Steel fiber reinforced concrete (SFRC) is a new type of composite material which can be poured and sprayed by adding an appropriate amount of short steel fiber into ordinary concrete. It has developed rapidly at home and abroad in recent years. It overcomes the shortcomings of low tensile strength, small ultimate elongation and brittle property of concrete. It has excellent properties such as tensile strength, bending resistance, shear resistance, crack resistance, fatigue resistance and high toughness. It has been applied in hydraulic engineering, road and bridge, construction and other engineering fields.

1. Development of steel fiber reinforced concrete
Fiber reinforced concrete (FRC) is the abbreviation of fiber reinforced concrete. It is usually a cement-based composite composed of cement paste, mortar or concrete and metal fiber, inorganic fiber or organic fiber reinforced materials. It is a new building material formed by uniformly dispersing short and fine fibers with high tensile strength, high ultimate elongation and high alkali resistance in the concrete matrix. Fiber in concrete can limit the generation of early cracks in concrete and the further expansion of cracks under the action of external force, effectively overcome the inherent defects such as low tensile strength, easy cracking and poor fatigue resistance of concrete, and greatly improve the performance of impermeability, waterproof, frost resistance and reinforcement protection of concrete. Fiber reinforced concrete, especially steel fiber reinforced concrete, has attracted more and more attention in academic and engineering circles in practical engineering because of its superior performance. 1907 Soviet expert B П. Hekpocab began to use metal fiber reinforced concrete; In 1910, H.F. Porter published a research report on short fiber reinforced concrete, suggesting that short steel fibers should be evenly dispersed in concrete to strengthen matrix materials; In 1911, Graham of the United States added steel fiber into ordinary concrete to improve the strength and stability of concrete; By the 1940s, the United States, Britain, France, Germany, Japan and other countries had done a lot of research on using steel fiber to improve the wear resistance and crack resistance of concrete, the manufacturing technology of steel fiber concrete, and improving the shape of steel fiber to improve the bonding strength between fiber and concrete matrix; In 1963, J.P. romualdi and G.B. Batson published a paper on the crack development mechanism of steel fiber confined concrete, and put forward the conclusion that the crack strength of steel fiber reinforced concrete is determined by the average spacing of steel fibers which plays an effective role in tensile stress (fiber spacing theory), thus starting the practical development stage of this new composite material. Up to now, with the popularization and application of steel fiber reinforced concrete, due to the different distribution of fibers in concrete, there are mainly four types: steel fiber reinforced concrete, hybrid fiber reinforced concrete, layered steel fiber reinforced concrete and layered hybrid fiber reinforced concrete.

2. Strengthening mechanism of steel fiber reinforced concrete
(1)Composite mechanics theory. The theory of composite mechanics is based on the theory of continuous fiber composites and combined with the distribution characteristics of steel fibers in concrete. In this theory, composites are regarded as two-phase composites with fiber as one phase and matrix as the other phase.
(2)Fiber spacing theory. Fiber spacing theory, also known as crack resistance theory, is proposed based on linear elastic fracture mechanics. This theory holds that the reinforcement effect of fibers is only related to the uniformly distributed fiber spacing (minimum spacing).

3. Analysis on development status of steel fiber reinforced concrete
1.Steel fiber reinforced concrete. Steel fiber reinforced concrete is a kind of relatively uniform and multi-directional reinforced concrete formed by adding a small amount of low carbon steel, stainless steel and FRP fibers into ordinary concrete. The mixing amount of steel fiber is generally 1% ~ 2% by volume, while 70 ~ 100kg steel fiber is mixed in each cubic meter of concrete by weight. The length of steel fiber should be 25 ~ 60mm, the diameter should be 0.25 ~ 1.25mm, and the best ratio of length to diameter should be 50 ~ 700. Compared with ordinary concrete, it can not only improve the tensile, shear, bending, wear and crack resistance, but also greatly enhance the fracture toughness and impact resistance of concrete, and significantly improve the fatigue resistance and durability of structure, especially the toughness can be increased by 10 ~ 20 times. The mechanical properties of steel fiber reinforced concrete and ordinary concrete are compared in China. When the content of steel fiber is 15% ~ 20% and the water cement ratio is 0.45, the tensile strength increases by 50% ~ 70%, the flexural strength increases by 120% ~ 180%, the impact strength increases by 10 ~ 20 times, the impact fatigue strength increases by 15 ~ 20 times, the flexural toughness increases by 14 ~ 20 times, and the wear resistance is also significantly improved. Therefore, steel fiber reinforced concrete has better physical and mechanical properties than plain concrete.

4. Hybrid fiber concrete
Relevant research data show that steel fiber does not significantly promote the compressive strength of concrete, or even reduce it; Compared with plain concrete, there are positive and negative (increase and decrease) or even intermediate views on the impermeability, wear resistance, impact and wear resistance of steel fiber reinforced concrete and the prevention of early plastic shrinkage of concrete. In addition, steel fiber reinforced concrete has some problems, such as large dosage, high price, rust and almost no resistance to burst caused by fire, which has affected its application to varying degrees. In recent years, some domestic and foreign scholars began to pay attention to hybrid fiber concrete (HFRC), trying to mix fibers with different properties and advantages, learn from each other, and give play to the "positive hybrid effect" at different levels and loading stages to enhance various properties of concrete, so as to meet the needs of different projects. However, with regard to its various mechanical properties, especially its fatigue deformation and fatigue damage, deformation development law and damage characteristics under static and dynamic loads and constant amplitude or variable amplitude cyclic loads, the optimal mixing amount and mixing proportion of fiber, the relationship between components of composite materials, strengthening effect and strengthening mechanism, anti fatigue performance, failure mechanism and construction technology, The problems of mix proportion design need to be further studied.

5. Layered steel fiber reinforced concrete
Monolithic fiber reinforced concrete is not easy to mix evenly, the fiber is easy to agglomerate, the amount of fiber is large, and the cost is relatively high, which affects its wide application. Through a large number of engineering practice and theoretical research, a new type of steel fiber structure, layer steel fiber reinforced concrete (LSFRC), is proposed. A small amount of steel fiber is evenly distributed on the upper and lower surfaces of the road slab, and the middle is still a plain concrete layer. The steel fiber in LSFRC is generally distributed manually or mechanically. The steel fiber is long, and the length diameter ratio is generally between 70 ~ 120, showing a two-dimensional distribution. Without affecting the mechanical properties, this material not only greatly reduces the amount of steel fiber, but also avoids the phenomenon of fiber agglomeration in the mixing of integral fiber reinforced concrete. In addition, the position of steel fiber layer in concrete has a great impact on the flexural strength of concrete. The reinforcement effect of steel fiber layer at the bottom of concrete is the best. With the position of steel fiber layer moving up, the reinforcement effect decreases significantly. The flexural strength of LSFRC is more than 35% higher than that of plain concrete with the same mix proportion, which is slightly lower than that of integral steel fiber reinforced concrete. However, LSFRC can save a lot of material cost, and there is no problem of difficult mixing. Therefore, LSFRC is a new material with good social and economic benefits and broad application prospects, which is worthy of popularization and application in pavement construction.

6. Layered hybrid fiber concrete
Layer hybrid fiber reinforced concrete (LHFRC) is a composite material formed by adding 0.1% polypropylene fiber on the basis of LSFRC and evenly distributing a large number of fine and short polypropylene fibers with high tensile strength and high ultimate elongation in the upper and lower steel fiber concrete and the plain concrete in the middle layer. It can overcome the weakness of LSFRC intermediate plain concrete layer and prevent the potential safety hazards after the surface steel fiber is worn out. LHFRC can significantly enhance the flexural strength of concrete. Compared with plain concrete, its flexural strength of plain concrete is increased by about 20%, and compared with LSFRC, its flexural strength is increased by 2.6%, but it has little effect on the flexural elastic modulus of concrete. The flexural elastic modulus of LHFRC is 1.3% higher than that of plain concrete and 0.3% lower than that of LSFRC. LHFRC can also significantly enhance the flexural toughness of concrete, and its flexural toughness index is about 8 times that of plain concrete and 1.3 times that of LSFRC. Moreover, due to the different performance of two or more fibers in LHFRC in concrete, according to the engineering needs, the positive hybrid effect of synthetic fiber and steel fiber in concrete can be used to greatly improve the ductility, durability, toughness, crack strength, flexural strength and tensile strength of the material, improve the material quality and prolong the service life of the material.

——Abstract (Shanxi architecture, Vol. 38, No. 11, Chen Huiqing)