An investigation of FRP-to-timber bonded interfaces

Abstract

Timber has been used as a construction material in civil infrastructure throughout the world for several millennia and it is still a popular construction material to this day. Degradation of timber due to mechanical and environmental actions, as well as possible higher loads, can necessitate the need for strengthening or repair. The external bonding of fibre-reinforced polymer (FRP) composites offers a viable solution. A lack of understanding of the strength and behaviour of FRP-to-timber bonded interfaces is, however, hindering the safe and rational design of FRP strengthening measures for timber structures.

The aim of this research project is to enhance understanding of the strength and behaviour of the bonded interface between timber and FRP. Important tangible outcomes of the project include the development of effective bonding systems as well as bond stress-slip models and bond strength model which quantify the bonded interface. In order to achieve these outcomes, an extensive experimental and analytical investigation is conducted. Tests are performed and reported on FRP-to-timber joints as well as FRP-strengthened beams. For the former, softwood (Pine), hardwood (Camphorwood) and glulam timber products have been tested. Variables include (i) externally bonded (EB) plates and near-surface mounted (NSM) plates, (ii) FRP plates formed in a wet lay-up procedure and pultrusion, (iii) bonded length of FRP, (iv) adhesive type, (v) adhesive thickness, (vi) timber species, and (vii) natural growth characteristics of the timber such as annual growth rings and knots. The concept of an effective bond length has been verified from the tests as well as effective procedures and materials for bonding FRP to softwoods and hardwoods. Models are also proposed and validated to quantify the bond strength and bond stress-slip relationships of the joint tests. The strengthening methods are then applied to glulam beams in order to observe the behaviour and strength of the bonding systems on a larger scale as well as on a system that bends. The bond strength model proposed from the joint tests is then assessed against the beam tests. Finally, conclusions are made on the entire program of study. Then, recommendations for future research are proposed.