Distributed optical-fibre strain sensing in reinforced concrete structures

Abstract

An innovative technique based on optical fibre sensing that allows continuous strain measurement has recently been introduced in structural health monitoring. Known as Brillouin Optical Time-Domain Reflectometry (BOTDR), this distributed sensing technique allows measurement of strain along the full length (up to 10 km) of a suitably installed optical fibre. The sensors can be wrapped around or embedded in structures, where a single optical fibre potentially replaces a very large number of point sensors. The installation of optical fibres in concrete structures involved several procedures such as pre-tensioning of cables, spot-glued or end-clamped onto steel rebars, and the use different types of commercially available optical cables. Such instrumentation techniques must be validated in terms of their measurement performance, which is the aim of this research. This was done through a series of well-instrumented uniaxial load tests of concrete columns. The loading of the structures were performed within the elastic range and later loaded up to failure. The test results revealed that all sensing cables of various types used in this study measured strains of about the same values (within the BOTDR accuracy of 30 microstrains) and were comparable with other independent instrumentation devices. Strain data from the two methods of attachment (spot-glued or end-clamped), either pre-tension or without pre-strain, generally did not indicate any dissimilarity between them. These findings have enabled the establishment of the best practice of field instrumentation consisting fibre optic sensors in the current exploration the use of end-clamping technique.