3D mesoscale discrete element modeling of hybrid fiber-reinforced concrete

To investigate the relationship of hybrid fiber volume friction on the mechanical properties and failure mechanisms of steel-polypropylene hybrid fiber reinforced concrete (HFRC), the study constructs a three-dimensional, five-phase mesoscale model using the Discrete Element Method (DEM), which consists of mortar, coarse aggregate, polypropylene fibers (PPF), steel fibers (SF), and the interfacial transition zone (ITZ). The axial compression test results of twelve cubic specimens with four different hybrid fiber combinations (0.5 % SF + 0.05 % PPF, 0.5 % SF + 0.1 % PPF, 1.0 % SF + 0.05 % PPF, 1.0 % SF + 0.1 % PPF) were used to validate the model's accuracy, examining the influence of fiber content on the mechanical performance and cracking features of HFRC. Directed by experimental mix proportions, the model employed cluster particle modules with realistic aggregate geometries and randomly distributed SFs to simulate concrete microstructure. Utilizing the Flat-Joint Model (FJM) in DEM software (PFC3D), two contact models are developed: one FJM with bi-nonlinear softening segments to describe the ITZ between steel fibers and PPF-reinforced mortar (PFRM); another FJM considering the volume fraction effect of SF to describe the ITZ between coarse aggregate and FRM. Both models were experimentally validated, proving their accuracy. The results indicate that the established DEM model can accurately simulate the stress-strain relationship and failure modes of HFRC under compression. The hybrid fibers increased the compressive strength by 2.13-17.33 % and reduced the cracking strain by 5.25-9.33 %. Furthermore, the computational findings reveal that PPF primarily affects the crack initiation phase, while SF plays a significant role in the overall crack propagation stage. This discovery holds significant importance for understanding and optimizing the structural performance of HFRC.