[Cob] finding stats

[Shannon Dealy]

On Wed, 31 Dec 1969, Ron Becker wrote:

> Hi Shannon, I am curious about the average sand content  in adobe blocks and 
> cob. [ as described in the Hand Sculpted House]   Are they generally 
> considered equal? Somehow I got the impression that cob had more sand. I 
> understand that the quality of sand can make a difference.

Adobes are more likely to simply use the soil at hand without modification 
(as was the case with traditional cob), providing there is sufficient 
clay content to bind it.  Modern cob tends to use more of a tailored mix 
which will usually have a higher sand content, preventing shrinkage 
cracks, increasing compressive strength, and possibly other effects such 
as improving water resistance.  Some sands make a better mix than others, 
such as sharp varying particle sizes when compared with beach sand, but 
even for compressive strength, straw plays a significant role (up to a 
point).  When you put any solid material in compression, the point at 
which it yields is the point at which the sides buckle outward, and a 
decent straw content will help to hold the sides in, keeping them from 
buckling outward and thereby increasing the compressive strength.

> I think straw plays the dominant role in shear situations but where 
> compression strength is concerned, it's the proper amount of good [ sharp, 
> and a variety of particle sizes ] sand  that makes talk about bond beams in 
> properly built cob walls nonsense.

Actually no, a bond beam is not about compressive strength, it is about 
"bonding" the walls together so they move as a single piece, which is 
really about straw quality and content which ties things together.  The 
reason cob has far less need for a bond beam is that (to a point, 
and unlike adobe) the straw interconnect makes cob it's own bond beam.

FWIW.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
                       |    Embedded Systems, Real-time, Device Drivers
Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
    or: (541) 929-4089 |                  www.deatech.com