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Re: Granular Layer over Vapor Retarder

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On 22 Feb 2005, at 19:56, GSKWY(--nospam--at) wrote:

> Lloyd,
> Why are you concerned that the material above the vapor retarder be 
> free-draining?  This is intended as a "blotter layer" that absorbs excess mixing water 
> from the concrete.  If the mix is free-draining this water is all just going 
> to go through and sit on top of the vapor retarder.  It will provide better 
> curing for the bottom of the slab if some of it is held against the bottom of the 
> slab.

This makes good sense.  I was thinking of the performance of this layer
should there be a flooding condition or infiltration of water, either from
above or below, through the vapor barrier.  If the granular layer becomes
saturated, then a free-draining soil will dry out fairly quickly, where
a layer with more fines will hold the water longer (maybe years) and be
in a condition to pose problems.

> In addition, having more smaller particles makes it easier to grade and 
> reduces the subgrade friction.

Particles passing the 100 sieve will give many of these properties without
risking the possibility of plasticity in the mix.

> I will admit that I can't comment on the relative compactability of something 
> with 3% fines (minus #200), versus 6% fines, versus 10%,  it all kind of 
> looks a like, but 3% is the limit for concrete aggregate in concrete subject to 
> abrasion per ASTM C 33.  The limit for concrete not subject to abrasion is 5%, 
> but I think even that is probably unnecessary strict for a fill course layer. 
> For those who don't remember elementary soil mechanics,  sieves 3/8 in. and 
> above are identified by the opening size,  sieves with openings smaller than 
> 3/8 in.. are identified by the number of openings per in., each way.  I.e. a #4 
> sieve has 4 openings each way for a total of 16 openings per square in.;  a 
> number 200 sieve has 200 openings each way, for a total of 40000 openings per 
> in.  A #200 sieve looks like metal cloth.  
> Something that is truly a silt will, by definition, be non-plastic.  Silt is 
> just very, very miniature rock particles.  Similarly,  the rock dust from 
> crushing will be non-plastic.  Clays are the result of chemical changes that alter 
> the original rock minerals. Some clays are more plastic than others, 
> depending on the mineral composition of the rock they were formed from, and how they 
> were altered.  Most silty soils have some clay in them, so they have some 
> plasticity.

This meets the definition of ASTM D-653 fairly closely on the silts being non-
plastic.  Under ASTM D-2487, USCS system, you can have a silt of high
plasticity, if it plots below the A-line but has a high LL.  I suspect that there
is a small percentage of rather highly plastic clays in the silt that gives it
this property, and that the silt particles are not, themselves, plastic in nature.

True, rock dust should be non-plastic.  If the rock is not contaminated 
with clay, then the fines shouldn't be a problem and there should be no 
Atterberg attributes with which you'd have to contend.  And in this case,
there should be little problem with minus #200 materials in a small percentage.

Take Care,
Lloyd Pack, P.E.

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