Self-Sensing Steel–FRP Composite Bars for Crack Monitoring and Mechanical Behavior Evaluation in Reinforced Concrete Members

Distributed fiber-optic sensors (DFOS), which are based on optical frequency-domain reflectometry (OFDR), provide high-resolution strain measurements and have promising application in structural health monitoring. This study introduces a novel steel fibe-reinforced polymer composite bar (SFCB) with self-sensing, structural reinforcement, and damage control features designed to evaluate the response and damage status of concrete members. Investigating the force transfer mechanism between SFCB and concrete is essential for understanding concrete cracking behavior and establishing a reliable damage evaluation approach. Initially, tension tests were conducted on SFCB concrete members to investigate the effects of cover depth and bonding mechanism (concrete type and surface treatment of the SFCB) on the end effects, along with a test procedure designed to effectively eliminate the end effects. The results indicate that the use of members with small cover depths, surface sandblasted SFCB, and geopolymer concrete (GPC) can reduce the impact of the end effects. The tracking and quantification of particular crack progressions were subsequently assessed through the integration of a digital image correlation (DIC) system and a DFOS system. Finally, based on the results from which the influence of end effects has been eliminated, a theoretical model for the response of SFCB concrete tension members was proposed, along with a model for damage variables that is independent of geometry and material behaviors.