Evaluation of FRP/wood adhesively bonded epoxy joints on environmental exposures

Abstract

This article describes the evaluation of the durability of joints composed of wood adherends with a bonded layer of fibre-reinforced polymer (FRP) fabric. Carbon and glass fibres in an epoxy matrix were studied. The main purpose of FRP usage with timber in the construction industry is generally to improve the stiffness/strength of reinforced members without any influence on their service-life or any environmental impact. From the perspective of the timber reinforcement process, optimal dimensional stability during moisture changes in wood should be one of the most important criteria for such joints. Therefore, FRP/wood joints were evaluated with regard to the influence of real external environmental conditions on the bondline over a period of 40 months. During exposure to these conditions, specimen failures and defects were continuously visually evaluated. The decisive factor in this evaluation was bond integrity, verified by the tensile shear strength of the FRP/wood joint. After the experimental study, it was noted that the first 20 months have a significant effect on bondline failure occurrences, which involve decreases in tensile shear strength. In the next 20 months, the FRP/wood bondlines resist other severe hygrothermal stresses without significant strength decreases. An additional observed parameter was the percentage of wood failure in the bonded area of single lap joints, which characterises the mode of failure of the bonded joint. To determine the influence of ageing on adhesive due to ultraviolet radiation and varying temperature, infrared absorption spectroscopy analysis was performed to reveal changes in the macromolecular structure of the epoxy adhesive. Findings showed that UV radiation had a significant influence on the degradation of the adhesive structure.

Keywords:

• durability
• FRP/wood joint
• epoxy adhesive
• long-term ageing test
• infrared absorption spectroscopy

Acknowledgement

The results were achieved while carrying out the research project No. P104/11/1557 ‘Complex Evaluation of Durability of Bonded Structural Timbers’, which is fully financially supported by GACR (Czech Science Foundation).