What are the methods of concrete construction and maintenance in winter?

The biggest difference between winter curing and normal temperature curing of concrete is that the poured concrete needs heat preservation, so that the concrete can still complete its hardening process in the positive temperature environment at the cold atmospheric temperature, thus achieving its due strength index.

On the one hand, it is poured in the cold air temperature environment, on the other hand, it is required to be cured in the positive temperature environment, which requires some measures to make the concrete really be cured in the positive temperature environment. In order to achieve this goal, a series of measures must be taken, which constitutes the method of concrete maintenance in winter. As for what kind of maintenance method to choose, it should be chosen according to the actual situation of the site.

(1) thermal storage method: cover the heat generated by heating concrete raw materials (except cement) and the heat generated by cement hydration with thermal insulation materials, and extend the cooling time of concrete to make the concrete reach or exceed the critical strength before freezing.

This method is widely used in winter construction of construction sites, but it should be noted that when using heat storage method for maintenance, the outdoor temperature should not exceed-15℃, otherwise the cooling time of concrete will be shortened, and the concrete will be frozen before reaching the critical strength, thus destroying the quality of concrete structure.

There are different insulation materials to choose from, such as straw curtain, vermiculite and perlite. However, due to the dry weather and strong wind in winter, there is a fire problem when using flammable insulation materials. At present, thermal insulation quilts made of non-woven fabrics and straws are used in some areas, and the effect is also good, but it must be noted that non-woven fabrics must be flame retardant materials.

No matter what kind of thermal insulation material is used, the thermal insulation structure must be tightly covered to ensure air tightness and water leakage. The insulation thickness at the edges and corners should be 2~3 times that of the general surface, and the temperature should be kept constant. For the wall concrete poured with composite steel formwork, the overall demolition scheme should be adopted. When the concrete reaches 1N/mm2, the side formwork can be gently separated from the concrete, and then closed to continue curing until the formwork is removed.

For the insulation layer covering the formwork and the concrete surface, wet materials should not be used, and the insulation material should not be directly covered on the wet concrete surface. The surface of newly poured concrete should be covered with a plastic film.

(2) Steam heating method: it is to use the heat energy of steam to heat and maintain the newly poured concrete. There are four ways: one way is to add a sealed cover plate on the formwork, and steam is introduced into the cover plate for heating, thus curing the concrete. This method is called steam jacket method, which is suitable for cast-in-place beams, slabs, frame structure walls and columns. Another method is to cover the surface of concrete members with plastic film or canvas film and introduce steam to cure concrete.

This method is called shed cover method, which is suitable for precast beams, slabs and underground foundation trenches. Another way is to install steam circulating pipes in or on the back of the formwork to heat and cure the concrete when pouring vertical structures such as large formwork concrete and column formwork concrete. This method is called hot die method, which is suitable for wall, column and frame structures.

The last method is to reserve a hole in the concrete member, introduce steam into the hole to heat and cure the concrete, and then inject mortar or fine stone concrete after curing to close the exhaust hole. This method is called internal steam ventilation method and is suitable for precast beams, columns, trusses, cast-in-place beams, columns and frame beams.

(3) Electric heating method: the electrodes are inserted into the newly poured concrete at certain intervals, and the electric energy is converted into heat energy by using the resistance of the concrete itself, and the concrete is heated and solidified. Various electric heaters can also be used. Such as resistance wire, electromagnetic induction heater, far infrared heater and electric blanket.

Because the electric heating method is to use electricity to heat, there will be many corresponding requirements, such as what kind of voltage to choose, the distance between the electrode and the steel bar in concrete and so on. , and the specification has specific requirements. When using it in practical work, you can read it in detail.

(4) Greenhouse method: For underground structural engineering and structural engineering with relatively concentrated concrete consumption, the greenhouse method is more suitable. A shed can be built around the concrete to be cured, so that the concrete can gradually harden in a warm environment to meet the design strength requirements.

Extended data

When using steam curing concrete, attention should be paid to:

1) When ordinary portland cement is used, the maximum curing temperature should not exceed 80℃, and when slag portland cement is used, it can be increased to 85~95℃, but when internal steam ventilation is used, the maximum heating temperature should not exceed 60℃.

2) When the whole structure is cured by steam, the dosage of cement should not exceed 350kg/m3, the water cement ratio should be 0.4~0.6, and the slump should not be greater than 50 mm ..

3) When curing concrete by steam heating, the condensed water should be removed and prevented from infiltrating into the foundation soil. When there is a steam outlet, the distance between the nozzle and the exposed surface of concrete should not be less than 300mm.

4) Steam curing concrete can be mixed with early strength agent or air-entraining water reducer, but it is not suitable to mix air-entraining agent or air-entraining water reducer.

5) Don't use bauxite cement for steam curing concrete, because this kind of cement loses too much strength after steam curing.

6) Steam curing method should use low-pressure saturated steam. When there is high-pressure steam on the construction site, it can only be used through a pressure reducing valve or a water supply device.