SUPERPAVE Digest 116
Topics covered in this issue include:
1) SMA asphalt cement content
by Eric Johnson <>
2) Re: SMA asphalt cement content - Reply
by Allen Cooley <>
3) Re: SMA asphalt cement content - Reply
by "André de Fortier Smit" <>
4) Re: SMA asphalt cement content - Reply
by rajib <>
5) SMA asphalt cement content -Reply
by George Thomas <>
6) Re: SMA asphalt cement content -Reply
by rajib <>
· To: Superpave list <>
· Subject: SMA asphalt cement content
· From: Eric Johnson <>
· Date: Tue, 06 Oct 1998 09:32:34 -0800
· CC: Bob Lewis <
We have recently designed an SMA with an indicated optimum of 5 1/4
percent asphalt cement. The gradation is up against the maximum limits
usually specified for each sieve. Our concern is that this asphalt
content appears to be too low and the mix is not a true SMA. Our
question is what is the lowest optimum asphalt anyone has specified for
an SMA and was the performance satisfactory?
Eric Johnson
Engineer of Tests
Alaska DOT/PF
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· To:
· Subject: Re: SMA asphalt cement content - Reply
· From: Allen Cooley <>
· Date: Tue, 06 Oct 1998 13:28:46 -0500
Eric,
To answer your question about minimum asphalt contents, the
recently completed NCHRP 9-8 study recommends a minimum asphalt content
of 6.0 percent if your combined aggregate bulk specific gravity is
2.75. It also says that you can reduce your minimum asphalt content by
0.1 percent for each increment of 0.05 for the combined aggregate bulk
specific gravity above 2.75 (e.g., if combined bulk specific gravity of
your aggregate is 2.85 then the minimum asphalt content is 5.8
percent). Likewise, you must increase your minimum asphalt content by
0.1 percent for each increment of 0.05 decrease in combined aggregate
bulk specific gravity (e.g. Gsb=2.60 then minimum asphalt content of 6.3
percent). These requirements were based on the volumetrics of SMA
mixtures.
I have a few questions for you though, you indicated that your
gradation as "up against the maximum limits." Are you using the SMA TWG
gradation requirements or some other requirement? By "up against the
maximum limits", do you mean the fine side of the gradation band? What
VMA does your mixture have? Is it above the recommended 17.0 percent?
What is the percent passing the No. 200 sieve (sorry, 0.075 mm sieve)?
Have you checked to see if you have stone-on-stone contact? All of
these factors plus maybe some others can affect your SMA design.
Hope some of this helps.
Allen Cooley
Research Engineer
National Center for Asphalt Technology (NCAT)
211 Ramsay Hall
Auburn University, AL 36849
Phone: (334) 844-6228
Fax: (334) 844-4485
e-mail:
· To:
· Subject: Re: SMA asphalt cement content - Reply
· From: "André de Fortier Smit" <>
· Date: Tue, 06 Oct 1998 15:23:56 -0500
I can't help but respond to the above question and reply.
What I see are recipes for SMA. Add a bit of this, not too much of that
etc. Why not use the following approach - may take a bit longer but who
knows- you may learn something and have greater confidence in SMA's in
the end.
1. Find out what an SMA is and what it is used for.
2. Study research papers (there are tons of them) on the WWW and in AAPT
and ICAP proceedings.
3. Experiment in the lab and in the field.
The binder content of an SMA (the mastic part thereof) is critical. Too
low and the durabilty is low, too high and the mix is prone to shear
failure. And then there is the skeleton structure of the larger
aggregate. Yes - a tricky mix, but one that is worth the effort. (Just
ask the Germans - they've been using it for decades)
· To:
· Subject: Re: SMA asphalt cement content - Reply
· From: rajib <>
· Date: Tue, 06 Oct 1998 16:42:23 -0400
Andre:
What Allen Cooley said about designing SMA is not from a recipe. Allen
and his colleagues at the National Center for Asphalt Technology (NCAT)
have been doing "tons" of work on SMA. Their recommendations are based
on volumetrics.
One of the most important concern regarding high binder content is
draindown of binder. With the right stone-on-stone contact skeleton you
should not get shear failure.
Even though the Germans have been doing SMA for decades, as far as I
know, their design is recipe design, whereas the design recommended by
NCAT, FHWA and NAPA in the United States is actually based on "tons" of
lab work and field experience.
Sincerely,
Rajib B. Mallick
Worcester Polytechnic Institute
André de Fortier Smit wrote:
· To:
· Subject: SMA asphalt cement content -Reply
· From: George Thomas <>
· Date: Tue, 06 Oct 1998 15:52:41 -0500
· Cc:
6 October 1998
Two comments on asphalt content of SMA:
1. Yes, a 5.25 % asphalt is a bit lower than is normally thought of as
typical for an SMA This would imply a VMA of 16, at an Air Void content
of 4 % or a VMA of 15, at an Air Void content of 3 % (This latter Air Void
content of 3 % is not recommenbed. According to Jim Anagnos, and
Jerry Huber the asphalt content should be around 6 to 6.5 %. According
to the FHWA TWG for SMA Model Spec the minimum VMA = 17%. This is
for an SMA whose nominal max aggregate size is 1/2". For a nominal
max aggregate size of 5/8", the min VMA should be about 16.5%.
2. The more fundamental question is not the asphalt content, but what is
the VMA? If you assume a lab mix design (aggregate dried in oven and
no question of moisture), the void space can contain only air and asphalt.
Air voids and VMA are both volumetric percent. Asphalt is customarily in
weight percent. To get everything into the same units, we convert
volumetric percent asphalt to weight percent using a conversion factor
of 2.28, based on an asphalt sp gr of 1.020. Slight variations of asphalt
sp gr have very little effect on this conversion factor.
The relation then becomes VMA = [(Wt % Asphalt) * 2.28] + Air Void %.
For Example: If you have a VMA of 15 %, and you are designing for a
4% Air Void, then 15.0 - 4 = 11% This 11% is the volumetric asphalt
percent. 11.0/2.28 = 4.8 Wt Percent. This is 'way too low. Say you
adjust your combined gradation (for a lower percent passing the #4
sieve) to a VMA of 18.0. Then you have 18.0% - 4% Air Voids = 14.0%
vol % asphalt. 14.0/2.28 = 6.1 Wt %. This is in the right range for SMA.
3. To get the high asphalt content (6 to 6.5%) you will normally have to
design for a VMA that is 1 to 1.5% above the usual min VMA.
Note that this little equation also works on dense graded mixes and
superpave mixes.
George Thomas
(817) 370-6841
· To:
· Subject: Re: SMA asphalt cement content -Reply
· From: rajib <>
· Date: Tue, 06 Oct 1998 17:07:21 -0400
George's comment on VMA is very true. VMA is absolutely the most
important thing. You can find whether you have stone-on-stone contact or
not by plotting VMA versus percent passing 4.75 mm. At the point where
stone-on-stone contact begins you should see an increase in VMA. The mix
should have a VMA corresponding to that condition. The VMA at this
condition is obviously more than in the case of dense graded mixes,
which have higher percentage passing the 4.75 mm sieve.
Sincerely,
Rajib B. Mallick
Worcester Polytechnic Institute