Effect of specimen geometry on the dynamic direct tensile responses of ultra-high-performance fiber-reinforced concrete

Direct tensile responses of ultra-high-performance fiber-reinforced concrete (UHPFRC) at high strain rates were investigated using a high-rate hydraulic universal testing machine (HR-UTM) with modified cylindrical specimens. The modified cylindrical UHPFRC specimens containing 2 vol.% steel fibers exhibited tensile strain-softening behavior even at static strain rates, in contrast to the tensile strain-hardening responses commonly reported in previous studies. The modified cylindrical UHPFRC specimens containing 0.5 and 2 vol.% steel fibers exhibited average tensile strengths of 8.8 and 10.0 MPa, respectively, at static strain rate ( ε ̇ =5.55×10 −4 s −1 ). As the strain rate increased from 5.55×10 −4 to 162.96 s −1 , the tensile strength of the specimen with 2 vol.% steel fibers increased from 10.0 to 17.1 MPa. Moreover, the elastic modulus of UHPFRC in direct tension increased from 59.4 to 124.7 GPa as the strain rate increased from 5.55×10 −4 to 99.53 s −1 . However, at strain rates exceeding 162.45 s −1 , accurately determining the tensile elastic modulus became difficult because of vibrations and early damage to strain gauges. Overall, the results demonstrate that specimen geometry plays a critical role in governing the dynamic tensile response of UHPFRC, emphasizing the need to consider geometric effects in material design and structural applications subjected to high strain rates. • HR-UTM used to test modified cylindrical UHPFRC specimens under direct tension. • Tensile behavior of UHPFRC was dependent on the specimen geometry. • Cylindrical specimens showed strain softening, unlike dog-bone-shaped specimens. • Direct tensile strength and elastic modulus increased notably with increasing strain rates. • Findings aid material design for construction and standardization of test methods.