During construction in winter, the water in the newly poured concrete is easy to freeze, causing its volume to increase to 9% of its original size, which seriously damages the internal structure of the concrete. Therefore, we must take some antifreeze measures during construction. Among the numerous antifreeze methods, adding concrete antifreeze to concrete is a method often used in construction.
Since the 1970s, my country's concrete antifreeze has developed rapidly, and there are many brands of antifreeze. If it is not selected properly, it will have a negative impact on the quality of concrete. When selecting antifreeze, be sure to choose the appropriate type of antifreeze according to the concrete with or without reinforcement. The current types of antifreeze can be roughly divided into three categories: chlorine salts, chlorine salts for rust prevention, and chlorine-free salts:
1. Chlorine salts
Chlorine-salt antifreeze is mainly an admixture composed of sodium chloride and calcium chloride as the main raw material and other early-strength components, water-reducing components, and air-entraining components. It has a good effect of reducing water, early strength and lowering the freezing point, but it has a rust-promoting effect on steel bars. It will cause the volume to expand twice as much as the original, causing cracks in the concrete, forming a vicious circle, and even destroying the concrete buildings. Therefore, chlorine salt antifreeze is only suitable for unreinforced concrete projects.
2. Chlorine salt antirust
This kind of antifreeze is mainly an admixture composed of chlorine salts and rust inhibitors, water-reducing components, early-strength components, and air-entraining components. It has water-reducing, early-strength, anti-freeze and anti-rust effects, and can be widely used in reinforced concrete projects.
3. Chlorine-free salts
This type of antifreeze is an admixture based on nitrite, nitrate, carbonate and urea. It has no corrosion effect on steel bars, but nitrite and nitrate can cause stress corrosion. Therefore, it cannot be used in prestressed concrete works, galvanized steel or reinforced concrete structures in contact with aluminum and iron. The antifreeze produced by urea is rarely used now because of the phenomenon of polluting the environment due to the large dosage. The antifreezes produced by regular manufacturers are often compound antifreezes. It can fully integrate the advantages of various components, which can reduce water, strengthen, and lower the freezing point, accelerate the hydration of cement under negative temperature conditions, improve early strength, and reduce the damage of freezing stress to concrete. Therefore, this kind of antifreeze The antifreeze effect is good, and it is an ideal antifreeze agent in winter concrete construction.
If the correct antifreeze is selected, it will also affect the quality of concrete if it is used improperly. Therefore, the following points should be paid attention to when using concrete antifreeze:
1. Ask the manufacturer to provide the product manual, the product appraisal report made by the government-designated department and the manufacturer's product certificate to prevent the purchase of unqualified products.
2. Within the specified temperature, it must be used according to the dosage provided by the manufacturer or the cement on the construction site should be used to determine the dosage through tests, and special personnel should be assigned to control the dosage.
3. During construction, the adaptable temperature of the admixture must be controlled according to the daily minimum temperature, so that it is safe to use.
4. When using liquid antifreeze, if there is precipitation, it should be stirred evenly before use. If the powder antifreeze is wet and agglomerated, it should be ground and passed through a 0.63mm sieve before use.
5. Do not use active aggregate or aggregate mixed with active silica in concrete construction. Because the antifreeze contains more potassium and sodium ions, the concrete will expand when it interacts with active silica during the hardening process of concrete, resulting in structural damage and reducing the usability and durability of concrete.
6. Under the condition that the construction requirements are met, the winter construction must use less water and strictly control the water-cement ratio according to the requirements.
7. When the temperature is lower than -5°C, hot water can be used to mix concrete. When the water temperature is higher than 65°C, the hot water should be mixed with the aggregate first, and then the cement should be mixed. When the temperature is lower than -10°C, the aggregate The greenhouse can be moved or heating measures can be taken.
8. For the concrete mixture mixed with antifreeze, the exit temperature should not be lower than 10°C, and the entry temperature should not be lower than 5°C.
9. Concrete mixed with antifreeze shall not be poured on the frozen foundation. If the foundation is frozen, insulation measures should be taken.
10. Before pouring concrete, the ice, snow and dirt on the formwork and steel bars should be removed, but the ice and snow should not be melted directly with steam to avoid refreezing.
11. The concrete mixed with antifreeze should be poured within 15 minutes when it is transported to the pouring place, and the exposed concrete surface should be covered with plastic film or thermal insulation material after pouring.
12. Concrete mixed with antifreeze must not be watered for maintenance under negative temperature conditions. The initial curing temperature should not be lower than the specified temperature of antifreeze, otherwise insulation measures should be taken.
13. When the difference between the concrete surface temperature and the ambient temperature is greater than 15°C after formwork removal, insulation materials should be used to cover and maintain.
In short, in concrete construction in winter, in addition to selecting suitable antifreeze and strictly following product instructions and specifications during construction, we also need to observe more, compare more, and analyze and summarize more when problems arise. , combined with a variety of preventive measures construction, in order to effectively prevent the phenomenon of concrete structure frost damage.
