Placing concrete at the correct temperature is critical for its durability and ultimate strength. This is because temperatures above the normal concrete curing range (32 °C) will not only reduce the workability of concrete but also cause a significant reduction in its ultimate strength.
Similarly, low ambient temperatures, below 5 °C, slow the hydration process considerably, i.e., the curing process. This may lead to a reduction in strength development and an extension of the construction schedule.
Moreover, low temperatures can lead to irreversible strength loss. Therefore, pouring concrete at the wrong temperature causes durability and strength issues which may require costly repair and rehabilitation.
What are the Risks of Pouring Concrete at a Wrong Temperature?
1. Slump Loss
If concrete is poured at a high temperature, the mixture loses moisture quickly, losing consistency, decreasing workability, leading to improper surface finish and lower ultimate strength.
For instance, if the temperature of the concrete mixture is raised 10 °C above the upper limit range, which is around 32 °C, the concrete can lose around 10% of its 28-day strength.
Moreover, in integrally colored concrete, variations in water content due to high temperature cause noticeable differences in concrete color between adjacent pours.
Surface drying is another potential problem due to hot weather, leading to concrete shrinkage and cracking. Site engineers can use a nomograph from ACI 395 to determine whether the evaporation rate is detrimental.
Air temperature, relative humidity, concrete temperature, and wind velocity should be known to read the evaporation rate on the nomograph as shown in Figure-1. If the evaporation rate exceeds 0.49 Kg/m2/hour, shrinkage cracking is likely to occur.
Therefore, high temperatures should be avoided while concrete is placed. Alternatively, necessary precautions should be taken to prevent the detrimental effects of high temperatures on concrete.
Crack Development
The high-temperature condition can put concrete at the risk of chipping, flaking, and cracking under pressure.
Slow Hydration Process
Low temperature slows down the hydration process considerably. Slow concrete curing may shift over time, resulting in serious structural damage to buildings and materials which are being supported.
Concrete strength development is further delayed if the concrete mixture contains ground granulated blast furnace slag or pulverized fuel ash.
Loss of Strength due to Freezing
Cold ambient temperatures can freeze moisture inside the concrete, leading to concrete damage due to the increased size of water. The concrete damage due to freezing can be immediately observed if it occurs on the concrete surface.
However, it is hard to notice when it is inside, and such concrete is highly likely to crumble when heavy loads are placed on the structure. The loss of strength is greater if freezing occurs during the first 24 hours after concrete placement, around half of the expected 28-day strength will be lost. Ultimately, freezing temperatures cause a loss of strength and durability.
FAQs
1. Losing Water Inside Concrete Mixture
2. Crack Development
3. Slow Hydration Process
4. Loss of Strength due to Freezing
The high-temperature condition can put concrete at the risk of chipping, flaking, and cracking under pressure.
Place concrete at a temperature not less than 10 °C; otherwise, curing processes will slow down, and freezing is highly likely. So, try to pour concrete during the night or early hours of the morning when the temperature falls.
Concrete curing requires both adequate moisture and correct temperature. The concrete temperature during curing should be above 5 °C; otherwise, the curing process will decrease substantially.
The duration of maintaining concrete at an appropriate temperature depends on the concrete type. For example, one day is enough for high early strength development concrete not exposed to freezing and thawing circles. However, concrete elements that carry sizable loads need around 20 days.
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Methods of concrete pouring as per the recommendation of ACI 304R-00