Concrete can be successfully placed, finished and cured in winter so long as you understand what affect low temperatures have on the fresh and early age concrete.
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.
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 responsibilitiesYou 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.
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, Wellington or Christchurch.)