Road Maintenance Solutions from One Source

by Glynn Holleran; and Jeffrey R. Reed, President, VSS, West Sacramento, Calif, USA

When working with a less-than-optimum asphalt, modifications can make the mix perfect for the slurry process 

As slurry surfacing becomes more popular in developing countries, issues of asphalt quality and its effect on the performance of slurry surfacing emulsions and mixes gets more attention. Often mix designers have little or no choice in the asphalt source. Most refineries run a wide slate of crudes, and processing methods vary. The variables of sources, types of crudes and processing methods affect the asphalt chemistry and, potentially, the emulsability.

This article briefly reviews aspects of asphalt chemistry and refining that contractors need to take into account when selecting asphalt material. In will also show measures contractors can take to improve the performance of poorer asphalt sources in slurry surfacing emulsions and mixes.

Review of asphalt chemistry

When a designer introduces a dispersing media, such as a high molecular weight aromatic oil, to asphalt, emulsability can improve. But, asphalt with high aromatic oil content may allow excessive solubility in the organic phase. This means the emulsifier would be depleted over time. Modifying the asphalt by air rectification can adjust composition to overcome this. If the asphalt has high paraffinic levels, it will be more difficult to emulsify, producing a slower cure. This may be adjusted with aromatic polar materials, such as carboxylic and napthenic acids.

Examples show some options

Oxidized base pitch
Researchers began with an experiment on oxidized base pitch. This asphalt was a blown pitch and showed higher oil contents than normal. The asphaltene content was high but acceptable. The emulsion passed most standard properties but had a high shear susceptibility. Storage stability degraded with time.
Manufacturing parameters were acceptable for the initial emulsions, but the emulsion became coarse. Increasing the emulsifier content did improve the basic storage stability, but shear susceptibility remained.
The emulsifier type was changed and a co-emulsifier used. This allowed an increase in stability, probably due to an increase in packing at the asphalt/water interface. As may be seen for the slurry mix, the cohesion was higher in the stable systems.

Straight run heavy crude
In the next situation, an asphalt from a heavy crude was used. The standard slurry emulsion was acceptable and gave good results. However, theMicro-surfacing systems with immadazoline emulsifiers were a problem, probably due to emulsifier compatibility.
The composition showed a high asphaltene content and low resin and oil contents. Two approaches were taken to modify the asphalt/emulsion: First, an aromatic oil was added to the asphalt - composition adjustment. Next the emulsifier was added to the asphalt - dispersion of asphaltenes. The result in both cases was a significant improvement in stability and a significant improvement in cohesion, wet stripping and wet track abrasion resistance in the final Micro-surfacing.

Straight run waxy crude
In the third situation, researchers experimented with straight run waxy crudes. The composition and experiment indicated no problems with the emulsion; however, the emulsion did have some trouble setting. Cohesion build-up and finished properties, such as displacement at higher temperatures, also experienced problems.
Carboxylic acid increased the break/cure rate and polymers improved the high and low temperature properties.

In the end, researchers learned that the complex chemistry of asphalt causes emulsion properties to vary. This variance can adversely affect the performance and application of slurry and Micro-surfacing properties. By addressing the chemical issues by emulsifier, additive or asphalt modification can allow even substandard asphalt to be used satisfactorily. 

Store Emulsions Safely Having emulsions on hand means storing them correctly to ensure they aren't contaminated or their structure compromised. As the Asphalt Institute points out, failure to follow safeguards will result in an unsatisfactory material when it's time to perform a job. The following chart lists the dos and don'ts of storing asphalt emulsions.
Storage of asphalt emulsions
Always be sure... to store the asphalt emulsion as you would fluid water, at a temperature between 50 and 185° F (10 and 85° C), depending on the use. to use hot water as the heating medium for storage tanks with heating coils. Low pressure or waste steam also may be used, with temperature controlled on the coil surface to not more than 185° F (85° C). to store at the temperature specified for the particular grade. For spray applications, the emulsions are stored at higher temperatures than for mixing with aggregate. For example, the higher viscosity rapid-set spray grades are stored at 125 to 185° F (50 to 85° C) since they are usually applied in this temperature range. The lower viscosity grades are stored at lower temperatures. that storage tanks are insulated. to use tall, vertical tanks that expose the least amount of surface area to the air, reducing the amount of "skinning" that occurs on the surface of the emulsion. to provide cathodic protection to avoid possible corrosion of tank walls and heating coils.	Never... permit the emulsified asphalt to be heated above 185° F (85° C). Elevated temperatures evaporate the water, resulting in an increase in viscosity and an asphalt layer in the tank. The materials can no longer be used as intended and it will be difficult to empty the tank. let the emulsion freeze. This breaks the emulsion, separating the asphalt from the water. The result will be two layers in the tank, neither suited for the intended use, and the tank will be difficult to clean. allow the temperature of the heating surface to exceed 212° F (100° C). This will cause premature breakdown of the emulsion on the heating surface. use forced air to agitate the emulsion. It may cause the emulsion to break. overmix the emulsion. Using large diameter, slow-turning propellers, about 3 feet (1 m) up from the tank bottom, can prevent a skin from forming on top of the emulsion. overpump the emulsion. Tanks may be circulated top to bottom with a pump, but this requires caution.
Suggested storage temperatures  for emulsified asphalts
Grade Temperature degrees F/degrees C Minimum Maximum RS-1 70/20 140/60 RS-2, CRS-1, CRS-2 125/50 185/85 SS-1, SS-1h, CSS-1, CSS-1h,  MS-1, HFMS-1 50/10 140/60 CMS-2, CMS-2h, MS-2, MS-2h,  HFMS-2, HFMS-2s 125/50 185/85 Information for this side bar was compiled from "A Basic Asphalt Emulsion Manual," Manual Series No. 19 from the Asphalt Institute, Lexington, KY.