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

Description of the research topic and tasks
The construction industry is facing increasing pressure to reduce the consumption of natural resources and to improve the sustainability of materials used in structural applications. In this context, the recycling of construction and demolition waste, especially concrete, is becoming more and more important. However, in many practical cases recycled concrete is still used in low-value applications, despite its significant potential as a structural material.
In recent years, increasing attention has been given to the concept of multi-recycling of concrete, where recycled aggregates are reused in successive generations. Especially in case of high-performance or fiber-reinforced concretes, the recycled material may still preserve relatively high mechanical performance and favourable durability properties. At the same time, the changes in material structure, such as increased porosity or modified interfacial transition zones, can influence the overall behaviour of the concrete in a complex way.
Parallel to these developments, Fibre Reinforced Polymer (FRP) reinforcement is also gaining increasing interest in structural engineering, mainly due to its corrosion resistance and long-term performance advantages. However, the interaction between FRP reinforcement and recycled concrete materials is still not well understood, particularly in the case of multi-recycled concrete, where the material structure may differ significantly from conventional concrete.
The main aim of the proposed research is to investigate the mechanical behaviour and durability related properties of recycled and multi-recycled concrete, with special focus on its interaction with FRP reinforcement. The study aims to better understand how the material composition, recycling history and microstructural characteristics influence the overall performance of such systems.
Special attention is given to the role of high-performance fiber-reinforced concrete (HPFRC) as a potential source of recycled aggregates, where residual fibers and dense microstructure may contribute to improved performance even after multiple recycling cycles. The research also considers how these characteristics affect the interaction mechanisms between concrete and FRP reinforcement, including but not limited to bond behaviour.
The research is planned to combine experimental investigations with analytical and numerical approaches, to provide a consistent description of the material behaviour and to support the possible structural application of recycled concrete materials.

Proposed research methodology
The PhD research is expected to follow a structured but flexible methodology, including the following main steps:
• Review and critical evaluation of the available international literature related to recycled concrete, multi-recycling concepts and FRP reinforced systems
• Identification of the main research gaps, especially regarding the interaction between FRP reinforcement and recycled concrete materials
• Design and preparation of different concrete mixtures, including:
◦ conventional reference concrete
◦ recycled aggregate concrete (RAC)
◦ multi-recycled concrete, including possibly HPFRC-based recycled aggregates
• Experimental investigation of material properties, including, but not limited to:
◦ compressive and tensile strength
◦ durability-related properties (e.g. water absorption, freeze-thaw resistance)
◦ microstructural characteristics (e.g. porosity, ITZ)
• Investigation of the interaction between FRP reinforcement and recycled concrete, including:
◦ evaluation of bond behaviour where relevant
◦ analysis of failure modes and crack development
• Study of the influence of recycling cycles and material composition on the overall behaviour
• Development of analytical approaches to describe the observed mechanisms
• Application of numerical modelling methods for the simulation of material and interaction behaviour
• Limited extension towards structural applications, if relevant
• Validation of the obtained results and comparison with conventional concrete systems
• Publication of the main findings in international journals and conferences