What is antifreeze? Admixtures that can harden concrete at negative temperatures and achieve sufficient antifreeze strength within a specified time are called antifreeze. 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.
1. Characteristics of antifreeze
Antifreeze has three characteristics according to its mode of action:
One type has a very low co-dissolution temperature after being mixed with water, 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 excessive temperature, it will still cause frost damage and reduce the final strength of the concrete.
The other type can not only lower the freezing point of water, but also seriously deform the lattice structure of ice containing this type of material, so that ice expansion stress cannot be formed and Destroy the structure of hydrated minerals and damage the strength of concrete, such as urea and methanol. 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.
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 interact with cement. Such as calcium chloride and potassium carbonate, the hydration reaction occurs to accelerate the setting and hardening of concrete, which is beneficial to the development of concrete strength.
It should be noted here that antifreeze and antifreeze components 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
Antifreeze containing chlorine salts is only suitable for products that do not contain Reinforced plain concrete, masonry mortar. Containing a sufficient amount of rust inhibitor can be used for general reinforced concrete but not for prestressed reinforced concrete. Chloride-free antifreeze is suitable for all kinds of concrete constructed in winter, whether it is ordinary reinforced concrete or prestressed concrete.
3. Main varieties and properties of antifreeze
Antifreeze is composed of antifreeze components, It is composed of several functional components such as water-reducing 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, the local characteristics of antifreeze are strong, but the antifreeze components used in it are But they are almost the same.
The antifreeze components in the admixture are:
① Nitrite includes sodium nitrite, Calcium nitrite, 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.
⑧ Low-carbon alcohols include methanol, ethanol, ethylene glycol, 1,2 propylene glycol, and glycerol.
⑨Salts of small molecular weight carboxylic acids include calcium formate, sodium acetate, calcium acetate, sodium propionate, calcium propionate, monohydrate
Calcium acetate.
4. Key points of antifreeze application technology
① Consider the growth of concrete strength in negative temperature environments .
②The amount of antifreeze.
③The amount of other components in the composite antifreeze.
④The amount of antifreeze used should comply with the following regulations: when the daily minimum temperature is -5℃, the concrete should use one layer of plastic film and two layers of straw bags or other substitutes When covering maintenance, early strength agent or early strength water-reducing agent can be used instead; when the daily minimum temperature is -10℃, -15℃, -20℃, when the above insulation measures are adopted, the specified temperature can be -5 respectively.
°C, -10°C and -15°C antifreeze.
⑤ When preparing and using antifreeze, attention should be paid: before preparing composite antifreeze, the active ingredients, moisture and insoluble content of each component of the antifreeze should be mastered, and the preparation should be calculated based on the effective solid content. 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.
⑥When 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 mixed , then add calcium chloride solution.
⑦ When products are supplied 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 first be processed Crush and then mix��. Finally, all the materials should be able to pass through the 0.63mm aperture sieve
.
⑧Containers storing liquid antifreeze should have insulation or heating equipment.
5. For concrete structural projects constructed in winter, the amount of concrete antifreeze should be selected according to the type of concrete antifreeze
① When using monomer antifreeze components, the maximum amount of antifreeze should not be greater than the following regulations: the amount of chlorine salts should not be greater than 7% of the mixing water; sodium nitrite, calcium nitrite, sodium nitrate, The dosage of 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 composite component concrete antifreeze, 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 when using concrete antifreeze
(1) Chloride antifreeze:
Antifreeze using chlorine salt (calcium chloride, sodium chloride) as the antifreeze component can be used in unreinforced concrete projects.
(2) Chlorine salt rust inhibitor antifreeze:
Use chlorine salt and rust inhibitor Antifreeze (sodium nitrite, etc.) as antifreeze component can be used in reinforced concrete projects, but it cannot be used in reinforced concrete of 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.
5) Structure using cold-drawn steel bars or cold-drawn low-carbon steel wire reinforcements.
6) Thin-walled structures of water supply and drainage structures, crane beams, roof trusses, drop hammer or forging hammer foundations of intermediate and heavy-duty working cranes and other structures.
7) Electrolytic workshops and structures within 100m of high-voltage DC power supply.
8) Structures close to high-voltage power sources, such as power stations and substations.
9) Prestressed concrete structure.
10) Concrete structure containing active aggregates.
(3) Chlorine-free salt antifreeze is an antifreeze that uses non-chlorine salt materials as antifreeze components. It 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 parts in contact with aluminum and iron. Reinforced concrete structure.
2) Contains 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 The materials are consistent with the specifications and proportioning orders, and the measuring instruments are inspected to ensure accurate measurement of raw materials. At the same time, a dedicated person is 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 standards and requirements. The quality inspector or tester of the mixing station should frequently check the materials and measurement conditions in the background.
8. Concrete maintenance and formwork removal
After the concrete is poured, if it is cold concrete, If the comprehensive heat storage method is chosen, the concrete should be insulated and cured and temperature measured according to the thermal calculation requirements of 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 exposed to freezing; the other group is used to test the concrete strength after 28 days of negative temperature to normal temperature curing. The test blocks maintained under the same conditions should be maintained 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.
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