In recent years, with the continuous development of highways in our country, the construction period has been continuously shortened, and it is very common for concrete structures to be constructed in winter. When the concrete has not reached its due strength, there is still a certain amount of water inside the concrete, and this water contains free water. After the ambient temperature reaches the freezing point of water, it promotes the transformation of the water phase into ice, resulting in the redistribution of water in the concrete and ice interlayer. The formation of this leads to a decrease in the cohesion of the interface between the concrete aggregate and the cement stone, causing volume expansion and leading to early freezing of the concrete. Antifreeze, as an important component of concrete to prevent early freezing, has been widely used in concrete construction in winter.
1 Mechanism and principle of antifreeze
1.1 Mechanism of antifreeze
(1) According to Ural’s Law. From the research, it can be found that the degree of reduction of the liquid phase freezing point of the concrete mixture material is directly related to the molar concentration of the antifreeze. By reducing the freezing point, the cement in the concrete structure of the material is directly converted at negative temperatures. Becomes liquid water, which is the mechanism of antifreeze application in winter environment construction.
(2) Ice crystal distortion theory. Pure water will freeze at 0°C, because hydrogen bonding causes the internal water molecules to aggregate into molecular aggregates, and the frost heaving ability will be relatively strong. After adding antifreeze to the aqueous solution, if the temperature is lower than 0°C, ice will precipitate. The presence of antifreeze will cause greater interference with hydrogen bonds. The precipitated ice crystals will be small and flocculent. At this time, Liquid phase water can be formed at the same time. From a macro perspective, the edges and corners are relaxed, and the frost heave stress will also be significantly reduced.
(3) Liquid-ash ratio balance theory. After adding antifreeze to concrete materials, when the temperature continues to drop to the liquid freezing point, ice crystals will precipitate. Under negative temperatures, the concentration of the solution increases due to the occurrence of ice precipitation. It will reach an equilibrium state under the new temperature. This is the coexistence state of ice and liquid. The content of the concrete structure still exists in the form of liquid water. It is necessary to In order for concrete to continue to liquefy, the content of the antifreeze will not change, and the liquid solution at the specified temperature will remain unchanged. After the cement hydration reaction, the water consumed will be supplied by ice. That is to achieve a liquid-ash ratio balance.
(4) Maturity theory. During the hydration process of cement materials, relatively complex chemical and physical changes will occur internally, which are easily affected by temperature. For concrete materials with specified mix ratio parameters, the structural strength has a functional relationship with curing temperature, time, etc. If the temperature during curing is relatively low, the hydration reaction rate will decrease, and the structural strength will seriously decrease.
1.2 Antifreeze Principle of Antifreeze
The action mechanism of antifreeze is directly affected by its internal components. Impact, the following are currently commonly used:
① Water-reducing ingredients. It can appropriately reduce the amount of water added during the mixing process, which can greatly reduce the total amount of moving ice and avoid the occurrence of frost heaving. At the same time, after the water-reducing component is dispersed, a large amount of water can be precipitated, eliminating inferior blisters, and can directly transform large particles into a crystal structure of small particles, which can effectively reduce frost heave pressure;
② Air-entraining ingredients. Integrating micron-sized small bubbles directly into concrete materials can, on the one hand, effectively remove and cut harmful pores and avoid serious crack spread during the frost heave process; on the other hand, an expansion reaction occurs by introducing a large number of bubbles, and the bubbles can absorb the expansion Stress can also reduce the occurrence of frost damage. When gas is introduced into concrete, its durability, frost heaving resistance and ability can be improved;
③Early strength component. In the construction process, subsequent construction can only be carried out after the concrete strength reaches the specified strength. During the final setting process of concrete, it is necessary to avoid structural damage caused by excessive external loads. The key function of antifreeze is to accelerate the setting of concrete, so that it can reach the critical anti-freeze strength in the shortest time, greatly increasing the hardening speed and avoiding structural damage;
④Antifreeze ingredients. Add a certain amount of inorganic salt as needed. The working principle is as follows: the freezing point of the antifreeze component (taking 2% NaNO2 as an example) solution can drop to -1.5°C. After the temperature reaches this temperature, the freezing side free water exists inside the pores. A freezing reaction will occur, part of the NaNO2 in the ice will be broken out, and the concentration of NaNO2 in the remaining unfrozen water will increase; after the temperature drops, other free water will continue to freeze, and the remaining free water will continue to freeze. The concentration will also increase. During the entire reaction process, until the lowest co-dissolution point of NaNO2 is reached, all free water can form ice.
The antifreeze mechanism and principle of antifreeze are formed by a variety of effects, and antifreeze does not produce better effects after being added. It must be Taking into account the specific construction conditions, it is necessary to select an antifreeze agent that meets the needs of the project to improve the frost resistance of concrete.
2 How to use antifreeze correctly
2.1 How to use antifreezeUsage temperature
Any antifreeze has its range of use, and you need to choose the appropriate type according to your needs. However, this temperature is not an accurate data and must be determined based on factors such as engineering conditions and construction environment. The antifreeze application temperature and anti-freeze critical strength should be used together. Before reaching the antifreeze application temperature, it should be ensured that the concrete strength can be greater than the anti-freeze critical temperature, so as to improve the use effect and avoid structural damage. The lower the temperature at which concrete can be used, the better the antifreeze effect. The concrete can reach the strength requirements in a longer period of time, which can greatly improve the critical anti-freeze strength performance.
The current use temperature of concrete antifreeze in my country is generally between -10℃~-15℃. If you want to lower the use temperature again, the antifreeze Formula design will be more difficult, and there will be more uncertain influencing factors. Therefore, it is not necessary that the application temperature of the antifreeze must be lower than the lowest temperature of the construction environment. Ultimately, what is important is that the critical strength performance can be achieved when the temperature is higher than the application temperature of the antifreeze.
2.2 Take covering and insulation measures
After the concrete pouring is completed, certain covering and insulation measures must be supplemented , slow down the loss of heat generated during the hydration process, ensure the internal temperature of the concrete, and achieve normal hydration of the concrete. At the same time, the thermal insulation of concrete also delays the time when concrete is exposed to negative temperatures to a certain extent, ensuring the normal development of the strength of concrete to achieve the necessary critical frost resistance strength when exposed to negative temperatures. Commonly used insulation methods include: using insulation boards such as benzene boards or extruded boards to insulate structural formwork, and covering the top surface of concrete pouring with quilts and geotextiles.
2.3 Good construction organization
Only by fully doing the work of mixing, vibrating and other aspects , can reflect the effect of antifreeze. Extending the mixing time by 30 minutes can make the materials more evenly mixed and fully mix the antifreeze to improve material performance and eliminate accident problems. Furthermore, the application of antifreeze requires optimal stirring time and vibrating time. Excessive stirring or vibrating will lead to a decrease in the amount, and insufficient stirring or vibrating will result in a large number of bubbles inside or uneven materials. The decline in material performance affects the anti-freeze effect. In addition, the mixing station must be equipped with insulation devices as needed and strictly limit the transportation distance to ensure that all links can proceed smoothly and the connection between each process will be smoother, which can effectively improve the concrete molding construction. Temperature can effectively extend the curing time, so that the concrete material can reach the critical temperature parameters of frost resistance as soon as possible.
3 Conclusion
Concrete antifreeze is a very important component that has a significant impact on structural performance. direct impact, so in-depth study of its application conditions is required. In recent years, Western countries have begun to increase their efforts in the development and application of concrete antifreeze. Research on thiocyanate has been relatively in-depth, and the American Society for Testing and Materials (ASTM) has issued some standards. When evaluating and selecting antifreeze, multiple factors need to be considered, not just lowering the freezing point. Therefore, concrete antifreeze has been widely used, and its theoretical and practical systems have become more complete.
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