Maximum allowable shrinkage crack width in PT structures?

[Tomfh]

What in your opinion is the maximum allowable crack width in a typical PT slab (say a post tensioned car park deck) before structural integrity may begin to be affected? I.e when is the shear and moment capacity across a crack starting to become compromised due to the gap? When does a shrinkage crack become a so called “structural crack”?

Are there any technical references for this?

 

[centondollar]

Crack width calculation (as done according to standards) is very simplified and not necessarily a very accurate endeavor, so I wouldn't be too worried about crack widths. If you want to model cracking with more accuracy, you need to do 3D modelling where rebar and concrete are coupled (with a reasonable factor for the interface bond, which may be hard to determine), contact is well defined, concrete has a pressure-dependent material model, steel has a non-linear material model, and so on. That's just another rabbit hole.

Make it simple: calculate the stress caused by uniform (or non-uniform) shrinkage in the cross-section. Then, prestress to cover that, dead loads and some live load, and aim to keep the cross-section decompressed (or with tensile stress below concrete tensile strength) at all times in serviceability limit state. Better yet, use concrete which shrinks less (usually high strength concrete where the autogeneous component is smaller). Even better, take notice of the fact that most shrinkage (autogeneous) typically occurs during and a short time after curing, which means that the full shrinkage strain is most often relevant only together with dead load and not with live load during service life of the structure.

Structural integrity in the sense of ultimate capacity will not be compromised by cracking since you have tendons, ordinary rebar and a wide flange to resist that.

 

[Tomfh]

I’m referring to real shrinkage cracks you might encounter and need to assess, be it as designer, or as a third party, etc. I’m not referring to theoretical maximum crack widths.

Eg you come across a full depth shrinkage crack in a PT slab that is 0.1mm wide. Most of us will dismiss it as being of no structural consequence. What about a 0.5mm shrinkage crack? A 1mm crack? When do you deem it worthy of further consideration regarding its impact on structural capacity?

 

[centondollar]

To that I would ask "What is there to do about it"? Unless the slab is deflecting excessively or showing signs of complete collapse, there is seldom any viable recourse to fix the problem so to say. I don't think the building owner will accept partial or complete demolishing over some cracks, so the typical best bet is to observe the situation (are deflections beyond expected values etc.), possibly install a monitoring system and then leave it be.

 

[Tomfh]

For larger structural cracks it is common to repair, eg to epoxy fill.

 

[centondollar]

Epoxy fill will not reinstate uncracked mechanical properties (I assumed you were worried about stiffness and strength), but yes, filling cracks to prevent intrusion of water, dirt, dust and chemicals is probably worth the effort.

 

[Tomfh]

I am talking about filling the crack to restore structural integrity, not just to keep it clean.

 

[Retrograde]

Generally 0.3mm is considered acceptable.

Refer to Table C2.3.3.1 in the commentary to AS3600.

 

[Tomfh]

The new commentary gives a range of limits from 0.2mm to 0.7mm (or less for water retaining structures). 0.7mm is recommend as the maximum where the crack "is in a sheltered environment and will not be visible".

 

[LittleWheels]

How do you get wider full-depth cracks in a post-tensioned slab? Doesn't the post-tensioning bring the concrete into compression under dead load? The width of the crack may vary through its depth, so the crack width at the surface should be wider than at the reinforcement/ tendons. How do you get >0.5mm cracks at the post-tensioning depth?

 

[centondollar]

I am talking about filling the crack to restore structural integrity, not just to keep it clean.

To my knowledge, it is not possible to restore structural integrity (if by this you mean serviceability concerns) by spraying epoxy into cracks. The filling will be incomplete and lost stiffness due to cracking will not be regained, since the epoxy filling does not undo the deformation which led to cracking.

If we use a more common definition of structural integrity, which encompasses capacity against failure, some cracks would not lead to loss of structural integrity as the ultimate limit state design is done by assuming a cracked section. The only exception is excessive deflection (which shouldn't be an issue in a PT slab), which may cause excessive curvature, bending moment and finally rupture the rebar and tendons.

 

[Tomfh]

How do you get wider full-depth cracks in a post-tensioned slab?

The slab shrinks, and the crack forms and opens.

How do you get >0.5mm cracks at the post-tensioning depth?

When the shrinkage forces overwhelm the tendon forces.

 

[Tomfh]

t is not possible to restore structural integrity (if by this you mean serviceability concerns) by spraying epoxy

It is routine to restore structural integrity of cracked concrete with epoxy (and other) materials. For cracks you typically inject thin epoxy to reinstate the cross section.

 

[centondollar]

It is a routine to restore structural integrity of cracked concrete with epoxy (and other) materials. For cracks you typically inject thin epoxy to reinstate the cross section.

I don't find this believable. Can you refer me to some report which summarizes research on this subject and proves that epoxy injections restore uncracked flexural stiffness?

Maybe I misunderstand what you meant by "structural integrity", but I am rather certain that after cracking, the only way to restore the cross-section to full stiffness and strength is to increase prestress force, usually with external tendons. The physical mechanism "joining together" epoxy and the faces of the concrete crack is adhesion, and that adhesion cannot "pull together" the crack, because the limiting factor is still the tensile strength of surrounding concrete.

 

[Enable]

centondollar: Epoxy injection is a long established protocol for restoring strength of cracked concrete elements where the cracks are non-moving. ACI has a field guide for such things. There is some debate about its effect on stiffness but there is literature suggesting that it does do a decent job at that as well (deformation that has already occurred due to loads that won't be removed is what it is though).

From ACI field guide

Conference Paper

 

[Tomfh]

Maybe I misunderstand what you meant by "structural integrity",

Strong enough to resist the loads.

A cross section with a full thickness crack cannot resist the same shear and bending loads as an uncracked cross section. Filling the crack with epoxy reinstates the flexural and shear load paths, ie reinstates the structural integrity.

My original question was how wide does a full thickness crack need to be before structural integrity is significantly affected.

 

[centondollar]

While you are right in theory, the capacity of a PT slab is designed by assuming a cracked cross-section, so a crack does not compromise structural integrity. Cracking is a problem related to serviceability, e.g., deflections and moisture intrusion, so the answer to your question is "it doesn't matter".

 

[rapt]

Centondollar,

That is a little simplistic.

The axial tension across the crack needs to be considered in estimating the capacity of the section. It will reduce the capacity. The fact that the cross-section is cracked does not make the axial tension disappear.

 

[centondollar]

What axial tension are you referring to? Axial tension from shrinkage? Sure, it can be added to the analysis, but it doesn't relate to my previous reply.

As for strength design of reinforced and prestressed concrete, the models are simplistic, but that's just the way it is.

 

[rapt]

Axial tension due to restraint to shrinkage!