Localized tension characterization of reinforced concrete flexural members using surface bonded distributed fiber optic sensors

Abstract Structural monitoring and assessment can be used to ensure the long‐term performance of structures and inform when maintenance is required by accounting for the degradation of material properties. A key force‐resisting mechanism in reinforced concrete that must be considered as part of an assessment and that can degrade over time is tension stiffening, which can affect stiffness and significantly influence the deflection of lightly reinforced members such as slabs. Distributed fiber optic sensing has proven to be an effective tool for monitoring structural behavior; however, limited research directly measuring local tensile strain distributions in between cracks on concrete surfaces exists. Yet such research could improve understanding of tension distributions for modeling of tension stiffening for assessment. To help characterize concrete tensile strain distributions, three reinforced concrete slab strips were instrumented with distributed fiber optic sensors (DFOS) on the longitudinal reinforcement and on the concrete surface in the flexural tension region after initial cracking in 4‐point bending. The results from the DFOS showed that crack spacing heavily influenced surface strain distributions. These surface strains were linked to the internal crack width decreasing with depth to the reinforcement caused by eccentric force transfer between the reinforcement and concrete, referred to as “unwrapping.” At locations where the surface was unaffected by unwrapping, the concrete strain distribution was parabolic and tensile over the length. A nonlinear finite element model that simulates force transfer between reinforcement and concrete through discrete reinforcement ribs and fracture mechanics was developed, which corroborated the measured surface strain distribution and could be used as an assessment tool.