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Post-Tensioned CMU

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Mark,

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)
Tucson, Arizona

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.