Budapest University of Technology and EconomicsFaculty of Civil Engineering - Since 1782

Description of the research topic and tasks
The interaction between reinforcement and concrete is one of the key aspects of reinforced concrete behaviour, where the bond is responsible for the force transfer and the composite action between the materials. Traditionally steel reinforcement is used in most of the cases, however in the last decades Fibre Reinforced Polymer (FRP) bars are getting increasing attention, mainly due to their corrosion resistance and also favourable mechanical properties in some applications.
At the same time, the bond behaviour of FRP bars is different compared to conventional steel reinforcement, which comes from the different material behaviour, anisotropy and also the surface characteristics. Because of this, the existing knowledge based mainly on steel reinforcement cannot be directly applied in all cases.
Although many research works already dealt with the bond behaviour of FRP bars, these are mostly focusing on short-term behaviour. Much less information is available regarding the time dependent and durability related bond mechanisms, especially when environmental effects are also present, such as temperature changes, moisture conditions, freeze-thaw cycles or alkaline environment. These effects can influence the long-term behaviour and also the serviceability performance of FRP reinforced concrete elements in a not negligible way.
The main goal of the proposed research is to improve the understanding of the time dependent bond behaviour of FRP bars in concrete, taking into account the possible degradation mechanisms coming from environmental effects, and to develop modelling approaches which can describe these phenomena in a realistic way.
The research is planned to be based on combined experimental, analytical and numerical work, where special attention is given to multiscale modelling aspects, trying to connect the behaviour on material level to the overall structural response.
Including also investigation of alternative and more sustainable FRP materials, like for example basalt or hybrid reinforcements, which are recently getting increasing attention both from research side and practical applications.

Proposed research methodology
The PhD research is expected to follow a structured, but at the same time flexible methodology, including the following main steps:
• Review and critical evaluation of the available international literature related to FRP-concrete bond behaviour, with special focus on durability and time dependent effects
• Identification of the main research gaps based on the literature and definition of the detailed research plan
• Preparation and execution of experimental investigations (e.g. pull-out tests and in some cases beam tests as well), considering:
◦ environmental effects (temperature, moisture, freeze-thaw, chemical exposure)
◦ time-dependent behaviour (aging, possible degradation)
◦ different FRP types (e.g. GFRP, BFRP, hybrid bars) and different surface characteristics
• Evaluation of the experimental results and development of local bond stress-slip relationships, also considering the effect of time and environment
• Development of analytical models to describe the bond behaviour and its degradation
• Application of numerical modelling approaches, including also multiscale aspects, where:
◦ smaller scale (material/interface level) mechanisms are connected
◦ to the global bond response
• Some limited extension towards structural level behaviour (mainly serviceability, like crack width, tension stiffening), if it is reasonable
• Validation of the developed models using the experimental results
• Publication of the main findings in international journals and conferences