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73
Mix designs should be developed using these materials. Artifi- 5.3.1 CAR Test Parameters
cially produced gradations should be avoided because there
may be an interaction between aggregate shape, texture, and Based on recently performed work, the following test param-
angularity and material packing. Coarse gradations should be eters are recommended for the CAR test:
used to better assess coarse aggregate properties, and fine gra-
dations should be produced to better assess fine aggregate prop- · Gradation,
erties. For a given aggregate-gradation combination, a mix The blended fine aggregate should be tested using the
design should be developed that meets all of the Superpave cri- same gradation expected in the fine aggregate fraction
teria and that produces a reasonable level of VMA. of the HMA (material passing the 4.75-mm sieve), and
Aggregate shape, texture, and angularity would be expected Since some agencies may wish to use the test to
to affect field compactability, rutting, and, possibly, the fatigue approve sources, as-received stockpile samples should
characteristics of HMA mixtures. NCAT has developed a
also be tested.
prototype HMA workability device (181). A device such as
· Samples should be blended with 3% moisture.
this may be able to assess the effects of aggregate shape, tex-
· Samples should be compacted with 50 Marshall blows
ture, and angularity on the workability of HMA. This vibra-
tory compactor has been used to successfully demonstrate the on one face.
increased compactability of warm asphalt mixes (182). Since
the vibratory compactor is a constant stress device, it can be
used to apply constant compaction energy to compare the 5.3.2 Laboratory Evaluation
compactability of various mixes designed using the same
number of gyrations. State agencies should be contacted to identify a range of
The rutting potential of the mixes should be assessed using aggregates having both good and bad performance. Efforts
a simple performance test such as a version of the Flow Num- should be made to identify several of the cubical, crushed
ber Test. In addition, rutting resistance may be assessed using materials that have AASHTO T304 uncompacted void con-
the Asphalt Pavement Analyzer, which has been used more tents between 43 and 45. Natural sands with similar uncom-
widely by agencies (to date). Fatigue can be evaluated using pacted voids content should also be identified. Additional test-
the beam fatigue test. Though there is a demonstrated shift ing, such as petrography and the tests from NCHRP Project
factor with field performance, fatigue results may be used to 4-30A applicable to fine aggregates, should be performed to
rank mixes. assess these "borderline" materials.
Fine gradations or gradations through the restricted zone
should be produced to better assess fine aggregate properties.
5.2.2 Field Evaluation Two coarse aggregate sources are recommended: partially
crushed gravel and crushed limestone. For a given aggregate-
If the Phase I laboratory evaluation indicates relationships
between aggregate shape, texture, and angularity and field gradation combination, a mix design should be developed that
performance, testing should be conducted on aggregates used meets all of the Superpave criteria and that produces a rea-
in actual field sections. Because of the difficulties in accu- sonable level of VMA.
rately determining traffic levels in field test sections, acceler- Rutting potential of the mixes should be assessed using a
ated loading facilities should be utilized as much as possible. simple performance test such as a version of the Flow Number
Sections should be selected to represent a range of aggregate Test. In addition, samples might be assessed using the Asphalt
types, climatic regions, and traffic loads. Pavement Analyzer, which has been used more widely by
agencies to date.
5.3 THE RELATIONSHIP BETWEEN THE
COMPACTED AGGREGATE RESISTANCE
TEST AND RUTTING PERFORMANCE OF HMA 5.3.3 Field Evaluation
The CAR test has been proposed as an alternate to AASHTO If the Phase I laboratory evaluation indicates relationships
T304. Testing has been conducted by a task group within between aggregate shape, texture, and angularity and field per-
TRB's Superpave Aggregate and Mixture Expert Task Group formance, testing should be conducted on aggregates used in
to further develop the procedure. However, little testing has actual field sections. Because of the difficulties in accurately
been conducted to relate the refined test procedure to field per- determining traffic levels in field test sections, accelerated
formance. A research study should be conducted on a national loading facilities should be utilized as much as possible. Sec-
scale to relate the CAR test to performance. Companion test- tions should be selected to represent a range of aggregate
ing should be performed with AASHTO T304. types, climatic regions, and traffic loads.
