Concrete construction in winter requires positive temperature pouring and positive temperature curing. The heating of raw materials, as well as the mixing, transportation, pouring and curing of concrete should be thermally calculated and constructed accordingly.
(1) Requirements for materials and material heating
①For preparing concrete during winter construction For cement, Portland cement and ordinary Portland cement with high activity and high hydration heat should be given priority. The type of cement used in steam curing should be determined through experiments. The strength of cement should not be less than 42.5 grade, and the water-cement ratio should not be greater than 0.6. Cement must not be heated directly. It must be transported to a greenhouse for storage 1 to 2 days before use. The temperature of the greenhouse should be above 5°C.
② Because the specific heat of water is about 5 times that of sand and stone aggregates, the method of heating water should be given priority when mixing concrete in winter, but the heating temperature must meet the specified values.
③ Aggregates are required to be cleaned and stored in advance to ensure that the aggregates are clean and free of frozen blocks, ice and snow. All winter aggregate storage sites should be located on higher ground without water accumulation.
④The temperature of sand and stone for concrete mixing during winter construction must meet the temperature required by thermal calculations. The heating method can be adapted to local conditions, but steam heating is the best method.
⑤ No matter which method is used to heat the raw materials, when designing the heating equipment, the maximum daily material consumption and the required temperature must be found first. According to the raw materials From the initial temperature and specific heat, find the total heat required. Also consider the heat loss during the heating process. With the total heat required, the type, size and number of heat sources can be decided.
(2) Mixing
Concrete should not be mixed in the open air. A greenhouse should be set up as much as possible, and large concrete mixers should be used first. capacity of the mixer to reduce heat loss from the concrete. Before blending, rinse the blender with hot water or steam. The mixing time of concrete is 50% longer than the specified time at normal temperature. Since mixing cement with water at about 80°C will cause sudden solidification, the order of material feeding is to first add water and sand into the mix, and then add cement. If it can be ensured that the hot water does not come into direct contact with the cement, the water can be heated to 100°C.
(3) Transportation
The transportation time and distance of concrete should ensure that the concrete does not segregate or lose Plasticity. The main measures are to reduce transportation time and distance; use large-volume transportation tools and properly insulate them.
(4) Pouring and curing
Before pouring concrete, the formwork and steel bars should be removed from the Freezing snow and dirt, try to pour concrete as quickly as possible to prevent excessive heat loss. The exit temperature of the concrete mixture should not be lower than 10°C, and the mold entry temperature should not be lower than 5°C. When using heating curing, the temperature of concrete before curing shall not be lower than 2℃.
When heating and curing the integral structure, the location of the construction joints should be set at a place where the temperature stress is small. When the heating temperature exceeds 40°C, temperature stress will inevitably occur inside the structure due to the high temperature. Therefore, the opinions of the design unit should be sought before construction and construction joints should be set at appropriate locations within the span. Leave the construction joints open, and immediately use 3 to 5 atmospheric pressure airflow to blow away the cement film, sewage and loose stones on the joint surface after the cement is finally set.
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