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This study investigates the axial load response of ultra-high-performance fiber-reinforced concrete (UHPFRC) columns with compressive strengths of 163 and 181 MPa (design strengths of 150 and 180 MPa). The UHPFRC used in this study had 1.5% of hybrid micro-steel fibers (1.0% of 19.5 mm fibers and 0.5% of 16.3 mm fibers) and did not contain coarse aggregate. A total of nine UHPFRC columns confined by transverse reinforcement with volumetric ratios of 0.9–9.9% and two different configurations (Types A and C) were tested under pure axial load to investigate the influence of these variables. The overall behavior of the UHPFRC columns was compared with the response of similar strength ultra-high-strength concrete (UHSC) columns having coarse aggregate. Test results showed a pronounced effect of the volumetric ratio of the transverse reinforcement on the confinement. Hybrid micro-steel fibers controlled brittle cover spalling very well and assisted the transverse confinement reinforcement after the peak load. Applicability of the confinement reinforcement equations in the current seismic design provisions for developing ductile behavior of the UHPFRC columns was investigated. The analytical study examined the ability of the existing high-strength concrete (HSC) confinement models for predicting the axial load response of the UHPFRC columns, and a prediction model that accounts for the effects of steel fibers and the stress-strain relationship of UHPFRC is proposed. [ABSTRACT FROM AUTHOR]

This study aims to investigate the effect of fiber orientation on the flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) under quasi-static and impact loadings. Two placement methods and two specimen sizes were considered to provide different fiber orientations, and image analysis was carried out to quantitatively evaluate the fiber orientation. Quasi-static flexural tests were performed according to ASTM standard, while impact tests were performed with two different potential energies (0.48 and 1.13 kJ) using a drop-weight impact test machine. Test results indicated that under quasi-static loading conditions, higher flexural strength, normalized deflection capacity, and toughness were obtained in the beams that contained better fiber orientation in the direction of the tensile load compared to those with poor fiber orientation; however, no obvious difference in the first-cracking properties was observed with respect to the fiber orientation. Under impact loading conditions, the flexural strength and energy absorption capacity were both increased with better fiber orientation, and a greater increase in the flexural strength with strain-rate was obtained in the beams with better fiber orientation at an identical potential energy (relative to their counterparts with poorer orientation). Therefore, providing good fiber orientation can be an effective way to improve the impact resistance of UHPFRC beams. [ABSTRACT FROM AUTHOR]

To investigate the effect of the reinforcement ratio on the flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) beams under impact loading, a total of four large-sized (200 × 270 × 2900 mm) beams were fabricated and tested using a drop-weight impact test machine. The incident kinetic energy and impact velocity were 4.2 kJ and 5.6 m/s, respectively. A higher reinforcement ratio exhibited lower maximum and residual deflections and better deflection recovery. The test results also indicate that the maximum crack width at a certain drop stage decreased with the reinforcement ratio. A nonlinear analytical model for predicting the impact behavior of a UHPFRC beam was developed using multi-layer sectional analysis and single-degree-of-freedom analysis, and the model was verified through comparison with the experimental results. [ABSTRACT FROM AUTHOR]

This study investigates the bond performance of steel and glass fiber-reinforced polymer (GFRP) rebars embedded in ultra-high-performance fiber-reinforced concrete (UHPFRC). The steel rebar showed 2.8–3.6 times higher bond strengths than the GFRP rebar and rebar yielding at embedment length of 2 times the rebar diameter. The bond failure of GFRP rebar occurred by delaminating resin and fiber in GFRP rebar, different to that of steel rebar (shearing off and crushing of concrete). For GFRP rebar, higher bond strength was obtained when a larger rebar diameter and a shorter embedment length were used, and reincrease of pull-out stress in softening branch was observed owing to wedging effect. Equations for normalized bond strength and development length of GFRP rebar embedded in UHPFRC with pull-out failure were suggested. In addition, analytical models for bond-slip response of GFRP rebar proposed in the literature were considered, and adequate parameters were derived from the present test data. [ABSTRACT FROM AUTHOR]