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RE: Granular Layer over Vapor Retarder

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On 21 Feb 2005, at 17:22, Sherman, William wrote:

> Gail, I agree with you in that the ACI 302 recommendations do not
> provide adequate information on gradations and enforceable
> specifications for fill to be placed above vapor retarders. I have been
> considering the use of "aggregate base course" material of the type
> typically used by DOT's for base course below pavements. This material
> is generally readily compactable and is well graded. 
> Based on a review of various DOT specifications for "aggregate base
> course", I have developed a proposed gradation specification below;
> however, the ranges for each sieve size have been adjusted to
> accommodate a range of gradations from various specifications, so this
> does not match any "single" DOT gradation. The intent is to have a well
> graded granular material that easily compacts. The maximum size is based
> on my own preference. Following is the proposed wording:  
> "A 4-in layer of readily compactable, well-graded, crushed rock or
> crushed gravel shall be placed over the vapor barrier before setting
> reinforcing steel for the concrete placement. The crushed rock or
> crushed gravel, combined with fine aggregates as needed, shall conform
> to the following gradations: 
> Sieve Designation     Percent Passing (by weight) 
> 1-inch                       100 
> No. 4                        30-70 
> No. 8 (or No. 10)        20-70 
> No. 40                      10-50
> No. 200                     3-15 
> The liquid limit of the material passing the No. 40 sieve shall not
> exceed 30 and the plasticity index shall not exceed 6." 
> I would like to get some feedback on this proposed gradation. 
> William C. Sherman, PE
> (Bill Sherman)
> CDM, Denver, CO
> Phone: 303-298-1311
> Fax: 303-293-8236
> email: shermanwc(--nospam--at)
> ________________________________
>  From: GSKWY(--nospam--at) [mailto:GSKWY(--nospam--at)] 
>  Sent: Monday, February 21, 2005 7:48 AM
>  To: seaint(--nospam--at)
>  Subject: Granular Layer over Vapor Retarder
>  A question came up last week about what to specify as a granular
> layer on top of a vapor retarder.  One thing I would very definitely not
> recommend specifying is a capillary break.  
>  Although you might find it recommended in an older publication
> or see it on older plans,  it is not something that is recommended in
> current literature.  A capillary break material is typically all the
> same size material,  somewhere between 3/8 and 3/4 in. It does not pack
> well and has a high coefficient of friction .  If you fill in the top
> with a layer of fine-grained material to reduce the friction, you no
> longer have a capillary break, so you can't call it out as one. 
>  However, the recommendations included in ACI 302 (and
> regurgitated in various other places such as the NRMCA document on vapor
> barriers and ASTM E 1745) are in my opinion worthless.  If not worse
> than worthless, because of the time wasted in reading them.
>  What I have seen used successfully, and I will note that this
> recommendation is not endorsed by the NSPE,  is what is often referred
> to as crusher-fines with the following gradation:
>  Sieve Size %    Passing 
>  3/8"                  100% 
>   #4                    90 - 100% 
>   #8                    55 - 80% 
>   #16                  40 - 70% 
>   #30                  25 - 50% 
>   #200                  6 - 15% 
>  This is also the gradation used by the Forest Service for hiking
> trails, as discussed in this article.   
> .html
>  The article cites a trail by NCAR - I worked on that trail, as
> well as one up by Winter Park.  Even with a little Wacker-Packer
> (jumping jack),  the material compacts quite well.  
>  ACI 302 does in fact mention the use of crusher fines,  but
> doesn't include any information about gradation, which is of little use.
> "Crusher fines" can mean anything, different suppliers can have quite
> different gradations.
>  The ACI 302 recommendation for a material over the vapor
> retarder is as follows:
>  "When a fill course is used over the vapor barrier/retarder, it
> should be a minimum of 4 in. of trimmable, compactible, granular fill
> (not sand), a so-called crusher-run material. Usually graded from 1-1/2
> to 2 in. down to rock dust is suitable. Following compaction, the
> surface can be choked off with a fine-grade material  to reduce friction
> between the base material and the slab.
>  If it is not practical to install a crusher-run material, the
> vapor barrier/retarder should be covered with at least 3 in. of
> fine-graded material such as crusher fines or manufactured sand."
>  To recommend a "trimmable" material be used is worthless;
> "trimmable" is a meaningless word.  Similarly,  it is of questionable
> value to recommend that the material be  "compactible".  All material is
> compactible unless it has a very high organic content, for example top
> soil.  Top soil is compressible,  i.e. when you put a weight on it, it
> will compress,  however when you remove the weight, it re-expands;  it
> is thus not compactible.
>  Someone with even half a brain might also question why two very
> different materials are being recommended.  I.e. something with a top
> size of 1-1/2 to 2 in.  versus a fine-graded [sic] material such as
> crusher fines or manufactured sand (top size of 1/4 in.)  
>  Someone with even half a brain might also question why in one
> paragraph it specifically states that sand should not be used, but in
> the next paragraph it recommends manufactured sand.   Generally
> manufactured sand is considered sand.  
>  Gail Kelley

My concern with both of these gradations is the amount of fines
in the mix.  I would want a more freely draining gradation.  I like
the sand option but don't really like a poorly graded mix as it does
not compact as well as a well graded mix.  The starting particle
sizes are good, I would just limit the smaller size so as to have
at least a non-plastic mix.  Non-plastic silts comprising no more
than 3% of the mix will give the desired compaction characteristics
without retaining moisture as much as a plastic mix will retain.

I realize that the problem with trying to exclude these fines or 
limit their inclusion  to a small percentage, is the labor that must 
go into creating the mix.  If the mix is a washed aggregate mix 
then it may give the desired reduction in fines, but if you wanted 
to take the mix straight from the crusher, then it's going to be more 
difficult to limit the fines in the mix. And with the higher labor 
costs to get a cleaner mix, the end user will have to pay more 
for the product.

Take care,
Lloyd Pack, P.E.

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