The idea of durability design of concrete for underground engineering

　　The underground engineering of rail transportation needs the durability of concrete, and the purpose of durability design is to ensure the durability of concrete and the overall project completion quality. For this reason, it is necessary to design a series of contents from the design, implementation, use management and maintenance of the project. The main process of design is to firstly design the overall concrete durability target, and then make specific design according to the basic principle of durability design, and analyze the specific measures according to the principle, and the specific measures are divided into the design of the thickness of the protective layer and the design of efficient concrete. On the basis of this, supplementary measures and engineering quality control are carried out, and the design is carried out in terms of repair and maintenance.

　　Current status of the development of durable concrete

　　1 The choice of additives

　　Additives are often used in the production of durable concrete. In the construction of underground projects for rail transportation, polycarboxylate water reducing agents with a value of 26 RP are generally selected to achieve the durability required for underground projects. In the development of durable concrete for underground projects, the polycarboxylate water reducing agent is directly connected to the polymer main chain, which improves the water reduction of the polycarboxylate water reducing agent. The amount of polycarboxylate water reducing agent is related to the water reducing capacity of the water reducing agent, the amount of 1% to 1.5%, the water reduction of concrete can reach the best, and the higher the amount of polycarboxylate water reducing agent, the stronger the water reduction. However, when the amount of polycarboxylate water reducing agent reaches 1.6% and above, the water reducing agent tends to saturate, water reduction can not continue to improve, the application of water reducing agent is not effective. In addition, in the process of using water reducing agent, the greater the proportion of water reducing agent, the strength of concrete will also increase.

　　2 Selection of concrete raw materials

　　Select raw materials with good adaptability of polycarboxylate water reducing agent, raw materials are mainly divided into cement, external admixtures and aggregates. The cement can choose silicate cement, which can have very high adaptability, and in the selection of external admixtures and aggregates, also choose S95 mineral powder and Class II fly ash, which can improve the durability of concrete, and in the selection of aggregates, there are two kinds of fine aggregates and coarse aggregates, fine aggregates are with certain fineness modulus, generally choose yellow sand, fineness modulus is 2.3. coarse aggregates are The choice of crushed stone, the diameter of the crushed stone is generally 25mm or less.

　　3 Concrete proportion

　　After the selection of admixtures and raw materials, the concrete should be proportioned, proportioning is the last link affecting the durability of concrete. In the process of construction of underground rail transit projects, how to maintain the durability of concrete, it requires a special proportion, generally in the construction of underground projects, the need to maintain the quality of the project, underground rail transit slump in 140mm ~ 180mm or so, at this time, should be mixed with mineral powder and fly ash, the use of water to control the amount of 165kg/m3. in order to improve the durability of concrete.

　　The current problems and the solution

　　1 Current problems

　　As the durability of durable concrete is affected by the admixtures, raw materials and matching ratio, the problems of its use also appear more in the above aspects, mainly in the use of high cost of admixtures, not widely used, such as polycarboxylic acid admixtures and poor adaptability of cement, concrete performance by the influence of aggregate fluctuations, durability of concrete early strength is lower than ordinary concrete, the structure of the site maintenance, and The problem of poor adaptation of polycarboxylic acid admixture to cement, the performance of concrete is affected by the fluctuation of aggregate, the early strength of durable concrete is lower than that of ordinary concrete, and the requirements for on-site maintenance and demoulding time are higher.

　　2 Poor adaptability of polycarboxylic acid admixtures and cement

　　In the process of making durable concrete, the use of polycarboxylic acid admixture is very important, and the effect of concrete durability is very much related to the adaptability of polycarboxylic acid admixture and cement. It can be seen that the most adaptable ratio should be polycarboxylic acid admixture 3 and cement 3. 27.92% can be achieved.

　　3 Influence of sand and gravel materials on the durability of concrete

　　Sand and gravel materials are very critical in the use of concrete and can directly affect the strength and durability of concrete. The fine aggregates of sand and gravel materials occupy about 30% of the concrete ratio, which shows the influence of sand and gravel materials in the durability of concrete. The influence of sand and gravel is divided into the following points.

First of all, the ratio of sand and gravel materials, the ratio of sand and gravel materials on the line of concrete tender pumpkin influence, aggregate grading is a direct impact on the early work performance of concrete and the key to the mechanical properties of hardening, the smaller the aggregate mixture of voids, the smaller the surface area of concrete, the better its work performance, and if the aggregate mixture of large voids, the larger the surface area of concrete, its work performance is also reduced.

　　Through experimental research found that the smaller the fineness modulus of sand and gravel, the worse the ability of its grading, even if the use of additives to improve the ratio, will also reduce the slump of concrete, resulting in the reduction of concrete and ease of concrete, it can be seen that the durability of concrete is reduced with the fineness modulus of sand and gravel.

In the mud content, if the mud content of sand and gravel, the greater the quality of the aggregate, the poorer the strength of concrete, while the same ratio, if the mud content is smaller, the stronger the workability and strength of concrete can be seen, the mud content of sand and gravel will directly affect the durability of concrete, if the use of high mud content of stone for concrete mixing, will cause a small slump, the mix of the compatibility of the problem, the mud content reaches the level of the concrete, the mix of the concrete, the mix of the concrete. Poor mud content to a certain extent, will cause the reduction of concrete strength, even if the use of admixtures at this time, can not change the durability of concrete, resulting in reduced durability of concrete, can not meet the use of concrete standards for rail transit underground projects, the quality of engineering construction brings hidden problems.