Behaviour of Bond Mechanism in Fibre Reinforced Polymer (FRP) Composites Externally Bonded to Timber
- Publication Type:
- Thesis
- Issue Date:
- 2019
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Timber has been extensively used in construction for many centuries due to a number of advantageous properties such as aesthetics, strength-to-weight ratio, fire performance and acoustic properties. Besides, timber is only one of few renewable construction materials that can be used in large quantities. There has been an increase in the use of timber in modern structures in recent times with the advent of engineered wood products and growing interest in the use of environmentally sustainable materials in construction. Timber structures may need to be repaired and/or strengthened due to a number of reasons, such as, degradation as a result of biological and/or physical hazards, loss of strength or damage due to overloading or to meet increased load demands due to change in functionality or to comply with new code requirements. Therefore, either entire structures or key components may require strengthening, rehabilitation or replacement to maintain or upgrade their structural integrity.
Whilst demolition and replacement of degraded structures is a straightforward solution, it is often costly and time-consuming. Recent studies and applications have demonstrated that Fibre Reinforced Polymer composites (FRP) can effectively and economically be used for new structures, as well as in the strengthening and retrofitting of existing civil infrastructure. FRP is a material with high stiffness and strength to weight ratio, high Young’s modulus and high fatigue performance. Moreover, additional advantageous properties of FRP such as being light in weight with superior corrosion resistance and flexibility in application make it a viable alternative to steel in reinforcing and/or repairing timber, especially in aggressive and extreme environments.
One of the most common problems associated with the use the externally bonded FRP sheets is the premature failure due to debonding which limits the full utilisation of the material strength of the FRP. Whilst the debonding mechanism in FRP bonded to concrete is well understood based on several previous studies, only limited attempts have been made to investigate the debonding behaviour of FRP bonded to timber. It is important to mention that there are some fundamental differences in the failure mechanism when FRP is bonded to timber compared to when it is bonded to concrete. Concrete is weak in tension; whilst tensile strength of timber is much higher. Therefore, the models which work for FRP-to-concrete bond may not work for when FRP is bonded to timber. As such, a knowledge gap on potential parameters that influence bond behaviour of FRP-to-timber interface exists. Therefore, a sound understanding of the behaviour of FRP-to-timber interfaces needs to be developed and consequently, further understanding of the bond is essential.
The main goal of this research was to identify and investigate the potential parameters affecting the behaviour of the bond between timber and FRP. To achieve these outcomes, an extensive experimental program followed by analytical and numerical investigation was carried out. Through the experimental program, the influence of potential factors such as bond width, bond length, material properties and geometries on the bond strength was investigated. Investigation of the bond parameters showed that the bond strength significantly increases with increase in bond width and timber tensile strength. In addition, bond length has a major impact on the bond strength; however, bond strength cannot increase further once the bond length exceeds the effective bond length.
Whilst a number of analytical methods exist to predict the bond behaviour of FPR-to-concrete interface, analytical solutions to determine the interface behaviour of FRP-to-timber have not been fully investigated. Furthermore, existing analytical models for FRP-to-timber joints have been mostly derived based on the theoretical proposals where concrete had been used as a substrate and therefore, these models do not correlate particularly well with the experimental results. Novel theoretical models are proposed in this study to quantify the bond length, bond strength, the strain distribution profile, slip profile and shear stress relationships for FRP-timber joints. A good correlation could be obtained between the proposed models and experimental results.
Numerical simulation of FRP-to-timber joint is one of the most neglected fields of research. Numerical simulation has been undertaken to gain a better understanding about the interface behaviour of FRP-to-timber joints, and also to evaluate the feasibility of FRP application bonded to timber. It was found that by employment of proper constitutive behaviour for materials, the bond behaviour can be successfully predicted by FEA models.
The outcomes of FRP-to-timber joint tests and the models developed for the joints were then scaled up to FRP-strengthened timber beams. Finally, a design procedure for an FRP strengthened timber beam was developed to design and accurately predict the flexural capacity of strengthened timber beams.
The experimental, analytical and numerical works presented in this dissertation lead to a number of conclusions which are expected to make a significant contribution for understanding and modelling of FRP strengthened timber beams.
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