Asphalt emulsions are an environmentally friendly, energy efficient and cost effective product for use in today’s paving and preservation efforts. But, what are they and how are they made?
What Is an Asphalt Emulsion?
An asphalt emulsion is liquid asphalt cement emulsified in water. It is composed of asphalt, water and an emulsifying agent. The emulsifying agent is sometimes called the surfactant, which is composed of large molecules. These soap molecules are like tadpoles, having a hydrocarbon tail soluble in liquid asphalt and an electrically charged or ionic head soluble in water. The asphalt particles are surrounded by the ionic charge, which causes the droplets to repel each other and stay suspended in the water.
All asphalt emulsions are designed to eventually break, or revert to asphalt and water. Some emulsions break by chemically destabilizing the surfactant, others by a simple evaporation of the water. Still others break by a combination of chemical destabilization and evaporation.
Today’s emulsion manufacturers have many new chemistries available to design the emulsion for the optimal break time and specific use.
How Are Emulsions Made?
Asphalt cement is the basic ingredient of asphalt emulsions. In most cases, it makes up from 50 to 75 percent of the emulsion. Some properties of the asphalt significantly affect the finished emulsion. But there is not an exact correlation between the properties and the ability to emulsify the asphalt. The properties of the asphalt cement do affect the performance of the residual asphalt on the road.
Water is the second ingredient in an asphalt emulsion. Minerals or other materials in water can affect the production of stable emulsions. Water that is suitable for drinking may not be suitable for asphalt emulsions.
The chemical used as an emulsifier greatly influences the properties of an asphalt emulsion. The emulsifier keeps the asphalt particles in suspension and controls the breaking time. It also determines whether the emulsion is cationic or anionic (or non-ionic). Chemical compatibility of the emulsifying agent with the asphalt cement is essential for producing a stable emulsion.
The emulsion is produced by separately metering the soap and water solution and hot liquid asphalt into a colloid mill at predetermined rates and temperatures. The colloid mill shears the asphalt into microscopic particles. The size of the particles affects the physical properties of the emulsion.
Sometimes additional ingredients, including latex, polymers, acids and other additives are fed into the emulsion to further modify its physical characteristics.
Names and Classifications of Emulsions
Emulsions are classified by their ionic charge. Cationic emulsions begin with a “C.” If there is no C, the emulsion is usually an anionic. The charge is important when designing an emulsion for compatibility with certain aggregates.
After the charge designation, the next set of letters describes how quickly an emulsion will set or coalesce to a continuous asphalt mass. The standard terms are RS (Rapid Set), MS (Medium Set), SS (Slow Set), and QS (Quick Set).
RS emulsions break rapidly and have little or no ability to mix with an aggregate. MS emulsions are designed to mix with aggregates, and are often called mixing grade emulsions. MS emulsions are used in cold recycling, cold and warm dense-graded aggregate mixes, patch mixes and other mixes.
SS and QS Emulsions
SS emulsions are designed to work with fine aggregates to allow for maximum mixing time and extended workability. They are the most stable emulsions and can be used in dense-graded aggregate bases, slurry seals, soil stabilization, asphalt surface courses and some recycling. SS emulsions can be diluted with water to reduce their viscosity so they can be used for tack coats, fog seals and dust palliatives. SS emulsions are also used as driveway sealers.
QS emulsions work well with fine aggregates but are designed to break faster than SS emulsions. QS emulsions are used in micro-resurfacing and slurry seal designs. The quick break allows for faster opening to traffic.
High Float Emulsions
An “HF” that precedes the setting time designation indicates a High Float emulsion. HF emulsions are designed so the emulsifier forms a gel structure in the asphalt residue. The thicker asphalt film allows these emulsions to perform in a wider temperature range. High Floats are used in chip seals, cold mixes and road mixes.
After the set designation, there is a series of numbers and letters that further describe the characteristics of the emulsions. The number 1 or 2 designates the viscosity of the emulsion, with the number 1 meaning lower viscosity and 2 meaning higher viscosity. If there is an h or s at the end of the name, the h indicates a harder base and the s for a softer asphalt base. For example, SS-1h is a slow setting emulsion with a lower viscosity made from a relatively hard base asphalt.
A “P” may be added to the set designation to show the presence of polymer in the emulsion. An “L” indicates the presence of latex polymer. For example, CRS-2P is a cationic, rapid setting emulsion having a higher viscosity and containing some polymer. Polymers and latex are used to add strength, elasticity, adhesion and durability to the pavement. Polymer asphalt emulsions can be less brittle at low temperatures to resist cracking and stiffer at high temperatures to resist rutting and bleeding. Polymers permit the application of micro surfacing in wheel-path ruts and other locations where multiple stone depths are required.
Adaptable and Flexible
The physical properties of emulsions can be modified to optimize storage, curing, mixing, traffic return and bonding strength. Emulsions readily accept additives such as polymers, latex, fillers, anti-strips, stabilizers and other modifiers to enhance the physical properties of both the emulsion and the cured asphalt.
An emulsion’s charge can be positive, negative or neutral to improve compatibility with available aggregates. Emulsions can be engineered for a wide variety of conditions, such as cooler temperatures or wet or dusty conditions.
Some other attributes of asphalt emulsions include:
- Producing no or minimal hydrocarbon emissions
- Requiring no added heat for many applications
- Readily coat damp aggregate surfaces
- Producing minimal fumes
- Being ideal for remote locations where there are no hot mix plants
- Greatly reducing needs for raw materials and energy
- Being used in a variety of recycling systems
- Reducing user delay costs
- Reducing overall life-cycle costs
- Improving skid resistance
- Reducing hydroplaning and improving visibility
- Costing less than thin hot mix asphalt overlays.
The many benefits and varied types of asphalt emulsions give pavement engineers the ability to select an effective treatment for every specific road need.
The Basic Asphalt Emulsion Manual (MS-19), a joint publication of the Asphalt Institute and the Asphalt Emulsion Manufacturers Association (AEMA), is recommended for anyone needing more information on asphalt emulsions. Much of the material in this article was taken from MS-19 and from AEMA’s booklet, Asphalt Emulsions.