Ultra-High Performance Hybrid Polyvinyl Alcohol-Polypropylene Fiber-Reinforced Cementitious Composites with Augmented Toughness and Strain-Hardening Behavior

A new generation of cementitious composite materials as ultra-high performance concrete (UHPC) possessing notable mechanical characteristics, has demonstrated a high potential to be widely used in practice. In this research, first, the compressive strengths of UHPC composites with various mixture designs and by exploitation of different curing conditions (water, autoclave, and heat curing) were investigated. Based on the results, it was perceived that the stepwise method proposed herein led the compressive strength of the samples to rise from 76 to 212 MPa exhibiting an augmentation of about 179%. Furthermore, to scrutinize the impact of fiber hybridization on the behavior of UHPC composites in flexure, polyvinyl alcohol (PVA) and modified polypropylene (PP) fibers with different volume fractions were incorporated into the cementitious matrix. The maximum increase in flexural strength owing to the contribution of added fibers was observed to be 128%. The previous studies have shown that hybrid PVA-PP has no effects on enhancement in the mechanical properties of engineered cementitious composites (ECC); however, through this research, it was figured out that such hybrid fibers have a remarkable influence on the improvement of flexural strength, toughness, and energy absorption capacity of UHPC. According to the load–deflection curves, it can be expressed that all the hybrid PVA-PP fiber-reinforced cementitious composites examined in this study possess strain-hardening properties associated with bending. Furthermore, the strain-hardening behavior in UHPFRC incorporating PVA and modified PP fibers is more pronounced in comparison to that of ECC. The results demonstrate that such hybridization is a promising technique and yields a composite material with superior mechanical characteristics.