Asphalt Concrete Base refers to an aggregate, sand, and asphalt cement mixture used as a base underneath more asphalt pavement courses, most commonly in a roadway application. It is often utilized as a partial substitute for standard aggregate base as its use can minimize the design thickness of aggregate base engineering requirements by two or three fold. Asphalt concrete base goes between the stone base and intermediate course.
Asphalt concrete base mixture typically includes larger aggregate sizes than standard Intermediate and Surface Course mixtures of asphalt. The most common aggregate composition for pavement base meets the following gradations for amount of crushed limestone material passing through different size sieves:
100% passing 2″ sieve
75-100% passing 1″ sieve
50-85% passing 1/2″ sieve
25-60% passing 4.75mm sieve
15-45% 2.36mm sieve
10-35% passing 1.18mm sieve
3-18% passing 300micrometer sieve
1-7% passing 75 micrometer sieve.
If not hand placed, the equipment required when placing Asphalt Base will be an asphalt paving machine or road widener and 2-3 tandem rollers. Asphalt Concrete Base can be placed and raked by hand for smaller areas such as drives or small parking lots, but the production will reduce dramatically.
For wide open mainline roads with minimal traffic, peak productivity can be achieved. For widened sections, drives, and parking lots with twists, islands, turns, bumpouts, intersections, etc, productivity can be dramatically reduced as these areas require considerable handwork and starting/stopping/slowing of the paver.
While still a critical part of the road section assembly, asphalt concrete base serves a more practical purpose of providing a ‘rough’ base for more finish courses of asphalt. For this reason it will not require as much horizontal, vertical, and aesthetic accuracy as the Intermediate and Surface courses. A few bumps, low spots, and high spots within reason can usually be compensated for later in the paving process. Most engineers and localities will permit a higher percentage RAP (Recycled Asphalt Product) in the mixture, commonly up to 40-50% of the total dry material volume.