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.
|Cold Weather Concreting|
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 bley providing heating enclosures.
- The addition of chemical admixtures.
Conventional Chemical Admixtures in Cold Weather Concrete
Conventional 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 Concrete
The 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 Admixtures
There are two groups of antifreeze admixtures that provide the characteristics of antifreeze and the accelerated setting and hardening properties.
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)
It is possible for the incorporation of other admixtures that contains superplasticizers to be incorporated with the antifreeze admixtures. The main advantage of such combination is that in totality there will be a reduction of water.
The water reduction will reduce the freezable free water content in the mix. This freezable water content is the one that serves as the heat sink for the heat liberated by the initial hydration reactions. This will hence reduce the number of antifreeze admixtures.
Selection of Antifreeze admixtures
The 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
A laboratory test must be carried out with the operating materials and the dosage of antifreeze admixtures that are intended to be used in the field.
The incorporation of other admixtures like retarders, superplasticizers with antifreeze admixtures is not restricted in cold weather concreting. The dosage of all the admixture that are used must be established experimentally.
Application and Advantages of Antifreeze admixtures
The antifreeze admixtures are technologically simple and beneficial for cold weather concreting. The admixture helps in improving the cohesiveness, cold joint minimizations, sand streaking, and plasticity. These are estimated to provide large cost saving than other methods of steam curing or concrete enclosures.
The combination of antifreeze agents with water reducing agents or air-entraining agents will help in increasing the resistance of concrete towards the frost action and corrosion.