Study On Pullout Mechanism Of Twisted Steel Fiber In High Performance Concrete

Twisted steel fiber(TSF)is a new steel fiber with distinguishable cross-section shape,and it was invented by Prof.Antonie E.Naaman from University of Michigan,USA.TSF has proven to significantly improve the mechanical properties of concrete,and thus it will advance the broad use of TSF reinforced concrete in engineering applications.In this paper,the interaction between TSF and concrete matrix and the TSF’s pullout micro-mechanisms were thoroughly understood by the comprehensive means of experimental study,theoretical analysis,and numerical simulation.The results were believed to provide the theoretical basis for the development and application of TSF in structural applications.The main research content and results are as follows:1.Based on the slump flow prediction model of steel fiber concrete,different mix proportions of self-compacted steel fiber reinforced concretes with various strengths were designed.Compressive strength test,concrete bend strength test,and "dogbone" tensile strength test were conducted to measure the basic mechanical properties of concretes embedded with hooked-end steel fibers(HSF)or TSF.Results show that the mechanical properties of TSF reinforced concrete were more superior than the ones of HSF concrete.2.Following a brief review of the pullout mechanical mechanisms of smooth steel fiber and HSF,the untwisting plasticity deformation mechanism special to the TSF reinforced concrete during the pullout test was analyzed.Subsequently,the analytical formulas of pullout forces of TSF with a rectangular cross-section and triangular cross-section were obtained,and then the theoretical pullout force values of TSF with different geometries used in the experiment and simulations were predicted.Results show that the theoretical pullout forces of TSF with a square cross-section,rectangular cross-section,and triangular cross-section was largest,the second largest,and smallest,respectively.This result was consistent with the simulation result.3.Pullout testing results of HSF show that the pullout force increased with the strength of concrete matrix,and the characteristics of pullout curves,pullout force peak,and attenuation position were in agreement with the theoretical analysis results.The influence of concrete matrix strength and the embedment length of fiber on the pullout results of TSF was studied Results show that(a)the concrete matrix strength could substantially increase the bonding strength of TSF in fiber reinforced concrete composites,indicating that the optical properties of TSF can be fulfilled in ultra-high performance concrete(UHPC);(b)the pullout curves can be simplified as a trilinear model consisting of an approximatively linear stage,a plastic"untwisting" stage,and a sudden dropping stage.The micro-XCT images showed apparent damage in the fiber-matrix interfacial transition zone for low strength concrete,but no evident damage in the fiber-UHPC channel after the fiber was pulled out,revealing the untwisting plasticity deformation mechanism special to the TSF reinforced UHPC.In addition,the value of the pullout force was independent of the fiber embedment length as long as it was longer than an effective untwisting length.4.A finite element analysis(FEA)model of the TSF pullout in TSF reinforced UHPC was established using ABAQUS.The computational efficiency was relatively high,and the mesh size was insensitive to the simulation results.The simulated pullout curves agreed well with the experimental results.Thus,the developed FEA model can effectively simulate the untwisting plasticity deformations of TSF.Parameter analysis was carried out to identify and select the optical parameters of TSF which were consistent with analytical results.This study can provide a reference for the design of geometrical parameters of TSF.5.Based on the analysis of TSF pullout characteristics and Cohesive Zone Model,an equivalent model of pullout force was developed to conduct the fracture simulation of TSF reinforced concrete using ABAQUS.The fiber stress values and multi-crack development process were explicitly simulated,followed by the two-dimensional Monte Carlo analysis for parameter analysis.The pullout force-displacement curves during the whole process agreed well with the experimental results.Based on the optimal analysis results of the geometrical parameters of TSF during the pullout test,the parameter analysis of fracture simulation of TSF during the pullout was conducted.