Bonded Post tension slab Cracking due to restraint

[aaronPTeng]

All,

I am interested on everyone's comments. I see alot of restraint cracking in post tension slabs which have not accounted for restraining elements by isolating them, adjusting the P/A or adding nominal reinforcement in these areas.

If you look at the capacity of the sections, they typically achieve the strength that is required, although it can be questionable in some cases due to a reduced P/A, Secondary cracking is common but in the main load path which the structural engineer has indicated can be OK from a strength perspective.

This level of cracking, besides it being a water proofing issue, ie causing leaks with large cracks what other issues can it cause? I personally do not believe it is the appropriate way to design however some will do it as they believe it is cheaper during construction (In some cases large amounts of epoxy are used later down the track to seal some of the cracks)
has anyone seen the long term effects of these types of design?. Does it create a Fire rating problem if the cracks are of certain size? RAPT your comments appreciated especially

Regards,

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[rowingengineer]

your post is a little unclear as to your question.

1. if your asking should restraints be taken into account during analysis? yes is the awnser, good restraint detailing is also important
2. if your asking if cracking in the secondary direction to the load path is an issue? i would asking what size crack and for what purpose.
3. if your concerned about a particular design by someone than be more specific with details.
4. cracking causing a fire rating issue would only be an issue if the cracks were very large.
5. expoxy sealing cracks is common for un expected cracks however i prefer a cement based injection system.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[aaronPTeng]

Rowing Correct, it isn't that clear, let me try again. My question is simply what is the problem with post tension slabs that have a great deal of restraint cracking due to this not being taken into consideration of the design. I have the following so far:

1. Depending on location most water proofing membranes will not be able to handle the size of cracking that occurs and water can pass through
2. Cracking can be a fire rating issue, Rowing what is your opinion on size of crack that would be a problem?
3. From an aesthetic point of view it does not look very good (clients perspective)
4. Cost of Flexible sealant/epoxy or cement based injection systems will cost money in maintenance of the building for the first 5-10 years until the majority of cracks are no longer "live"

What are other items I can add to the list?

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[hokie66]

It is incumbent upon the design engineer to consider all restraints, and to design accordingly. There are a number of approaches to this, but restraint cannot be ignored, and crack control reinforcement is essential and dependent of the degree of the restraints.

I don't know about the fire rating issues, but the watertightness and aesthetics/occupant perception are certainly biggies. A parking deck with big cracks can't be maintained without water leaking to lower levels, which can damage vehicles below. Membranes can only span narrow cracks. Fillers are a continuing maintenance issue, and do not solve the perception that the slab is defective.

 

[aaronPTeng]

Hokie, I agree 100%.

I believe and I must stress this is only a personal opinion meaning no offense to anyone, but a competent engineer would design for restraint. But I do have situations of this being ignored by others on the bases they can save $50 - $100 per unit in a large unit development from not putting any steel in to handle restraining effects, or not isolated this elements permanently or only temporary.

In my eyes its not he right way to go. However after a few years when you get called in on some of these developments as the owners and residences are worried the only real answer I can say is this is due to restraint and you have to keep (through multiple methods available) seal the cracks. and as they are live you can use some form of flexible sealant but it will occur over the first 5 - 10 years of the building life.



"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[rapt]

I had posted a longer reply but Eng-Tips decided to throw a hissy fit and it was lost, so here is the abbreviated version as Hokie66 has since covered a lot of it in briefer fashion!

The main problem in banded slabs is that the tendons parallel to the bands are usually sized to provide about a maximum of 1MPa P/A. This will not provide good crack control if there is any restraint.
In in a band beam structure the differential shrinkage between the band and the slab along with the "secondary stress" in the slab which are normally ignored in assuming one way action and for a roof the stresses from temperature differential plus any possible restraint to shortening due to shrinkage and temperature variation are often sufficient to crack a slab with 1MPa P/A.
Once it is cracked, tendons at 2-3m spacing provide no crack control. Maximum, spacing of bonded steel for crack control is 300mm. The secondary direction minimum reinforcement rules in AS3600 do not specifically say this but they logically require the code maximum spacing to provide the crack control they suggest they provide. So to achieve any acceptable degree of crack control in any slab that has restraint stresses the reinforcement must be at maximum 300mm centres!

And most slabs will experience restraint stresses from either differential shrinkage or vertical element restraint.

Where there are significant restraint effects from vertical elements, the slab must at least be released from these restraints temporarily until the prestressing operations are completed (pour strips) to at least get the P/A into the concrete! Then the effects of restraint must be considered as well.

 

[aaronPTeng]

Rapt,
agree,

But, If you had a client come to you (could be a building owner) and say "what is wrong with my building, why is it cracking" (For example a basement perimeter wall is tied in and no conventional internal reinforcement besides a 1.2m strip around the out side of N12's) what would you say? its restraint cracking patch it and re patch it and it will eventually stop?

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[aaronPTeng]

Note I didn't design the building in the first place (As I try and make an effort to avoid this problem)

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[rowingengineer]

I wouldn't get keen to blame the design engineer for cracking in the slab off the bat. The care and attention I see given to isolation joints ect these days is fairly minimal by all sub contractors including the general contractor. I have spent many an hour making people redo joint's to ensure dowels are straight and not angled. If I could get these back I fairly sure I could take a nice long holiday.

As for an owner coming and asking why is my building cracking often I find it that the owner if complaining about a planned joint. I would need more info before I pinned the design engineer. What if he specified non-shrinkage concrete and that wasn't delivered, who do you pin? (before anyone points out that the beast if fictitious I say Geopolymer)

I once had a complaint about cracking, and it turned out to be the changes in the color of concrete each side of a pour strip. Complaints don't always mean errors and don't forget it is the size of the crack we are limiting not the fact that one could develop.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[dcarr82775]

Isolating from a vertical element is easier said than done. I have seen similar cracking in slabs specifically detailed to 'isolate' the vertical element from the slab as those that were not. Could just be the typical concrete sub in the US isn't very good.

 

[aaronPTeng]

Rowin, not to come across rude, but I am looking at restraint cracks, not joints etc. I am not out to try and judge the original engineer however it is not how I would design,

in the case where you have frequent restraint cracking > 1mm. What would you then say.

The aim of the post is not to identify the types of cracks or anything, it is more about we all have seen poorly detailed structures failing to take into account restraining effects. which in PT structures leads to larger cracks. Besides just patching them are there any real issues with these? (Again I do not design this way).



"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[rowingengineer]

arronPTeng,
Sorry I make no apologies for being a stickler, open discussion without abuse works for me.

I still see this as a leading question:
"it is more about we all have seen poorly detailed structures failing to take into account restraining effects. which in PT structures leads to larger cracks"

I have reviewed a few PT structural designs over my time and I would struggle to put a large % of the jobs as poorly detail with regard to restraint by the Pt designer, I could however list heaps of other differences. I would put a fair amount of blame with the main structural consultancy. Generally There is two structural engineers engaged with neither having well defined scopes for the iteration of restraint. Hence why I hate just putting slabs as PT by others. If I had to pick one to punish I would pick the one whom decided it was too hard to design and hand balled it without first getting a good layout sorted.

I also disagree be I very rarely see the correlation between "poorly detailed structures" V's "PT structures leads to larger cracks". What I see these days is young engineers working above there experience level.

and my favorite pet hate for this week is: I hate the way people that are engineers point at other engineers for issues with a building, rather than looking at the full team.

most projects have
- project manager
- builder
- constructor
- supplier
- D&C structural engineer
- Main structural consultant
- other people that are also involved.

All whom have large inputs to the quality of the final building. I hold everyone equally responsible. If the design is incorrect and don't have enough details then the builder should put his hand up or as I would put it " put his balls on the line" and raise the issue with everyone until it is fixed. The builder is meant to have enough experience to be able to make this call. So does the project manager and many others.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."