Crack rebar combine with bending 3

[Tommy385]

I would like to hear some opinions (code independent) about why is bending reinforcement not adding up (or you do add it up)to already calculated cracking reinforcement (caused by shrinking). Talking firstly about foundation slabs.
Thank.

 

[Tommy385]

I ignored the bending because the question ist mostly about shrinkage and adding up effect with bending.

Few interesting views: rebar preventing cracks, rebar reduces cracks, rebar causes cracks with interaction with soil, lets add up creep and restrains... and there is a nice topic if somebody wants to write a paper.
Thank you for your answers and opinions.

 

[rapt]

Hotrod,

Sorry, too early in the morning to think straight.

Yes, the reinforcement goes into compression until the concrete cracks either due to flexure or restraint effects.

 

[BridgeSmith]

"so, do we all agree now at least that after cracking (caused by shrinking) the reinforcement within cracks is in tension and within concrete in commpresion?"

I still think it's the opposite. As the concrete tries to shrink, the internal restraint of the reinforcement creates tension in the concrete and compression in the steel. If the concrete was in compression, it wouldn't crack.

 

[rapt]

I thought that was what we were agreeing on, As the concrete tries to shrink, the internal restraint of the reinforcement creates tension in the concrete and compression in the steel.

If the concrete cracks due to the tension in it, whether from the reinforcement induced shrinkage restraint of tension from another source such as flexure or external restraint, then there is no longer tension in the concrete at the crack and and tension force remaining at the crack must be taken by the only thing left across the crack which is the reinforcement

 

[BridgeSmith]

To circle back to the OP, there is no need to provide more reinforcing for shrinkage in addition to the flexural reinforcement because shrinkage would cause compression in the reinforcement, and flexure causes tension in the reinforcement.

 

[hokie66]

HotRod10,
I think your last statement is dead wrong. Shrinkage only causes reinforcement compression in uncracked concrete. Crack control reinforcement is for controlling the widths of cracks, and the reinforcement at that stage is in tension.

 

[BridgeSmith]

I thought that once the concrete cracks, the stress in the concrete due to shrinkage would be relieved, and there would be no stress in the steel due to shrinkage.

 

[rapt]

HotRod10

Shrinkage only causes compression in the reinforcement if the member is unrestrained. If it is restrained it then causes tension in the concrete and the concrete cracks and that tension can increase the reinforcement tension and crack width.

Restraint would normally be external, eg friction with the ground in a slab on grade or connections to other elements that will restrain shortening in the concrete. Even having a suspended slab with beams can result in shrinkage restraint in the slab due to differential shrinkage rates between the thinner slab and the deeper beam section. But I have seen a case where a very huigh steel percentage caused the concrete to crack due purely to internal restraint. In that case, the reinforcement was then in tension to control the crack.

The minimum S&T reinforcement in the codes is not normally additive to the minimum flexural reinforcement. But if there is restraint, those minimums will not necessarily provide adequate crack control. The strain/stress conditions need to be checked accounting for flexure and axial tension caused by restrained shortening (shortening could be from temperature change or shrinkage).

And if there is full restraint the Australian code would require much higher minimum reinforcement than the normal about .002 in ACI. It would be about .0045 in the flexural direction and that would only give about .4mm crack width. To achieve better would require up to .0075 minimum or more as a reinforcement minimum. These are about 75% of the amount required for S&T in a non-flexural restrained member.

 

[Toby43]

A GIVEN amount of reinforcement in a concrete section/member has an "ability" to control cracking AND provides the section/member with an ultimate bending capacity. If this GIVEN amount of reinforcement satisfies both these performance criteria then it is adequate.
If shrinkage effects are deemed to affect the bending capacity of the section/member, then it must be taken into account. If bending reinforcement is designed to yield and strain well beyond yield, then the shrinkage effects would be negligible (i.e reinforcement strains associated with shrinkage are vastly smaller than strains associated with bending failure). If the ultimate bending capacity was reached prior to yield of the reinforcement (brittle - not good), then shrinkage strains would need to be accounted for - indeed it is my opinion such an analysis would be in vain, as our ability to predict all the "elastic" effects/strains in real structures (usually indeterminate to some degree - my experience is with buildings, bridges may be different) is limited (differential settlement, restraint (internal/external), construction sequencing etc...) Thus a ductile (and redundant) structure is more reliable than a brittle one (no matter how "sophisticated" the elastic analysis is).
In summary the reinforcement requirements for crack control and ultimate bending are not additive if yield can be developed in the reinforcement prior to ultimate failure, if this can't be achieved, then good luck...

Toby

 

[WARose]

Not that it means anything.....but I checked several reinforced concrete texts I have on this issue and none of them indicate that these forces are additive. It is an interesting issue though. (That I have thought about myself.)

 

[BridgeSmith]

In the situation you describe (external friction or restraint), rapt, I agree with you. The OP did mention "foundation slabs", so I definitely get where you're coming from. I was isolating the stresses due to shrinkage alone, assuming that the external forces from restraint and flexure could and would be calculated and included by superposition to get the net forces and stresses in the reinforcement.

If you're saying there is localized tension in the steel at the crack locations, I can see that as at least possible. I don't know enough about interaction of the concrete and steel at the localized area of a cracked concrete section to disagree on that point.

Over the full length of the reinforcement, however, strain compatibility dictates that unless the strain in the steel increases (the reinforcing gets longer), there cannot be tension stress in the reinforcement. In a restrained beam or slab, the external restraint and the reinforcing steel are both working against the force generated by the concrete trying to shrink, trying to return the beam or slab to its original length (and the original length of the steel), but generally expanding beyond that original length. No stretching of the steel means no tension in the steel overall.

 

[Tommy385]

@WARose I actually found it in german commentary to EC2, for "normal case" is the addition not necessary, higher requirement should be used (so long the strain between cracks stays under 0,8 promille.

@HotRod10
on the right side of picture are strains after cracking:
strain in concrete
strain in steel
bond stresses

 

[BridgeSmith]

So there is localized tension in the reinforcement at the crack locations, giving way to compression at locations away from the crack. Ok, I'm convinced of that much, anyway.

Sorry for the lengthy detour folks. The question still remains (at least in my mind) of what happens when the cracked section is subsequently subjected to flexural or axial tension. Does the flexural tension override (replace) the effects of cracking due to shrinkage, directly add to it, or something in between?

 

[rapt]

WARose,

The individual effects of the 2 conditions are not additive.

Both effects should be applied at the same time to determine the overall combined stress state. The combined result will normally be somewhere above the larger of the 2 and below the sum of the 2 when analysed together.

 

[Tommy385]

@rapt
I came to same conclusion. At ULS concrete is fully cracked with many micro crackes that are reducing stress in steel caused from shrinking in "big cracks". I believe that I will find same explanation if I find time to look up some papers.( If I do I will post it).

I thank you all for your thoughts and discussion.