15 May 2001
offers much more pertinent information about the properties of a PMAB than conventional grading.
Many state highway agencies have produced specifications for local use of PMABs that are
modifications of current conventional tests. However, these specifications are often built around specific
asphalts and specific polymers, are not of general use, and can cause problems when asphalt or
polymer sources change. The SHRP PG binder specification was originally intended to be a "blind"
specification where all binder materials (modified and unmodified) would be evaluated on the basis of
properties that relate directly to performance. However, many commercial asphalt modifiers cannot be
properly evaluated using some of the SHRP aging practices due to phase separation and some of the
performance-related properties may not be applicable to some PMABs. Additional research is underway
to address these shortcomings. In the interim, use of SHRP PG on polymer-modified asphalts must be
used with caution. The direct tension test must be employed according to current use practice (as of
March, 1998, this is the horizontal test arrangement using deicing fluid as the bath medium and metal
binder molds) on all PMABs to verify the low temperature grade determined by bending beam
rheometry. The modified asphalts must be shown to not be prone to phase separation or gross
morphology changes during the SHRP performance grading procedures. This may be addressed by
applying phase separation testing (ASTM D-5892) to determine the ring and ball softening point
difference between upper and lower sections of heated tubes of binder. A guideline is that this
difference should be no more than 4EC. In general, the maximum range of temperatures that a typical
unmodified asphalt (non air-blown) demonstrates in SHRP grading is between 80 and 90EC. For
instance, a typical AC-20 or AC-30 viscosity graded asphalt may yield a PG58-22 or PG64-22 with a
temperature use range of 80 and 86EC, respectively. To extend this range past 90EC, modifiers
(primarily polymers) are added to asphalt binder. A typical polymer modified AC-20 or AC-30 will yield a
PG70-22 or PG76-22 depending on the amount of polymer added. However, some modifiers may
extend both the high and low temperature grades. Softer asphalt grades (such as an AC-5 or AC-10)
combined with a polymer modifier can be used to yield a binder with better low temperature properties
for a given region while maintaining the necessary high temperature properties.
b. Aggregates. Aggregates for use in hot mix asphalt should be clean, hard, and durable. Angular
aggregates provide more stable hot mix asphalt mixtures than do rounded aggregates.
(1) Sieve analysis. Aggregates to be used in a paving mix, as listed in table 2-1, should be
subjected to a sieve analysis. An experienced engineer can obtain information from an aggregate's
grading curve concerning the suitability of the aggregate for a paving mix, the quantity of asphalt cement
required, and whether mineral filler should be added. Sieve analyses of fine and coarse aggregates
shall be conducted according to ASTM C 136.
(2) Specific gravity. Specific gravity values for aggregates used in paving mixture are
sometimes required in the computation of percent voids total mix and percent voids filled with asphalt in
the compacted specimens. Criteria have been established to specify limiting values for these void
properties. Therefore, specific gravity values must be carefully determined following specified
procedures to insure that the criteria are properly applied. Two different methods can be used for
determination of the theoretical maximum specific gravity of a mixture. The selection of the appropriate
test procedure depends in part on the water absorption of each aggregate blend.
(a) Apparent specific gravity of aggregate. The apparent specific gravity of the fine and
coarse aggregate can be used to compute the theoretical maximum specific gravity with aggregate
blends showing water absorption of less than 2.5 percent. The apparent specific gravity shall be
determined as described in ASTM C 127 for coarse aggregate, ASTM C 128 for fine aggregate, and
ASTM C 188 or D 854 (whichever is applicable) for mineral filler. Properly weighted values, based on
the amount of each type of material in a given blend, should be used in computations subsequently