ENGINEERED CEMENTITIOUS COMPOSITES FOR UNBONDED POST-TENSIONED BEAMS

Although post-tensioned concrete members with unbonded strands are popular for rapid construction, longer spans, and replaceability, they have lower flexural strength than their counterparts with bonded strands due to lack of strain-compatibility. With tensile strain hardening and tensile strain ductility, Engineering Cementitious Composites (ECC), a class of fiber reinforced cementitious materials, can improve flexure strength of members with unbonded post-tensioning. This research investigated the impacts of ECC on flexural behavior of post-tensioned concrete beams with unbonded strands using nonlinear analyses. The analyses utilize fiber-based beam-column elements and principles of strain and displacement compatibility and were validated by tests from the literature. Three ECC-strand bond types were investigated: bonded (benchmark case), bonded and unbonded strands (called combined strands), and unbonded strands. All analyses were performed for conventional concrete (CC) and ECC for comparison. Moment-deflection relationships, strand stress at flexure strength, neutral axis depth, ultimate curvature distribution along the beam length and failure modes were obtained from the analyses. The results showed that the use of ECC increased cracking moment and the flexural strength but had little effect on stiffness regardless of the ECC-strand bond type. This improvement in strength is attributed to fibers that provide tensile resistance after cracking, in a way similar to bonded steel reinforcement contributing to tensile resistance. ECC increased deformation capacity of beams with all ECC-strand bond types, but this increase was the most significant for beams with combined strands. Similarly, ECC increased the stress in unbonded strands at flexure strength for beams with combined strands, allowing the unbonded strands to yield. For beams with only unbonded strands, the increase in unbonded strand stress was small. ECC did not have a significant impact on bonded strand stress at flexure strength.