Cracked PT Slab Reinforcing
Cracked PT Slab Reinforcing
(OP)
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?
Any thoughts on what could be causing this crack?






RE: Cracked PT Slab Reinforcing
RE: Cracked PT Slab Reinforcing
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.
RE: Cracked PT Slab Reinforcing
RE: Cracked PT Slab Reinforcing
BA
RE: Cracked PT Slab Reinforcing
RE: Cracked PT Slab Reinforcing
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!
RE: Cracked PT Slab Reinforcing
-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...
RE: Cracked PT Slab Reinforcing
RE: Cracked PT Slab Reinforcing
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.
RE: Cracked PT Slab Reinforcing
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!
RE: Cracked PT Slab Reinforcing
RE: Cracked PT Slab Reinforcing
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.
RE: Cracked PT Slab Reinforcing
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.
RE: Cracked PT Slab Reinforcing
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.
RE: Cracked PT Slab Reinforcing
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