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aesthetic can be designed with UHPC without loss of
loadbearing capacity. Although the raw materials for the
manufacture of UHPC are more expensive than those required
for normal-strength concretes, much less are required.
The dense structure, moreover provides an improved
corrosion protection so that a clearly higher service
life and less maintenance costs for the structural components
are to be expected.
Shear resistance of UHPC prestressed concrete
girders
Shear resistance tests on prestressed concrete girders
made of UHPC have been carried out on behalf of the German
Research Foundation (DFG) at the Institute of Concrete
Structures at RWTH Aachen since 2005. In the
42 experiments conducted so far, the infl uence of the fi ber
concrete on the prestress, the shear slenderness and local
additional reinforcement was investigated both on solid
girders and girders with web openings. During the tests,
the number and position of the web openings were systematically
varied.
The experiments were performed on the UHPC crosssection
shown in Fig. 1. The shear resistance was markedly
increased through the addition of the fi bers (Fig. 2).
Already the addition of 0.9 volumetric percent led to an
increase in the bearing capacity from 149 kN (T2b) to
326 kN (T5a). Through the addition of 2.5 volumetric percent,
a value of 408 kN (T3b) was attained. In order to obtain
loadbearing capacities of this order in normal-strength
concrete, the girder height would have to be at least doubled
and a shear reinforcement provided (Abb. 1).
Infl uence of openings
Small openings for conduits with a diameter of d/2 have
long become standard practice for roof trusses. The infl uence
of the openings on the shear resistance has been systematically
investigated in several test series so thata
structural design depending on the number, position and
diameter of the openings can be performed. Furthermore,
various forms of additional reinforcement adjacent to the
openings were tested and their bearing capacity determined
in load tests.
Outlook
First prototype application in the form of footbridges of
UHPC have already been realized in Germany. Based on
the fi ndings gained through the shear resistance tests, optimized
prestressed concrete girds with and without openings
can now also be cost-effi ciently realized.
Shear force/Querkraft [kN]
without fi bers
ohne Fasern
Defl ection/Durchbiegung [mm]
Podium 6
Fig. 2 Load-deformation curves resulting from shear resistance
tests performed on prestressed concrete girders made of UHPC
with diff erent fi ber contents.
Abb. 2 Last-Durchbiegungs-Kurven von Querkraftversuchen an
Spannbetonträgern aus UHPC mit unterschiedlichen Fasergehalten.
Einfl uss von Öff nungen
Kleine Öff nungen für Leitungsführungen mit einem
Durchmesser bis d/2 sind gängige Praxis bei Dachbindern.
Der Einfl uss der Öff nungen auf die Querkrafttragfähigkeit
wurde in mehreren Versuchsreihen systematisch
untersucht, so dass eine ingenieurmäßige
Bemessung abhängig von Anzahl, Lage und Durchmesser
der Öff nungen vorgenommen werden kann. Zudem
wurden verschiedene Formen von Zulagebewehrung neben
den Öff nungen getestet und deren Traglaststeigerung
ermittelt.
Ausblick
Erste Prototyp-Anwendungen in Form von Fußgängerbrücken
aus UHPC sind in Deutschland bereits ausgeführt.
Mit den vorliegenden Erkenntnissen aus den Querkraftversuchen
werden jetzt auch optimierte Spannbetonbinder
mit und ohne Öff nungen wirtschaftlich
realisierbar.