In order to shorten the construction period, concrete construction must be carried out in winter. However, when concrete construction is carried out in winter, it is easily affected by temperature, which directly affects the overall quality of the project. This is mainly because the form of water will change in low temperature environment in winter, which directly affects the quality of building concrete. Therefore, the construction quality must be strictly controlled when concrete construction is carried out on site. With the in-depth research on concrete construction, it is found that a period of pre-curing time before a new round of concrete pouring can greatly reduce the solid phase in concrete and increase the liquid phase, thus improving the hydration speed of cement and helping to improve the quality of concrete construction in winter. Practice shows that only by adopting appropriate construction techniques and methods can the quality of concrete construction in winter be improved. This paper discusses the key technologies of concrete construction in winter.
1 characteristics and preparation of concrete construction in winter
1. 1 Characteristics of concrete construction in winter
First, due to the low temperature in winter, the strength of concrete increases slowly. In practical engineering, in order to make the strength of concrete reach the standard strength quickly, some thermal insulation measures have been taken. Secondly, for the maintenance of concrete, it is necessary to carry out environmental maintenance at room temperature, which increases the construction cost and makes the construction process more complicated. Thirdly, some quality problems often appear in poured concrete, and most of them will appear in the spring melting period or later period, which has obvious lag. Fourthly, concrete construction in winter will not only encounter the problem of temperature change, but also the problem of water loss, which is caused by the high wind speed in winter.
1.2 Preparation for concrete construction in winter
1.2. 1 technical preparation when formulating relevant construction technical schemes and measures, it is necessary to ensure both construction quality and production safety, and the main contents to be considered should include materials needed for construction, mobilization and rural power plan, equipment deployment, etc. 1.2.2 after the production preparation schedule is determined, it is necessary to plan the construction tasks and site preparation, including equipment insulation and anti-freezing, site leveling and temporary road setting. 1.2.3 resource preparation This link needs to organize admixture materials, thermal insulation materials, construction instruments, do a good job in the inspection and retest of raw materials, and do a good job in the mixture ratio of materials.
2 key construction technology of concrete construction in winter
2. 1 Construction temperature control technology
2. 1. 1 concrete block-by-block layering technology Considering that the concrete pouring area is generally large, in order to avoid the high internal temperature of concrete during on-site construction, it is necessary to pour block by block, and post-pouring belts should be set between concrete blocks to facilitate subsequent construction. When the thickness of concrete to be poured is too thick, it should be poured in stages and layers, and the thickness of each layer should be controlled at about 2m as far as possible. It should be noted that the concrete pouring time of post-cast strip should be carried out after the main concrete temperature is basically stable, so as to avoid joint cracking and affect the construction quality. 2. 1.2 Controlling the pouring temperature of concrete The so-called "pouring temperature" refers to the temperature of concrete after pumping and vibrating after coming out of the mixer, and the "pouring temperature" in the construction site is generally controlled within 28℃. In order to control the pouring temperature of concrete, we need to start from the following aspects: First, we need to control the initial temperature of raw materials before the concrete is opened. The specific operation is that the relevant technicians first measure the initial temperature of each raw material, calculate the discharge temperature of these raw materials, and finally estimate the pouring temperature; Second, because the pouring temperature will be affected by many factors, in practical work, its temperature will often be high or low. When the pouring temperature exceeds the standard value, corresponding measures will be taken, such as covering the concrete pump pipe with straw bags and sprinkling water to cool down. 2. 1.3 When controlling the intermittent concrete pouring of each layer, it is necessary to pay attention to the intermittent concrete pouring between layers. The practice shows that the effect is better when the pouring gap is 5 ~ 7d days, mainly because the internal temperature of bottom concrete reaches the peak and shows a downward trend during this time period. It is worth noting that when the pouring interval exceeds 7d, it is necessary to check the concrete and take corresponding temperature control measures in order to ensure the good combination of the front and back layers of concrete.
2.2 Choose the correct mixture ratio
First, the concrete constructed in winter should have high frost resistance, which is an important means to improve the frost resistance of concrete: first, choose the appropriate cement type, which is generally used in the construction site. Cement is characterized by high hydration heat, so the strength of early release is the greatest; The second is to reduce the water-cement ratio as much as possible, and the specific operation is to increase the dosage of cement, thus increasing the hydration heat to a certain extent, which is convenient to shorten the time required for concrete to reach the critical strength; Thirdly, in the construction site, relevant technicians will also use air entraining agent to improve the frost resistance of concrete. Its principle is to use air entraining agent to generate a certain number of bubbles in concrete under the condition of constant concrete mix ratio, so as to increase the volume of cement slurry, which can not only improve the fluidity of mixture, but also improve the cohesiveness and water retention of concrete to a certain extent.
2.3 Heating and heat preservation of raw materials
In practical engineering, due to the low construction temperature in winter, raw materials need to be heated and insulated, and the following aspects should be paid attention to in the treatment process: First, the temperature of sand and gravel aggregate used should be kept above 0℃, and once the heating water cannot meet the mixing temperature, the aggregate should be evenly heated to ensure that the temperature of the aggregate is within a certain range, which is convenient for construction; Second, the temperature of construction water should not be higher than 80℃. Too high or too low water temperature will directly affect the construction quality. At the same time, direct contact between cement and hot water should be avoided. The specific operation is to add aggregate and hot water first, and then add cement after mixing evenly. Three, the construction site can also use aggregate heating, but the heating temperature should not be higher than 60℃; Fourth, cement cannot be directly heated. When cement needs to be heated, it can be preheated in greenhouse.
2.4 concrete mixing and transportation
First, in order to reduce the heat loss in the process of material transportation, not only should the concrete mixing site be set near the construction site, but also the containers for transportation should be selected correctly, including the shape, size and insulation measures of the containers. Second, before concrete pouring, must ensure that there is no sundry on the template and steel bar. Thirdly, for the concrete used, the frost resistance of the concrete itself should be measured before pouring, and corresponding measures should be taken to prevent frost and keep warm. Fourthly, for projects that need continuous construction, effective measures should be taken to ensure that the concrete reaches the required strength within the specified time. Fifth, strictly control the mixing time of concrete. Too long or too short mixing time will greatly affect the construction quality of concrete. See table 1 for the shortest concrete mixing time. According to table 1, when determining the mixing time of concrete, it needs to be determined according to the slump of concrete itself, the type of mixer used and the mixing capacity.
3 Conclusion
Because winter construction will encounter not only temperature problems, but also wind speed problems, various problems often occur in concrete construction in winter, which makes the process of quality control in winter construction more complicated. In the actual construction process, we should focus on controlling the temperature of concrete. Practice shows that it is necessary not only to strictly control the construction temperature of concrete, but also to choose effective winter concrete construction techniques and methods. Only in this way can the winter construction quality of concrete be improved as a whole.
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