Multipoint Measurement of Early Age Shrinkage in Low w/c Ratio Mortars by using Fiber Bragg Gratings

In this study, a systematic sensing method has been proposed and developed to measure the early age shrinkage in low w/c ratio mortars. Fiber Bragg grating (FBG) strain and temperature sensors have been installed using a specialized installation method and utilized for measuring the internal strains and temperatures in a cylindrical mortar specimen. To achieve multipoint measurement of strains and temperature, two groups of FBG sensors with different initial wavelengths connected with two optical lines were taken as strain and temperature sensor series respectively. To keep the FBG vertical, copper tubes, as the channels for distributing the optical lines with FBGs, were employed during casting. Finally, the copper tube protecting the FBG strain sensors was removed slowly from the specimen while the FBG temperature sensors were kept in the other copper tube through the whole testing process. In this way, shrinkage strains and temperatures were collected at the beginning of the setting. It is found that there is no significant difference between the temperature values measured at different elevations in the specimen.