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How to apply concrete antifreeze in winter construction?

The main purpose of using antifreeze in winter concrete construction is to prevent freezing damage, allowing the poured concrete to continue hardening at negative temperatures and achieve the required design strength. In winter construction, the negative temperature method is used to mix antifreeze with concrete. Compared with the heating method commonly used in winter construction, it requires simpler equipment, lower investment, saves energy, and is easier to use.

The frost resistance mechanism of concrete mixed with antifreeze agent

Causes of concrete freezing and anti-freezing principle of antifreeze agents

Concrete is a porous material that is susceptible to freezing damage under three important conditions: temperature, Water, and the condition of the pores in the internal structure of the concrete. Without any of these three conditions, it cannot cause damage. During and after hardening, concrete forms many pores of different sizes and interconnected capillary channels. These pores and capillary channels are filled with Water. When the temperature drops below zero, the water phase transforms into ice, expands in volume, and causes frost damage.

Composite antifreeze generally consists of antifreeze, an early-strength agent, a water-reducing agent, and an air-entraining agent. Among them, antifreeze lowers the freezing point of the liquid phase in concrete, allowing some non-freezing water to remain at a certain negative temperature, ensuring the concrete does not suffer freezing damage and achieves the expected strength within a certain period of time.

Early strength agent promotes the generation of early strength of concrete, enabling it to reach critical strength early to prevent frost damage.

Water-reducing agents not only maintain the workability required for concrete and reduce water consumption, but also have a dispersing effect, making cement into small, separated individual particles that are evenly distributed in Water. This improves the pore structure of concrete, reduces the amount of Water that freezes in concrete, and makes the frozen grain size smaller and more dispersed, thereby reducing the destructive effect of ice.

The air-entraining agent introduces air into the concrete during the mixing process, forming many closed micropores to alleviate the stress damage caused by ice frost heave and improve the early frost resistance of the concrete.

Main Performance and Technical Indicators of Antifreeze

Commonly used antifreeze agents include NON-F concrete composite antifreeze admixture and MN-F concrete composite antifreeze admixture.

Principles to be followed in the selection of antifreeze agents

When the daily minimum temperature is -5 ℃, when the concrete is covered and cured with one layer of plastic film and two layers of straw bags or other substitutes, an early strength agent or early strength water reducer can be used instead of an antifreeze agent.
When the daily minimum temperature is -59 ℃, -10 ℃, and -15 ℃, and the above insulation measures are still used, antifreeze agents with specified temperatures of -5 ℃, -10 ℃, and -15 ℃ can be used, respectively. At the same time, implement the various frozen application regulations in the current Chinese standard GB50204-92.
The selected antifreeze must be approved by the construction administrative department at or above the provincial level. Before using antifreeze in engineering, conduct simulation tests based on the technical indicators in the user manual to determine the concrete mix proportion and avoid quality problems caused by improper dosage.

Mix design of concrete with antifreeze agent

Adopt the relevant test standards and laboratory methods in JC415-92 Concrete Antifreeze Standard.
The sand content of antifreeze concrete mixed with air-entraining components should be reduced by 2% -3% compared to when not mixed. The dosage of antifreeze should be determined through testing based on the construction temperature.
The water-cement ratio of concrete should be 0.5-0.6 for C20 concrete and 0.35-0.45 for C20 concrete.
The cement dosage for C20 and C40 concrete should not be less than 300 kg/m3, and the cement dosage for important load-bearing and thin-walled structures can be increased by 10%.

Application examples of concrete mixed with antifreeze agent

The compressor foundation pile for a certain company’s 480k t/a cracking and renovation project is a drilled pile type. The C40 concrete is mixed with a composite early-strength antifreeze agent, NON-F, at a dosage of 10% (based on cement mass). The test pile requires a 30-day curing period before static load testing can be carried out. Construction can only be carried out after the test is qualified. Winter construction will begin in early March, with outdoor temperatures ranging from 12-29 ℃. The freezing depth is only 1.6m due to the microclimate in the device area.

Because the heat-absorbing concrete strength is reduced in frozen soil, large drilling tools are used to drill through the frozen soil into the concrete piles, increasing the pile diameter by 20 cm compared to the original pile. For some frozen-soil piles, wooden molds separate the concrete from the frozen soil. When the ground temperature below the frozen layer is 6-7 ℃, concrete piles cannot suffer from freezing damage.

The 3D compressive strength of the test block is 15.2 MPa, which exceeds the requirement of the critical strength of concrete for freezing at 3.5MPa.
The 7-day compressive strength of the test block is 29.45 MPa, reaching 73% of the design strength and exceeding the product’s technical specifications by 50%.
Actual quasi-curing for 7 days: compressive strength of 38.2 MPa, reaching 95% of the design strength.

Precautions for winter concrete construction with antifreeze agent

The raw materials for concrete mixed with antifreeze must meet the winter construction requirements. The cement used should preferably be Portland cement or ordinary Portland cement, with a cement grade not lower than 425 #, and the use of high alumina cement is strictly prohibited.
Pay attention to the mixing method when using antifreeze. For antifreeze containing insoluble substances or salts with low solubility, they must be added together with cement in powder form. When preparing a solution for use, it should be thoroughly dissolved and stirred evenly, and the concentration and amount added each time should be controlled. When using composite antifreeze agents, their solubility should be considered. If they are not miscible, prepare them as separate solutions and add them to the concrete separately. To accelerate dissolution, prepare a solution using hot Water at 40-60 ℃ and add it separately to the concrete. If antifreeze is added in powder form, it should be finely ground and sieved through a 0.63mm sieve before use if it becomes damp and clumps.
Strictly control the dosage. According to the daily minimum temperature during the construction period of -10 ℃, -15 ℃, and -20 ℃, antifreeze agents with specified temperatures of -5 ℃, -10 ℃, and -15 ℃ can be used, respectively. The dosage of different antifreeze agents varies greatly. Inaccurate dosage significantly affects concrete performance. Excessive dosage can cause the concrete to set too quickly, leading to construction difficulties, severe salt precipitation on component surfaces, and reduced external decoration quality. Excessive dosage actually reduces concrete strength, and even after curing at normal temperature, strength does not increase. If the dosage is insufficient, the concrete structure will freeze and fail. The industry is gradually shifting toward low-alkali content and low dosage, which helps prevent alkali-aggregate reaction damage in concrete.
The mixing time of antifreeze concrete should be extended by 50% compared to without antifreeze, to ensure that the antifreeze is evenly distributed in the concrete and the concrete strength is consistent. Shorten transportation and watering time as much as possible. To improve early concrete strength, the concrete formwork temperature shall not be lower than -5 ℃.
Concrete with air-entraining agent in composite antifreeze should be cured using the negative temperature method instead of steam curing. Steam curing not only reduces strength but also lowers durability. After pouring the concrete, immediately cover the exposed surface and do not water it. The initial maintenance temperature shall not be lower than the antifreeze’s specified temperature. When the concrete temperature drops below the antifreeze agent’s specified temperature, the concrete’s critical frost-resistance strength should reach 3.5 MPa; otherwise, insulation measures must be taken.

Conclusion

Using the NON-F composite antifreeze agent ensures that concrete construction is not affected by freezing damage at temperatures above 15 ℃, supporting the construction period and delivering good quality and economic benefits.