Repairing Post-tension slab

[JohnB001]

Hello, can anyone guide me in the right direction here.
I am working on a project where the building is facing the beach and the garage slab (post-tension), is cracked and some areas have a delaminating portion Of concrete completely fallen and leaving the tendon exposed.this area is the edge of the slab facing the beach, which was caused by corrosion

See pictures..

I am to provide a repair plan for the slab.
Please mention any reference in the code that can be useful or good tips from your experience.
I have worked on column and beam repair previously but never a post-tension slav, which I know it is a different ball-game.

Also any ideas what are the tiny holes in the slab? Any purpose?

 

[structSU10]

If the tendon anchorage is compromised it may need to be locked off, shoring provided, and the anchorage rebuilt. There is a document by the PTI/ICRI (PTI DC80.3-12) that goes into some repair issues to keep in mind.

 

[rapt]

How thick is the delaminated piece. It looks very thin.

And the slab also looks very thin. Minimum for post tensioned would normally be 5 - 6" thick.

Are you sure it is post-tensioned, or is it a thin pretensioned precast decking panel with a thin topping/wearing surface?

Holes could be old injection repair holes to re-bond the topping to the slab!

 

[hokie66]

rapt's comments raise a fundamental question. Are the photos of the top or bottom of the slab? I thought the bottom, he seems to think the top. I can't figure how the photos show the slab thickness.

 

[Ingenuity]

The engineering of construction documentation (CD's) for PT slab tendon repairs requires experience. without it you may get get 'crucified' by the repair contractor and/or the building owner. Seek to engage an engineer who has such experience.

Also, before you can author CD's you need to undertake some field work so that estimated quantities of repairs can be determined, and understand if partial or full length stand replacement is necessary, new anchorages required, where can splices be placed etc. Presumably you have copies of the original structural framing drawings.

The corner/edge crack in you first photo is not what I see as typical PT tendon corrosion damage.

The 'holes' look to me like vertical rebar dowels that have been cut off. Hard to tell from the photo.

What is the vintage of the building - when was it constructed?

Also, it is possible that the original SE drawings may not be how is was constructed. In my market, from the 70's through 80's it was common for buildings to be designed as say precast pretensioned but re-designed by a contractor's engineer and built as PT slab, or vise-versa. This may have happened in your market too. Like rapt states, it may be pretensioned! Very important to conduct some thorough field work.

The reference stated by structSU10 PTI/ICRI (PTI DC80.3-12) Guide for Evaluation and Repair of Unbonded PT Concrete Structures is worth purchasing.

 

[Ingenuity]

rapt's comments raise a fundamental question. Are the photos of the top or bottom of the slab? I thought the bottom, he seems to think the top. I can't figure how the photos show the slab thickness.

hokie66 raises a good point. I first 'read' the photo to show the top of slab, but I think you are right - it is the bottom of slab.

Perspective:

 

[Ingenuity]

Looking at the second photo - assuming it is the slab bottom - leads me to think this framing may be precast pre-tensioned, as per this markup:

Hence, the importance of thorough field work!

 

[hokie66]

Drain holes make sense. But whatever, that nasty broken chunk has spalled due to corrosion of steel inside the concrete. Maybe no waterproofing on a balcony above?

 

[JohnB001]

Assuming this is precast prestressed slab, the building is only 3 stories so I won't be surprised if it is not a pt slab. I could not find any as-builts though. But are there any way to repair a precast pretension slab if that's what it turn out to be?

 

[Ingenuity]

But whatever, that nasty broken chunk has spalled due to corrosion of steel inside the concrete

I agree. Looking less like a PT slab with that crack/spall damage, in my experience, but a field review will determine.

 

[Ingenuity]

Assuming this is precast prestressed slab, the building is only 3 stories so I won't be surprised if it is not a pt slab. I could not find any as-builts though. But are there any way to repair a precast pretension slab if that's what it turn out to be?

Yes, it is very possible to repair a pretensioned concrete system with that type of damage. However, if there is stand damage (corrosion etc.) to the end of the plank, then any concrete chipping around that damaged strand will cause 'development length' issues with adjacent undamaged strand (of the same strand). If there is damage to the midspan segment of a damaged strand then it can be re-stressed (using GRABB-IT devices) but it is very cumbersome so often supplemental reinforcement or FRP is used as a repair technique.

It all depend on the extent of damage. Again, a thorough field review, often with exploratory probing etc,, is essential.

 

[JohnB001]

 

[rapt]

Optical illusion!

More photos help!

 

[JohnB001]

Agree lol, were you able to tell by the extra photos?

 

[Tomfh]

Definitely looks like planks, and yes could be drain holes.

As for the spalling, you need to knock it off to see what’s going on.

 

[JohnB001]

So 0 possibility for it to be a PT Slab?
I thought even PT Slabs can be poured in lifts, therefore the panel line in between is there.
Also i thought these holes are where the post-tensioning tendons are anchored.
But anyway, I am no expert on the subject. Appreciate your opinions!

 

[JohnB001]

I just found one more picture that might give us all the answers.

 

[Tomfh]

Sure looks like corroded planks. The bars along the edge have corroded, breaking open the void/cell in places.

 

[JohnB001]

So methods of repair in this case would be super tricky. Would impact the whole plank.

 

[rapt]

The last photos and gap in the damaged area appears to show a bad quality concrete wearing slab above and rotten timbers between that and the precast slab panels.

Is the part where the concrete has broken off part of a plank or an edge insitu infill strip?

And the damage appears to be different in the 2 photos. Is one much later than the other?