Flexural Members with High-Strength Reinforcement: Behavior and Code Implications

High-strength steel reinforcement provides various benefits to the concrete construction industry, including a more efficient use of high-performance concrete, reduction of reinforcing bar congestion, and materials savings. Prior to the 2013 interim revisions of the AASHTO LRFD Bridge Design Specifications, the value of reinforcing steel yield strength used in design was limited to being no greater than 517 MPa (exceptions were permitted with owner approval for cases with a yield strength of less than 414 MPa). In 2007, National Cooperative Highway Research Program Project 12-77 was initiated to evaluate the AASHTO specifications with respect to the use of high-strength reinforcing steel and other grades of reinforcing steel having no discernible yield plateau. Among the objectives of this project was the investigation of ductility and crack control of flexural members using high-strength reinforcement. This research led to a number of recommendations that were subsequently incorporated into the 2013 interim revisions of the specifications. The flexural behavior and design of members reinforced with high-strength steel are presented. This paper also provides the background information for the AASHTO specification revisions related to strength reduction factors for flexure. The research demonstrates that the strain limits for high-strength reinforcement must be changed to achieve the curvature ductility comparable to that implicit with the current use of Grade 414 reinforcing steel. Moreover, the service load stresses in steel should be limited to 60% of the yield strength.