post tensioned slab blow ou

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post tensioned slab blow out
hi,
i have an 8" post tensioned slab under construction, the contractor called and stated that the slab anchorage area was blown out, it crushed the concrete slab edge for about 3'x5' area.  this happened in 6 hours after stressing.
i have checked the design and it was satisfactory, the end anchorage details were just typical post tensioned slab detail provided by the pt supplier, i.e. backing bars, and some top bars.  the reinforcement was inspected by building inspector before pour.  
in your professional opinion, what would have caused the slab blow out?  please shed some lights on this, because i am quite nervous under this situation.
thank you in advance for your kind advice.
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lutein:  i assume you are saying that concrete was crushed and/or buckled around one of the post tensioning jack or anchor.  my first guess would be that the concrete was not the right mix or not fully cured or both.  try to find out what the applied load was.  is there a concrete coupon available to test?  was the concrete properly installed ?
i sympathize with yu, i would be nervous also.  i suggest you get to the site as quickly as possible and see for yourself.
good luck
dave   
dave
thank you very much for your response.
you are correct, the concrete was crushed and/or buckled around one of the post tensioning jack or anchor.
the concrete strength appears to be ok, i.e. 75% strength @ stressing.  
i suspect that the anchor may not be installed exactly @ the mid depth of slab, do you think this might be the problem?
issues of professional liability are not clearly defined in such instances.  fingers, typically, will be pointed in order to recover costs for physical damages and contract damages for delays.  but you can be prepared.
1. lock up all your project files and restrict access.
2. make a list of all contractors, subs, suppliers, vendors, and inspectors.
3. notify your insurance carrier.
4. if you鈥檝e working bare, have an attorney review your project files for an opinion of your exposure.
5. go on-site and photograph the damage and any areas that were successful.  do not discuss, offer opinions or speculation to anyone while on-site.  if pressed, say you鈥檙e looking into the incident.
6. don鈥檛 offer anymore opinions on this forum or any other forums.
7. notify the owner of the incident 鈥?offer no opinions.  say you鈥檙e looking into the incident.
8. have an outside se review you project contract, calcs and documents for standard of care.
9. were you on-site prior to the incident?  if so, review your notes and contacts as well as any opinions expressed and to whom.
10. try to do everything from this point forward in writing.  if you must talk on the phone or to other directly, make a contact report and keep it with your files.
11. if this becomes a mess consider arbitration rather than litigation.
don鈥檛 be nervous.  prepare for the worst and expect the best.
lutein:  olddog04 has some good ponts.  were all the rods tensioned, or was this the first and only?  is the 75% strength based on tables or is it actually calculated from a coupon?  how far off center is the post tension rod?  is there other rebar in the slab?  how about the actual applied load as opposed to the specified load?  is the post tensioning system equipment in good condition and claibrated?  is only onme side dmaged or is the opposite side also damaged? call me old fashioned, but i still suspect a problem with the concrete and/or the methods.
out of curiosity what are the parameters such as the concrete area, concrete type/strength, the applied load, the cure time, etc?
can you post some photos?
regards
dave
thanks for the advices
i will be on site friday and take some photos.
this is one more reason for me to dislike post tension.
lutfi
lutfi - isn't is something - each of us has a pet peeve structural system that we just can't quite embrace.  mine is hollow core slab systems.
keep in mind proper compaction of concrete around the anchor area is also needed.
i don't mind hc at all.
i often times think about hundreds of kips of force stored in a cable; what will happen if the strands break up! it would be a night mare.
lutfi
lutein,
a common cause of failure is the improper consolidation of concrete in the anchorage zone.  if a honeycomb or rock pocket of concrete is left behind the tendon anchors (either the live or dead end) then obviously the load placed on the anchors will cause the honeycomb or rock pocket to consolidate: concrete failure and tendons either sink in or burst out.  the extent of the failure would be a function of the size of the honeycomb or rock pocket.  sometimes tendon anchors will not break through honeycombed concrete until the forms are removed from the underside of an elevated slab.
another common cause of failure is low concrete strength around the anchors.   just because the cylinder breaks came up to the required 70% does not mean that the slab is the same strength or that concrete strength is the same at the edges as it is in the middle.  if the concrete strength is low and the tendon anchors were placed lower than mid-depth well then you can see that that is going to be a problem.
i鈥檇 be looking into how the cylinders used as field breaks were cured to make sure i got results that reflect actual field conditions.  how many yards was the pour?  how many sets of cylinders were made?  was the concrete tech aci certified and if not why not?  aci 318 requires aci certified techs.  
also, be on the look out for reverse curvature of the tendons between the low point and the stressing anchor, improperly placed backup bars, and improperly installed tendon anchors (angled instead of perpendicular to eos).
inspection by the building inspector, how worthless is that?  is he/she checking everything that an icc reinforced concrete special inspector would check, or everything that an aci concrete construction special inspector would check, or everything that an engineer of record would check?  probably not.  is the contractor performing his contractually required qa/qc?  probably not.  maybe the owner needs to rethink the required inspections on this project and have qualified, certified, independent inspections prior to placements of concrete.