken to ensure that the surface temperature of the concrete at the time of placing is not less than 5°C for a succeeding period of at least: 

 4 days when the cement used in the concrete is ordinary Portland cement 

 2 days when the cement used in the concrete is rapid hardening Portland cement. 

Such precautions may include the following: 

 Warming the aggregates and heating the water, provided that the temperature of either does not exceed 60°C. Water and aggregates shall be mixed for a period sufficiently long for them to acquire a uniform temperature before cement is added. 

 Completely surrounding the freshly placed concrete with a cover and heating the enclosed air, which shall be kept moist. Draughts of hot or dry air shall not be directed at surfaces. 

 Insulating the formwork and finished concrete surfaces. 

 Providing screens to protect the concrete from air currents. 

The Contractor shall provide the Engineer with details of the precautions he proposes to take to protect the concrete from the effects of low temperatures and with details of the methods he proposes to use assess the correct timing at which such protection may be removed. No concreting shall be done in cold weather prior to the approval the Engineer for the proposed measures. 

## Concreting in Unfavourable Weather 

Concreting shall not be permitted during heavy rain or snowfall, or when the air temperature falls be- low 2°C, or when the concrete temperature rises above 32°C. When the air temperature exceeds 25°C, concreting shall only be permitted after special precautions, approved by the Engineer, have been tak- en to prevent early setting of the concrete, such as lowering the temperature of the water to be used in the mix or by means of a cooling-system, keeping the aggregates and shutters continuously sprayed with water and erection of temporary sun shades over the working area. During concreting operations the temperature of the placed concrete shall be recorded. 

## Compaction of Concrete 

The Contractor shall regard the compacting of the concrete to be of fundamental importance. A water- tight concrete of maximum density and strength shall be obtained. 

Concrete shall be thoroughly compacted during the operation of placing and shall be thoroughly worked around the reinforcement and embedded fixtures and into corners of the formwork and moulds. 

Mechanical vibrators shall be of the immersion type with a frequency of not less than 6000 vibrations per minute and as approved by the Engineer. A sufficient number of vibrators shall be used to handle the maximum rate of concrete production with a 50% allowance for standby units during any period of concreting. All operators handling vibrators shall be trained in their operation. 

Vibrators shall be inserted into the uncompact concrete vertically and at regular intervals. Where the uncompact concrete is in a layer above freshly compacted concrete, the vibrator shall be allowed to penetrate vertically for about 100 mm into the previous layer. Vibrators shall be withdrawn slowly from the mass of concrete so as to leave no voids. Internal type vibrators shall not be placed in the concrete in a random or haphazard manner nor shall concrete be moved from one part of the work to another by means of the vibrators. 

