What is Asphalt Emulsion?
An asphalt emulsion is a stable dispersion of asphalt droplets in water, held in suspension by an emulsifying agent. It has three components: asphalt, water and the emulsifying agent. The FAA specification uses the term “emulsified asphalt” for this material.
Breaking and setting
An emulsion changes from a liquid to a binder when it breaks. Breaking is the separation of the water from the asphalt. Setting is closely related to how soon the emulsion breaks.
Types
The emulsifying agent gives the droplets an electrical charge. A grade with a C at the start, such as CRS-1, is cationic. Its droplets are positively charged. A grade without the C is normally anionic. Its droplets are negatively charged.
The setting grades are:
- RS (rapid set). Breaks quickly. Used in spray work such as chip seals and sand seals.
- MS (medium set). Mixed with graded aggregate for cold mixes and cold recycling.
- SS (slow set). Gives the longest mixing time. Used for tack coats, fog seals and slurry seals.
- QS (quick set). Used in microsurfacing and slurry seals when the surface must cure fast enough to reopen to traffic.
Use on airfield pavement
The FAA standard specification for airport construction, AC 150/5370-10H, has these items that use emulsified asphalt:
- Prime coat (Item P-602). A coat on a prepared base course. It protects the aggregate base from raveling under construction traffic and helps bond it to the first asphalt layer.
- Tack coat (Item P-603). A coat that promotes bonding between the underlying surface and a new asphalt layer. The emulsion must not be diluted. For existing asphalt, the residual rate is 0.04 to 0.07 gal/SY (0.18 to 0.32 L/m²). The emulsion rate is 0.06 to 0.11 gal/SY (0.27 to 0.50 L/m²).
- Seal coat (Item P-608). A surface treatment of an emulsion of natural and refined asphalt materials, water and a polymer additive. It needs aggregate on runways and high-speed taxiways. The concentrated emulsion must have a positive particle charge and at least 57% residue.
- Slurry seal (Item P-626). A mixture of emulsified asphalt, polymer, mineral aggregate and water. The FAA limits its use by airplane weight. It is not approved for runways at airports that serve airplanes over 30,000 lb (13,600 kg).
The FAA maintenance circular AC 150/5380-6C also lists the tack coat, and its use on the cut sides of an existing pavement before patching.
Pavement condition
Item P-608 says that the performance of a seal coat depends on the pavement condition at the time of application. The condition survey gives the type, amount and severity of the distresses. It also gives the pavement condition index (PCI). A typical asphalt pavement for a seal coat has a structural condition index deduct value of less than 10. Its PCI is more than 60. Pavements in fair or better condition, as defined in ASTM D5340, can take a seal coat.
Frequently Asked Questions
- What does it mean when an emulsion breaks?
- Breaking is the separation of the water from the asphalt. Setting is closely related to how soon an emulsion breaks. Rapid-setting grades break quickly, and slow-setting grades give the longest mixing time.
- Can a contractor dilute the emulsion for an airfield tack coat?
- Not under the FAA standard specification. Item P-603 in AC 150/5370-10H says the emulsified asphalt for a tack coat shall not be diluted. Other agencies can have other rules, so check the specification of the project.
- What weather limits apply to an emulsion tack coat or prime coat on an airfield?
- Item P-602 and Item P-603 allow application only on a dry surface in weather without fog or rain. The air temperature must be 10 °C (50 °F) or more. It must not be below 2 °C (35 °F) in the 12 hours before application. The Resident Project Representative may waive the temperature limits. Item P-608 seal coat has stricter limits. Air and pavement must be at 16 °C (60 °F) or more and rising. No rain may be expected within 8 hours.
- What do the letters in a grade such as CRS-1 mean?
- A C at the start means the emulsion is cationic, which means its droplets are positively charged. Without the C, the emulsion is normally anionic and negatively charged. The next letters give the setting speed. RS is rapid set, MS is medium set, SS is slow set, and QS is quick set.
References
- asphalt-magazine-emulsions-101
- Asphalt Emulsions 101 Asphalt Magazine (Asphalt Institute)
- faa-ac-150-5370-10h
- AC 150/5370-10H, Standard Specifications for Construction of Airports FAA
- faa-ac-150-5380-6c
- AC 150/5380-6C, Guidelines and Procedures for Maintenance of Airport Pavements FAA
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