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Cracked P/T wedges
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MVPs
(Structural)
(OP)
18 May 05 13:44After tensioning tendons for a large bridge project we received reports of radial cracking appearing in numerous steel anchor wedges. Some wedges only had one crack, some had multiple cracks. The cracks are visible on the exposed ends, and run from the inside diameter to the outside diameter, and I assume they continue longitudinally into the body of the wedge. No slippage has evidently occurred in the strands (verified by measurement of the strand ends), and we don't know when the cracks first occurred - no one was paying attention all last week while the tensioning was performed, and the cracks were first noted on Monday.We have conflicting information on the seriousness of this problem. This is second-hand on my part, but I was informed that the P/T installation manual from the Post-Tensioning Institute says that these types of cracks "frequently" occur. No mention is made of what "frequently" means as a percentage of the total project. Or, what action, if any, needs to be taken.Other (maybe) perinent information: The strands were being stressed with a short stroke jack; the duct voids will all be grouted after the final post-tensioning.Question 1: If the voids are grouted after post-tensioning, do the (cracked) wedges actually perform any function after the grout has set?Question 2: If the wedges are actually required, do the cracks make any difference to the future performance of the structure?Question 3: What percentage of cracked wedges is acceptable before they should be replaced?Our initial reaction is to replace the cracked wedges, but we are wary of construction claims.Thanks for any help.Steve
Steve R. Yates
City of Portland Oregon
Office of Transportation Engineering
(Civil/Environmental)
19 May 05 00:03Couple of questions before getting out the PTI manual. 1. Have the ends of the strands been cut, if so you probably cannot put a jack on to destress and replace the tendons. 2. If the tendons are meant to be grouted why has this not been done after one week?
Zambo
(Structural)
(OP)
19 May 05 10:54Hi, Zambo, thanks for the reply -No, the strands have not been cut yet, so detensioning would not be a problem.As for the time delay in grouting, the contractor completed the tensioning late Friday afternoon (05/13), and was preparing for grouting earlier this week. After the cracked wedges were discovered on Monday (05/16), we shut him down so we could gather more information.Steve
Steve R. Yates
City of Portland Oregon
Office of Transportation Engineering
(Structural)
19 May 05 12:01Question 1: If the voids are grouted after post-tensioning, do the (cracked) wedges actually perform any function after the grout has set?
Yes, the wedges perform the most important function - they secure the tendons. As the assessment of the quality and effectiveness of the grouting is unpredictable, it should be assumed that the anchors are carrying all post tensioning force at all time. The only reason for grouting is corrosion protection.
Question 2: If the wedges are actually required, do the cracks make any difference to the future performance of the structure?
I'm not familiar with the type of anchors you are using – some Internet link of photograph will help. In general, longitudinal micro cracks, parallel to the strands, should not affect their wedging capability. As long as the strands are not slipping, I think re-wedging is not necessary.
Question 3: What percentage of cracked wedges is acceptable before they should be replaced?
High percentage of cracked wedges will be indicative of some problem with the material (say above 20%), or with the geometry of the wedges and rings. Is it cast steel, surface hardened?
(Structural)
(OP)
19 May 05 13:16Hi wiktor -Thanks for the reply. The anchors are VSL type ES, and the wedges are identified as such on page 2 of this link to VSL:If you imagine looking directly on end at one of these wedges, you will have two wider radial lines defining the two halves of the wedge assembly. Our cracks are also radial, and extend from ID to OD - in some cases we have more than one crack per wedge.I hope this helps.Steve
Steve R. Yates
City of Portland Oregon
Office of Transportation Engineering
(Civil/Environmental)
19 May 05 20:00I had a look in the PTI post-tensioning manual but could find no reference to cracked wedges. If the strands were slipping I would have expected at least one to have disappeared right out of the streesing block and into the duct by now, it does seem that the system is working. However, there must be a geometry problem with either the diameter of the strand, size of the wedges or holes in the stressing block. To get an answer I think you would have to get VSL to look at samples of the system and also check test reports to see if they have records of cracked wedges during testing.
Destressing and replacing the wedges will not be straightforward as there will be a double bite of the wedges, or even the original bite position will end up within the tensioned length of strand.
I think the best option is to receive a statement from VSL that the cracked wedges are acceptable. Then you could try a few lift off tests on individual strands to verify that there has been no slippage and the force in the tendons is still acceptable.
Zambo
(Structural)
(OP)
20 May 05 10:55My thanks to Zambo and wiktor for their replies. To close the loop for you both, it was decided that we will accept the cracked wedges and proceed with the project. We have requested a written statement from our engineers (presumably with backup from VSL) attesting that the cracked wedges are not a structural or serviceability concern. The decision and resolution was made based on a combination of further Internet research, discussions with VSL, our design engineers,and with various other sources within local and state agencies, and with colleagues within the private sector. Again, your input was appreciated and most helpful. Thank you.Steve
Steve R. Yates
City of Portland Oregon
Office of Transportation Engineering
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