At the International Conference on Concrete Durability held in 1991, Professor Metha pointed out that the damage to concrete structures caused by freezing damage is second only to the damage to concrete structures caused by corrosion of steel bars, ranking highest in the damage to concrete structures. The second reason is that frost damage has seriously affected the durability of concrete structures.
There are two types of frost damage to concrete. One is that the concrete that has been fully hardened undergoes freezing damage when it undergoes alternating changes in positive and negative temperatures. The other is that the concrete is frozen in the early stage. Among them, the early freezing damage of concrete is mainly due to the freezing volume expansion of free water in the concrete that has not yet participated in the hydration reaction before the fresh concrete reaches the critical compressive strength. At this time, the cement has not yet been fully hydrated, and the gelation that plays a buffering and regulating role is The holes have not yet fully formed, causing frost heave damage to the concrete. In addition, the setting and hardening process of concrete is also interrupted by the freezing of the mixing water, which hinders the formation of the cement's microstructure, causing serious damage to the internal structure of the concrete and causing irrecoverable strength loss.
At present, in terms of improving the frost damage resistance of fresh concrete, in addition to focusing on measures such as insulation and maintenance of re-poured concrete, we mainly rely on adding concrete antifreeze to enhance the resistance of fresh concrete to frost damage. Frost resistance of concrete. The common concrete antifreeze on the market now mainly consists of four parts: antifreeze component, early strength component, air entrainment component, and water reducing component. The specific mechanism of action of each component is as follows:
1 Antifreeze components and their antifreeze mechanism
1.1 Antifreeze components
The main function of the antifreeze component is to significantly reduce the freezing point of the liquid phase in the concrete, so that the unhydrated mixing water inside the concrete remains in a liquid state under negative temperature, allowing the hydration reaction of the cement to continue. , to maintain the strength growth rate of concrete. At the same time, the antifreeze component and the mixing water are mixed and co-dissolved, which significantly reduces the freezing point of free water in the concrete, avoids the formation of ice crystals in the concrete, makes it unable to generate frost heave stress, and prevents frost heave damage caused by ice crystals to the concrete structure. Currently commonly used concrete antifreeze components mainly include alcohols and some inorganic salts. Through research by Wang Xidong, Wang Shumin and others, under low temperature conditions, although the hydration rate of cement is significantly lower than that at normal temperature, the addition of antifreeze components allows the concrete to always have a certain amount of liquid water participating in the cement during the continuous cooling process. of hydration.
1.2 Early strength component
The early strength component in concrete antifreeze can accelerate the development of cement The hydration speed accelerates the setting and hardening of concrete, reduces the amount of free water inside the concrete, and reduces the chance of freezing damage caused by free water. At the same time, the addition of early strength components can make the concrete reach the critical frost resistance strength as soon as possible. The critical frost resistance strength of concrete means that when the concrete reaches a certain strength value (critical strength), it can have a certain resistance strength, and low temperature will no longer cause damage to the concrete. According to the research of Li Wenbin and others, adding antifreeze with an early strength component can ensure that concrete reaches the critical freezing strength as soon as possible under low temperature, thereby enhancing the frost resistance of concrete.
1.3 Air-entraining component
The interconnected micro-pores formed by the air-entraining agent in concrete Acts as a "buffer" to absorb the expansion stress of ice crystals and reduce pressure. During the freezing process of concrete, these pores can prevent or inhibit the formation of tiny ice bodies in the cement slurry. The key factor that affects the frost resistance of concrete is the degree to which the air-entraining agent introduces bubbles to be evenly distributed in the cement stone, that is, the spacing and number of bubbles, rather than the total air-entraining amount.
1.4 Water-reducing component
The water-cement ratio directly affects the porosity and pore structure of concrete. When the water-cement ratio is small, the density of concrete after hardening is high, the freezable free water existing inside the concrete is reduced, and the frost resistance is improved. As the water-cement ratio increases, not only does the total volume of saturated water pores increase, but the average pore diameter also increases. The frost heave pressure and seepage pressure generated during the freezing and thawing process are greater, and the frost resistance of concrete will be reduced. The water-reducing component in antifreeze can reduce the amount of mixing water, thereby reducing the total amount of free water, fundamentally reducing the content of freezable ice and eliminating the internal causes of frost heaving. Through the dispersion of water-reducing components, the wrapped water is released, inferior blisters are eliminated, and coarse ice crystals are converted into fine ice crystals, optimizing the cement hydration environment and reducing frost heave pressure.
2 Precautions for compounding antifreeze
At present, most concrete production companies directly use compounding Composite antifreeze that contains antifreeze, early strength, air entrainment, water reduction and other components can not only reduce the labor input of concrete production enterprises, but also facilitate the quality control of the antifreeze itself and concrete production. Although the compounding of antifreeze involves mixing several components, attention should still be paid to the following aspects:
1) During the compounding process of antifreeze, Pay attention to the amount of moisture introduced. If the amount of water in the antifreeze is high and the effective components are low, it will easily cause the antifreeze itself to freeze, so high-concentration products should be preferred.
2) Pay attention to controlling anti-freeze,The dosage of some inorganic salts in the early strength component. Because many inorganic salts are both antifreeze and setting accelerators, excessive dosage will not only affect the water reduction rate of the composite antifreeze and the slump retention of concrete, but also have a greater impact on the durability of the concrete structure.
3) Pay attention to determine the appropriate antifreeze and early strength components of inorganic salts to avoid crystallization and precipitation at low temperatures, clogging pipelines and affecting concrete production.
4) When using organic matter as an antifreeze component, the dosage should not be too large, otherwise it will affect the later strength of the concrete.
5) Control the chloride ion content and alkali content in the antifreeze to prevent any impact on the durability of concrete.
In short, the compounding of antifreeze is not simply mixing several different components together. It should be verified through experiments whether the effect of the compounding of various components is superimposed or not. To offset each other, the impact of each component on the performance of concrete should be fully considered based on the concrete raw materials, mix ratio, technical requirements, etc.
3 Effect of antifreeze on concrete properties
3.1 Workability of fresh concrete
Most antifreezes, especially early-strength antifreezes, will accelerate the hydration reaction rate of cement clinker minerals and make the liquid phase thicker. Therefore, the addition of antifreeze can improve negative temperature. Bleeding phenomenon of concrete under certain conditions. Early-strength antifreeze will shorten the setting time of concrete and is conducive to condensation and hardening under negative temperature conditions. However, it should be used with caution in commercial concrete transported over long distances.
3.2 intensity
According to research by Yang Yingzi and others, in a negative temperature environment, freezing is harmful to The damage caused to the internal structure of mortar and cement stone is small, but it will affect the early strength and later strength of concrete. This is mainly because when fresh concrete is formed, part of the water in the vibrated concrete is concentrated at the bottom of the coarse aggregate. Formation of water bladder. When fresh concrete is frozen, the phase change of water causes expansion, resulting in the "debonding" of the coarse aggregate and cement mortar. Even after returning to normal temperature, it cannot heal well and shows a large loss of strength. When antifreeze is added, due to water reduction (reducing the water-cement ratio, that is, reducing the thickness of the water film layer), air entrainment (relieving frost heave stress), early strength (promoting the formation of early structure), antifreeze (reducing the liquid The comprehensive effects of such factors as phase freezing point) greatly reduce the damage to the internal structure of concrete under negative temperature. Therefore, the strength of concrete mixed with antifreeze is significantly higher than that of benchmark concrete under negative temperature and negative temperature to normal temperature conditions. It can be seen that the important role of antifreeze is to improve the interface transition zone of negative temperature concrete.
4 Precautions for the use of antifreeze
The antifreeze used in concrete should be based on the specific temperature Adjust the amount of antifreeze in a timely manner according to fluctuating conditions to ensure the antifreeze and early strength effects of the antifreeze. The amount of antifreeze should be controlled within a certain range, because part of the antifreeze and early strength components in the antifreeze directly participate in the hydration process of cement. If the amount of antifreeze is too much, it will affect the durability of the concrete structure. Therefore, if the temperature is too low and exceeds the application range of antifreeze, attention should still be paid to insulation and maintenance measures for newly poured concrete to avoid freezing damage to the concrete.
5 Summary
Each component in the antifreeze will change the frost damage resistance of concrete from different angles. The rational use of antifreeze can effectively reduce the occurrence of early concrete diseases, significantly improve the construction speed of winter construction and the early strength of concrete, shorten the construction period, and improve the quality of the project.
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