Cob Re: Particle Size Distribution

[John Hall]

Otherfish at aol.com wrote:
> 
> to john hall
> so what are the relevant particle sizes?
> clay?
> silt?
> sand?
> 
> thanks
> otherfishHello Otherfish.

The clay is the binding agent (cement). Without it, you can't have cob. 
 Like cement, it is not supposed to make up any more than the voids left 
by the coarser particles. If it does then you get shrinkage (everything 
else, silt sand and gravel is inertly stable, clay is not).  The optimun 
mix is to get a well graded inert element (everything down from medium 
gravel to silt) to give as dense as possible a "skeleton", and then the 
clay should just fill the interstices (voids). 

If you overdo the clay, you get settlement and shrinkage.  If you 
underdo it, you lose potential strength, weather resistance etc.  That's 
why its important.

Of course you do not need to be quite so scientific about it.  Simple 
site trials can tell you how your cob will behave.  Mix up a few 
variable (known) batches, compact a sample from each in a biscuit tin or 
similar (say a cubic foot), tip it out and leave it to cure.  Measure 
with a tape (and weigh if you want to know declining moisture content) 
each sample on a regular basis until cured. Once you've got an idea of 
how much each sample is going to shrink, hit each one with a hammer and 
see how strong it is. Then choose your optimum mix, with respect to both 
shrinkage and strength.   

Very few sites will produce "optimum' grading with respect to gravel, 
sand, silt, and clay, but the most important concern is to ensure the 
clay content is as correct as is practically possible (assuming you are 
really serious about constructing long-term buildings out of the stuff).

Only trying to be helpful, 

Best Regards,

John (lived in cob for at least half his life) Hall