Tensile characteristics of ultra-high performance fiber reinforced concrete with and without longitudinal steel rebars

This study presents an experimental evaluation of ultra-high-performance fiber-reinforced concrete specimens with and without longitudinal reinforcement under direct tensile loading. The study variables are (i) volume fraction of fibers (1.0% and 2.0%), (ii) type of steel fibers (straight and hooked end), and (iii) longitudinal steel reinforcement ratio of 0.0% and 1.2%. All the specimens are tested using a servo-controlled fatigue testing machine in a displacement control mode. The changes in displacement were monitored using a linear variable displacement transducer and a digital image correlation technique. The strain profile at different loading stages is presented to identify the crack evolution process. Test results show that the average localized strain ranges from 0.2% to 0.36%, with corresponding crack widths of 0.3 mm to 0.6 mm. A uniaxial tensile stress-strain model is proposed based on the test results and literature database. The longitudinal steel reinforced specimens show both stiffening and strengthening effects. Tension-stiffened specimens with 1.0% fibers failed at a higher strain due to the formation of multiple macro cracks. In the specimens with 2.0% fibers, the rebar fractured in a brittle manner due to crack localization. A higher longitudinal reinforcement ratio is needed to effectively utilize UHPFRC under tension-dominant loads.