[Ferro List] Fwd: micro-cracking, fibers

[Marc de Piolenc]

Dear listmates,

I'm getting a little worried by some of the list traffic about
microcracking and fibers, so I'm chiming in to remind us of some
fundamental facts that seem to be getting lost in the discussion.

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.

Marc de Piolenc
Iligan City, Philippines