[Ferro List] Microcracking Expansive & Non-Shrink

[Christopher Glasspool]

Marc & All,
What about where the fibered mortar is used only as a thin (5mm or less) skin, and not engineerd to be part of the stress loads for the total  ferrocement sandwhich? I had thought that PVA-ECC would make a great tough - durable, flexible coating for normal steel mesh ferrocement. Do you think this wouldn't work?

Don't all regular portland cements shrink to some degree? The history of fc work doesn't preclude these, and I don't know of any problems, do you? I'm thinking that when you say non-shrink you mean low-water mixes with proper curing and no calcium admixtures, true?

In U.S.A. we only have one type of expansive cement that I'm aware of (Type K, made by CTS), and it would seem to be only available in bulk orders. It has a special mineral admixture (ettenrite), and I assume it to be fairly expensive.

I know of one pre-packaged spec mix to meet the requirements of non -shrink, and that is the Quickcrete non-shrink grout mix. It comes in two specs, one for structural-working applications and has a 8,000 psi cure, and looks to be a good pre-bagged fc mortar (which some list memebers had been looking for previously), the other non-shrink Quickcrete grout looks to be for compressive use only with a 10,000 p.s.i. cure, and should be too brittle for fc work. Not only is the first referenced good possibly for fc projects, but might make a good control for side by side comparisons to local or customized mixes, like for Tom's Crud Test for example. 

Any ideas on how to simply test for exceptable shrinkage amounts, and what would that be? Does anyone know of a admixture that decreases shrinkage? 
Thanks, chris

Slightly sniped:
Regarding microcracking, it is a given under load, but also nothing to
worry about. Cured cement mortar has a high tensile modulus, but low
tensile strength - hence it cracks under tensile strain. This is not a
problem because, in ferro, mortar acts ONLY IN COMPRESSION. As long as
the cracking stays "micro," there are very few applications that
cannot tolerate it.

Unfortunately, some people who regard it as a problem are advocating
the addition of high-modulus fibers to the cement-mortar matrix, which
completely defeats the purpose of the low  modulus, high strength
steel mesh because load is never transferred to the steel mesh until
the fibers fail - at which point failure of the complete composite is
likely to occur catastrophically and without warning.

In the very few situations where microcracking is not permissible, the
solution is to use a slightly expansive mortar mix; this will preload
the steel reinforcement in tension and even microcracking will not
occur until the tensile preload is exceeded. A mortar that shrinks
when curing is not acceptable in any ferro application, for obvious
reasons.


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