Page 120 - MODELING OF ASPHALT CONCRETE

98 Cha pte r F o u r

agreement between the phase angles measured in axial and shear conditions. The SST
device measured phase angle values ranging from 12 to 70°, while the axial testing gave
values ranging between 12 and 42°.
It can be hypothesized that two possibilities contribute to these findings: the SST
device may have control problems at very high and low test temperatures, or the
difference in the specimen loading mode may contribute to the different material
responses. In compressive axial loading, the average stress amplitude is always larger
than zero, but in the SST device, the average shear strain and stress amplitude are zero
due to the shear straining through zero, as Fig. 4-4 shows. This may cause compressive
testing to be more influenced by the aggregate skeleton effect (which is elastic) in the
higher test temperatures, causing lower phase angle values and higher modulus values.
Also material inhomogeneity and anisotropy may cause some differences.

Stiffness as the Asphalt Mix Performance Indicator

Introduction
A new mix design procedure, the Superpave volumetric mix design, was developed in
the Strategic Highway Research Program (SHRP) in the mid-1990s. However, unlike the
Marshall mix design method, the new Superpave volumetric mix design procedure did
not include any mechanical test to check the mixture performance after the volumetric
part of the design procedure had been completed. Experience from the implementation
process over recent years has shown that the volumetric mix design procedure without
a performance test is inadequate for ensuring acceptable mix performance. Work leading
to the development of a simple performance test has been in progress through the
NCHRP 9-19 project: “Superpave Support and Performance Models Management,”
Task-C at ASU (Pellinen and Witczak, 2002a).
The focus of the proposed simple performance test (SPT) has been to measure a
fundamental engineering material property that can be linked back to the advanced
material characterization measurements that are needed for a detailed distress analysis.
The three main asphalt mixture distresses considered in the design process are permanent
deformation, fatigue cracking, and thermal cracking. The main objective of the overall
NCHRP 9-19 Task-C research effort was to recommend from several candidate tests the
most promising fundamental SPT for use with the Superpave volumetric mix design
procedure. The potential simple performance tests that have been studied can be
categorized as stiffness-related tests, deformability tests, and cracking tests.

Stiffness-related Tests: Recommendations for SPT
The laboratory test program for stiffness-related test by Pellinen (2001) and Pellinen
and Witczak (2002a) included duplicating mixtures from three different experimental
test sites in the United States. These sites were the MnRoad, FHWA-ALF, and WesTrack
sites. All mixtures from these sites were dense graded mixtures. However, two stone
mastic asphalt (SMA) mixtures and two dense graded mixtures from the Finnish
Asphalt Pavements Research Project (ASTO) were also studied. ASTO project was
conducted from 1987 to 1992 including comprehensive laboratory and field research
effort on Finnish asphalt mixtures (Saarela, 1993). The ASTO test specimens were
fabricated by the Technical Research Center of Finland and tested at ASU.
Testing was conducted using two replicate specimens. Specimen instrumentation
was as shown in Fig. 4-6. Each specimen was tested in an increasing order of temperature