4. AGGREGATES.
a. Suitability. Basic or alkaline rocks (limestone, dolomite), provide
better adhesion with asphaltic films than do acid or silicious rocks (granite,
quartzite). Where acid rocks are used, the addition of an antistripping agent
or hydrated lime may be required.
b. Coarse Aggregate. Coarse aggregates should be clean, hard, and durable.
In asphaltic concrete mixtures, crushed rock is preferable for its higher
stability and performance. Other requirements for coarse aggregate are
contained in ASTM D692.
c. Fine Aggregate. Fine aggregate for bituminous concrete mixes may be
composed of naturally occurring sand or of aggregate particles produced from
crushed stone or crushed gravel. Fine aggregates should otherwise conform to
ASTM D1073.
d. Mineral Filler. In bituminous concrete mixtures, mineral filler should
be limestone dust, portland cement, or other similar inert materials. At least
two-thirds of the material passing the No. 200 sieve in a bituminous mix
should be nonplastic material meeting the requirements of ASTM D242.
5. BITUMINOUS MIX DESIGN.
a. Hot-Mix Asphalt Concrete. Use the Marshall mix design method for de-
signing hot-mix asphalt concrete mixture. Detailed instructions for the design
procedure are contained in DA TM 5-822-8. For test methods, see MIL-STD 620.
(1) Asphalt Cement. Use penetration grade, AC viscosity grade, or AR
viscosity grade of asphalt cement in the mix design. Figure 9 provides
recommended grades for each area of the United States. These recommendations
should be tempered by local practice. In areas where only the viscosity grades
are available, determine those sources having acceptable penetrations for use
in the project.
(2) Aggregates. Use aggregates in the mix design which meet the
requirements of ASTM D692 and D1073 for coarse and fine aggregates,
respectively. Mineral filler, when required, should conform to ASTM D242.
Aggregates used for mix design should be identical to those anticipated to be
used in construction. Gradation of aggregates should conform either to ASTM
D1663 or Asphalt Binder & Wearing Courses for Flexible Pavement, NAVFAC TS-
02681. In general, the maximum aggregate size for wearing courses should not
exceed 3/4 inches. For binder and intermediate courses, the maximum aggregate
size should not exceed two-thirds the course thickness.
(3) Marshall Requirements. Use the 75-blow compaction procedures for
designing primary roads and streets. For secondary roads, streets, and parking
areas, use the 50-blow procedure. Follow the procedures given in DA TM
5-822-8 for preparing and testing the trial mixes. Criteria for determining
the optimum bitumen content and the adequacy of the mix are given in Tables 11
and 12. A minimum bitumen content of 5 percent is recommended.
(4) Optional VMA Method. This optional method of mix design utilizes the
concept of the voids in the mineral aggregate (VMA) to determine the optimum
bitumen content. The VMA represents the volume of voids in the compacted
aggregate mixture and includes the percent of air voids plus the effective
asphalt content expressed as a percent of the total volume. The mix design
method requires that a minimum volume of voids be contained in the aggregate
mix in order to accommodate sufficient bitumen for stability and durability.
Procedures for computing the VMA are given in Mix Design Method for Asphalt
Concrete and Other Hot-Mix Types, The Asphalt Institute Publication MS-2.
Criteria for minimum percent VMA are given in Table 13. In using the design
procedure, no correction is required for highly absorbent aggregate.
b. Mixed-In-Place Bituminous Pavement. Mixed-in-place bituminous pavements
can be used on secondary and other low-volume roads. The pavement should
consist of one layer with compacted thickness of 2 inches. For recommended
limits for aggregate gradation and bitumen contents, see Table 14. For
recommended grades of bitumen, see Table 10.
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