DIGEST 2006 - Sabita

Or more appropriately, a lone thar
bleating on the mountaintop?
Sabita's August 2006 edition of
Asphalt News has a short article
on research done by NCAT (USA)
on Lift Thickness versus Nominal
Maximum Aggregate Size
(t/NMAS). (NMAS = 1 size larger
than the first sieve to retain more
than 10%). This extensive
research can be found on the
following website:
www.eng.auburn.edu/centre/ncat/
reports.
The report says it all in these
“deep and meaningfuls":
• Mixes with larger maximum
size aggregates have larger
voids that are more likely to
be inter-connected, resulting
in higher permeability;
• As density decreases,
permeability increases;
• As thickness decreases,
permeability increases;
• Therefore mats that are
thicker, with higher
density and smaller
maximum size aggregate
will be less permeable.
Figure 1 is a chart from this
research and shows the effect of
t/NMAS on permeability. It is a
trend curve for a variety of
fine-graded mixes, and NCAT's
recommendation is:
t/NMAS should be at least 3:1
(Note the “at least")! Our
traditional mix is 1.9:1, which
from Figure 1 indicates it may be
60% more permeable than if it
was 3:1.
Oh dear, is our sacred cow in
danger of becoming mincemeat?
Figure 1
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