SURFACING:
Most park roads are bituminous concrete over a crushed stone
subbase; the surface course may be a cold mix or hot sheet asphalt
depending on availability of materials and climate. Big Bend has
much heat, sand, and hard aggregate; Yellowstone has much cold,
travertine, little aggregate. Crushed stone chips are apread over
a tack coat to give the road surface a color and texture which blends
into the immediate landscape hues. The South Rim Drive at Grand
Canyon is surfaced with red chips; the roads at Craters of the Moon
are surfaced with black chips. I prefer crushed gravel to rounded
gravel (of uniform size) because it adheres to the tack coat longer.
The angularity of particles exposes more surface area to be enclosed
in road tar than is enclosed in rounded gravel particles.
Last summer our Park Engineer in Yellowstone used Asphalt
Emulsion rather than road oil in applying a chip seal; the reason
being the high cost of oil and gasoline. It worked as well as oil,
we thought, and it will be interesting to see how the road came
through the winter.
