Fibre Orientation in Steel-Fibre- Reinforced Concrete: Quantification methods and influence of formwork surface and reinforcement bars in structural elements

Abstract

The effectiveness of fibres greatly depends on their orientation with regard to the principal tensile stress direction. Consequently, the ability to control fibre orientation during casting would increase the effectiveness of fibres and reduce the necessary dosage. Understanding fibre orientation mechanisms is therefore important. The theoretical part of the thesis introduces the fibre orientation mechanisms and definitions, presents the standards and guidelines for structural design with respect to fibre orientation, and gives an overview of methods for the detection of fibre orientation and the assessment of the fibre amount and its distribution in FRC elements. The methods are divided into two groups: direct and indirect. Direct methods are based on direct observation of fibres (e.g. visual), while indirect methods include measuring values like conductivity, permeability and AC impedance which can then be related to fibre orientation or fibre amount. The experimental part of the thesis describes three cases in which fibre orientation mechanisms and the influence of formwork, reinforcement bars and element type on fibre orientation were investigated in steel-fibre-reinforced concrete slabs or walls cast from a single point. In the first case, slabs were cast using three different horizontal mould roughnesses: oiled laminated plywood, oiled ordinary wood surface, and glued sand surface. In the second case, slabs with three reinforcement bar solutions were cast: non-reinforced, unidirectionally reinforced, and grid-reinforced. In the third case, two wall elements were cast in order to determine the fibre distribution and orientation in walls and the effect of formwork tie bars. The data for analysis was obtained from small specimens. Beams were sawn from the slabs or walls in all three studies, and in the first case study cores were also drilled. The fibre orientation and distribution were characterised using the following methods: manual fibre counting at sawn cross sections, image analysis of sawn and polished cross sections, and Xray Computed Tomography (CT) with subsequent image and computer analysis, in addition to the prior numerical flow simulation used as part of the test planning. Sawn beams were also put through 3 or 4-point bending tests. When the results from the different fibre orientation and distribution detection methods were compared, a good correlation was found between the manual fibre counting, image analysis, and CT scan analysis. The fibre orientation expressed by the fibre orientation factor and by the mean cosine of the out-of-plane angles did not correlate very well, however, because the fibre orientation factor is influenced by local variations in fibre content. The experimental results showed that the surface roughness of the formwork had considerable influence on fibre orientation, and that the reinforcement bar layout had a great influence on both fibre orientation and distribution in both vertical and horizontal directions. In the wall elements, the fibre orientation showed large variation in general, and the formwork tie bars affected local fibre distribution and orientation. The beam bending tests demonstrated the strong effect of fibre distribution and orientation on residual flexural tensile strength. The beams sawn from slabs cast in slip-surface formwork or from wall elements varied greatly in residual flexural strength, while slabs cast in moulds with a rough surface or in the presence of reinforcement bars had considerably less variation, because fibre orientation in the lower layers was much more isotropic. Comparison of residual flexural tensile strength results showed that the four-point bending test generally gave lower values than the three-point bending test in accordance with EN 14651. In recent years, there has been good progress in the development of standards and guidelines for the design and execution of SFRC, but the findings of this research are that fibre orientation needs to be incorporated in a more consistent way.