Post-tensioned slab repair 1

[kmead]

I am being tasked with coming up with a solution to repair a post-tensioned slab that has exposed and corroded tendons. The slab is 8 inches thick but I know nothing about the reinforcing / tendon location...etc. There are no existing drawings anywhere even in the city records, so I'm on my own as far as that's concerned.
My thinking in approaching this, is to have an inspection company x-ray the slab and determine the tendon layout, then based on that, maybe use Fiber Reinforced Polymer to reinforce the slab. While all this sounds easy, the difficulty lies in the fact that the slab is about 12 feet wide by about 200 feet long, and there are 10 levels with various degrees of slab damage.
So, my questions are:
1. Is my thinking legitimate? does FRP replace steel tendons?
2. Do I need to x-ray every square foot of the slab? or just a representative area?
3. Without any existing drawings or any information about the design, does x-raying the slab even do any good?

I am as you can tell not experienced with post-tension concrete and I am planning on teaming up with someone who does, but I just wanted to get a head start and find out some thoughts about this. So, any help would be appreciated.

 

[SkiisAndBikes]

kmead,

Some preliminary thoughts/questions on what could become a complex rehabilitation.

1. What is the cause of the corrosion? De-icing salt or other?
2. Bonded or unbounded tendons?
3. What is the condition of the grease, if unbounded? Emulsified? Any water in sheaths?
4. Any tendons already broken? Have any of the tendons been tested insitu to determine if they are broken?
5. Only 12 feet wide? Please confirm. Seems very skinny. What is the slab's use?
6. In my experience, provided you are familiar with the drape of a typical tendon, using a covermeter is a much cheaper, accurate and sound method for locating tendons. X-ray would be super expensive to locate them all, plus the area would have to be 100% clear during the x-ray operation. Perhaps use X-ray to spot check, although I think that would be a waste of money based upon my past experience. Perhaps maybe to get a picture of a anchor location. Locate with covermeter and paint each tendon location with paint on the top surface at each negative moment location, then connect the dots. Same could be done on the soffit, but likely not required.
7. If considering FRP, how are you going to justify the required fire rating?
8. I have been involved in projects with unbounded tendons where the old tendons are removed and replaced. Concrete delamination repairs are completed at the same time. Repairs are phased to minimize shoring requirements. This can be a complex process that is best suited to someone with experience (contractor and engineer).
9. I have also been involved with projects where flat steel plate was added to the structure, however, this was just to take a small percentage of the original prestressing force.
10. If it is not in too bad shape, acoustical monitoring could be considered to provide future data on tendon breaks.


11. If a chloride laden environment, consider a thin type waterproofing membrane after all the repairs are completed.

 

[rapt]

You have the fire rating problems mentioned above. Also, how will the FRP replicate the PT effects on crack control and deflection.

 

[kmead]

SkiisAndBikes,
1. The building is on the beach so it's salt air corrosion.
2. I don't know since as I mentioned I don't have any drawings.
3. I don't have that information either
4. No tendons have been broken as far as I could visually see, but I was hoping the x-ray would reveal the broken tendons if any.
5. MY apologies, it's 120 feet.
6. Great points! I will discuss the covermeter with the inspection company.
7. Quite frankly, I did not think about that, and that might be the deal breaker for FRP.

Thank you for the thorough replies, I appreciate it. But as I read your questions, I realize that it may not be something I want to tackle considering my lack of experience with PT concrete. It is however good to have learned a few things about it.

Thank you and rapt for your responses.

 

[SkiisAndBikes]

kmead,

Re 2. and 3., this information can be determined from an investigation, although the investigation has to be done quite delicately. After locating a tendon, typically on the underside (so midspan so it is close to the underside of the concrete), get an experienced contractor to carefully jackhammer (15lb max) the cover concrete away. Visual inspection first on the tendon sheath. Then if plastic sheath, cut open with knife to examine grease, see if any water comes out and then a hammer test (ASTM I think?) to see if tendons are still tensioned. If you determine at this point that the tendon ducts are bonded, then I think you have much less of a potential problem.

Re 4. You might get lucky and find a broken tendon with X-ray, but it will be a bit like finding a needle in a hay stack.

Finding a sub-consultant with the speciality experience and teaming up with them might be a great way to learn for the next project.

 

[kmead]

Great points. Thanks again

 

[Ingenuity]

Based upon the photo your attached, your PT tendons are UNBONDED and paper-wrapped. Probably dates from mid 60's to early 70's.

Such an antiquated system is plagued with problems of water ingress and systemic corrosion.

You are going to need to undertake a detailed condition survey, inclusive of scanning (GPR or x-ray), invasive probing, tendon testing, etc. and the likely repair/strengthening will entail strand removal and replacment (both difficult and costly with paper-wrapped systems) and/or supplemental strengthening. FRP and external PT solutions present fire-related issues.

Worst case is selective or full demolition. I was involved with a paper-system to a parking deck that was so bad that full demo and replacment was less expensive than repair and continued maintenance.

Good luck.

 

[kmead]

Ingenuity,
All the more reasons for me not to get involved in this project.
Thank you all for the great replies

 

[Ingenuity]

kmead,

Should you ever delve into the 'dark side' of PT evalaution and repairs, this document is worth purchasing and reading:

Guide for Evaluation & Repair of Unbonded Post-Tensioned Concrete Structures​

Available here Link and also at the ICRI website too.