Freezing of newly poured concrete in cold weather conditions can be a major issue for contractors and engineers. The reason being, freezing can cause significant and lasting damage, which may not be resolved through subsequent curing, leading to an unsatisfactory final product. As a result, it is crucial to comprehend the adverse effects of concrete freezing and take necessary measures to prevent it.
What to do if Freshly Placed Concrete Freezes in Cold Weather?
When freshly poured concrete is exposed to freezing temperatures, the water within it expands nine times, leading to an increase in concrete porosity and a decrease in its strength. This increase in water volume is the primary cause of damage to the concrete. If the concrete is subjected to freezing during the few hours following its placement or before it achieves a compressive strength of approximately 3.5 MPa, it can lose up to half of its strength. However, if the concrete has reached a strength of around 3.5 MPa and there are no external sources of water available, it will not suffer permanent damage due to freezing.
To prevent damage to concrete due to freezing, it is essential to protect it soon after it has been placed, compacted, and finished until the majority of mixing water in the concrete has been consumed by the hydration process, and the risk of freezing has been reduced. This means protecting the concrete from freezing during the first 24-48 hours and maintaining the appropriate curing temperature until the concrete attains a compressive strength of 3.5 MPa. The permeability of hardened concrete increases, and its strength declines when the formation of crystal ice in concrete leads to the expansion of cement paste.
The minimum temperature required for continuous curing of concrete is around 5 degrees Celsius, but this depends on the section size and air temperature. Concrete requires little or no external water supply during cold weather curing unless it is within heated protective enclosures. If freshly placed concrete gains a minimum strength of 24.5 to 27.5 MPa, it can withstand multiple freezing and thawing cycles. Contractors can use accelerated set concrete obtained by incorporating chemical admixtures, reducing the water-cement ratio, increasing the cement content, reducing the quantity of supplementary cementitious materials, or using Type III Portland cement (high early strength cement).
Table-1: Minimum Temperatures Based on Section Size and Air Temperature Needed for Curing of Concrete
| Section size, minimum dimension, mm | ||||
| < 300 | 300 to 900 | 900 to 1800 | > 1800 | |
| Air temperature, oC | Minimum concrete temperature as placed and maintained, oC | |||
| – | 13 | 10 | 7 | 5 |
| Minimum concrete temperature as mixed for indicated air temperature*, oC | ||||
| -1 | 16 | 13 | 10 | 7 |
| -18 to -1 | 18 | 16 | 13 | 10 |
| Below -18 | 21 | 18 | 16 | 13 |
FAQs
What happens if concrete freezes during curing?
When concrete is exposed to freezing temperatures, it can suffer from both immediate and permanent damage. Even after the concrete thaws out, the damage may remain, and any attempts to cure the concrete may not lead to the desired strength. This is a significant concern as the strength of concrete is crucial to its ability to withstand the stresses it will face during its lifetime. Therefore, it is essential to take measures to protect concrete from freezing and to ensure that it is properly cured to achieve the desired strength.
How cold is too cold for concrete to set?
According to the ACI 306R-10 guidelines, the process of hydrating concrete is not significant when the temperature falls below 5°C. This means that the amount of water absorption and chemical reaction that takes place between the water and cement in the concrete mix is considered to be minimal at temperatures below 5°C. Therefore, the hydration process that leads to the hardening and strength development of concrete is likely to be very slow or may not occur at all when the temperature is below this threshold. It is important for builders and contractors to consider this information when working with concrete in cold weather conditions to ensure the best possible results for their projects.
How can you tell if concrete is frozen?
When concrete freezes at a young age, it is possible to observe imprints of ice crystals on its surface. These imprints are caused by the freezing of the moisture present in the concrete mix. When the temperature drops below freezing point, the moisture in the mix turns into ice crystals and expands, leading to the creation of small voids and cracks in the concrete surface.
As the concrete begins to harden, the imprints of the ice crystals become more pronounced and visible on its surface. These imprints can vary in size and shape, depending on the type of concrete mix used and the weather conditions at the time of pouring.
Observing these imprints can be useful for identifying potential issues with the concrete’s strength and durability. For example, large or irregularly shaped imprints could indicate that the concrete has not cured properly, which could lead to structural problems in the future.
In summary, imprints of ice crystals can be observed on the surface of concrete that has frozen at an early age. These imprints are caused by the freezing of moisture in the mix and can provide insights into the strength and durability of the concrete.
What is the best temperature to place concrete?
The optimal temperature range for pouring concrete is between 5 and 15 degrees Celsius.
What is the temperature at which damages due to concrete freezing occurs?
Freezing damage in concrete can occur when the temperature drops below 0°C.