Ekkehard Fehling

Ultra-High-Performance Concrete: Research, Development and Application in Europe

One of the breakthroughs in concrete technology is ultra-high-performance concrete (UHPC) with a steel like compressive strength of up to 250 N/mm2 and a remarkable increase in durability compared even with high-performance concrete. In combination with steel fibers it is now possible to design sustainable filigree, lightweight concrete constructions with or even without additional reinforcement. Wide span girders, bridges, shells and high rise towers are ideal applications widening the range of concrete applications by far. In addition, e.g. to some pedestrian bridges heavily trafficked road bridges have been built in France and in the Netherlands. Bridges are already under construction in Germany as well. A wide range of new concrete formulations has been developed to cover an increasing number of applications. Technical recommendations have recently been published in France and Germany that cover material as well as design aspects. The paper will report on the state of research and application of UHPC in Europe, on material and design aspects of UHPC and will present the state-of-the-art based on an international Symposium on UHPC held in Kassel in 2004.

Tensile Behavior of Ultra-High-Performance Concrete Reinforced with Reinforcing Bars and Fibers: Minimizing Fiber Content

When combining conventional reinforcement with fibers, differences in the structural behavior arise compared to reinforced and prestressed concrete. This fact is of special interest for ultra-high-performance concretes (UHPCs) because fibers are added to these concretes generally to improve ductility. With regard to durability, the positive influence of the fibers on the crack widths is significant. Especially for enhanced durability requirements, an essential improvement compared to reinforced concrete (RC) can be achieved by combined reinforcement. The mechanical relationships required for the analysis of the crack formation process of the two reinforcing elements—reinforcing bars and fibers—are presented in this paper. They are linked with each other considering equilibrium and compatibility. The derived relationships are validated experimentally. Because of its universal formulation, the proposed model is not limited to UHPC but is generally applicable to all types of concrete.

Design of an Innovative Composite Bridge Combining Reinforced Concrete with Ultra-High Performance Fibre Reinforced Concrete

A novel concept is applied for the design and construction of an innovative 46m long overpass road bridge. The principle of this design concept is to use Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) to “harden” only those zones where the structure is exposed to severe environmental and high mechanical actions, i.e. top surface of the deck slab, kerb elements and the zone above the middle support including a unique UHPFRC hinge. All other parts of the bridge structure remain in conventional reinforced and prestressed concrete as these parts are subjected to moderate exposure. This conceptual idea (originally developed for the rehabilitation of existing concrete structures) combines efficiently protection and resistance functions. The resulting bridge has significantly improved structural resistance and durability. By the time of writing this paper, the bridge was in the tendering phase.

Compression-Tension Strength of Reinforced and Fiber-Reinforced Concrete

The compressive strength of concrete can substantially decrease in relation to the uniaxial compressive strength by transverse tension and cracking. This holds true for plain and reinforced concrete. The question of the biaxial compression-tension strength of reinforced concrete has been examined over the past 40 years by numerous scientists. Their results, however, vary considerably and lead to contradictory conclusions. Accordingly, in national standards, very different calculation rules can be found on this subject, whereby the provided reductions differ up to a factor of 2 for the same application. Based on the authors’ own experimental investigations and a critical review and classification of former test series that are regarded as trendsetting, a proposal for the reduction of the compressive strength of cracked reinforced concrete is developed. For the first time, the influence of fibers in addition to bar reinforcement is also considered.

Design of First Hybrid UHPC-Steel Bridge across the River Fulda in Kassel, Germany

A 132 m long hybrid ultra-high performance concrete (UHPC) steel bridge with 6 spans is currently being built across the river Fulda in Kassel, Germany. UHPC offers supreme durability characteristics and, hence, has been selected for the replacement of an existing, damaged timber structure. The novel structural concept consists of a hybrid steel – UHPC truss structure and precast UHPC plates for the bridge deck. The deck plates are glued to the upper chords of the truss structure. For the transfer of shear forces from the truss diagonals (steel tubes) to the UHPC chords, prestressed bolted friction connections are used. The paper describes the conceptual design, the final design, accompanying tests and the erection of the bridge structure. A monitoring system will be installed in order to gather practical experience with the new materials and the novel structural concept.

Effect of Steel Fiber on the Behavior of Circular Steel Tube Confined UHPC Columns Under Axial Loading

This paper is aimed at presenting an experimental investigation on a total of 12 circular ultra high performance (UHPC) concrete filled steel tube short and intermediate columns under axial loading on only the concrete core. The influence of steel fibers with 1% and 2% by volume was examined by means of some performance indices including the strength and strain enhancement index (SI and SE), the ductility index (DI) and the strength ratio (SR). It was found that the use of steel tube as a confinement method for UHPC columns provides a significant improvement in both terms of the strength and the ductility, thereby restricting the extremely brittle failure of UHPC. In addition, there is no noticeable influence of steel fibers on the strength and ductility of test columns. However, the presence of steel fibers affects to the failure mode and slightly increases the ductility of the intermediate columns.

Tragfähigkeit von Biegebauteilen aus UHPC

Ultrahochfester Beton (UHPC) ist ein innovativer Baustoff mit einer hohen Druckfestigkeit und guten Dauerhaftigkeitseigenschaften. Durch Kombination von Stabstahl- und Stahlfaserbewehrung konnen Zugkrafte effektiv aufgenommen und gleichzeitig sprodes Versagen verhindert werden. Zur Ermittlung der Traglast von Balken werden, aufbauend auf den an Dehnkorpern gewonnenen Erkenntnissen, die Faserwirkung und die Interaktion der Bewehrungselemente fur den masgebenden gerissenen Querschnitt in einem eigenen Berechnungsansatz beschrieben. Eigene Versuchsergebnisse wie auch das mechanisch begrundete Modell zeigen, dass die Duktilitat im Traglastbereich mit steigendem Fasergehalt abnimmt. Darauf basierend wird die anzusetzende Mindestbewehrung diskutiert und ein Ingenieurmodell zur Bemessung im Grenzzustand der Tragfahigkeit vorgestellt.

Design for Serviceability of Ultra High Performance Concrete Structures

The combination of steel bar reinforcement with fibers causes differences in the structural behavior compared to reinforced and prestressed concrete. This fact is of special interest for ultra-high-performance concretes (UHPC), because fibers are added to these concretes generally to improve ductility. Based on the well-known relations concerning calculation of crack width for reinforced concrete, the influence of the fibers on the cracking process is introduced considering equilibrium and compatibility. For the calculation of deformations a modification of the stress-strain relationship is derived considering the tension stiffening due to the contribution of fibers within the cracked sections and in between them. Because of their mechanical basis, the equations are not limited to UHPC but generally applicable to all types of concrete.