Distributed Detection and Quantification of Cracks in Operating Large Bridges

Abstract

A bridge monitoring approach is proposed for the distributed detection and quantification of crack opening displacements (CODs) along the lengths of large bridges. The proposed method does not require a priori knowledge of traffic loads and is applicable under normal operational conditions of bridges. A damage index is introduced, using the difference between crack-induced strain peak values and bridge base strains caused by moving loads. The applicability of the proposed approach for quantification of the detected cracks is demonstrated in terms of CODs at a number of locations in a multispan bridge. Six field tests were performed on a 332 m, five-span precast post-tensioned concrete box girder bridge for this purpose. The bridge was tested under operational conditions; thus, random daily traffic loads were employed as external forces. Two distributed optical fiber sensors based on the Brillouin scattering system were adhered to the external and internal surfaces of the box girder bridge. Fiber Bragg grating crack sensors were also employed for verification of the COD measurements, based on direct acquisition of distributed strains over the 332-m length of the bridge. By using this approach, it is possible to detect and quantify cracks with CODs larger than 5 µm.