Page 60 - Mechanics of Asphalt Microstructure and Micromechanics

Mechanical Proper ties of Constituents 53

2.4.9 Indirect Tensile Test (IDT)
The IDT procedure is provided by the AASHTO T 322 standard and measures the creep
compliance and strength of asphalt mixtures using indirect tensile loading techniques
at intermediate to low temperatures ( 20°C). It analyzes mixtures for low-temperature
cracking. In this test, a compressive load is applied across the diametrical axis of a cy-
lindrical specimen. Due to the mechanics of the test, a nearly uniform state of tensile
stress is placed across the diametrical plane. The testing machine consists of a closed-
loop servo-hydraulic, or mechanical screw system capable of applying static loads as
low as 5 N. A complete analysis of the test results requires that creep compliance and
tensile strength be measured at temperatures varying from –10°C to 20°C.

2.4.10 Four-Point Bend Beam Test for Anisotropic Properties
This test is similar to the standard three-point bend test except the locations of two con-
centrated loads are not at one-third of the span, but 95 mm away from the support (Li
and Wang, 2008). The location of the concentrated load allows enough space to attach
the extensometers between loading rollers. During a test, load (measured with a 2-kilo-
newton cell), compressive deformation (strain) on the top of the beam, and tensile de-
formation (strain) at the bottom of the beam are recorded every 0.1 s. These parameters
are measured with strain-gauge extensometers with a gauge length of 25 mm attached
at the mid-span of the beam. The load is applied to the beam at a rate of 5 N/s so that
the target peak load can be reached in a reasonable amount of time without introducing
significant creep deformation.

2.4.11 True Triaxial (Cubical) Test
This flexible boundary type of device was originally developed by for multiaxial test-
ing of rock materials. The stress-controlled cubical testing device consists of six main
components: (1) a steel frame, (2) six wall assemblies, (3) a deformation measuring
system, (4) a stress application and control system, (5) six rigid membranes, and (6) a
data acquisition and process control system (DA/PCS) (Wang et al., 2005). The frame
functions are: (1) to apply external multiaxial load to the cubical asphalt concrete spec-
imen, and (2) to serve as the reaction structure for the application of the total normal
stresses to the top, bottom, and lateral faces of the cubical asphalt concrete specimen.
Pressure limits are 132.5 MPa for uniaxial pressures and 64.52 MPa for a hydrostatic
loading condition. The deformation of the cubical HMA specimens (100 mm long) is
measured at three points on the top, bottom, and each of the four lateral faces using
three LVDTs per face. Typically, the cubical specimens are loaded in isotropic com-
pression (IC) in each direction followed by triaxial compression (TC), triaxial exten-
sion (TE), simple shear (SS), conventional triaxial compression (CTC), conventional
trixaxial extension (CTE), and cyclic CTE. Elastic moduli and Poisson’s ratios are cal-
culated from experimental data.

2.4.12 Gyratory Testing Machine (GTM)
The GTM was developed by the Corps of Engineers as an effective tool in the evalua-
tion of asphalt mixture quality (Roberts et al., 1996). It can also be used for mix design
and shear strength measurement during compaction. This machine can compact as-
phalt mixtures using a kneading process that simulates the action of rollers during
pavement compaction. Parameters such as the vertical pressure, gyration angle, and