The setting of concrete involves a series of complex chemical reactions that are very sensitive to temperature. This reaction, under normal conditions, gives off a considerable amount of heat. If it is so cold that the reaction slows down, the reaction and heat evolution is delayed – consequently, the strength gain will be drastically reduced.

The use of admixtures can over-ride some of the negative affects of low temperatures and enable you to continue to place concrete in low temperatures, hence winter concreting.

So what is a “low” temperature?

In New Zealand, we are constrained in what we can and can’t do in the construction industry by the Building Code and the referenced documents that support the Code. The one we are interested in at the moment is NZS 3109. This Standard is quite specific in what are the permissible temperatures for concrete placement. This document is tied to the Building Code through NZS 3604, the key document for the house building industry in New Zealand. You are bound to satisfy the clauses in these documents, failure to do so could have significant consequences for you if things do not go to plan.

The clauses that refer to this are found in Section 7 of NZS 3109 (Concrete placing, curing and finishing.)

Specifically, Clause 7.2 .1 (Unfavourable Conditions) says that “concrete shall not be placed on frozen ground, nor shall concrete be placed in unfavourable conditions as defined in 7.2.2 which may be detrimental to the quality and finish of the concrete in the structure unless adequate precautions have been taken.” NZS 3109 may be purchased from Standards New Zealand (visit www.standards.co.nz).

Unfavourable conditions?

The unfavourable conditions include temperatures below 5 degrees on a falling thermometer, or 2 degrees on a rising thermometer, or where it becomes impractical to work and finish the concrete adequately.

The precautions listed in the Standard include the use of air entrainers, using low slump concrete, using admixtures (accelerators and water reducers,) increasing cement contents, using some form of frost protection, and avoiding frozen ground.

Your responsibilities

You are all contractually responsible ultimately to the homeowner who is paying the bill. The lines of responsibility may vary though. For instance, if you are the placer, you are responsible to the builder who is in turn responsible to the owner. There is no shirking this line of accountability for the parties involved.

If you knowingly place concrete in the temperature conditions explained above, it is implicit that you have considered the risks and taken precautions to prevent damage to the slab. In the event of a problem later on, you may be held to account for your actions and asked to justify them in a court. There, the test of “reasonableness” will be applied (with the benefit of hindsight too).

It is risky to presume that a defence based upon the use of, say, air entrainment alone, will impress a judge when he is faced with an extensively spalled surface of a path, driveway or slab. Expert evidence will sink your argument very quickly.

If you are asked to place concrete in very cold weather, it would be prudent to get the instruction confirmed in writing. If you are the builder taking a risk, be prepared to face the financial consequences of deciding not to wait.

PRACTICAL SITE MEASURES

Planning

1. Be organized.  Get the gear in place together with the labour, in time.
2. Check the weather. A sunny afternoon will mean low overnight temperatures with the risk of a frost. If snow is forecast, get out your skis and put away the wheelbarrow!
3. Consider using low slump (80mm) concrete. This concrete has a lower water content, it will bleed less and have a shorter setting time.
4. Order concrete with accelerator in it.
5. Consider using a higher grade of concrete. The extra cement will cause the concrete to set up faster.

On the job

1. Do not place concrete on frozen ground. If snow is forecast cover the sub-base to protect it or delay the pour.
2. Check the predicted temperatures for the day, and decide whether or not to proceed if temperatures are falling below 5 degrees, or if a frost is predicted overnight. If a decision is made to proceed get the covers necessary to protect the concrete from freezing.

Placement

1. Place the concrete as early as practicable in the morning to give plenty of time to finish the surface in daylight.
2. Do not wet up the concrete. Ensure the slump is not above 80 mm.
3. Do not attempt to finish the concrete until all bleed water has evaporated. This can take some time on cold windless days.
4. It may be necessary to remove the bleed water on cold windless days. Whatever system you use take great care not to mix the water into the top of the concrete. This will lead to a soft dusty surface.
5. Do not spread cement onto the surface to dry up the top. This will lead to dusting and excessive crazing of the surface
6. Do not overwork slabs that have had a delayed set due to low temperature. (Be careful that delayed finishing does not destroy the surface after final set has occurred).

Curing considerations

1. If icy or frosty conditions are expected immediately after finishing operations have been completed, do not use water to cure the top surface as this will freeze and disrupt the top surface before it has had a chance to develop any strength.
2. Cover the slab with straw and plastic to ensure the top of the slab remains above freezing point. Be aware that straw can discolour the top of the slab if it gets wet. If this is a concern consider using polystyrene or some other insulation mat instead.
3. After the concrete has passed through its first night, remove the insulation and spray the concrete with a curing compound. If more severe weather is expected replace the insulation for at least the duration of the cold spell for good measure (or four days as a reasonable rule of thumb).
4. Do not seal freshly placed concrete unless the sealer is specifically designed for application to new concrete at low or very low temperatures.
5. If the concrete is warm after the use of the insulation, carefully remove the insulation in a way that will not drop the temperature of the concrete more than 20 degrees over a 24hour period. This will avoid the risk of thermal shock cracking developing.

Conclusions

Winter conditions place additional risks on the concrete placing industry. Forward planning and ordering the right concrete for the day can minimize these risks; BUT be prepared to call a job off until temperatures increase to above 5 degrees. The costs associated with this time delay are insignificant when compared to remedial work or full replacement costs later on.

Allied Concrete staff are more than happy to help you with any problems or enquiries.

For more information or assistance, please don’t hesitate to call. Your call will be automatically connected to our nearest plant. (Calls from mobile phones will be directed for Auckland or Christchurch.)

DISCLAIMER: Allied Concrete has endeavoured to present the best possible information. However we accept no responsibility for the application of the principles discussed.