🕑 Reading time: 1 minuteFreezing reduces strength of concrete by 20 to 40 % when fresh concrete is subjected to freezing. Antifreeze admixtures of concrete, its properties and uses in cold weather concreting are discussed. The resistance of the fresh concrete against the freeze and thaw cycle is given by the durability factor which is also lowered by 40 to 60%. There is 70% decrease in the bond between the reinforcement and the concrete that is normally cured. Hence it is very essential during the concreting in cold weather conditions to ensure that the concrete will not undergo freezing in its plastic state. There are two methods for carrying out cold weather concreting:
- Provision of normal ambient temperatures for the concrete. This can be done through the heating of the concrete ingredients or by providing heating enclosures.
- The addition of chemical admixtures.
- Conventional Chemical Admixtures in Cold Weather Concrete
- Antifreeze Admixtures for Concrete
Conventional Chemical Admixtures in Cold Weather ConcreteConventional concrete used calcium chloride as accelerating admixtures to offset the retarding effects of slow hydration of concrete in low temperatures. This admixture is not effective below the freezing temperatures. This is found to be a drawback in the conventional form of admixtures. Hence, for arctic weather conditions, special admixtures are necessary. One such is antifreeze admixtures.
Antifreeze Admixtures for ConcreteThe antifreeze admixtures affect the physical condition of the mix water used in the concreting. These can depress the freezing point of the water to a large extent and can be used in temperatures lesser than -30 degrees Celsius. This can enable the extension of the period of the construction activity.
Chemical Composition and Action of Antifreeze AdmixturesThere are two groups of antifreeze admixtures that provide the characteristics of antifreeze and the accelerated setting and hardening properties. They are: 1. First Group: This includes chemicals, weak electrolytes, sodium nitrite, sodium chloride and non-electrolytic organic compounds which lower the freezing point of the water used in the concrete. But these group acts as weak accelerators to promote the setting and hardening. 2. Second Group These include binary as well as ternary admixtures which contains potash and additives based on calcium chloride, sodium nitrite, calcium chloride with sodium nitrite, calcium nitrite -nitrate-urea and other chemicals. These have effective antifreeze properties and accelerating property to promote the setting and hardening. These are used in larger dosages compared to that of conventional admixtures. One such example is the use of 8% of sodium nitrite to keep the liquid at a temperature of -15-degree Celsius. These admixtures function by lowering the liquid phase freezing point and by accelerating the cement hydration at the freezing temperatures. Based on the dosage in the mixture, the non-chloride admixture enables the mix (concrete or the mortar) to be placed at sub-freezing temperatures. This hence reduces the need of protective measures required during the cold weathering works. The method improves the quality of the concrete and as it facilitates early setting, early stripping of formworks can also be carried out. This helps in the reuse of the form within a small duration and hence speed up the construction. The table-1 shows the significant difference is strength gain at 3, 7 and 28 days for plain concrete and antifreeze admixture used concrete.
Table.1: Concrete Compressive strength with and without antifreeze admixture(As per Ratinov and Rosenburg)
|Property||Plain Concrete||Freeze-protection Admixture|
|Set time (-4 degree Celsius)||-||-|
|Compressive strength (MPa)|
|-4 degree Celsius (3 days)||3.4||9.24|
|-10 degree Celsius (7 days)||8.3||39.3|
|-10 degree Celsius (28 days)||18.1||49.9|
Selection of Antifreeze admixturesThe factors based on which the selection of antifreeze admixtures is carried out are:
- The type of structure
- The operating Conditions
- Protecting methods employed in winter concreting
- Cement brand and aggregate types