Size-dependent impact resistance of ultra-high-performance fiber-reinforced concrete beams

•Flexural strength of UHPFRC decreases with increasing specimen size.•Using twisted or long straight fibers improves static flexural performance of UHPFRC with short straight fibers.•Deflection-hardening response of UHPFRC under impact is successfully captured.•Long straight steel fiber is most effective in improving impact resistance of UHPFRC.•Strain-rate is proper to analyze DIF on fLOP of UHPFRC excluding size effect.

This study examines the rate dependent flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) beams with three different sizes. Two different loading rates (static and impact), fiber aspect ratios (lf/df of 65 and 100), and fiber types (straight and twisted) were considered. Test results indicated that the static flexural performance, including the flexural strength and toughness, were improved by increasing the fiber aspect ratio or through the use of twisted steel fibers. The static flexural strength clearly decreased with an increase in specimen size due to a decrease in the number of fibers at the crack surface. The use of straight steel fibers with a higher aspect ratio of 100 provided the best impact resistance in terms of the highest post-cracking flexural strengths and the largest normalized energy dissipation rates, compared to those of twisted steel fibers and straight steel fibers with a reduced aspect ratio of 65. Thus, the use the straight steel fibers with high aspect ratios was recommended to improve the impact resistance of UHPFRC. Dynamic increase factor (DIF) on the flexural strength of UHPFRC beams was properly investigated with strain-rate, regardless of specimen size. In addition, there were no effects with regard to the fiber aspect ratio and type on the relationship between the DIF of the first-cracking flexural strength and the stress- (or strain-) rate.