Title: Experimental Investigation of Hybrid Concrete Elements with Varying Fiber Reinforcement under Concentrated Load
Author(s): Plückelmann, S.; Breitenbücher, R.
Publication: Symposium Paper
Appears on pages(s): 422-431
Keywords: Steel fiber reinforcement, hybrid reinforcement, splitting stresses, bearing behavior
In special cases, concrete members are exposed to high locally concentrated loadings. Such
concentrated loadings lead to a multi-dimensional stress state beneath the loaded area. Due to
the load diffusion, large splitting tensile stresses are generated in the upper regions of the
concrete member (i.e. St. Venant disturbance zone) and spread along directions perpendicular
to the load. In order to resist these splitting tensile stresses, the state of the art is to reinforce
concrete members with transverse steel reinforcement. An alternative approach is to add steel
fibers to the concrete matrix. However, regarding economic concerns it may not appropriate
to reinforce the entire concrete member with an adequate high amount of steel fibers, rather
only those zones where high splitting stresses are expected.
The main objective of the presented experimental study was to investigate the load-bearing
and fracture behavior of hybrid concrete elements with splitting fiber reinforcement under
concentrated load. For this purpose, in a first step, hybrid specimens were produced
containing both plain and fiber concretes. The reference specimens consisted exclusively of
plain concrete, while the hybrid specimens were partially strengthened with various types of
steel fibers only in the St. Venant disturbance zone, instead of a full range fiber
reinforcement. The thickness of the reinforcement layer was varied in order to determine the
optimal configuration of fiber reinforcement. Taking into account the influence of the casting
direction on the fiber orientation and consequently on the bearing and fracture behavior, the
hybrid specimens were cast either in standing or in lying molds by means of a “wet-on-wet”
casting technique. These hybrid elements were then tested under concentrated load.
The test results showed that under concentric loads the maximum bearing capacity of the
hybrid specimens increased progressively with growing thickness of the fiber reinforced
concrete layer. In contrast to the plain concrete specimens, the fiber reinforcement led to a
remarkable improvement in the post-cracking ductility. Compared to the fully reinforced
specimens, the hybrid specimens that were only reinforced in the St. Venant disturbance zone
exhibited - besides an almost identical bearing capacity - a similar local behavior in the postcracking
zone. Furthermore, a significant impact of the casting direction on the bearing as
well as fracture behavior could be proved.