Long-Term BOTDA Baseline Assessment of a Fiber Optic Sensing Textile on a Composite Bridge Girder

This paper presents a six-year assessment of the baseline comparability of a Brillouin optical time-domain analysis (BOTDA)-based sensing textile installed in a composite bridge girder under unloaded field conditions. Six monitoring campaigns collected from 2020 to 2025 were analyzed using campaign-temperature-corrected strain profiles. The known physical segmentation of the sensing path was used to evaluate cross-campaign behavior through segment-wise mean deviation, root-mean-square deviation (RMSD), percentile-based deviation, and position-wise drift-rate metrics. The results show clear segment-dependent variability, with larger deviations occurring mainly in the non-structural lead and return segments than in the bonded structural sensing sections. In the 2025 campaign, the combined bonded structural segments exhibited an RMSD of 292 µε and a 95th-percentile absolute deviation of 508 µε. Their mean drift rate was approximately −11 µε/year, with a 95th-percentile absolute drift rate of approximately 101 µε/year. These results indicate better long-term baseline repeatability within the bonded sensing path than in the non-structural portions of the measurement chain. However, because temperature correction was based on campaign-average surface measurements rather than a distributed temperature profile, residual thermal effects cannot be fully separated from the observed long-term variation.