What is Raveling?

Raveling is a pavement distress in which aggregate particles come loose from the surface. FAA AC 150/5380-6C defines it as “the wearing away of the pavement surface caused by the dislodging of aggregate particles”. The Long-Term Pavement Performance (LTPP) manual of the US Federal Highway Administration (FHWA) adds the loss of asphalt binder.

Definition

The sources word the definition in four ways. The FAA PAVER manual for asphalt airfields says raveling is the dislodging of coarse aggregate particles from the pavement surface. FHWA says it is the dislodging of aggregate particles and the loss of asphalt binder. Pavement Interactive calls it the progressive disintegration of a hot mix asphalt (HMA) layer from the surface downward. NCHRP spells the word ravelling and treats it with weathering as one progressive deterioration.

How raveling develops

Raveling starts when the aggregate loses its bond to the binder. FAA AC 150/5380-6C says that raveling may show that the asphalt binder aged and hardened a lot. As raveling continues, larger pieces break free. The pavement then has a rough and jagged surface.

A raveled surface also holds water and loose debris. NCHRP says that deteriorated areas can collect enough water to cause hydroplaning or wheel spray. WSDOT says that raveling is a good sign that the pavement is permeable. It also says that an unsealed raveling pavement continues to ravel, age and harden much faster than normal.

Causes

The FAA AC groups raveling with the disintegration distresses of asphalt pavement. It lists these causes of disintegration:

• Climate.
• Insufficient compaction of the surface.
• Too little asphalt binder in the mix.
• Loss of adhesion between the asphalt coating and the aggregate.
• Severe overheating of the mix.

NCHRP and Pavement Interactive give more detail on the loss of bond.

• Hardened binder. NCHRP lists hardening of the asphalt cement. It also says that high air voids bring faster aging and more raveling. A thicker asphalt film slows the aging.
• Dust on the aggregate. Dust interferes with the adhesion of the binder.
• Segregation. Local areas of the mix lack fine aggregate particles. The binder then holds the coarse particles at few contact points.
• Low density. Poor compaction leaves too little cohesion in the mix.
• Oil spillage. NCHRP classes the softening of the surface and the dislodging of aggregate by oil as raveling.

WSDOT adds the mix design and the construction. These can all lead to raveling:

• A mix with too little asphalt.
• Overheating of the mix during manufacture.
• A mix that gets cold during paving.
• Dirty aggregate.
• Paving late in the season.

Moisture is a separate path. The AC defines asphalt stripping as the stripping of the binder from the aggregate by moisture that enters the mix. The AC lists it with the other types of disintegration.

Traffic and equipment can also dislodge aggregate. Pavement Interactive names studded tires, snowplow blades and tracked vehicles. The PAVER manual counts mechanical damage from hook drags, tire rims or snowplows as high severity raveling.

Raveling and similar distresses

• Weathering. Weathering is the loss of the binder and the fine aggregate matrix. Raveling is the loss of coarse aggregate. The PAVER manual does not record weathering (called surface wear) on an area where it records medium or high severity raveling.
• Polished aggregate. FHWA defines it as the surface binder worn away to expose coarse aggregate. The LTPP manual lists raveling and polished aggregate as two separate surface defects.
• Asphalt stripping. Stripping is the loss of the bond between binder and aggregate. Raveling is the loss of the aggregate from the surface.
• Pothole. A pothole is a disruption in the surface where a portion of the pavement material has broken away, leaving a hole. The AC says that fatigue of the surface causes most potholes.

Raveling on concrete pavement

On concrete pavement, FAA AC 150/5380-6C uses the term scaling for the disintegration and loss of the wearing surface. It defines disintegration of concrete as the breaking up of the pavement into small, loose pieces, including the dislodging of aggregate particles. It lists scaling as the first category. The PAVER manual rates scaling at medium severity when coarse aggregate comes loose from the surface. FHWA gives the normal depth of scaling as 3 to 13 mm.

Both FAA AC 150/5380-6C and the PAVER manual also use the word raveling for concrete. They call corner spalling “the raveling or breakdown of the slab” near a corner.

How raveling is rated

The PAVER manual for asphalt surfaced airfields rates dense mix asphalt on three severity levels. If the inspector is in doubt, the inspector counts the missing coarse aggregate particles in three representative areas of 1 square meter.

Each severity level applies if one of its two conditions is true. The manual measures raveling as the area of surface affected.

The manual has separate severity levels for two other surfaces.

• Slurry seal or coal tar over dense mix. Low is a scaled area under 1 percent. Medium is 1 to 10 percent. High is over 10 percent. For coal tar, pattern cracks under 1/4 inch (6 mm) wide are low and cracks of 1/4 inch or wider are medium. A peeling surface is high.
• Porous friction course. The manual uses a sample of 1 square foot (1/10 square meter). Low is 5 to 20 missing aggregate pieces. Medium is 21 to 40. High is over 40. The manual also counts missing aggregate clusters.

The FHWA LTPP manual works in a different way. It records raveling as distress 13, in square meters, with no severity levels. It notes that raveling shows possible problems with the mix. It says that raveling should not be rated on chip seals.

Raveling and FOD on an airfield

FAA AC 150/5380-6C says that a pavement distress can generate loose material that may strike aircraft propellers or be ingested into jet engines. It says that a raveled surface, with its rough and jagged look, can be a significant source of FOD. It also says that all pavement inspections should address FOD.

EASA guidance for the inspection of paved areas lists looseness of aggregate in asphalt surfaces as a sign of damage to look for. The same guidance says that asphalt and concrete pavements may be the most common source of FOD on an aerodrome. The PAVER survey records the area of raveling.

What to do about raveling

Find the root cause first. Pavement Interactive says to investigate a raveled pavement to find why it failed. The repair depends on the extent.

• Small areas. Remove the raveled pavement and patch.
• Large areas. If the raveling shows a general failure of the asphalt layer, remove the damaged pavement and overlay.
• Roads with raveling. WSDOT says that a fog seal is the best treatment. It seals the surface and slows the fast deterioration. If the raveling becomes severe, a chip seal may work. A sand seal may suit a road with more raveling.

Table 6-1 of FAA AC 150/5380-6C gives a surface treatment or an overlay as the repair for weathering and oxidation of flexible pavement. The table does not list raveling by name. AC 150/5320-6G says that surface treatments work better the sooner they are applied. It says that the first one is often applied 5 years after construction.

FAA specifications limit where a fog seal can go. Item P-632 describes asphalt pavement rejuvenation, commonly called a fog seal. It is for pavements with low to moderate weathered surfaces, as ASTM D5340 defines them. The item is for pavements on which aircraft do not operate, such as shoulders, overruns and roads. It may also serve airport pavements, except runways and acute-angled exit taxiways, at airports with airplanes of less than 60,000 lb (27,216 kg). Item P-608 is an emulsified asphalt seal coat for taxiways and runways. It uses a suitable aggregate to keep the surface friction.