Experimental analysis on the workmanship factors and performance of fibre optic infrastructure monitoring cables embedded in concrete structures

Abstract

Structural health monitoring helps to protect infrastructure by guiding timely interventions. Distributed Fibre Optic Sensing (DFOS) is one such monitoring technique with significant advantages such as continuous measurement capability, small size and lightweight. This is the first time introducing the Fibre Optic Monitoring technology to Sri Lanka. A recently developed FOS technique, Brillouin Optical Frequency Domain Analysis (BOFDA), provides advantages such as smaller sized analyser unit and faster measurements, however, it lacks experimental evidence on the measurement accuracy. FOS, in general, also has unestablished workmanship practices on pre-straining and placing of fibre optic cables. This study is particularly aimed at BOFDA-based instrument calibration and verification of the accuracy of the BOFDA-based strain and temperature measurements. Firstly, laboratory experiments were performed to investigate the accuracy the measurements provided by the BOFDA-based DFOS system using the strain cable calibration test setup and temperature cable calibration test setup. These calibration tests indicated that actual maximum spatial resolution of the selected BOFDA unit is in the range of 0.4 - 0.6 m. The strain and temperature measurements provided by BOFDA were found to be reliable, since constant strain and temperature coefficients were observed. The workmanship practices were examined using a set of four-point beam bending tests with different configurations of FO cable locations and pre-tensioning levels. The results showcases that the location of the fibre cables, whether on the surface or embedded in concrete either near or away from the reinforcement, had no significant effect on the accuracy of strain measurements. However, contrarily to the practice, clear evidence was found on the need for pre-straining of strain cables to obtain accurate compressive strain measurements. This research therefore showcases the reliability of BOFDA technique and encourage future fibre optic monitoring deployments to ensure pre-straining of fibre optic cables as a good workmanship practice.