permaculture

[EFMonaco <EFMonaco@compuserve.com>]

John wrote:
>........ about surface bonding cement (as described by Rob Roy, used for
his dry-stacked masonry underground house wall construction technique)...<

A little more on surface bonding cement....
The first I ever heard about it, they were claiming you could dry stack CMU
(Concrete Masonry Units, aka/ concrete blocks) blocks, without mortar, then
trowel this stuff on both sides of the wall and the result will be as
strong as mortared CMU construction.     (As a sidebar, this doesn't
include internal reinforcing where you fill the certain cells of the CMU
with rebar and grout both vertically for  column loads and horizontally for
wind loads.  Around here they call this reinforced masonry.  In Florida,
they form a concrete tie-beam around the top of the wall (plus at
mid-points if it's tall) and fill it full of reinforced concrete.  They
call this block and tie-beam construction.  It's stronger than reinforced
masonry but takes more time to form the tie beams.)
Anyhow, I didn't follow this closely but it appeared from the literature
that it had worked.  The problem I have with it is this.  Once I was
jacking up an old house to rebuild the foundation (talk about a type 1
error), and I was using an hydraulic jack.  As we jacked up a bit, we'd
shove in cribbing, but some of the points we had to jack were several feet
off the ground so we had to get the jack up in the air.  In a few cases we
used dry stacked concrete blocks, which we shouldn't have done.  As we
jacked on top of these they started to crush and crack, whereas if we
jacked on wood, it didn't.  We tried putting a piece of 2x8 on top of the
block first because I thought the jack was inducing point stresses on the
block.  The result was that the block didn't crack where the jack was, it
cracked between the blocks.  In trying to understand this, I realized that
the block, being rigid and brittle, would induce point stresses where one
tiny piece of irregular surface touched another tiny piece of irregular
surface on the next block.  The wood, provided a cushion so this wouldn't
happen.  Mortar does the same thing.  We often think of mortar holding the
stones, bricks or blocks together, but in reality it holds them apart, just
enough to transfer the loads to the next one, all the way to the ground. 
When you dry stack blocks, or anything else, you don't get this cushioning
effect, so I just can't understand how the surface bonding cement can
provide the cushion and take up the loads, even if it does.  Bottom line: I
would never use a drystacked system if there were significant loads
involved.  I just don't trust it not to fail, especially given the
imperfections of construction.

What I think it's great for is repair.  I was working on a project where
they were drilling some rock with an hydraulic rock hammer attached to a
backhoe (a hoe ram), when a neighbor said we cracked his foundation walls,
which were brick.  We ended up coating the walls with surface bonding
cement and the guy was tickled pink.  We also did his barn for PR's sake.
I used it in a basement to coat below-grade masonry (some brick, some
block), then coated that with paint mixed with cement.  It sealed pretty
well and looked like stucco.  I also used it to patch stucco, raking out
the crack first.  Those fibers are great!  You can also trowel it on in
layer after layer to sculpture something; it sticks to itself.  Great
stuff.  If you're putting it on something like old brick, you have to make
sure the bricks are clean.  In my case, I had to scrape the brick with a
wire brush and chip off paint with a bushhammer in some cases.  I didn't
get all of it off, but there was enough loose mortar that I raked out to
create a gazillion nooks and crannies to shove the surface bonding cement
into before smoothing out a final coat.

Gene Monaco