Elongation of Post Tension Cables 4

[swthwdy]

Our home is built on expansive soil in the Houston, TX area.
Our builder 'forgot' to have the post tension cables in our foundation tested for elongation prior to the final stressing.
The PT CO just did the final stressing and even overlooked stressing at least one cable.
Should the PT CO knowingly have done a final stressing without benefit of elongation measurement and report?
We already have much differential settlement damage in our 6 year old house. Our Engineers believe there is a poorly compacted foundation pad and no engineering, site preparation plan or inspection by a licensed inspector has been produced. The builder says they throw the plans away as soon as the subdivision is completed, although they still build the same house in different areas.
What are some other problems we may encounter in the future?
I guess what I really want to know is: Should the PT CO have final stressed the cables without knowledge of elongation report on the PT Cables?

 

[techmaximus]

Are you saying that the PT Co performed the stressing of the cables 6 YEARS after the slab was placed?



SCET - Techmaximus

 

[swthwdy]

No,the house is 6 years old, the concrete was 20 days when the cables were stressed.
Should the PT CO have known the elongation of the cables prior to final stress?
The PT Co wants to rethread the unstressed and broken cable, however there is no plan that shows exactly where the anchored end is at the other end.
Do PT cables loose any elongation in a 6 year period, or would they still stress @ 7000 psi?

 

[MotorCity]

swthwdy,

A few things to consider:

The EOR should have calculated the expected elongation which should be field verified by the contractor prior to final stressing.

The cables will lose some of the elongation due to relaxation of the strands. 7000 psi seems low for a final stress in the cable. Is that the additional cable stress or the final cable stress?

The most important consideration would be stressing any of the cables after you have experienced significant differential settlement. If you stress the cables, you could introduce an eccentric load which could create total disaster.

Good luck

 

[swthwdy]

Thank you so much MotorCity for your much needed clarifications.
Yes, the 7000 psi is the final stress according to the PT CO written report.
Thank you again.

 

[techmaximus]

In your first post you asked, “Should the PT CO knowingly have done a final stressing without benefit of elongation measurement and report?”

The answer is NO. First of all there is only one stressing. The PT Co should have measured the elongation of the tendons at the time of stressing to determine if the required force was placed onto the tendon.

You ask, “Should the PT CO have known the elongation of the cables prior to final stress?”

The answer is YES. The PT Co should have calculated the expected elongation prior to stressing. All of this information is on the PT Shop Drawings. The PT Co should have verified that the actual elongation was ±7% of the expected elongation at the time of stressing.

However, what they probably did was stress the tendons to the jacking machines calibrate PSI. They then just assumed that the correct elongation was met and moved on.

You write, “The PT Co wants to rethread the unstressed and broken cable, however there is no plan that shows exactly where the anchored end is at the other end.”

The tendon may not be “broken”. It is likely that it simply pulled out of the dead-end anchor at the time of stressing. It sounds like what they want to do is chip out concrete where they believe the dead-end anchor is until they find the dead-end anchor. Who knows how much concrete they will have to chip out in order to find the dead-end anchor: a little or a lot. Once the dead-end anchor is found, they can pull out the tendon. They would then have to chip out concrete at the live end far enough in to allow the placement of a new live end anchor. A new tendon can be run through the existing hole. Anchors would then be placed at both ends (either one live and one dead or two live) of the new tendon. The chipped out areas would then be formed up and re-poured. Once the patch hardens to +/= 3000 psi the tendon can be stressed.

As MotorCity mentioned, if differential settlement has occurred where this tendon runs, stressing it could be disastrous. Only allow this if the profile of the tendon will be unchanged from when the structure was originally placed.

You also asked, “Do PT cables loose any elongation in a 6 year period, or would they still stress @ 7000 psi?”

It is possible that the tendon was stressed by a jack that was calibrated to 7000psi. That means that this is the force required by that particular jacking machine to impart the required number of kips onto the tendon. Can you clarify that? This tendon was probably stressed to 7000psi on that machine and that resulted in a load of 27 kips being placed on that tendon.

Do tendons relax? Yes. Do they relax enough in 6 years to allow settlement? No. I mean you didn't buy a home with a life expectancy of six years.

If the structure has differential settlement where other PT tendons exist, the structure may already possess potentially deadly eccentric loads. It may be just a matter of time before disaster strikes. It may be necessary to de-tension all of the tendons, jack the structure back into its correct place, and then re-tension the tendons.

BTW, expansive soils only expand when the moisture content rises. Your real problem may be the killer Gs: Grade, Gutters, and Ground Water. If the killer Gs are allowing the moisture content of the soil to rise, well you have another set of problems to fix.

The Post Tension Institute at

http://www.ptistore.com

has several books that you and your lawyer will want to refer to often when you get your so-called builder on the witness stand.

Is your builder required to fix this differential settlement under a Home Buyers Warranty claim? If so, require him to hire an engineer to recommend a solution and them require independent monitoring by a testing firm or the engineer to verify that the engineer’s solution is carried out to the letter. If you have a Home Buyers Warranty and you have not filled a claim do so at once. DO NOT allow work to proceed that the builder and the PT Co cook up by themselves as their concern is likely self preservation and not your home.




SCET - Techmaximus

 

[swthwdy]

Thank you Techmaximus for your informative, expert, eloquent and truthful reply.
My husband and I have been in litigation in District Court, since 1999, no 10 year warranty involved.
We are seniors who always felt the production builder and the PT CO tried hard to knowingly cover up what they had not done to follow codes, standards, rules and/or regulations.
Here in TX., home builders are not licensed, there are no inspections of new construction in the Counties outside of City limits, therefore the builders ignore codes, cut corners, cover up defects and go merrily on their way, leaving behind thousands of homes which may one day literally fall apart as ours is.
It was not until early this week that we deposed the PT CO and began to put the scenario of what should have been done together. Of course now the PT CO wants to settle, but rest assured I am one tough Grandma, and would rather have the Jury give me nothing, than settle with either the builder or the PT CO and continue to allow them to hide their deceitful building practices.
Thank you again.

 

[techmaximus]

I have to tell that I would have an engineer develop a “Cadillac Fix” and settle.

((Cadillac Fix + Lawyer’s Fees) x 1.25) + Ten Year Warranty = Settlement






SCET - Techmaximus

 

[swthwdy]

And what might a "Cadillac Fix" involve?
The County Appraisal District has already devalued the house over 50% and said they would have taken it to 0% except for the fact that we are living in it.
The 10 year warranty costs as much or more to place a claim, and prove defects, than a Court case, then you face a group of arbitrators in the builder's back pocket.
The builder wanted to 'float" an unlevel upstairs floor and not disclose when we sold the house.

Thanks again Techmaximus. What part of the country do you reside?

 

[RodbusterRon]

Hi all,
I do agree with tech maximus, except for his statement that cables are only stressed once. I am an Ironworker foreman in Boston, and have worked with rebar and post tension cables since 1985. Cables are often stressed in stages. Sometimes as many as 3 times on single strand pulls. Most common is 50% on all cables in the first pull. Then a second pull to 100%. Many times cables are pulled in alternating sets. (i.e 1-3, 2-4). What we do is place a piece of plywood, cut 4" wide, over the cable ends to be pulled and paint a distinct line on the cables so as to measure them after the final pull. Once the ram and pump (jacking machine as tech puts it) does it's final pull the elongation is measured. A normal pull is 7500 psi. I have done many, many beachfront condos in Florida where the cables were pulled only once (in theory), and then only to 6200. In most single pull cases, there must be 2 pulls performed because the ram runs out of room to pull to full elongation.
I think your problem may not be a PT CO only problem. Cables are meant to flex, more so than rebar. So if your soil compaction was not up to snuff, the cables may not be entirely to blame. I've never heard of cables being used in a ground slab. Most ground slabs, especially on unstable soil, would be of heavy rebar and concrete construction. Cables are used in upper floors of buildings, so as to run through column lines. When they are stressed, cables pull the slab up at the column points. Thus, if there are no pilings driven into the ground for cable groupings to pass through, I fail to see what good cables will do. Maybe a look into the engineering design company would be interesting also. Also look into the company that did the land prep (excavators). There may be blame to be placed in areas you may not have thought of. Don't just look at the PT cables. I hope this helps you, and good luck.

 

[FalsePrecision]

RodBuster,
I don't know what you mean by, cables aren't used in ground slabs. Post tension in ground slabs is very common.

 

[Dinosaur]

One possible fix for this problem could involve installing helical ground anchors and jacking the foundation up and back to a level condition.

There are companies that specialize in restoring homes that suffer damage due to differential settlement sometimes caused by poor soils under the foundation.

Good Luck - Ed

 

[RodbusterRon]

False,
Well, I don't know about houses, but I've never done A post tension slab on a high rise ground slab. Most of those slabs were somewhere around 4-6 feet thick with #11 rebar. I've done ground slabs with mesh or rebar only. The way cables are designed, unless you are running groups of cables through a column or caison, I fail to see how it does any good. Please fill me in. The slab is lifted into place by the cables exerting downward pressure on the column lines. So theoretically if there is nothing for the cable to anchor itself to, to hold the slab level, I can never see how they can do what they are designed to do, given their flex properties. I tend to think this is something a house builder came up with to save money and time. Because PT cables take less time to install, less time to tie into place, and are cheaper to use because you use less cable than rebar.

 

[swthwdy]

Thank you for all of the suggestions. Another thought is what may happen if trying to level the foundation, to the walls, ceilings and floors that have been built to the original highs and lows of the foundation. We now do have cracking of walls and ceilings, windows that won't open, eaves etc. exterior, that are coming apart, and more problems.

 

[RodbusterRon]

I think if you try to straighten that slab without the benefits of piles driven under it, you are fighting a losing battle. I'm not sure what helical ground anchors are, like Dinosaur said, but that may help. Think of it as building a suspension bridge with no towers to support the main cables, and the roadway just hanging in mid-air. Of course that is an extreme example, but you get the idea. I think the whole reason your house was devalued to (really)0% was because the whole design in general is just plain wrong. I think unknowledgeable architects and building inspectors are to blame in the first place. I would challenge the design first, then go after the materials and installation practices. Again, Good luck. I hope I've helped.

Ron

 

[FalsePrecision]

1) I would think it would be easy to find the dead ends. Use a magnemometer to detect steel, and look for the grout plug where the anchorage wedges exist.
2) However, if the tendons are replaced and re-tensioned, I think it will have very little effect to repair the slab.
3) The helical anchors may be feasible where the slab is settled down.
4) In general, it is very hard to get justice. Reminds me of how my wife lost her house to mold because no one would tell the truth (no honesty in this world). I had a house built that had severe stucco cracking, and everyone from the Builder to the local Building department lied like used car salesmen. Sometimes you just have to take the loss, and get on with life.

 

[techmaximus]

RodbusterRon Yes, I agree with what you wrote about staged stressing. I have seen many of these as well. I misinterpreted what she, the homeowner, wrote; I thought that she meant that they stressed the tendons, left, and then came back at a much later date.

Just to clarify, when you say 7500 psi, you mean gauge pressure, right? The stress on the tendon is higher, no? On the high rise building that I am working on right now we have two stressing jacks. One of the jacks is calibrated to a gauge pressure 5300 psi and the other to 5400 psi. But each jack stresses the tendons to 27 kips.

FYI, the tendons in the case of the original post are used in the SOG to resist the forces induced by expansive soils. She lives outside of Houston, TX where these kinds of reactive soils are a problem. Rather than undercut and then import non-reactive soils they use PT slabs on ground. You are right about the soil compaction; however, for a different reason that you may think. If expansive soils are compacted on the dry side of optimum moisture content it tends to expand more that it would have had it been compacted on the wet side of optimum moisture content. The expansion is compounded in either case if the soil is not compacted to the proper density. Additionally, the last thing one wants with structures built on expansive soils is a rise in the moisture content like from ground water, improperly draining gutters, or incorrect grade. We call these the killer Gs as these sources of moisture will only cause the soil to expand. This is, of course, especially true if the soil is not fully compacted. We don’t have much expansive soils in our area so I do not what degree it would require but I believe the higher the better, like 98 or 100% at 2 or 3% over optimum moisture content. I seriously doubt is one could trust a home builder to get a phases (sub-grade prep, placing rebar and tendons, stressing) of this kind of construction right without intense, strict third party inspection that was working for and being for by the owner.

FalsePrecision

You may not be able to use a magnemometer to detect the dead ends of the tendons because the slab should have edge bars and the tendon anchors should have backup bars place over and under them. You are correct in that it would be easy to pick out the live ends due to the plugs.

However, the home owner wrote that the walls were built on a foundation that was already in the process of heaving. She wrote, “the walls, ceilings and floors that have been built to the original highs and lows of the foundation”. Straitening out the foundation at this late date will be even more disastrous. This house probably needs to be condemned, destroyed, and rebuilt.


SCET - Techmaximus

 

[FalsePrecision]

Maybe it would be possible to shore up the house and demolition the slab from under it?

 

[RodbusterRon]

Thanks Tech for the informative lesson on soil properties. Yes I mean Gauge pressure. Yes the overall stress on the cable is higher. I assume that this slab was built right on dirt, much like a monolithic slab. I'm curious as to why, if soil in the area tends to be sketchy, no piles were drilled. I would think if you had caisons to run cable groupings through, you could undermine that house 50' if you wanted to, with NO ill effects. Anyway, this sounds like a bad situation by all means.

Ron