Winter is a season when concrete engineering quality accidents are more frequent. There are many cold winds, low temperatures, and large temperature differences between day and night. Concrete is prone to two phenomena:
① Slow setting phenomenon: affects the construction progress of the project;
② Frost heave damage: has obvious hysteresis, often appears in the early or late spring thaw, causing the reduction of concrete strength, impermeability and durability, affecting the quality of the project, and is difficult to handle. In order to avoid the above phenomenon as much as possible, I would like to talk about my personal views on the precautions for winter construction of concrete mixing stations for reference by industry insiders.
1.Selection of raw materials and their temperature control
(1).Cement
Cement should preferably be silicate cement, ordinary silicate cement and early-strength cement to improve the early strength of concrete and reduce the frequency of frost damage.
(2).Sand, stone
Stock sand should be used first because it has a lower relative moisture content, produces fewer frozen blocks, and has relatively stable performance. If new sand is introduced in winter, the moisture content of the incoming sand should be strictly required (controlled below 4%) to reduce the probability of freezing and the frequency of adjusting the mix ratio during production to ensure the quality of concrete. Stone has a low relative moisture content and a low probability of producing frozen blocks, so it should be protected from rain and snow. In order to ensure the normal use of aggregates, it is recommended that the sand and stone on site be covered with sand nets or felt to protect them from rain and snow and to play a certain role in heat preservation. The stacking height of yellow sand and crushed stone should be appropriately increased. If the material is sufficient, it is appropriate to stack 5~8m to reduce the frozen layer and protect the internal temperature.
(3).Mixing water
Mixing water is drinking water. Hot water should be used for mixing during winter construction. The water temperature should be controlled at around 60℃, but not higher than 80℃.
(4).Antifreeze (antifreeze pumping agent, hereinafter referred to as antifreeze)
Antifreeze contains antifreeze components, which can lower the freezing point of water and allow concrete to continue to hydrate under negative temperature conditions. Many concrete mixing stations reduce the amount of admixtures in order to reduce the cost of concrete, which is not desirable. If the concrete is frozen due to insufficient antifreeze or too low temperature, it will still cause frost damage, reducing the strength and durability of the concrete. Therefore, when we detect and use admixtures, we must strictly control the performance and amount of antifreeze. In order to pursue economic benefits and reduce costs, many admixture factories add a large amount of chloride salts to replace the amount of effective ingredients such as nitrite and triethanolamine during the compounding of antifreeze admixtures. This will invisibly increase the content of chloride ions in concrete, which will have a serious impact on the durability of concrete. Therefore, when the antifreeze enters the factory, it is necessary not only to test the water reduction rate, compressive strength ratio, air content, setting time, etc., but also to measure the chloride ion content. Before storing antifreeze, the additive tank should be kept clean and leak-proof. If the additive tank originally stored room temperature additives, in order to avoid confusion between two additives with different properties, the original additives or additive residues in the tank should be cleaned before storing antifreeze.
(5).Admixture
Due to the different fineness of fly ash, mineral powder and other admixtures, micro-aggregate effect occurs in concrete, which improves the compactness of concrete, that is, improves the strength, frost resistance and durability of concrete, but fly ash and mineral powder can prolong the hydration time of cement. Therefore, the amount of admixtures should be appropriately reduced during winter construction.
2.Temperature control of equipment
In order to save costs, many mixing stations have relatively simple equipment and facilities. After entering the winter construction period, the equipment cannot operate normally, which not only wastes a lot of manpower, material and financial resources, but also delays time. The untimely supply affects the company's reputation. In order to ensure the normal operation of the equipment and the normal production of concrete, the aggregate bin discharge port, admixture tank, and mixer should be closed, and heating points (stoves, electric heaters) should be set. The operation room should also be equipped with electric heaters while having air conditioning. Boiler heating should be used if conditions permit, and the heating pipeline should run through the aggregate bin discharge port, admixture shed, water tank, mixer shed and operation room, which not only saves time and labor, but also better guarantees the concrete outlet temperature and working performance.
For the mixer trucks under the company's jurisdiction, insulation blankets are used before winter construction to keep the mixer trucks warm to ensure that the temperature of the concrete does not drop during transportation. After loading the mixer truck, it is strictly forbidden to add water to the truck at will to prevent the quality of the delivery from being affected by changes in the water-cement ratio of the concrete during transportation. After rinsing the mixer truck, the water in the tank should be drained in time to prevent quality risks caused by ice.
4.Control of production process
The test personnel test the moisture content and temperature of sand and stone every day, and the number of tests should be increased in rainy and snowy weather. According to the climate conditions in the north, concrete only needs to be mixed with hot water, and the aggregate does not need to be heated. The quality inspectors must check the temperature of the mixing water before each opening. It must not be higher than 80℃ to prevent false setting of concrete; it must not be lower than 40℃ to ensure that the temperature of the concrete entering the mold is not lower than 5℃. Strictly control the dosage of antifreeze and extend the mixing time of concrete to ensure the uniformity of concrete.
5.Quality control of concrete after delivery
The following five points should be paid attention to in the quality control of concrete after it leaves the site.
(1) Quality inspectors shall be arranged at the construction site of the project to be responsible for the on-site vehicle allocation and concrete quality control. The unloading time of the concrete shall be compressed as soon as possible to ensure the temperature of the concrete entering the mold.
(2) Before pouring concrete, the quality inspectors at the construction site shall check whether there are frozen blocks or snow on the surface of the formwork, which may affect the quality of the concrete.
(3) After the concrete is poured, the construction unit shall be notified in time to cover the concrete with plastic film and blankets to ensure that the concrete is not frozen before the initial setting; during winter construction, the concrete shall not be watered for maintenance, and before the concrete strength reaches 1.2MPa, it shall not be stepped on, supported or loaded.
(4) After the concrete is poured, the quality inspectors shall promptly remind the tank truck driver to unload the water used to clean the concrete mixer truck.
(5) If the difference between the surface temperature of the concrete after demolding and the ambient temperature is greater than 20℃, insulation covering maintenance measures shall be taken.
6.Conclusion
The quality of concrete requires not only strict control of the mixing plant, but also the support and close cooperation of the construction unit. In order to ensure the quality of the project, each construction unit must meet the requirements of GB50204 and JGJ104 for winter concrete construction. This will avoid quality risks of premixed concrete during winter construction and work together to ensure the quality of the project.
