Use of Unstressed Seven-Wire Strands as Longitudinal Reinforcement of Concrete Beams

Six full-scale beams using either only steel bars, mixed steel bars and seven-wire steel strands, or only seven-wire steel strands as longitudinal reinforcement were tested under four-point bending The bars were ASTM A706 Grade 60 (420 MPa) steel deformed bars, and the strands were ASTM A416 Grade 270 (1860 MPa) seven-wire steel strands. Test results showed that average crack spacing and width increased, while equivalent stiffness, flexural overstrength, and ductility decreased with increasing replacement of bars with strands. The decrease in ductility was mainly due to the reduced strain capacity of strands in tension and earlier buckling (bulging) of strands in compression compared to bars. The equivalent stress block method with the proposed models for strands provided conservative predictions for all the beams. The moment-curvature analysis method with the proposed models for strands produced moment-curvature relationships fairly close to measured responses, including the flexural overstrength and the failure mode. Keywords: beams; ductility; flexural overstrength; longitudinal reinforcement; moment capacity; moment-curvature analysis; stiffness; unstressed seven-wire strands.