Winter construction is very common in northern my country. If effective measures are not taken or improper measures are taken, concrete is prone to freezing damage, causing engineering accidents. Among the winter construction methods of concrete projects, adding antifreeze is a good choice. By adding antifreeze, the winter construction process can be simplified, the project quality can be guaranteed, energy and additional facilities can be saved, and the project cost of winter construction can be reduced. Therefore, understanding the mechanism and rules of concrete freezing damage, and being familiar with the mechanism and usage of concrete antifreeze are of great significance to ensuring project quality and achieving balanced construction throughout the year.
1 Analysis of the freezing damage mechanism of concrete
The reason why the concrete mixture can gradually solidify and harden after pouring until The ultimate strength is obtained due to the hydration of the cement. In addition to being related to the concrete composition materials and mix ratio, the cement hydration rate is also closely related to the external temperature. When the temperature rises, hydration accelerates and strength growth accelerates. When the temperature drops to 0°C, part of the water present in the concrete freezes, causing the cement hydration to slow down and the strength growth to slow down accordingly. As the temperature continues to decrease, when the water in the concrete completely turns into ice, the hydration of the cement basically stops, and the strength of the concrete will no longer increase. After water turns into ice, its volume increases by about 9%, and at the same time it generates an expansion stress of about 2.5MPa. This stress is often greater than the initial strength value formed inside the concrete, causing the concrete to be damaged to varying degrees. In addition, when the water turns into ice, larger ice particles will be produced on the surface of the aggregate and steel bars, which will weaken the bonding force between the cement slurry and the aggregates and steel bars. After the ice melts, various voids will be formed inside the concrete, reducing the durability of the concrete.
During winter construction, when the concrete strength reaches a certain limit value, since the structure has been initially formed and has the ability to resist frost heave damage, the concrete will not be frozen again. Be damaged by freezing, this strength is called the critical frost resistance strength of concrete. The proposal of critical anti-freeze strength is a major breakthrough in the theory of concrete winter construction and is also an important basis for formulating concrete winter construction measures. The key to concrete winter construction is to make the concrete reach critical frost resistance strength as soon as possible. The function of antifreeze is to lower the freezing point of the mixture and refine the ice crystals. The concrete maintains a certain amount of liquid water at negative temperatures, allowing the cement to slowly hydrate and the concrete to reach critical anti-freeze strength.
2 The components and mechanism of antifreeze
Antifreeze is based on the mechanism of concrete freezing damage, combined with Theories such as critical anti-freeze strength, optimal ice formation rate, and ice crystal morphological transformation are developed by summarizing long-term winter construction practices and are generally composed of four components.
1) Antifreeze component
Antifreeze component is a kind of antifreeze component that makes the concrete mixture at negative temperatures Chemicals that are not susceptible to freezing damage in the environment. Many inorganic salts and organic substances have antifreeze properties. Most inorganic salt water solutions (such as sodium nitrite, sodium chloride) have a very low eutectic temperature with water, which can lower the freezing point of water and allow concrete to still hydrate at negative temperatures. When using antifreeze, the concentration of inorganic salts must be maintained high enough, otherwise freezing damage will still occur. At present, in addition to the application of inorganic salts as antifreeze components, organic antifreeze components (such as organic alcohols, triethanolamine, etc.) have also begun to be widely used. Organic components can not only lower the freezing point of water, but also seriously deform the crystal lattice structure of ice, preventing the formation of frost heave stress, thus destroying the hydrated mineral structure and damaging the strength of concrete.
2) Early strength component
The main function of early strength component is to accelerate the setting and hardening of concrete , so that it can reach the critical strength of frost resistance as soon as possible and enhance the ability of concrete to resist frost damage. More importantly, the acceleration of the early hydration reaction can turn most of the free water in the concrete into non-freezing combined water, thereby reducing the freezing damage of the concrete.
3) Water-reducing component
The main function of the water-reducing component is: reducing 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. The water-reducing component can release wrapped water, eliminate inferior blisters, convert coarse ice crystals into fine ice crystals, optimize the cement hydration environment, and reduce frost expansion pressure. In other words, reducing water means improving strength, which means improving the frost resistance of concrete. It also increases the density of concrete, greatly reduces the freezing damage caused by free water to concrete, refines capillary pores, and contributes to reducing liquid The phase freezing point has a promoting effect.
4) Air-entraining components
Add an appropriate amount of air-entraining components to mix the concrete A large number of uniformly distributed, stable and closed polar microbubbles are introduced during the process. This can not only cut and seal the connected pores in the concrete, reduce the crack expansion during frost heaving, but also act as an expansion "buffer" to absorb the expansion stress of ice crystals and reduce frost damage.
3 Correct use of antifreeze
There are many methods for concrete winter construction, and generally priority should be given Choose the heat storage method. If the heat storage method cannot meet the requirements after thermal calculation, you should consider adding antifreeze. Adding antifreeze is often combined with thermal storage methodsApplication, using the initial heat of concrete and the heat released by cement during the hydration process, to accelerate the increase in concrete strength. When antifreeze is added, the durability of concrete will be affected to varying degrees. The amount of admixture should be reduced as much as possible on the premise that the initial curing temperature of the concrete can reach the critical strength of freezing.
The requirements of the structure itself should also be considered, such as chlorine salts that corrode steel bars. In reinforced concrete projects and prestressed concrete projects, chloride-free antifreeze can be used. In structures used in high humidity and high temperature environments, and in structures that are in contact with corrosive media such as acids and alkalis, chlorine salt rust-inhibiting antifreeze is used.
The selection of antifreeze must correctly understand the "usage temperature". Any antifreeze product that meets the standard has a clear "use temperature", that is, the concrete must reach the critical strength of anti-freeze before the ambient temperature drops to the "use temperature", otherwise the concrete may be frozen. The lower the "use temperature" of concrete, the better the antifreeze effect of the antifreeze. The concrete has more time to increase its strength, and the possibility of reaching critical frost resistance strength is greatly increased.
Only by deeply understanding the freezing mechanism of concrete and the action mechanism of antifreeze, can various types of antifreeze be used efficiently and correctly, speed up the construction progress, and ensure Engineering Quality.
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