What is a Chip Seal Surface Treatment?

A chip seal is a pavement surface treatment that covers a sprayed asphalt binder with a layer of aggregate chips.

A chip seal is also written “chipseal”. FAA Item P-609 calls it a chip seal coat. FHWA calls it a chip seal or seal coat. It is a bituminous surface treatment, not a structural layer (FHWA-HRT-14-065). Crews apply it to an existing asphalt pavement or to an older bituminous surface treatment.

What a chip seal does

FHWA lists three uses. A chip seal seals fine cracks, it keeps water out of the pavement, and it sustains or improves friction. FHWA gives a typical macrotexture depth of more than 1.0 mm for a chip seal. For comparison, it gives 0.3 to 0.6 mm for a slurry seal.

The FAA lists seal coating as preventive maintenance (AC 150/5380-6C, section 1.1). AC 150/5380-6C, Table 6-1, lists a surface treatment as a repair for weathering and oxidation and for loss of skid resistance. Surface treatments work better the sooner crews apply them. The first one is often applied 5 years after construction (AC 150/5320-6G, section 4.2).

An FHWA study of 13 chip seal projects in five states found a pavement life extension of 3 to 8 years (FHWA-HIF-10-020, Table 7). These were road projects, so an airfield may see other lives.

A chip seal does not add strength. FHWA says chip seals should not go on pavements with structural distress. Alligator cracking from a failed base needs removal and replacement (AC 150/5380-6C, Table 6-1).

Single, double and triple chip seals

A single chip seal has one application of binder and chips. Double and triple chip seals add more. FHWA says the layers may use different aggregate sizes. Item P-609 allows up to three applications and sets a quantity for each.

The note to P-609 says to use the lower rates for fine aggregate and the higher rates for coarse aggregate. It also says to use more asphalt if the pavement is highly oxidized, badly cracked or coarse. The RPR sets the exact amounts. The RPR is the resident project representative.

Materials

Binder

The binder is the sprayed asphalt that holds the chips. FHWA says it is typically a polymer-modified asphalt emulsion. It can also be a liquid asphalt such as asphalt cement or cutback, including rubberized asphalt. FHWA lists emulsion-based, asphalt rubber and hot applied chip seals. Item P-609 allows asphalt cement or emulsified asphalt. The contractor gives the RPR the manufacturer’s certificate of analysis before the emulsion goes on the pavement (P-609-2.2).

Aggregate

Item P-609 sets the quality of the chips. The aggregate must meet these limits:

• Loss of 40 percent at most in the Los Angeles abrasion test (ASTM C131).
• Soundness loss after 5 cycles of 12 percent at most with sodium sulfate or 18 percent at most with magnesium sulfate (ASTM C88).
• Clay lumps and friable particles of 0.3 percent at most (ASTM C142).
• At least 90 percent by weight of particles with two fractured faces (ASTM D5821).
• Flat, elongated, or flat and elongated particles of 8 percent at most at a 3:1 ratio (ASTM D4791).

P-609 gives three gradations. For Aggregate No. 1, 100 percent passes the 1 inch (25.0 mm) sieve. For No. 2, 100 percent passes the 3/4 inch (19.0 mm) sieve. For No. 3, 100 percent passes the 1/2 inch (12.5 mm) sieve. FHWA says the chips should be close to the same size, clean and free of excess fines.

How crews build a chip seal

Item P-609 sets the order of work:

1. Clean the surface so that it is free of dust, dirt, grease and vegetation (P-609-3.3). Repair cracks first under Item P-101.
2. Spray the binder with a pressure distributor at the specified rate and temperature (P-609-3.5).
3. Spread the aggregate straight away over the binder (P-609-3.6).
4. Roll the aggregate straight after spreading. Rolling continues until no more aggregate goes into the surface.
5. Wait before the next application. It must not start until the last one sets, and not before 24 hours pass.
6. Keep traffic off for at least 24 hours (P-609-3.9). Just before opening, roll the whole area with a self-propelled pneumatic-tired roller.

P-609-3.1 sets the weather limit. Work does not start when the air is below 60 °F (16 °C). It does not start when the surface is below 70 °F (21 °C). The RPR can direct otherwise. The contractor must also have the distributor calibrated in the same construction season (P-609-3.2).

The FHWA highway checklist gives more detail. Treat these values as road practice, not as FAA requirements:

• The first roller pass comes within 2 minutes of the aggregate for an emulsion.
• Rolling is complete before the emulsion breaks, and within 15 minutes at most.
• An emulsion needs at least 3 roller passes at 3 mph (5 km/h) or less.
• The final sweep is no later than the morning after placement.
• The checklist starts work at an air and surface temperature of 50 °F (10 °C) and rising. It suspends work above a pavement temperature of 140 °F (60 °C). It says to avoid work in wind over 20 mph (32 km/h).

Which pavements suit a chip seal

The FHWA checklist asks the inspector to review the candidate pavement first:

• Limit existing rutting to less than 3/8 in (about 10 mm) deep.
• Find the type, amount and severity of cracks. Fill or seal cracks wider than 1/4 in (about 6 mm) before the chip seal (see crack sealing).
• Check for bleeding or flushing.
• Do not use a chip seal on a pavement with structural distress.

A chip-sealed pavement may get a later overlay. FHWA says a thin HMA overlay may go over an existing surface treatment (FHWA-HRT-14-065). AC 150/5320-6G, 4.9.5.1, lists a double chip seal as one type of interlayer under an overlay. It adds that an interlayer does not remove the need to fill cracks in the old pavement.

FHWA lists bleeding or flushing on the old surface as a point to check before the work starts. Rutting is a separate distress, and the rutting page explains its types.

Common defects

The FHWA checklist links each defect to a cause:

Embedment shows if the binder rate was right. FHWA says to consider a lower asphalt rate if embedment is more than 80 percent. It says to consider a higher rate if embedment is less than 50 percent.

Chip seal on an airfield

Chips can come loose. FHWA reports flying chips and broken windshields on roads. The FAA P-609 note says the method may leave excessive loose aggregate. For this reason the FAA only recommends it on pavements other than airfield pavements.

Loose stone is foreign object debris (FOD). ICAO Annex 14, 10.2.1, asks aerodromes to inspect movement area surfaces to avoid and remove FOD. Recommended Practice 10.2.9 asks for taxiway shoulders free of loose stones that turbine engines could ingest. AC 150/5380-6C says that loose material from distress may strike propellers or enter jet engines. It says that all pavement inspections should address this.

P-609 allows a chip seal coat on tertiary airfield pavements such as overruns. It is not for areas with routine turbo-prop or jet traffic. It may be used at general aviation airports that serve airplanes of 30,000 lb (13,600 kg) or less. Use elsewhere needs a modification to standards under FAA Order 5300.1.

Chip seal, seal coat, slurry seal and microsurfacing

These treatments have different layers and different rules on an airfield.

In a cape seal, the slurry seal covers the chip seal after it cures. The slurry seal locks the chips in place, which limits chip loss and gives a smoother surface (FHWA). P-609 says this combination may be an option where small aircraft operate. It uses Item P-626 for the slurry seal.

Item P-623 is a different product. The FAA approves it for all pavements except runways that serve airplanes of 30,000 lb (13,600 kg) or less. With FAA concurrence the engineer may specify it for airports with airplanes under 60,000 lb (27,216 kg). Runways and acute-angled exit taxiways stay excluded. The seal coat page and the slurry seal page cover these treatments in full.

P-623 applies to pavement in fair or better condition under ASTM D5340. The note gives a typical candidate as a structural condition index deduct value below 10 and a PCI of 60 or more. A pavement condition survey therefore decides whether a spray seal is an option.

Chip seal, crack sealing and joint sealing

These three treatments seal a pavement against water, but they cover different parts of it.

The choice follows the defect. A chip seal suits a pavement with fine cracks and no structural distress. FHWA sets crack sealing for cracks 5 to 19 mm wide with less than 25 percent edge deterioration. FHWA sets joint resealing for pavement where sealant damage is systemic and the pavement is otherwise well performing.

The treatments can follow each other. The chip seal checklist says to fill or seal cracks wider than 1/4 in (about 6 mm) before the chip seal goes on. On an airfield, the FAA limits the chip seal itself. Item P-605 covers sealant for cracks and joints, and Item P-604 covers compression seals. See crack sealing and joint sealant .

Inspecting a chip-sealed pavement

A chip-sealed pavement needs a visual check for loose chips, bleeding, streaking and aggregate loss at joints. These are the defects in the table above. On an airfield, the sweep and the re-inspection matter most, because loose chips are FOD (AC 150/5380-6C, chapter 4).

For a drone survey of a pavement, TarmacView plans the flight and captures the photos. Crack detection and the ASTM D5340 rating are in development.