Modification of the wavelength-strain coefficient of FBG for the prediction of steel bar corrosion embedded in concrete

The corrosion of steel bar embedded in concrete can be detected from the Bragg wavelength shift of the fiber Bragg gratings (FBGs) winding around the steel bar. Previously, when the steel bar in concrete is surrounded by the FBG, the corrosion, which is proportional to the expansion of the bar cross-section, was predicted simply from the well known relationship of the Bragg wavelength shift and the only axial strain of the FBG. However, as the FBG is embedded in the concrete, stresses from all directions will apply to it. In this case, the Bragg wavelength change will attribute to stresses coming from the axial strain as well as the lateral strain. Therefore, the relation between the Bragg wavelength shift and the axial strain of FBG should be modified. We use a cylindrical and elastic model to analyze the strain distribution of a steel bar embedded in the concrete, and obtained a new relation between the Bragg wavelength shift of the FBG wrapped around the steel bar and the axial strain of the fiber. With this modified coefficient the corrosion of the steel bar embedded in the concrete can be predicted more accurately.

► We model a coiling FBG on the steel bar embedded in the concrete cylinder for detecting the corrosion of steel bar.
► The magnitude of the stress (strain) of the FBG and the concrete cylinder were obtained by finite element analysis.
► A new formula was derived to calculate the coefficient of the wavelength shift and the axial strain of FBG.
► The conversion coefficient between Bragg wavelength and axial strain of FBG should be modified to 0.829.