Cob Straw in "STUCCO" or "ADOBE"

[Shannon C. Dealy]

On Sat, 3 Apr 1999, Charlie Duncan wrote:

[snip]
>   OK, I'm confused.  
> 
>  Why IF [if] your clay and sand mixture is correct do you need to add straw?  

Depends on what your needs are.  Here is what each of the ingredients buys
you:

   Clay: Binds everything together.  Expands when wet, shrinks as it drys.
         Ideal mixtures would have just enough clay to completely coat
         the surface of each grain of sand/each piece of fiber (straw) and
         perhaps just a bit more to fill in gaps between the fibers and
         sand grains.  Low clay content will not hold together properly,
         high clay content will tend to have shrinkage cracks.

   Sand: Provides compressive strength, doesn't expand or shrink.
         Maximizing sand content reduces shrinkage cracks because it
         won't shrink.  Low sand content will have poor compressive
         strength, particularly if the clay has any moisture.  High sand
         content will only cause problems if it is at the expense of
         using sufficient clay and straw to meet your other requirements.

   Straw:Provides tensile strength, keeps things from pulling apart, and
         makes the dried mixture less brittle.  Reduces cracking by
         preventing shrinkage from pulling areas apart.  Low straw
         content is more susceptible to damage from impacts or
         earthquakes.  Since in an ideal mix, the straw is primarily
         displacing sand, high straw content will reduce compressive
         strength.


Shannon C. Dealy     |                    DeaTech Research Inc.
dealy at deatech.com    |               - Custom Software Development -
                     |         Embedded Systems, Real-time, Device Drivers
                     |       Networking, Server Configuration/Administration
                     |            Scientific & Engineering Applications
                     |                       www.deatech.com