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RE: Alternatives to Post-Tensioning Construction

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Post tensioning systems have come a long way since the 70's.  A typical
modern system includes complete factory-encasement of the strands in a
waterproof grease and then an extruded plastic sheath and either of the two
main types of anchorage available:

1) standard anchors which are bare castings and provide minimum protection
to the gripper zone and the transition between the anchor and the strand.

2) fully encapsulated, electrically isolated anchors that are completely
covered in a thick plastic coat, have a grease cap over the gripper pocket
and a plastic trumpet that fully engages and seals the strand sheathing to
the anchor sheathing.

A properly detailed PT system will provide excellent performance over the
life span of the structure.  In areas where salt exposure is a problem the
typical system recommended is the fully encapsulated one.  Further
protection can be provided with proper specification of low-shrinkage,
high-density concrete containing fly ash and/or silica fume.

The durability of any concrete structure, PT reinforced or otherwise, is
predicated on proper and timely maintenance coupled with good detailing and
construction practices.  The reasons previously stated by others on the list
note the benefits of PT.  I would however try to locate penetrations early
on in the design process and only locate conduit parallel with and in the
slab. Plumbing should be kept out of the decks for maintenance reasons if
nothing else.

Nicholas Blackburn, PE

ps: On a related note what software does the List use for PT design.  We use
PT Data typically but I am familiar with Floor and ADAPT PT.

-----Original Message-----
From: h.d.richardson(--nospam--at) [mailto:h.d.richardson(--nospam--at)]
Sent: Wednesday, May 09, 2001 10:23 AM
To: seaint(--nospam--at)
Subject: Re: Alternatives to Post-Tensioning Construction

Fellow engineers,

	I am astonished by what I am reading!

	In Calgary during the late 1970s, up to 1981, most residential and
office buildings being built were of post-tensioning construction. 
There were literally a few hundred of them built.  Serious structural
problems have since developed with these buildings.  Breaking of the
cables as a result of corrosion/oxidation is the culprit.  Water
infiltration of the system seems to be a major contributing factor.

	Fortunately for me, during that period of time I was working for
and similar employers, hence, I have no personal involvement in the
problem at the design stage.

	Presently, there are engineers doing a good "bread and butter"
doing annual structural inspections locating broken cables; and there
are contractors specializing in replacing broken cables.  It's a lot
like replacing light bulbs or, as one mechanic I used to know years ago
would say "like fixing Fords".  It's difficult to sell a post-tensioned
building in Calgary at the present time; most buyers are aware of the
high cost of maintaining the structure.  I doubt if there has been a
post-tensioned building built in Calgary in the last 20 years.

	I am curious to know how the corrosion problem is being dealt with
other locations at the present time.  Moisture infiltration has to be a
problem, especially in coastal climates like Seattle.  I hope the thread
of the present discussion will extend to that topic.

				Regards to all

				H. Daryl Richardson

Paul Crocker wrote:
> Of the several dozen residential project I am aware of having been done in
> the last few years in the Seattle area, any that were too tall to be wood
> were P.T. flat plates.  P.T. typically allows a shallower floor system
> steel framing or mild steel slabs.  The P.T. also tends to reduce
> problems that can occur in mild steel slabs.  Ducts and conduits can be
> burried in the slab, which is an advantage overall even if it is not
> something I enjoy as an engineer.  Openings are not very hard to deal with
> if placed *before* the slab is poured.  It becomes more difficult to place
> openings afterwards, however, as it is not a great idea to hit cables,
> although a few will get broken by careless plumbers from time to time.
> that anticipate cutting a large number of openings in the slab throughout
> the building's life are typically not P.T.
> Paul Crocker, P.E.

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