In order to ensure the workability and pumpability of concrete and prevent the occurrence of freezing damage during the winter construction of concrete, it is necessary to base on different engineering environments, different parts, and different pouring methods etc. Select the concrete mix ratio for winter construction. At the same time, antifreeze or concrete antifreeze pumping agent should also be used to reduce the frost heave stress inside the concrete and prevent the formation of small ice crystals inside the concrete. Concrete antifreeze is an admixture used in construction under negative temperature conditions in winter. At the specified temperature, it can significantly reduce the freezing point of concrete, refine ice crystals, prevent the liquid phase of concrete from freezing or maintain a certain amount of liquid water, ensuring that the cement The hydration effect can achieve the expected strength within a certain period of time, and the concrete will not be damaged by frost heaving at low temperatures.
(1) Components of antifreeze
Composite antifreeze pumping currently used by concrete companies Agent, its components are generally composed of antifreeze, early strength, water reduction and air entrainment. Among them: (1) Antifreeze mainly reduces the freezing point of the liquid phase in concrete; (2) The dosage of early strength agent increases the early strength of concrete so that it can reach or exceed the critical anti-freeze strength of concrete as soon as possible. (3) Water-reducing agents can reduce the water consumption of concrete and reduce frost damage to concrete. (4) Air-entraining agents can alleviate the frost heaving stress damage of ice and improve the anti-early freezing damage performance of concrete.
(2) On-site inspection of antifreeze
Whether antifreeze or antifreeze pumping agent is used, Before use, check whether its quality meets the technical requirements of JC475-2004 "Concrete Antifreeze Agent" and JG/T377-2012 "Concrete Antifreeze Pumping Agent".
(1) Before antifreeze enters the factory, the manufacturer's factory certificate, test report, CCC certification (scanned copy) and other documents should be inspected first. Instructions generally include: product name, main antifreeze components and alkali content, range of use, specified temperature, dosage, storage conditions, validity period, usage methods and precautions, etc., focusing on antifreeze components, dosage, suitable temperature and use method. The certificate should be marked with: production date, batch number, company name and address, inspection conclusion, company quality inspection seal, signature of quality inspection personnel, etc.
(2) Sampling and testing of antifreeze should be carried out according to 50T/batch. Antifreeze sampling should be representative. For liquid antifreeze, attention should be paid to sampling from the upper, middle and lower layers respectively. The amount of each batch of samples should be no less than the amount of antifreeze required for 0.15t of cement specified in JC475-2004. Carry out routine project tests in accordance with the standards. After the test is completed, they will be sealed and stored for half a year, in case of disputes, they will be submitted to the nationally designated inspection agency for re-inspection or arbitration.
(3) Test the homogeneity index of the antifreeze pumping agent according to the specification JC/T377-2012. Liquid antifreeze focuses on testing the solid content, density, and pH. value and chloride ion content and detect the amount of ammonia released. Concrete mixed with antifreeze should be tested for water reduction rate, air content, setting time difference, slump change over time for 1 hour, compressive strength ratio for 7 days of negative temperature curing, and strength loss rate ratio for 50 freeze-thaw cycles, etc. Antifreeze contains an appropriate amount of air-entraining components, which can alleviate concrete stress damage caused by frost heave stress. Through the detection of 50 freeze-thaw strength loss rates, the main purpose is to prevent the adverse effects on strength caused by improper selection of antifreeze components, early strength components, air entrainment components, etc. in concrete antifreeze.
(4) Different varieties and models of admixtures should be clearly marked, stored separately, and marked with rain and sun protection measures. Observe the antifreeze pumping agent entering the factory to ensure that it is uniform in color and free from turbidity, stratified precipitation, and crystallization. In addition, samples must be taken and placed in a freezer at a specified temperature to observe their changes. If liquid admixtures appear to crystallize, precipitate, etc., they can only be used after passing the performance test; powdered admixtures should be prevented from getting wet and agglomerating. If there are any agglomerations, they should be crushed until they pass the 0.63mm sieve after passing the performance test before use. .
(5) In addition to testing in accordance with specifications, testing can also be conducted based on experience and practical testing. For example, the pure pulp detection method (observation of freezing point method), that is, first, conduct a pure pulp test according to the recommended dosage and observe the loss over time, then put the remaining pure pulp into a paper cup and place it in a negative temperature environment to observe whether If there is freezing, find the freezing point. Observe the same condition test block method. Place the concrete specimens made according to the production mix ratio outdoors, wrap them with insulation materials such as felt cloth, and observe whether there are traces of ice on the surface of the specimens.
(3) Problems in the application process of antifreeze
(1) Crystallization of antifreeze (Sedimentation) problem
During the winter concrete production process, attention should be paid to preventing the crystallization of liquid antifreeze. The reasons for the crystallization phenomenon of liquid antifreeze can generally be divided into two types. One is that the solubility of the components in the antifreeze decreases as the temperature decreases and crystals precipitate, such as sodium sulfate in naphthalene-based water reducing agents. The other is the antifreeze. A certain component in the agent is incompatible with other components and crystallizes, such as using polycarboxylate water-reducing agent with other components. When the antifreeze precipitates and becomes sticky, and crystals precipitate, it will not only seriously affect the homogeneity and use effect of the antifreeze, but may even block the pipeline of the admixture and affect the mixing.The production of soil.
(2) Freezing point problem of liquid antifreeze
When using liquid antifreeze, Sometimes there is ice formation before reaching the specified temperature for use, so the quality is often doubted. More than 50% of the water in the actual liquid antifreeze pumping agent will begin to form ice crystals at a certain low temperature. Although freezing does not change its chemical composition, it may have an adverse effect on the effect of the admixture. Therefore, it is necessary to properly insulate the antifreeze pumping agent to keep it in a solution state, especially when using low-concentration polycarboxylate water-reducing agents.
(3) The test conditions in the standard are quite different from the production practice conditions
JG/ The test methods stipulated in some main test items in T377-2012 "Antifreeze Pumping Agent for Concrete" stipulate that the benchmark cement and sand, gravel, and water that meet the standard requirements should be used for testing, resulting in admixtures that pass the standard method. If used in production, it will not meet the requirements
. In the antifreeze standards, the tested concrete is tested at a constant negative temperature, which is different from the natural ambient temperature conditions. Different types of antifreeze have different effects on the growth of concrete strength at negative temperatures. At the same time, the hardening temperature of concrete also affects the growth of concrete strength, that is, under positive temperature, constant negative temperature, and under alternating positive and negative temperature conditions, the strength of concrete All will change. Therefore, different varieties or different dosages of antifreeze should be selected according to changes in concrete hardening temperature, ambient temperature and requirements for concrete strength.
(4) Antifreeze dosage problem
At present, the use of antifreeze pumping agents generally follows The cementitious material is added in a weight ratio, and the amount of antifreeze is adjusted according to changes in temperature, without taking into account the influence of the design strength grade of concrete, cement type, etc. This mixing method is questionable. During actual use, we have to make adjustments according to different project parts and different concrete strength levels. Especially for thin-walled structures, protective layers, cushions or low-strength concrete, the amount of antifreeze should be appropriately increased.
(5) Winter concrete mix ratio
When designing the winter concrete mix ratio, appropriate attention should be paid to Increasing the amount of cement per cubic meter of concrete and reducing the amount of mineral powder and fly ash will help improve the early strength of concrete. The amount of mineral powder is controlled at 10% to 15% of the cementitious material, and is not mixed below C20. The higher the strength level of the concrete, the dosage can be increased appropriately. The amount of fly ash is controlled at 10% to 30% of the cementitious material. The use temperature of antifreeze pumping agent should be determined based on the lowest temperature recorded in the local area and based on the test results.
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