[Strawbale]Re: testing and relevant parameters

Rene Dalmeijer rene.dalmeijer at hetnet...
Tue Sep 14 10:04:43 CEST 2004


You have indeed mentioned many parameters that could be of interest. There 
are also some prerequisites ie the bales should conform to these otherwise 
you should not even consider using them ie they are the potential cause of 
a premature demise of the SB wall.
- Never been rained on after baling the bales should look and also smell 
clean, bright yellow any greying shows that the bales are unfit for use 
(the colour off course varies depending on the type of straw)
- Moisture content 15%-20%
- The strings should be tight, no loose strings. Broken strings means that 
inferior quality strings have been used and most probably there will be a 
big spread in bale quality.
- Long straw: at least 200mm and the average higher.
- Squareness of bales: This is of much less importance but does vastly 
improve build ability and therefore the ease with which a structurally 
sound wall can be built.

Practice has shown that the following characteristics are of importance:

1) Bale density:
This is by far the most important factor regarding build ability and load 
bearing capacity, all other parameters are of much less importance. I 
always prescribe at least 115kg/m^3 more is better also with regard to 
insulation value.
2) Plaster bond:
A good bond between the plaster and the bales this means for earth high 
clay content ie the first layers should crack. A good penetration of the 
first layer of plaster I personally therefore prefer French dip to hand 
plastering the first layer with earth plasters. Spray on is also good but 
make sure the clay content is  high typically 1:1 clay to sand
3) Pre-compressing:
After the bales are stacked it is good to pre-compress the bales 4% also in 
the case of non load bearing. This gives a much more stable surface to 
plaster thus avoiding cracking due to later movements and settling/creep.
4) Connections:
The use of suitable open weave(5>mm) mesh fabrics like hemp, reed, (or 
plastic)etc pasted into the base layer. This fabric is much more effective 
at transferring loads from the plaster to surrounding structure or 
openings. I therefore hardly use pins, if I do these are bamboo or wood.
5) Wall/plaster footing:
The plaster should begin and end with a proper supporting edge specifically 
at the foot of the wall but also at the top. This is necessary because the 
plaster carries the load and not the bales otherwise the plaster will have 
a tendency to peel off the bales at the footing and top plate under 
sufficiently high loads or movement of the structure.

Coming back to your list of parameters the above are to my experience the 
most important considerations to the success of a SB wall. The biggest 
variable I think that we do not measure but base on experience are the 
mechanical properties of the plaster. I think this is an area where there 
is a possibility to improve on by making SB building efforts verifiable via 

Already quite extensive testing has been done on SB walls. As far as I know 
very few of the tests give mechanical details of the plaster involved. This 
includes the test we did for determining the acoustic sound insulation. 
(Mea Culpa) I think it is necessary to determine field measurable 
parameters for determining the mechanical properties of the plaster. This 
will also require testing to determine what the effect is of the measured 
properties of the plaster are on the whole wall system. To put it other 
wise we have to determine what field measurements should be done on the 
plaster relevant to determining the performance of the whole wall system. I 
think what is required is a minimum test cube failure load for the plaster. 
Other tests like water erosion and abrasion could also be considered.

Many of the other factors Menno mentions are already part of the present CA 
and PIMA codes. Some parameters Menno mentions are not in the codes because 
until so far they have proved to be irrelevant to the performance of the 
whole wall system.

I have focused the above on the use of Earth clay plasters as these show 
the greatest variability but the other options should also be measured. To 
determine required minimum parameters for these options.

At 07:36 AM 9/12/04, you wrote:
>  Variables may include type of straw (wheat, rice
>etc), straw
>length, straw moisture content, straw density, type of
>binding (metal,
>sisal, twine, polypropylene etc.), number of bindings
>(2,3), bale size, bale
>orientation (flat, on edge), construction bond
>(running bond, etc.), wall
>even-ness and plumb, pinning (material, method and
>placement - if any), mesh
>(type, gauge and placement (if any), render
>(materials, material mix,
>thickness, consistency, setting conditions, curing
>conditions, drying
>conditions, quality of bond, skill of application,
>number of coats, method
>of application - hand, pump, spray), wall layout
>(unsupported wall length),
>number and placement of penetrations, bracings (if
>any), buttresses (if
>any), strappings (if any), top plate and bottom plate
>design etc.,etc. etc..


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