Structural health monitoring of a composite bridge using Bragg grating sensors. Part 1: Evaluation of adhesives and protection systems for the optical sensors

The present paper is part of a series of two concerning the structural health monitoring of a road bridge using optical fibre Bragg grating sensors. This bridge named the West Mill Bridge is entirely made of glass and carbon fibre composite materials. The use of Bragg grating sensors was made possible by the development of a bonding methodology and appropriate protection systems for the sensors. The optical sensors were successfully implemented on site and they all survived the construction and installation of the bridge. This first paper focuses on the evaluation of the adhesives and protection system of the sensors. Pullout tests on optical fibres bonded on bridge coupons by the selected adhesives were carried out to evaluate the influence of dry and wet conditions on the fibre bonding. Cyanoacrylate and epoxy adhesives were used in this study. The durability of the sensor protection system that was developed and implemented on the composite bridge was investigated by immersing protected optical sensors in water. The strain development on the sample surfaces during immersion was continuously monitored whereas the integrity of the sensor bonding line was evaluated by performing three-point bending tests at specific time intervals. The first study highlighted that failure mechanism and load of the fibres bonded on bridge coupons were not influenced by the environmental conditions, and that stress concentrations developed during curing of the epoxy adhesive were the reason for lower failure loads and altered failure mechanism. The second study demonstrated that the protection system isolated effectively the sensors from the water, and also that swelling of the bridge samples due to water absorption by the composite material was recorded.