Asphalt Surface Course refers to an aggregate, sand, and asphalt cement mixture used as the final surface course placed in a lift (often above the intermediate asphalt pavement course), in a roadway application, driveways, parking lots, and trails.
Asphalt surface course mixture typically includes smaller aggregate sizes than Intermediate and Base Courses of asphalt. Most engineers and localities will permit minimal amount of (commonly 0-20% of dry weight) RAP (Recycled Asphalt Product) in the mixture. RAP is known to not perform and bind the mixture together as efficiently as new fresh asphalt binder, but when used in moderation can be a great recycling measure and can also reduce the production cost of the material.
The most common aggregate composition for asphalt surface course meets the following gradations for amount of crushed limestone material passing through different size sieves:
100% passing 3/8″ sieve
90-100% passing 4.75mm sieve
65-100% passing 2.36mm sieve
40-85% passing 1.18mm sieve
20-60% passing 600micrometer sieve
7-40% passing 300micrometer sieve
0-20% passing 150micrometer sieve
0-10% passing 75 micrometer sieve.
A critical part of the road section assembly, asphalt surface course serves as the final ‘wearing’ course for traffic and requires a level, true longitudinal placement and often has a transverse cross slope from the crown of the road to the shoulder. Some consider the surface course primarily aesthetic with little to no structural value, simply providing a final wearing surface. But most applications still require considerable horizontal and vertical accuracy. Bumps, low spots, and high spots cannot be compensated for as the surface course is the final paving process.
For larger quantities and areas, the equipment required when placing Asphalt Surface Course will be an asphalt paving machine (or road widener paving machine for widening or trench repair) and 1-3 tandem rollers. The crew required will be 6-7 workers composed of the following: 2-3 operators for paver (one on top and 1-2 down low), 2 operators for rollers, and 1-2 laborers for hand raking and finish work.
Asphalt Surface Course can be done without a paving machine for smaller areas such as drives or small parking lots, but the ‘in-place’ production will reduce dramatically as the material will require hand or small equipment placement. 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.