Effects of single and hybrid fiber incorporation on the toughness, mechanical properties and microstructure of fiber-reinforced mortar

In order to study the influence of fiber types and their ℓ/d ratios on the mechanical and microstructural properties of fiber reinforced mortar (FRM), four-point bending test and compressive test were carried out on FRM prisms to study their flexural toughness and mechanical strength at the ages of 3 and 28 days. Additionally, mercury intrusion porosimetry and electron scanning microscopy were applied to study the mechanisms responsible for the mechanical performance and mesoscopic failure modes of two types of polypropylene (PP) fiber, termed PP1 and PP2, and a polyvinyl alcohol (PVA) fiber, and their hybrids, used as mortar reinforcement. The results indicated that the improvement in the mortar's equivalent flexural toughness due to fiber incorporation followed the order: PP1 > PP2 > PVA, that in the equivalent flexural strength followed the order: PVA > PP2 > PP1. The PP1 fibers loosened the fiber-matrix interfacial transition zone (ITZ) and weakened the bond, resulting in easy fiber pull out. The ITZ of hydrophilic PVA fiber was relatively dense. The moduli, fracture elongations, and surface properties of PVA and PP1 fibers differed, resulting in a lack of synchronization in limiting FRM crack propagation at 3 days, as each fiber contributed to stress resistance at different stages. However, they exhibited complementary characteristics. PVA fibers restricted the initiation and development of early cracks, encouraging the formation of multiple cracks, while PP1 fibers prevented unstable crack growth in the strain-softening stage of FRM.