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Stumped.....Post Tension Slab Decomposition

Stumped.....Post Tension Slab Decomposition

Stumped.....Post Tension Slab Decomposition

(OP)
I don't have a lot of experience with post tension slabs, yet came across a potential client with a series of post tension slabs yesterday with problems.

The slabs from the 'flat work' (sidewalks and courtyard surfaces) of a 12 building apartment complex. The slabs apparently began to flake and disintegrate as early as 3 years after initial construction.
80% of the distress is located on, or within 12" of a post tension tenon. The slabs are approximately 4 to 5" thick. Distress is not consistent in shape. In some areas it is a round, or oval pattern. In other areas the distress follows 80% of the tenon path. The remaining 20% of the distress is the slab corners just peeling off.

The HOA coated the entire system with an epoxy substrate system. The system has been bubbling and scaling since it was installed. The distress is in the localized area described above.

The distress areas are not consistent with surface drainage. The HOA diligently shovels the walks. Standing water seldom occurs. The one area that had 'massive' ice flow issues, is the least destroyed.

I'm thinking it is a concrete flaw. The concrete seems to be consistently flaking and scaling. I'm thinking the post tension strands have higher stress in their local, making the flawed concrete in these areas go first.

The end goal is to get the system to stop bubbling so there isn't this tripping hazard.
Any thoughts?

2nd part of the problem.....how do you fix this? As stated, not a lot of experience with post tension. How do you demo a post tension slab safely? The slab has support columns (floor and roof) that land ON the slab in locations.

RE: Stumped.....Post Tension Slab Decomposition

Could you post a few photos?

I wonder if the tendons were not placed in a straight, flat line and thus created some localized flexure or stress in the slab adjacent to them.

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RE: Stumped.....Post Tension Slab Decomposition

Sounds a lot like the surface was over troweled too early in the setting process.

RE: Stumped.....Post Tension Slab Decomposition

Quote (OP)

The slabs from the 'flat work' (sidewalks and courtyard surfaces)

Quote (OP)

The slabs are approximately 4 to 5" thick.

So these slabs are not primary structure then? Just hardscape? I've never encountered such a system.

Quote (OP)

The slab has support columns (floor and roof) that land ON the slab in locations.

Really? That sounds less like a hardscape-only element then. I is confused.

Quote (JAE)

I wonder if the tendons were not placed in a straight, flat line and thus created some localized flexure or stress in the slab adjacent to them.

This is my thinking as well. You'd get some degree of drape due to construction imperfections and, presumably, slope changes. Where that drape would be causing the tendons to push upwards into the 2" slab top cover, you've got added potential for spalling.

Quote (OP)

2nd part of the problem.....how do you fix this?

If PT drape is the issue, I can't think of a more reasonable fix than demo and replace. You could patch up the damaged concrete but there would be no guarantee that you wouldn't just see more of the same in the future. In the unlikely event that it would be tolerable, I suppose that you could add a topping to the system, bonded or mechanically tied down, to try to prevent future bursting/spalling. Chances are you've got the same problem on the underside of the slab as well.

If the slab is sandwiched between columns in some locations, which sounds weird as heck, I imagine that you'll just have to find a way to let some of that remain in place.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Stumped.....Post Tension Slab Decomposition

So this is a PT SOG, right?

Are there high levels of sulphates in the soils or groundwater?

If so, cue in the punch-list attorneys from CA, AZ, NV etc!

RE: Stumped.....Post Tension Slab Decomposition

Quote (bigmig)

The HOA coated the entire system with an epoxy substrate system. The system has been bubbling and scaling since it was installed.

So they installed an epoxy coating system to the entire top surface of the slab, then damage starts occurring? Seems they have a moisture gradient within the SOG, driving out through the top surface, hence the "bubbling and scaling".

Was an impervious membrane installed under the SOG during original construction?

RE: Stumped.....Post Tension Slab Decomposition

Were the surfaces salted during winters?

RE: Stumped.....Post Tension Slab Decomposition

Interesting problem.

The first thing that came to my mind was expansive steel corrosion of the post tension cable, especially if the cable was not plastic coated or if it was improperly sealed with lubricant or other protective coating. The corrosions could have come from salting the walkways as mentioned by Chicopee or from sulphates (or other types of salt compounds) in the soil as mentioned by Ingenuity.

When I have observed expansive steel corrosion there is a very definitive crack which parallels the reinforcement steel. The fact that there is sluffing/flaking and disintegration of the concrete suggests to me the concrete was placed with a very high water content. It might be worth the time and effort to pull and test a couple of core samples to determine the integrity of the concrete.

RE: Stumped.....Post Tension Slab Decomposition

(OP)
Sorry for the delay in responding to everyone's questions.

I don't have photos because there is not much to see. The epoxy coating is covering everything.

1. Tenons were most likely draped. The tenons are wrapped in a plastic sheathe. There is no way to confirm if they are set straight from a plan view as they are covered.
2. Icing salts are used.
3. The slabs are definitely post tension. In certain areas, the slabs appear to extend into the building interior.....sort of like they just poured one gigantic slab for everything (interior walls and sidewalks). That is a huge problem from a removal point of view.
4. The sulfate level is a good question. The manager said there are areas on site where they cannot keep the grass green....regardless of watering effort. This would indicate some type of hyper percolation ability, or high sulfates.
5. The site is located approximately 15 to 20 feet above the water table. The river (stream by most standards) is less than 1/4 mile away.
6. We will discover if they put a membrane below the slab and see if it was perforated or not.
7. Some of the damage is not at the tenons....it occurs at the slab edges. They are just sloughing off in flakes and spall.
8. The epoxy is satisfactorily adhered. What is breaking is the concrete to concrete surface.

The randomness is just confusing, but points towards a concrete defect....like troweling to much or too early and over hydrating the surface.

RE: Stumped.....Post Tension Slab Decomposition

Time for field sampling and petrographic testing. Scan the slab areas before taking core samples.

RE: Stumped.....Post Tension Slab Decomposition

I doubt the tendons are deliberately drapped in a 4" exposed slab on ground, except possibly where they go into the main building area if they do as you suggest. Not sufficient depth to provide any realistic drape and also no reason if it is simply a slab on ground.

There will be local higher stress at the anchorages, but nor at other points along the tendons.

Sounds like rubbish concrete to me. Have you done cores and tested the concrete?

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