The micro–mechanical mechanism of multiple cracking behavior induced by interface strengthening for UHPC

The multiple cracking behavior of ultra–high–performance concrete (UHPC) is strongly dependent on the bridging of cracks by fibers and the spatial strength distribution of matrix. A good match between fiber bridging properties and matrix tensile strength can significantly improve the utilization of fibers and matrix. In this study, a micromechanics based theoretical model is proposed, which can match the matrix tensile strength with the fiber–matrix interface bonding strength. The spatial strength distribution of the matrix and the stress transfer between the fiber and the matrix is considered in the model. Bridge stresses, additional stresses, stress transfer distances, and pulley forces at the crack face for various crack openings are derived. Based on the calculation results, the interfacial bonding strength is determined to be 14 MPa which match the common condition of UHPC (L_f=13mm,d_f=0.2mm, 2vol%,andσ_m=5.5MPa). Dense multiple cracks were observed in the designed tensile samples, and the peak strength and peak strain increase 53.3% (13.91 MPa) and 70.6% (2900με), respectively. The applicability of the model is validated by comparing the simulation results and the experimental results in previous investigations at the same paraments (τ_f=5.5MPa,σ_m=5.5MPa). With the proposed model, the mechanism of fiber–matrix interface enhancement contributing to the multiple cracking behavior can be understood. The results of this study could provide fundamental insights for the design of UHPC for various ductility and strength requirements.