"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:
Percent Passing (by weight)
No. 4 30-70
No. 8 (or No. 10) 20-70
No. 40 10-50
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.
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
90 - 100%
55 - 80%
40 - 70%
25 - 50%
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
"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.