Critical
Evaluation of Using the SUPERPAVE Volumetric Mixture Design Procedure
for Modified Binders
Research by
Dong-Woo Cho/
Prof. Hussain Bahia
Dept of Civil & Environmental Engineering
Asphalt
concrete is the composite material composed of aggregate and asphalt
binder to pave driving ways. It also can be categorized as flexible
pavement due to asphalt binder. Most roads have been paved with
asphalt concrete because asphalt concrete provides smooth driving,
easy construction and maintenance, and 100% recycling.
There
are three different distresses to give detrimental effect to asphalt
concrete:
1) Rutting is its permanent deformation due to heavy traffic.
2) Fatigue cracking is based on accumulated repeated traffic.
3) Thermal cracking includes low-temperature cracking and thermal
fatigue cracking.
Crystallization by low temperature causes fracture failure and
it is called thermal cracking. Thermal fatigue cracking based
on asphalt aging or hardening is very similar with fatigue cracking.
These distresses are the indicators to evaluate the performance
of asphalt binder or asphalt concrete and are all based on temperature
change. Because conventional asphalt binder is very susceptible
to temperature change, the research for modified binder has been
studied and the approach to evaluate the modified binder performance
has been developed.
This
study was conducted to examine the possible interference of modified
binders with the standard procedure for volumetric mixture design
used in the Superpave system. Sensitivity of volumetric properties
and moisture damage performance was evaluated using four binders
modified with two different technologies (Polymer and no-additive
modifications).
The
study included three tasks to evaluate effects of compaction temperatures
(in the range of 72C to 148C), effects of vertical pressure (in
the range of 200 kPa to 600kPa), and moisture damage according
to the AASHTO T283 procedure (is a testing procedure for moisture
damage test).
The
Zero Shear Viscosity concepts were used to estimate mixing and
compaction temperatures, compaction energy index was used to study
changes in compaction effort, and cohesion and adhesion of binders
were measured. The results indicate that temperature effects,
within a reasonable range, are somewhat marginal and that unless
temperatures are reduced to below 80 C, the effects on volumetric
properties are rather small. It was also found that using a target
viscosity of 1.5 Pa•s for mixing and 3.0 Pa•s for compaction
can provide very reasonable compaction temperatures, particularly
if Zero Shear Viscosity is used. These criteria for temperatures
worked well for the grades and type of modification used in the
study.
The
vertical stress used in the gyratory compactor showed very significant
effects on volumetric properties. Changing pressure from 600 kPa
to 300 kPa resulted in 3.0 to 4.0 % increase in air voids, which
is more significant than changing viscosity by more than 10 times.
It is therefore postulated that the focus on keeping temperature
high during compaction in the lab and in the field could be un-founded.
With increasing pressure during compaction in the lab or weight
of roller in the field, significant changes in density could be
achieved. Moisture damage does not appear to be affected significantly
by the variation in binder viscosity profiles for the binders
and aggregates used in this study. Because of the limited sample
size, more work is needed in this area to study effect of mixing
and compaction
temperatures on moisture damage.
Key:
Pa
= Pascal = N/m^2 = Newton per square meter (a unit of pressure
or stress)
Pa
* s = Pascal times second (a unit of viscosity)
kPa
= kilo Pascal = a thousand times of Pascal (a unit of pressure
or stress)
