Rotation capacity of plastic hinges in mixed reinforced UHPC flexural members

• Applicant: Professor Dr.-Ing. Torsten Leutbecher
• Subject Area: Structural Engineering, Building Informatics and Construction Operation
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 551949869

The rotation capacity of plastic hinge regions characterises the ductility of reinforced/prestressed concrete beams and enables the redistribution of forces in statically indeterminate structures. Existing models for predicting the rotation capacity of reinforced/prestressed concrete beams can hardly be transferred and applied to UHPC beams, since UHPC beams are generally reinforced with steel fibres. When steel fibres and bar reinforcement are combined, the tensile forces in cracks are transferred by the interaction of both types of reinforcement. After pronounced multiple crack formation with typically crack spacing of only a few millimetres, the further increase in deformation concentrates in a few, often just a single crack once the elastic limit of the bar reinforcement is exceeded. This so-called crack localisation defines a third failure mode for mixed reinforced UHPC beams, besides the two classical failure modes concrete compression failure and tensile failure of the longitudinal reinforcement. To predict the rotation capacity of plastic hinges in mixed reinforced UHPC beams, knowledge of the moment-curvature relationship is a prerequisite considering the possibility of crack localisation. The aim of this research project is to develop a mechanically based, analytical engineering model to determine the rotation capacity of plastic hinges dominated by flexural cracks in mixed reinforced UHPC beams. The modelling is based on the hypothesis that crack localisation is not triggered by reaching a specific strain value, but rather a specific crack width. Thus, the initiation of crack localisation is expected to depend especially on the parameters controlling crack formation, such as the ratio of longitudinal reinforcement, the rebar diameter, the cross-sectional height and the stress-crack opening behaviour of the steel fibre reinforced UHPC. Furthermore, the heterogeneity of fibre distribution/orientation may influence the rotation capacity of plastic hinge regions. Difficulties in verifying these hypotheses and modelling based on them are caused due to the lack of well-documented experimental data. Since most studies in the literature only focus on the prediction of the ultimate moment resistance of the beam but not on its deformation capacity, the deformations and strains were only rarely recorded within the relevant plastic regions. There is also hardly any data available on the fibre distribution/orientation. However, this information is necessary to reliably determine the contribution of the steel fibre reinforced UHPC in a flexural crack. Therefore, crack development, strain distribution, and fibre distribution/orientation are planned to be comprehensively characterized in the experimental campaign of the project using various methods. On that basis the engineering model will be developed and validated.

• DFG Programme: Research Grants