From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Wed, 21 Feb 2001 09:33:17 -0500
As I see it, post-tensioning of cmu has been an extrapolation of
post-tensioning of concrete and, in fact, some of the basic precepts of
concrete prestressing (either pre- or post-tensioning) seem to have been
forgotten along the way.
Prestress loses due creep and shrinkage have a significant effect on the
residual prestress in the masonry or concrete and creep is a function of
masonry assembly or concrete strength. To the best of my knowledge, no
long-term creep tests on prestressed masonry have been conducted and even
the tests submitted to ICBO-ES for P/T masonry fences were not required to
include long-term creep tests.
While I will not design prestressed concrete with f'c less than 3500 psi (and
generally not less than 4000 psi), I have seen prestressed concrete designed
based on f'c = 2000 psi (P/T slab-on-grade). In this case, I would venture
to say that after creep and shrinkage takes place, there is *no* prestress
remaining in the concrete.
When I was in school and prestressed concrete was still in its infancy in the
U.S. (1950's) and a technical writing course was required, I wrote a paper on
the development of prestressed concrete. Prestressed concrete actually
preceeded reinforced concrete, being first used in the late 1800's for
concrete hitching posts to which horses could be tied. (A threaded rod
embedded in concrete.) Because of the lack of high strength steels,
structural use of prestressed concrete was delayed for half a century as the
creep and shrinkage of concrete offset the maximum stress that mild steel
could be subjected to.
In a word, I would avoid using prestressed masonry until more research is
performed on the long-term creep and shrinkage effects. And I definitely
would not use mild steel as a prestressing component in any case.
A. Roger Turk, P.E.(Structural)
Mark Webster wrote:
. > Has anybody tried using post-tensioning strands instead of mild
. > reinforcement for CMU vertical reinforcement? Are there any proprietary
. > systems out there which use this approach? Is it necessary to grout the
. > cells if post-tensioning is used?
. > I'm trying to determine whether this is a viable option for CMU
. > reinforcement.