Abstract:
People have summarized a lot of antifreeze experience in engineering practice. This discussion summarizes the winter construction of concrete. Several methods and three characteristics of the action mechanism of antifreeze are described, and the impact of antifreeze on concrete properties such as fluidity, bleeding, setting time, and strength are also expounded. It is pointed out that concrete mixed with antifreeze can achieve the same quality level as concrete cured at room temperature, and greatly reduces the energy consumption of winter construction and saves costs.
1 Overview
The country's "Winter Construction Regulations for Construction Projects" (JGJ/T104-2011) The regulations stipulate that according to local meteorological data statistics for many years, winter construction will begin when the daily average outdoor temperature is stable below 5°C for 5 consecutive days. According to this regulation, the northwest region where we are located generally undergoes winter construction during construction, and whether antifreeze is added during winter construction of concrete. How much impact antifreeze has on the performance of concrete is a matter for our construction and supervision engineering technicians working on site. It is necessary to study and consider things carefully based on site conditions, construction conditions, management level, etc.
2 Winter Construction Methods
For a long time, people have summarized many effective methods through engineering practice Antifreeze experience, winter concrete construction can be mainly attributed to the following methods.
2.1 Heat storage method
Mainly heat the raw materials when mixing concrete, such as sand And the mixing water is preheated before mixing the concrete. The maximum temperature of the sand should not exceed 60°C, and the maximum temperature of the water should not exceed 80°C, because the cement will produce false setting when the temperature is too high. In this way, the concrete will still have a certain temperature after transportation and vibration. If the heat storage and insulation are done well, the temperature of the concrete can be maintained at the mold entry temperature for a period of time after pouring. This pre-added heat and the hydration heat of the cement are used to slowly cool the concrete and gradually harden during the cooling process to achieve the expected strength requirements.
2.2 Warm shed method
Build an insulated and heated shed at the construction site, and use artificial heating to make the shed The internal air is kept at a positive temperature, so that the pouring and curing of concrete are all carried out in the shed, creating a positive temperature environment for concrete construction. The construction operation of this method is no different from that at normal temperature. The quality of the concrete is reliably guaranteed and it is not prone to freezing damage. However, the disadvantage is that this method is more expensive.
2.3 Steam heating method
This method uses the moist heat of steam to heat the concrete to make the concrete Hardens quickly, accelerating the acquisition of required strength. Although the steam heating method has high heat, high humidity and low cost, it is a complex problem to ensure high-quality concrete products. The most critical issue is to choose a reasonable steam curing system, otherwise the quality of the concrete cannot be guaranteed. .
2.4 Electric heating method
This method is to use electric energy as a heat source to maintain concrete. Specific implementation methods include electrode heating, electric blanket heating, power frequency eddy current heating, etc. The purpose is to convert electrical energy into thermal energy and increase the temperature of concrete. The electrothermal method has special effects on concrete structures that are required to reach the design strength as quickly as possible in a relatively short period of time.
2.5 Comprehensive thermal storage method
Comprehensive thermal storage method is a combination of antifreeze and thermal storage method The construction method used is a relatively simple and economical method, which has been widely used in winter construction in recent years. Compared with heating methods such as electric heating method, steam method, greenhouse method, far infrared method, etc., it can not only save winter application costs, but also reduce the consumption of manpower and material resources, reduce concrete construction links, and achieve better technical and economic results.
3 Modes of Action of Antifreeze
Research has proven that antifreeze has three modes of action. One type has a very low co-dissolution temperature after being mixed with water, and has the function of lowering the freezing point of water and allowing concrete to still hydrate at negative temperatures, such as sodium nitrite and sodium chloride. The other type can not only lower the freezing point of water, but also seriously deform the crystal lattice structure of ice containing such substances, so that it cannot form frost heave stress to destroy the hydrated mineral structure and damage the strength of concrete, such as urea and methanol. . The third category is that although its aqueous solution has a very low co-dissolution temperature, it cannot significantly reduce the freezing point of water in concrete. Its function is to directly react with cement to accelerate the condensation and hardening of concrete, which is conducive to the development of concrete strength. Such as calcium chloride and potassium carbonate.
In winter construction, concrete mixed with antifreeze can be hardened at negative temperatures without heating, and can ultimately reach the same quality level as concrete cured at room temperature. , greatly reducing the energy consumption of winter construction and saving costs.
4 Effect of antifreeze on concrete properties
4.1 Fluidity
Most antifreezes have a certain plasticizing effect. Under the condition of unchanged fluidity, they can reduce the water-cement ratio by more than 10%. Domestic antifreezes are widely used.�� is composed of antifreeze components and water-reducing agents, and often shows a superposition effect. For example, the combination of nitrate and naphthalene-based water-reducing agents or carbonate and lignosulfonate can significantly improve the flow of negative temperature concrete. properties or reduce the amount of antifreeze.
4.2 Bleeding
Most antifreeze will not promote the bleeding of negative temperature concrete and make the mixing Material segregation, because most antifreeze will accelerate the hydration reaction of cement clinker minerals and make the liquid phase thicker, which can improve the bleeding phenomenon of negative temperature concrete. However, antifreeze components such as urea, ammonia, and organic alcohols have a certain retarding effect and tend to promote bleeding in high-fluidity concrete. Appropriately increasing the sand rate can improve the bleeding phenomenon.
4.3 Condensation time
Early strength antifreeze (such as potassium carbonate, calcium chloride, etc.) It tends to shorten the setting time of concrete, so it is conducive to the setting and hardening of negative temperature concrete. However, it should be used with caution in commercial concrete transported over long distances, or used in combination with other admixtures.
4.4 Strength
The effect of antifreeze on the strength of concrete, in addition to the type of antifreeze and the mixing In addition to the quantity, it is also closely related to factors such as the freezing time and freezing temperature of the concrete.
Research shows that the mechanical properties of negative temperature concrete mixed with antifreeze are significantly better than those of negative temperature concrete without antifreeze. For example, if liquid antifreeze mixed with ethylene glycol and water-reducing agent is added at a dosage of 2.5% of the cementitious material, the early strength of concrete can be increased by 30% to 40%, and the later strength will increase by about 20%.
4.5 Elastic modulus
The elastic modulus of concrete mixed with antifreeze and the elastic modulus of benchmark concrete There is no significant difference in quantity.
4.6 Effect of antifreeze on concrete durability
Study shows that antifreeze can increase negative temperature Regarding the durability of concrete, for example, adding antifreeze compounded with salts can significantly improve the impermeability of negative temperature concrete; adding antifreeze compounded with organic matter can significantly improve the frost resistance and carbonation resistance of negative temperature concrete.
5 Conclusion
After we understand the impact of adding antifreeze on the performance of concrete, we face You will be aware of the complex construction environment, and use theory to effectively guide construction in accordance with the requirements of the new national industry standard "Construction Regulations for Winter Period of Construction Projects" (JGJ/T104-2011).
With the development of urban infrastructure construction in my country in the past ten years, concrete winter construction has gradually become more common. Traditional antifreeze can no longer meet the high-strength and high-strength requirements of modern winter construction. Durability, rapid construction and environmental protection requirements. Some non-toxic, environmentally friendly and economical high-performance composites that take into account the durability requirements of concrete do not contain chlorine salts, have low alkali content, are non-toxic (does not contain nitrite), and do not produce toxic gases (does not contain urea and ammonium salts). Antifreeze is the future development direction of antifreeze. However, if admixtures are used improperly, such as adding admixtures without product identification certificates or adding them blindly without carefully reading the operating requirements of the admixture product instructions during operation, the expected results will often not be achieved, and quality accidents may even occur. Therefore, continuous learning, strict compliance with relevant national norms and procedures, and keeping pace with the times to understand the performance of new products are eternal topics for our engineering and technical personnel.
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