Excess camber growth

[bridgebuster]

We have a small bridge under construction, 60' long x 60' wide. The superstructure is prestressed adjacent box beams. 10 box beams being erected in two stages. Stage 1 is finished. The fabricator is shipping the beams tomorrow morning; they're supposed to be erected tomorrow night. A little bit of a problem. One beam has a camber of 5 1/8"; the theoretical camber due to prestress and growth is 2 1/2". I think we have a solution to get the post-tensioning ducts to line up, but what's bothering me is the excess camber.

I don't have a lot of experience with prestress. I can see a measured camber was 25% higher than theoretical as the high end but 100% is odd, to say the least. Maybe Eci was very low and the QA inspector missed it? Any thoughts?

 

[BridgeSmith]

That generally happens when the precaster gets in a hurry and releases the strands and transfers the prestress to the concrete too early. Either that, or the concrete didn't cure properly. If the girders have harped strands, I suppose it's also possible they screwed those up (failed hold-down or not adequately stressed).

I don't know how your contract is written, but that would be way outside the specified camber tolerance for us, and therefore would be cause for rejection of the girders. That's probably what would happen if they showed up at the site like that.

Excess camber would likely produce excess tension in the concrete of the top flange, which will severely shorten the superstructure service life where salt is used.

 

[bridgebuster]

Thanks Rod.

No draped strands. The camber tolerance is +/- 3/4" along the box & 3/4" between adjacent units. However, the owner's plant inspector is only flagging the tolerance between adjacent boxes.

 

[BridgeSmith]

I would hold them to the 3/4" overall, and reject any that don't make it. Most likely they just got in a hurry to release, but it's possible it didn't cure properly. Did they test/report the concrete strength at release?

 

[bridgebuster]

According to the owner's records, the camber 5 days after transfer was 2" (per design it was a little over 1 1/2" at transfer); within tolerance and possibly the last time it was checked. The beam was cast a little over a year ago. I don't know if they'll share any other info with us. Kind of strange to go from 2" to 5+". Today, the owner decided to reject the beam. While discussing a solution to make the tendons fit, they didn't realize the 5+" of camber was way out of tolerance. The worst part, the contractor was planning to erect the beams tonight; doesn't look like they'll finish the bridge by July.

 

[rapt]

Creep due to very high compression stress in the bottom half from the prestress would cause significant upward camber over 12 months. It is calcuable.

 

[jorton]

That is a long time to sit in the yard on dunnage. Were they supported at the ends? I'm guessing not. The smaller self-weight moment can cause the camber to be much higher than anticipated.

 

[bridgebuster]

Supposedly they were stacked 3 high, sitting on 12x12's near the beam ends. Only 1 out of 12 has this abnormal camber.

 

[cooperDBM]

If there was a net tension at the top cracking of the odd girder could see a sudden increase in the camber growth.

 

[wrantler]

Is it possible the odd one was cast 1 yr ago and the rest are recent? Are they trying to use an old extra girder they had lying around?

 

[bridgebuster]

They were all cast at the same time. I talked with someone recently about this, he said the answer might be found in a petrographic analysis. The owner wasn't receptive. The owner accepted the beam. We told the contractor to place it at the middle, where the overlay is deepest and enlarge the opening for the transverse tendon.