Characterisation of strain distribution in concrete-filled steel tubular columns via distributed fibre optic sensing technology

Abstract

In a concrete-filled steel tube (CFST), concrete is under passive confinement. Until recently, it has been a challenging task to capture the axial and hoop strain developments in the passively confined concrete using conventional instrumentation methods. Distributed fibre optic sensors (DFOS) offers a new option to tackle this issue, especially with Optical Frequency Domain Reflectometry (OFDR) technology that enables strain measurements in real time, with high spatial resolution and improved precision. This paper presents an experimental study to demonstrate the feasibility and accuracy of OFDR-based DFOS to measure the axial and hoop strains in CFST. The installation procedures including choice of adhesives for DFOS are presented via validation tests on steel plates, steel H-sections, hollow steel tubes and unconfined concrete cylinders, where DFOS measurements are compared with strain gauge data. The technique is then used to monitor the individual steel and concrete elements in CFST. Unlike strain gauges that only provide pointwise measurements, DFOS has the ability to pinpoint strain localisation effects in the steel and confined concrete. Consequently, the test results reveal unprecedented information regarding the development of axial and hoop strains in the CFST, as well as the interaction between the steel tube and confined concrete at different load levels. Recommendations on DFOS installation techniques are presented in this paper, while the experimental findings will provide useful reference for future developments of empirical stress-strain models for CFST.