Crack Propagation Model Of UHPC Based On Fiber Anti-cracking Mechanism

Ultra-high performance concrete(UHPC)has been widely used in bridge repair and reinforcement and new bridge structures due to its ultra-high strength,toughness and durability.In order to meet engineering needs,it is necessary to study the effects of various steel fiber parameters on UHPC tensile properties from a meso level,clarify the strengthening and toughening mechanism of steel fibers,and establish reasonable and effective numerical simulation methods to predict the fracture behavior of UHPC.The article uses a combination of theoretical analysis and finite element simulation to study the fracture performance of UHPC at the meso level.First,based on the principle of composite material mechanics,the UHPC fracture surface softening curve is derived,and the optimization and fracture calculation analysis show that the model can be better applied to the UHPC matrix.Based on this model,the influence of the fracture height ratio on the fracture performance of UHPC is studied,and it is concluded that with the increase of the fracture height ratio,the crack initiation load decreases approximately linearly,and the destabilization load decreases gradually.The single fiber drawing was simulated,and the influence of interface strength,fiber length,fiber diameter and interface elastic modulus on the mechanical properties of fiber drawing was studied.It is concluded that interface debonding is a gradual process from outside to inside.For fibers in the undebonded section,the interfacial shear stress is distributed exponentially.When the debonding is close to the free end,the phenomenon of "two-way debonding" appears.With the increase of interface strength,the increase of fiber length and the decrease of fiber diameter,the energy dissipation of fiber crack resistance on the fracture surface of the matrix under the same fiber content increases,but the change of the interface elastic modulus has no effect on the fiber crack resistance performance.Based on the Monte Carlo method,a UHPC mesoscopic failure model considering random fiber distribution is established.This model can effectively characterize the multicrack propagation in the UHPC uniaxial tension process and the shear failure under uniaxial compression.The influence of fiber content,fiber length and fiber diameter on the uniaxial compression performance of UHPC is studied.It is concluded that as the fiber content increases,fiber length increases and fiber diameter decreases,UHPC compressive strength and toughness increase,but the toughness is relatively high.The increase in strength is more obvious.The effects of multi-parameters on UHPC tensile properties and crack propagation paths are studied,and it is concluded that compared with passing through,cracks tend to "pass" fibers,and the larger the fiber inclination angle,the easier it is to "pass".Before the matrix cracks,the larger the fiber inclination angle,the smaller the force.With the increase of the fiber content,the increase of the fiber length and the decrease of the fiber diameter,the crack propagation path becomes more complicated,and the fiber’s crack resistance is enhanced.The interface strength and the change of matrix strength basically have no effect on the main crack propagation path,but it will increase the matrix toughness.The cracking stress and elastic modulus increase with the increase of fiber content,and decrease with the increase of fiber diameter and fiber inclination,but they are not affected by the change of fiber length and interface strength.The peak stress increases with the increase of fiber content,fiber length and interface strength,and decreases with the increase of fiber diameter.The tensile mechanical properties of UHPC are the best when the fiber inclination is in the range of 15～30°.The research conclusions of this paper can provide references for the optimization design of UHPC materials.