Application of FBG-based sensors in build environment

Abstract

Fibre Bragg grating (FBG)-based optical fibre sensors have found numerous applications in various industries over nearly 20 years. This is due to their many advantageous features, which include those that are typical of fibre-optic sensors (FOSs), for example being light weight, compact, having immunity to electromagnetic fields and an ability to operate in harsh environment. In addition to that, FBG-based sensors offer intrinsic multiplexing capability and they operate using wavelength-encoded signals which are insensitive to the intensity variations that normally affect the performance of intensity-based FOSs. One particular sector that is developing an interest in the use of FBG sensing technology is the civil engineering industry, where FBG-based sensors have begun to be used as physical sensors in a variety of civil structures, many of which are built using reinforced concrete. These sensors are used as part of structural health monitoring (SHM) systems which are effective in detecting possible damage in civil structures as a result of physical loading and corrosion which can lead to deformation, crack formation, fatigue and overloading. However, such sensors are incapable of providing information pertaining to chemical attacks that cause the degradation of the concrete stmcture before physical damages have been detected or observed. Arising from this, the aim of this thesis is to address the technology gap identified through the exploration and development of a series of FBG-based sensors for structural health monitoring and required by industry. The primary objective of the work is to explore the use of polymer- coated FBGs as sensors in the cementious matrix to monitor moisture which is considered as a main agent involved in most of the chemical attacks, by transporting corrosive ions in concrete structures. The sensing concept used in this work exploits the inherent characteristics of the FBG and is based on the strain effect induced in the FBG through the swelling of the polymer coating. A direct indication of the moisture level is given by the shift of the Bragg wavelength caused by the expansion of the sensing material. The context of the work, both technical and applications- focused and the development and characterisation of the sensors are discussed extensively in this thesis. The effectiveness of the sensors developed was evaluated through a series of tests performed under various harsh conditions using a range of concrete specimens with different porosity and mix composition, designed in collaboration with civil engineers. Results of the work are reported and relevant conclusions drawn for the use of this technology in the civil engineering sector, as well as incorporating a discussion of future trends and potential wider applications