What is Asphalt Cement?

The asphalt cement is the petroleum-derived binder that holds the aggregate together in hot-mix asphalt (HMA). The FAA describes the asphalt surface course of a flexible airfield pavement as aggregates “bound together with asphalt cement or other bituminous binders”. Current FAA specifications call the material “asphalt binder”. In practice both names describe the same product.

How binder is graded

Three grading systems exist, each with an ASTM specification:

• Penetration grading (ASTM D946) measures the binder at 25 °C.
• Viscosity grading (ASTM D3381) measures it at 60 °C.
• Performance grading (ASTM D6373) relates the binder to the maximum and minimum pavement design temperatures. A grade such as PG 64-22 carries the high and the low design temperature.

D6373 binder may be refined from crude petroleum, with or without modifiers. The FAA notes that a PG spread between the high and low temperature of 92 or more typically means the binder is modified.

What the FAA specifies for airfields

Item P-401 in AC 150/5370-10H covers surface courses of flexible airfield pavements for aircraft above 30,000 lb (13,600 kg). The binder must conform to ASTM D6373. The engineer picks the base PG for the climate. The engineer then raises the high-temperature grade for traffic:

The same item adjusts the grade for recycled material. Up to 20 percent reclaimed asphalt pavement (RAP) changes nothing. Above 20 to 30 percent RAP, the binder is one grade softer, so PG 64-22 becomes PG 58-28. Unmodified binder must not exceed 325 °F (160 °C) when it meets the aggregate. Modified binder must not exceed 350 °F (175 °C).

Item P-404 is a surface course for areas that need a fuel-resistant surface, where fuel spills reach the pavement. Its binder is PG 82-28 or 88-22 with an elastic recovery of at least 85 percent. Once the binder passes the fuel resistance tests, the FAA names it PG 82-28FR or 88-22FR.

What binder shows on the surface

Several asphalt distresses in AC 150/5380-6C and the PAVER manual trace back to the binder:

• Raveling is the loss of aggregate particles. It may indicate that the binder aged and hardened significantly. Loose pieces become foreign object debris (FOD).
• Weathering is the wearing away of the binder and fine aggregate matrix from the surface.
• Stripping is the loss of binder from the aggregate through moisture.
• Jet blast erosion is a darkened area of burned or carbonized binder.
• Bleeding is a shiny, sticky film caused by too much binder or too few air voids. Extensive bleeding can severely reduce skid resistance.

An inspection records these surface distresses. The binder grade itself comes from the specification and the certificate of analysis for the mix, not from the surface.