Tips for using concrete in cold weather

When should concrete be laid?

Different procedures apply depending on when concrete is laid, with particular caution needed when the air temperature falls below 10°C.

AS 1379:2007 Specification and Supply of Concrete requires that concrete temperature at the point of delivery should be between 5°C and 35°C, and that discharge of all the concrete in a batch shall be completed within 90 minutes of commencing mixing.

When the air temperature is below 10°C, you will need to ensure an acceptable minimum concrete temperature at the point of delivery and a time limit of 90 minutes for complete dispatch of the concrete. It may be increased to ensure the concrete will gain the required strength for its application.

Impact on concrete behaviour

Low temperatures can have several effects on concrete behaviour:

• Extended setting times - The time for bleedwater evaporation will increase, which will then extend the time on the job, and consequently increase job costs. It’s important not to try to speed up the setting time by adding cement or cement/sand mixtures to use up excess water 
• Concrete strength - It takes longer for concrete to gain strength during cold temperatures – consequently, formwork removal should be delayed
• Cracking - The extent of cracking may be increased, as lower concrete strength may be inadequate to resist drying shrinkage
• Freezing - At low temperatures concrete is vulnerable to freezing both before and after it has set. As a general rule concrete must be protected from freezing for 24 hours after placement. If concrete does freeze the potential for strength gain is reduced. The extent of the damage caused may depend on the age and strength of the concrete when it freezes.

Minimising the effects of low temperatures

To minimise the effects of low temperatures and increase the rate at which concrete gains strength, consider the following:

• Increase the quantity of cement
• Use the correct type of cement. HE (High Early) strength cements will gain strength more rapidly than general purpose cement
• Order lower slump content in the concrete. Having less water in the mix will increase the rate of strength gain
• Reduce the time between mixing and placing
• The use of admixtures will help the concrete in reducing the setting time and accelerating the strength gain. Contact your supplier for the appropriate admixtures to be used
• Have the batch plant use hot water in the mix. This will help accelerate the rate at which the cement hydrates.

Take precautions on site

When a sudden and unexpected frost occurs or where the air temperature falls below 5°C the following precautions should be taken:

• Protect against winds and frost where possible
• Insulate formwork so the heat that is generated in the first 24 hours by the placement of the cement is retained within the concrete. Timber formwork is a reasonable thermal insulator and is sufficient for moderately cold conditions. Metal formwork will offer little or no protection from the cold if not properly insulated
• Delay stripping of formwork and leave it in place as long as practicable to protect from frost
• All concrete should be cured to ensure it achieves its main strength and durability. Use an insulation blanket, such as plastic sheeting or a tarp, to help the process of curing during cold weather concreting. The blanket should be properly lapped at the joints to ensure wind tightness. Newly released concrete from insulated formwork or heated enclosures should never be saturated in cold water. The temperature of the concrete surface should also be allowed to drop slowly to avoid thermal cracking.

To find out more, contact HIA’s Building Services team.

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