A corrosion threshold-controllable sensing system of Fe-C coated long period fiber gratings for life-cycle mass loss measurement of steel bars with strain and temperature compensation

In this study, a corrosion threshold-controllable sensing system of long period fiber gratings (LPFG) is developed and validated for life-cycle monitoring of steel bars in corrosive environments. Three Fe-C coated LPFG sensors with two bare LPFG sensors in LP₀₆ and LP₀₇ modes for strain and temperature compensation were multiplexed and deployed inside three miniature, coaxial steel tubes to measure three (long-term in years) critical mass losses through the penetration of tube walls and their corresponding (short-term in hours) corrosion rates in the life span of steel bars. The strain/temperature and mass loss measurements are based on the changes in grating period and refractive index of surrounding medium, respectively. Thermal/mechanical loading and accelerated corrosion tests were conducted to validate the functionality, sensitivity, accuracy, and robustness of the proposed sensing system. Since both the steel tube and Fe-C layer represent the material composition of steel bars in the context of corrosion, the mass loss correlation among any two of the steel tube, Fe-C layer and steel bar is independent of the test conditions such as the current density and sample length, and thus applicable to engineering practices. The outer tube can notably delay and decelerate the corrosion process of its inner steel tube due to the reduced current effect.