Concreting over winter

Concrete may be one of the most reliable materials in construction, but winter conditions can quickly test even the best-laid plans.

 

While New Zealand benefits from a relatively moderate climate, cold snaps, overnight frosts, and fluctuating temperatures still pose real risks to concrete performance on site.

 

Understanding how low temperatures affect concrete and how to manage those risks is essential for maintaining quality, durability, and compliance during the colder months.

 

Standards such as NZS 3109 set out clear guidance on when conditions are unsuitable for placing concrete.

 

These include when the ground is frozen, when temperatures fall below 5°C and continue to drop, and when temperatures sit below 2°C even if they are rising.

 

These thresholds reflect the impact cold weather has on the hydration process, the chemical reaction that allows concrete to set and gain strength.

 

As temperatures decrease, hydration slows significantly, increasing the risk of poor performance if conditions are not properly managed.

 

If fresh concrete freezes before it has set, the water within the mix expands as it turns to ice, disrupting the internal structure and effectively pausing hydration.

 

While the reaction can resume if temperatures rise, the structural integrity may already be compromised.

 

The period immediately after setting is also critical. If concrete has hardened but has not yet developed sufficient strength, freezing can lead to internal cracking and long-term durability issues.

 

Surface freezing can also cause delamination, weakening the top layer. In general, concrete needs to reach a strength of around 3.5 MPa before it can safely withstand a freeze-thaw cycle.

 

Despite these risks, winter concreting can be carried out successfully with the right approach. One of the most important considerations is ensuring that concrete is never placed on frozen ground.

 

In most cases, it is practical to delay a pour until ground temperatures rise or to protect the surface overnight using insulating materials such as straw or thermal coverings to prevent freezing.

 

Material selection and mix design also play a role. Air-entrained concrete can improve resistance to freeze-thaw cycles once adequate strength has been achieved.

 

Set accelerators may be used to reduce the time to the final set, allowing earlier finishing and protection, but they must be used with care.

 

Their effectiveness is limited at very low temperatures, and overdosing can have the opposite effect, delaying the set.

 

Non-chloride accelerators are recommended for reinforced concrete to avoid corrosion risks.

 

It is also important to respond to actual site conditions rather than relying on seasonal assumptions.

 

Winter can still bring warm, dry days, which increase the risk of rapid moisture loss and plastic cracking.

 

The indiscriminate use of accelerators or other additives without considering these conditions can lead to avoidable issues.

 

Protecting early-age concrete is critical in cold weather. Significant temperature swings between day and night can create thermal stress, resulting in cracking.

 

Covering freshly placed concrete with polythene or insulating materials can help reduce heat loss and minimise the effects of wind chill.

 

Where freezing conditions are expected before sufficient strength has been reached, proper insulation is essential.

 

Beyond temperature control, curing practices become even more important in winter. Concrete that dries out too quickly can suffer from reduced strength and increased surface defects, even in cold conditions.

 

Maintaining adequate moisture through curing compounds, coverings, or light water application helps ensure the hydration process continues as intended.

 

Extending curing times during winter is often necessary, as strength gain is slower compared to warmer months.

 

Planning and communication across the project team also play a significant role in successful winter pours.

 

Monitoring weather forecasts, scheduling pours during the warmest parts of the day, and coordinating with suppliers can help avoid unnecessary risks.

 

Ready-mix providers can often adjust mix temperatures or recommend suitable admixtures based on local conditions, making early engagement a practical advantage.

 

Care must also be taken to avoid thermal shock. Removing formwork too early in cold conditions can expose warm concrete to sudden temperature drops, potentially causing surface cracking.

 

Following recommended stripping times remains important, particularly when there is a large temperature difference between the concrete and the surrounding environment.

 

Using warmer materials can also assist in cold conditions. Heated water and well-stored aggregates can help increase the initial temperature of the mix, promoting faster setting.

 

However, this must be balanced with safety considerations, including limiting water temperatures and ensuring safe handling practices on site.

 

Finally, heating methods should be carefully managed. Unventilated heaters can release carbon dioxide, which reacts with the concrete surface and can leave it weak and dusty. Any heating used should be indirect and well-ventilated to avoid these issues.

 

With the right planning and precautions, high-quality concrete can be achieved throughout winter.

 

Rather than avoiding cold-weather pours altogether, understanding the risks and adapting to conditions allows projects to continue without compromising on performance.