Strain-derived anchorage mobilization and failure-mode transition in FRCM-strengthened reinforced concrete beams

Four-point bending tests were conducted on reinforced concrete beams strengthened with an epoxy-impregnated carbon-grid fabric-reinforced cementitious matrix (FRCM) system. The specimens had the same nominal cross section, reinforcement details, and shear span, while the FRCM anchorage length was varied as 0.5, 1.0, 1.5, and 2.0 times the beam depth. The effects of anchorage length were examined using load–deflection responses, final damage observations, and measured strain distributions in the tensile reinforcement and FRCM layer. The measured FRCM strains were further used to evaluate the effective anchorage length, anchorage mobilization ratio, and transfer-demand indices. All strengthened beams exhibited higher cracking, yielding, and peak loads than the corresponding control beams. However, the strength increase was not proportional to the provided anchorage length. Within the tested series, shorter anchorage lengths showed localized FRCM strain concentration near the loading points, whereas longer lengths showed wider strain-mobilized regions. The different damage modes are interpreted as specimen-specific trends rather than direct effects of anchorage length. These results indicate that the effectiveness of FRCM strengthening should be evaluated not only by the provided bonded length, but also by the actual strain-mobilized region and transfer-demand distribution in the anchorage zone. The proposed strain-derived indices are presented as empirical interpretation tools for the tested specimens, rather than as a general design model.