[Cob] Building Community

[Shannon C. Dealy]

On Thu, 18 Nov 2004, Stuart DeJong wrote:

> Could some people throw some numbers at me for cob
> construction?  Is there any available hard data that I
> could reference?  I would like to collect some solid
> information that is rather detailed.  Thanks

You don't specify what kind of numbers you are looking for:

Structural: Information such as tensile, compressive, and shear strength
         as well as modulus of rupture, is going to be very much mix
         dependent, not just the sand / clay / straw ratio you use (which
         by themselves could easily alter some of these values by 50% to
         100%), but the type of clay and/or fiber you use as well as the
         mix of grain sizes in the sand could significantly alter these
         values as well.  So for structural information, you will need to
         make some test batches and perform tests on them, though some
         fairly simple tests could probably be devised to give you a ball
         park figure.  Once you have some numbers and a feel for how the
         materials interact, you should be able to adjust the mix to get
         something closer to your goals.  If you just want absolute worst
         case numbers, adobe numbers would probably make a good starting
         point (cob should easily beat the numbers for any standard adobe
         mix across the board), though the people doing the testing work
         at the University of British Columbia in Canada might have some
         better information (there was a posting about this work roughly
         two months ago on this list).

R-value: There have been a wide range of values claimed, and it can
         be difficult to correctly measure R-value for high thermal
         mass materials, particularly something as rough and variable as
         cob.  I would generally go with something in the 0.35 to 0.5 per
         inch range for a conventional cob mix (10% straw more or less).
         For reference, stucco is 0.20/inch, most hardwoods are 0.91/inch,
         and wall board is 0.45/0.5 inch (and alot of that value comes
         from the paper coating).  Given the above, the claims some people
         have made in the past of 1+/inch for cob are clearly not
         reasonable, and are most likely an artifact of the thermal
         mass distorting the measurements.  Of course, mass is also an
         important consideration in the thermal design of your building.

Construction time:
         This does not take into account time for drying, carpentry
         work, etc. (which can be substantial), but based on personal
         experience, an experienced adult male cobber (me) in reasonably
         good physical condition (5'10", 190 lbs), working alone, can
         manually mix and apply about one cubic yard of cob in an eight
         hour day.  Mechanical mixing can of course accelerate the process
         somewhat.

I realize this probably doesn't answer your question, but ultimately, no
more meaningful answer can be given without defining a series of
"standard" cob mixes, taking measurements, and either finding a way to
match a local mix you can make to one of the "standards", or a
series of formulas that can be used to account for variations between
different mixes.

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