Aging infrastructure requires reliable and efficient monitoring solutions to detect damage before structural failure occurs. Traditional sensing methods are often impractical due to extensive cabling and installation complexity. Externally Bonded Reinforcement (EBR) using fiber-reinforced polymer (FRP) composites is widely applied to strengthen damaged concrete structures, creating an opportunity to integrate sensing functionality directly into the strengthening layer.

This study explores the use of the conductive carbon layer within a mechanochromic FRP composite as a damage-sensing network for FRP-retrofitted concrete beams. Electrical resistance measurements were used to relate the fractional change in resistance (FCR) to structural behavior under flexural loading. The embedded carbon fiber network acts as a distributed sensor capable of detecting strain and cracking within the composite–concrete system.

Experimental results show a clear correlation between applied load, crack development, and changes in electrical resistance. The carbon fiber network exhibits a very high sensitivity to mechanical deformation, with a gauge factor far exceeding that of conventional metal strain gauges. This high sensitivity enables the detection of early-stage damage at low strain levels. In well-bonded specimens, consistent electrical response patterns were observed, including characteristic resistance changes prior to failure, indicating the potential for early warning of critical damage. Poor bonding conditions, however, resulted in inconsistent electrical behavior and reduced reliability.

Overall, the findings demonstrate that electrical resistance measurements in conductive carbon fiber composites offer a promising, integrated approach for global structural health monitoring of FRP-strengthened concrete structures, particularly when adequate bond quality is ensured.