Bond behavior of high-performance fiber reinforced concrete (HPFRC) under direct tension pullout

Bond is the complex interaction that occurs at the interface of reinforcement and concrete, enabling force transfer between the two materials. The bonded interface is highly sensitive to any passive confinement that occurs normal to the contact area. The internal confinement that occurs in high-performance fiber reinforced concrete enhances the bond strength to such an extent that very short anchorage lengths suffice to develop a standard steel reinforcing bar. In order to evaluate and quantify the bond strength of bars embedded in novel cementitious materials, a combined experimental and analytical study was conducted using 13 specimens fabricated with high-performance concrete materials containing either steel or PVA fibers. Bond specimens were tested in a direct tension pullout (DTP) setup in order to quantify a lower bound for bond strength of bars in high performance fiber reinforced cementitious (FRC) matrices with tension-hardening properties, where the bar is developed in a tension stress field. The test setup was selected so as to minimize unaccountable confinement owing to support conditions and to eliminate out-of-plane effects due to flexural curvature. Study variables included the bar cover thickness and the type of FRC matrix used. The behavior of the test-specimens was analyzed in detail using nonlinear finite element modeling and an extensive database of published bond tests in high-performance strain-hardening cementitious matrices. Strain fields obtained from digital image correlation (DIC) were used to verify the kinematic hypotheses traditionally used in solving the field equations of bond (i.e. relationship of bar and concrete strains with the rate of slip along the anchorage), and to study the effect of enhanced fracture energy supplied by the fibers on bond mechanics. Striking improvements were observed on the measured HPFRC and ECC bond strength and failure mode over bond of conventional (unconfined) concrete. Direct comparison with existing pullout setups in the literature shows that bond strength measurements of the DTP formulation are more conservative and illustrate clearly the effect of bar cover on bond strength.