| Summary: | Nowadays, high strength concrete (HSC) is gradually gaining popularity as a material used in the construction of structural elements for economical and technical reasons. HSC exhibits more brittle behaviour in comparison to normal strength concrete when subjected to compression. Ductility is a design index especially in seismic prone area and is affected by variation of neutral axis depth. The definition of the neutral axis depth is important in order to obtain the rectangular stress block in the analysis of a concrete section. This paper presents an experimental study on the evolution of the neutral axis's depth and bending stiffness variation with the ductility at bending on HSC beams. Tests on 9 HSC beams based on the American Concrete Institute (ACI) code with variable tensile bar percentages ( rmin , 0.2rb , b 0.3r , b 0.4r , b 0.5r , b b b b 0.75r ,0.85r ,r ,1.2r ) are presented. The test involved loading the beams incrementally until failure occurs. The load-neutral axis depth diagram and the loadsection stiffness diagrams were drawn. The results illustrate that with an increase in the tensile bar percentage, the neutral axis depth will increase at the ULS such that it causes brittle failure in compression with lower tensile bar percentages at the same applied load. Also the variation of bending stiffness is opposed to the variation of ductility.
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