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Possible freezing of freshly placed concrete in cold weather conditions is a primary concern for contractors and engineers. This is because freezing can cause immediate and permanent damage, and the subsequent curing may not help gain the desired concrete strength. Therefore, it is necessary to understand the detrimental effects of concrete freezing and how to avoid it.
What to do if Freshly Placed Concrete Freezes in Cold Weather?
- When freshly poured concrete freezes, the volume of water increases nine times, increasing concrete porosity and reducing strength. The increase in water volume is the primary cause of the concrete damage.
- Concrete can lose around half of its strength if it suffers from freezing during the few hours after its placement or before concrete achieves a compressive strength of around 3.5 MPa.
- Concrete will not suffer permanent damage due to freezing provided that it has reached a strength of around 3.5 MPa and external sources of water are not available.
- Concrete can achieve a strength of approximately 3.5 MPa within 24 to 48 hours if the concrete mixture is well-proportioned and curing temperature is maintained (10 degrees Celsius).
- The formation of crystal ice in concrete leads to the expansion of cement paste. As a result, the permeability of hardened concrete increases, and its strength declines.
- Protect concrete soon after its placement, compaction, and finishing process until the hydration process consumes the majority of mixing water in concrete and reduces the risk of freezing. In other words, protect concrete from freezing during the first 24-48 hours and maintain the right curing temperature till concrete attains the compressive strength of 3.5 MPa.
- The minimum temperature needed for continuous curing of concrete is around 5 degrees celsius. However, this is influenced by section size and air temperature (see Table-1).
- If concrete reaches a strength of 3.5 MPa and is not exposed to external water sources, it can resist one freeze and thaw cycle without damage.
- Concrete needs 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, water-cement ratio reduction, increasing 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|
|Minimum concrete temperature as mixed for indicated air temperature*, oC|
|-18 to -1||18||16||13||10|
Freezing can cause both immediate and permanent damage, and the subsequent curing may not help gain the desired concrete strength.
Hydration of concrete below 5°C is negligible as per ACI 306R-10.
Imprints of ice crystals can be seen on the concrete surface if it is frozen at an early age.
The best temperature is suitable for pouring concrete ranges from 5 to 15°C
Freezing damage may occur when the concrete temperature drops below 0°C.