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Laboratory Testing the NCHRP Projects 9-25 and 9-31 mixtures, the concept
of resistivity was developed and appeared to relate very
As part of NCHRP Projects 9-25 and 9-31, a wide range of well to the results of the RSCH test. To further verify
laboratory tests was performed on a variety of HMA. The lab- these results, field data from three sources was compiled
oratory tests were designed to provide information concern- and analyzed:
ing the rut resistance, fatigue resistance, permeability, and
resistance to age hardening of the mixtures studied. The most 1. The MnRoad test track, as discussed in numerous
important of the procedures performed as part of this reports by the Minnesota DOT and the University of
research included the following tests: Minnesota (5);
2. The NCAT test track, as documented by Brown et al. (6);
· RSCH testing using the Superpave shear tester, at 58°C and
and
64°C (AASHTO T320-03); 3. The WesTrack project, as documented in FHWA's Perfor-
· Uniaxial fatigue testing at 10 Hz and 4°C and 20°C, which
mance of Coarse-Graded Mixes at WesTrack--Premature
includes an initial measurement of the complex modulus Rutting (2).
|E*| (16); and
· Mixture permeability, using the Florida permeability test
The approach in analyzing the uniaxial fatigue results
(Florida Test Method FM 5-565). involved a further development and simplification of con-
tinuum damage theory. Because of the variability in fatigue
The mixtures tested were made using eight different aggre- data, the relatively small amount of testing performed as
gates and gradations: part of NCHRP Projects 9-25 and 9-31 and because of the
novelty of the approach, further verification of the results
· A 9.5-mm limestone from Virginia, coarse and fine
was desired. This verification was performed by applying
gradations; the same analytical approach to fatigue data gathered dur-
· A 19-mm gravel from Pennsylvania, coarse and dense
ing the Strategic Highway Research Program (SHRP),
gradations; as summarized in SHRP Report SHRP-A-404: Fatigue
· A 19-mm limestone from Kentucky, coarse and dense
Response of Asphalt-Aggregate Mixes (15). Although these
gradations; and data were gathered using flexural fatigue tests, continuum
· A 12.5-mm granite from California, dense and fine
damage theory predicts that the damage in the extreme
gradations. fiber at the test conclusion for this procedure should be
constant and can thus be related to the results of uniaxial
All of these mixtures were combined with a performance tests.
grade (PG) 64-22 binder. Some were also combined with a The permeability tests performed during this research were
PG 58-28 binder and/or an air blown PG 76-16 binder. In very limited. This occurred for two reasons--(1) the air void
most cases, the design gyration level was 100, but for some content of the mixtures was relatively low, resulting in low
mixtures, Ndesign was 75. All of the California granite mixtures permeability values and (2) permeability tests performed in
were designed using 125 gyrations. Early in the project, an the laboratory will usually show lower values than those
attempt was made to design and evaluate some mixtures at determined from field cores. Therefore, most of the speci-
50 gyrations, but these were typically very weak and difficult mens tested during NCHRP Projects 9-25 and 9-31 showed
to test; further testing of these mixtures was therefore aban- very low or zero permeability. To better understand the rela-
doned. All mixtures were made using three binder contents: tionship between mixture composition and permeability,
the optimum binder content, optimum -1%, and optimum data from the Florida permeability study was included in this
+1% (by total mix weight). The materials used represented a analysis (3).
range of aggregate types, gradations, binder grades, and mix- Because understanding the extent and scope of the data
ture compositions. used in developing the performance models developed dur-
ing this research is essential to interpreting the findings pre-
sented in this chapter, the external data sets summarized
Analysis of Other Data Sets
above are discussed in more detail in the sections below.
As discussed later in this short report, some of the find- This unfortunately increases the length and complexity of
ings made during the research appeared very promising, this report, but makes clear the fact that the findings are
but somewhat controversial. Therefore, in several cases ver- based on a much more robust set of data than that which
ification of the findings was attempted using data sets from was collected during testing performed under NCHRP Proj-
other research projects. In evaluating the rut resistance of ects 9-25 and 9-31.
