High strength fibre reinforced concrete

Static and fatigue behaviour in bending

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Recently, a number of high strength and ultra high strength steel fibre concretes have been developed. Since these materials seem very suitable for structures that might be prone to fatigue failure, such as bridge decks, the understanding of the static and fatigue bending behaviour is vital. In order to evaluate the bending behaviour of high and ultra high strength concretes, an experimental and analytical approach was followed. Four mixtures were chosen, two ultra high strength concretes, with a compressive strength of more than 200 MPa, and two high strength concretes, with a compressive strength of 120 MPa. The main testing method was the four point bending test on un-notched beam specimens of dimension 125/125/1000 mm. The beams were tested at a span of 750 mm, loaded in their third points. First a series of static bending tests were performed, followed by a number of fatigue bending tests under different values of the upper load level. A point of further attention was the fibre alignment and dispersion: this was examined with image analysis on photographs of beam cross-sections taken at the fracture surface. The tests showed that the mixtures were deflection hardening materials and also strain hardening materials in tension. The mixture with the best workability had the lowest scatter in the static and fatigue behaviour, and this highlights the effect of the fresh state properties on the material behaviour in the hardened state. The fibre count showed that even though a direct relation was found between the number of fibres in the critical cross-section and the flexural strength of the beam in static loading, such a clear relation was not found in fatigue loading. Also, while in plain concrete the static load-displacement curve has been reported to function as an envelope curve for fatigue displacements, this was not valid for the flexural tests of this study. A general conclusion derived from the fatigue tests of the mixtures in this study, is that the fatigue regulations, as used for normal strength concrete, remain suitable for a safe fatigue design with high and ultra high strength concretes. The experimental results were confirmed with an analytical model, the multi-layer model. The tensile material input parameters were obtained from uniaxial tensile tests. Further, the same multi layer model was adapted and used for the fatigue behaviour. A suitable material input relation is proposed that contains a gradual stiffness and strength decrease with increasing number of load repetitions. With this approach, all three stages of the deformation evolution of a fatigue experiment can be modelled. The proposed model can be easily implemented into existing finite element codes, and can therefore be used for fatigue verifications of larger structural elements of various shapes.