Behaviour of steel fibre reinforced high performance concrete under biaxial loading conditions.
PhD thesis, University of Glasgow.
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This thesis includes an experimental investigation of the behaviour of steel fibre reinforced high performance concrete (SFRHPC) under biaxial loading conditions. Also included are constitutive models to enable numerical predictions of the strength behaviour of such a material.
Within the experimental stage a large biaxial test machine was designed and manufactured. The load capacity of each axis was 2000 kN. Special thought was given to the load platen system because of the friction which occurs between the platen system and the concrete specimen. Brush bearing platens and solid steel blocks with and without Teflon friction reducing pads were tried. Because the brush platen and the Teflon pads were constantly damaged during testing, solid steel block platens were finally used. For tests where tension loads were involved, experiments were carried out with dog-bone shaped specimens and specimens glued on to the platens. Finally, the tension loads were transferred through threaded rods cast into the specimens and connected to the machine patterns with screws.
Modern control schemes and high speed data acquisition systems were used to monitor the material response and to collect experimental stress and strain results. The principal deformations were monitored and the crack patterns and failure modes examined. Failure envelopes were developed based on the strength data for each fibre variable. The load capacity of SFRHPC under biaxial load conditions was found to be larger than for plain HPC for all fibre types and volume fractions. The stress-strain recordings indicated a linear behaviour almost up to failure. The examined failure mode between plain and steel fibre HPC was similar and all specimens failed very suddenly with a splitting failure type.
The test variables included four different types of hooked ended steel fibres with different fibre volume fractions from plain to 2% in 0.5% steps. The specimens were tested under the entire range of stress combinations including uniaxial compression and tension and biaxial compression-compression and compression-tension. As a result the specimen still failed in an explosive manner but the pieces were still connected together by fibres after failure. The biaxial strength compared to the uniaxial strength increased more, as observed with normal strength concrete. With the addition of fibres the biaxial strength behaviour of HPC was almost the same as for plain normal strength concrete. The deformation characteristics of plain and SFRHPC showed a linear behaviour up to a higher stress than normal strength concrete. In fact the linear limit was almost as high as the failure load.
The examined strength data was used to model the biaxial strength envelopes of HPC and SFRHPC using different methods. These included the Ottosen failure criterion and the Willam and Warnke failure criterion.
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