With the rapid development of social economy, a large number of urban subway projects have been launched one after another. In subway construction, concrete is the most widely used building material, and its technical quality is also developing rapidly. As a substance to improve and adjust the performance of concrete, concrete admixture has been widely used in subway construction because of its high requirements for concrete performance. Due to the strict requirements of subway engineering and different geological characteristics, subway engineering requires high durability, adjustable setting and light weight for concrete performance. At the same time, due to the subway construction period and other factors, concrete is also required to have the characteristics of easy molding and convenient maintenance. Such standards require that concrete must use additives in order to have various characteristics and meet the needs of subway engineering. At present, concrete admixtures mainly include water reducer, getter, early strength agent and retarder. The scientific use of concrete admixture is helpful to improve the overall quality of subway engineering.
1 Classification and function of concrete admixture
1. 1 Classification of concrete admixtures
In mixing, concrete is more stable, and the concrete additives to improve the performance of concrete are mainly water reducer, air entraining agent and pumping agent. Admixtures for adjusting the setting time of concrete and increasing or decreasing the setting speed of concrete mainly include early strength agent, retarder and accelerator. The concrete admixtures that make the performance of concrete more stable and improve the impermeability and crack resistance of concrete mainly include expansion agent, antifreeze and colorant. Concrete additives, such as rust inhibitor, air entraining agent, waterproof agent, etc., can reduce concrete pressure, protect building steel bars and improve the service performance and life of building steel bars.
1.2 Influence of several main concrete admixtures on concrete
Most concrete admixtures can improve the performance of concrete without adverse side effects. In the process of transportation, the performance of admixture is relatively stable, and it is not easily affected by the environment, so it will not adversely affect the materials and structures such as building steel bars during use. Concrete admixture is relatively safe and has little impact on the environment. 1.2. 1 water reducer. Adding a proper amount of water reducing agent to concrete can increase the slump of concrete and make the fresh concrete have better workability. Moreover, the bleeding water of concrete will be greatly reduced after adding water reducer, and the frost resistance and impermeability of concrete will be greatly improved. In order to avoid the slump loss of concrete and the economy of production cost, mixing method and batching method are generally used to mix water reducer into concrete. But pay attention to the dosage of water reducing agent in concrete. Too much mixing will lead to too long hardening and setting time of concrete, resulting in insufficient early strength of concrete and failure to meet construction standards. 1.2.2 air entraining agent. With the addition of air entraining agent, a large number of bubbles are produced in concrete, which improves the working performance of concrete and greatly reduces the possibility of bleeding and segregation. At the same time, the setting time of concrete is greatly shortened and the strength of concrete is also improved. 1.2.3 antifreeze. Adding antifreeze to concrete can reduce the freezing point of concrete, ensure the hydration of cement and make concrete reach the expected strength quickly.
2 the application of concrete admixture in subway engineering
2. 1 Selection of additives
In subway engineering, the type of concrete admixture should be selected according to the actual construction needs and technical standards, and the materials used, such as the proportion of concrete, the type of cement or the type of gravel, should be considered. Before the actual construction, we should choose the same materials as the actual construction, carry out the trial mixing test of concrete admixture, and adjust the type and proportion of admixture according to the test results. Only after the test results meet the construction requirements and technical standards can they be put into actual construction.
2.2 Additive dosage and adding method
The choice, dosage and mixing method of concrete admixture will have a great influence on the practical performance of concrete. The dosage of concrete admixture should be determined according to cement type, mineral composition, admixture and fineness of concrete. Considering the particularity of subway engineering, the dosage of concrete admixture is also different from that of general construction engineering. Generally, the dosage of organic retarder is 0.03%~0. 1%, inorganic early strength agent is 1%~2%, water reducer is 0. 1%~0.3%, naphthalene series water reducer is 0.2%~ 1.8%, and accelerator is 2. In order to ensure engineering application and prevent mistakes, before putting into practical use, the dosage and dosage of additives should be tested according to technical standards and construction requirements. The way of adding concrete admixture also has a great influence on the use of admixture. Different additives are added in different ways. Sometimes it is necessary to innovate the mixing method according to the actual subway project. In a word, the purpose of adopting different mixing methods is to obtain better concrete performance and meet the actual construction needs. Take water reducer as an example, there are four kinds of mixing methods: one is to mix water first, which is called the first mixing method; The second is to add water reducer while mixing concrete, which is called synchronous method; 3. During the mixing process, the water reducing agent is added 2-3 minutes after the concrete is completely mixed, which is the post-mixing method; Fourth, after the concrete is mixed, it should be added for many times and stirred for many times. This method is called stagnant water method. The performance indexes of concrete obtained by different mixing methods will be very different, and the mixing method needs to be selected according to the actual situation in different use scenarios. At the same time, in the admixture, we should pay attention to the type, dosage and mixing time of cement to avoid the adverse effects of admixture on cement and ultimately affect the performance of concrete.
2.3 the proportion of concrete admixture
In order to ensure the construction quality of subway engineering, before using concrete admixture, the proportion of admixture and cement must be fully tested to check whether the proportion used can meet the engineering standards. According to the test results, adjust the proportion and dosage of cement, sand and admixture. For example, mixing common water reducer into cement needs to reduce the total mass of cement by 5%~8%, while using superplasticizer needs to reduce the total mass of cement by 8%~20%. In the process of mixing, the dosage of cement, water fly ash, sand and other materials should be accurately calculated to control the performance of concrete additives, save costs to the maximum extent, reduce waste and improve the efficiency of subway construction.
2.4 curing of admixture concrete
In subway engineering, the maintenance of admixture concrete is also an important work. The construction period of subway project is long. After entering the winter, in order to make the concrete mixed with antifreeze reach the required strength as soon as possible, the concrete should be covered and insulated to prevent the damage of low temperature to the concrete, causing engineering losses and delaying the construction period and efficiency. Anti-crack concrete and expansive agent concrete are used to prevent shrinkage compensation. Water can be saved. When using additives to produce steam curing concrete products, a reasonable and scientific steam curing system must be adopted to prevent unreasonable steam curing from affecting the products. At the same time, it is necessary to water the concrete in the later period of steam curing, which can ensure the maximum performance and strength of concrete and lay a good foundation for subway construction.
3 Admixture concrete in the use of subway projects often appear problems and solutions
3. 1 concrete cracks and solutions
Concrete cracks are common problems in the construction of subway projects in northern winter. The reason for cracks is that the atmospheric causes of concrete are too different from the surface temperature, and concrete cracks are easy to occur if the thermal insulation and maintenance work is not in place. In order to solve the problem of concrete cracks, heat preservation should be carried out immediately after the initial setting surface of concrete. The general method is to cover the film and keep warm with a quilt to prevent cracking due to excessive temperature difference. By setting up a closed greenhouse, concrete can also be kept warm, and a fire can be made in the greenhouse to raise the temperature of the greenhouse with heating equipment. After thermal insulation, the gap between concrete temperature and ambient temperature is narrowed, which effectively avoids cracks. At the same time, strengthen the monitoring and control of concrete mold temperature, air temperature, concrete surface and internal temperature, so that the temperature difference between concrete surface and concrete interior is always within a safe range, and concrete will not crack due to excessive temperature difference.
3.2 concrete segregation bleeding and fly ash floating and solutions
Concrete segregation and bleeding is also a common problem in subway engineering. The reason for this problem is generally that there is a weighing error when adding concrete admixture or too little judgment on the water content of sand, resulting in too much water or water reducer. The solution is to regularly check the metering device of the mixing station and pay attention to detecting the moisture content of sand. In the process of concrete vibrating, the problem of fly ash floating up often occurs, and black powdery substances are produced on the surface of concrete. In view of the floating problem of fly ash, the vibrating process of concrete should be strictly controlled to prevent excessive vibration and segregation.
3.3 pouring concrete slump loss is too fast and its solution
During the high temperature construction in summer, the slump loss is easy to occur too fast. According to the actual construction experience, the reason for the problem is that the slump loss of concrete is accelerated due to high temperature. The slump loss of concrete at low temperature is very small. When the temperature exceeds 30℃, the slump loss of concrete is greatly accelerated. At the same time, the water content of sand is too high, and the adsorption of polycarboxylic acid water reducer is strong. The fineness of modified silicon powder is greater than that of concrete, and the adsorption of polycarboxylic acid water reducer will make slump loss too fast. The main methods to deal with the rapid slump loss of concrete are as follows: avoid high temperature time as much as possible when pouring concrete, and carry out construction when the temperature drops at night; Strictly select sand to reduce the silt content of sand.
4 development direction of concrete admixture
At present, construction projects are becoming more and more complicated, and the requirements for building materials are getting higher and higher. The market demand for concrete admixtures is increasing, but the requirements for the performance and quality of concrete admixtures will also increase. The following points should be paid attention to in the future development of concrete admixtures: strengthening the in-depth research of concrete admixtures, continuously improving the performance indexes of admixtures, improving the production technology of admixtures, and realizing the serialization and diversified development of concrete admixtures; On the premise of ensuring the performance of additives, reduce the production cost of additives, improve production methods and processes to reduce production costs and enhance market competitiveness; Vigorously develop new and efficient additives. Highlight the composite effect, so that compared with conventional additives, it has more functions, stronger comprehensive performance and wider application scope.
5 concluding remarks
Concrete admixture not only plays an important role in subway construction, but also has important significance in most engineering projects. The research and development of concrete admixture is a complex system engineering. The construction unit should closely combine practice and theory, conduct in-depth research, and constantly solve the problems found in actual use, so as to contribute to the improvement of the performance of concrete admixture products and the research and development of new admixture products.
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