Improvement of Torsional Resistance in Ultra-High Performance Fibre Reinforced Concrete Beams
This research highlights the effect of concrete cover on the behavior of ultra-high performance fiber reinforced concrete rectangular solid beams under pure torsion. The main parameter in this research is the thickness of concrete cover which is varying between 21 and 52 mm. For this purpose, four under-reinforced ultra-high performance fibre reinforced concrete rectangular solid beams were cast and they were tested under pure equilibrium torsion. The test results verified that the torsional resistance at peak and crack loads were improved up to 113% and 134%, respectively, of the estimated value based on thin walled tube theory. Moreover, all of the twisting angle at ultimate load and shear strain in concrete were found to decrease up to 64.9%, 40.1%, respectively. In addition, both strain in longitudinal reinforcement and strain in stirrup were reduced up to 50%. The space truss analogy was modified to be compatible with this parameter effect. The modified model showed that it has a good agreement with test results.
Ultra-high performance fibre reinforced concrete (UHPFRC) is an especial kind of concrete which has high compressive and relatively high tensile strength. However, this concrete has a high value of brittleness index in comparison with fibrous normal and high strength concrete. Therefore, cracking in this type of concrete could be occured and propagated suddenly. The torsional resistance in UHPFRC beams is provided by concrete prior to cracking and reinforcements plus fibres at post cracking stages because the concrete carries tensile stress in the pre-crack stage and reinforcements with fibres carry tensile stress in the post cracking stage.
The contribution of the concrete, fibre and reinforcements is influenced by using of UHPFRC. The torsional resistance provided by fibres is improved due to high volume fraction ratio in the concrete. Furthermore, the amount of stress in the reinforcement in post cracking stage is reduced due to thick concrete cover. The thickness of concrete cover is more, the stress in reinforcements are less. Therefore, the contribution of reinforcements to resist torsion after cracking reduced.
The most important point in torsional resistance in non-fibrous concrete beams is the thickness of concrete cover. The thickness of concrete cover is the inflection point for the torsional resistance. When concrete beams have a small thickness, the torsional resistance of the beams tends to be over-strengthened and vice versa.
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