What is a Wearing Course (Surface Course)?

The wearing course is the top layer of a pavement structure that carries aircraft traffic directly. It is also called the surface course.

Where the wearing course sits

AC 150/5320-6G names four parts of a pavement structure: the surface course, the base course, the subbase course and the subgrade (1.6). In a flexible pavement, each layer protects the layer below it (3.15.1).

Pavement Interactive says the surface course is the layer in contact with traffic. It normally holds the highest quality materials. The surface course can be one layer or several asphalt sublayers:

• Wearing course. The top sublayer. It takes the traffic wear and can be removed and replaced when it is worn.
• Binder course. The sublayer below the wearing course. It is also called the intermediate course. It supplies most of the asphalt thickness, and its main job is to spread the load.

AC 150/5370-10H says that Item P-403 may be used for asphalt binder courses. The base course below the binder course is a separate layer. See the glossary entry on the base course.

Types of wearing course

• Dense-graded hot mix asphalt (HMA). Pavement Interactive calls it a well-graded mix for general use, suitable for all layers. On an airfield, this is FAA Item P-401. The P-401 gradations are well graded from coarse to fine.
• Stone matrix asphalt (SMA). A gap-graded HMA that gives stone-on-stone contact in the mix. Pavement Interactive says it is almost exclusively a surface course mix on high-volume US roads.
• Open-graded friction course. A thin, permeable asphalt layer. FHWA says its macrotexture helps drainage at the tire and pavement contact, and reduces hydroplaning and spray. FHWA reports a service life of 7 to 10 years, and says it may ravel and shove at intersections. On an airfield, the FAA calls the equivalent a porous friction course (PFC).
• Portland cement concrete (PCC). In a rigid pavement, the slab forms the surface course. This is FAA Item P-501.
• Gravel. On an unpaved road, the wearing course is surface gravel. The Gravel Roads Maintenance and Design Manual describes the mix. Surface gravel needs stone for strength, sand to fill the voids and plastic fines to bind the surface. Base gravel needs more large stone and few fines. The manual gives one state specification as an example. In it, surface gravel has 4 to 15 percent fines and a plasticity index of 4 to 12. Base gravel has 3 to 12 percent fines and a plasticity index of 0 to 6. The manual says to follow the specification of your own state.

Material and thickness

In a flexible pavement, the surface course is asphalt mix, FAA Item P-401. In a rigid pavement, it is the cement concrete slab, Item P-501. The slab carries the load and forms the surface. The FAA says to use P-401 for pavements that serve aircraft heavier than 60,000 lb (27,215 kg). Item P-403 may be used for 60,000 lb or less.

AC 150/5320-6G, Table 3-3, gives the minimum asphalt surface thickness. It is 3 in (75 mm) for pavements that serve aircraft under 60,000 lb, and 4 in (100 mm) for heavier aircraft. Table 3-4 gives a minimum of 6 in (150 mm) for the concrete surface. The structural design sets the real thickness.

Each lift of asphalt also has a minimum. P-401 Table 2 recommends a minimum construction lift thickness for each gradation. It is 3 in (75 mm) for gradation 1, 2 in (50 mm) for gradation 2 and 1-1/2 in (38 mm) for gradation 3. Gradation 3 is meant for leveling courses. Pavement Interactive gives a rule of thumb to avoid mat tearing. The compacted lift should be at least twice the maximum aggregate size, or three times the nominal maximum aggregate size.

Some areas get spills of fuel, hydraulic fluid or other solvents. Aircraft fueling positions and maintenance areas are examples. The FAA says to use a solvent-resistant surface there, such as P-501, P-404 or P-629 (AC 150/5320-6G, 3.15.3.4).

What the layer does

The asphalt wearing course does three jobs:

• It limits the penetration of surface water into the base course.
• It gives a smooth, skid-resistant surface free from loose particles that could become FOD.
• It resists the shear stresses from aircraft wheel loads.

A concrete slab also gives a skid-resistant surface and prevents surface water from reaching the subbase.

How it is laid and compacted

P-401 requires a self-propelled paver with a heated screed. The paver spreads the mix to the specified thickness, smoothness and grade. P-401 does not allow the asphalt on a wet surface. It also sets a minimum temperature for the underlying course. The limit is 40 °F (4 °C) for a mat 3 in (75 mm) thick or more. It is 45 °F (7 °C) for a mat thicker than 2 in (50 mm) and thinner than 3 in (75 mm). The resident project representative (RPR) may waive the temperature limit. All other requirements, including compaction, still apply.

Rollers follow the paver. P-401 requires self-propelled rollers. Crews roll the mat until it has a uniform texture, the correct grade and cross-section, and the required field density. The roller wheels are kept moist to stop the asphalt from sticking.

TxDOT describes the usual roller train:

• Breakdown roller. The first roller behind the screed. It is usually a vibratory steel-wheel roller.
• Intermediate roller. Used if more compaction is needed. It is usually a pneumatic-tire roller.
• Finish roller. The last roller. It is almost always a static steel-wheel roller, for smoothness.

The breakdown speed for a static steel-wheel or pneumatic roller is 2.0 to 3.5 mph. For a vibratory steel-wheel roller it is 2.0 to 3.0 mph. Successive passes overlap by at least 6 in (15 cm).

Compaction decides how long the layer lasts. Pavement Interactive says that dense-graded mixes should not exceed 8 percent air voids or fall below 3 percent. Too many voids let water in and cause raveling. Too few make the mix unstable. P-401 accepts a lot on air voids, surface course mat density and joint density. Table 5 sets these limits. Total mix air voids are 2.0 to 5.0 percent. Surface course mat density is 92.8 percent minimum. Joint density is 90.5 percent minimum.

Joints and smoothness matter as well. In P-401, the longitudinal joint of one course is offset at least 1 ft (30 cm) from the joint in the course below. The joint in the top course lies at the centerline of a crowned pavement. Transverse joints are offset at least 10 ft (3 m). The contractor tests smoothness with a 12 ft (3.7 m) straightedge. The limit is a variance of less than 1/4 in (6 mm) in 12 ft. The contractor must correct a deviation that traps more than 1/4 in (6 mm) of water on the final surface course. The methods are diamond grinding or replacement of the surface course to its full depth.

Surface requirements

EASA CS ADR-DSN.B.090 says a runway surface should have no irregularities that impair friction or affect take-off or landing. A paved runway should be built or resurfaced to give friction at or above the minimum friction level. ICAO Annex 14 Volume I, 3.1.26, and EASA both give a texture depth. A new surface should have an average texture depth of not less than 1.0 mm.

EASA GM1 ADR-DSN.B.090 gives a standard of construction for evenness. Tested with a 3 m straight-edge in any direction, the finished wearing course has no deviation greater than 3 mm. Both EASA and ICAO say to take care with smoothness when runway lights or drainage grilles are set into the surface.

Two scales of texture matter. Microtexture is the texture of the individual stones. It is the main component of skid resistance at slow speeds. Macrotexture is the texture among the stones. EASA says macrotexture is the major factor for drainage capacity at high speed. EASA also says to choose crushed material that resists polishing, and it names the polished stone value (PSV) as the measure. A new asphalt surface is usually quite smooth because of the rolling. AC 150/5320-12C, paragraph 2-4, lists ways to improve it: proper mix design, porous friction courses, chip seals, slurry seals and saw-cut grooves.

EASA GM1 ADR-DSN.B.191 gives the measures of macrotexture. The volumetric method gives a mean texture depth (MTD) and is valid from 0.25 to 5 mm. The profile method gives a mean profile depth (MPD) and is valid from 0 to 5 mm. The two values differ, so a transformation equation is needed to relate MPD to MTD. EASA also gives the ESDU scale, which grades runway texture from A (0.10 to 0.14 mm) to E (1.01 to 2.54 mm).

Grooving shortens the drainage path. The FAA standard groove is 1/4 in by 1/4 in, with 1-1/2 in between centers. Grooves wear under traffic. Take a 1,500 ft (457 m) length of runway. If 40 percent of its grooves are 1/8 in (3 mm) or less in depth or width, their effect against hydroplaning is much reduced. The FAA then asks for corrective action.

A porous friction course (PFC) is a thin open-graded HMA overlay, 3/4 to 1-1/2 in (25 to 40 mm) thick. The FAA recommends against it on runways with more than 91 turbojet arrivals per day per runway end. Rubber deposits can plug its voids.

Friction levels

AC 150/5320-12C sets three friction levels: minimum, maintenance planning and new design or construction. The values depend on the friction measuring device and the test speed. For a Mu Meter at 40 mph the values are 0.42, 0.52 and 0.72. The survey interval depends on the daily turbojet landings per runway end. It runs from 1 year for fewer than 15 landings to 1 week for more than 210.

Distress

The PAVER manual for asphalt airfields lists 17 distress types. These are the ones that start in the wearing course:

• Bleeding (42): a shiny, sticky film of bituminous material on the surface. Excess asphalt or low air voids cause it. It is recorded when it is enough to reduce skid resistance.
• Polished aggregate (51): the aggregate above the asphalt is very small or has no rough, angular particles. Repeated traffic causes it.
• Raveling (52): coarse aggregate particles come out of the surface.
• Weathering (57): the asphalt binder and the fine aggregate matrix wear away from the surface. PAVER does not record it where medium or high severity raveling is recorded.
• Slippage cracking (55): crescent-shaped cracks that braking or turning wheels cause. A low-strength surface mix or a poor bond with the layer below causes them.
• Jet blast erosion (46): burned or carbonized binder. Burned areas can be up to about 1/2 in (13 mm) deep.

Rutting is a distress of the whole structure, and the wearing course can be the layer that ruts. PAVER also lists it (53). Scaling (70) is the matching distress of a concrete wearing course. The PAVER concrete manual calls it the delamination or disintegration of the slab surface. Over-finishing, lack of curing, inadequate air entrainment and freezing before the concrete gains strength cause it.

You cannot judge texture and friction by eye. AC 150/5320-12C says that research shows visual evaluations of friction are not reliable. It also says that visual inspections cannot identify poor texture, and that texture depth is found only by direct measurement. A visual survey, including a survey from a drone, records the surface distresses above. It does not replace a friction test or a texture measurement.

Replacing the layer

AC 150/5320-6G calls the removal and replacement of part or all of a flexible wearing surface rehabilitation. A mill and overlay often gives significant extra functional and structural life. Removing and replacing any layer below the surface course is reconstruction.

The AC uses the pavement condition index (PCI) as a guide. A PCI above 70 marks a candidate for maintenance. A PCI from 55 to 70 marks a candidate for rehabilitation. A PCI below 55 marks a possible candidate for reconstruction. The AC adds that the distress type can justify another choice.

An overlay can also correct non-structural problems, such as the crown, the longitudinal profile or skid resistance. Before milling, take cores to find the thickness and condition of the surface. If milling leaves less than 2 in (50 mm) of surface course, the thin layer is likely to delaminate under construction traffic (4.8). Profile milling and a leveling course correct surface irregularities such as shoving, rutting and bleeding (4.10.1.2).

Maintenance

The FAA recommends rubber removal at intervals from 2 years (fewer than 15 landings) to 2 months (more than 210). A chip seal or a slurry seal is a temporary way to restore friction. A slurry seal usually lasts 2 to 5 years and is an interim measure until an overlay. A fog seal can substantially reduce friction in the first year.