Cold Weather Concreting (Nat)

When specifying concrete it is important to take into account the season and the time of placement, as there is a different ordering and placement procedure between hot and cold weather. Precautions especially need to be taken when the air temperature falls below 10°C.

AS 1379-2007 Specification and Supply of Concrete requires that concrete temperature at 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 min from the commencement of mixing.

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

EFFECTS OF LOW TEMPERATURES

Lower temperatures can have a number of effects on the behaviour of concrete.

Extended Setting Times

The time for bleedwater evaporation will increase, which will then extend the time on the job, and consequently increasing the job costs. It is important not to try to speed up the setting time by adding cement or cement / sand mixtures to use up excess water.

Concrete Strength

In lower temperatures the concrete strength is achieved considerably slower and consequently formwork removal should be delayed.

Cracking

The extent of cracking maybe increased, as a lower concrete strength maybe inadequate to resist drying shrinkage.

Freezing

At lower 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 hrs until after placement. If concrete does freeze the potential for strength gain is reduced. Extent of this damage may vary upon the age and strength of the concrete when it freezes.

MINIMISING THE EFFECTS OF LOW TEMPERATURES

To minimise the effects of low temperatures and to increase the rate of which the concrete gains strength, the following should be considered.

• 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 appropriate admixtures to be used.
• Have the batch plant use hot water in the mix, this will help accelerate the rate of which the cement hydrates.

PRECAUTIONS TO BE TAKEN ON SITE

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

• Protection from the cold, including cold or frozen ground, winds and frost.
• Insulate formwork so the heat which is generated in the first 24 hrs by the placement of the cement is retained within the concrete. Timber formwork is a reasonable thermal insulator and would be 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 in place as long as practical to protect from frost.
• All concrete should be cured to ensure it achieves its main strength and durability. To help the process of curing in cold weather concreting, an insulation blanket should be used such as plastic sheeting or tarps. The blanket should be properly lapped at joints to insure 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.

For further information HIA members can contact HIA’s Building Services staff on 1300 650 620 or hia_technical@hia.com.au.

  If you would like to become a HIA member, contact 1300 650 620 or enquiry@hia.com.au

Ref No. NAT MAT 08-12

The above is intended to provide general information in summary form. The contents do not constitute specific advice and should not be relied upon as such. Formal specific advice should be sought by members with respect to particular matters before taking action.  ABN 99 004 631 752