DON’T GET CAUGHT COLD

We are fortunate in New Zealand to live in a moderate temperature environment, but in many places morning frosts during winter are common

In the winter months, there is a range of issues to be considered when concreting. NZS 3109 Concrete Construction provides a definition of unfavourable cold conditions when concrete should not be placed.

These are:

• When the ground is frozen.
• At temperatures below 5°C with temperature descending.
• At temperatures below 2°C with temperature ascending.

So, what is the reasoning behind the above temperature limitations? Let’s first consider what happens to concrete at 0°C.

If concrete is frozen before it sets, the hydration of the cement is suspended, partly because the water within the mix freezes and expands, preventing the reaction from continuing. If the temperature rises, and the concrete is vibrated to remove the pores created by the ice, the cement hydration should continue, producing satisfactory concrete. This only remains true however, if the concrete is re-vibrated after the ice has thawed.

If the concrete has set but not reached sufficient strength, freezing can result in internal cracking and loss of strength. If the freezing only occurs on the surface, it may result in delamination of the top surface.

After concrete has attained a strength of approximately 3.5 MPa, it is usually considered to have sufficient strength to resist a freeze thaw cycle.

The requirements of NZS 3109 give some protection against potential freezing of the concrete before it has attained sufficient strength. As a specifier or placer of concrete, what can you do to ensure a fit-for-purpose project in the cold winter months?

The best solution is to understand the fundamental principles of concreting in the cold so that the most appropriate solution can be applied to your project. The following are some general comments that are worth considering.

Never place concrete on frozen ground

In New Zealand, it should be possible to wait until the ground temperature rises before placing concrete. Alternatively, the ground can be protected overnight with straw or a similar material to prevent freezing.

Use air-entrained concrete

The advantage of air entrainment is that it gives the concrete superior freeze/thaw resistance when it has reached sufficient strength. The concrete will still however, need to be protected from freezing until it reaches at least 3.5 MPa.

Consider the use of set accelerators

These will decrease the time to final set, meaning the concrete can be finished and potentially protected earlier. When the concrete is reinforced, it is recommended that non-chloride accelerators are used. Be aware that accelerators have limited effectiveness when mix temperatures are below 5-8°C. Also, be aware that overdosing with some set accelerators can in fact retard set, so ensure that the dose rate is that recommended by the admixture manufacturer.

There is a tendency in some areas to add set accelerators simply because it is winter, rather than based on the expected temperatures during placing and finishing. It is possible to get very warm, low humidity days in winter. The indiscriminate use of accelerators can lead to problems with premature setting or plastic cracking.

High Early Strength (HE) cement could possibly be a consideration, although it is generally not stocked at ready mixed concrete plants.

Protect the freshly placed concrete

In winter, there can be large changes in the ambient day and night temperatures. This can lead to restrained early thermal contraction. The use of early entry saw cuts, or tooled joints, can prevent the formation of ugly random cracking. Some winter cracking suggests that the top surface has chilled relative to the body of the concrete, resulting in surface random cracking. Although the insulation value of polythene is negligible, covering with polythene can potentially minimise the wind chill effect and prevent this type of cracking. If freezing conditions are expected and the concrete is unlikely to have attained a strength of 3.5 MPa, insulate the concrete.

Prevent thermal shock

If the temperature is cold and the concrete warm, there is a risk that the removal of formwork can result in surface cracking. (A temperature differential of 20°C or more is commonly quoted as the range across which this phenomenon can occur). Always follow the minimum formwork stripping times specified in NZS 3109.

Use warm materials to make the concrete

The use of hot water and aggregates that have been stored in bins can mean that the concrete mix temperature is elevated, resulting in a faster setting time. Talk to your local ready mixed concrete supplier for options that are practical in your area. Never use water above 70°C, and be aware of the safety requirements of using hot water on a construction site.

Do not use unventilated heaters

The CO2 given off by some heaters can react with the concrete surface, producing a dusty, weak surface. Always make sure the exhaust gases are ventilated, and do not aim the burner directly at the concrete.

With some simple precautions, it is relatively easy to obtain high-quality concrete all year round.

Concrete NZ is aligned with the New Zealand Concrete Contractors Association (NZCCA). To be a member of the NZCCA requires a high standard of knowledge and practical experience. To find out more visit www.nzconcretecontractors.org.nz