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that passes the 4.75-mm (No. 4) sieve and is retained on appears to be no relationship between LA abrasion and
the 2.36-mm (No. 8 sieve). It does not appear that any long-term abrasion or wear of the pavement surface.
of the current consensus property tests can be used to · LA abrasion is specified by 96% of responding agencies.
address this size fraction. · A maximum LA abrasion loss of 40% is the most com-
· Uncompacted voids in fine aggregate, AASHTO T304, mon specification level.
appears to be a reasonable screening test for fine aggre-
gate blends. Numerous other tests have been investi-
gated, but to date none consistently show a better rela- 6.2.2 Sulfate Soundness
tionship with performance.
· Of agencies that specify AASHTO T304 or an equiva- The research revealed the following about sulfate soundness:
lent, 51% have adopted the specification criteria recom-
mended for the Superpave mix design system; 21% of · Of the responding agencies, 66% specify sodium sul-
state agencies specify more stringent criteria. fate soundness and 31% specify magnesium sulfate
· There are materials with uncompacted voids contents in soundness.
the range of 43% to 45% that test as false negatives. · A maximum sodium sulfate soundness loss of 12% is
These materials have a demonstrated history of field specified by the majority of the agencies using that
performance under high traffic even though they do not procedure.
meet the uncompacted voids content specifications for · Magnesium sulfate soundness loss and micro-deval abra-
high traffic. sion loss are highly correlated. The micro-deval test is
· Research should be conducted to relate the CAR test to also related to abrasion of particles in the pavement.
the rutting performance of HMA. Borderline aggregates · The micro-deval test is more precise than the sulfate
that fail the current fine aggregate uncompacted void soundness tests.
content specifications for high traffic but that provide · The micro-deval test should replace sulfate soundness
good performance should be investigated in-depth as test for measuring aggregates' resistance to abrasion, wet-
part of this study. ting and drying, and slaking. Research may need to be
· Of state agencies, 46% continue to limit natural sand conducted to identify specifications for specific aggre-
content by specifications. Limits between 10% and 15% gate types, similar to those used by Ontario.
are most common. · States prone to freeze-thaw cycles should consider a
freeze-thaw test, such as AASHTO T103, in addition to
the micro-deval abrasion loss
6.1.4 Sand Equivalent
No recent research has been able to corroborate the rela- 6.3 GRADATION
tionship between clay-like particles, identified by the sand
equivalent test, and moisture damage in the laboratory. How- The restricted zone--included in the original Superpave
ever, the phenomenon that produces this type of failure may mix design system--was demonstrated to be unnecessary. It
be difficult to duplicate in the laboratory. has been removed from most current Superpave specifications.
The methylene blue value appears to be a better indicator Accelerated testing at the 2000 NCAT Test Track indicates
of harmful clays in fine aggregate than is the sand equivalent no difference in the rutting performance of coarse-graded ver-
test. There is concern by some agencies that the test is not sus fine-graded Superpave mixtures.
suitable for routine specifications.
6.4 AGGREGATE PRODUCTION
6.2 SOURCE PROPERTIES
Following are the conclusions reached regarding source
Following are the conclusions reached regarding source properties:
properties.
· In addition to crusher type, aggregate particle shape can
be improved by
6.2.1 LA Abrasion Running the crusher with a full or choked feed cavity
to promote interparticle crushing.
The research revealed the following about LA abrasion: Operating crushers in closed circuits where a recircu-
lating feed can be used to fill the crusher cavity.
· LA abrasion is related to aggregate breakdown during Reducing the reduction ratio, reducing the feed size,
handling, mixing, placement, and compaction. There or increasing the circulating load.
