Technical information

2-12
2.3.2 HMA Complex Modulus
Complex modulus testing of the NCAT asphalt mixtures was conducted under the
direction of Dr. Terhi Pellinen at Purdue University. A detailed description of the work
is presented in a report by Barde and Cardone (2004) which can be found in Appendix
C. The materials were sampled in loose state from the delivery trucks during
construction and then shipped to Purdue University. Specimens were prepared and
tested in accordance with the NCHRP 1-37A protocol (NCHRP, 2002).
Initially, mixtures were heated to compaction temperature in the oven; a
temperature of 155 °C was selected for mixes 1 and 3, and a temperature of 145 °C was
selected for mixes 2 and 4. Next, specimens were prepared using a gyratory compactor.
For each mixture type, four specimens were prepared for testing (i.e., a total of 16) by
targeting an air void content of 7%. The first two specimens, denoted by ‘a’ and ‘b’ in
the report, were compacted with assumed correction factors (i.e., ratio between sample
bulk specific gravity that is measured using saturated surface-day method and bulk
specific gravity that is calculated based on sample height in the gyratory mold). The
next two specimens, denoted by ‘c’ and ‘d’ in the report, were prepared with calibrated
correction factors after studying the resulting densities of the first two.
The gyratory compacted pills were further cored to produce cylindrical
specimens 4 in. (102 mm) in diameter and 6 in. (152 mm) in height. The final air void
content for the 16 samples varied in the range of 6.8 to 8.2%. Complex modulus tests
were done on each of the 16 samples in stress-controlled uniaxial compression mode
with a Haversine load pattern. Six different frequencies were employed: 25, 10, 5, 1, 0.5
and 0.1 Hz; applied in descending order. The frequency sweep was executed five times
for five different temperature levels: -10, 4.4, 21.1, 37.8 and 54.4 °C; applied in
ascending order.
A dummy specimen was used to verify that the target temperature level was
reached. The testing was carried out using a UTM 25 with a 25 kN load cell. Stress
levels were adjusted in order to maintain the values of both permanent and recoverable
strain within specified protocol limits. In an effort to minimize edge effects, latex sheets
covered with silicon grease were inserted between loading system and specimen at both