1. When the temperature is equal to or lower than -5℃, it is recommended to use concrete mixed with antifreeze enhancer, otherwise the strength of concrete may be affected by too low temperature.
2. Do a good job of concrete curing, and immediately after pouring, cover it with film and one or two straw bags for heat preservation and curing. Even the concrete mixed with antifreeze should be cured as required.
3. According to relevant specifications, the strength of cast-in-place concrete members should be calculated according to the daily equivalent age of 600℃, not 28 days.
4. In order to prevent freezing injury, it is forbidden to add water to ready-mixed concrete.
5. In winter, the temperature is low, the air is dry, and the strength of concrete increases slowly, so the time of formwork removal should be extended appropriately. After form removal, concrete shall be cured immediately.
In winter, the ambient temperature is low, the hydration reaction of cement is slow, and the early strength of concrete increases slowly or even not, which is easy to cause the fresh concrete to freeze. The appearance of frozen concrete is peeling and slag dropping, and the surface is seriously dehydrated, which seriously affects the strength of concrete. Even if the temperature rises later, the strength of early frozen concrete is difficult to meet the design requirements. Adding early-strength antifreeze into fresh concrete can effectively lower the freezing point, accelerate the early hydration of cement, improve the early strength of concrete, and effectively prevent the early freezing of concrete. Due to the recent low temperature, it is suggested to adopt early-strength anti-freezing concrete in the near future to ensure that the early concrete will not be frozen, and at the same time, the formwork removal time will be appropriately extended to minimize the unfavorable factors of low-temperature conditions on concrete, so as to successfully pass the main acceptance in the future.
Second, the maintenance method
After concrete pouring, if it is not cured in time, the water in concrete will evaporate too quickly, which will lead to dehydration, so that the cement particles that have formed gel can not be fully hydrated and lack sufficient cohesive force, thus causing the flaky or powdery surface of concrete to fall off. In addition, when concrete does not have enough strength, premature evaporation of water will also produce large shrinkage deformation and drying cracks. Therefore, the initial curing after concrete pouring is very important, and the concrete should be cured immediately after the final setting. The curing time of concrete is affected by engineering structure, cement varieties, environmental and climatic conditions, concrete mix ratio and other factors. Considering the particularity of winter construction environment, it is suggested to adopt curing agent with mold curing brush. If portland cement, ordinary portland cement or slag portland cement are used, the curing time at room temperature should not be less than 7d; The curing time should not be less than14d; When a large amount of mineral admixture or retarded admixture is used; For projects with durability requirements such as frost resistance and impermeability, the maintenance time should not be less than 14d.
(1) Formwork with formwork can not only play a good role in thermal insulation, but also avoid a large temperature difference between the concrete surface and the external environment. Especially when the temperature drops suddenly, the formwork removal time should be strictly controlled, and other insulation measures should be taken when necessary. The formwork is in close contact with concrete to avoid water evaporation and has a certain moisturizing effect. Mass concrete needs formwork curing 14 days, and ordinary concrete structure needs formwork curing for 7 days. The template maintenance operation is convenient and simple, but the template takes a long time, which is not conducive to the turnover of the template. For projects with high construction period requirements, if formwork maintenance cannot be adopted, other maintenance measures should be considered.
(2) Water conservation is a common maintenance method, and the construction operation is simple. However, the average daily temperature in winter is lower than 5℃, which is not suitable for water conservation.
(3) Brush curing agent to cure concrete structure. After the formwork is removed, brush curing agent on its surface to prevent water loss and play a role in water conservation. When painting curing agent, it should be painted evenly, and each coat should be painted vertically and horizontally. Check the integrity of curing agent coating frequently to keep the concrete surface wet. However, it should be noted that the curing agent is easy to crack due to external influence, which will affect the moisturizing effect. In addition, different types of curing agents have great influence on the curing effect, which also puts forward higher requirements for the use and selection of curing agents.
(4) Film-covered curing After the concrete is cast or dismantled, the plastic film is covered on the surface of the concrete to prevent the concrete from losing water, which is beneficial to the hydration of the concrete and ensures the moisturizing curing effect. When pasting the film, it should be noted that the plastic films should overlap well to ensure that the films are tightly closed, and the integrity of the film should be checked frequently to ensure that there are condensed water droplets inside the film. The film covered with plastic film is not easy to be fixed and damaged, resulting in poor maintenance effect.