Basic Theory of Steel Fiber Reinforced Concrete

Steel fiber in concrete mainly restricts the expansion of concrete cracks, so that its tensile, flexural and shear strength are significantly improved compared with ordinary concrete. Its impact resistance, fatigue resistance, post-cracking toughness and durability are also greatly improved. It can make the concrete that was originally a brittle material into a composite material with certain plastic properties.

Ⅰ. The fiber spacing theory of steel fiber reinforced concrete

The fiber spacing theory, also known as the fiber crack resistance theory, was proposed by J.P.Romualdi and J.B.Batson in 1963. According to the theory of linear elastic fracture mechanics, the theory explains the restraint effect of fibers on the occurrence and development of cracks. It is believed that in order to increase the tensile strength of concrete, a brittle material with internal defects, it is necessary to reduce the size of internal defects as much as possible and improve toughness. Reduce the stress intensity and concentration at the crack tip. Therefore, steel fibers are used to connect the cracks. When tensioned, the fibers that cross the cracks transfer the load to the upper and lower surfaces of the cracks, so that the materials at the cracks can continue to bear the load. In this way, the stress concentration at the edge of the hole is alleviated due to the occurrence of cracks. With the increase in the number of bridging crack fibers, the smaller the distance between the steel fibers, the greater the stress concentration at the crack tip and the greater the reverse stress field on the crack tip. When the number of fibers increases to densely distributed in the cracks, the stress concentration will disappear, further indicating the fiber's anti-cracking effect, that is, in the process of composite material structure formation and stress failure, it effectively improves the composite material's crack resistance before and after stress. The ability of initiation and expansion can achieve the purpose of strengthening and toughening concrete by steel fiber.

Ⅱ. The shear lag theory of interfacial stress transfer in steel fiber reinforced concrete

In steel fiber concrete, the cement matrix structure around the steel fiber is different from its own structure, that is, there is an interface layer between the steel fiber and the matrix. The performance of steel fiber concrete mainly depends on the performance of the concrete matrix, the content of steel fiber and the characteristics of the interface between them. It is assumed that the interface is a thin layer with negligible thickness, but with certain mechanical properties. When a load is applied to the steel fiber concrete, the load is generally applied to the low elastic matrix first, and then part of the load is transferred to the high elastic modulus fiber through the fiber-matrix interface, so that the fiber and the matrix share the load so as to enhance the bearing weight.