What is Hot Mix Asphalt (HMA)?

Hot Mix Asphalt (HMA)

Hot mix asphalt (HMA) is a paving mixture of aggregate and asphalt binder that is mixed in a plant and compacted while hot.

What it is

HMA is a blend of asphalt binder and well-graded aggregate. A plant mixes it, and crews place and compact it while it is hot. Airfield owners use it for new pavement, patches and overlays. On a flexible pavement it is the surface course. The FAA says this course keeps surface water out of the base and resists aircraft loads. It also gives a smooth surface without loose particles and a skid-resistant surface.

FAA specification

FAA Item P-401 is the specification for the HMA surface course of pavements for aircraft heavier than 30,000 lb (13,600 kg). Item P-403 covers other courses, lighter aircraft, roads, shoulders and blast pads. The specification lets the engineer choose a Marshall or a gyratory mix design.

The default design values in Table 1 of P-401 are:

• 3.5% air voids.
• 75 blows or gyrations for airports that serve aircraft over 60,000 lb, and 50 for lighter aircraft.
• A tensile strength ratio of at least 80.
• A rut depth of less than 10 mm at 4,000 passes in the Asphalt Pavement Analyzer (APA). Where no APA is available, a Hamburg wheel test is an alternative. Neither test is required for aircraft under 60,000 lb.

The engineer selects the asphalt binder as a Performance Grade (PG) under ASTM D6373.

Production and placement

P-401 limits the temperature of the materials. Unmodified binder must not exceed 160°C (325°F) when it is added to the aggregate, and modified binder must not exceed 175°C (350°F). The aggregate must not exceed 175°C.

The contractor must not place asphalt on a wet surface. The underlying course must be at least 4°C for a mat 3 in (7.5 cm) thick or more. For a mat between 2 and 3 in, the minimum is 7°C. The resident project representative may waive the temperature limit, but all other requirements stay. If a joint stays exposed for more than 4 hours or cools below 80°C, the contractor cuts it back before placing the next lane.

Acceptance

P-401 accepts each lot on air voids and density, tested on cores. Air voids in the total mix must be between 2.0% and 5.0%. Table 5 sets lower density limits of 92.8% of theoretical maximum density (TMD) for a surface mat and 90.5% for a joint. A lot is acceptable at 90 PWL (percentage within limits) or more.

Fuel-resistant variant

Fuel softens asphalt. The FAA states that continuous fuel or oil spillage softens an HMA surface. Item P-404 is a hot mix for surface courses where fuel spills. Its binder must pass fuel-resistance tests and is graded PG 82-28FR or 88-22FR.

Condition checks on HMA

AC 150/5380-6C lists these distresses of HMA:

• Rutting: a depression in the wheel path from permanent deformation of a pavement layer.
• Asphalt stripping: moisture strips the binder from the aggregate.
• Fuel or oil spillage damage.

EASA guidance for aerodrome inspection includes a check for cracking and looseness of aggregate in asphalt surfaces. EASA also notes that asphalt and concrete pavements may be the most common source of foreign object debris (FOD) on an aerodrome.