Paving field answers

Crusher run, also called dense-graded aggregate, DGA, GAB, 21A, 411, or 3/4 minus, is a crushed-stone blend running from a top size around 3/4 in down through sand to fines. The continuous range of sizes lets it compact into a tight, interlocked layer, which makes it the standard material for road base and gravel-road surfacing.

Rolling stops doing useful work once the mat cools to its cessation temperature, commonly cited near 175 to 185°F for many dense-graded HMA mixes. Below that the aggregate locks and the roller only smooths the surface. Polymer-modified and stiffer binders differ, so confirm the stop temperature for your mix and agency spec.

Density controls the air voids, and air voids decide how fast a pavement ages. Below about 92 percent of Gmm the voids interconnect, water and air get in, and the mat ravels and cracks early. Agency research ties each extra 1 percent of air voids to roughly 10 percent less service life.

Alligator cracking is fatigue failure from repeated traffic load on a structure that cannot carry it: a base too thin, too weak, saturated, or never compacted. It shows as interconnected cracks in the wheelpath. Because it is structural, it needs a full-depth patch and base repair, not a sealcoat or a thin overlay.

A residential asphalt driveway commonly runs 2 to 3 in of compacted asphalt over 6 to 8 in of compacted aggregate base. Two inches is a working minimum for cars, while 3 in adds durability and handles the occasional truck or RV. The base depth matters as much as the mat, and the spec and soil set the real numbers.

A mill and overlay resurfaces asphalt by grinding off the worn top layer with a cold-milling machine, then paving a new lift over the milled, swept, and tacked surface. It removes the failed surface, holds the original grade so ties-in stay flush, and renews ride on a pavement whose base is still sound.

Asphalt milling, or cold planing, is grinding off a measured thickness of existing pavement with a rotating drum of carbide teeth, loading the cuttings onto a conveyor and into a haul truck. It is done before an overlay to fix grade and remove the failed surface, and the millings are recycled as RAP.

Superpave, short for Superior Performing Asphalt Pavements, is the asphalt mix design system most agencies use, developed under the Strategic Highway Research Program. It picks a performance-graded binder for the site climate and traffic, builds the aggregate gradation to control points, and designs to volumetric targets using a gyratory compactor instead of the older Marshall hammer.

Surface asphalt is the fine, dense top lift that rides the tires, sheds water, and resists skidding, built from small 9.5 to 12.5 mm stone. Base asphalt is the bottom lift built from the largest stone, 25 mm and up, for structure. The surface seals and wears; the base carries and spreads the load.

Asphalt thickness depends on the traffic and the subgrade, so there is no single number. A car-only parking lot can run a couple of inches over modest base, while a truck route over soft soil needs a full structural design. Size it to the design ESALs and the subgrade strength using the agency method, never by habit.