Keywords: structural website construction; 1; ; ; ; ; ; ; ; ; ; The essence of fiber action is1.1; After years of popularization, Dura fiber, a polypropylene anti-crack fiber, has been successfully applied in 1000 projects in more than 20 provinces, municipalities and autonomous regions. Mainly used in roads, bridges, airports, subways, civil buildings, engineering, prefabricated components, thermal insulation materials, dry powder mortar and so on. Such as special sections of expressway toll stations; Soft base pavement; Pavement of large bridges, overpasses and elevated roads; Bridge maintenance; Highway repair; Basement floor, side walls and retaining walls of buildings; Outdoor and indoor parking lots and driveways; Airport apron and hangar; The roof and ceiling of the master; Floor slab and stair slab; Mass concrete such as transfer floor girder, super-large beam and column, radiation-proof wall of linear accelerator, oil depot foundation, spillway pier, petrochemical coke tower frame, wind power tower foundation, etc. High strength concrete filled steel tubular column: thin-walled structure; Equipment foundation; Swimming pool, water storage pool, sewage pool; Sewage pipes and cable pipes; Tennis courts and basketball courts; Large garbage dump; Nuclear waste landfills and nuclear waste storage containers; Roads in residential areas; Plastering of internal and external walls of industrial and civil buildings; Interior decoration; Scouring and grinding concrete in canals, flood discharge tunnels, etc. Water conservancy dike; Underground foundation of subway and light rail; Tunnel; Culverts and slope protection; Workshop, bridge reinforcement and maintenance. There are many important large-scale and typical projects such as Shenzhen Civic Center, Shenzhen Convention and Exhibition Center, Shenzhen Metro, Shenzhen Swimming and Diving Hall, Chongqing Chaotianmen Square, Chongqing Yuhai Wangdi Square, Chongqing World Trade Center, Chongqing Airport, Chongqing Yu 'ao Bridge, Chongqing huanghuayuan bridge, Chongqing Shibanpo Bridge, Guangzhou New Airport, Guangzhou Metro, Guangzhou New China Tower, Guangzhou Minghui Mall, Guangzhou Jiazheng Square, Beijing-Zhuhai Expressway and Hubei Publishing. Since 1999 waterproof experts wrote polypropylene fiber represented by Dura fiber into the Atlas of Building Waterproof Structure in Shenzhen, Guangzhou, Beijing and other places have adopted the specifications, dosage and practices of Dura fiber in lightweight panel wall engineering and thermal insulation engineering on the basis of a large number of engineering practice data and expert argumentation, and incorporated the use of polypropylene fiber into local technical regulations. Since then, the engineering practice has been continuously expanded, and many other brands of engineering fibers have been widely popularized and applied, which has opened up a good development momentum for synthetic fiber concrete in China. Dura fiber has been successfully applied in different types of projects and different regional climatic conditions. What is the essence of synthetic fiber for engineering? How to treat the function of synthetic fiber? With the fierce competition of many brands of synthetic fibers in engineering, many problems have been raised on this issue. Some manufacturers are one-sided in promoting the function of fiber. It seems that as long as fibers are added to concrete/mortar, cracks will no longer exist, which violates the mechanism of fiber action and ignores the individual conditions of specific projects. The main object of synthetic fiber solution is the early primary cracks of concrete, and it is wrong to exaggerate the inhibitory effect of synthetic fiber on cracks infinitely. In fact, adding fiber into concrete/mortar can only prevent the cracking of non-structural cracks, and it is impossible to completely eliminate them. 1.2 ; The main feature of mixing microfibers in concrete/mortar is its crack resistance. In fact, because the fiber with relatively high elastic modulus is mixed into the concrete, its essence is to reduce the brittleness of concrete as much as possible, thus solving some problems caused by the inherent defects of concrete-easy to crack due to brittleness, which plays an important role in improving the internal structure of concrete/mortar. This effect is different from general reinforcement, but a fundamental improvement of the defects of concrete/mortar itself. These include effectively increasing the toughness of concrete; Reduce cracks and improve impermeability; Reduce cracks and delay the corrosion of steel bars; Reduce the chemical erosion of concrete structures; Enhance frost resistance and reduce the damage of concrete structure; Reduce concrete bleeding and improve the quality of surface concrete; Reduce cracks, improve wear resistance and impact resistance. This influence is not reflected by several strength indexes, but by many indexes, especially the durability of concrete. Synthetic fiber concrete has become the real significance of keen research in domestic theoretical circles, and it also lies in how to truly reveal and measure the essence of fiber's action on concrete. It is precisely because we revealed the essence of its function in the process of popularizing Dura fiber that the application of Dura fiber gradually shifted from simple external wall treatment to many important structural parts with high technical difficulty, crack resistance, impermeability, wear resistance, impact resistance and high seismic requirements. Representative ones are: C50 concrete rigid self-waterproof structure of the main structure in front of Guangzhou Metro Park Station, C30S8 large-scale pumping concrete construction of the basement floor and external wall of Shenzhen Citizen Center with a depth of 23m, with a total height of168m, which is used for cracking resistance of C50 concrete of KTL joists and ring beams of box transfer floor. The total amount of concrete in Shenzhen Baoan Gymnasium is 35,000 m3, and the total amount of Durafiber concrete is 1700 m3, which is used for basement floor, beam and slab, prestressed beam and slab, retaining wall, fire pool, post-pouring belt, etc. The impermeability is improved by 60-80%, and good engineering effect is achieved. In the project of TCL Industrial Research Building in Shenzhen, in order to meet the needs of prestressed concrete structure design, C60 concrete with Dura fiber crack resistance was used for cantilever beam, which increased the tensile strength by about 50% compared with ordinary C40 concrete. The transfer floor of Shenzhen Children's Palace's Juvenile Mountain Back Garden adopts steel-concrete composite structure, with complex beam-column joints, large steel content and difficult concrete pouring. Dura fiber is added to ensure the quality of concrete. The C70 and C80 concrete-filled steel tubular columns in the basement of Guangzhou New China Building and the raft with a thickness of 600mm and 8000m2 are crack-resistant. The radiation protection walls of linear accelerators in Zhengzhou, Xinxiang and Wuhan hospitals in Henan Province are crack-resistant. Chongqing, Shenzhen, Beijing, Wuhan and other places swimming, diving pool crack prevention, seepage prevention. A large number of highway toll stations such as Beijing-Zhuhai Expressway and Guangzhou New Airport Expressway are wear-resistant and impact-resistant. Anti-crack and anti-seepage engineering of underground foundation of Guangzhou and Shenzhen subways. Crack resistance and impermeability of complex structures of several super-large basements in Guangzhou, Shenzhen and Wuhan. Chongqing, Gansu, Jiangsu, Heilongjiang, Jilin, Guangdong, Henan, Jiangxi, Hubei and other places have a large number of bridge deck pavement and bridge stress columns and box girders. Anti-crack engineering of mass concrete with a large number of transfer floors in various places. Anti-cracking project of large-volume frame of petrochemical coke tower in Hunan, Xinjiang and Jiangsu. Application of mass concrete and anti-wear and anti-seepage concrete in many water conservancy projects in Inner Mongolia, Henan and Hunan. Numerous successful application examples have verified the role of synthetic fiber in concrete, and it has been welcomed by the engineering community as an indispensable additive material for concrete crack resistance. Concrete is the most widely used building material and the most important structural material in engineering. Reinforced concrete structure has become the most widely used structural form in the world. At present, China is carrying out unprecedented large-scale infrastructure construction, but many concrete structures, including bridges, roads, tunnels, ports, dams, buildings and so on. During or shortly after construction, obvious cracks appeared, which affected the durability of appearance and structure in erosion, and also affected the use function of some structures, exposing serious durability problems, and its service life was lower than the design life standard. Only by seriously solving the durability problems of various concrete structures can we make full use of resources. Only by extending the service life of all kinds of buildings as much as possible, delaying the threat to structural safety caused by the passage of time and ensuring their normal use can we save the reconstruction and repair costs as much as possible. Popularizing synthetic fiber concrete in concrete structures can not only solve some problems brought by the development of buildings to tall, large and complex structures, but also become an important means to solve the durability of structures. 2; ; ; ; ; ; ; ; ; ; The condition of fiber action is 2.1; The action conditions of fibers can be understood from the outside and inside of fibers. 2. 1. 1; Appearance: it can be understood from two aspects: the shape of fiber in concrete/mortar and the relationship between fiber and aggregate. Whether fibers can be randomly and evenly distributed in concrete/mortar is the key to whether fibers can play a role. No matter how to explain the mechanism of fiber action, it is necessary to ensure the uniform and random distribution of fiber in concrete/mortar in order to play a role. Micro-cracks are blocked by fibers in the development process, which consumes energy and is difficult to develop further, thus blocking stress and achieving anti-cracking effect. Because the fiber surface is treated by different activators in the production process, the fiber is evenly dispersed when it meets water, and the external force is mixed with various aggregates of concrete, further combining the fiber with various aggregates. Dura fiber is easy to disperse evenly, which is recognized by everyone who has used this product. We usually put a small amount of fiber into clear water in a transparent cup and stir it. It can be intuitively found that Dura fiber is scattered in a three-dimensional suspension state, and it will not change much after a long time. However, some similar products may disperse after stirring, but will soon float into a flocculent layer. The fiber reflecting the latter situation is not easy to disperse evenly in the actual preparation process of concrete/mortar. This observation method is similar to the method of "fiber high stability rate" proposed by some people. [1] Because the density of polypropylene fiber is less than that of water and fiber surfactant, the fiber dispersed in water will gradually show obvious delamination and segregation due to buoyancy and surface activation energy. Put different brands of short fibers into a measuring cup, stir and let stand, measure the suspension height at different time periods, compare their stability, and judge the dispersion of fibers. The grip of fiber on aggregate is another key to whether it can play a role. Fibers can keep aggregate as much as possible to avoid being pulled out when stressed. Different fibers have different standards. Under the microscope, we can see that they hold different aggregates. If there is no loss of slump of concrete after adding fiber, the fiber will either be poorly dispersed or poorly held, and the function of fiber is impossible to talk about. 2. 1.2; The function of fiber lies in its mechanical properties. Such as tensile strength, tensile limit, fiber uniformity, acid and alkali corrosion resistance, ultraviolet aging ability, etc. According to fiber experts, the tensile strength is inversely proportional to the tensile limit. This relationship should be appropriate, and it is not the extremely high tensile strength of fibers that can produce high crack resistance. Fibers undergo tensile deformation during stretching. The ratio is not appropriate, and the tensile strength can't meet the requirements. Of course, due to manufacturing constraints, the data can only meet the requirements as much as possible. The tensile strength of polypropylene fiber is too large, which may lead to increased brittleness. If the tensile limit is too large, the fibers in concrete/mortar may not be able to control cracks during deformation. It is understood that the tensile limit of Dura fiber is about 15%, which is close to natural fiber and needs certain control technology to produce it. The modification of fiber is also shown in this respect. The tensile limit index is also an index to measure whether the fiber crack resistance can really realize its function. 2. 1.3 ; To truly understand the characteristics, advantages and disadvantages of each material, it is not feasible to emphasize that one material excludes another. Materials are constantly changing, so we should constantly know and use new materials. Only by giving full play to the composite effect of materials can we comprehensively solve the problems encountered in engineering. For example, the mixture of steel fiber with high elastic modulus and polypropylene with low elastic modulus can play different roles in the process of concrete failure. Because of its large quantity and performance characteristics, polypropylene fiber mainly restricts the early primary cracks and micro-cracks of concrete and plays a role under low tensile stress; The amount of steel fiber is small, but it has obvious reinforcement effect and obvious effect on preventing macro cracks. Two kinds of fibers can inhibit the generation and propagation of concrete cracks at different stages, improve the tensile strength and flexural strength of concrete, and combine the energy absorption advantages of two kinds of fibers with different elastic modulus to produce synergistic effect on internal defects of concrete, which can not only effectively strengthen but also effectively toughen. Another example is adding fly ash or silicon powder in the application of hydraulic concrete to increase impact resistance, wear resistance and crack resistance. According to the proportioning test of the Experimental Center of the Yellow River Committee, when 20% fly ash and Dura fiber are added, the wear resistance is improved by 6- 18% when the dosage is 0.6/0.9/10.2 kg/m3 respectively. The experiment of Nanjing Institute of Water Sciences proves that adding polypropylene fiber and silica powder can effectively improve the wear resistance of concrete by 33-58%. [2] We also proved this point in the practice of the spillway tunnel project of Halaqin Reservoir in Inner Mongolia.