Emulsified Asphalt

Emulsified asphalt is simply a suspension of small asphalt cement globules in water, which is assisted by an emulsifying agent (such as soap). The emulsifying agent assists by imparting an electrical charge to the surface of the asphalt cement globules so that they do not coalesce (Roberts et al., 1996¹). Emulsions are used because they effectively reduce asphalt viscosity for lower temperature uses (tack coats, fog seals, slurry seals, bituminous surface treatments (BST), stabilization material). Emulsions are typically either anionic (asphalt droplets are negatively charged) or cationic (asphalt particles are positively charged).

Generally, emulsions appear as a thick brown liquid when initially applied (Figure 1). When the asphalt cement starts to adhere to the surrounding material (aggregate, existing surface, subgrade, etc.) the color changes from brown to black (Figure 2) and the emulsion is said to have "broken" (Figure 3). As water begins to evaporate, the emulsion begins to behave more and more like pure asphalt cement. Once all the water has evaporated, the emulsion is said to have "set". The time required to break and set depends upon the type of emulsion, the application rate, the temperature of the surface onto which it is applied and environmental conditions (TRB, 2000²). Under most circumstances, an emulsion will set in about 1 to 2 hours (TRB, 2000²). ASTM D 3628 contains guidance on selection and use of emulsified asphalt.

Production and Composition

Asphalt and water are emulsified in a colloid mill, whose spinning discs shear the hot asphalt into fine droplets that disperse in water containing the emulsifying agent. Some water can end up inside the asphalt droplets, making a "water-in-oil-in-water," or double, emulsion. Polymers and chemical stabilizers may also be added for the desired properties. Asphalt is normally the largest component, about 40 to 70 percent of the emulsion; water makes up about 30 to 50 percent, and other ingredients such as solvents, polymers and other modifiers up to about 10 percent.

Stability and Breaking

Because asphalt and water do not mix naturally, the droplets in an emulsion always tend to coalesce or to bond to other materials. Paving uses rely on this: once the emulsion is applied, the water leaves and the asphalt attaches to the aggregate or existing surface, which is what happens when a tack coat breaks and sets. Until then the emulsion has to stay stable. Emulsions that may be stored for any time contain emulsifying and stabilizing chemicals that give all the droplets the same electrical charge, positive (cationic) or negative (anionic), so that they repel one another. Cationic emulsions are the most common type in paving.

Classification

Emulsions are graded by how quickly they set, rapid-setting (RS), medium-setting (MS) or slow-setting (SS), and by charge: cationic grades carry a "C" prefix (for example CRS or CSS), while anionic grades have no prefix. An "h" or "s" suffix indicates that a hard or soft base asphalt was used. Some anionic emulsions are designated "HF" (high float); chemicals give the asphalt a thicker film, which stays on aggregate rather than draining away.

Besides the uses listed above, emulsions are used in chip and scrub seals, microsurfacing, and in some processes for producing warm mix asphalt.

@misc{pavint-emulsified-asphalt,
  author = {{Pavement Interactive}},
  title = {Emulsified Asphalt},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/materials/asphalt/emulsified-asphalt/}},
  note = {Accessed [date]}
}