Guideline for the Design and Use of Asphalt Pavements for ...

LIQUID ASPHALT
Cutback Asphalt
Cutback asphalts are liquid asphalts which are manufactured by adding (cutting back) petroleum solvents
(also called cutter stock or diluents) to asphalt cements. They are made to reduce the asphalt viscosity for
lower application temperatures. Application to aggregate or pavement causes the solvent to escape by
evaporation, thus leaving the asphalt cement residue on the surface. Based on the relative rate of
evaporation, cutback asphalts are classified into three types: Rapid Cure (RC), Medium Cure (MC) and
Slow Cure (SC).
The type of distillate (solvent) used in their production determines the grade of the cutback asphalt. Rapid
Cure grades are typically blended with light, highly volatile diluents, such as naphtha, that will evaporate
quickly and leave a hard, viscous-base asphalt to function with the aggregate on the road. Medium Cure
grades are “cutback” with a less volatile kerosene-type of solvent which evaporates more slowly leaving a
base asphalt of medium hardness or viscosity. Slow-Curing blends contain a low-volatility fuel-oil type
solvent and thus require the longest curing period. They leave a soft, low-viscosity asphalt on the aggregate.
Slow Cure grades are not used as much as the Rapid Cure and Medium Cure grades as it typically takes
many months for these heavy diluents to evaporate and cure. The various sub-grades within a grade of
cutback asphalts are determined by the amount of solvent used in their production.
Some of the types and characteristics of cutback asphalts are also noted in Table 1-1. The table also lists
some of the available grades of cutbacks.
The use of cutback asphalt and its availability has been greatly reduced due to environmental restrictions
(solvents evaporating into the atmosphere, causing air pollution).
Emulsified Asphalt
An asphalt emulsion is produced by combining asphalt cement, water and an emulsifying agent through a
high-shear device called a colloid mill. Hot AC is pumped into the colloid mill where it is sheared into very
small droplets, typically within the range of one to ten microns in diameter. Simultaneously, an emulsifying
agent, dispersed in water, is pumped into the colloid mill. The emulsifying agent is both hydrophilic (likes
water) and lipophilic (likes oil/asphalt). The lipophilic part attaches itself to the asphalt particle and the
hydrophilic side suspends these particles in the water medium. From the colloid mill, the asphalt emulsion is
then directed to its respective storage tank. The asphalt cement makes up from 55 to 70 percent of the
emulsion. Most emulsions are made with asphalt in the 100-250 penetration range.
As usage and grade dictate, asphalt emulsions may contain additional stabilizers, coating enhancers, antistripping
agents, solvents and break control agents that can be either co-milled during production or postadded
into the emulsion.
There are three categories of emulsified asphalt, Anionic, Cationic and Nonionic. The emulsifying agents
impart minute electrical charges on the emulsion droplets. If the droplet charge is negative, the emulsion is
Anionic. A positively charged emulsion is a Cationic emulsion. Nonionic emulsions are neutral, and their
droplets have no electrical charge. Most asphalt emulsions used in the paving industry are either Anionic or
Cationic. Nonionic emulsions are seldom used.