1. 1 polypropylene anti-crack fiber-Dura fiber has been successfully applied in more than 1000 projects in more than 20 provinces, municipalities and autonomous regions. Mainly used in roads, bridges, airports, subways, industrial and civil buildings, water conservancy projects, 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 pipeline, communication cable pipeline; 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 microfiber is mixed into concrete/mortar, which mainly shows its crack resistance. In fact, because fibers with relatively high elastic modulus are mixed into concrete, the essence of its function is to reduce the brittleness of concrete as much as possible, thus solving some problems caused by inherent defects of concrete, such as easy cracking, 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 the construction, obvious cracks appeared, which affected the appearance and durability of the structure running in corrosive environment, 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. Conditions of fiber action
2. 1 The action condition of fiber can be understood from the external and internal aspects of fiber.
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 electron 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. The function of1.2 fiber depends on the mechanical properties of the fiber itself. 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 the limitation of data, 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.
For another example, in the engineering practice of Dura fiber, there are still many problems of crack resistance and impermeability of concrete that are solved together with UEA. For example, the deck pavement of Wuxi Xicheng Canal Bridge and Yixing Xijiu Bridge, C40P 12 and C50P 12 concrete in the basement of Hubei Publishing Culture City, C60P 12 concrete in the basement of the surgical building of Union Medical College Hospital, and C60P 12 concrete in the large-scale transfer floor of Chongqing World Trade Center, etc. The super-long floor structure of Guangzhou New International Airport Terminal adopts C40 fly ash compensation fiber concrete, which is composed of fly ash, micro-expansive agent and Dura fiber. The anti-cracking effects of the three materials complement each other and give full play to their respective strengths. The construction effect of more than 40,000 m3 concrete is good.
To promote a technology and product, we must adhere to a scientific attitude and seek truth from facts. If we rely on exaggerated publicity, this new technology and new product will have no vitality. At present, there are many synthetic fiber brands applied and promoted in domestic construction projects, both imported and domestic, among which the good and the bad are mixed and vary greatly. Because the production of engineering fiber belongs to the chemical fiber industry, the user is the engineering industry. Users are not familiar with chemical fiber production and can't judge the fiber quality intuitively, but they are generally concerned about whether it can be evenly dispersed. Dispersion is superficial and intuitive, but from the internal quality of fiber, such as tensile strength, tensile limit and holding power, we can't draw a conclusion directly by observation. If you find the problem after using it, it's too late!
Of course, the role of polypropylene fiber in concrete is still an auxiliary role in the final analysis. We usually call it secondary stiffener. Of course, concrete needs normal maintenance in the construction process, and we can't think that there will be no cracks with fiber and ignore normal maintenance. This point needs no elaboration.
3. Appropriate fiber content
3. 1 What is the suitable fiber content? Judging from the material of synthetic fiber, it is beneficial and harmless to concrete in a broad sense because of its acid and alkali resistance and physical toughening and strengthening. However, the theoretical and engineering circles have different opinions on the appropriate dosage. Judging from the nature of the role of synthetic fibers in concrete, as long as synthetic fibers are added, the inherent defect of concrete-brittleness will be solved to varying degrees, so there is nothing wrong with adding them anyway. It seems that it is better to solve a range change with fuzzy theory. But from the point of view of design and construction, there must be quantitative indicators. In addition, considering the workability of concrete, it is necessary to adapt to the relevant requirements of construction.
The content of fiber should be determined according to the purpose of use, and the specific proportion of concrete, aggregate and other additives should be considered. At present, the recommended dosage data of most manufacturers mostly come from the experimental conclusion data of fiber volume dosage of 0.05% and 0. 1% in the theoretical circle. Because the specific gravity of fibers made of different materials is different, it is of course different to calculate the volume content and convert it into the fiber weight content. During the application of Dura fiber in more than 0/000 practical projects in China, according to different requirements, the ratio of concrete/mortar has been 0.5, 0.67, 0.7, 0.8, 0.9, 65,438+0.0, 65,438+0.3, 65,438+0.36 per cubic meter. Generally speaking, the content of crack resistance, impermeability and wear resistance is relatively high; In the use area, the content of ultra-fine sand is relatively high; There should be a special proportion of engineering parts and uses with special requirements. High doses are not necessarily suitable for general engineering applications, except for special purposes.
At present, there is no national technical specification, the engineering application is in the exploration stage, and the theoretical basis of fiber content difference is insufficient. The most reliable way to determine the fiber content is to try and compare according to the engineering purpose. Especially for the structural parts of large-scale projects, it is more necessary to carefully and prudently carry out the trial matching and performance comparison of various fiber contents in order to choose the best scheme.
Aiming at the leakage problem of granite basin bottom in the 35 000 m2 reservoir pool of Henan Huilong Pumped Storage Power Station Project, the comparative tests of 7 proportions of 6. 5438+0. 400 m2 shotcrete were carried out, and the optimum dosage and construction scheme of Dura fiber were determined. The large-area shotcrete construction used in this project is rare in China.
The basement of Jiazheng Commercial Plaza in Guangzhou covers an area of 43,000 square meters, which is not only a large-area ultra-long and ultra-wide structure, but also a complex floor structure, with a floor height difference of about 3.6m, complex engineering geological conditions and abundant groundwater. At the same time, two subway boxes arc through the floor, and three large arc transfer ground beams are designed between the floor and the subway boxes. The design requirements are C35S8, local S 12, no expansion joint, post-pouring belt and reinforcement belt, which require higher crack resistance, impermeability, volume stability, durability, hydration heat and pumpability of concrete. After the demonstration of five trial matching schemes, the Dura fiber of 0.7kg/m3 was determined. After careful construction, although the floor was placed in the open air for more than half a year, it experienced the influence of winter and summer and the change of groundwater level. However, after repeated inspection by all parties, no harmful cracks and leakage were found, and the construction and concrete quality fully met the design requirements.
3.2 Inferior fiber will not only strengthen and toughen concrete, but will have a negative impact. Some people think that poor quality products can make up for their effectiveness by increasing the dosage. If the dosage of inferior products is increased, there will be serious problems such as caking, honeycomb and hollowing, which will not be used at all. At present, there has been a lesson, that is, the poor quality of fiber mixing has caused serious problems in concrete construction, and we have to make up our minds to blow it up. The development of synthetic fiber concrete is to solve the problem of optimizing engineering under new conditions and ensure a long-term solution. Especially for water conservancy and hydropower projects, it has special significance. Appropriate adjustment of fiber content should pay attention to both theory and experience. It is a scientific attitude to combine empirical data with theoretical data to adjust and select the appropriate dose for different engineering sites.
3.3 Fully collect and provide engineering data and laboratory data, and prepare national technical specifications as soon as possible to guide engineering design and application. As we all know, the formulation of a specification is not an easy task, which requires a lot of time, money, data and engineering practice verification. Synthetic fiber concrete is a new technology, which has been successfully applied in many projects, and the empirical data is also of popularization significance at present. The key now is not what value the data takes, but whether it is recognized that this technology can solve the problems that need to be solved in the current project. If it doesn't need to be solved, no matter how good the technology is, there is no need to apply it. It is said that we should "prescribe the right medicine."
4 indicators considering the role of synthetic fibers
The biggest change of fiber to the mechanical properties of concrete is not to improve its compressive strength, flexural strength and other strength indicators, but to greatly improve the fracture energy and ductility of concrete. Using the general indexes of concrete compressive strength and tensile strength to measure synthetic fiber concrete can not fully reflect the improvement of fiber on the basic properties of concrete. As mentioned above, the role of synthetic fiber in concrete/mortar is comprehensive, and it has the dual functions of strengthening and toughening in the case of low content (volume content is about 0. 1%). Especially for high-strength concrete mixed with synthetic fibers, its toughening effect is particularly prominent. Judging from a large number of laboratory data and engineering test data, the compressive strength index has improved and decreased. However, from the perspective of the change amplitude and absolute value, it is not enough to affect the original design requirements of concrete. The reason why some strength indexes are sometimes improved is also to solve the additional effects brought by some cracks.
At present, because the research and application history of synthetic fiber concrete in China is short, and people are used to measuring the performance level of materials by the compressive strength or flexural strength of specimens, so they pay more attention to strengthening indicators and less attention to toughening indicators that can truly reflect the role of fibers, such as bending and tensile toughness, fatigue strength and fatigue life. If the consideration index of the role of synthetic fiber in concrete stays on the original understanding, it will not be conducive to the promotion of synthetic fiber concrete. The toughness index should be quantified, and a unified technical specification suitable for the national conditions should be formulated as soon as possible, so that synthetic fiber concrete can be truly popularized and used in a large area. Fortunately, the theoretical circle has begun to study this aspect. Some experts and scholars have begun to study and consider the toughening effect of synthetic fibers on concrete from the bending toughness index and residual bending strength. Recently, experts from the National Academic Committee of Fiber Reinforced Concrete obtained the complete stress-strain curve of polypropylene fiber reinforced concrete through direct tensile test. When the dosage of Dura fiber concrete is 0.5- 1.3 kg/m3, the tensile strength, ultimate tensile strain and breaking energy of Dura fiber concrete are increased by 12-20%, 37-49% and 33-68% respectively.
5 convenience of synthetic fiber concrete preparation
A large number of engineering practices have proved that the preparation of polypropylene fiber concrete is simple and easy.
(1) Simple physical reinforcement will not have any conflict with various aggregates, admixtures and cement used to prepare concrete. No need to change the original design scale;
(2) There is no special requirement for mixing equipment, and it is easy to operate in the commercial concrete mixing station or on site;
(3) There are no special requirements for the construction technology, no special training is needed, and workers are easy to master.
No matter how good the product is, it is difficult to popularize it if it is complicated in practical use and operation. High-quality fiber lies not only in the realization of function, but also in its simple operation and suitability for large-scale construction. Dura fiber can be popularized rapidly, which is also related to its simple and convenient concrete preparation. Dura fiber is packed in degradable paper bags, which is easy to disperse. When the medium-sized and large-sized mixer is stirring, it can be directly put into stirring without tearing the paper bag. Portable mixer can be put into the mixer at one time according to the dosage, and it is not necessary to shake it off manually. All engineers who have prepared Durafiber concrete/mortar have a deep understanding of this. How can we meet the requirements of large-scale construction by sending people to sprinkle the fiber into the mixer little by little? In order to prevent uneven or difficult mixing and prevent floc agglomeration from being observed directly by naked eyes, some units even reduce the fiber content at will in the process of preparing concrete. It should be said that the realization of functions and the preparation of concrete/mortar are not allowed. The slump of the mixture mixed with fiber will be lost, but the workability of concrete with volume ratio of 0.05-0. 1% can still easily meet the requirements of pumping and pouring, and there should be no special requirements for concrete preparation.
At present, the quality of construction workers in China's construction industry is generally not high, and the construction management is still relatively extensive. For large-scale construction of synthetic fiber concrete, it must be easy to prepare, otherwise subjective problems will affect the preparation and use effect of synthetic fiber concrete, and then affect the large-scale promotion of this technology.
Synthetic fiber concrete has been developed abroad for more than 20 years, and it began to develop in China after the mid-1990s, and was gradually accepted by the engineering community. China is a big country that produces and uses cement concrete, and the application of synthetic fiber concrete has just started. The real popularization and development of synthetic fiber concrete ultimately depends on low-cost domestic fibers. The real improvement of various engineering synthetic fiber products must be based on technological progress. Any quick success or even fraud will affect the healthy development of synthetic fiber concrete, a new technology.
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