Key words: fiber reinforced concrete; Steel fiber; Synthetic fiber; Compound; High performance concrete; sustainable development
0. preface
In less than 200 years since the birth of 1824 cement, the concrete material itself has experienced a continuous development process, from low strength and single variety to high strength and light weight concrete family. However, ordinary concrete material itself has some defects such as low tensile strength, poor ductility, brittle failure under tensile stress or impact load, and its durability such as freeze-thaw cycle, shrinkage resistance and wear resistance is also poor, which greatly affects and hinders the further application of concrete. At the beginning of the 20th century, some people began to mix chopped fibers into concrete to make fiber concrete, which improved the tensile strength and brittleness of concrete. Therefore, fiber reinforced concrete has been widely developed and applied in the following decades. Before the 1990s, the development of concrete simply pursued strength, but the appearance of fiber concrete showed that it developed to high performance and became "green" concrete in 2 1 century. The problems of resources, energy and environment have always been faced by China. Fiber reinforced concrete has superior performance, but it needs more cement and consumes a lot of resources and energy. How to make fiber reinforced concrete take the road of sustainable development remains to be solved.
1. Development of fiber reinforced concrete
Cement-based materials are man-made building materials with a history of 170 years. Strength has always been the main performance index of concrete as an important structural material, and high strength of concrete has been the direction of concrete research and development for many years. However, the inherent weakness of concrete [3]- poor tensile strength, bending strength, impact resistance, explosion resistance and toughness, still limits its advantages, and with the continuous improvement of concrete strength, this weakness is more and more prominent. It is generally believed that the higher the strength of concrete, the worse the toughness, the higher the brittleness, and the smaller the ductility and crack resistance, which brings security risks to the seismic performance of the structure. Therefore, many scholars have been exploring ways to improve the above properties of concrete for a long time. Fiber reinforced concrete is one of the most widely studied and applied important ways in recent years. At present, there are mainly two kinds of fiber concrete: one is short fiber concrete with high elastic modulus, and its representative fiber is steel fiber; The second type is short fiber reinforced concrete with low elastic modulus, and the representative fibers are polypropylene and nylon fibers.
Fiber reinforced concrete is a kind of cement-based composite material with cement slurry, mortar and coarse aggregate as base materials and metal materials, inorganic materials or organic fibers as reinforcement materials. It is a new building material formed by uniformly dispersing short and thin fibers with high tensile strength, high ultimate elongation and high alkali resistance in concrete matrix. The development of fiber reinforced concrete began at the beginning of the twentieth century, especially the research and application of steel fiber reinforced concrete was the earliest and most extensive. As early as 19 10, H. F. Porter [7] of the United States proposed adding short steel fiber into cement and concrete to improve its tensile strength, and published the first paper on steel fiber reinforced concrete. 19 1 1 year, Graham of the United States officially mixed steel fiber into concrete, which preliminarily verified its superiority. 1963, American scholar Romuldi expounded the reinforcement function and mechanism of steel fiber theoretically, which laid a theoretical foundation for the further research and development of steel fiber reinforced concrete and made it leap from the small-scale exploration and experiment stage to the new stage of large-scale development. The United States held special reports on fiber reinforced concrete in 1990 and 199 1, which officially kicked off the research and application of fiber reinforced concrete. From 65438 to 0995, South Korea held a special report meeting on fiber reinforced cement concrete, and from 65438 to 0996, the third international report meeting on cement concrete was held in Beijing, China, indicating that the research and application of fiber reinforced concrete has been internationalized. In foreign countries, fiber reinforced cement concrete composites have been widely used in non-load-bearing components. Domestic research started late. At first, Shanghai Synthetic Fiber Research Institute studied the reinforcement effect of nylon staple fiber on cement concrete, and Anhui Wan Wei Company used vinylon to reinforce concrete.
At present, many developing countries have begun to research and manufacture low-cost fiber cement products with plant fibers as reinforcements [8], and a series of high-performance fiber reinforced cement-based composites have come out one after another, and have been initially applied to pilot projects [9]. Wu Zhongwei, an academician of China Academy of Engineering [1] pointed out that composite is one of the main ways of material development, and the technical idea of composite is super-superposition effect, that is, 1+2 >: 3. Fiber reinforcement plays an important role in composite. 2 1 century, fiber reinforced concrete should develop in the direction of environmental protection, economy and high performance, so that the excellent performance of fiber reinforced concrete will bring greater social and economic benefits to mankind.
2. Mechanical properties of fiber reinforced concrete
The function of fiber in cement matrix is mainly reflected in three aspects: reinforcement, crack prevention and toughening.
2. 1 enhancement
The tensile capacity of concrete is only about one tenth of the compressive capacity, and it often presents brittle failure under external load. It can be clearly seen from the concrete splitting test that the ordinary concrete block suddenly breaks into two pieces when reaching the ultimate load, which belongs to brittle failure (Figure 1). The addition of fiber can effectively improve the tensile strength of concrete. When cracks appear in the matrix concrete, part of the load is transferred to the fiber, thus improving the tensile capacity of the concrete. The failure process of splitting tensile strength test of fiber-reinforced concrete specimen shows good pseudo-ductility. When the ultimate tensile strength is reached, it is not suddenly broken in half, but many microcracks propagate along the main crack near the center line, but the whole block is always bound by fibers (Figure 2).
2.2 Anti-crack effect
When the cement matrix is in plastic state, it is easy to produce micro-cracks, while in the hardening process, shrinkage cracks will expand due to water loss, resulting in new cracks. Adding fiber into cement matrix can prevent the expansion of original cracks and delay the occurrence of cracks, so that the impermeability and frost resistance of composite materials are significantly improved compared with the matrix. After the ordinary cement base material hardens, when the load reaches the cracking load of the base material, the base material cracks rapidly and spreads rapidly along the main crack, leading to brittle fracture across the beam section. However, due to the existence of a large number of chopped fibers, fiber reinforced concrete can still bear the load after the matrix cracks, which makes fiber reinforced concrete show high ductility and has certain signs before failure.
Zhang Yilun and Deng Zongcai respectively studied the early crack resistance of polypropylene fiber and polyacrylonitrile fiber. The experimental results show that the anti-cracking effect of fiber on concrete and mortar is remarkable, and the effect is more obvious with the increase of fiber content in a certain range. Due to the low water cement ratio, the autogenous shrinkage of high performance concrete mainly occurs in the early stage, which will lead to a large number of micro-cracks on the concrete surface. Ba Hengjing [12] found that the autogenous shrinkage of 1 day accounts for 50%-60% of the autogenous shrinkage in 28 days, which is the main reason for the early cracking of high performance concrete. Adding a certain amount of fiber into high performance concrete is an ideal method to solve early cracking. Pang Xinfeng, Tsinghua University [14] studied the influence of modified polyacrylonitrile fiber on the early cracking resistance of high performance concrete by the method of plate test [13], and found that modified polyacrylonitrile fiber and mortar can be well bonded together, and adding 0. 12% fiber can obviously inhibit the early cracking of concrete.
2.3 Improve the deformability of concrete (toughening effect)
When fiber reinforced concrete is in tension (bending), even though a large number of cracks have appeared in the matrix, it can still bear a certain load and has pseudo-ductility, thus obviously improving the toughness and impact resistance of composite materials. This pseudo-ductility of fiber reinforced concrete makes its deformation ability much higher than that of ordinary concrete. Han Rong et al. [15] studied the tensile strain of steel fiber reinforced concrete by contrast test. The test results show that the stress-strain curve of steel fiber reinforced concrete has obvious descending section and good pseudo-ductility.
The toughness of concrete materials, that is, the deformation performance and energy absorption capacity of concrete materials, is very important to concrete structures, especially to improve the seismic capacity of structures, which is of great significance. The research shows that [16], the addition of fiber can significantly improve the flexural toughness of concrete, and with the increase of fiber content, the flexural toughness index and residual strength index of concrete are increasing. In the Wenchuan earthquake in China, many houses were brittle, which did not leave enough time for people to escape. The application of fiber reinforced concrete will be an important means to improve the seismic capacity of structures.
3. The application of fiber concrete
Fiber reinforced concrete has developed rapidly in 100 years since its appearance and is widely used in engineering field. Mainly the application research of steel fiber concrete, carbon fiber concrete and synthetic fiber concrete.
3. 1 steel fiber concrete
Steel fiber reinforced concrete first appeared in the early 20th century, and metal fiber reinforced concrete was used in Russia from 65438 to 0907. 19 10, American H.F.Porter published a research report on short steel fiber concrete.1910, American Grhama mixed steel fiber into ordinary concrete. After decades of development, the theory of steel fiber reinforced concrete [17] has basically matured. A large number of experimental studies show that steel fiber reinforced concrete has ordinary and better mechanical properties and is widely used in engineering. At present, steel fiber reinforced concrete is widely used in several fields: (1) building engineering [19-20]: mainly used for roof waterproofing; The connection between frame structure and frame column of high-rise building improves the seismic capacity of the structure; The roof of Run Shao Yifu Gymnasium of Harbin Institute of Technology is made of steel fiber concrete. (2) Road and Bridge [2 1]: 1997 The Jiazhuqing tunnel of Nanning-Kunming Railway successfully passed through the large fracture and large deformation section due to the selection of steel fiber concrete structure. (3) Water conservancy project: Steel fiber has been applied in Nanyahe asbestos secondary power station, a tributary of Dadu River, and the impact resistance and wear resistance of steel fiber concrete have been well reflected.
However, when the volume ratio of steel fiber exceeds a certain range, it is easy to agglomerate, and the price of steel fiber is relatively expensive, which increases the construction cost and limits the application of steel fiber concrete.
3.2 carbon fiber reinforced concrete
Carbon fiber, which was developed in 1960s, has high elastic modulus and tensile capacity and is generally used for structural reinforcement [22][23]. Carbon fiber has better performance than steel in rigidity and strength. Compared with the benchmark cement-based composite, the cement-based composite with 3% carbon fiber volume content has twice the elastic modulus and five times the tensile strength [24]. However, due to the high price of carbon fiber, the application of chopped fiber in concrete is limited, and it is widely used to strengthen bridges and houses in the form of sheets.
3.3 Synthetic fiber concrete
There are many kinds of synthetic fibers, including polypropylene fiber, polyacrylonitrile fiber and polyvinyl alcohol fiber. Because of its low price, stable chemical properties and excellent tensile properties, it has been widely studied and applied abroad since the 1980s, especially polypropylene fiber. In 1990s, polypropylene fiber was used in Guangzhou-Foshan Expressway Project, Wuhan Yangtze River Bridge Deck Project and Ningbo Baixi Reservoir Concrete Face Rockfill Dam Project. There are many researches on the mechanical properties of synthetic fiber concrete at home and abroad [26][27][28], which improve a lot of data for the application of synthetic fiber concrete. Synthetic fiber has a high cost performance, so it has developed rapidly in China. Mainly used in construction, bridge paving, swimming pools and other projects, the effect is very good.
3.4 Hybrid fiber concrete
Hybrid fiber concrete is a new composite material which combines two or more fibers into cement matrix reasonably, and produces a new composite material which can not only give full play to the advantages of different fibers, but also reflect their synergistic effect. As early as 1970s, Walton[29] and Majumdar abroad began to study the mixed fiber soil, and then more researchers carried out this research.
The research on hybrid fibers in China was relatively late, and there were some related reports until the 1970s. At present, hybrid fiber concrete is a research hotspot, and related achievements have been made. It is not enough to use only one kind of fiber to improve the strength and ductility of concrete, because the greater the volume ratio of fiber, the worse its dispersibility, but it will not play a role in strengthening and toughening. Adding fibers with different properties into concrete can give full play to the properties of various fibers, strengthen and toughen them at different stress stages and different structural levels, which reflects the hybrid effect of the two. Hybrid fiber concrete is a kind of multiphase composite material with many interfaces, and its design theory is not perfect. In recent years, str oven[32] and Nandakumar[33] have applied fracture mechanics to study the reinforcement theory, but further research is needed. The engineering application of hybrid fiber concrete is not much, which has a great relationship with the current research progress. With the deepening of research, the application prospect of hybrid fiber concrete is very good.
3.5 Research on Local Strengthening Members of Fiber Reinforced Concrete
Adding fiber to ordinary concrete can obviously improve the mechanical properties of concrete, but it also increases the cost of concrete. In order to improve the mechanical properties of concrete and minimize the project cost, the concept of local reinforcement is put forward, that is, fiber reinforced concrete is used in the stressed parts of components to improve the working performance of components. Based on the concept of "good steel is used on the blade", fiber reinforced concrete members are being further studied and some preliminary research results have been obtained. The research shows that the local reinforcement of special parts with large bending distance with fiber can improve the bearing capacity of members, reduce the pressure of over-dense steel bars, increase the span of members and reduce the project cost, which is undoubtedly of great engineering significance.
At home and abroad, there is not much research on fiber reinforced concrete local reinforcement members, and the research object is steel fiber. As early as 1980' s, there was research in this field abroad, and some achievements were made [39][40], mainly beams and columns. The use of steel fiber reinforced concrete in beam-column joints not only saves shear reinforcement, but also improves the shear and bending capacity of the joints, improves the fatigue resistance of the joints, and presents good ductility when damaged. Since then, Naaman[38] and others have studied the seismic joints of prestressed concrete structures. The results show that the ductility and energy absorption capacity of this kind of steel fiber locally reinforced joint are obviously higher than that of concrete joint, and it can play a good plastic hinge role in earthquake. Swamy[ 18] has done the local reinforcement test of fiber reinforced concrete for beams, and has carried out the local reinforcement of steel fiber reinforced concrete on the concrete surface in compression zone, tension zone and tension zone. The test results show that the local reinforcement effect of steel fiber reinforced concrete is very obvious. The initial cracking load is increased by 10%, the crack width is reduced, the number is increased, and the deflection is greatly reduced, which shows that the local reinforcement of steel fiber greatly improves the stiffness of the beam. However, his research is limited, and his conclusion needs to be further improved and supplemented. Chinese scholars Gao Danying and Zhao Jun [35-37] continued to study this aspect, and obtained the law of partially strengthening beams with steel fiber concrete: adding steel fiber within 0.3h can improve the cracking moment of members, and the reinforcement effect is consistent with that of adding steel fiber to the whole section. For local reinforced concrete beams with steel bars and steel fibers, when the layer thickness of steel fiber concrete is greater than or equal to hf ≥0.3 h, the steel bars can be completely cut off.
The calculation method of plane steel fiber reinforced concrete beam is used to calculate the cracking moment Mfcr. The reinforcement effect of this kind of fiber reinforced concrete beam is the same as that of fiber reinforcement, but the amount of fiber is greatly reduced and the cost is also greatly reduced.
In China, steel fiber reinforced concrete (SFRC) local strengthening members have been applied in engineering, generally applied to joints of frames, pile tips, key parts of sleepers and so on. , and achieved certain economic benefits.
4. Existing problems
Although fiber reinforced concrete can improve the mechanical properties of concrete to a great extent, the fluidity of concrete in concrete will be reduced due to the large specific surface area of fiber, and more cement slurry is needed to wrap the fiber, which will increase the cement consumption of a single concrete. China is a developing country. According to Wu Zhongwei's estimation, China needs 800 million tons of cement in 20 10, which is close to12 of the world output at that time. Such a huge cement industry is unbearable in terms of energy consumption, material consumption and especially environmental burden. In addition, producing such a large amount of cement will bring great pressure to the environment. The combination of fiber and high performance concrete will be the development trend of concrete in the future: on the one hand, it can solve the early cracking problem of high performance concrete, on the other hand, due to the existence of various fine mineral admixtures, it can improve the working performance of fiber concrete. In order to make the development of concrete in China embark on the road of sustainable development, the development of concrete should be carried out in the following aspects:
(1) Actively develop high-performance concrete, improve the working performance of fiber concrete, reduce the amount of cement, reasonably use industrial waste residue to replace part of cement, and reduce environmental pollution.
(2) Actively explore new materials, compounding is an important means to improve the performance of concrete, improve the scientific level of concrete from the theoretical research of compounding, explore the compounding of cement, cheap fiber and mineral admixture, and give full play to the superposition effect and technical and economic benefits.
(3) Strengthen research methods and combine experiments with theoretical analysis. The research on the mechanical properties of new materials is often carried out through experiments, which leads to the separation of material mechanical analysis and structural mechanical analysis. Therefore, new materials should be applied in the structural model, and software simulation analysis should be carried out to verify the accuracy of the test results, and the theoretical calculation formula should be correctly deduced to better guide the engineering application.
(4) Strengthen the theoretical research on the application of fiber reinforced concrete structures. Fiber reinforced concrete has been applied in engineering. Zhao Jun and others have done some research in this field, but only in steel fiber reinforced concrete. The author is currently studying the local reinforcement of hybrid fiber reinforced concrete. Local fiber reinforced concrete members can greatly reduce fiber consumption, reduce project cost and improve concrete performance, which has great economic benefits.
Chinese researchers should consider the relationship between resources, energy and environment while improving the working performance and mechanical properties of fiber reinforced concrete, so as to make the development of fiber reinforced concrete embark on the road of sustainable development.