Concrete Spalling to ends of pretensioned beams
Concrete Spalling to ends of pretensioned beams
(OP)
I'm undertaking a study to investigate a common defect on road bridges. The bridges are two-span with precast pretensioned concrete box beams, transversely post-tensioned. The beams are simply-supported, fixed at the centre support and free to move and the abutment ends. The prestressed strands at the the free-end of the beams have corroded and the expansion stresses have caused the concrete to spall. The likely cause of this defect is failure of the expansion joints directly above, allowing the ingress of water. The ends of the strands are debonded. It is not yet known what the extend of the corrosion along the strands is. I have attached a photo to show this defect.
My questions are as follows:
1.Has anyone come across this defect?
2.Can anybody point me to any literature which is specific to this problem, i.e. research studies, journals, etc.?
3.Has anyone any thoughts as to what the implications of this defect may be, i.e. loss of bond, loss of strength, etc
My thanks in advance
My questions are as follows:
1.Has anyone come across this defect?
2.Can anybody point me to any literature which is specific to this problem, i.e. research studies, journals, etc.?
3.Has anyone any thoughts as to what the implications of this defect may be, i.e. loss of bond, loss of strength, etc
My thanks in advance





RE: Concrete Spalling to ends of pretensioned beams
Try to find the post mortem study on the Pennslyvania I70 box beam collapse. I believe that study concluded that if you see a longitudinal crack along the bottom of a prestressed box beam, you need to assume the strand is completely corroded, and maybe also the next interior one.
RE: Concrete Spalling to ends of pretensioned beams
RE: Concrete Spalling to ends of pretensioned beams
It is difficult to scale from the picture but I don't think there would be anymore than 1-1/4" cover to the reinforcement.
There are admixtures that can be used to slow the ingress of chlorides but the most effective method is to increase the cover. Wouldn't use any less than 2" for a precast girder in an aggressive environment.
RE: Concrete Spalling to ends of pretensioned beams
You're looking at the bottom edge of the end of the beam in front of the abutment wall. The pretensioned strand is longitudinal.
You have to assume that the corrosion extends the full length of the debonding sleeves. It's a tough situation.
Nowadays we wouldn't leave strand cut to the end of the beam, and the number and location of debonded strand would be restricted. Some jurisdictions don't allow debonded strand at open beam ends at all.
RE: Concrete Spalling to ends of pretensioned beams
RE: Concrete Spalling to ends of pretensioned beams
I am more used to post-tensioned box beam where this wouldn't happen as the anchorages are recessed from the end of the beam and then protected by either concrete cover or epoxy.
In this case I expect the debonded strands should have been cut back, say 75mm, to form a socket that could have been filled to provide corrosion protection.
I agree that it has to be assumed the corrosion extends the full depth of the debonded length.
Can access be made through the abutment end wall?
RE: Concrete Spalling to ends of pretensioned beams
RE: Concrete Spalling to ends of pretensioned beams
You couldn't expect epoxy to protect the ends of flush cut strands for long, expecially if they are debonded. Agree with Zambo, but I think access will preclude doing that now.
RE: Concrete Spalling to ends of pretensioned beams
RE: Concrete Spalling to ends of pretensioned beams
If the girders are judged to be OK then you could consider removing the ballast wall and turning the abutment into a semi-integral abutment. This will encase the ends of the girders. New bearings would probably be required to take the extra weight. Judging from the current bearing and use of box beams I assume the spans aren't large. This may be the best long term solution.
Removing the ballast wall, encasing the girder ends, and rebuilding a new ballast wall corbelled off the back of the abutment seat may be an option.
Lastly, and least effectively, you could remove the top of the ballast wall and extend the deck over the ballast wall. The deck could bear on compressible material on top of the ballast wall. The approach slab, if present, could be seated on the end of the deck. This eliminates the joint but doesn't do much else to protect the girder ends.
RE: Concrete Spalling to ends of pretensioned beams
Would it be fair to say that if only the debonded strands are corroded this will NOT undermine the structural integrity of the beam? Also what testing techniques are available to determine how far along the beam the strands are corroded?
RE: Concrete Spalling to ends of pretensioned beams
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I am still concerned over the treatment of the cut pre-stressing stand. I was driving past some precast units getting erected today and it didn't appear that the cut strands had anything special applied for treatment other than coating the exposed end with epoxy. Logging on to the local department of transportation website, this is how the note was written for the treatment of cut strand:
AFTER TRANSFER OF PRESTRESS, STRANDS SHALL BE CUT FLUSH WITH THE END OF THE [UNIT] AND EXPOSED STRANDS SEALED AGAINST CORROSION BY THE APPLICATION OF EPOXY RESIN.
RE: Concrete Spalling to ends of pretensioned beams
RE: Concrete Spalling to ends of pretensioned beams
I don't think you can assume that the corrosion of any steel, unbonded or bonded, doesn't damage the beam. When steel corrodes, it expands, thus applying splitting forces which lead to spalling of concrete.
RE: Concrete Spalling to ends of pretensioned beams
But if the only access is as seen on the photo then the first step is to assure yourself that there is no damage to the bonded length of the tendons. If that can be done then it may be possible to seal the exposed corroded ends to stop the damage where it is now.
What about grit blasting from above to remove the worst of the corrosion. Then make a formwork out of 1mm steel plate which can be lowered down. Seal around the form (the difficult part) and then pour epoxy into the form which should work its way along the debonded length of tendon by capillary action.
Are there multispan bridges where the beams aren't against an abutment? Is the situation similar there, or is there more space to work?
RE: Concrete Spalling to ends of pretensioned beams
I asked the question about the location of the photo, and cooperDBM said that the corroded strands are at the bottom edge.
RE: Concrete Spalling to ends of pretensioned beams
RE: Concrete Spalling to ends of pretensioned beams
ahh yes... you're right. Also from the OP I didn't note that they are 2 span bridges but the corrosion problem only exists at the abutments.
I didn't really look closely at the photo before apart from the corroded ends of strands - now the more I look at it the more I can't see which edge has spalled off. Is that bearings we can see?
RE: Concrete Spalling to ends of pretensioned beams
The beam ends at the pier are typically cast into a diaphragm. If there isn't sufficient positive moment connection between the beams ends (at the bottom of the beams) over the pier the beams can pull away from the cast-in-place diaphragm at the bottom due to camber growth and thermal gradients. This can let water get at the exposed strands, though it's not nearly as bad as at the abutments (and there's no leaking expansion joint). Don't know if that's the case here (Irocco ?)
The bearings that you can see are steel laminated elastomeric bearings. Typical for this kind of bridge.
RE: Concrete Spalling to ends of pretensioned beams
2. Michigan Tech has done a recent study for Michigan DOT on the problem. See link below. Good reading. MDOT also did a study on repairs for deteriorated AASHTO I-beam ends.
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3. This problem could lead to loss of flexural or shear capacity depending on how far it extends beyond the beam end.