【Summary Description】
The late autumn is about to pass, the leaves are falling, and the air is rustling, and winter has come. If the concrete construction is not prepared and prevented in advance before the arrival of winter, it will inevitably affect the quality and progress of the project, and if the measures are improper, it will also bring hidden dangers to the quality of the project.
(1) Requirements for ready-mixed concrete
Concrete raw material quality, mix ratio design, material feeding measurement and control during mixing, discharge temperature and heat preservation during transportation, as well as ensuring the temperature of concrete entering the mold, etc., all need to be requested in advance with the mixing station. During the construction, the concrete manufacturer should be notified in time to master the various properties of the concrete, and the two parties should cooperate to do a good job in the winter construction of the concrete.
(2) Requirements for concrete mix ratio
Due to the low temperature in winter, in order to increase the growth rate of concrete strength as soon as possible, the concrete mix ratio should be adjusted appropriately, and a certain early strength agent and antifreeze agent should be added. Mixing ratio changes must be tried before use. The purpose of adding early-strength and anti-freezing composite admixtures to concrete (anti-freezing performance reaches -15°C):
①In order to improve the early strength of concrete in the covering and heat preservation stage, the freezing critical strength of concrete should be reached as soon as possible (when ordinary concrete is prepared with Portland cement or ordinary Portland cement, it should be 30% of the designed concrete strength standard value) and demolition The strength required by the mold;
② Prevent the concrete from freezing during sudden cooling;
③ In order to make the strength of concrete continue to increase after the removal of insulation.
(3) Requirements for concrete raw materials
(1) The cement strength grade in the concrete mix ratio shall not be lower than 42.5, and the amount of cementitious materials shall not be less than 300kg/m³. Ensure that the water-cement ratio is not greater than 0.5, and deduct the water brought in by the aggregate and antifreeze solution from the mixing water.
(2) The mixing station is required to heat concrete raw materials: because the specific heat of water is about 5 times that of sand and stone aggregates, the method of heating water should be preferred when mixing concrete in winter, but the heating temperature of water should not be higher than 80°. The use of warm water mixing can prevent the concrete from dissipating too quickly and the surface freezing. The aggregate used for mixing concrete should be clean and free from ice and snow blocks and other easily cracked objects. The storage site for aggregate in winter should be on a high terrain and not on the ground. Places with accumulated water; cement shall not be directly heated, and shall be transported into a greenhouse for storage 1 to 2 days before use. The temperature of the greenhouse should be above 5°C.
(3) Ensure that the temperature of the concrete transported to the construction site is above 15 °C, so as to ensure that the temperature of the concrete entering the mold is controlled above 6 °C, and an antifreeze agent with antifreeze performance of -15 °C is added. Chloride antifreeze should not be used in reinforced concrete.
(4) Concrete mixing and transportation

(1) Rinse the mixer with hot water before mixing at the commercial concrete station. The mixing time of concrete is 50% longer than the specified time at room temperature.
(2) Due to the sudden setting phenomenon when cement is mixed with water at about 80°C, when the material is put in, water and sand should be mixed first, and then cement is added.
(3) After the concrete leaves the station, it should be transported to the pouring site in time. During transportation, care should be taken to prevent concrete heat loss, surface freezing, concrete segregation, loss of cement mortar, and slump changes. The tank truck for transportation should be wrapped with thermal insulation materials to minimize the heat loss during transportation, and to ensure that the temperature of the concrete out of the tank during transportation should generally not drop by more than 5 to 6 °C per hour.
(4) Construction site

(1) Insulation measures at the construction site
The scaffolding around the construction floor is used as a wind-shielding measure. The enclosure range is generally within the range of two floors, mainly at the operation floor and the next layer of concrete curing layer. Color strips are used under the floor to block, and the separation lines from the elevator entrance, stair entrance, door and window, post-casting belt, etc. to the construction joint area are blocked.
(2) Concrete pouring
Before the concrete is poured, it is necessary to remove the ice, snow and dirt on the formwork and steel bars, especially the snow and ice at the bottom of the vertical members must be checked again before the concrete is poured. The concrete pump should be set up with a shed and sealed with colored strips to keep the concrete insulated. All pump pipes should be insulated by wrapping the black quilt first, and then wrapping it with a layer of plastic film. Before pouring the concrete construction joints, the cement film and loose stones should be removed, wet and rinsed, and the temperature of the original concrete at the joints should be higher than 2 °C. Continue to pour concrete after a layer of mortar with the same composition as the mortar in the concrete. When pouring concrete in winter, it is necessary to strengthen the vibration of the concrete to improve the compactness of the concrete as much as possible. Vibrating concrete in winter must be mechanically vibrated, and the vibrating time should be longer than that at room temperature.
(3) Test block retention method
In addition to providing standard strength data for the acceptance of concrete projects, concrete test blocks in winter construction also provide a basis for removing thermal insulation and removing formwork. Therefore, when making test blocks, it should be implemented in accordance with the relevant provisions of GB 50204-2015 “Code for Acceptance of Construction Quality of Concrete Structure Engineering”. The number of test blocks is set to three groups, and they are maintained under the same conditions as the construction site. One group is used to test the strength of concrete before freezing to determine the duration of concrete insulation and maintenance, one group is used to test the strength of the demolition of the load-bearing floor; the other group is a 28d standard curing test block. The maintenance conditions of the test block with the same conditions must be consistent with the maintenance conditions of the construction site structure.
(4) Demolition conditions of concrete
The formwork can be removed when the concrete is cooled to 5°C and exceeds the critical strength and meets the requirements of normal temperature concrete formwork removal (the temperature of the concrete is measured by a thermometer). Beam and slab formwork can be dismantled after the concrete strength reaches 75% of the design strength for spans ≤8m, and can be dismantled after the concrete strength reaches 100% of the design strength for spans >8m. Rear can be removed.
(5) Concrete curing measures

The upper part of the roof and beam concrete is insulated by covering the surface of the newly poured concrete with a plastic sheet and then a layer of straw. For weak parts such as corners or windward surfaces, straw mats should be added and lap joints should be made. If the outdoor daily average temperature is 0°C and below, cover the surface of the poured concrete with a layer of plastic film to prevent the covering such as straw from getting wet, then cover the surrounding of the concrete with straw, and finally cover the pit mouth with a layer of plastic to prevent moisture. Enhance thermal insulation to ensure the quality of concrete. The temperature of the concrete entering the mold should be controlled above 6 °C; the initial curing temperature after concrete pouring should not be lower than 5 °C. If the above temperature cannot be reached, the curing water temperature should be increased to meet the requirements.

Post time: Jan-14-2022