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Additional Stresses Due To Creep & Shrinkage
2

Additional Stresses Due To Creep & Shrinkage

Additional Stresses Due To Creep & Shrinkage

(OP)
Hi

Need a little help here to understand long-term creep and shrinkage behavior. Is my current understanding below acceptable?

Due to creep and shrinkage, there are

(1) Additional strain and curvature.
(2) No additional internal stresses in concrete if there is no restraint.
(3) Additional restraint stresses in bonded reinforcement against creep and shrinkage which will help reduce long-term effects.

My question is for strength (ultimate) calculation, how do we take those additional stresses in bonded reinforcement into consideration?

Thanks in advanced for kindness.




RE: Additional Stresses Due To Creep & Shrinkage

Hi,

(1) Yes to additional strain. The definitions of shrinkage and creep are generally that they are changes to the long-term concrete strain.

Probably yes to additional curvature (even though it may be small and negligible in certain circumstances).


(2) Generally there are additional stresses in the concrete. External restraint to shrinkage will put the concrete in tension and this often causes the concrete to crack, leading to the requirement for 'shrinkage reinforcement'.

The effect of creep is usually to transfer stress from the concrete to the steel reinforcement, so the concrete becomes less stressed over time.


(3) The bonded reinforcement generally 'brakes' (reduces) the shrinkage and creep strain compared with unreinforced concrete. Whether that is good depends on the details and which aspect of concrete design you are considering. Eg heavy reinforcement may cause shrinkage warping which changes the deflection for good or ill, or cracking.


(4) Creep and shrinkage are usually ignored in determining the ultimate strength of a concrete member. For example, consider a column in compression. Due to shrinkage and creep, the steel reinforcement carries more than its fair share of the load at service loads. When the ultimate load occurs, the reinforcement yields so takes no further load and the concrete is then required to carry the remainder until both concrete and steel have failed. Design codes require transverse reinforcement (stirrups/ligatures/ties) to prevent the reinforcement from buckling rather than yielding.


All of the above is in general terms. As always, there will be exceptions depending on the circumstances.

RE: Additional Stresses Due To Creep & Shrinkage

(OP)
Steveh49,
Thanks and just want to clear my uncertainty further a bit.smile
(2)

Quote:

External restraint to shrinkage will put the concrete in tension
So if there is no external restraint, there is no internal stresses in concrete?

Quote:

The effect of creep is usually to transfer stress from the concrete to the steel reinforcement, so the concrete becomes less stressed over time.
If there is no reinforcement or very minimum compression reinforcement (provided just to hold shear links - rebar cage), concrete will subject to this additional creep stresses?

(4)

Quote:

For example, consider a column in compression. Due to shrinkage and creep, the steel reinforcement carries more than its fair share of the load at service loads. When the ultimate load occurs, the reinforcement yields so takes no further load and the concrete is then required to carry the remainder until both concrete and steel have failed.

So @ ultimate, reinforcement yields, concrete will carry those additional stresses? But for our concrete strength design @ ultimate, how do we cater for those additional load? I haven't seen any ultimate strength calculation relating to those creep and shrinkage effects. Is it just there is no creep & shrinkage effect on strength calculation.

Thanks



RE: Additional Stresses Due To Creep & Shrinkage

Hi again,
So if there is no external restraint, there is no internal stresses in concrete?

If there's no external restraint and no bonded reinforcement. If reinforced concrete shrinks, the concrete tries to get shorter but the steel reinforcement doesn't. Since they're bonded, they have to end up at the same length and this means that the steel goes into compression while the concrete goes into tension. The steel compression force and concrete tension force are equal in magnitude to there's no nett force.


If there is no reinforcement or very minimum compression reinforcement (provided just to hold shear links - rebar cage), concrete will subject to this additional creep stresses?

If you apply a load to unreinforced concrete, it will creep over time (the strain increases) but there is no change in the stress because the force carried by the concrete (concrete area * concrete stress) has to equal the applied load at all times.


So @ ultimate, reinforcement yields, concrete will carry those additional stresses? But for our concrete strength design @ ultimate, how do we cater for those additional load?

When the reinforcement yields, it still carries the load that caused it to yield. So the concrete in this column only needs to carry the applied compression force minus the steel yield load (steel yield load = steel area * yield stress).


I haven't seen any ultimate strength calculation relating to those creep and shrinkage effects. Is it just there is no creep & shrinkage effect on strength calculation.

Just ignore creep and shrinkage when calculating the ultimate capacity of the concrete. But creep and shrinkage can cause additional load that you need to consider because if they increase deflection then they will increase P-Delta moments in compression members. Design codes usually include a factor in the slender column moment magnification to account for creep.

RE: Additional Stresses Due To Creep & Shrinkage

steveh49,

RE Shrinkage, there can be stress induced if the shrinkage is not the same on all faces, eg bottom surface on permanent metal decking has much reduced shrinkage from that surface so a varying shrinkage over the depth of the section which will induce warping and stress in a continuous member.

RE stress under creep, the peak compression stress will normally reduce over time. The force couple must be maintained, but the stress distribution will change.

RE: Additional Stresses Due To Creep & Shrinkage

(OP)
Hi Rapt,

Quote:

Shrinkage, there can be stress induced if the shrinkage is not the same on all faces.

So if the shrinkage is uniform over the cross section, there is no stress induced? BTW, how does RAPT software consider the warping shrinkage?

Quote:

stress under creep, the peak compression stress will normally reduce over time.

Is it because of the increase in N.A depth (compression stress block depth) over time?

Thanks

RE: Additional Stresses Due To Creep & Shrinkage

varying shrinkage over the depth of the section which will induce warping and stress in a continuous member
Agreed. The shrinkage (warping) is restrained by the member continuity.

RE stress under creep, the peak compression stress will normally reduce over time. The force couple must be maintained, but the stress distribution will change.
Are you talking about an unrestrained (statically determinate) unreinforced concrete member with uniform creep throughout?

However, if the subject is reinforced concrete, agreed again - the concrete tends to shed load to the steel.

RE: Additional Stresses Due To Creep & Shrinkage

RE Creep,

With bending stress under creep, the peak stress reduces and the neutral axis depth increases to maintain the same compression force. There is a good chance there is no compression steel!

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