Removing concrete above existing post tensioned tendon in slab

[shacked]

I am working on a remodel for a custom home with an existing post tensioned concrete slab on grade. There are no existing plans so I don't know if it is bonded or un-bonded. Also the contractor cored a hole through the slab and it is 12 inches thick and the tendons are located in the center, 6 inches below the top of concrete.

The Architect wants to create a 2" deep depression in the top of the existing slab for a shower. This is located right at the edge of the exterior footing where the tendons are anchored. See attached pdf.

I have some reservations about specifying this modification on my plans, but I do not have a lot of experience with P.T. concrete slabs.

Your thoughts much appreciated?

 

[Ingenuity]

If the PT SOG is in the US then 99.99% chance it is UNbonded PT.

A 12" PT SOG is very thick. Are you sure the contractor did not core through an internal stiffening 'rib' that is 12" thick, rather than the slab itself?

A 2" depression in close proximity to anchorage is problematic.

I would GPR scan the slab and locate the tendons and spacing (and will also approximately verify the slab thickness), and based upon on the plan dimensions of the shower setdown, determine how many tendons are in this area.

If the slab is indeed 12" thick, a 2" depression may be okay with respect to the tendons.

I would proceed only after more investigation work.

 

[strguy11]

I agree with Ingenuity. It most likely is unbonded, and 12" SOG is very thick. Is any part of the tendon visible? How was it determined they are 6" deep? Bonded Tendons are normally in a corrugated duct, which unbonded are in a smaller smooth plastic sheathing.

 

[shacked]

Thanks for your input guys.

According to the contractor when they scanned it they said it was 6" below the top of concrete.

I am having my doubts if this is actually a pt conc slab since it is 12" thick? This is according to the contractor so I don't know how reliable this info is. I will have the architect look at the City for existing plans like he should have in the first place.

Thanks for the info strguy11.

 

[shacked]

So, an update on this situation. I spoke with the contractor today and they did verify that the existing slab is post tensioned. They dug down on the outside of the footing and located the grout filled anchor location for the tendons. Also he had the slab scanned and the tendons are located at the center of the slab, which is 10 to 12 inches thick throughout.

Now in order to re-locate plumbing he wants to cut a 5inch deep slot in the slab in order to run the plumbing through then dowel to each side of the slab and infill with concrete.
Obviously the cut will create about 5 locations where the concrete coverage above the pt tendon is 1 inch.

I don't think that this is a good idea, but I'm just curious as to anyone elses opinion that may have more experience than myself with pt concrete slabs on grade.

Also, I do not have a soils report so I don not know the type of soil.(Obviously expansive or poor soil)

Thanks

 

[Ingenuity]

I was recently involved in commercial PT SOG repairs where the previous tenant trenched the SOG and cut many tendons (some 300 ft long) that was not discovered until the owner's new tenant revealed the issues. More than 50 tendons had to be repaired at considerable cost.

Trenching to a depth with only 1" cover to the tendon/s is problematic - the tendons are NOT perfectly flat (nor straight) so deviations in top cover vary, especially with SOG because the tendons are usually only supported about every 3' and a 1/2" dia tendon is somewhat flexible when placing the concrete. In the case above, many tendons were sitting on the ground with zero bottom cover, and many had less than 1" top cover.

I would NOT permit any saw cutting, until all trenching was complete (to 'tidy up' the chipping and avoid feather-edges). All trenching must be done by 25 lb MAX chipping tools. No trenching within 36" of any edge. If the trenching is of considerable length (say <8 ft), it may have to be staged - do trench #1 chipping, place pipe and concrete, then continue with adjacent and connected trench #2 etc. Otherwise you are loosing large lengths of sectional area that was previously compressed due to P/A, that you will not recover.

As I am sure you are aware, if you agree to the 'trenching' and something goes wrong, you take the responsibility - which is why the contractor is asking for your 'blessing'. Maybe you can get the owner to indemnify you (in writing) if the owner is insistent on the plumbing trenching. With no geotech report/info you are at added risk, IMO. Custom home re-model sort of implies wealthy client, who has access to costly attorneys.

 

[shacked]

Thanks for the information. Ya I realize that is why he is contacting me, so it is on me.

So then the only solution would be to sawcut and chip out square sections of concrete between each of the tendons, then hand trench into the soil for the placement of plumbing. Then re-pour concrete and dowel into the existing slab.

I am assuming that as you chip away the concrete on each side of the tendon you would want to leave a certain amount on concrete on each side of the tendon, but how is this determined?

Do you know where I would be able to learn more about working with existing post tensioned concrete slabs?