Mechanical Behaviour and Durability of FRP-to-steel Adhesively-bonded Joints

Abstract

During the last two decades, fiber-reinforced polymer (FRP) bridge decks have been increasingly used as a competitive alternative for wood, concrete and orthotropic steel decks, due to their various advantages: light-weight, good corrosion resistance, low maintenance cost and rapid installation for minimizing the traffic disturbing time. These advantages meet critical needs for rehabilitation and new construction of pedestrian and highway bridges. To be cost effective, FRP decks are usually supported by steel girders. For the connection between FRP decks and steel girders, adhesive bonding technique is usually considered as a preferable connecting method, which can reduce construction time, save weight by eliminating fasteners, allow more uniform load transfer, achieve better adaption to the brittle and anisotropic nature of FRP materials and provide higher joint efficiency. Despite the fact that FRP bridge decks and adhesive joints are already in service in many FRP-steel composite bridges, mechanical behaviour and long-term performance are still not clearly understood, which results in more conservative designs of the FRP-steel composite bridges. To compensate this lack, the overall aim of this project is to investigate mechanical behaviours (in terms of strength and stiffness) of adhesively-bonded joints between FRP bridge decks and steel girders, as well as durability of these adhesively-bonded joints. As to the first aspect, considering the distribution of traffic loads in the longitudinal and transverse directions of bridges, the adhesive-bonded joints have been experimentally studied under six loading conditions, including tensile loading, shear loading and four combining ratios of tensile and shear loading. A specific tensile-shear loading device was designed and then employed to offer six different angle loading conditions. Different surface pretreatment methods (acetone (AC), sand paper (SP) and sand blasting (SB)) were compared with regard to influences on the stiffness, load-bearing capacity, failure mode and interfacial bonding quality of adhesive joints. A Finite Element (FE) model was developed to simulate the stress distribution throughout the adhesive joints under different loading conditions, which proved that the failure of joints was induced by combination of both tensile and shear stress peaks. The edge zone (approximately 10mm from the ends of the adhesive layer) was the most sensitive area to initiate the failure, where both the shear stress peak and the tensile stress singularity were located. Another critical aspect of this research is to characterize the durability of FRP-to-steel adhesively-bonded joints under both temperature and moisture effects. The influence of hydrothermal environmental aging on the mechanical behaviours of adhesive joints has been studied and compared with the un-aged adhesive joints. The shear-tensile failure criterions of hydrothermal aged and un-aged adhesive joints were addressed. To better understand the moisture effects, the moisture diffusion process in FRP composite materials was characterized. Subsequently, the hydrothermal degradation on the flexural and interlaminar properties of FRP laminates was addressed. A coupled hygro-mechanical FE model was developed to analyse the enviroment-dependent mechanical behaviours of FRP lanimates. This FE model was first validated by test results of flexural tests and subsequently employed in an inverse parameter identification method to determine the elastic interlaminar shear modulus of FRP laminates. Predictive equations for environment-dependent mechanical properties (flexural and interlaminar) of FRP laminates were sustained by using the least square method for the curve fitting. Results of this research can contribute to the development of a design code of FRP-steel composite bridges. They can also be used as a reference information for understanding mechanical behaviours and durability of FRP-to-steel adhesively bonded joints for other applications in civil engineering field, such as strengthening of steel structures using FRP composite materials.