Exploring the shear behaviour of fibre-reinforced concrete

Author: Nicola Gehri
Language: English
external page DOI: 10.3929/ethz-b-000677209

Abstract

It is widely acknowledged that the addition of fibres can improve the mechanical behaviour of reinforced concrete and, thus, fibres may partially replace conventional reinforcement. Furthermore, the substitution of manually assembled and placed reinforcing bars with an equivalent dosage of fibres directly added to the concrete mix can yield considerable economical and ecological benefits. However, due to the often unfavourable softening behaviour after cracking of the concrete when subjected to tension, in practice, the application of fibre-reinforced concrete is nowadays essentially limited to structurally low-demanding or secondary elements. On the other hand, fibres offer substantial potential as shear reinforcement: various experiments have shown that even a moderate fibre dosage can prevent brittle shear failures in beams without conventional transverse steel reinforcement. Nevertheless, the theoretical understanding of the effectiveness of fibres as shear reinforcement is still limited and tests representative for real structures are scarce. To this end, extensive experimental and theoretical investigations of the shear behaviour of fibre-reinforced concrete were carried out in this thesis. To gain a better understanding of the structural response of fibre-reinforced concrete, the crack behaviour of fibre-reinforced girder web elements was investigated in detail. Comprehensive knowledge of the crack pattern and the crack width and slip is of particular importance in fibre-reinforced concrete, as the stresses transferred across cracks by the fibres are directly related to the crack kinematics.

The first part of this thesis addresses the development of an innovative, refined measurement technique for the automated and reliable extraction of the crack pattern and crack kinematics based on digital image correlation measurements. This technique offers detailed information of the crack behaviour, particularly in large-scale experiments with complex crack patterns, where conventional crack measurement methods are insufficient. Moreover, an approach is proposed for the determination of characteristic crack properties in homogeneous large-scale panel experiments directly from digital image correlation measurements, providing highly valuable information for the development and validation of sound mechanical models.

The second part of this thesis explores the shear behaviour of fibre-reinforced concrete through a combination of experimental and theoretical investigations. To this end, large-scale shear tests on fibre-reinforced concrete panels that are representative of girder web elements were conducted in the Large Universal Shell Element Tester at ETH Zurich. The tests were instrumented with the refined crack measurement technique developed in the first part of the thesis, which provided deep insight into the structural behaviour of fibre-reinforced concrete members subjected to shear. The obtained results build the basis for the development of a mechanically sound model for fibre-reinforced concrete subjected to shear and the derivation of practical design recommendations that aim at fostering the use of fibre-reinforced concrete in future structural applications.