A study on the tensile fracture behavior of polypropylene fiber reinforced concrete based on a microscale model

Abstract

Polypropylene fibers are distributed in concrete with varying shapes and quantities, which is not conducive to microscopic modeling and analysis. This study aims to establish an effective and efficient crack calculation model suitable for polypropylene fiber-reinforced concrete (PFRC) to reveal the reinforcement mechanism of polypropylene fiber-reinforced concrete. Firstly, from a microscale perspective, PFRC was regarded as a three-phase heterogeneous material comprising aggregate, mortar (with embedded polypropylene fibers), and interfacial transition zone (ITZ). Three-point bending and splitting tensile tests were conducted on mortar-aggregate composite and mortar specimens with the fiber volume fraction as a variable. Furthermore, Peak stress ratio and fracture energy ratio were introduced to characterize the modification effect of polypropylene fibers. The influence of polypropylene fibers on the mechanical properties of mortar and ITZ was investigated, and the modification mechanism of polypropylene fibers on concrete was analyzed from a microscale perspective. Finally, a comprehensive microscale fiber-reinforced concrete cracking finite element model was established based on the results of microscale experiments using two-dimensional image recognition and three-dimensional numerical simulation methods. The research results show that PP fibers can significantly enhance the mechanical properties of mortar, with flexural and tensile strength increased by 9.6–15.0% and 9.5–21.9%, and the maximum fracture energy increased by 88.7%. The hydrophobicity of PP fibers limits the elimination of bubbles, increases the porosity of ITZ, reduces the bonding strength of ITZ, and after maximum weakening, the flexural strength, tensile strength, and fracture energy of ITZ are only 74.3%, 79.1%, and 37.6% of those without fiber doping. The microanalysis model established based on this can effectively describe the relevant concrete tensile fracture process indicators. The ratio of the stress peak calculated by the 3D model to the test is 0.997, while the ratio of the stress peak calculated by the 2D model to the test is only 0.706. The simulation effect of the 3D model is closer to reality. The influence of ITZ on the peak stress of concrete is more significant than that of mortar, and improving the weakening effect of PP fibers on the bonding strength of ITZ is the key to improving the macroscopic mechanical properties of PFRC.