What a Mix Design Is
An asphalt mix design specifies the recipe of an asphalt mixture: the gradation (size distribution) of the aggregate, the binder grade, the binder content, and the air voids in the compacted mix. Different mixes are designed for different applications — surface course, binder course, base course, light-duty, heavy-duty. Specifying the right mix design matters.
Illinois Mix Design Naming
Illinois Department of Transportation uses a naming convention based on aggregate gradation and design air voids. IL-9.5 means a mix with maximum 9.5mm aggregate, designed at standard air voids. IL-12.5 means maximum 12.5mm aggregate. N-50 / N-70 / N-90 / N-105 indicates the design gyration count (the number of gyrations a Superpave gyratory compactor uses to design the mix). Higher N-number = heavier traffic capacity.
Common Mixes for Commercial Parking Lots
IL-9.5, N-50: Standard light-duty surface course. Used for most commercial parking lots — retail, office, HOA. IL-9.5, N-70: Slightly heavier-duty surface for medium-traffic commercial. IL-12.5, N-50 or N-70: Binder course (the layer below the surface course) for most commercial work. IL-12.5, N-90: Heavy-duty mix for industrial yards and high-stress applications. SuperPave PG 64-22 binder is standard for Chicagoland surface mixes.
Surface Course vs. Binder Course
Standard commercial pavement is paved in two lifts: a binder course (the rougher, denser bottom layer) and a surface course (the smoother, denser top layer). Total thickness for light-duty: 1.5 inches binder + 1.5 inches surface = 3 inches asphalt. Heavy-duty: 2 inches binder + 2 inches surface = 4 inches asphalt. Industrial: 2.5 inches binder + 2.5 inches surface = 5 inches asphalt.
Polymer-Modified Mixes
Polymer-modified asphalt mixes use a polymer-modified binder instead of standard PG 64-22. Polymers improve high-temperature performance (less rutting), low-temperature flexibility (less cracking), and overall durability. Common in heavy-traffic commercial applications: drive-throughs, dumpster pad approaches, intermodal entrances. Costs about 10 to 20% more per ton than standard mix.
Binder Grade
Asphalt binder is graded by performance temperatures. PG 64-22 is the Chicagoland standard — performs at 64°C high temperature and -22°C low temperature. PG 70-22 (modified) is used for heavier traffic or hotter applications. PG 76-22 is heavy-duty industrial. The right binder grade matches the climate and traffic. Wrong binder grade causes premature failure.
CA-6 Aggregate Base
Below the asphalt is the aggregate base — typically CA-6 (a graded crushed stone). Standard light-duty: 6 inches CA-6 over compacted sub-grade. Heavy-duty: 10 to 12 inches CA-6, sometimes with geotextile fabric for sub-grade reinforcement. CA-6 needs to be properly placed in lifts and compacted to 95% standard Proctor density.
Why Mix Specification Matters
Cheap asphalt is cheap because it skimps on something — binder content, aggregate quality, or thickness. Properly specified asphalt that meets IDOT mix design and is properly compacted will last 20 to 30 years. Cheap asphalt fails in 5 to 10 years. The cost difference upfront is small. The cost difference over time is enormous.
How to Verify You’re Getting the Right Mix
Itemized estimates name the mix design (IL-9.5, N-50, etc.), the binder grade (PG 64-22), the thickness, and the application rate (tons per cubic yard or pounds per square yard). Ask the contractor for the mix design ticket from the asphalt plant when material is delivered — this verifies what you actually got. Real commercial contractors provide this without hesitation.
Get a Free Commercial Paving Estimate
Need help with a commercial paving project? Every estimate is free, on-site, and itemized. Most on-site estimate within 48 hours, with on-site estimate within 48 hours. Use the form on this page or call (630) 555-PAVE.
Get a Free Commercial Paving Estimate
Need help with a commercial paving project? Every estimate is free, on-site, and itemized. Use the form on this page or call (630) 555-PAVE.