Specialty Emulsions
Composition of Asphalt Emulsions
An asphalt emulsion consists of three basic ingredients: asphalt, water, and an emulsifying agent. On some occasions the emulsifying agent may contain a stabilizer.
It is well known that water and asphalt will not mix, except under carefully controlled conditions using highly specialized equipment and chemical additives. The blending of asphalt cement and water is somewhat akin to an auto mechanic trying to wash grease from his hands with water only. It is not until a detergent or soapy agent of some type is used that grease can be successfully removed. The soap particles surround the globules of grease, break the surface tension that holds them, and allow them to be washed away. Some of the same physical and chemical principles apply in the formulation, production, and use of asphalt emulsion.
The object is to make a dispersion of the asphalt cement in water, stable enough for pumping, prolonged storage, and mixing. Furthermore, the emulsion should break down quickly after contact with aggregate in a mixer, or after spraying on the roadbed. Upon curing, the residual asphalt retains all of the adhesion, durability, and water-resistance of the asphalt cement from which it was produced.
Classification
Asphalt emulsions are divided into three categories: anionic, cationic and nonionic. In practice, the first two types are ordinarily used in roadway construction and maintenance. Nonionics, however, may be more widely used as emulsion technology advances. The anionic and cationic classes refer to the electrical charges surrounding the asphalt particles. This identification system stems from one of the basic laws of electricity-like charges repel one another and unlike charges attract. When two poles (an anode and cathode) are immersed in a liquid and an electric current is passed through, the anode becomes positively charged and the cathode becomes negatively charged. If a current is passed through an emulsion containing negatively charged particles of asphalt they will migrate to the anode. Hence, the emulsion is referred to as anionic. Conversely, positively charged asphalt particles will move to the cathode and the emulsion is known as cationic. With nonionic emulsions, the asphalt particles are neutral and therefore do not migrate to either pole.
Emulsions are further classified on the basis of how quickly the asphalt will coalesce; i.e., revert to asphalt cement. The terms RS, MS, and SS have been adopted to simplify and standardize this classification. They are relative terms only and mean rapid-setting, medium-setting, and slow-setting. The tendency to coalesce is closely related to the mixing of an emulsion. An RS emulsion has little or no ability to mix with an aggregate, an MS emulsion is expected to mix with coarse but not fine aggregate, and an SS emulsion is designed to mix with fine aggregate.
The emulsions are further subdivided by a series of numbers related to viscosity of the emulsions and hardness of the base asphalt cements. The letter "C" in front of the emulsion type denotes cationic. The absence of the "C" denotes anionic or nonionic. For example, RS-1 is anionic or nonionic and CRS-1 is cationic.
Four grades of high-float medium-setting anionic emulsions, designated HFMS, have been added to standard American Society for Testing and Materials (ASTM) specifications. These grades are used primarily in cold and hot plant mixes, coarse aggregate seal coats, and road mixes. High float emulsions have a specific quality that permits a thicker film coating without danger of runoff.
A quick-set type of emulsion (QS) has been developed for slurry seals. Its use is rapidly increasing as the unique quick-setting property solves one of the major problems associated with the use of slurry seals.
Standard specifications for quick-set emulsions are under development. Additionally, some emulsions are made with the water dispersed in asphalt, usually a cutback. As these so-called "inverted emulsions" are seldom used, they are not discussed in this manual.
Specifications
ASTM and the American Association of State Highway and Transportation Officials (AASHTO) have developed standard specifications for the following grades of emulsions:
Grades of Emulsions
| EMULSIFIED ASPHALT | CATIONIC EMULSIFIED ASPHALT |
| RS-1 | CRS-1 |
| RS-2 | CRS-2 |
| MS-1 | --- |
| MS-2 | CMS-2 |
| MS-2h | CMS-2h |
| HFMS-1 | --- |
| HFMS-2 | --- |
| HFMS-2h | --- |
| HFMS-2s | --- |
| SS-1 | CSS-1 |
| SS-1h | CSS-1h |
The "h" that follows certain grades simply means that a harder base asphalt is used. The "HF" preceding some of the MS grades indicates high-float, as measured by the Float Test (ASTM D 139 or AASHTO 50). High-float emulsions have a quality, imparted by the addition of certain chemicals, that permits a thicker asphalt film on the aggregate particles with minimum probability of drainage. Some user agencies specify an additional cationic sand-mixing grade designated CMS-2s, which contains more solvent that other cationic grades. All grades in this lengthy list of emulsions may not be stocked by most producers. Communication and planning between user and producer helps facilitate service and supply of a given grade.
The specifications for emulsified asphalts (ASTM D 977 and AASHTO M 140) make no mention of a solvent in the emulsion. CRS- and CMS- cationic emulsion specifications (ASTM D 2397, AASHTO M 208), on the other hand, permit solvent but restrict the amount.
General Uses of Emulsified Asphalts are given in Table V-1, Chapter V. Standard specifications for emulsified asphalt carry ASTM Designations D 977 and D 2397 and AASHTO Designations M 140 and M 208. For convenience, the basic requirements of these specifications are given in Tables II-1 and II-2.
Variables Affecting Asphalt Emulsion
There are many factors that affect the production, storage, use, and performance of an asphalt emulsion. It would be hard to single out any one as being that most significant. But, among the variables having a significant effect are:
1. Chemical properties of the base asphalt cement
2. Hardness and quantity of the base asphalt cement
3. Asphalt particle size in the emulsion
4. Type and concentration of the emulsifying agent
5. Manufacturing conditions such as temperatures, pressures, and shear
6. The ionic charge on the emulsion particles
7. The order of addition of the ingredients
8. The type of equipment used in manufacturing the emulsion
9. The property of the emulsifying agent
10. The addition of chemical modifiers.
The above factors can be varied to suit the available aggregates or to suit construction conditions. It is always advisable to consult the emulsion supplier with respect to a particular asphalt-aggregate combination as there are few absolute rules that will work the same under all conditions.
An examination of the three main constituents-asphalt, water, and emulsifier-is essential to an understanding of why asphalt emulsions work as they do.