Page 51 - Mechanics of Asphalt Microstructure and Micromechanics

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44 Ch a p t e r w o

repeated for a series of values of R, s m and f. The same specimens as for creep recovery
tests were used and the same conditioning was applied.

2.1.9.12 Pulse Train Tests
To simulate pavement loading conditions, intermittent identical tensile stress pulses
tests with a trapezoidal shape function of time are performed on dumbbell-shaped
specimens. The purpose of this test is to investigate the relationship between the single
load/unload behavior analyzed through the creep and creep recovery tests. During the
test, a constant maximum stress s p in each trapezoidal stress pulse is applied for a time
·
period Δ p with a loading and unloading rate s = 4s p /Δ p . Also, tests at –5, 0, 10, and 20°C
are performed with varoius time periods Δ p between consecutive trapezoidal pulses at
a predetermined s p .

2.1.9.13 Bulge Test
The bulge test was first introduced for the study of thin film mechanical properties
(Vinci and Vlassak, 1996). In the first version of the test, a thin film was secured over a
circular orifice and a uniform pressure applied to one side of the film to measure the
deflection of the film as a function of the applied pressure. The pressure-deflection
data were then converted into a stress-strain curve for the film. In this test, the results
are very sensitive to small variations in the dimensions of the sample as the occurrence
of small compressive stresses causes the film to buckle or wrinkle due to its small
bending stiffness.

2.1.9.14 Spherical Indentation Test
The spherical indentation response of bitumen was investigated by Ossa et al. (2005b)
using small cylindrical specimens (60 50 mm) tested in a hydraulic machine. Spheri-
cal indenters with diameters of 15 or 40 mm were used and indentation was performed
to a depth less than 2 mm in a temperature-controlled chamber at 0°C. Cyclic and creep
recovery indentations were conducted to model the asphalt response based on effective
eff
stress (s ) and effective strain (e ) concepts. Measured indentation load (F) versus
eff
indentation depth (h) and the indentation depth (h) versus time response were used for
comparison with model predictions.

2.2 Aggregates Properties

2.2.1 Mechanical Properties
As indicated at the beginning of the chapter, aggregates are about 80% by volume of AC.
The properties of aggregates certainly have an important influence on AC. In addition,
the mineral composition and surface characteristics of aggregates also affect the bond-
ing between aggregates and binder. In this section, typical mechanical properties of ag-
gregates including Young’s modulus, Poisson’s ratio, and strength are presented. Real-
istic mechanical models of aggregates are also discussed. Methods to quantify surface
characteristics such as aggregate shape, angularity, and texture are briefly discussed.
Abrasion properties using different methods are also discussed. The abrasion properties
affect the surface characteristics and the friction behavior of pavements.
Considering that typical aggregates (of high quality) have a strength of about 3.0
MPa and above, and the stress that pavement (highway pavement) can experience is
about 0.5 MPa, it can be concluded that most of the aggregates should be in the linear