elongation of post tension cables

huangyhg

elongation of post tension cables
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?
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are you saying that the pt co performed the stressing of the cables 6 years after the slab was placed?
scet - techmaximus
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?
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
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.
in your first post you asked, 鈥淪hould the pt co knowingly have done a final stressing without benefit of elongation measurement and report?鈥?br>
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, 鈥淪hould the pt co have known the elongation of the cables prior to final stress?鈥?br>
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 卤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, 鈥淭he 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.鈥?br>
the tendon may not be 鈥渂roken鈥?  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, 鈥淒o pt cables loose any elongation in a 6 year period, or would they still stress @ 7000 psi?鈥?br>
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.
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.  
i have to tell that i would have an engineer develop a 鈥淐adillac fix鈥?and settle.
((cadillac fix + lawyer鈥檚 fees) x 1.25) + ten year warranty = settlement

scet - techmaximus
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?
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.
rodbuster,
i don't know what you mean by, cables aren't used in ground slabs. post tension in ground slabs is very common.