Page 106 - Mechanics of Asphalt Microstructure and Micromechanics

CHAPTER4

Experimental Methods

to Characterize the

Heterogeneous Strain Field

4.1 Introduction
Due to the heterogeneity of asphalt concrete (AC), its strain field is not uniform, even if
it is subjected to a uniform stress field. Permanent deformations in AC pavements are
not only a function of the magnitude and number of repetitions of external loading, but
also are influenced by the characteristics and spatial arrangement of the particles in the
mass. How individual particle motions (micro) relate to the global response (macro) is
important in micro-mechanical studies that relate “micro” measures to “macro” re-
sponse. Macro-response refers to the measurement that assumes that the AC is a homo-
geneous material whose response can be quantified by global measurements. However,
since AC is composed of aggregates and asphalt binder, and the aggregates virtually do
not deform under typical tire loading, interpretation using conventional strain mea-
surements is inappropriate. Since the actual deformation takes place almost solely in
the mastic (the mixture of asphalt binder and small particles), permanent deformation
is localized in the mastic. In addition, fatigue cracking often initiates from the interfaces
between the mastic and the larger aggregate particles. Accordingly, measurement of the
permanent strains in the mastic may be a better measure for exploring the mechanisms
of permanent deformation and fatigue cracking. The mastic strain measure is likely to
be more relevant in that the mastic is more homogeneous than AC. In this chapter,
methods which use imaging techniques and pattern recognition to measure the perma-
nent strains in the mixture and the mastic are presented. Application of the procedure
to the measurements of the strains developed in a specimen used in an asphalt pave-
ment analyzer (Georgia Loaded Wheel Tester, GLWT) is described. The importance of
these measurements is that they provide approaches to verify mechanics principles and
computational simulation methods, even at the microscopic scales.
Strains are typically computed from measured displacements. For the measure-
ment of the displacement of a point, a reference point and the displacement of the
point relative to the reference point are required. The methods for measuring displace-
ments generally fall into one of two fundamentally different techniques: invasive and

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