Jack Rowe jackrowe at compuserve.com
Tue Sep 29 21:04:08 EDT 1998

Ferro-cement is any type of steel and concrete or mortar composite. In the
case of cisterns, the mix used is more of a mortar (or stucco) since: 1) it
has no rocks, and 2) it has lime and extra cement added for workability and
to aid in adhesiveness to already-cured mortar (this is basically a
bricklayer's mortar, made with mortar cement instead of regular cement).
Cement:Sand mixes traditionally range from 1:2 to 1:5 (1:5 is way too weak
and porous). The mix goes from a pile of wet, muddy sand to a paste
consistency when it has enough cement in the mix. A little extra cement is
ok but a lot extra is wasteful, increases shrinking during curing/drying
and detracts from strength. 

The steel (remesh, rebar, chicken wire) reinforces the shell basically only
in the tensile direction. The rounder the shell is, the more-evenly spread
are the stresses of the water in the tank, and the more the stresses are
carried along the length of the steel (its greatest strength) instead of
trying to bend it. Middle-Eastern masons build accurate domes and cylinders
by tying their brick-laying hand with a non-stretchy cord to a center stake
(dome) or pole (cylinder), then placing each brick at the end of their
reach as they go (neat pictures in SHELTER, Lloyd Kahn, ISBN 067976948X).

Cement doesn't dry, but hydrates so it's important that it not dry out for
a week or more (preferrably a month) while it's curing. Evaporation of
large excesses of water not used in hydration can cause porosity, so use a
minimum of water for adequate workability for highest strength and
water-proofness. Cement isn't toxic in the conventional sense, but has the
two qualities Dan mentioned that seem important, 1) the alkaline cement
lowers solubility of metals (important) and 2) there's the possibility of
some of said metals being right there in the cement... but they might not
dissolve readily, or might precipitate back out after acidic rain was
neutralized by the cement. 

We've made cisterns in two applications, the remesh/stucco wire armature
stiffened by bracing and the wrapped rebar. The first coat can develop
small cracks, which is why the outside should be done first then the second
coat done after the first is rigid (2 - 3 days). Actually, I really like
Marsha's report of the Brazilian man who uses adobes to make a rigid form
-- much better quality end-result. I'll bet the adobes don't even have to
be the best. Mud-stuccoing the adobe form to a nice roundness would
maximize strength and minimize stucco needed. Stuccoing the cistern onto
the inside of the form would allow use of the Arab masons' idea for the
purpose of stucooing the interior mud surface of the forms into a nice,
smooth curve before applying the cement stucco, and would allow the cement
to shrink a little as it cures. 

Since cement stucco mix is a mortar and sticks so well to itself, 2 layer
construction is probably pretty good as long as the inner coat is of high
quality. But, as Dan mentions one-piece construction is ideal if applying
the one layer can be accomplished with absolutely no flexing or cracking.
Seems the adobe form would be a great way to accomplish this. If cracking
occurs or is likely, a final inner coat would be called for. 

Snow? What's snow?

Jack at 30 degrees north

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