No abstract

Adding steel fibers to concrete improves the post-cracking tensile strength of the composite material due to fibers bridging the cracks. The residual performance of the material is influenced by fiber type, content and orientation with respect to the crack plane. The latter is a main issue in fiber-reinforced concrete elements, since it significantly influences the structural behavior. The aim of this research is to develop a tailor-made composite material and casting method to orient fibers in order to optimize the performance of the material for structural applications. To this aim, a mechanized concreting device that induces such preferred fiber orientation is designed and fabricated. It uses vibration and a series of narrow channels to guide and orient fibers. A composite with oriented fibers is produced using a hybrid system of macro and micro fibers and high-performance concrete. From the same concrete batch, specimens are cast both with and without the fiber orientation device, obtaining different levels of fiber orientation. Three-point bending tests are performed to measure and compare the residual tensile strength capacities with standard specimens cast according to EN 14651. Elements with favorable fiber orientation show a significant increase in residual tensile strength with respect to the standard beams. Finally, computed tomography and an electromagnetic induction method are employed to better assess the orientation and distribution of fibers in the beams. Their results are in good agreement and enable to link the residual tensile strength parameters with fiber orientation.

No abstract

Tunnels in unfavorable ground conditions are exposed to high internal stresses due to localized loading at the outer face. Addition of extra layers of compressible material as a countermeasure needs more material and greater excavation volumes which is in conflict to the global goals of CO2 reduction. Thus, two alternative types of segments with higher bearing capacities are proposed that allow for lining thickness reduction. They are derived from multi‐load topology optimization considering relevant load cases during construction and in service. The first design incorporates centered recesses that maximize segmental stiffness. The second shifts the recesses to the outer face increasing their potential to absorb deformation. Two prototypes of each type are fabricated from high performance steel fiber reinforced concrete. Experiments show that a concrete reduction of up to 55.2% is achieved with respect to a conventional design. The volume savings give space for layers of radially compressible material.