Research On The Ductility Of The Steel Fiber Reinforced Concrete And Its Application In The Underground Structure Calculation

The steel fiber reinforced concrete (SFRC) has a higher tensile strength, bending strength, resistance to fracture, ductility and resistance to impact compared with normal concrete, which the concept of the concrete being the brittle material may be changed. In the application of tunnel supporting system, the use of SFRC can decrease the thickness of lining, substitute welded wire reinforcement, decrease the construction and maintenance cost, improve the impermeability,cracking resistance of the lining, and increase the life span of the structure.The research of the SFRC about the mechanics was focused on the theory of strength in the past; however, there were not enough research on ductility and cracking resistance. This paper investigated the ductility of the SFRC after cracking by four points bending tests. The digital camera technology is used in the test to measure the relationship between crack width w and crack height h'. Furthermore, the load and crack width data is used to obtain stress-strain softening curve, reflecting that the stress of concrete decreases with the increase of crack width. By the stress-strain softening curve and calculation formula proposed in this paper, the bending capacity of steel fiber concrete beam can be derived from the crack width. According the testing analysis, it was discovered that the typical tensile strain softening curve can be described with straight line segments defined by four key points. The method to determine the line segments based on experiment data was proposed.After cracking of the SFRC, the steel fiber can provide certain tensile stress on the crack section, which is analogous to the function of the reinforcement in reinforced concrete. However, the tensile stress of the steel fiber decreases with the increase of the crack width while the tensile stress of the reinforcement increases with the increase of the crack width. Based on the tensile strain softening curve above, the capacity of the SFRC under the combination of axle force and bending moment was conducted and from which obtains the equation of bending moment capacity. At last that equation is used to get N-M interaction curves under the action of specified axle force and under extreme capacity condition. The experimental and analytical results indicate that after cracking and under certain crack range, stress redistribution on crack section can increase the force arm of bending, which increase bending capacity. The bending capacity of crack section also increases with the increasing of axle force. Therefore, it theoretically explains the phenomenon that bending capacity does not decrease but increase after cracking.Compression-shear failure is one of the most common failure modes of the SFRC. The compression-shear failure of the SFRC was studied in this paper by varying compressive force and fiber contents of SFRC. It was observed from the test data that the obvious softening effect of the SFRC after failure can be characterized by a bilinear softening curve. The compression shear strength of SFRC is proven to be function of compressive stress and steel fiber contents. However, the effects of these two factors are independent. Finally, the calculation formula of compression shear strength was proposed based on the test phenomena and data, and verified with a finite element model.The compression bending failure mode and the compression shear failure mode of the SFRC lining were analyzed by 2D and 3D FE models in ADINA. By introducing stress-strain softening curve and shear softening curve, the finite element software can not only simulate the crack of steel fiber concrete lining, but also can simulate the character of subsequent bearing capacity after the lining crack. So that method improves the hypothesis of the linear elastic calculation of the tunnel lining material, and indicates the ductility of steel fiber concrete material from the calculation results. It was found that the spread SFRC has enough ductility and have the resistance to the load from surrounding rock after cracking. Therefore the spread SFRC is a viable system for the initial supporting for the underground structure construction.