ASPHALTopcs | Summer 2017 | VOL 30 | NO 2

mitigating this problem. The asphalt
binder is specifically selected for the
mix design at intersections (PGAC
requirements are normally one-grade
higher for heavy traffic conditions or
for pavement where there are heavy
slow-moving vehicles, and even
two-grades higher for extremely
heavy truck or bus traffic). The PGAC
is commonly enhanced or polymermodified
to achieve these grades.
Aggregate properties need to
be considered just as carefully.
The aggregate structure must be
capable of carrying the load and
developing a high degree of stoneto-stone
interlock to resist shearing.
The need for consistent gradations
is well recognized, but there also
must be consistency in particle
shape, texture and absorption.
Both coarse and fine aggregate
must be angular to provide the
interlock required for rut resistance.
Plastic deformation is similar to
slope failure. Rounded aggregates,
with a low angle of repose, tend to
act like ball bearings and therefore
have much less shear resistance.
Crushed materials, which stay up
in a stockpile, have the necessary
internal friction to resist shear.
Fortunately, Ontario has excellent
aggregate sources that can meet
these quality requirements.
Recent developments in crushing
equipment and techniques have
also helped to improve aggregate
angularity and cubicity. In many
cases, washing, crushing and other
secondary processing can improve
marginal aggregates.
Air voids are particularly important
and should neither be too low
nor too high. Air voids are the air
spaces or pockets of air that occur
in the compacted mix; mixes are
typically designed at four per cent
and produced between three and
five per cent. Mixes with excessively
low air voids can lead to flushing and
potentially result in rutting.
On the other hand, mixes with
high air voids are more prone
to durability problems. If the air
void content is too high, the voids
become interconnected which
allows the passage of air and water
resulting in poor durability. Keep
in mind that density and air void
content are directly related – the
higher the density (or compaction)
the lower percentage of air voids.
Pavements are typically compacted
to eight per cent air voids in-place
(or 92 per cent maximum relative
density). A proper design will
provide sufficient air voids for
stability and sufficient asphalt
cement and volumetric properties
for good long-term performance.
Improper construction techniques
can also lead to rutting. Consolidation-type
rutting occurs when the
pavement has not been sufficiently
compacted. Under traffic loads, the
aggregate particles will reorient
themselves into a denser condition.
Therefore, it may require more effort
to get the necessary compaction
with some of the stiffer mixes (with
enhanced PGAC and aggregate
properties) used for intersection
improvements.
Aggregates are, of course, only
one component of a hot mix (even
if they do make up 95 per cent of
the pavement). It is not just the
characteristics of each component
of the mix, but also the interplay
of those components that provides
high-quality pavement.
Improved asphalt intersections
can be built with minimal delay
for motorists, and while the initial
cost may be higher than the more
conventional asphalt mixes, the
dramatically improved performance
will result in a lower life-cycle cost.
The final product will be a long
lasting, cost-effective, smooth
asphalt intersection and the
answer to one of our biggest
municipal pavement design
challenges.
1. Road Safety in Canada, Government of Canada -
www.tc.gc.ca/roadsafety2011
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