15 May 2001
New Mexico, Oklahoma, and Utah have natural rock-asphalt deposits where paving material is produced
commercially. The character and quantity of the aggregate and asphalt in the material varies among the
different deposits and sometimes varies within the same deposit. Rock asphalt pavement is prepared by
blending into the natural asphalt a crushed impregnated limestone or sand-stone or a combination of the
two in proper proportions to produce a properly graded mixture with a specified asphalt content. The
natural rock asphalt must be enriched (that is, more asphalt must be added to the mixture) if the material
contains insufficient asphalt in its natural state to produce a satisfactory mixture. Hot mixes are
sometimes produced by heating crushed limestone impregnated with relatively hard asphalt, alone or
with added sand, and mixing with additional asphalt cement in a conventional plant.
a. Advantages and disadvantages. The advantages and disadvantages are the same as those for
plant-mix cold-laid asphalt mixtures. In addition the use of rock-asphalt pavement reduces cost because
this mixture already contains binder material.
b. Uses. Rock-asphalt pavements are used for roads and streets not subjected to traffic by tracked
vehicles. Rock asphalts are sometimes used as the aggregate in slurry seals, but only predominantly
sandstone rock asphalts should be used in slurry since some limestone rock asphalts polish under traffic
and thus produce a slick pavement surface.
PLANT-MIX COLD-LAID ASPHALT PAVEMENTS.
a. General. Cold-mix asphalt pavements can be used as a low-cost surface for low-volume roads
or as a base course for high-volume roads and airfields. While cold mixes do not provide pavements
with the same quality of hot mixes, cold mixes do perform satisfactorily for the purposes intended. Cold
mixes can be stored for several months or more and are very useful for minor patching. Because fuel is
not needed to heat cold mixes, these pavements can generally be constructed at lower costs than hot
mixes. Some disadvantages of using cold mixes are that a lower density is usually obtained in cold
mixes than in the construction of hot mixes and that a curing period is needed to allow water or volatiles
to evaporate so that a satisfactory shear strength is obtained.
(1) Preliminary work. The first step in designing a paving mixture is to make a survey to insure
that the materials needed are available in suitable quantities and their use is economically feasible in the
pavement construction. Sufficient samples of material should be obtained during the survey to
accomplish the tests described later. Materials normally required for the paving mix are coarse
aggregate, fine aggregate, mineral filler, and bitumen.
(2) Sampling. Test reports reflecting the results of sampling and testing of the aggregates and
bituminous materials will be prepared. A gradation analysis must be conducted on the aggregates to
determine whether the aggregates can be blended to meet the contract gradation specifications.
Representative samples of materials must be furnished for laboratory testing. Large samples must be
divided into sizes usable for testing in the laboratory, in a way that will represent field conditions.
Sufficient quantities of materials will be obtained at the time of sampling to meet the ASTM requirements
and for laboratory pavement design tests subsequently described. Normally, aggregates that will
produce 90 kilograms (200 pounds) of the desired gradation and 19 liters (5 gallons) of bitumen will be
sufficient for these tests.
(1) Tests on aggregates.