ASPHALTopics | Spring 2018 | VOL 31 | NO 1

TECHNICALLY
SPEAKING
by Vince Aurilio & David Newcomb
The merits of balanced mix design
There have been a lot of changes or advancements in
asphalt technology over the last fifteen to twenty years.
We have also seen a number of significant changes in
understanding the materials used to produce hot mix
asphalt (HMA). However, there are still a number of
challenges to the continuous improvement of asphalt
mixture performance. The current methods used to
perform volumetric mixture designs are only indirectly
related to the quality of the material, and its time to
re-evaluate how we do mix designs.
The 1960s and 1970s were an era where fatigue
cracking was somewhat common. In response, changes
to structural designs were made that resulted in thicker
HMA layers, and mixture designs were adjusted to allow
for more asphalt in the mixtures. In the 1980s, rutting
(permanent deformation) was a major problem across
the U.S. and Canada. A number of key changes were
made to address this issue such as the introduction
of aggregate shape and texture, the adoption of PG
binder grading, and rut testing. Rutting is now not as
troublesome as it used to be. However, there are still
more performance issues to be addressed.
The focus has shifted back to solving the issues of
lack of durability and premature cracking in asphalt
pavements. There have also been a number of important
materials changes, including increased use of RAP/RAS,
the switch to Performance Graded Asphalt Binders, and
the necessity to modify asphalt cement using different
processes, some of which may have contributed to the
present issues. In addition, we have seen changes to the
mix type selection and/or the mix design process.
The introduction of Superpave and mixes like stone
mastic asphalt (SMA) and more recently warm mix asphalt
(WMA) have been innovative and have provided our
industry with more tools. However, as these technologies
evolved, many of our approaches to mix design and
quality control have remained in the pre-1980s in terms
of how we design and test mixtures. When our materials
CRACKING
RUTTING
40 OAPC | ASPHALTOPICS
MIX
DESIGN