Foundation: should i be concerned?

[wosat]

Greetings Inspector Gurus!

I am not an inspector, but house hunting has increased my interest in the whole process. The house we're bidding on is great, but the foundation has features that make me nervous! All you have to do is listen to the radio around here to know that foundation problems are common in this area (North Texas).

I've done a lot of searching and reading about foundations (this site is great), but a few days of reading does not an expert make. Of course, we will be getting the house inspected as soon as we are allowed to, but hearing some preliminary opinions from you guys would be most appreciated.

The house was built in 2000, and I believe (correct me if I'm wrong) it has a post-tension foundation, slab on grade, that has been poured in two steps. The corners do have some wedge-shaped cracks (which I've learned is not uncommon), but what really concerns me is what appears to be exposed cable-ends. Adding to my worry, there is obvious patching material that has been applied. In one section, the patching is so thick and bumpy, it makes me wonder what the heck has been covered up. I've read a bunch of posts here, but I haven't seen anything like this.

Anyone care to voice an opinion? Once we sign the contract, our option period is only 10 days, so I'd like to start the process with a little knowledge. Plus, I've found the process of reading about foundations and soil and such to be quite interesting.

Thanks in advance,

- Chris

[David P. Valley]

We don't have post tension slabs in Massachusetts, but I know Jeff Pope can help you out on your questions.

He'll be here shortly...

[Gerry Beaumont]

Hi to all,

Chris, post tensioned slabs do require specialist knowledge to inspect, especially if there is any signs of urrent failure, and yes the cable ends should be sealed.

I think what you are calling prior repairs maybe in fact the sealing of the cables after they had been tensioned.

I am far from an expert on this process, but the basics of it are that the foundation is built up using a "grid" of thickened concrete beams running in both directions within the slab, and thickened perimeter footings, after the curing has taken place the cables are then tensioned effectively pulling the bottom of the slab inwards and producing a crown effect in the slab.

From what I understand the biggest failures are from moisture entering the slab and rusting the cables thus releasing the tensioning, and from poor landscape drainage allowing expansive soils to move differentially under the slab.

Some parts of the country have a lot of this type of construction particularly the DFW are of Texas. Make sure you use a home inspector very familiar with this type of construction and if there are any signs of failure seek the opinion of a structural engineer specialising in this area.

Regards

Gerry

[Jeffrey R. Pope]

Good morning. Thanks for directing me here Dave.

First and foremost, any exposed cable is a defect - period. They must be completely sealed at both ends. If not, they will eventually fail.

Second, I'm not sure what you mean by "poured in two steps." PT slabs (generally speaking) must be a monolithic pour - a single pour with no cold joints.

The only obvious problems that can be seen in the photos are the exposed cables and poor quality workmanship with regards to properly sealing the cable ends.

Gerry FYI: proper cable installation requires a poly sleeve for the entire length of the cable so no part is ever exposed to direct contact with the concrete.

What happens is the sleeves get nicked, cut, and punctured during installation. . . This sets up the “latent defect” process. . .

Barry, this is not true. Concrete has no deteriorating effect on steel cables. The greased and sleeved cable is a relatively new product (early '70's). They are very popular in PT concrete for slabs and parking structures.

Still today, the most efficient and strongest system is multi-cable systems where "dry strand" is used in conduit (ranging from 4" to 12") and grouted with high strength grout after stressing.

[Barry Adair]

I inspect North East Texas residential and commercial and am very familiar with PTC-SOG foundations. www.adairinspection.com

An onsite complete inspection of interior and exterior features is necessary to determine a conclusive response for any type foundation.
I will render NO opinion based on a few photos. An inspector evaluates all associated conditions, and then renders his/her opinion with confidence and assurance.

Corner shear is common in this area mostly cosmetic but can affect the proper support of masonry work in some instances, or can be a sign of other problems.

The “bumpy” coating is called “parge” this is generally applied to “dress” the exposed perimeter footing where surfaces are uneven from form movement, blow-outs where concrete was not worked properly to remove all air pockets “aggregate exposed”, or other anomalies in the exposed face surface of the exposed perimeter footing.

ALL tendon ends must be pre-treated then sealed with a compatible cementitious coating to avoid elements exposure and corrosion.

Proper tendon installation requires a greased poly sleeve for the entire length of the tendon so NO part is ever exposed to direct contact with the concrete and so No internal corrosion occurs. What happens is the sleeves get nicked, cut, and punctured during installation or are not properly terminated at both ends. This sets up the, once poured, “latent defect” process that cannot be detected until failure occurs.

http://www.vsl.net/downloads/what-pt.pdf

Please call me to discuss all of your inspection needs. 214-328-8331

[Gerry Beaumont]

Hi to all,

Thanks Jeff & Barry, I knew others would be better versed on these than me.

Regards

Gerry

[Barry Adair]

Jeff,

Please provide further "concrete" information to back up your statement. I am always willing to learn.

All I have inspected have been greased and why are the majority seen today greased systems if "dry strand" is better?

[Barry Adair]

Jeff,

Concrete is the conduit for corrosion.

Is not direct concrete contact an issue for proper tensioning, does affect cable integrity by transferring moisture/water, mineral salts, and does have properties within that expedite corrosion.

True or False

[Jeffrey R. Pope]

Originally Posted by badair

All I have inspected have been greased and why are the majority seen today greased systems if "dry strand" is better?

Most (if not all) residential applications are "grease and wrap." It's an "unbonded" system.

The sleeve and grease keeps the concrete from bonding to the cable allowing them to be stressed uniformly. The process of the unbonded system is faster and generally less expensive (labor wise).

Granted, exposed cables can create problems during the stressing process, but unless the slab is in a really corrosive environment, minor exposures are not critical.

The biggest difference between the bonded and unbonded systems when finished is that the unbonded system (greased) is always "live." Failure in a live cable can be destructive.

The bonded system requires much thicker concrete (generally used in freeway bridges, dams, and other heavy construction). Multiple cables (I've placed as many as 30 in one conduit) are stressed and then grouted, "bonding" them into position.

[Barry Adair]

I have to go inspect.
i'll view responses later.
Thanks to all and I appreciate your input, Jeff, always learning keep up the great work!

[Barry Adair]

Jeff,

Please indulge me the liberty to advise staying focused.

My responses were toward residential PTC-SOG foundations to assist a non-inspector.

Your vast experience in other construction realms is appreciated by me, but may confuse the actual issues and the less experienced inspectors and homebuyers.

I am still interested in any documentation you can provide as I too am occasionally employed in these non-residential realms. This may bring back some hopefully fond memories. http://www.fhwa.dot.gov/bridge/pt/pt01.cfm

“i love to drive fast and take chances”

Everyone,

As a phase (pre-pour) inspector pertaining to residential PTC tendons I look for:

• Unsecured or improperly installed anchorages and temporary plastic wedge cups
• All tensioning terminations through forms
• Greased sheaths and ends
• Penetrations, tears, or breaks in the moisture barrier (poly around here)
• Penetrations, tears, cuts, or breaks in the tendon sleeves (polyethylene sheath)
• Tendon location per plans
• Chair placement

What else should I be looking for?

Out of hundreds, possibly thousands of tendons viewed I’ve only had one replaced by the builder before the pour.
I’ve had 1 callback for failure, after the fact. They weren’t blaming they wanted to let me know what I had written and photo documented would be used as ammunition by their “representative” and wanted to know “if I had anything further to add.” “NO!” was the correct answer.

Anything wrong with this installation through the beam, over water supplies and under waste pipe then back up and out the other form, in your opinion?

BakerPTC.JPG

[Jeffrey R. Pope]

Awesome link, thanks for the trip down memory lane. The largest segmental bridge I ever worked on was the H-3 freeway on the island of Oahu. . . two bridges over a mile long each and two tunnels over a mile long each. Those were the days. . .

Back to residential PT. The picture doesn't give enough information for a conclusive determination, however, at the very least, the parabolic curve of the tendon has been compromised.

It may be that it is mid span at its lowest point (which would be proper), but the pipes (where the chair is) throw off the curve and will likely be broken when the cable is stressed.

As an inspector, you are right to look for damaged sheathing. Generally speaking, duct tape is usually sufficient to seal minor breaks or tears. Many times we used a "split sheath" to cover damaged areas, or where the sheathing was cut too short at the live end. The point is, the concrete (in and of itself) will not deteriorate the cable.

In a highly corrosive environment (near the beaches or where roads are salted for example), corrosive agents can be introduced through hairline cracks during movement. It becomes more important in these areas to ensure the sheathing is intact and that hammered-on "grease packs" are used to seal the wedges at both ends.

The most critical thing (IMHO) to look for is a damaged cable - kinked cables or broken strands. These will certainly fail.

[Peter Foxe Smothers]

Hello all-

My brother in law works for a very large foundation company here in town that pours & tensions almost every slab here in Shreveport. Their company has developed and patented a way to sleeve the cables in plastic. Basically they run the cable through a machine that coats the cable completely in melted plastic.

[Jeffrey R. Pope]

Originally Posted by psmothers

Basically they run the cable through a machine that coats the cable completely in melted plastic.

That has been the method for many, many years (even before they were used in residential applications). The original unbonded system was to grease the cables and then wrap them in paper. They were originally referred to as grease & wrap cables. Otherwise, they were dry strand.

[wosat]

Wow. Thanks to everyone who replied.

Thanks for the correction, Jeff. I saw what I thought was a horizontal seem from a two-stage pour, but I guess it's just an impression from the forms.

We've decided to get a level B engineering report in addition to a full home inspection. It's a small price to pay.

As part of my research, I've even checked the federal soil survey for information about the soil the house is built on -- turns out to have a linear expansion index of 3-5%, which is as good as it gets around here (way better than the Houston Black they have to deal in other parts of the state).

Assuming the deal goes through, the addition of a french drain in an area that seems to suffer from poor drainage and re-routing of the downspouts farther from the house are both things that we are assuming will happen -- pending advice from the professionals of course.

Thanks again for your comments,

- Chris