permaculture

["Eugene F. Monaco" <EFMonaco@compuserve.com>]

Message text written by INTERNET:permaculture@listserv.oit.unc.edu
>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).<

On reinforcing steel:
Steel is an energy intensive material, you have to resign yourself to that
if you use it.  So is portland cement.  You have to resign yourself to that
too.  Having done that you should consider some points if you're building
for 50 years or more.
First is salt content of the mortar mix.  The salt corrodes the reinforcing
steel and causes it to explode the concrete off of it.  This happens in
roadway applications that are salted during snow season.   I've also seen
reinforced concrete beams in St. Thomas spawled, being structurally
unstable, that were made with beach sand (marginally washed, high salt
content).  My worry with ferro-cement cisterns is that this can happen,
particularrly in a marine or alkaline environment, even though it'll take
20 or 30 years.  The point here is to wash your sand well and make sure the
water is not saline.  Alkaline environments in the SW are perhaps
borderline.

The next thing one can do is to use epoxy-coated reinforcing mesh, which
you can get.  It's a little more expensive, but you're not using very much.
 It's good insurance, again resigning yourself to the fact that you're
using a toxic material if ingested.  The manufacturers of this stuff point
out that it should be considered criminal negligence against public
agencies that build roads without this stuff, knowing that the roads are
going to tear up in  a few short years.  Mollison always said it's okay to
use high-tech, high-energy materials so long as they last longer than, well
a long time.

I've often wondered about bamboo reinfocing because it doesn't corrode,
although I have no direct experience with it in concrete applications; I
wonder if it will rot, and how long it'll last under given conditions. 
Contrast that with the non-coated reinforcing mesh and see what makes
better sense.  I think that the Romans always designed their structures in
compression (using vaults and arches), to avoid the tensile issue.  This
brings up the idea of putting the tension ring on the outside so you can
get to it.  I think they call this a barrel.

Gene Monaco