Post Tensioned Slab On Grade 1

[BS2]

Background: I have run across an existing building with a post tensioned slab on grade spanning between grade beams and belled piers bearing approximately 10'-12' on bedrock. The building is three stories, approximately 25 years old, is configured in plan view as an "X" shape, and is relatively large - think college dormitory.

The contractor was trenching through the slab in one room of the building to install plumbing and cut several tendons during the process. I was told there was not a void under the slab and the soil borings appear to indicate decent soils at least in this portion of the building. Other portions of the building may have had 5'-6' of fill which may be the reason for the post tensioning and grade beam system.

Question: Is retensioning the tendons possible? Feasible? Necessary? Problematic? I am not too worried about the vertical support of the slab due to the soil bearing beneath the slab. I am more curious about potential cracking or other unforeseen consequences of retensioning the tendons or even leaving them untensioned.

Any insight would be appreciated. Thanks!

 

[Canuck65]

Are the tendons bonded or unbonded? I have worked on several projects where we removed and replaced tendons that were unbonded. A bit of a process, but doable. If they are bonded, then it becomes more problematic.

 

[BS2]

Good point, I should have specified. The tendons are unbonded. The contractor would prefer not to retension if possible.

 

[Ingenuity]

In 2018 we repaired 45+ unbonded tendons to an existing PT SOG project where a plumbing contractor saw cut the SOG for new piping in a re-purposed box store - surprisingly similar to your project!

SOG tendon repairs are typically easier than suspended slab tendons repairs because there is no drape so the locations where splicing hardware can be installed etc is more flexible, and tendons are typically in singles not grouped.

But, access for re-stressing typically require center-stressing anchorages, unless new strand segments are installed with access to the existing edge anchorages.

As far as is it necessary to repair...the EOR usually decides, but often owner's take the position of "if you broke it, you fix it".

 

[JoelTXCive]

Do you have access to the live & dead end of the cable?

If so, sometimes you get lucky and a new cable can be re-thread through the old conduit sleeve fairly easily.

I've done this repair on residential houses before.

The cable company just uses their jack in reverse and threads a well greased cable through and monitors the distance. If the cable gets stuck, then you know where to bust out the chipping hammers at. You chip a small hole and fix whatever is preventing the cable from advancing.

 

[rapt]

Ingenuity,

Would be interesting to see the crack pattern in the short direction if they did not add any bonded reinforcement in the long direction of the infill.

 

[hokie66]

One crack per tendon, I think.

 

[rapt]

Hokie,

At least that will limit the crack widths, having them that close together!

 

[Ingenuity]

Would be interesting to see the crack pattern in the short direction if they did not add any bonded reinforcement in the long direction of the infill.

Yes, not too sure. We were not the EoR. We did the tendon-related repairs only - rebar and concrete by others.

I did hear that the concrete contractor had difficulties pumping the designated mix and had to resort to 'wheelbarrow brigade' to get from the transit mixer to the work areas - total of 30 yards!!

I do know that 48 hours after concrete placement we stressed the tendons and there were no cracks then. That 1 long tendon provides near 90 psi of P/A to the 'inner-parts' of the slab strip, albeit with a very eccentric force, and assuming you neglect restraint effects, etc.

PT SOG design "in my parts" is real sketchy, in my opinion. This one, more so, because it has perimeter 4' high retaining walls on 3 sides, resulting in the SOG being on compacted fill, and the PEMB trust is taken via the PT SOG in some fashion.

 

[BS2]

Thanks for the replies thus far.

I am not the EOR. I was asked by the contractor about this situation. The owner is willing to accept the opinion and recommendations of a PE. The obvious and default answer is to repair the tendons. However, I wasn't sure if anyone had experience not repairing the tendons. It sounds like most engineers recommend repairing the tendons and there is little evidence about the effects of not repairing them.

I believe that there is good access to both sides of the tendons in the short direction. Only 1 or two of these is cut. I am not sure if there is access to both sides in the long direction where there are several cut cables. The complexity of this repair will greatly depend on gaining the proper access.

 

[rapt]

Ingenuity,

But the force from that 1 strand will spread over several metres width (assuming 30 degree distribution either side of the line of the strand), so actual P/A will be much lower.

And the infill is full restrained against shrinkage by the connecting slabs and the subgrade. If they cured it properly you would not expect much shrinkage in 48 hours so not expect cracks then. But 12 - 24 months later would be interesting!

 

[Canuck65]

There are instances where tendons may not be repaired, or not repaired immediately. There are several multi storey towers in my area that have unbonded tendons and have issues. Years ago acoustical monitoring was setup so that tendon breaks could be identified. Generally they EOR team can tolerate the odd localized tendon break but when there are several that accumulate in an area, then tendon replacement is required. This is under 24 hour monitoring by the computerized acoustical system and has significant oversight from the EOR team. And .. this is the result of long term deterioration, not a contractor mistake. I cannot imagine any owner accepting less than full repair from the contractor.

It is a good reminder to the engineering community that not all ground level slabs are non-structural. Do your background review before working on existing buildings, before contractors sawcut into these slabs. Last year we were called into a project where the contractor had sawcut long trenches in the 'SOG' for new plumbing for a new restaurant. Building was supported on piles due to poor soils and the slab-on-grade was structurally reinforced. Expensive repair that could have been avoided.

 

[Gregorymeyers]

It should be built again because the fault can go further, and there will be much more to rebuild. It is better to prevent this in advance. Because I was a student and I wrote an essay on this topic. The way I got it from here [URL unfurl="true"]https://edubirdie.com/research-paper-editing-service[/url]. I found out that this kind of work needs a lot of attention, and if everything is not done correctly and after technology, it will have a very negative effect in years. Paper about engineering and other research you can find on that site. There is a lot of interesting information there.