Page 418 - Mechanics of Asphalt Microstructure and Micromechanics

T
410 Ch a p t e r w e l v e

specimens subjected to tension, tension-compression, and compression. He found that
the measured values of dynamic modulus in tension and tension-compression were
around 50% of that in compression at 37.8°C (100°F), and the difference decreases as the
testing temperature decreases. Khanal and Mamlouk (1995) repeated all dynamic modu-
lus tests that Kallas had done and static compression and tension tests, and they reached
conclusions similar to those of Kallas. With different modulus in tension and compres-
sion, they found a reduction in the tensile strain at the bottom of the AC layer and the
vertical compressive strain on the top of the subgrade. Christensen and Bonaquist (2004)
reported that the compliance values in tension and compression are not the same, even at
low temperatures of –20°C, –10°C, and 0°C.
All the above research efforts show that the difference between modulus in tension
and modulus in compression is lower at lower-testing temperatures. Daniel and La-
chance (2005) conducted dynamic modulus tests in tension and compression on asphalt
mixtures with 15%, 25%, and 40% reclaimed asphalt pavement (RAP). They found at
20°C there was no difference between modulus in tension and compression for the con-
trol mixtures (with no RAP) and mixture with 40% RAP; while the modulus in compres-
sion was larger than that in tension for a mixture with 15% and 25% RAP. The data also
indicates that when the loading frequency increases, both moduli in tension and com-
pression increase, as does the difference between moduli in tension and compression.
This means at the same loading frequency, the difference of moduli in tension and com-
pression increases as temperature decreases, which is a different observation from other
research. Note that this was observed on asphalt mixtures with RAP. Table 12.3 presents
a summary of the research reviewed.
In addition, many research efforts (Christensen and Bonaquist, 2004; Von Quitas et
al., 1982; Bonaquist et al., 1986) were devoted to the comparison of dynamic moduli
from compression/tension to that of the indirect tensile test (IDT) (AASHTO T322). It
is well known that the modulus calculated according to the IDT is based on the as-
sumption of the same modulus in tension and compression. Once the bi-modularity of
asphalt mixture is considered, the IDT modulus thus calculated is in fact neither a

Moduli Ratio
Authors Type of Test (Compression to Tension)
Secor and Creep test on a bend beam N/A (larger difference at higher
Monisomith temperature)
Kallas Compression, tension, and alternate 1.3 at 25°C;
dynamic modulus test 2 at 37 °C
Khanal and Quasi-static compression and tension 0.86~1.37 at 5°C;
Mamlouk by a ramping load; compression, 1.10~1.45 at 25°C;
tension and alternate dynamic 1.50~2.54 at 40°C
modulus test
Christensen and Uniaxial compression and tension Around 2 at temperatures from
Bonaquist* creep test; the IDT test –20°C to 0°C
(larger difference at higher
temperature)
Daniel and Compression and tension dynamic N/A (larger difference at higher
Lachance modulus test frequency)
* In this research, the compliance was actually measured, instead of modulus.
TABLE 12.3 Research on the ratio of compressive modulus to tensile modulus of asphalt mixture.