DIGEST 2006 - Sabita

ackup wearing course in the
event of dolerite aggregate
becoming unavailable during the
contract. It also provided the ideal
opportunity to develop the mix
with quartzite aggregate as part of
the preparation of the GPTRW rut
resistance mixes.
Design methodology
(a) Fundamental design
methodology
The fundamental principles of
compositional and volumetric
asphalt design 1) 2) 3) , as developed
on various high performance
pavement structures since 2001
are shown in Table 1 and were
used to obtain specific
performance properties for these
mixes.
The compositional and volumetric
design methodology adopted
utilises the following processes to
arrive at optional mixes in
accordance with target
performance requirements:
• Best aggregate sources in
terms of desired aggregate
properties (e.g. shape,
i.e.cubical to slightly flaky;
hardness according to COLTO
criteria; consistency) are
used;
• Initial grading is optimised for
50mm thickness, starting
with a selected or specified
nominal aggregate size
(13.2mm) and filler to yield
filler/binder ratio in the
region of 1.3, and the
in-between points on the
grading log curve smoothed.
Also the fraction passing the
2.36mm screen (primary
control sieve) is set at a level
that ensures a “coarse
graded” mix more or less
coinciding with the coarse
boundary of the conventional
grading envelope;
• Aggregate proportioning is
carried out to meet the target
grading;
• Marshall volumetric design is
then carried out to check
VMA, VIM and binder/film
thickness relationships;
• The proportion passing the
2.36mm screen is then
adjusted to modify the VMA
until volumetric requirements
are met. It was noted that a
2.5% reduction in the
percentage passing the
2.36mm screen generally
resulted in a 1% increase in
VMA (at the same binder and
filler contents);
• VMA is then adjusted, until
binder film thickness, VIM’s,
and gyratory voids meet
volumetric/compositional
targets. The aim is always to
arrive at a cost-effective mix,
by adopting the minimum
binder content (or VMA for
that matter) that will satisfy
durability and fatigue criteria;
• Performance tests are then
carried out followed, if
necessary, by a process of
iteration until performance
criteria are met.
(b) Verification with other mix
design methodologies
(i) PRADO
The implementation of the Belgian
Road Research Centre
104