| dc.description.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. | nb_NO |
