Have you ever wondered how concreting activities are carried out in the cold? Is there a difference between concreting in regular and cold weather?
Definition of Cold Weather Concreting
Cold weather concreting is defined as any concreting activity performed at or below 5°C ambient temperature.
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Effect of cold weather on concrete
The hydration of cement gives concrete its strength. Cement hydration is a temperature-dependent process. The rate of hydration is faster at hot temperatures and slower at cold temperatures.
It has been shown that cement hydration happens even at temperatures as low as -10 degrees Celsius.
What happens when fresh concrete's water freezes?
The concrete acts similarly to water-logged soil that is prone to heaving. The freezing of the mixing water increases the overall volume of the concrete.
Because cement cannot hydrate in the absence of water, the setting and hardening of fresh concrete will be delayed because there is no water available for chemical reactions.
Cement hydration is what gives concrete its strength. The hydration of cement is a temperature-dependent process. The rate of hydration is faster in hot weather and slower in cold weather.
It has been demonstrated that cement hydration occurs at temperatures as low as -10 degrees Celsius.
What happens when the water in fresh concrete freezes?
The concrete behaves similarly to heaving soggy earth. The freezing of the mixing water increases the volume of the concrete overall.
Fresh concrete will take longer to set and harden since there is no water available for chemical reactions because cement cannot hydrate in the absence of water. If the concrete has grown enough strength, it will withstand freezing temperatures without damage. It would not freeze because vast amounts of water would have mixed with cement or be situated in microscopic pores.
Concreting in Cold Weather
For the cold weather conditions in India, good concrete can be generated primarily by conserving the heat of cement hydration with insulations of the concrete surface and formworks.
Air entrainment of concrete is required in settings where alternate freezing and thawing during service are expected. At 28 days, the compressive strength of such air-entrained concrete shall not be less than 90% of that of a reference mix without air entrainment.
Because of its strong heat conductivity, normal-weight concrete should not be used in cold weather. It is preferable to use lightweight concrete with low thermal conductivity. The hydration of cement in lightweight aggregate concrete effectively prevents water from freezing.
Type of Cement and Admixtures
Using cement that hydrates faster and at a higher temperature is preferable.
In cold weather, rapid hardening cement or cement with a high rate of heat development, i.e., having high C3S and C3A contents and rich mixes with a low water-cement ratio, is favorable.
Accelerating admixtures can be used to speed up the setting time of concrete. Chlorides corrode steel and should be avoided if steel is included in concrete. It should also be avoided when requiring sulfate-resistant concrete.
Heating of Mixing Ingredients
In chilly weather, the mixing materials can be heated to achieve the desired temperature.
Water is easily heated, but it should not be heated above 60 to 80 degrees Celsius. The hot water allows the concrete to set quickly. We should avoid putting the cement in contact with hot water. As a result, the order in which the mixed elements are fed into the mixer should be determined.
If boiling the water alone is insufficient to raise the temperature of the concrete, the aggregates should be heated as well. Heating aggregate above 52 degrees Celsius is not advised.
The temperature of the mixed materials must be kept under control. The temperature of the final concrete should be estimated in advance using the IS CODE formula.
The objective of controlling the temperature of the resulting concrete is to ensure that
- The temperature of concrete is high enough to prevent freezing but not so high that the setting occurs.
- High temperatures reduce strength and workability and may cause thermal contraction.
Placement of Concrete in Cold Weather
The concrete should not be placed on ice. If possible, the formwork should be heated as well.
The concrete must be kept from freezing after placement for the Pre Hardening Period.
The pre-hardening period is the time during which the temperature of freshly laid concrete must be kept above freezing.
Insulation for concrete poured in cold weather comes in a variety of forms. The insulation should be removed in such a way that there is no rapid temperature change at the surface.
An Alternative Solution
An alternate solution is to decrease the freezing point of the mixing water well below 0-degree Celsius by using anti-freezing admixtures such as Potassium carbonate (Potash), calcium nitrate and sodium nitrite, etc.