Influence of a Built-In Ultra-Weak Fiber Bragg Grating Sensor on Its Interfacial Properties with Asphalt Mixture

Ultra-weak fiber Bragg grating (UWFBG) sensors are increasingly applied in asphalt pavement monitoring; however, the quantitative criteria for their vertical placement based on deformation coordination remain insufficient. This study investigates the deformation coordination mechanism between UWFBG sensors and the asphalt mixture under different vertical embedding positions. Three mesoscale finite element beam models with sensors embedded at the top (T), middle (M), and bottom (B) positions were established to simulate the lateral strain field evolution, core lateral tensile strain response of the UWFBG sensor, and interfacial mechanical behavior under three-point bending loading. To quantitatively evaluate the deformation compatibility, a weighted deformation coordination index was constructed by integrating the lateral tensile strain change rate of the UWFBG core (representing strain response sensitivity), the interface damage degree, and the interface opening displacement. A weight sensitivity analysis was performed to ensure the consistency of the result ranking. The results demonstrate that the vertical embedding position of the UWFBG sensor not only affects its own lateral tensile strain response, but also alters the lateral strain redistribution within the asphalt mixture beam, the migration of the neutral surface, and the damage development at the UWFBG sensor–asphalt mixture interface. The UWFBG sensor embedded at the bottom (B) position induces the most pronounced tensile strain amplification and neutral surface migration in the surrounding asphalt mixture, whereas the sensors embedded at the middle (M) and top (T) positions exhibit faster degradation of the UWFBG sensor–asphalt mixture interface or limited strain amplification, resulting in lower deformation coordination levels. Overall, the bottom-embedded configuration exhibits the strongest strain amplification, with the highest peak lateral tensile strain of the UWFBG core. The deformation coordination index (Ic) of the bottom configuration at the later loading stage is approximately 0.42, which is higher than that of the middle (0.37) and top (0.31) configurations. The consistent ranking under different weight combinations confirms the robustness of the evaluation work and identifies the bottom-embedding configuration as the most favorable arrangement for strain monitoring.