Concreting in Hot Weather

The properties of concrete that may be affected by hot weather include:

Consistence – The slump/flow/slump flow of concrete reduces more rapidly. Adding water to improve the consistence shall decrease the concretes compressive strength, potentially increase permeability and ultimately affect the durability of the structure.

Setting time – As concrete temperatures increase the setting time is reduced and thus the time to place, compact and finish the concrete is reduced.

Plastic Shrinkage Cracking – The hot weather shall accelerate the loss of moisture from the surface and therefore increase the risk of plastic shrinkage cracking.

Thermal Cracking – Concrete is at risk of thermal cracking when it is first placed and the heat of hydration raises the temperature of the interior of the concrete. Changes in temperature of the concrete may also result in cracking particularly where concrete is placed on a hot day followed by a cool night.

Surface Finish – With the increased rate of hydration the surface of the concrete shall ‘dry’ quicker, leading to premature finish being applied – trapping an amount of bleed water, this bleed water may result in debonding of the top surface and subsequent flaking/de-lamination

Minimising the effect of hot weather:

Admixtures:

Use of admixtures to maintain water/cement ratio thus enabling increases in consistence.

• Increase dose of a lignosulfonate based plasticiser to use as a set retarder
• Use of a retarder to use as a set retarder
• Use of admixture combinations to ensure consistence retention of concrete
• Use of S/Plasticiser to reduce the total cement content (Heat of hydration is proportional to the total cement content)

Planning:

1. The Use of CEM I increases the rate and heat of hydration. Potentially reducing the time available for placing and finishing.
2. Concreting for cooler parts of the day shall be beneficial.
3. Risk of cold joints increases during hot weather.