Load-Bearing Behaviour of Interfaces between Normal Strength Concrete and Ultra-High Performance Concrete under Fatigue Loading

• Applicant: Professor Dr.-Ing. Vincent Oettel
• Subject Area: Structural Engineering, Building Informatics and Construction Operation
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 575543319

Many reinforced concrete and prestressed concrete bridges need to be replaced or strengthened in the long term due to the constantly increasing volume of traffic and the high proportion of heavy-load vehicle traffic. Maintaining existing bridges by suitable strengthening measures is almost always the most sustainable, economical and rapid solution. One promising strengthening measure is the subsequent addition of a concrete topping made of steel fiber reinforced ultra-high performance concrete (UHPC). Due to its very high strength and high resistance, a thin UHPC layer can be used as a directly driven-on roadway slab. By dispensing with a road surface, the dead weight of the superstructure is (almost) not increased, so that no higher stresses occur in the superstructure, in the substructures and in the foundation soil. Assuming a rigid bond slip behavior between the normal strength concrete (NSC) and the UHPC top concrete layer, a monolithic component behavior is achieved and the cross-section can be dimensioned accordingly. Rigid bond slip behavior can be provided by adhesive bond and friction as well as by an interface reinforcement that may be arranged. As UHPC has a high adhesive tensile strength, complex interface reinforcement is often not required from a structural point of view, which significantly reducing the time and cost of the strengthening measure. However, it must be taken into account that bridges are exposed to high fatigue loads, so that the fatigue resistance of the interface between NSC and UHPC is of decisive importance for the effectiveness of the strengthening measure. As very few studies have been carried out to date, it is currently unclear to what extent cyclic loads affect the bond behavior between NSC and UHPC. The degradation behavior of UHPC-NSC interfaces under cyclic loading is currently not even rudimentarily understood. The project will therefore conduct fundamental investigations into the damage processes and mechanisms of UHPC-NSC composite joints (without interface reinforcement) under fatigue loading. The investigations are carried out using systematically graded experimental tests on small and large format specimens, which include tensile, shear and beam tests under cyclic loading. The tests are documented using mechanical measurement methods and imaging techniques, which are then evaluated and analyzed. These results are supplemented by numerical investigations (FEM). Based on the results obtained and the results available in the literature, an approach for the fatigue resistance of UHPC-NSC interfaces is to be derived. The objective of this research project is to record, understand, describe and predict the degradation behavior of UHPC-NSC interfaces under cyclic loading.

• DFG Programme: Research Grants