Ratios Soil:Cement
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Ratios of Portland Cement to Aggregate
The most frequent question seems to be about the ratio of the mix for soil cement. 

There is no certain answer.  The "general" answer is that almost always the aggregate:cement ration will be between 5:1 and 10:1.  The specific answer depends upon all the considerations of the specific application, of which there are several applications in any building project.

The first step in any proposed use of soil-cement is finding the best possible "soil", if you will.  Regular concrete uses clean sand and gravel and portland cement, "clean" meaning washed or free from vegetative matter or humus.  Adequately "clean" aggregate for soil cement is most likely found in the bed of washes thru one's property.  Surface earth will generally be high in humus and related undesireable stuff like roots, twigs, leaves and gung, all of which will both weaken the mix and be a real nuisance when trying to finish a concrete slab.

An excavation below the surface or into a hillside (same thing, isn't it?) may very well find "clean" materials only a foot or two beneath the surface.

The other undesirable element in aggregate is excessive CLAY.  Clay expands and contracts excessively with the varying moisture content.  We all have probably walked over lands where the surface is "sticky" or "greasy" when wet, but when dry is riddled with cracks -- that'll be excessive clay.

Clay particles are very fine-grained, which can be helpful in troweling a smooth finish on a slab, or obtaining a smooth surface on a poured wall.  However, as it is wet in the concrete mix and then shrinks as the concrete sets and dries, too much clay is unacceptable.  In a typical poured wall with rebar reinforcement, hairline cracking will eventually appear to delineate each and every rebar within the wall.  This is always a cosmetic problem, but may also become a structural deficiency.

The free and easy way of seeing the percentage of clay in a proposed aggregate is to fill a glass or jar with water, dump in the aggregate, shake well and leave to settle overnight.  In the morning you'll see the clay as the bottom layer, and that any also-undesireable vegetative stuff and humus is at the top.  Comparing  samples from different locations will reveal the best you have.
To help somewhat more, tho, here are a few considerations and GENERAL guidelines:
  • Desired or necessary strength.  These are determined by the particular application.  As a GENERAL rule, using STANDARD sand/gravel aggregate, footing concrete is 2500 psi (about 6:1) and slab concrete is 3000 psi (about 5:1).
  • In conjunction with the above is the imposed load, making a demand called Compressive Strength.   Obviously one will have problems with crumbling concrete if a weak mix were used in footings when placing concrete walls 2-3 feet thick and 8-12 feet high upon them.  Obviously the same would apply to the lower portions of those walls.  However, each upward wall-course has less load imposed upon it, so the necessary strength decreases as the tiers are poured, and the mix-ration might be decreased correspondingly.
  • Concrete weighs 144 pounds per cubic foot.  Thus a footing supporting a 10-foot column of 1 square foot must support 1,440 pounds.  At a level 3 feet below the top of the wall, the imposed load of the remaining wall pour is only 144x3 = 432 pounds.  But the considerations are hardly that simple, because even the very top tier of concrete must support the roof plus the wind & snow-loads upon the roof!
  • The imposed load must be considered in designing a concrete slab.  'Tis enough to say that obviously a slab subject to the loads of heavy vehicles must be stronger than one subject only to foot-traffic.  This will also impact the necessary thickness of the slab, as well as the mix-ratio!
A "home-brew" test can be done easily by using a number of large styrofoam coffee-cups, placing mixes at 5:1, 6:1, 7:1, etc.  (You might as well quit at 10:1 because after that you'll have nothing but junk mix anyway).  Use minimal water - only enough to allow thorough mising.  Let your samples set a few days, then see what it takes to break them, or apply load until they either break or crumble.  If you want to be sophisticated about it and can deal with the math, you can calculate the psi strength from that.  Regardless, you'll quickly gain a sense of the difference between good serviceable concrete and "junk".  Applying common sense to your new knowledge may be sufficient to accomplish a good home poured of your soil-cement.
The bottom line::
  • Most building today falls under building codes, mostly the Uniform Building Code, and requires a building permit and periodic inspection.  Talk first to your building inspector, because you want to know first-hand just what requirements they expect you to meet.  It is always well to establish some rapport with the inspector(s).  More importantly, though, most building inspectors come from a long construction background, and may be be willing to share a wealth of advice and construction tips from that repertoire.  Certainly your rather unusual approach to building should be a welcome change from their day-to-day routine of dealing with standard blocks, bricks and sticks.
  • Portland cement, reinforcing, delivery, and hired machinery and labor is not cheap.  Certainly your own time is also valuable.  A good consulting engineer will probably save you more than his/her fee by calculating the correct reinforcing and mix-ratio, and usually also knows the ropes about suppliers, test facilities, subcontractors and all such.  Doing it all yourself may be "possible" but not necessarily all that smart!  
copyright 1998 - 07 July 2006 by Ron Klotz-Zellhoefer, SolarSense Designs, Arizona & New Mexico

 Permission is granted and welcomed for personal application only.