According to the “Winter Construction Regulations for Construction Projects” (JGJ104), winter construction begins when the outdoor daily average temperature is stable below 5℃ for 5 consecutive days; when the outdoor daily average temperature is stable for 5 consecutive days Winter construction will be canceled when the temperature is higher than 5℃. If no effective measures or improper measures are taken during the winter construction of the main structure, the concrete will suffer freezing damage, which will cause accidents to the quality of the project. Therefore, antifreeze must be used correctly according to the mechanism and rules of concrete freezing damage, which is of great significance to ensuring the quality of the project.
Admixtures that can harden concrete at negative temperatures and achieve sufficient antifreeze strength within a specified time are called antifreeze agents. Concrete mixed with antifreeze can be hardened at negative temperatures without the need for heating, ultimately reaching the same quality level as concrete cured at room temperature. During winter construction of the main structure, antifreeze must be correctly understood and used to ensure the construction quality of the main structure.
1 Characteristics of antifreeze
Antifreeze has three characteristics according to its mode of action:
1) After being mixed with water, it has a very low co-dissolution temperature, which can lower the freezing point of water and allow concrete to still hydrate at negative temperatures. Such as sodium nitrite, sodium chloride. However, once the concrete freezes due to insufficient dosage or too low temperature, freezing damage will still occur and the final strength of the concrete will be reduced.
2) It can not only lower the freezing point of water, but also seriously deform the lattice structure of ice containing this type of material, so it cannot form ice expansion stress and destroy water. Chemical mineral structures, such as urea and methanol, can damage the strength of concrete. When the dosage is insufficient, the strength of concrete stops growing at negative temperatures, but it has no effect on the final strength after turning to positive temperatures.
3) 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. Such as calcium chloride and potassium carbonate, which accelerate the setting and hardening of concrete through chemical reaction and are conducive to the development of concrete strength. It should be noted here that antifreeze and antifreeze group are not the same concept. Antifreeze is a type of admixture, which consists of water-reducing components, antifreeze components, air-entraining components, and sometimes early-strength components. Its function is to not only harden the concrete at negative temperatures, but also enable it to eventually reach the quality level of concrete cured at normal temperature. The antifreeze component refers to a chemical substance that protects the concrete mixture from freezing damage in negative temperature environments.
2 Scope of application of antifreeze
1) Chloride salt antifreeze can be used in concrete projects, Reinforced concrete projects are strictly prohibited from being used in prestressed concrete projects, and should comply with the provisions of GBJ50119 “Technical Specifications for the Application of Concrete Admixtures”; chlorine salt rust-inhibiting antifreeze can be used in concrete and reinforced concrete projects, but is strictly prohibited from being used in prestressed concrete projects. , and should comply with the provisions of GBJ50119 “Technical Specifications for the Application of Concrete Admixtures”; inorganic salt antifreezes such as nitrite and nitrate are strictly prohibited from being used in prestressed concrete and concrete structures in contact with galvanized steel.
2) Organic compound antifreeze can be used in concrete engineering, reinforced concrete engineering and prestressed concrete engineering.
3) Organic compound and inorganic salt composite antifreeze and composite antifreeze can be used in concrete engineering, reinforced concrete engineering and prestressed concrete engineering.
Antifreeze containing harmful ingredients such as hexavalent chromium salts and nitrites is strictly prohibited from being used in drinking water projects and parts in contact with food, and is strictly prohibited from consumption.
Antifreezes containing ammonium nitrate, urea and other antifreeze that produce pungent odors must not be used in office, residential and other construction projects.
4) For hydraulic engineering, bridges and concrete projects with special freeze-thaw resistance requirements, the type and dosage of antifreeze should be determined through tests.
3 Main varieties and properties of antifreeze
Antifreeze is composed of antifreeze components, It is composed of several functional components such as water agent and air-entraining agent. The percentage content of each component varies with the winter temperature change characteristics of the area where it is used. Therefore, antifreeze has strong local characteristics, but the antifreeze components used are similar.
The antifreeze components in the admixture are: ① Nitrite includes sodium nitrite, calcium nitrite, and potassium nitrite. ② Nitrates include sodium nitrate and calcium nitrate. ③Carbonates include potassium carbonate. ④Sulfates include sodium sulfate, calcium sulfate, and sodium thiosulfate. ⑤Chlorine salts include sodium chloride and calcium chloride. ⑥Ammonia.
⑦Urea. ⑧Low carbon alcohols include methanol, ethanol, ethylene glycol, 1,2 propylene glycol, and glycerin. ⑨ Salts of small molecular weight carboxylic acids include calcium formate, sodium acetate, calcium acetate, sodium propionate, calcium propionate, and calcium acetate monohydrate.
4 Technical Key Points for Antifreeze Application
1) The strength of concrete in a negative temperature environment should be considered increase.
2) The amount of antifreeze should be properly considered.
3) The dosage of other components in the composite antifreeze should be properly considered.
4) The amount of antifreeze used should comply with the following regulations:
When the daily minimum temperature is -5°C and the concrete is covered and maintained with a layer of plastic film and two layers of straw bags or other substitutes, early strength agent or early strength water reducing agent can be used instead; When the daily minimum temperatures are -10°C, -15°C, and -20°C, and the above insulation measures are adopted, antifreeze with specified temperatures of -5°C, -10°C, and -15°C can be used respectively.
5) When preparing and using antifreeze, you should pay attention to:
Before preparing compound antifreeze, you should master The active ingredients, moisture and insoluble content of each component of the antifreeze should be calculated based on the effective solid content when preparing. When preparing the compound antifreeze solution, it should be stirred evenly. If there is freezing or precipitation, the solutions should be prepared separately and added to the mixer respectively. There should be no precipitation, suspended matter, or flocculation. The above phenomenon indicates improper formulation. When certain components interact, the component must be found and replaced.
6) When calcium chloride is used in combination with air-entraining agent or air-entraining water-reducing agent, the air-entraining agent or air-entraining water-reducing agent should be added first, and then stirred Finally, add calcium chloride solution.
7) When supplying products in the form of powder, the smallest component should be carefully processed during production so that it can be evenly dispersed in the largest component. Coarse-grained raw materials must be prepared first. Crush and then mix. Finally, all should be able to pass through the 0.63mm aperture sieve.
8) Containers storing liquid antifreeze should have insulation or heating equipment.
5 Dosage requirements for antifreeze used in winter construction
Concrete structure projects constructed in winter, The amount of concrete antifreeze should be selected according to the type of concrete antifreeze.
① When using a single antifreeze component, the maximum amount of antifreeze shall not be greater than the following regulations: the amount of chlorine salts shall not be greater than 7% of the mixing water; The dosage of sodium nitrite, calcium nitrite, sodium nitrate, and calcium nitrate shall not exceed 5% of the weight of cement; the dosage of urea shall not exceed 4% of the weight of cement; and the dosage of potassium carbonate shall not exceed 10% of the weight of cement.
② When using concrete antifreeze with composite components, the selection should be based on the temperature, dosage and process requirements specified in the product manual. It is strictly prohibited to adjust the dosage at will. .
6 Precautions for using concrete antifreeze
1) Chloride antifreeze: use chlorine Salt (calcium chloride, sodium chloride) is used as an antifreeze component and can be used in unreinforced concrete projects.
2) Chloride salt rust-inhibiting antifreeze: Antifreeze using chlorine salt and rust inhibitor (sodium nitrite, etc.) as antifreeze components can be used for reinforced concrete Engineering, but shall not be used for reinforced concrete in the following structures.
① Structures used in environments with relative humidity greater than 80%, structures in areas where the water level rises and falls, open-air structures or structures that are often exposed to water. ② Structures in contact with galvanized steel or aluminum iron, as well as structures with exposed embedded iron parts without protective measures. ③ Structures in contact with corrosive media containing acid, alkali or sulfuric acid. ④ Structures that are often exposed to ambient temperatures above 60°C. ⑤Structures using cold-drawn steel bars or cold-drawn low-carbon steel wire reinforcements. ⑥Thin-walled structures of water supply and drainage structures, crane beams, roof trusses, drop hammer or forging hammer foundations of intermediate and heavy working cranes and other structures. ⑦Electrolysis workshop and structures within 100m from high-voltage DC power supply. ⑧Structures close to high-voltage power sources, such as power stations and substations. ⑨Prestressed concrete structure. ⑩Concrete structures containing active aggregates.
3) Chlorine-free salt antifreeze is an antifreeze that uses non-chlorine salt materials as antifreeze components and can be used in reinforced concrete projects and prestressed concrete projects. However, antifreeze containing the following antifreeze component materials shall not be used in the following structures.
① Nitrate, nitrite, and carbonate admixtures shall not be used in prestressed concrete projects, as well as galvanized steel or steel bars in contact with aluminum and iron. Concrete structure. ②Contains toxic antifreezes such as hexavalent chromium salts and nitrites. It is strictly prohibited to be used in drinking water projects and food contact parts.
7 Mixing of concrete mixed with antifreeze
Before mixing concrete, you should check whether the background preparation is consistent with The materials required by the specifications and the proportioning list must be consistent, and the measuring instruments must be inspected to ensure accurate measurement of raw materials. At the same time, a dedicated person shall be arranged to be responsible for the delivery of antifreeze. The mixing time of concrete mixed with antifreeze should be 50% longer than the usual requirements or implemented in accordance with relevant specifications and requirements. The quality inspector or tester of the mixing station should frequently check the materials and measurement conditions in the background.
8 Curing and form removal of concrete
After the concrete is poured, if it is cold concrete, then Insulation and curing are not required; if the comprehensive thermal storage method is selected, the concrete should be insulated and cured and temperature measured according to the requirements of thermal calculation for winter construction to prevent the concrete from freezing. At the same time, at least two more sets of test blocks cured under the same conditions should be made compared to normal temperature curing. One group is used to test the critical strength of concrete when frozen; the other group is used to test the concrete strength after 28 days of negative temperature to normal temperature curing. The test blocks cured under the same conditions should be cured under the same conditions as the concrete structure. No one may remove the formwork or remove the insulation measures without permission before receiving the formwork removal notice and insulation removal notice from the project department.
No one is allowed to remove the formwork and remove the insulation measures without prior notice of insulation.
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