Headed and High-Strength Shear Reinforcement in Concrete Members

Abstract

Results are reported from 29 large-scale tests of reinforced concrete beams designed to investigate the use of Grade 80 (550) steel and headed deformed bars as shear reinforcement, both of which are expected to alleviate reinforcement congestion and improve constructability when used in heavily reinforced members. The specimens were tested under three-point bending with a shear span-to-effective depth ratio of 3. Shear reinforcement consisted of either stirrups or headed deformed bars made with Grade 60 or 80 (420 or 550) steel. Headed bars were evaluated both engaging and not engaging the longitudinal reinforcement to determine whether such engagement is necessary for acceptable performance. Other variables included beam depth, shear reinforcement size and spacing, longitudinal reinforcement ratio, and concrete compressive strength. Test results showed that when headed deformed shear reinforcement (of diameter db) is placed with at least 6db of side cover and having at least one longitudinal bar within the side cover, or when headed reinforcement is engaged with longitudinal bars at each end, beam shear strength was similar to that of companion specimens with stirrups. Specimens designed for the same nominal shear strength, Vn, using either Grade 60 or 80 (420 or 550) shear reinforcement exhibited similar shear strengths and crack widths at service-level loads (taken as 0.6Vn). Test results indicate there may be cause to reevaluate the ACI Building Code minimum for ρtfytm of 50 psi (0.34 MPa): 9 out of 12 specimens with ρtfytm < 90 psi (0.62 MPa) had measured shear strengths less than Vn calculated using measured material properties, whereas only 2 out of 13 specimens with ρtfytm > 90 psi (0.62 MPa) did. The statistically significant difference between these groups persisted when Vn was based on the number of adequately anchored stirrup legs intercepted by the failure surface.