Durability and Long-term Performance of Adhesively Bonded FRP/steel Joints

Doctoral thesis, 2017

Fibre reinforced polymer (FRP) composites offer excellent properties, such as high specific strength and stiffness, corrosion resistance and light weight. Over the past four decades, FRPs have been increasingly used for strengthening and repair of bridge structures, and more recently, in the manufacture of whole/hybrid FRP bridges. Although, the short-term behaviour of FRP/steel bonded joints has been extensively studied, the subject of the long-term performance and durability has not been researched to the same degree. Today, uncertainties regarding the durability aspects of adhesively bonded FRP/steel joints present a major obstacle to their growing application. This thesis aims to deepen the understanding of the structural effects of environmental exposure conditions relevant to bridges on bonded FRP/steel joints, with a focus on predicting the mechanical response of aged joints. Firstly, to map out the research needs, a comprehensive state-of-the-art literature review was carried out, and the most important identified knowledge-gaps were pursued for research. An extensive experimental programme was conducted including long-term testing of bonded FRP/steel joints that were subjected to various temperature ranges, humidity levels, and cyclic exposure scenarios. Among other factors, the effects of adhesive layer thickness and type of FRP material were investigated. The results showed the importance of FRP permeability on moisture and damage distribution profile in bonded joints. In addition, freeze–thaw cycles were found to have no unfavourable effects on the strength of dry or preconditioned joints. Complementary material characterization tests were also conducted to study moisture diffusion kinetics. These results underlined the importance of considering the exposure history for prediction and design purposes. Furthermore, the dependency of cohesive laws of the adhesive material on environmental exposure was investigated using an innovative approach based on open-face specimens in conjunction with the J-integral analysis. FE simulations were incorporated to predict the mechanical response of joints after environmental ageing. Firstly, the applicability of the cohesive zone modelling approach to strength prediction of bonded FRP/steel joints was investigated. The results confirmed the accuracy of the predictions provided that the variation of failure modes were taken into account. Moreover, the minimum required overlap length was found to be directly proportional to the shape of cohesive laws. This finding, in combination with the environmental-dependent cohesive laws, can be employed in the design phase to ensure sufficient anchorage length after in-service exposure. Lastly, sequentially coupled moisture diffusion–fracture analysis were found to provide reasonable predictions of the mechanical behaviour of environmentally aged joints. This study provides the basis for durability-related experimental characterisation methods and predictive modelling of adhesively bonded FRP/steel joints.