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Post tensioned slab blow out
2

Post tensioned slab blow out

Post tensioned slab blow out

(OP)
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.

RE: Post tensioned slab blow out

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    

RE: Post tensioned slab blow out

(OP)
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?

RE: Post tensioned slab blow out

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’ve 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’re looking into the incident.
6. Don’t offer anymore opinions on this forum or any other forums.
7. Notify the owner of the incident – offer no opinions.  Say you’re 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’t be nervous.  Prepare for the worst and expect the best.

RE: Post tensioned slab blow out

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

RE: Post tensioned slab blow out

(OP)
Thanks for the advices
I will be on site Friday and take some photos.

RE: Post tensioned slab blow out

This is one more reason for me to dislike post tension.

Lutfi
www.cdeco.com

RE: Post tensioned slab blow out

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.

RE: Post tensioned slab blow out

Keep in mind proper compaction of concrete around the anchor area is also needed.

RE: Post tensioned slab blow out

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
www.cdeco.com

RE: Post tensioned slab blow out

2
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’d 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.

RE: Post tensioned slab blow out

I would expect the specification for concrete strength at the time of stressing to be 28N/mm2 or 30N/mm2 minimum, especially if this is a standard detail.

Honeycombed concrete, voids or a poorly carried out repair around the cast in anchorage would be the first place to look for a defect.

You don't say if the tendon is a single strand or a number of strands. If a number of strands then the anti-bursting rebar around the anchorage then it should be checked that the correct rebar detail has been used for the system.

Check the prestressing records including the jack calibration to ensure that over stressing was not carried out.

It is also possible that the anchorage was not correctly aligned with the duct, this is more likely to have caused wires to break than the anchorage to fail. However, the forces from a misaligned anchorage/duct are considerable.

Zambo

RE: Post tensioned slab blow out

One more item to consider along with the advice above is the age of the concrete.  I was taught that the Modulus of Elasticity comes up more slowly than strength.  This is why the post-tensioned bridges in California have a concrete strength and age requirement.  A low E value may lead to all kinds of things.


Lufti,

I've seen the removal of earthquake-damaged spans, and when the P/S strands are cut...  nothing happens.  Most likely becuase they're fully grouted.  Debonded strands/rods may be a different story (that's why you NEVER stand behind the jack or the anchor).  Having seen P/T bridges go up and come down, they're pretty robust so I'm confortable with the system.  Non-bridge applications?  Well,...

Regards,




RE: Post tensioned slab blow out

Lutein,

I would agree with the majority of the suggested reasons why there was a failure at the anchorage zone. I have seen several situations like this, having been a design engineer for a PT supply/design company. I assume that the PT strand is a 1/2" nominial dia tendon. Most likley your problem was due to the honeycombing behind the anchroage zone as Techmaximus had indicated. Were the tendons banded in the area that blew out, or was it a single tendon? Be sure to have the structure remain shored until repair of the tendon is complete.

 


  

RE: Post tensioned slab blow out

(OP)
Thank you for your very kind and benificial feedbacks - I really appreciate this.
I went to the jobsite and talked to the contractor, and they also admitted that they did see some honeycombing when they start chipping out the slab @ that area.
You guys are good expert on this!
Thanks !!!

RE: Post tensioned slab blow out

The members have provided a great deal of insight as to what might have contributed to your "blow out" problem.  I have seen PT contractors use 'field cured cylinders' to determine when PT operations should commence.  There can be huge differences between the strength of field cylinders and the concrete member being stressed.  Some contractors have a tendency to cure the cylinders alot more favorably than how the concrete is being cured.  We had cylinders showing the concrete was 6300 psi, but cores show the strength to be only 4000 psi.  Why such a difference?  Cold Weather and a lack of protection and winter heat.  Concrete does not gain much strength when the ambient temperature gets below zero Celcius. The best solution is to use ASTM C1074 Maturity Systems to more accurately measure the in-situ strength of concrete.  

Fred J. Croen, RSM
Engius, LLC
Boston, MA
www: engius.com

RE: Post tensioned slab blow out

(OP)
This is a very helpful thread and I really appreciate all the inputs, advices, and professional opinions on this blow out condition that I have met.
After a series of site observation on the blow out condition, we have concluded that:
1. This particular blow out on this project was caused by inproper consolidation, because honey-comb and air voids were found on the concrete sample at the blow out area.
2. Due to the congestion of rebars at the anchorage zone, the contractor claimed that it was 'difficult' to perform vibration.
3. Stressing under inssuficient concrete strength 'may' be one of the reasons as well.  Although the test results came out to be satisfactory, but, I totally agreed with Mr.Croen's take on concrete testing method.

 

RE: Post tensioned slab blow out

Croen, I have one of your intelliRock devices on my desk.  I think we are working a job later this fall/winter on which the EOR has specified their use for the exact reasons you mentioned.  I'll let you know how they work out.

Techmaximus

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