Better Roads Staff
“The source of the charge is the emulsifier, as well as ionizable components in the asphalt itself,” James says. “These small charges on the droplets normally provide an electrostatic barrier to their close approach to each other (like charges repel). However, when two droplets do achieve enough energy to overcome this barrier and approach closely then they adhere to each other (flocculate). This flocculation may sometimes be reversed by agitation, dilution or addition of more emulsifier.”
That’s why it’s important to use asphalt emulsions soon after they are produced. “Over a period of time the water layer between droplets in a floccule will thin and the droplets will coalesce,” James says. “The coalescence cannot be reversed. Factors that force the droplets together such as settlement under gravity, evaporation of the water, shear or freezing will accelerate the flocculation and coalescence process, as does anything that reduces the charge on the droplets. Lower viscosity asphalts coalesce more rapidly than high viscosity asphalts.”
An “alphabet soup” of letters designate the types of asphalt emulsions that are available, but it need not be confusing, as the letters correspond to the attributes of the emulsion.
Cationic emulsions begin with a “C.” If there is no C, the emulsion is usually an anionic, reports the Asphalt Institute (AI).
Emulsified asphalts come in rapid-, medium-, and slow-setting grades for different applications and are developed through the use of different emulsifying agents and the addition of some solvents. Still, their asphalt droplets particles will be either anionic or cationic. Rapid-setting emulsions are used mostly for chip sealing, while the medium- and slow-setting grades are used for emulsions mixes for recycling, fog seals or tack coats placed in advance of asphalt lifts.
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,” according to the Institute. “MS emulsions are used in cold recycling, cold and warm dense-graded aggregate mixes, patch mixes and other mixes.”
SS emulsions are designed to work with fine aggregates to allow for maximum mixing time and extended workability, AI says. “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 surfacing and slurry seal designs. The quick break allows for faster opening to traffic.
An HF that precedes the setting time designation indicates a high float emulsion. Polymer-modified HF emulsions are made with a special family of emulsifying agents that leaves a gel structure behind in the asphalt residue, and were developed for low volume roads in areas where a graded cover aggregate is to be used, according to the Iowa Highway Research Board (IHRB).
“High float emulsions are also quite effective when used with somewhat dusty aggregates because they provide a thicker asphalt film on the aggregate and the aggregate can penetrate much more uniformly,” the IHRB says. “This is because high float emulsions are slightly anionic (sets slower than most cationic emulsions), and there is a small amount of solvents in them that act as a cutter in penetrating the dust. A thicker asphalt film coats the aggregate; therefore, high float emulsions do not flow and drain as readily as conventional emulsions.”
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