During winter construction, when the average temperature is less than or equal to 5℃, the strength of concrete develops slowly. When the minimum temperature is between -1 and -2°C, if effective insulation measures are not taken, the newly poured concrete may suffer from freezing damage. Once it is frozen, it will cause great damage to the concrete. The use of antifreeze in winter, coupled with good insulation and maintenance, is a practical and effective measure to reduce early concrete diseases and improve the antifreeze effect of concrete. It can ensure that the durability and strength of concrete meet construction requirements and ensure the quality of concrete projects.
The main action mechanisms of antifreeze in concrete are as follows.
(1) Ice crystal distortion theory
When pure water freezes at 0°C, the water molecules will converge to form a molecular aggregate with strong frost heaving force due to the effect of hydrogen bonds. After adding antifreeze to water, ice will slowly precipitate when the temperature drops below its freezing point. However, the hydrogen bonds between water molecules will be interfered by the antifreeze molecules and will precipitate into a flocculent structure, which is very soft macroscopically. Small ice crystals that significantly reduce frost heave stress. For example, NaNO2, Na2SO4, Ca(NO3)2, etc. have strong ability to reduce frost heave force.
(2) Ural's Law
The concentration of antifreeze will reduce the liquid phase in the concrete mixture The freezing point plays a certain role. Only by lowering the freezing point of the liquid phase can the concrete have liquid water to participate in cement hydration under negative temperature conditions. This is concrete that uses antifreeze. The concrete can still hydrate under negative temperature conditions and improve the strength of the concrete.
(3) Liquid-cement ratio balance theory
When the temperature drops to the freezing point of the liquid phase in concrete , the concrete mixture containing antifreeze will gradually begin to freeze. In the liquid phase under a negative temperature environment, the freezing point decreases as the concentration of ice precipitation increases, and then reaches a state of coexistence of ice and liquid under this negative temperature condition. It is the new dynamic balance. At this time, the liquid water in the concrete can ensure that the cement continues to hydrate, the antifreeze concentration remains unchanged, and the liquid phase concentration remains unchanged. The water generated by melting ice supplements the water needed for cement hydration, which is the liquid-cement ratio balance.
(4) Maturity theory
The influence of temperature on cement hydration is extremely significant, and the mix ratio For a certain concrete, as the curing temperature decreases, the cement hydration rate slows down and the growth rate of concrete strength gradually decreases. Therefore, the development of concrete strength is not only related to curing time, but also has a great relationship with curing temperature. For example, the maturity coefficient is 1 at 20°C, but only 0.12 at -10°C. The concrete maturity during winter concrete construction acceptance is calculated based on 600℃·d, and is stipulated not to exceed 60 days. For many projects that do not take insulation measures, it is still difficult to reach the specified maturity of 600℃·d even if it exceeds 60 days. Springback detection The intensity is often low.
In winter concrete engineering construction, in order to prevent concrete from suffering freezing damage, antifreeze is often added to reduce the freezing point temperature of liquid water in concrete, ensure that there is liquid water inside the concrete that can satisfy hydration, and reduce the damage caused by low temperature. damage. Most of the currently widely used antifreezes are composite antifreezes, which have the advantages of small slump loss, high water reduction rate, high early strength, and moderate setting time. Its main components include the following.
(1) Antifreeze component
Its function is to destroy the crystal form of ice, lower the liquid freezing point of water, maintain the liquid water required for the cement hydration reaction inside the concrete, and avoid freezing damage to the concrete. Reach early strength as quickly as possible. The use of antifreeze components can first lower the freezing point of water and ensure that the hydration reaction of concrete continues at negative temperatures, such as sodium chloride, sodium nitrite, etc.; secondly, it can also change the lattice shape of ice and reduce frost heave stress. , to avoid damage to concrete strength, such as urea, methanol, etc. In addition, some antifreeze agents cannot reduce the concentration of water in concrete.Ct·（W/C）·dt (2)
Some construction workers believe that the antifreeze added to the concrete already has antifreeze capabilities and does not require maintenance. From the above analysis of the action mechanism of antifreeze, it is necessary and necessary to carry out thermal insulation maintenance during the construction process. Make concrete reach critical strength as early as possible to reduce the root cause of concrete freezing damage. According to national regulations, before the concrete poured under negative temperature conditions in winter is frozen, the concrete shall not be lower than "30% of the design concrete strength standard for concrete mixed with Portland cement or ordinary Portland cement, and slag Portland cement" The mixed concrete is 40% of the designed concrete strength standard. When the temperature of the concrete mixed with antifreeze drops below the specified temperature of the antifreeze, its strength should not be lower than 3.5~5.0MPa, that is, the concrete reaches a certain strength. value (critical strength), it can have a certain resistance strength. Low temperature will no longer cause damage to the concrete, and the strength will increase to the design strength after the temperature becomes normal.
Therefore, during the winter concrete project construction process, other winter construction measures should still be adopted after using antifreeze, such as the greenhouse method, comprehensive thermal storage method, etc.
Under the alternating positive and negative temperature conditions of -5℃~+5℃ daily temperature, early strength agent or early strength water reducing agent can be used, and cover should be used after concrete pouring Plastic film + one layer of straw bags or other substitutes for covering and maintenance; when the daily temperature is -5℃~-10℃, antifreeze with a specified temperature of -5℃ can be used. After concrete pouring, cover with plastic film + two layers of straw bags or Cover and maintain with other substitutes; when the lowest temperature is -15°C, use antifreeze with a specified temperature of -15°C, and use insulation measures in a timely manner for concrete curing after concrete pouring.
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