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Road Science: The Chemistry of Asphalt Emulsions.

RoadScience by Tom Kuennen, Contributing Editor A SPECIAL SERIES The Chemistry of Road Building Materials Spreading the Wealth Asphalt emulsions mix oil with water A s pavement preservation techniques tighten their beachheads in state, county and municipal road agencies, new attention is focused on surface treatments, thin asphalt surfacings and the asphalt emulsions that make them possible. Asphalt emulsions – mixtures of liquid asphalt and water – make expensive liquid asphalt go farther, covering far more square yardage and aggregate surface area for asphalt pavement preservation surfacings than possible with thicker lifts of hot or warm mix asphalt, all things being equal. While they don’t add structural value, these thin surfacings aren’t intended to. Instead they protect aging asphalt pavements, seal cracks, retain aggregate, waterproof pavement structure, enhance friction, and with rejuvenators, give new life to an aged, oxidized asphalt surface. Emulsions used in full depth reclamation (FDR), though, can add structural value to a pavement structure. “Asphalt emulsions are essential to all aspects of pave- 18 June 2012 Better Roads ment preservation, be they recycling, micro surfacing, chip seals, or crack sealing,” says Mike Buckingham, director of pavement preservation, COLAS, Inc., and 2012 president, FP2 Inc. “Almost everything involved in pavement preservation is somewhat tied to emulsions.” Asphalt emulsions make all this possible, yet they have a physio-chemical basis that in some ways is more complicated than plain bituminous pavements. Everyone knows oil and water don’t mix. That is, unless they are emulsiﬁed, and the chemistry of the necessary emulsiﬁers is key to the successful application of asphalt surface treatments. An asphalt emulsion is a homogeneous mixture of two insoluble substances, oil and water. In it particles of liquid asphalt (in the dispersed phase) are surrounded by molecules of water (the continuous phase). They are an emulsion is not a solution, which is a homogeneous mixture of two substances that are soluble with each other. Instead an emulsion is much like a solution, but as the two substances won’t dissolve into each other, small particles of one substance must be created that will become surrounded by the other substance. Milk is one example of a liquid emulsion, in which globules of milk fat are suspended in liquid. Smoke is an example of an emulsion in which solid particles (carbon) are suspended in a mixture of gases (air). Believe it or not, liquid asphalt is an emulsion. “Asphalt binder itself is an emulsion, even before you emulsify it,” says Andy Bickford, applications chemist, MeadWestvaco Corp., at the Asphalt Emulsion Technologies Workshop held in St. Louis in November 2011 by the Asphalt Emulsion Manufacturers Association. “Asphalt is composed of small crystal particles called asphaltenes, which are suspended in a continuous oily liquid phase generally classiﬁed as maltenes, meeting the deﬁnition of an emulsion. It’s the balance of the hard crystalline particles vs. the oil phase that determines things like asphalt grade and hardness.” Why Emulsify Asphalt? Because asphalt generally is solid at ambient temperatures, its viscosity must be lowered in order for it to be used. Energy can be added to the material to heat and liquefy it; solvents can be added to liquefy it; or it can be emulsiﬁed. “When you emulsify asphalt, you are creating a vehicle that will transport asphalt without having to heat it, or use

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