Fracture behavior of cement mortar reinforced by hybrid composite fiber consisting of CaCO3 whiskers and PVA-steel hybrid fibers

We added CaCO3 whiskers into polyvinyl alcohol (PVA)-steel hybrid fiber system to obtain multiscale hybrid fiber reinforced cement mortar (MHFRC). Three-point bending (3-p-b) tests were carried out on 64 notched beams to investigate fracture behavior of MHFRC. Double K Fracture Criterion (DKFC) and Work Fracture Method (WFM) were employed to obtain fracture parameters. Influence of the volume fraction of CaCO3 whiskers, content of PVA-steel hybrid fiber and water/ cement ratio (w/c) on fracture parameters are discussed. Addition of composite fibers consisting of CaCO3 whiskers and PVA-steel hybrid fibers could improve fracture behavior of MHFRC. As the content of CaCO3 whiskers increases, fracture parameters first increase and then decrease. Content of PVA-steel fibers also affect fracture behavior of the matrix. Thus, an optimum ratio between CaCO3 whiskers and PVA and steel fibers contents exist that provide the best fracture performance of cement matrix: 1 vol% of CaCO3 whiskers, 0.5 vol% of PVA fibers and 1.5 vol% of steel fibers (S15P05W10). Influence of the w/c value is also discussed. Fracture toughness (K-IC) and fracture energy (G(F)) of S15P05W10 group decreased as w/c values increased. The synergy of fibers in S15P05W10 was evaluated quantitatively, and the results indicated positive synergy effect on unstable fracture toughness (K-IC(un)) and fracture energy (G(F)) in cement matrix with higher w/c values. Negative synergy was observed for the initial fracture toughness (K-IC(ini)) Comprehensive reinforcing index (RIv) was introduced as the characteristic parameters of the hybrid fibers. Fracture parameters increased first and then decreased as RIv increased. Furthermore, the microscope morphologies of CaCO3 whiskers, PVA and steel fibers in the cement matrix were shown. These results helped to establish microscopic reinforcing mechanism of the hybrid fibers in the cement matrix. Based on the experiment results, empirical formulas, which taking into account fiber factor RIv and matrix factor w/c, were proposed to calculate fracture parameters of MHFRC.