permaculture

[EFMonaco <EFMonaco@compuserve.com>]

Folks, I wanted to add my two cents worth on concrete and tanks, from an
engineer's perspective:
Concrete is great in compression, but sucks in tension.  Concrete that can
withstand compressive forces of 3000 psi (lb. per square inch) will resist
tensile forces only to about 600 psi , 1/5 the compressive resistance.  You
don't want to go to all the trouble of building something and then have it
fail in tension. The reinforcing material is added to resist the tensile
forces with the hope that the whole mixture and matrix will bond together. 
That's why conventional rebar is bumped or deformed, so it doesn't pull out
of the concrete.  Having said this, it's important to understand where the
tensile forces occur.  This is the subject of structural analysis.  In a
tank it works like this:
Pressure on the sidewall of a tank is given as the weight (or density) of
the fluid times the depth.  If water weighs 62.4 lb. per cubic foot (pcf),
which it does, then the pressure on the sidewall at a depth of one foot
would be 62.4 pcf x 1 ft or 62.4 lb. per square foot (psf).  At two feet of
depth it would be 62.4 pcf x 2 ft = 124.8 psf, and so on.  As you can see,
a 5' deep tank has sidewall pressure at the bottom of 62.4 pcf x 5 = 312
psf, whereas at 10' deep the pressure would be double, 624 psf.  This
pressure is trying to force the tank apart at the bottom.  Note at the top,
the pressure is zero.  This pushing apart gives rise to tensile forces that
have to be resisted by your materials for the tank to stay in one piece. 
Note also that the pressure is the same at the bottom whether the tank is 1
ft. wide, 10 ft wide or a mile wide.  It's the depth that counts and
controls the design.
So, if a 10' deep tank has sidewall pressure of 624 psf, that's equivalent
to about 4 psi (624 psf / 144 square inches per square foot).  That doesn't
seem like much for our 3000 psi concrete that can withstand 600 psi in
tensile forces.  Add the weight of the concrete above (another 5 psi) and
you've got maybe 10 psi worth of forces acting on concrete that can
withstand 600 psi, if it could.  Trowled-on cement might resist only a
third or fifth of that, say 100 psi.  Still, theoretically, our 10' tank
could hold the water in it with ten times the strength required, if only
the world was static.  What happens in reality, as we know, is that soils
settle, the wind blows, the sun shines, temperatures rise and fall,
materials expand and contract, shit happens, and the tank might crack.  Why
take the chance?  But nevertheless, this gives credence to the notion that
you don't really need any reinforcing from a structural perspective.  The
lath works to keep the cement up in the air while it cures, and adds a lot
of strength.  But rusting is a big deal and needs to be seriously
considered.
I've seen buildings in St. Thomas where the concrete beams had chunks
busted out of them the size of filing cabinets because the rebar rusted and
exploded the concrete.  The theory there was that they used beach sand in
the mix, which had salts in it, even though they tried to wash it first. 
This occurred in approximately 25 yrs time.  On highway construction where
the roads are salted in winter the rebar corrodes and explodes the concrete
causing potholes and bridge failures.  This also happens in marine
environments.  Not to be put out of business, the rebar industry came up
with epoxy coated rebar which they hope will last longer.  This developed
to the point where lawsuits were filed against highway departments and
engineering firms that failed to specify epoxy coated rebar; the charge
being willful negligence; they knew or should have known that the structure
would fail.  Now a days, you will see virtually all highway and bridge
construction using epoxy coated rebar.
The same thing happens with wire mesh.  It will rust and explode in
alkaline environments.  We see this in sidewalks all over.  Folks still use
it in sidewalks because often nobody cares if a sidewalk breaks apart after
30 or so years; it's going to get busted up anyhow in most cases, but still
the principle holds.
The use of fiberglass fibers in a mix adds tensile strength to the mix and
eliminates the need for wire mesh.  I don't know the exact numbers offhand
but it probably doubles it (tensile strength).  It also works great for
resistance to thermal expansion cycles, which is like a Chinese water
torture on concrete.  Galvanized lath will obviously resist rusting orders
of magnitude greater than steel wire mesh.  Chicken wire rusts in a
heartbeat.  Plant-based lath like bamboo would work to get the cement up in
the air and would provide good tensile strength.  If the tank stayed wet it
probably wouldn't rot too fast, but even if it did it'd be better than
steel mesh.  Simply put, using steel in concrete is a time bomb exploding
slowly over time.  Slow it down enough and it doesn't pose too much of a
hardship; otherwise, watch out!
Trowling cement containing fiberglass fibers is no problem, you hardly know
it's there.  I'd use it before I'd use steel mesh.  And I definitely
wouldn't drive my non-epoxy coated rebars into the ground unless it's a
swamp-like reducing environment, or if I didn't care if the tank fell apart
after 10 or 15 years.
Gene Monaco