What is a Seal Coat?

A seal coat is a thin layer of asphalt emulsion that is sprayed on an existing asphalt pavement as preventive maintenance.

What the word means

On an airfield, a seal coat is a sprayed asphalt emulsion, sometimes with a sand cover. FAA Item P-608 says that the terms seal coat, asphalt sealer and asphalt material are interchangeable. On roads, FHWA uses the same words for a chip seal. This page covers the airfield seal coat first, then the road meaning.

FAA AC 150/5380-6C lists seal coating with the typical preventive and regular maintenance work on airport pavement. It says that timely maintenance supports good friction and limits foreign object debris (FOD). AC 150/5320-6G adds that a flexible pavement needs routine crack sealing, seal coats and small patches to reach its full life.

Seal coat types in FAA specifications

FAA item	What it is	Where it may go
P-608 Emulsified Asphalt Seal Coat	Asphalt emulsion with polymer and gilsonite. The contractor applies it with or without a sand cover.	Taxiways and runways with a suitable aggregate for friction. Secondary and tertiary pavements with or without aggregate.
P-608-R Rapid Cure Seal Coat	A rapid cure version of P-608.	The same pavements as P-608. It suits night work and short closures.
P-623 Emulsified Asphalt Spray Seal Coat	A polymer modified asphalt emulsion in two or three coats.	All pavements, except runways, that serve airplanes of 30,000 lb (13,600 kg) or less. Pavements without aircraft.
P-626 Emulsified Asphalt Slurry Seal	A mixture of emulsified asphalt, polymer, aggregate and water.	All pavements on general aviation airports that serve airplanes of 30,000 lb or less. Not runways at larger airports.
P-629 Coal Tar Emulsion	A coal tar emulsion as a micro-surface, sand slurry seal or spray seal.	Apron areas that need a fuel resistant coating. Local rules may ban coal tar products.

For P-623 the Engineer, with FAA concurrence, may also specify the item for airports that serve airplanes under 60,000 lb (27,216 kg). Runways and acute-angled exit taxiways stay excluded. P-626 follows a similar rule for airports that serve airplanes of 60,000 lb or less, and it excludes runways.

P-608 has a specific rule on aggregate. It is required on runways and high-speed taxiways. It is recommended on all taxiways and not recommended on shoulders.

What a seal coat is made of

A seal coat is an emulsion. The FAA specifications set these properties.

P-608. The emulsion is natural and refined asphalt, water and polymer. The residue has at least 20 percent gilsonite and at least 1 percent polymer. It has no coal tar and no tall oil pitch. The concentrate has at least 57 percent residue. The sand cover is angular manufactured specialty sand with a Mohs hardness of 6 to 8.
P-623. The emulsion has a density of 9 to 12 lb/gal (1.0 to 1.5 g/mL) at 25 °C. It has at least 44 percent residue and at most 56 percent water. The ash content of the residue is 30 to 40 percent. The polymer is at least 3 percent of the weight of the asphalt binder.

The density in P-623 is the density of the liquid product. It is not a density of the cured layer.

P-629 says that thermoplastic coal tar emulsion gives a fuel resistant surface where fuel spills occur. It also says that more states and local authorities now prohibit coal tar products.

How thick it is

The FAA specifies a seal coat by the amount of material per area. It gives no layer thickness. The engineer picks the rate for the surface texture, the porosity and the age of the pavement.

Item	Application rate
P-623, two coats	0.14 to 0.20 gal/yd2 for the first coat, 0.10 to 0.20 for the second, and 0.30 minimum in total
P-623, three coats	0.14 to 0.20, 0.10 to 0.20 and 0.08 to 0.15 gal/yd2, and 0.30 to 0.55 in total
P-608, 1:1 dilution	0.10 to 0.17 gal/yd2 (0.45 to 0.77 L/m2) of emulsion and 0.20 to 0.50 lb/yd2 (0.11 to 0.27 kg/m2) of sand
P-608, 2:1 dilution	0.08 to 0.17 gal/yd2 (0.36 to 0.77 L/m2) of emulsion and 0.20 to 0.50 lb/yd2 of sand

P-608 recommends a 1:1 dilution of emulsion to water for most applications. It recommends 2:1 for grooved, rough or coarse surfaces, and for pavement that is highly oxidized or badly cracked. Additional P-623 coats may not exceed 0.20 gal/yd2 each.

Which pavement is a candidate

The pavement must be in fair or better condition, with low to moderate weathering. ASTM D5340 or the PASER manual in AC 150/5320-17 gives these ratings. P-608 and P-623 describe a typical candidate as a pavement with a structural condition index (SCI) deduct value below 10. The Pavement Condition Index (PCI) is 60 or higher in P-623 and above 60 in P-608.

What a seal coat does not repair

A seal coat is not a structural layer. FHWA says this about chip seals, and says that a slurry seal gives no structural benefit. FAA AC 150/5380-6C lists a different repair for alligator cracking, which is to remove and replace the damaged pavement. The seal coat specifications also send crack repair to Item P-101, paragraph 101-3.6, before the seal coat.

For weathering and oxidation, and for loss of skid resistance, AC 150/5380-6C lists a surface treatment as the repair. The PAVER manual for asphalt airfields defines these surface distresses:

Weathering is the wearing away of the asphalt binder and fine aggregate matrix.
Raveling is the dislodging of coarse aggregate particles. Low severity raveling has little or no FOD potential.
Bleeding is a shiny film of bituminous material. The manual notes it when it reduces skid resistance.

Application and weather limits

Both P-623 and P-608 apply the seal coat only to a dry, clean surface. Both bar work when rain is expected within 8 hours of the end of the work. The temperature limits differ.

P-623: air and pavement above 10 °C (50 °F) and rising, and expected to stay above 10 °C for 24 hours.
P-608: air and pavement at or above 16 °C (60 °F) and rising. The contractor does not apply the coat if traffic opens within 72 hours and the pavement may exceed 130 °F (54 °C) in that time.

AC 150/5380-6C says that seal coats and other surface treatments need warm, dry weather for best results.

Before full production the contractor builds a control strip to set the application rate. P-623 asks for a control strip of at least 250 square yards. Traffic stays off a P-623 seal coat until it has thoroughly cured, about 24 hours. For P-608 the Engineer’s note gives a curing time of generally 4 to 24 hours.

Runways

For a runway or high-speed exit taxiway, P-608 asks for control strips. Each strip is at least 300 feet (90 m) long and 12 feet (3.6 m) wide. The contractor measures the existing skid resistance with continuous friction measuring equipment. Aircraft stay off the strips until the friction meets the maintenance planning friction levels. These are in AC 150/5320-12, Table 3-2, at 40 and 60 mph (65 and 95 km/h) wet.

Lights, signs and markings

The contractor covers edge lights, informational signs, retro-reflective markings and in-pavement duct markers against overspray. A fixture that cannot be cleaned to the satisfaction of the Resident Project Representative is replaced at the contractor’s cost.

Seal coat in road construction

In road work, a seal coat is a chip seal. FHWA says that a chip seal starts with a bituminous membrane on the existing pavement. A layer of aggregate chips goes on top and rollers embed it in the binder. The membrane is normally a polymer-modified asphalt emulsion, but it can be a liquid asphalt such as asphalt cement or cutback.

A chip seal seals fine cracks and keeps water out of the pavement structure. FHWA lists three limits. It is not a structural layer, it is susceptible to chip loss, and it is not common on heavily traveled urban roadways.

Slurry seal and seal coat

A slurry seal is not a sprayed seal coat. FHWA describes it as a mixture of emulsified asphalt, water, fine aggregate and mineral filler. The crew spreads or screeds it in a thin layer. It seals the surface from water, fills surface cracks and voids, and improves friction and appearance.

Microsurfacing is similar. FHWA calls it typically more durable and used on higher volume roadways. It normally uses a polymer-modified asphalt emulsion. FHWA says it mainly mitigates raveling and oxidation, and it can fill shallow to moderate ruts.

In airfield inspection

A seal coat is a decision that follows a pavement condition survey. The survey records the type, amount and severity of the distresses and gives the Pavement Condition Index. P-608 and P-623 both say that the performance of a seal coat depends on the pavement condition at the time of application. TarmacView plans pavement survey flights and captures the photos today. Crack detection and the ASTM D5340 rating are in development and offered as early access.