[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: GSBN:Interesting test results and some more.
- To: GSBN GSBN@...
- Subject: Re: GSBN:Interesting test results and some more.
- From: John Glassford huffnpuff@...
- Date: Tue, 14 Dec 2004 09:22:54 +1100
- Reply-to: "GSBN" GSBN@...
- Sender: "GSBN" GSBN@...
G 'day Chris
Some very interesting work is being done by Mike Faine and Dr John Zhang
at the Uni of Western Sydney. You should write to Mike and get a copy of
his latest paper. I copy here their summary but it makes more sense if
you read the whole paper.
Mike and Dr John are doing some great work on straw bale and especially
witth earthen renders.
I still prefer a combination of earthen renders/sand/chaff for the first
two coats to 40mm and then finish with a mix of earth and lime and chaff
for the finish coat to 50mm. I find Maudies Magical Mix to be extremely
strong, highly water repellent and can be coloured with oxides just like
a pure lime plaster. Also and just as important there is no need to make
the hydrated lime into a putty, I just use the powder lime straight out
of the bag. The first time I did this was three years ago and it is
working a treat, no cracking no moisture problems on an exposed wall..
In our last job we used hydrated plaster lime and sand for all three
coats straight out of the bag in a powder form and we pumped this mix
on. So far no cracking has occurred in over 450 sqaure metres of straw
bale wall. The final coat was a premix with lime/sand/oxide made up for
us in a factory in Melbourne which we applied by hand. The hydrated lime
we used was from Melbourne and it was of a very high quality. If we are
fair dinkum about getting straw bale into main stream we have to use
methods such as we used in Ballarat. It was very cost effective and very
fast which pleased the professional builder no end.
Kind regards The Straw Wolf.
<a target="_blank" href="http://www.maps.coolamonrotary.com/">http://www.maps.coolamonrotary.com/</a>
Here is Michael and Dr John's Conclusions to their latest testing:
5. Recommendation and conclusions
From these tests there are a number of aspects worth considering. The
first relates to the construction detail of a straw bale walls. It goes
without saying that achieving the best pre-compression of the bales (by
whatever method) is very important. Pinning of the bales by using
reinforcing rod isoverkill but some pinning is required as the wall
progresses for practical reasons (stability) particularly for the
The location of the top plate in relation to the render finish is
another important practical aspect. The two early tests allowed for
loading of the straw bales in compression, as the top plates did not
directly bear on the render finish. At the safe working load of 40kN the
deformation (average) for the two-storey wall (cement render) is about
5mm and for the earth rendered wall about 8mm. This is quite tolerable
but suggests careful detailing of the finish of the render against
adjacent surfaces (floor or slab) to avoid cracking of the finish.
The second aspect worth considering is that of the structural behaviour
of the test walls. The safe working load limit of 40 kN is approximately
10 kN above the limits set by the Pima and California Codes. These codes
may be very conservative and further testing of straw bale walls is
warranted to determine type characteristics suitable for inclusion
into building codes. At the ultimate failure load of 73 kN (for the
two-storey wall) some interesting comments can be made about straw bale
walls. Once this load limit has been reached straw bale walls compress
unimpressively, in fact the failure mode was unspectacular.
It is also clear that a cement render based wall will carry slightly
more load than an earth render wall (not having any cement component).
However the earth based render was easier to apply direct to the straw
wall and does not require a layer of wire mesh to be applied over the
wall. This saves both time and cost.
It can be said, from the last test that it is beneficial to load the
strawbale and the render material directly by supporting the render at
the bottom plate with a slightly wider top plate ladder frame at the top.
All the tests record a differential settling of the wall(s) (when
comparing left and right sides) with the average figure for
settlement being used for calculations. Constructing the walls as
straight and perpendicular as possible and in plane is definitely an
issue of construction quality and constructability. Care needs to be
taken to avoid using bales that may induce some rotation to the wall
The addition of the slaked lime to the mix design for the final coat
(for the earth rendered wall test 2) may or may not be warranted. Some
authors argue that this helps workability and has self healing
characteristics to guard against any micro-cracking and possible ingress
Further work needs to be done on both the composition of the render mix
design and the interaction of the straw to render bond to determine
exactly how this behaves as a construction material. The bond between
the straw and the render was observed to be very strong, particularly at
the time of demolition. These tests have shown that this material is
suitable for constructing walls for residential purposes from a
structural point of view.
Mike Faine - Lecturer UWS
School of Construction Property & Planning
Locked Bag 1797
Penrith South DC NSW 1797
Uni Office phone (02) 98524318 Uni fax (02) 9852 4300