Bursting Reinforcing Repair

[strguy11]

I received a phone call today from a restoration contractor asking me to review some cut reinforcing on a bridge expansion joint replacement project. After a review of the pictures, it appears that the rebar in question may be the bursting reinforcing around the anchorage of a multi-strand bonded tendon. (I have yet to receive all of the drawings in the area to verify, but this is what it appears to be).

My question is whether anyone has come across something like this before and conceptually how you would go about repairing. This

 

[KootK]

A colleague is working through a similar issue at the moment. I don't have an answer but I do have some thoughts:

1) The bursting reinforcement design case was fairly new concrete. You have mature concrete that is presumably much stronger.

2) What one needs is a way to evaluate bursting when there is no reinforcement present. There's got to be a method out there some place but I'm not familiar with it. Once could make something up utilizing concrete tension stresses but I'd feel a lot better about an established procedure.

3) To some degree, the anchorage is already load tested for bursting by virtue of the fact that it hasn't burst yet.

4) Conceptually, all that I can think of for a fix is to provide a plate top and bottom with pre-tensioned through bolts near the anchorages. That's not likely to be an attractive alternative of course. I'd also worry that the repair would initiate bursting.



The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[strguy11]

I contacted VSL and they indicated that since is was a grouted/bonded tendon, in theory the bursting reinforcing is required during the initial stressing operations, before the grouting is done. After that, in theory the anchors could be cut (not recommended) and the tendon would still have its force due to the bond between the tendon and the grout. There would be an initial length from the cut point for a distance to where the full effective force is gained (sort of like a development) length.

Since this was at the end of a span (over a column) and there was no local signs of movement I elected not to fix it. Based on my calculations for the development length, within this area the stresses check out even assuming the loss of one of the tendons. (There are multiple tendons in this girder).

 

[KootK]

Thanks for passing this along strguy11. Sorry that I didn't pick up on the fact that it was a bonded situation from the get go. Are all PT bridges bonded? That would make sense from a durability perspective.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[rapt]

I would not believe everything I hear from a PT company design office, especially the one you contacted if that is the quality of the response!

The force is still being applied there after the grouting is completed. So the bursting forces are still there. The bond will provide any increase in force due to strain in the concrete. But that is also assumed to be transferred to the end anchorage. It cannot be transferred to the free end of a tendon if the anchorage is removed.

And the anchorage is providing the anchorage at the ends. If there was no anchorage, the bond will build up the force over a long length (3-4m) with little or no force at the end.

You cannot cut an anchorage from the end of a bonded tendon and assume that the force will still be maintained from bond let alone carry the increased force from flexural strain!

 

[strguy11]

Rapt -

Thanks for your post. If you had actually read my entire post, I still checked the beam based on a full loss of tendon within the development zone, so i no i didnt rely solely on their information.

 

[rapt]

strguy11

I did read it all. Just making sure that it was understood by anyone listening that the response you received was not very good and that all should be wary of responses from such groups. I am sure if the VSL person had checked with his Technical group in Europe, they would have provided a different response!