Effect of Beam Length on Bond-Dependent Coefficient (kb) value for GFRP reinforced beams

This study investigates the influence of beam span length on the bond-dependent coefficient (kb) for GFRP-reinforced concrete beams—a key parameter in predicting crack widths under service loads. Design codes such as ACI 440.1R and CSA S806 adopt a fixed kb value (typically 1.4), based on limited experimental data, which may overlook the effects of span-dependent bond behavior. To examine this, six full-scale beams with spans of 1950 mm, 2500 mm, and 3050 mm were tested under four-point bending. Crack widths and FRP bar strains were recorded to calculate kb values. Results showed a clear variation in kb with span length: shorter spans yielded higher kb values due to localized bond stress, while longer spans showed lower values associated with more uniform stress distribution. Although limited in scope, the results suggest a consistent trend between span length and kb, indicating the possibility of developing correlations to estimate kb values for longer spans from reduced-span tests. These findings support that further research with expanded datasets is recommended to establish generalized relationships for use in design and code calibration.