An asphalt emulsion consists of three basic components: asphalt, water and an emulsifying agent.
An asphalt emulsion is a stable dispersion of asphalt cement droplets in water, which can be pumped, stored and mixed with aggregates.
An emulsion should “break” quickly upon contact with aggregate when mixed or after being sprayed. Breaking is the separation of the water from the asphalt. After breaking and curing, the asphalt residue has the adhesion, durability and water-resistance properties of the original base asphalt.
Classifications and Additives
The American Society for Testing and Materials (ASTM) and the American Association of State Highway and Transportation Officials (AASHTO) has standardized how emulsions are identified. The first designation is based on electrical charge. Cationic (positively charged) emulsions are identified with a “C.” If the “C” is not included, the emulsion is normally anionic (negatively charged). There are a few nonionic emulsions, but they are not widely used.
After the charge designation in an emulsion classification, there is information about the “setting” rate. Setting is closely related to how soon an emulsion breaks. The standardized designations are RS (rapid set), MS (medium set), SS (slow set), and QS (quick set). RS emulsions break rapidly and are typically used for spray applications, such as chip seals and sand seals. MS emulsions are designed to be mixed with graded aggregates. MS emulsions are used in making cold mixes and in cold recycling applications. SS emulsions are designed for maximum mixing time and workability. They are the most stable emulsions and are used for tack or bond coats, fog sealing as well as in slurry seals and some asphalt surface courses. QS emulsions are designed for use in micro-surfacing and slurry seals when a quick curing time is needed for opening to traffic.
The addition of “P” in the emulsion classification indicates polymer has been added to the emulsion. “L” means a latex polymer has been used. Polymers are added to enhance strength, adhesion, and durability. Numbers in an emulsion’s classification refer to the viscosity of the emulsion. A “2” (for example, RS-2) is more viscous than a “1” (RS-1). An “h” indicates harder base asphalt was used (SS-1h). “HF” or high float emulsions use a gel structure to allow a thicker asphalt film.
Storage, Handling and Testing
Emulsions are made in specialized plants, usually including a colloid mill. The asphalt is sheared into tiny droplets (1 to 10 microns, smaller than a human hair) and blended with the other components. Emulsions have particular storage and handling requirements.
Some important handling guidelines include:
Emulsions are extremely versatile materials. They are used for preventative and corrective maintenance activities on both asphalt and concrete pavements. They are used in stabilizing and reclaiming base courses, for constructing structural layers within a pavement and for various surface treatments. Emulsions can be used in recycling old, deteriorated pavements.
Some of the specific uses of emulsions include:
Tack (also known as bond) coats