Strain Rate Effect on High Performance Fiber Reinforced Cementitious Composites Using Slip Hardening High Strength Steel Fibers.

Abstract

The objective of this research is to develop an understanding of the high strain rate response of High Performance Fiber Reinforced Cementitious Composites (HPFRCC). The research is divided into four parts. In the first, HPFRCC with high tensile strength (>10MPa) and ductility (>0.5%) is developed by using slip hardening fibers within a high strength mortar. Two types of fibers, twisted and hooked, are used in volume fractions ranging from 1 to 2%. The large slip capacity of twisted fibers during pullout generates large pullout energy (large equivalent bond strength), and thus leads to high strain capacity composites with multiple micro-cracks. In the second part, experiments are performed to investigate the effect of strain rate on fiber pullout and composite response. The rate sensitivity of HPFRCC in tension depends on fiber type, volume fraction and matrix strength (or composition). As the strain rate increases, HPFRCC with twisted fibers exhibits a pronounced, beneficial strain rate effect, i.e. a higher tensile strength is achieved with no reduction in strain capacity. In contrast, HPFRCC with hooked fiber show no clear strain rate effect. In the third part of this work, a new impact test system that employs suddenly released elastic strain energy is developed to enable impact testing for cementitious composites with large-sized specimen. A prototype system that was simulated and built is only 1.5m in height and can generate a high rate impact pulse. Compared to current impact test system, the new setup is inexpensive, small, portable, safe and easy to operate. Finally, the source of strength enhancement for cement-based materials under high rate compressive loadings was investigated through computational simulation models. The observed strain rate effect or mortar under compression is primarily, but not totally, due to lateral inertial effects under high rate loading and the pressure dependent nature of cementitious materials. The test and simulation results show that it is possible to develop a high performance cementitious composite with 1% to 2% volume fraction of fibers that has high energy absorption capacity and that can therefore be used to mitigate the effect of extreme loading such as earthquakes, impact, and blast.