Concrete Spalling to ends of pretensioned beams

[lrocco]

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

 

[graybeach]

This is pretty common. Water gets in to the space around the debonded strand, and the resulting corrosion causes the spall. If it really is confined to the end, there isn't any loss of strength. But it probably is not confined to the end.

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.

 

[hokie66]

For someone who is not a bridge guy, could you explain the perspective of your photo? Is this the bottom corner of the outside box beam?

 

[rscassar]

Is it possible that the bridge is located in an aggressive environment with insufficient cover to the reinforcement. The ingress of chlorides has corroded the reinforcement and promoted spalling.

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.

 

[cooperDBM]

Hokie,

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.

 

[hokie66]

Thank, cooperDBM. Yes, leaving the reinforcement unprotected is certainly a bad idea. Corrosion causes spalling...loss of concrete integrity...failure.

 

[Zambo]

Good explanation cooperDBM.

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?

 

[rscassar]

I would expect the specification (from the department of transportation) to request that strands be cut flush with the end of the unit and that the strand and area immediately surrounding the cut strand be treated with a suitable epoxy. You could ask the department of transportation what quality assurance programs they have in-place to ensure that this process took place.

 

[hokie66]

kikflip,

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.

 

[bridgebuster]

This is a common problem with older prestressed bridges; take a ride on I-80 in New Jersey. You really can't do anything to fix it other than slap some mortar on the exposed surfaces. Also, you might want to investigate for alkalai silica reaction since the coarse aggreagate is river gravel.

 

[cooperDBM]

I don't see a quick, cheap, and effective solution in the narrow space available, so ...

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.

 

[lrocco]

Thanks for some great advice, there's a lot to consider.

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?

 

[rscassar]

Best thing to do is to find someone in your area who specializes in concrete testing equipment. I have never specified or recommended this equipment. I am also interested in what equipment other people specify when needing to establish the extent of corrosion of reinforcement.

http://www.abbeyspares.co.uk/pdfs/Canin+ Sales Flyer_E Low.pdf

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.

 

[lrocco]

According to BS8110-1-1997 Cl.4.12.3.2, for Unbonded strands (assuming a debonded strand is classed as unbonded?) there's a need for nominal cover to tendons, not to be less than 25mm

 

[hokie66]

lrocco,
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.

 

[Zambo]

All the tendon ends should be at the top of the beam. The best way to achieve a repair would be to get at the end of the beams either by removing concrete from the abutment end wall of from above.

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?

 

[hokie66]

Zambo,
I asked the question about the location of the photo, and cooperDBM said that the corroded strands are at the bottom edge.

 

[cooperDBM]

The strands in pretensioned box beams like this are generally straight, not harped, which is why they're at the bottom. The debonding is used to reduce the tensile stresses at the top (as harping would have done).

 

[Zambo]

hokie66,

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?

 

[cooperDBM]

Zambo,

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.