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.

 

[jayrod12]

Why wouldn't you think it could perform both duties at the same time? I don't have any hard research handy, but my opinion is that at service level loads, the combined stress in the reinforcing from loading and temperature and shrinkage is still much less than the stresses at failure.

 

[BridgeSmith]

Cracking reinforcement sees very little stress - just a slight bit of compression as the concrete shrinks before the it cracks. It's only purpose is to make the concrete crack uniformly, i.e. many small dispersed crack instead of a few big ones. Reinforcement that resists bending moments in will accomplish the task of controlling cracking without affecting its ability to perform its primary function.

I've never even heard it proposed that the area of reinforcement required for crack control, shrinkage, etc. be in addition to that required for bending design.

 

[Tommy385]

@HotRod10
I am not combining it either, just wanna hear some thoughts.
During the shrinking after the tension strength of concrete is overcome cracks are opening and due the strain reinforcement within cracks is seeing some tension not compression?
My code (EC2) is there pretty straightforward, I need to cover full tension strength at the time of cracking with reinforcement, to control the width of cracks. So within cracks rebar is seeing some tension and within concrete compression.
Is there something wrong with my thinking?

 

[BridgeSmith]

If it says you need to provide steel equal to the full tension strength of the concrete, that would be in line with how I am thinking about the mechanics. The concrete shrinks, producing tension in the concrete, which is restrained (or balanced) by compression in the steel. Where the concrete cracks, the tension in the concrete is relieved at that point and so is the balancing compression in the steel.

Even if it doesn't crack, the tension in the concrete will eventually be at least partially relieved due to creep of the concrete. I haven't calculated it for cast-in-place concrete, but based on my limited exposure to prestressed concrete, and the magnitude of the prestress losses I've seen, I'm very confident that the stresses due to shrinkage in CIP would be fairly small to begin with.

 

[MotorCity]

I usually provide a layer of shrinkage reinforcement perpendicular to the layer of flexural reinforcement. However, I have on occasion combined the required area for both into a single layer of reinforcing.

I believe this to be on the conservative side because generally, the maximum shrinkage forces and flexural forces do not occur simultaneously. Concrete cannot support the design loads when it is in the plastic state when most of the shrinkage occurs.

 

[hokie66]

HotRod10,
Reinforcement to control cracks is not in compression. Before cracking initiates, both the concrete and steel are in tension, and when the cracking occurs, the entire tensile force is transferred to the steel.

 

[Tommy385]

@hokie66:agreed
Would like to hear your thoughts on the topic. Should this be combined with flexural reinforcement?
And if not, why not?

 

[hokie66]

You don't need to add crack control reinforcement to flexural reinforcement. The greater requirement of the two should be used. Crack control is a serviceability issue, while flexure is a strength issue. Sometimes, as in water retaining structures, the crack control requirement is greater than the flexural requirement.

 

[IDS]

As stated by others, at the Ultimate Limit State (strength design) the reinforcement is yielded, and any effects of concrete shrinkage are totally negligible, if not precisely zero.

On the other hand shrinkage does significantly reduce the cracking moment, and hence increases deflections and crack widths. If either of these control the reinforcement design then the effects of shrinkage must be considered, and will affect the required reinforcement area.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BridgeSmith]

hokie66, your answer isn't making sense to me. How can both the steel and concrete be in tension due to shrinkage, which is an internal stress? Equilibrium demands that if both are in tension, there either has to be external compression applied, or the beam will expand until the stress dissipates, right? The only way it satisfies equilibrium is if one is in tension and the other in compression, or if there is no stress in either one.

 

[IDS]

HotRod10 - we are talking about combined shrinkage and bending. Before cracking the stress in the tensile steel will be reduced by the shrinkage, and it could even be in compression for a small applied moment and/or high shrinkage strains, but after cracking the tensile reinforcement at the cracks will carry the entire tensile force.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hokie66]

Concrete is not stressed by shrinkage. It is stressed when it shrinks while restrained. If you placed concrete on a frictionless surface, if such a thing existed, the volume would just change without stressing the concrete. That is why we try to decrease restraint insofar as possible.

Now, the reinforcement does contribute somewhat to restraint, and that is internal restraint, but because the area of reinforcement is typically very small in relation to the area of the concrete, the restraining effect of reinforcement is normally neglected. The role for reinforcement in resisting restraint shrinkage cracking is to tie the two sides of cracks together, thus tension.

 

[rapt]

HotRod10,

Basic statics explains it.

if the concrete has no reinforcement and is free to shrink, it shortens and no stresses are induced.

When there is reinforcement in the concrete bonded it it, the reinforcement does not shrink so it partly restrains the concrete from shrinking. So the concrete goes into tension due to this restraint as it cannot shrink as much as it wants to. And the reinforcement goes into tension because the concrete still shrinks, but not by as much as the free shrinkage as it is partly restrained. And the concrete and reinforcement tension forces balance each other.

if the concrete cracks due to this tension and it the reinforcement is not at the centre of the concrete then it gets even more complicated and depends on the amount of external restraint and the reinforcement eccentricity.

 

[IDS]

Now, the reinforcement does contribute somewhat to restraint, and that is internal restraint, but because the area of reinforcement is typically very small in relation to the area of the concrete, the restraining effect of reinforcement is normally neglected.

I disagree with that. The reinforcement has a large effect on the reduction in cracking moment, so it should be taken into account for serviceability design.


Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BridgeSmith]

I agree with you, as far as you went. However, the reinforcing doesn't want to shrink with the concrete, so it resists the shortening of the concrete, which initially puts the steel in compression, inducing a corresponding tension in to concrete. When the concrete cracks, at least some of the stress in the concrete is relieved, so less tension in the concrete and less compression in the reinforcement.


I agree.


That's where you lose me. The concrete and steel are bonded, so if the concrete shrinks, the steel has to shrink along with it. That puts the reinforcement in compression.

 

[Tommy385]

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?
(since no external forces we have to have equilibrium)

 

[IDS]

Tommy 385 - the thread is titled "Crack rebar combine with bending", so there are external forces, and in general the bending would result in the entire length of tension reinforcement being in tension, after cracking.

For an unrestrained slab supported on a frictionless surface there would be no stress in the reinforcement at cracks, and the reinforcement between cracks would still be in compression.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Tommy385]

@IDS on a frictionless surface concrete wouldnt be cracking from shrinking at all.
(I know whats the thread title because I wrote it)

 

[IDS]

@IDS on a frictionless surface concrete wouldnt be cracking from shrinking at all.

The reinforcement restrains the shrinkage, so an unrestrained reinforced slab certainly can crack.

I know you wrote the title, but what you wrote ignored the bending.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/