ASPHALTopics | Fall 2020 | VOL 33 | NO 3

Figure 1. Fluorescence microscopy images of different asphalts containing the same amount of SBS showing increasing
compatibility from left to right. Images taken on the micrometer scale.
The concentration of SBS used also influences the
performance of the pavement. Generally, increasing
the amount of SBS used will lead to better performance
if you have good compatibility. At low concentrations
(0 to 2 per cent), there is not enough polymer in the asphalt
cement to induce a significant elastomeric response. At
moderate concentrations (2 to 4 per cent), which could be
considered typical for Ontario pavements, the stiffness
increases and the asphalt cement becomes noticeably
elastomeric. In the 0 to 4 per cent SBS range, the SBS
is typically dispersed through the asphalt, but as the
concentration goes above 4 per cent, a phase
inversion begins.
The 4 to 6 per cent SBS range can suffer from issues
with instability as a result of polymer and asphalt rich
regions. However, when the concentration increases to
6 to 8 per cent, the phase inversion completes, and the
asphalt cement is stable again with very good performance
(referred to as Highly Modified Asphalt or HiMA). The addition
of 6 to 8 per cent SBS can be quite expensive though and could
be reserved for specific applications. The 2 to 4 per cent range
ensures an improvement in performance
and can be relatively cost effective. It
should also be noted that the specific
values of the ranges are highly
dependent on the compatibility
and use of a cross-linking agent.
low strain and assume the behaviour is linear (response
is proportional to the input). The addition of SBS means
that the behaviour is no longer linear and these tests can
mischaracterize the change in performance.
AASHTO M 332 uses the Multiple Stress Creep Recovery
(MSCR) test which applies a much larger strain and has proven
to be a better way to evaluate the rutting characteristics
and the elastomeric nature of PMAC. This strain is more
indicative of strains observed in the pavement. The percent
recovery portion of the test also gives a good estimate of
the SBS concentration. As the SBS concentration increases,
the strain tolerance increases, and the cracking resistance
generally improves. In this way, an increase in the percent
recovery can be indicative of better cracking resistance
even though the test is designed to evaluate rutting
resistance. The elastomeric behaviour will be present at
all temperatures. You can see in Figure 2 below that there is
a linear relationship between SBS and the percent recovery.
All three asphalt cements here show a similar relationship.
The Double Edge Notched Tension (DENT) test and Extended
Bending Beam Rheometer (ExBBR) test both operate in the
TIPS FOR EVALUATING
PERFORMANCE
Evaluating SBS-modified asphalt
cement can be tricky. The PGAC
system was originally designed around
evaluating unmodified asphalt and as
a result does not properly characterize
the increase in performance observed
in the field. The traditional PGAC tests
such as the Dynamic Shear Rheometer
(high and intermediate temperature
grade) and the Bending Beam Rheometer
(low temperature grade) operate with
Figure 2. Relationship between SBS content and MSCR recovery.
36 OAPC | ASPHALTOPICS