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GSBN:Clay plastered wall passes 1-hour test!

Hi all,

Since Bruce is on a plane out of San Antonio and David, Matts, and
Ben are cooling their heels and various other body parts by going
tubing down the Guadalupe River, I get to tell you a bit about
today's fire test.

Rule #1: Don't move a clay plastered wall once you've built it.  When
we arrived at the lab prior to the tests, we discovered that a 1/8 to
1/4 inch crack had opened up on the non-fire side of the clay
plastered wall.  Since the wall had been moved to make room for other
testing in the lab, we assumed that the cracking probably occured
during the move.  The cracks were patched with some of the same
material less than 24 hours before the test took place.

When the wall was rolled up against the oven, several more cracks
appeared on the unheated side.  These were not patched.  The heated
side appeared to be intact.

Based on what we saw yesterday - specifically, the plaster pretty
much hanging on by virtue of the stucco wire - we had decided that
we'd aim for a minimum of a one hour test, and make a final decision
at about 50 minutes at to whether we thought the wall would survive a
second hour of furnace time.  If it was doing real well and it was a
pretty sure bet that we'd make two hours, we'd go for it, but if it
was questionable we'd do the safe thing and stop at one hour and get
a fire rating for unstaked, unreinforced, clay plastered bales laid
flat in a load bearing wall.  We felt that overall that it would be
more beneficial to homeowners trying to get insurance to get the one
hour rating than to take the chance and maybe get no rating.

The clay plaster performed quite well, in some ways better than the
cement/lime of yesterday's test.

Moisture levels in this wall were lower than in the cement/lime wall.
The highest reading David got was 14%, with the majority being below
what he could measure (down to 11% or so).  None of us were
particularly surprised as the cement/lime was kept misted for the
first two weeks and the clay wasn't, and because of the higher vapor
permeability of clay.

Because we had put a 600psi load on the wall by means of hydraulic
jacks pushing up from the bottom, the bottom edge of the assembly was
exposed to the fire.  The wall was built with a piece of ply at the
bottom edge to act as a 'brick ledge' for the plaster, and that ply
of course caught fire pretty quickly.

About ten minutes into the test, a small chunk of the second coat of
plaster came loose at the bottom of the wall - a triangle about 8-10
inches on a side. The base coat at that location stood up well for
the rest of the test.

Cracking of the plaster (other than that one flake) did not occur
until later in the test.  I'm not sure of the exact time, but it was
at least a half hour into it, as opposed to twenty minutes into the
cement/lime plaster.  Another thing we noticed was that the clay
plaster took a longer time to heat up.  We were able to observe the
fire side through several view ports, and the light we were viewing
with was the glow of the materials inside.  The clay took a lot
longer before it got to the glowing point.

The first cracks that opened up on the heated side occurred within a
foot or so of the crack on the exterior.  These continued to expand,
large amounts of flame came from them, and eventually (maybe 45
minutes into the test) a several square foot section of plaster fell
away, exposing the straw beneath.  The raw bales did not just burst
into flame as we expected - in fact, we could see several straws just
hanging away from the bale, apparently not burning, even though they
were surrounded by fire.

At that point we still felt we *might* be able to make two hours.  At
about the 55 minute mark, we noticed that the straw just inside the
crack on the exterior plaster was visibly burning, and we chose to
end the test.  Chances of the back wall standing up to the water
pressure at any place that had been charred all the way through were
slim.  And even though the burning spot we saw was outside the
official test area (the wall was 12' high by 14' wide, and the oven
only 10' square), we figured that was a pretty strong indication that
the test area of the wall wouldn't make another full hour.

The hose stream portion of the test was uneventful (as much as you
can call pounding water on the wall can be considered uneventful).
As with yesterday's test, the remaining plaster (most of which was
still intact) and the charred straw was quickly washed away, and the
uncharred straw just soaked up everything that was thrown at it.

Upon cutting out some of the area on the exterior where we had
observed the straw burning, we found one channel right along a joint
between three bales had been burned through.  And that was all.  The
straw on either side of that roughly 3-4" diameter burned cavity was
unscathed.  Again, this should surprise nobody, as we know that the
joints between the bales are our weak spots.  We had been pretty
compulsive about stuffing the joints with straw, but unlike
yesterday's wall where we had also stuffed in some cob prior to
plastering, we let the sprayer do the stuffing of clay here.  We also
feel that if we had patched the crack in the exterior plaster the
fire would not have gotten as much oxygen and probably never would
have gotten to the back wall.

So Rule #2 is to compulsively stuff the cracks.

Dang... I just looked before posting and see that David got his post
out before I did.  Well, now you have more details, anyway.

(how was the water, David?)

Bill Christensen
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