In winter construction of concrete, especially in cold areas, in order to prevent early freezing of concrete, it is generally required that concrete should have a higher pouring temperature. For mass concrete, although the surface temperature is low after pouring, the internal temperature rises due to the hydration heat of cement. In order to reduce the internal and external temperature difference and the foundation temperature difference, the lower the pouring temperature, the better. When the pouring temperature exceeds the allowable value, the heat preservation measures after pouring should be strengthened to reduce the temperature difference between inside and outside. According to Code for Construction of Hydraulic Concrete (SDJ- 207- 82), the pouring temperature of concrete should meet the design requirements. The pouring temperature of mass concrete should not be lower than 3℃ in mild areas and 5℃ in cold areas, generally 5℃~8℃.

1. Early winter concrete construction

The temperature is positive during the day and negative at night. At this stage, the construction focuses on the heat preservation of aggregate and the temperature control during pouring. Most of the aggregates used in the current project are natural river pebbles. Before winter construction, all aggregates shall be prepared, piled up in a centralized way, and covered with geomembrane for thermal insulation to reduce temperature loss. Concrete pouring shall be carried out in the daytime as far as possible, so as to improve the pouring temperature of concrete, and the concrete shall be mixed with hot water, so as to minimize the number of transshipment and speed up the pouring. When the outdoor temperature is kept above -5℃, early strength water reducer (different types of concrete early strength water reducer, see the instruction) can be added. Ensure that the early strength of concrete can rise normally. After pouring, the concrete is partially covered with geomembrane or straw curtain to prevent the concrete from being frozen at an early stage.

2. Late stage of concrete winter construction

When the average temperature drops below zero, the pouring and curing conditions of concrete cannot be met, and the working conditions of workers deteriorate, which will affect the quality and construction period of concrete. In order to ensure the smooth progress of the project and the normal pouring of concrete, greenhouses should be built at this stage.

2. 1 shed erection: according to the specific project scale and the situation of the project site, the shed is built with a one-time fixed shed, that is, a shelf is built at the edge of the excavation area with scaffolding poles, the top cover is bound manually with scaffolding poles, and geomembrane is laid on it, so that the pouring bin surface, mixing equipment and used sand and gravel are all covered under the shed, and the inside is heated by a boiler.

2.2 Preparation before pouring: During concrete construction of general building structures in winter, the dirt on the formwork and steel bars should be removed before pouring to prevent the concrete from freezing on the container. Windproof and antifreezing measures should be taken. Once the concrete is frozen, it should be heated and stirred twice to make it have suitable workability before pouring. Horizontal construction joints should not be retained in concrete and reinforced concrete structures of construction projects. If there are any construction joints, the cement film and loose stones on the surface should be removed before continuing pouring, so that the surface is moist and washed clean, so that the surface temperature of the old concrete is higher than +2℃, and then a layer of cement or mortar is laid before continuing pouring concrete. "Chiseling" and foundation cleaning of concrete surface: After the mass concrete of water conservancy and hydropower projects reaches its final setting in normal temperature season, it is generally "chiseled" with a feng shui gun, and the mixed liquid of high-pressure water and high-pressure wind is used to wash the concrete surface to wash off the cement film on the concrete surface and expose the stones.

In winter, this method is limited, because a large amount of water used for washing will wet the insulation material and affect the insulation effect. Combined with the actual construction of this project in the greenhouse, when the concrete reaches the strength of about 2.0Mpa after pouring, use a toothed steel rake to cut hair. Cleaning the base surface: firstly, manually sweep the sundries on the concrete surface with a broom, and then rinse them with water. Heating of old concrete: Under severe cold conditions, the surface temperature (including the side surface) of old concrete exposed for a long time is usually negative. The depth and value of negative temperature vary with exposure time. When pouring new concrete in these parts, if the heating is not enough or the pouring temperature is not enough, the joints between the new and old concrete will freeze at an early stage.

In order to prevent the temperature of old concrete from being too low, the concrete surface can be covered with a straw curtain for thermal insulation after concrete pouring, and the next new concrete pouring can be properly heated.

3. Heating of concrete mixture

By heating the concrete material, after mixing, transporting and pouring, the concrete still has considerable heat and suitable temperature, is not frozen, and hardens under certain curing conditions, and gradually increases to the required strength. Specific requirements, there can be no frozen blocks in sand aggregate. The concrete mixture should have a certain temperature and should not be frozen during transportation, pouring and vibrating.

3. 1 heating of mixing water: water, sand and coarse aggregate in concrete mixing materials constructed in winter generally need to be heated. When heating, water should be given priority to. The heating temperature of water generally does not exceed 80℃, and the limit can reach 100℃, but the feeding sequence should be arranged. Heated water cannot directly contact with cement, so as not to cause rapid coagulation and "false coagulation" of cement. Water heating mode and heating equipment, considering the scale of the project and the situation of the construction site, adopt hot water boiler for heating.

3.2 When water cannot meet the requirements of concrete mixing temperature when heated to the highest temperature, heating aggregate should be considered. The aggregate used in this project is heated by direct steam heating.

3.3 Admixture heating: Admixtures cannot be directly heated by steam, but should be heated in a dilution barrel. Hot water should be used to dilute the admixture, and the temperature of hot water should not lose the effect of the admixture.

3.4 It is not allowed to heat cement in any way.

4. Concrete pouring into the warehouse requires that the concrete warehousing method is related to the equipment conditions of the whole project. But the particularity of concrete construction in winter should be considered as much as possible. When selecting concrete warehousing equipment, the convenience of winter construction should be considered. When concrete is put into storage, it is best to put it into storage at one time. Concrete warehousing can realize continuity, speed up supply, shorten time, reduce temperature loss of concrete and avoid surface freezing.

5. Use antifreeze. Adding antifreeze to concrete in winter construction can make concrete harden at negative temperature and achieve sufficient strength within the specified time, so that concrete can finally reach the same quality level as normal temperature maintenance, which is also one of the concrete construction methods in winter. The content of antifreeze admixture or the concentration of concrete aqueous solution changes with the pouring temperature and curing temperature of concrete. The lower the pouring and curing temperature of concrete, the higher the solution concentration is needed.

6. Maintenance of concrete After pouring, it is necessary to have a certain curing period to gradually increase its strength. Under normal temperature construction conditions, it is enough to keep the concrete surface moist by sprinkling water, but it is difficult to do this under winter construction conditions, so the ambient temperature is particularly important for concrete maintenance in winter construction. Combined with the engineering practice, because the engineering working surface is completely covered under the greenhouse and heated by the boiler, after the concrete is poured, a layer of grass curtain can be covered on the concrete surface to play the role of heat preservation, and water can be sprayed to keep the surface moist. Every link of concrete construction in winter should be closely linked, so all departments should coordinate with each other in the construction process, department leaders should always check and ask questions, and on-site construction personnel should play an active role to make the project hand over satisfactory products to the owner under unfavorable construction conditions.