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RE: Granular Layer over Vapor Retarder

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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.  
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