Better Roads May 2014 21
friction. Once they are moved into contact with each other,
angular aggregates produce high pavement strength. More
force and heavier compactors are needed to overcome the
internal friction between the fractured faces of crushed
stone aggregate. Most high-traffi c mix designs specify the
use of crushed aggregates with a certain number and shape
of fractured faces.
The role of thickness
No matter what type of mix or what type of project, an
important factor is the ratio between the size of the largest
aggregate in the mix and the thickness of the layer. This ratio
strongly affects the ability of the mat to accept compaction
energy and reach the specifi ed density.
For example, a layer thickness of 10 cm (4 inch) and a
maximum aggregate size of 25 mm (1 inch) are relatively
easy to compact. You can use high compaction forces and
not worry about damaging aggregates. With a 4:1 ratio,
there is ample space for aggregate movement allowing the
aggregates to re-orient. Many public works departments
specify a 3:1 ratio as a minimum for high-traffi c mix design.
When the ratio of layer thickness to aggregate size is less
than 3:1, the compaction process is much more diffi cult. In
particular, when there are angular aggregates in the layer, it
is likely that those aggregates will not move into the proper
compacted orientation without being damaged.
The compactor operator is more likely to notice drum
bouncing or the appearance of uncoated rock at the surface of
the mat. Lack of density and damaged aggregates will lead to
premature mat failure. When the ratio of layer thickness to ag-
gregate size is less than 3:1, low compaction energy is needed.
In summary, crews will always need to monitor tempera-
tures.
But, don't forget that what happens before the mix even
reaches the jobsite has an impact, too.
Article and photos courtesy of Cat Paving.
(Editor's note: This excerpt fi rst appeared in "The Cat Paving Products Guide
to Asphalt Compaction." The book is available on Amazon.com and through
participating Cat dealers.)
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