Pouring concrete in cold weather can slow setting and delay early strength gain. The concrete may still be suitable, but cold conditions affect how quickly it hardens, when shutters can be removed, and when the structure can be loaded or opened to traffic. In winter, the real risk is assuming normal timing still applies.
What this guide covers
- Why cold weather slows concrete setting and early strength
- What happens under standard cure and cold cure conditions
- Why temperature for concrete curing matters in winter
- Which site factors matter before placing concrete in cold weather
- How shutters, shade, wind and concrete thickness affect temperature
- What fly ash comparison in the Tip actually shows
- What site teams should check before a winter concrete pour
Why cold weather matters for concrete strength
Cold weather matters because concrete gains strength more slowly when the concrete temperature and surrounding conditions are low.
Concrete hardens through cement hydration. When temperature drops, hydration slows down. This does not automatically mean the concrete is defective, but it does mean the site team should expect slower setting, slower early strength gain and more caution before stripping, loading or continuing with follow-on work.
The practical warning point in Tip 3 is around 5°C. Under very cold curing conditions, early strength may be too low to measure at 24 hours. That affects site decisions because the concrete may look placed and finished, but still not have developed the early strength the programme expects.
Cold weather can affect:
- setting time
- early strength
- shutter stripping
- loading and trafficking
- cube results
- curing expectations
- strength gain in extended mixes
- follow-on trades and programme timing
The key issue is not only the air temperature when the truck arrives. The concrete must be protected during the hours and days after placing, especially overnight.
Standard cure and cold cure are not the same result
Standard cure and cold cure can produce very different strength results from the same concrete mix.
Standard cure refers to cubes stripped and cured for 28 days between 22°C and 25°C, as per SABS. Cold cure refers to cubes cured for 1 day at 5°C and then 27 days at 10°C at 90% relative humidity.
That comparison matters because it shows what cold conditions do to strength gain.
| Curing condition | Source condition | Practical site meaning |
|---|---|---|
| Standard cure | 28 days between 22°C and 25°C | Gives normal reference strength development |
| Cold cure | 1 day at 5°C, then 27 days at 10°C and 90% RH | Slows early strength and reduces 28-day result |
| 24-hour cold cure | Initial strength could not be measured | Stripping or early load may be unsafe too soon |
| Cold shaded exposure | Can be much colder than sunny areas | Site temperature may vary across the same project |
This is why temperature for concrete curing matters. In cold weather, the problem is not only whether concrete can be placed. The problem is whether it can gain strength quickly enough for the next site decision.
Fly Ash Concrete vs Standard Concrete in Cold Weather
Tip 3 compares a standard cement mix with a fly ash mix under cold-weather conditions.
The comparison is useful because it shows that the fly ash mix did not perform dramatically worse than the cement-only mix. Both mixes had very low early strength under cold conditions, and both reached similar 28-day results.
| Result age | Cement mix | Fly ash mix | What it shows |
|---|---|---|---|
| 1 day | Could not measure | Could not measure | Both mixes had insufficient 24-hour strength |
| 2 days | 5.5 MPa | 4.5 MPa | The cement mix was slightly ahead at early age |
| 3 days | 9.0 MPa | 8.0 MPa | Both mixes were gaining strength slowly |
| 7 days | 15.0 MPa | 14.0 MPa | The fly ash mix remained close to the cement mix |
| 28 days | 27.0 MPa | 26.5 MPa | Final results were almost the same |
In this cold-weather comparison, the performance difference between the cement mix and the Fly Ash mix was less than 1 MPa.
That matters because fly ash is often blamed too quickly when early strength is slow. The bigger issue is not the fly ash itself. The bigger issue is cold-weather strength development. Both mixes were affected by low temperature, and both needed more time before being judged.
This does not mean fly ash has no effect on concrete behaviour. It means the site team should not assume fly ash is automatically the cause of poor winter performance. The better question is whether the concrete has had enough time and suitable curing conditions to develop strength.
What should be checked before placing concrete in cold weather?
Before placing concrete in cold weather, check the expected temperature, exposure, formwork, mix type and follow-on programme.
The site team should not only ask, “Can we pour today?” The better question is, “Can this concrete be protected long enough to reach the strength we need before the next site activity?”
| Site check | Why it matters |
|---|---|
| Placing temperature of concrete | Concrete that starts colder may gain strength more slowly |
| Ambient temperature | Low surrounding temperature slows hydration and setting |
| Wind and exposure | Exposed concrete can cool faster |
| Sun or shade | Shaded areas may be much colder than sunny areas |
| Steel or timber formwork | Cold shutters can reduce concrete temperature |
| Section thickness or volume | Narrow sections can lose heat quickly |
| Workability of the mix | High-slump mixes may be more prone to cold curing effects |
| Binder content and W/C ratio | Mix design influences heat and strength development |
| Site check | Why it matters |
|---|---|
| Temperature trend | Falling afternoon or night temperatures increase risk |
| Stripping and loading programme | Cold concrete may need longer before it can carry load |
Expert site tip
Do not plan a winter pour around the warmest part of the day only. Check the coldest part of the concrete’s curing period. A slab, column or beam that cools overnight may need longer protection before it is stripped, loaded or opened to traffic.
How shutters and formwork affect cold-weather strength
Shutters and formwork can help or hurt, depending on their temperature and the element being cast.
Cold shutters can reduce concrete temperature significantly, especially in shutter boxes of narrow cross section, such as 200 mm x 200 mm. This matters because narrow elements do not hold heat in the same way as larger mass pours.
A cold steel or timber form can draw heat from the fresh concrete. If the element is also exposed to wind, shade or falling night temperatures, early strength gain can slow further.
This has a direct programme effect. Shutters may need to remain in place longer, and early stripping should not be based on normal warm-weather assumptions.
The practical decision is simple: if the formwork and concrete are cold, allow more time before treating the element as ready.
When cold weather becomes a programme risk
Cold weather becomes a programme risk when the site assumes normal strength timing still applies.
The issue is not simply “pouring in winter”. The mistakes are made when planning stripping, loading, trafficking or follow-on work as if the concrete gained strength under warm standard-cure conditions.
| Site assumption | Why it is risky | Better decision |
|---|---|---|
| “It was placed, so it is ready” | Placing does not prove strength gain | Check curing conditions and timing |
| “The day temperature was fine” | Overnight temperature may be the real issue | Check the coldest expected period |
| “The sunny side looks fine” | Shaded areas may be 10°C to 15°C colder | Judge each exposure separately |
| “We always strip after the same time” | Cold cure delays early strength | Allow longer before stripping |
| “Fly ash is the problem” | The Tip shows both mixes were affected | Treat low temperature as the main risk |
Cold weather changes the timing of strength gain.
Practical cold-weather site precautions
The best cold-weather precaution is to plan protection before the concrete arrives.
Good site practice is not complicated. Reduce unnecessary heat loss, protect concrete from cold exposure and allow more time where early strength may be delayed.
Practical precautions include:
- Check forecast temperatures before ordering
- Pay attention to night temperatures, not only daytime temperatures
- Consider shade, wind and exposure of the element being cast
- Avoid placing where temperatures may fall too low without protection
- Protect concrete from cold wind and rapid cooling
- Keep shutters in place longer where early strength may be delayed
- Avoid early loading or trafficking until strength gain is suitable
- Do not assume normal summer stripping times apply
- Discuss cold-weather concerns with the concrete supplier before the pour
- Allow for slower strength gain in cold cure conditions
When the temperature drops, the site programme must give the concrete more time to cure.
Questions site teams ask after a cold-weather pour
Can concrete be poured in cold weather?
Yes, concrete can be poured in cold weather if conditions are suitable and the concrete can be protected while it gains early strength. The decision should be based on temperature, exposure, element size, mix type and the follow-on programme.
What temperature is too cold for concrete?
Tip 3 highlights 5°C as a practical warning point. At this temperature, strength gain becomes very slow, and in the cold-cure comparison, the 24-hour strength could not be measured because the concrete was only just setting.
Does cold weather mean concrete will be weaker?
Not automatically. Cold weather mainly slows strength gain. However, the Tip 3 cold-cure comparison showed about 35% lower strength at 28 days when compared with standard cure conditions, so cold exposure must be taken seriously.
Should shutters stay on longer in cold weather?
Yes, shutters may need to stay in place longer because cold concrete gains early strength more slowly. This is especially important for narrow elements, shaded areas or concrete exposed to cold shutters and falling night temperatures.
Does fly ash concrete perform worse than cement-only concrete in cold weather?
Not necessarily. In the Tip 3 comparison, both the CEM I 42,5 mix and the 30% fly ash mix were seriously affected by low temperature. The main lesson is to manage cold curing and early strength expectations, not to assume fly ash is always the cause.
One thing to remember
Cold weather does not automatically mean bad concrete. It means slower strength gain. When pouring concrete in cold weather, plan around the coldest expected conditions, protect the concrete after placing and allow enough time before stripping, loading or trafficking.
Looking for more practical concrete guidance? Browse Quantum’s Technical Tips library.