What are the requirements for cement, aggregate, and concrete pouring during winter construction?
Concrete construction in winter, especially in cold areas, generally requires a higher pouring temperature of concrete to prevent early freezing of concrete. For large-volume concrete, although its surface temperature is low after pouring, the internal temperature rises due to the heat of cement hydration. In order to reduce the temperature difference between inside and outside and the foundation temperature, the lower the pouring temperature, the more advantageous it is. If the pouring temperature exceeds the allowable value, insulation measures after pouring should be strengthened to reduce the temperature difference between inside and outside. According to the "Hydraulic Concrete Construction Specifications" (SDJ-207-82), the pouring temperature of concrete should meet the design requirements. The pouring temperature of large-volume concrete should not be lower than 3℃ in mild areas, and should not be lower than 5℃ in cold areas, generally 5℃~8℃.
1. Early stage of concrete construction in winter
The temperature during the day is positive and the temperature at night is negative. The focus of this stage of construction is the insulation of aggregates and temperature control during the pouring process. Most of the aggregates used in the current project are natural river pebbles. Before entering the winter construction period, all the aggregates will be prepared, stacked centrally, and covered with geomembranes for thermal insulation to reduce temperature loss. Concrete pouring should be carried out during the day as much as possible to increase the pouring temperature of the concrete. Mix the concrete with hot water and minimize the number of reverse operations to speed up the pouring. When the outdoor temperature remains above -5°C, early-strength water-reducing admixture can be added (for different types of concrete early-strength water-reducing admixtures, please refer to the instructions for usage details). To ensure that the early strength of concrete can rise normally. After pouring, the concrete part should be covered with geomembrane or straw curtain for thermal insulation to prevent early freezing of the concrete.
2. Late winter construction of concrete
When the average daily temperature drops below zero, concrete pouring and curing conditions cannot be met, and workers' working conditions deteriorate, which will affect the quality and construction period of the concrete. In order to ensure the smooth progress of the project and the normal pouring of concrete, the construction method of setting up a greenhouse should be used at this stage.
2.1 Erection of the greenhouse: According to the scale of the specific project and the on-site conditions of the project, the erection of the greenhouse is a one-time fixed greenhouse, that is, using scaffolding poles to build a shelf at the edge within the excavated area. The top cover is tied manually with scaffolding poles, and a geomembrane is laid on it. The pouring bin surface, mixing equipment, and all the sand and gravel materials used are all covered under the greenhouse, and the interior is heated with a boiler.
2.2 Preparation work before pouring: In winter concrete construction of general building structure projects, dirt on the formwork and steel bars is required to be removed before pouring to prevent the concrete from freezing on the container. Windproof and anti-freeze measures must be installed. Once the concrete is frozen, secondary heating and mixing must be carried out to make it suitable for workability before pouring. Horizontal construction joints should not be left in concrete and reinforced concrete structures of construction projects. If there are construction joints, before continuing pouring, the cement film and loose stones on the surface should be removed, the surface should be moistened and washed clean, so that the surface temperature of the old concrete is higher than +2℃, and then a layer of cement or mortar should be laid before continuing. Pouring concrete. "Chipping" and cleaning the concrete surface: For large-volume concrete in water conservancy and hydropower projects, after it is poured in the normal temperature season and reaches final setting, a feng shui gun is generally used to "blast" the surface of the concrete with a mixture of high-pressure water and high-pressure wind. The cement film on the surface of the concrete is washed away, exposing the stones.
In winter, this method is limited because the large amount of water used for flushing will wet the insulation material, affecting the insulation effect. Based on the actual construction of this project in a greenhouse, when the concrete reaches a strength of about 2.0Mpa after pouring, use a toothed steel rake to roughen it. To clean the base surface, first use a manual broom to sweep away the debris on the concrete surface, and then rinse it with water. Heating of old concrete: Under severe cold conditions, the surface temperature (including the sides) of old concrete exposed for a long time is usually negative. The depth and magnitude of negative temperatures vary with the length of exposure. When new concrete is poured in these parts, if it is not heated enough or the pouring temperature is not sufficient, the joints between the old and new concrete will suffer from early freezing.
In order to prevent the temperature of the old concrete from being too low, the concrete surface can be covered with straw curtains to keep it warm after the concrete is poured, and it can be heated appropriately when the next new concrete is poured.
3. Heating of concrete mixture
By heating the concrete material, the concrete will retain considerable heat and have an appropriate temperature after mixing, transportation and pouring. It will not be damaged by freezing, and will harden under certain maintenance conditions and gradually grow to the required strength.
Specific requirements are that there should be no frozen blocks in the sand and gravel aggregate. The concrete mixture should have a certain temperature and should not be frozen during transportation, pouring and vibration.
3.1 Heating of mixing water: The water, sand and coarse aggregate in concrete mixing materials for winter construction generally need to be heated. When heating, water should be heated first. The heating temperature of water generally does not exceed 80°C, and the limit can reach 100°C, but the order of feeding should be arranged. The heated water cannot be in direct contact with the cement to avoid rapid setting and "false setting" of the cement. As for the water heating method and heating equipment, taking into account the scale of the project and the conditions of the construction site, hot water boilers are used for heating.
3.2 When heating water to the highest temperature cannot meet the requirements of concrete mixing temperature, heating of aggregates should be considered. The sand and gravel materials used in this project are heated by steam direct heating method.
3.3 Heating of admixtures: Admixtures must not be heated directly with steam. Heating of admixtures should be carried out in a dilution barrel. Hot water should be used to dilute admixtures. The temperature of the hot water should not be higher than the temperature of the admixtures that would cause the loss of admixtures. The effect is limited.
3.4 Cement is not allowed to be heated in any way.
4. Concrete pouring and warehousing requirements. The concrete warehousing method is related to the conditions of the entire project construction equipment, 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 taken into account. It is better to put concrete into the warehouse at once. It can achieve continuity, speed up the supply of materials and shorten the time. This can reduce the temperature loss of concrete and avoid surface freezing.
5. Use of antifreeze. Antifreeze is added to concrete during winter construction so that the concrete can harden at negative temperatures and reach sufficient strength within the specified time, so that the concrete can eventually reach the same level as normal temperature curing. With the same level of quality, this is also one of the best winter construction methods for concrete. The amount of antifreeze admixture or the concentration of the aqueous solution used to prepare concrete varies with the pouring temperature and curing temperature of the concrete. The lower the temperature at which concrete is poured and cured, the higher the solution concentration required.
6. Curing of concrete. After concrete is poured, it needs 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 for curing. But under normal temperature construction conditions, It is difficult to achieve this under winter construction conditions, so the ambient temperature is particularly important for the health of concrete in winter construction. Based on the actual project, since the entire working surface of the project is covered under the greenhouse and heated by a boiler, after the concrete is poured, a layer of straw curtain can be covered on the concrete surface for thermal insulation, and water can be sprinkled to keep the surface moist. All aspects of concrete construction in winter must be closely connected, so during the construction process, various departments must coordinate with each other, department leaders must always check and ask questions, and on-site construction personnel must play an active role to ensure that the project delivers satisfactory results to the owner under adverse construction conditions. product.