When the ambient temperature drops to about 0 ℃, some special technical measures must be taken for commercial concrete construction. This is common sense in the construction industry. How to formulate construction measures for the causes of freezing damage is not a simple problem. The following interview combines some experience in specific work and refers to relevant literature to put forward some opinions. There are many winter application measures for commercial concrete. The comprehensive heat storage method is commonly used in North China. The following will focus on the anti-freeze mechanism of the comprehensive heat storage method based on the causes of freezing damage.
1 Frost resistance critical strength
1. 1. During winter construction, when the strength of commercial concrete When it reaches a certain limit value, since the structure has been initially formed and has the ability to resist frost heave damage, the commercial concrete will not be damaged by frost again when it is frozen. This strength is called the frost resistance critical strength of commercial concrete. The proposal of critical frost resistance strength is a major breakthrough in the winter construction theory of commercial concrete, and is also an important basis for formulating winter construction measures for commercial concrete. The key to winter construction of commercial concrete is to make the commercial concrete reach the critical frost resistance strength as soon as possible.
1. 2. A large number of tests and practices have shown that the critical frost resistance strength of commercial concrete is related to cement type, water-cement ratio, cooling rate and other factors, and is generally The critical frost resistance strengths of commercial concrete and commercial concrete mixed with antifreeze are also different, and their values can be determined according to specifications. Generally speaking, the critical frost resistance strength of commercial concrete after adding antifreeze is slightly lower (relatively blank). This is because the water content of commercial concrete after adding antifreeze decreases, the ice crystals become more dispersed and weak, and the frost heave effect is weakened. for the sake of.
2 Antifreeze principle of antifreeze
Antifreeze is based on the freezing damage mechanism of commercial concrete, combined with antifreeze It is developed based on theories such as critical strength, optimal ice formation rate, and ice crystal form transformation, and summarizes long-term winter construction practices. It is generally composed of four components, and their functions are described as follows:
2. 1. Early strength component: The next process of construction can only be carried out after the strength is reached. In the initial stage of final setting of commercial concrete, large vibrations caused by construction loads on the floor slab should be avoided. The main function is to accelerate the setting and hardening of commercial concrete so that it can reach the critical strength of frost resistance as soon as possible; after reaching the critical strength, it can speed up the hardening speed of commercial concrete and overcome the slow strength growth caused by negative and low temperatures.
2. 2. Air-entraining component: Introduce micron-sized tiny bubbles (beneficial bubbles) into the commercial concrete. Its functions: 1) Cut and seal the commercial concrete. Connected pores (harmful pores) to reduce crack expansion during frost heaving; 2) The large number of bubbles introduced act as an expansion "buffer", absorbing the expansion stress of ice crystals and reducing frost damage. Introducing 3.5% gas into commercial concrete can digest 6.6% volume expansion. In the mature stage, it can improve freeze-thaw resistance and durability.
2. 3. Water-reducing component: Its functions: 1) Reduce mixing water, thereby reducing the total amount of free water and fundamentally reducing the content of freezable ice. (But a certain ice content should also be maintained) to eliminate the internal causes of frost heaving; 2) Through the dispersion of water-reducing components, the wrapped water is released, eliminating inferior blisters, converting coarse ice crystals into fine ice crystals, optimizing the cement hydration environment, and reducing swelling freezing pressure.
2. 4. Antifreeze components are mostly inorganic salts that have the effect of lowering the freezing point. The functions can be summarized as follows: antifreeze components (NaNO2, 2%) (Example) The freezing point of the aqueous solution is about - 1. 5 ℃. When the temperature drops to - 1. 5 ℃, the free water in the pores near the frozen side begins to freeze, and the inorganic salts in the ice body break out, and the salt in the remaining free water The concentration becomes larger (the freezing point further decreases); when the temperature continues to drop (such as - 5 ℃), some free water near the freezing side freezes again, and the concentration of the remaining free water continues to increase... and this process continues until Nitrite
The lowest co-dissolution point of sodium nitrite appears, and all free water in the pores freezes into ice. It can be seen that the function of the antifreeze component is to maintain a certain amount of liquid water (supercooled water) in the commercial concrete during the continuous cooling process, so that the cement hydration can continue (although the hydration rate at this time is already much faster than normal temperature). slow down).
It can be seen that the antifreeze mechanism of antifreeze is comprehensive and is a comprehensive expression of multiple effects. There is no antifreeze that works instantly. "Antifreeze" is only the final effect. It is achieved through the joint action of early strength, air entrainment, water reduction, antifreeze and other factors. Moreover, the effect of antifreeze is also related to the construction conditions of the project. Therefore, the winter construction of commercial concrete is a typical system project and must be considered comprehensively.
3 Correct use of antifreeze
3. 1. Correct understanding of "usage temperature"
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Any antifreeze product that meets the standard has a clear "use temperature" (such as - 15 ℃, - 20 ℃). The use temperature means "allowed "The temperature of commercial concrete construction" is not wrong, but it should be understood in conjunction with the critical frost resistance strength of commercial concrete. That is, before the ambient temperature drops to the "use temperature" of the admixture, commercial concrete must reach the critical frost resistance strength. This discussion��Concrete is safe, otherwise commercial concrete may be frozen. The lower the service temperature of commercial concrete, the better the antifreeze effect of the antifreeze. The commercial concrete has more time (including negative temperature areas) to increase its strength, thereby greatly increasing the possibility of reaching critical anti-freeze strength.
The current use temperature of commercial concrete antifreeze produced in China is mostly between - 10 ℃ ~ - 15 ℃ (can be applied to the daily minimum - 15 ℃ ~ - 20 ° C), the lower the temperature, the more difficult it is to design the antifreeze formula, and the uncertain factors also increase. From this perspective, in most cases, there is no need to require that the use temperature of the antifreeze must be lower than the lowest ambient temperature during construction. The key is to make commercial concrete reach critical frost resistance strength before the temperature drops to the use temperature of antifreeze.
3. 2. Take covering and insulation measures
The basic approach of the comprehensive thermal storage method is to cover and heat Add antifreeze and heat water and gravel if necessary. The function of covering is to make the cement hydration heat and raw material heating heat stay inside the commercial concrete for a longer time, and it is very important to extend the positive temperature hydration time of the cement as much as possible. The better the insulation is, the longer it takes for the commercial concrete to drop to the "use temperature" of the admixture, and the more time it takes to reach the critical frost resistance strength. In order to achieve this effect, the type and thickness of the covering material should be determined through thermal calculations in combination with the use temperature of the admixture, critical anti-freeze strength, ambient temperature and other factors.
3. 3. Good construction organization
The effect of antifreeze must be achieved through good mixing , Vibration is achieved, and the mixing is extended for 30 minutes to make the admixtures more fully mixed. Uneven mixing of the admixtures may even cause accidents. Furthermore, antifreeze has an optimal mixing time and optimal vibrating time. Excessive use will reduce the amount of air entrainment, while insufficient use will cause the bubbles to be unevenly distributed and even produce coarse and inferior bubbles, which are detrimental to antifreeze. In addition, measures such as minimizing the transportation distance, erecting insulation sheds at commercial concrete stations, wrapping conveyor pipes with insulation jackets, covering rack trucks with insulation quilts, and compacting process connections are all aimed at increasing the mold entry temperature of commercial concrete and extending the positive temperature curing time, as soon as possible. Reach critical frost resistance strength.
3. 4. Thermal calculation
Thermal calculation mainly refers to the "Thermal calculation" proposed by Wu Zhendong of Hunan University "Wu Zhendong formula" is used. It has many uses and is mainly used for verification calculations in North China. The key points are:
1) Calculate the mold entry temperature of commercial concrete, T, based on raw materials and ambient temperature;
">2) Calculate the time required for the commercial concrete to drop from this temperature to the temperature specified by the antifreeze, h;
3) Calculate the above curing time according to the maturity formula The strength that commercial concrete can achieve, MPa;
4) Compare whether the strength is greater than the critical frost resistance strength, and determine whether the winter application plan is feasible;
5) During construction, leave a test block with the same conditions, and test its actual strength 1 day before the ambient temperature drops to the antifreeze usage temperature to see if it reaches the critical anti-freeze strength.
3. 5. Master the dosage of antifreeze
Some people regard the dosage on the instruction manual as The base dosage should be adjusted up or down according to the actual temperature during construction. This approach is risky. Generally speaking, the dosage and operating temperature of relatively regular antifreeze products correspond one-to-one during formula design, and there is no adjustment problem. Most components of antifreeze have problems with optimal dosage. The scope of application is very narrow, and the relationship between dosage and efficacy is not linear. For example, if 3% is mixed into 4%, each component will increase by 33%, which is likely to cause a decrease in strength (about 5% to 10%) due to an increase in gas content; an increase in Na2SO4 and NaNO3 will lead to an increase in alkali content, which will affect the durability. unfavorable. If it changes from 3% to 2%, the efficacy will definitely not be reduced by 33%, but more. This is true not only for antifreeze, but also for other admixtures.
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