As per my previous concepts, the ductility of concrete in uni-axial compression decreases as the compressive strength increases.(As from the stress strain diagram, a low strength concrete fails at higher strain levels)
But when we talk about the structural elements i.e, Flexure members, Compression members numerous sources on the internet say " That the ductility of the concrete increases with the increase in the compressive strength" What could be a good justification to this? Please answer in the light of following:
1) Is it because we have improved shear strength attributed with higher strength?
2) The curvature ductility increases with the increase in compressive (Fig Attached below)
So can someone argue, that a column in pure axial state , its ductility decreases, but when we have some eccentricity ( i.e, bending) the ductility increases
Thanks in Advance!
If you draw the stress-strain curve of any material and calculate the area under the curve that represents the modulus of toughness which is the energy absorbed by the material per unit volume. This modulus is nearly constant even with changing the components of concrete, hence if you increase the strength (the vertical axis) you will find a decrease in the maximum strain representing ductility (the horizontal axis) as the area beneath the curve should not significantly change from one concrete grade to the other.
is the curvature ductility curve provided for a non reinforced beam? usually those type of experiments are conducted for reinforced members. The first figure is for sure a test on non-reinforced concrete beam in axial compression.
@Sami , Curvature Ductility is usually used in context of structural ductility i.e, of the members typically with reinforcing elements, since its a ratio of curvature at ultimate moment to that of the yield.
The question however still remains how does the increase/decrease in compressive strength affect the ductility of concrete ?
I think it has something to do with reinforcement. As concrete strength increases, tension controlled failure of tension reinforcement increases. In other words, steel is being activated more thoroughly as concrete becomes capable of taking higher stress. So the enhanced 'ductility' or curvature is in fact a result of steel reinforcement. I think a better way of comparing curvature ductility experiment with uniaxial compression experiment would be to conduct both tests on reinforced concrete samples.
regards
^Yes the ductility is enhanced, by virtue of enhanced tensile stress of concrete.
However, I disagree with "More activated reinforcement" in case of high strength concrete, the reinforcement comes into action only once the modulus of rupture is reached, i,e, once the cracks are formed, the reinforcement on the tension face becomes effective in carrying tension. Before that, tensile stresses in steel are low or negligible.
Concrete becomes brittle with the grade and with the addition of silica fumes and polymer impregnation.
@Usman: lol this is becoming a rather protracted discussion. modulus of rupture will be reached long before the yield in tension steel. So cracks will definitely be there on the tension face for tension steel to be activated. As for the question whether concrete becomes more brittle at higher grade? - yes it does. I suspect that the reason why curvature at ultimate failure is higher for higher grade concrete is because its doubly reinforced at both faces. so as concrete fc increases, it allows more bending due to ability to take higher stresses. Also, the cracks that develop are being arrested by reinforcement. There is however the possibility of concrete burst at higher strength. I may be wrong on this.
It is obvious that when you include reinforcement in concrete, then increase in concrete strength can effectively be utilized. Yes, steel shall play its role only after formation of micro-cracks in concrete. But, added elongation of steel shall increase the ductility of the composite material without which concrete would fail as soon as the rupture cracks form.
^ Thanks for your answer. However, the real question perhaps was why a higher grade concrete will provide more curvature than a lower grade one (assuming similar reinforcement arrangements, grade etc). I would like to add that a higher grade concrete has a much smaller stress block. This reduces the neutral axis depth from compression face and results in a bigger tension triangle. A higher strain in tension and compression steel becomes possible which leads to enhanced curvature.
Usman Mahmood ... Please check references below.. hope it will be helpful for you…
Regards…
https://espace.library.uq.edu.au/view/UQ:343235/UQ343235_OA.pdf
https://hub.hku.hk/bitstream/10722/71627/1/content.pdf?accept=1
http://dspace.unimap.edu.my/xmlui/bitstream/handle/123456789/13561/033-037_effect%20of%20material.pdf?sequence=1
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