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METHOD OF IMPROVEMENT OF SLURRY SURFACING PERFORMANCE G. Holleran, Vice President, Valley Slurry Seal Company, USA Jeffrey R. Reed, President, Valley Slurry Seal Company, USA Back to VSS Technology Papers Library ABSTRACT: 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 becomes of greater importance. Often there is little or no choice on the asphalt source. Most refineries run a wide slate of crudes, and processing methods vary. All of these variables affect the asphalt chemistry and potentially the emulsability. This paper reviews aspects of asphalt chemistry and refining that need to be taken into account in asphalt selection, and measures that can be taken to improve the performance of poorer asphalt sources in slurry surfacing emulsions and mixes. 1. BASICS OF ASPHALT CHEMISTRY 1.1 Asphalt Is Mixture Asphalt is a mixture of a range of hydrocarbon materials. It is the chemical composition of asphalt that determines its properties in whatever end use that is chosen. This is because asphalt is made up of polar and non polar compounds in complex association. The interaction of polar compounds determines asphalt structure and mechanical properties. The chemistry of the asphalt produced depends on two main parameters: the crude source and the manufacturing process. Crudes are classified as heavy, medium or light, depending on the level of bituminous materials. 1.2 Elemental and Molecular Composition Asphalt is a derivative of organic materials produced in processes that have spanned millions of years. They are not a single chemical species but a complex mixture of organic molecules that vary widely in composition from non polar saturated hydrocarbons to highly polar, highly condensed ring systems. They also contain trace amounts of metals, mainly vanadium and nickel. Although all asphalts are predominantly carbon and hydrogen most of the molecules contain at least one hetero (S,N,O) atom. Because the hetero atoms often impart functionality and polarity, they have a disproportionate effect on the properties. Correlation with discrete chemical groups are close to impossible due to the huge diversity. For this reason, asphalt is usually considered as consisting of fractions. This is usually based on polarity and there are several ways of expressing this. In SHRP, it is associated and non associated phases or more traditionally asphaltenes, resin and oils. This can be used to conceptualize the systems as either sol/gel systems or a continuum of species from non polar to polar. This can be represented on a single diagram with an "ideal area" defined for the crudes used. 2. MANUFACTURING METHODS AND THEIR EFFECTS Five primary methods are used to manufacture asphalt.
The method used will thus have an impact on the emulsability, and the chemical interaction that the asphalt may have with a given stone surface. 3. ASPHALT TYPE AND CHEMISTRY AND THE SLURRY SYSTEM 3.1 Emulsification and Emulsifiers For a slurry system, the asphalt chemistry must be compatible with that of the asphalt and the aggregate. 3.2 Methods Of Adjustment 3.2.1 Manufacturing Method Asphalts that form unstable emulsions become unstable by the processes of flocculation and coalescence. That is, the particles come together and form larger particles. Sedimentation has a direct relationship to particle size (inverse square) thus finer particles are more stable. Further, they form films on aggregate more easily, allowing better adhesion and more rapid build up in cohesion. A high shear mill system, lower viscosity binder, addition of kerosene or other diluent will achieve this end. 3.2.2 Emulsifier Emulsifiers which give a good balance of solubility in both phases and which can pack densely at the asphalt water interface will give more stable emulsions. This may require the asphalt to be chemically altered, or methods such as doping of the asphalt with emulsifier to be used. As the slurry surfacing characteristics are very dependant on the emulsifier type, then it must be chosen often from a limited range and adjusted with co-emulsifiers to achieve the desired results. 3.2.3 Asphalt Modification |
 Figure 1 Asphalt Compositions |
Manufacturing parameters were acceptable for the initial emulsions, but the emulsion coarsened. Increasing the emulsifier content did improve basic storage stability but the shear susceptibility remained.
The asphaltic material was the only one, so emulsifier type was changed and a co-emulsifier used. This allowed an increase in stability probably by 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. Traffic times were similar. (figures 2,3,4,5,6)
 Figure 2 Stability Of Blown pitch emulsions |
 Figure 3 Shear Susceptibility in blown pitch emulsions |
 Figure 4 Field Performance |
 Figure 5 Wet Track Abrasion Mixes blown pitch |
 Figure 6 Loaded wheel tests Mixes blown pitch |
4.1.1 Straight Run Heavy Crude
In this situation,the standard slurry emulsion was acceptable and gave good results. However the microsurfacing systems with immadazoline emulsifiers were a problem. Probably due to emulsifier compatibility.
The composition, (figure 1) showed a high asphaltene content and lower resin and oil. Two approaches were taken: aromatic oil was added to the asphalt (composition adjustment) and part of 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 microsurfacing.(figures 4, 7, 8,9, 10)
 Figure 7 Stability Heavy Crude emulsions(stripping test) |
 Figure 8 Shear Susceptibility heavy crude emulsions |
 Figure 9 Cohesion build in Mixes - Heavy Crude Base |
 Figure 10 Wet Track Abrasion Tests Mixes Heavy Base |
4.1.2 Straight Run Waxy Crude
The composition and experiment indicated no problems with the emulsion.(figure 1,11) However, problems were experienced with some of the emulsion setting, cohesion build up and finished properties such as WTAT and displacement at higher temperatures. Carboxylic acid increased break/cure rate and polymers improved high and low temperature properties. (figures 4,12,13,14)
 Figure 11 Stability of Waxy Base emulsions |
 Figure 12 Cohesion Build Mixes with Waxy Base |
 Figure 13 Wet Track Abrasion Mixes Waxy Base |
 Figure 14 LWT Mixes Waxy Base |
4.1.3 PPA (PD tar) blended asphalt
PPA is high in resins (figure 1) this allows the emulsifier to migrate into the binder over time, reducing stability. (figure 15)
 Figure 15 Stability PDA base Emulsions |
Slurry made with this material was generally acceptable (figure 4) however some thermal susceptibility was noticed under hot conditions.
A peptising agent in the asphalt was tried but was not successful, blending with other refinery fractions did work. (figure 4, 16,17)
 Figure 16 Wet Track Abrasion Mixes PDA Base |
 Figure 17 LWT Mixes PDA Base |
5. CONCLUSIONS
- Asphalt chemistry is complex and varies from crude to crude.
- Emulsion properties vary with this chemistry.
- Slurry and microsurfacing properties can be adversely affected in a performance and application sense.
- Addressing the chemical issues by emulsifier, additives or asphalt modification can allow even poor asphalts to be used satisfactorily.
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