Research focus

Design and construction with ultra-high performance concrete (UHPC)

Funding and duration

German Research Foundation (DFG), LE 3824-5-1
Duration: 2025 to 2027

Researcher

Sina Yüksel, M.Sc.

Project description

The rotational capacity of plastic zones characterizes the ductility of reinforced concrete/prestressed concrete bending beams and enables the redistribution of stresses in statically indeterminate structures. It is not easy to transfer existing models for predicting the rotational capacity of reinforced/prestressed concrete bending beams to UHPFRC bending beams, as UHPFRC bending beams are generally additionally reinforced with steel fibers.

When combining steel fibers and bar reinforcement, the tensile forces in cracks are transferred by the interaction of both types of reinforcement. After pronounced multiple crack formation with cracks at intervals of only a few millimetres in some cases, a concentration of the further increase in deformation on a few, often only a single crack is observed after the elasticity limit of the bar reinforcement is exceeded. This so-called deformation localization describes a third failure mode for combined reinforced UHPFRC bending beams, in addition to the two classic failure modes of concrete compression failure and tensile failure of the longitudinal reinforcement. Knowledge of the moment-curvature relationship, taking into account the possibility of deformation localization, is a prerequisite for predicting the rotational capacity of plastic joints in combined reinforced UHPFRC bending beams.

The aim of this research project is to develop a mechanically based, analytical engineering model to determine the rotational capacity of plastic bending crack hinges in combined reinforced UHPFRC bending beams. The modeling is based on the hypothesis that the deformation localization is not triggered by reaching a certain strain value, but a certain crack width, so that the occurrence of the deformation localization depends significantly on the variables controlling the crack formation, such as the longitudinal reinforcement ratio, the bar diameter, the cross-section height and the stress-crack opening behavior of the steel fiber reinforced UHPFRC. Furthermore, the heterogeneity of fiber distribution/orientation is expected to influence the rotational capacity of plastic zones.

Difficulties in testing these hypotheses and building a model based on them are caused by the lack of well-documented experimental data. Since most of the studies in the literature only focused on the prediction of the moment resistance of the bending beam, but not on the deformation capacity, deformations and strains were rarely recorded over a large area. Similarly, there is hardly any data on fiber distribution/orientation. However, this information is necessary to reliably determine the contribution of the steel fiber-reinforced UHPFRC in the flexural crack. In the experimental work planned in the project, crack development, strain distribution and fibre distribution/orientation are therefore to be comprehensively characterized using various methods and the engineering model developed and validated on this basis.