Cracked PT Slab Reinforcing 1

[larsacious]

I am investigating a rather large crack in a PT parking structure (see attached photos). It is believed that this crack has existed since the time of the post tensioning which occured approximately 30 years ago. The photo is of the east end of the crack and the crack runs west for approximately 60' then moves approximately 15' south and runs for another 60' before terminating. At the east end there is a continuous concrete wall that appears to be poured monolithic with columns. Originally, I thought this concrete wall was creating restraint and that the PT force was not getting into the slab. If this were the case I would have expected the crack to close up within 30' from the east end and instead the crack runs approximately 120'.

Any thoughts on what could be causing this crack?

 

[hokie66]

It's a direct tension crack caused by restraint to shrinkage, but without a plan of the slab and restraining elements, it is difficult to picture exactly the reason why the slab broke where it did.

 

[larsacious]

hokie66,

I think you are right. The thing that was puzzling me was why the crack didn't close up at the other end where there is no restraint(i.e. the wall is only on the one end). After further thought I believe the PT force is not getting into the slab for the entire length of the building. The crack location is near the center of the building and the strong axis of the columns is in this direction as well. The concrete wall at the east end of the ramp is where the most restraint occurs and is thus the location where the crack propogated.

Now the fun part...how to reinforce the slab. My thoughts here are to use fiber wrap reinforcing strips with fire proofing in this bay. Do you know of any other alternatives to this approach? I thought about external post tensioning but have some concerns with detailing/fireproofing of the exposed apparatus.

 

[hokie66]

From your description, I can't venture a guess as to whether the slab would require reinforcement or not. In many cases, direct tension cracks form where all that is required is to fill the crack.

 

[BAretired]

The concrete each side of the crack has no prestress in it for a considerable distance. The only way it can resist applied loads is by load balancing of the cables plus any conventional reinforcement it may have. This may be adequate for service loads, but from the standpoint of ultimate capacity, the slab would seem to be deficient in the vicinity of the crack.

BA

 

[hokie66]

Trying to get a feel for the structural arrangement. Looks like precast beams spanning about 60', and a one way cast in place slab spanning about 25' and supported on the beams. So the crack looks to be perpendicular to the span and about 5' or 6' from the supporting beam. Correct or close? Bonded or unbonded cables? Likely unbonded, as direct tension cracking can more easily concentrate along a single line with unbonded PT. 0.020" is not a wide crack, only about 0.5 mm, but it looks wider in the photos.

 

[rapt]

Agreed the crack looks wider than .02". Also that a complete slab layout is needed for further comment.

A reinforcement layout would also be helpful. ACI rules for minimum reinforcement in unbonded slabs have nominal start and end locations for top and bottom reinforcement that give very little overlap of the reinforcement at about 1/6th of the span (they are very bad rules!!). Looks like about this location. If this is the case there may be no bonded reinforcement through the crack, thus being why it has opened up so much.

But it also looks like there is some vertical separation either side of the crack, with the longer side lowest. Again what would happen if there is no bonded reinfrocement overlapping at this point. If this is the case, simple crack filling or fiber strips will not be sufficient as shear transfer is needed!

 

[larsacious]

@hokie66: You are correct about the span arrangement. It is a one way slab spanning to beams which typically span 60'. The post tensioning cables are unbonded. The crack occurs approximately 7' from the face of the beam (at the location where there is no mild steel reinforcing). I mispoke before and the crack width is 0.075" not 0.020". After studying the plans further I determined the following:
-The span where the crack is located is 25' and 7" thick.
-The adjacent spans are 17' and taper from 7" to 5.5".
-The remaining spans are 17' and 5.5".
-I analyzed the service level stresses here and determined that the specified effective prestress should keep the bottom slab from cracking.

@rapt: The mild reinforcing does not over lap as you suggested. I agree that crack filling/fiber reinforcing may not be adequate. There needs to be shear transfer across this crack and I am not confident that epoxy injection will solve this issue. Do you think external post tensioning will be the most effective approach?

I was hoping to post the plans today but unfortunately I was not able to find the time. I will post plans first thing in the morning.

Thank you everyone for your comments...

 

[larsacious]

Here is the original plan with some of my comments...

 

[rapt]

This minimum reinforcment extent problem has been recognised for years (by some at least, but obviously not the ACI PT experts). I discussed it with an International consultant who was looking at cracking in a US unbonded PT building over 20 years ago. The probelm he was investigating was caused by exactly the same thing. Always extend bottom reinforcement the full length of the span and lap it at supports to help both with this and temperature differential cracking in roof slabs and beams!

So there are 2 parts to the repair if this is the case, cosmetic to seal the crack and structural to provide adequate shear transfer across the crack.

As it is providing a load bearing function and is not just cosmetic, whatever you do has to be fire rated! External prestress would have to be fully encased.

Steel beams may be another option.

The crack width is unlikely to increase after 30 years due to shrinkage as all shrinkage has occured. But it will depend on temperatures. If the repair to the crack itself to seal it is done at the hottest time of year, there will still be significant shortening to the coldest time of year so the crack would open up again and ruin the cosmetic repair.

 

[larsacious]

Thanks rapt! You confirmed the approach that i proposed today. I am suggesting that we use steel purlins spaced at approximately 10 ft o/c and let the slab span in the perpendicalur direction by utilizing the temperature and shrinkage tendons. The connections will be with horizontally slotted holes to allow movement and the biggest challenge will be to avoid hitting the beam tendons with the anchor bolts. I plan on protecting the beams with spray on fireproofing.

I considered using external post tensioning but my fear is that by compressing this span i will put the adjacent slabs in tension and create similar cracks in those bays.

I completely agree that there should be a lap in the top and bottom reinforcing even though the code doesn't require it.

Situations like this are scary!

 

[hokie66]

I agree with using steel beams to reinforce the cracked slab. The beams look to be precast, as I think I can see joints over the columns. There is a reference on the plan to precast details. Are those beam details available to help with locating the drilled in bolts? I would tend to use tees bolted to the beams, then bolt the beams to the tees. Much easier to erect that way than with connections welded to the beams.

 

[larsacious]

@hokie66: The beams are post tensioned. The facade is precast and that is what the details are for. I have access to the PT shop drawings which I intend to use so that I can show the tendon profile with the beam connection locations superimposed on to help avoid hitting tendons. I will also require that the contractor has the beams scanned prior to drilling the anchors.

Thank you for the WT recommendation but I think you misunderstood my intent. I plan on having the angles welded to the plate and bolted to the beam. I envision them installing the beams with the assitance of a forklift so this connection should be pretty easy for them to erect.

 

[hokie66]

I suppose I didn't understand your intent as to the connection, as you didn't mention a plate or angles. Sounds like you intend a plate with a single web angle, then bolted to the beam web. I didn't mean a WT, but rather a fabricated tee. Essentially the same as yours.

Just for curiosity's sake, what are the dark lines on the beams at the columns in your first photos? They made me think the beams were precast.

 

[larsacious]

hokie66: Your right. I did not mention how the connection would be done here. I did write it up in my summary to the client today earlier and I guess I starting to lose my mind!

The dark lines are just shadows being cast. The formwork was not the prettiest out here and the beam and column side forms generally did not line up at the columns.

 

[BAretired]

The main crack occurs near one inflection point, then crosses over to the other inflection point. The cracked slab spans 25'-6" between beams, 50% greater than adjacent spans which are each 17'-0". The added reinforcement in the 25'-6" span is #5x8'-6" @ 14" top each end and #4x8'-6" @ 14" bottom. None of these bars extends far enough to intersect the crack, so they were ineffective in preventing it.

Remedial steel beams under the cracked span are probably the simplest solution. They could be connected to the side of the prestressed beams using bolts in shear or through the slab near the beam using hanger bolts in tension. I tend to favor the latter detail because I believe it is more economical, but that would have to be examined in more detail.

BA