Torsten Leutbecher

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.

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.

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.