How many PSI are needed to make compressed earth block?

Does anyone know how many psi are needed to make a compressed earth block? assume a reasonable proportion of lime is used in the mixture. The reason I am asking...a friend has offered me the loan of his tractor in the winter, while he is not using it. He has a backhoe on it - I was wondering if I could get around buying a machine and simply using the backhoe - with an appropriately shaped "foot" to stomp on the soil in a mould. He has a medium sized tractor - I think it is about 80 hp.

Tom, it might help to think of adobes and CEBs not so much as two different types of block, but more like two different spots on a spectrum.

A CEB is just an adobe that was made drier. It holds together because it's pressed, not so much because it dried that way.

And so the practical answer to your question is, you can almost definitely make that work, depending on how wet your mix is.

But I'm not sure that using the backhoe for the compressing would save you a ton of time or work. Using it for the digging definitely would! Maybe even for the mixing. Maybe even for the scooping of your mix out into position over your molds, where you're only moving it by hand a foot or two from the bucket into the molds. But I suspect not so much for the compression.

Here's a little snippet from Gernot Minke's "Building With Earth" on strength (pdf of the whole book, free for download in the lower right there):

Preparation
The compressive strength of a mix is affected
by the type and amount of preparation,
as well as by the proportion of water used
in the preparation, a fact that is neither wellknown
nor well-researched.
At the Institute for Building Technology of
the Swiss Federal Institute of Technology in
Zurich and at the BRL, it was proven that a
slightly moist loam, when free from lumps
and compacted in a soil block press, usually
has a smaller compressive strength than the
same loam combined with sufficient water,
mixed by hand, and then simply thrown into
a mould (as is done when making adobes).
In one experiment at the BRL, handmade
adobes had, on an average, a compressive
strength 19% higher than if produced in a
soil block press which imparted a pressure
of 20 kg/cm2 to the material. The belief of
many researchers and practitioners that
pressing in a soil block press leads to an
increase of compressive strength may only
be true for limited cases. As a rule, it is not.

In other words, you may not even gain anything by compressing them!

thanks! I love this forum! But i am guessing that for those who use some cement (perish the thought!) heat made from compression might help the chemical reaction along that comes with adding some water to cement...maybe this is why people think compression is necessary.

Hello Tom,

Several "adapted" hydraulic forms of compression devices have been fitted to tractors for this purpose in some commercial applications. For the most part a simple "Cinva Ram" or similar manual earth block devices will do more work with less energy expended for a single or low output production venue.

CEB are a bit different than just adobe by itself, though similar (identical?) machines can make them. Heat has nothing to do with the process of creating these block and because only a limited amount of "activator" e.g. lime, opc, or other pozzolanic is employed, the massive pressure is vital requirement...not heat per se, though many of these systems will not work as well in cold weather.

Hope that was of some help,

j

You may learn more from the Open Source Ecology people and their videos, drawings and descriptions. They have refined their CEB machine quite a bit. Here's a link you may find interesting for it: http://opensourceecology.org/portfolio/ceb-press/

There are several videos out there on assembly and builds of the OSE CEB machine. Enjoy!