SHRP-A-379 traducir

9. LABORATORY EVALUATION OF AGGREGATE
9.1 The SUPERPAVE ® mix design system requires measurement of the gradation,
apparent specific gravity, bulk specific gravity, sand equivalent, Los Angeles abrasion,
soundness, and deleterious material content of all aggregates, both unmodified and modified.
Empirical guidelines are presented that relate several of these parameters to pavement
performance. These tests are not considered performance-based for purposes of judging the
effect of modification, and as such, are not covered by this practice. Modifications to the
aggregate are evaluated strictly in terms of changes to the performance properties of the
modified paving mix.
10. LABORATORY EVALUATION OF PAVING MIXES
10.1 The ultimate benefits of the use of modifiers are demonstrated by their effect on
the performance characteristics of modified paving mixes. The SUPERPAVE ® system
addresses the primary pavement distress modes of permanent deformation (rutting), fatigue
cracking, and low-temperature cracking. It also accounts for the effects of aging and
moisture in the development of these distresses. Environmental conditions and traffic are also
explicitly considered.
10.2 The asphalt binder makes up only about 5 weight percent of a paving mix. As a
rule, the magnitude of the change in performance attributable to the modifier is smaller for
the paving mix than for the asphalt binder. Moreover, the role of the asphalt binder, whether
modified or not, in influencing pavement performance varies with the type of distress
investigated. About 40% of the resistance of a paving mix to permanent deformation is
attributable to properties of the asphalt binder; in contrast, binder properties can account for
about 60% of a mix's resistance to fatigue cracking and as much as 90% of its resistance to
low-temperature cracking.
10.3 The SUPERPAVE ® accelerated mix performance tests and mix design system
are the key tools for laboratory evaluation of modified paving mixes. As in the case of
modified asphalt binders, the SUPERPAVE ®performance-based mix specification provides
an objective measure against which the ability of specific modifiers to enhance pavement
performance may be gauged.
10.4 Performance Testing of Modified Paving Mixes--This section provides a method
for determining the performance characteristics of modified paving mixes, comparing them
with criteria contained in the SUPERPAVE ®performance-based mix specification, and
estimating their influence on long-term pavement performance. Correlation of these
performance characteristics with those of conventional paving mixes (or of other modified
mixes commonly employed by the specifying agency) permits an impartial decision on the
benefits of using a new modifier in a particular situation.
NOTE 14.--The SUPERPAVE ® pavement performance prediction models were used to develop the
SUPERPAVE ® mix specification limits. Since the mix design system incorporates accelerated performance tests that
215