The question of how particle size influences the quality of aggregates and asphalt is the focus of a new paper in the journal Construction and Building Materials. Scientists from the University of Arkansas’s Department of Civil Engineering have conducted the research.
Study: Refining particle size specification for asphalt emulsion.Image Credit: Andrew Ostry/Shutterstock.com
Asphalt Emulsions
Asphalt emulsions have been in use for a century, with Hugh Alan Mackay’s 1922 patent considered to be the start of the technology. At first, only anionic emulsions were developed for commercial use until the introduction of cationic emulsions in the 1950s. Anionic emulsions were limited to soil stability applications.
Cationic emulsions possess benefits including better breaking behavior and compatibility with aggregates. These emulsions have largely replaced anionic variants, and by the 1980s they dominated the market. Improved performance and a shorter return to traffic are made possible using cationic emulsions.
By the mid-1980s, research produced polymer binders, which further improved the performance of asphalt emulsions. Superior properties compared to unmodified emulsions provide improved aggregate retention and rutting and bleeding resistance. Three million tons of emulsion were produced in the US in 2018, accounting for 5-10% of total asphalt binder consumption.
Surface treatments are the main use of asphalt emulsions, but they are used in a wide range of applications for pavement preservation and remediation. The use of asphalt emulsions significantly improves the quality, durability, and lifespan of pavements.
Another use for emulsions is in sprays to seal chips in pavements. Best practices can extend the life of a pavement by up to 7 years without the need for extensive maintenance. The use of emulsion sprays to seal chips improves skid resistance, renews pavements that become aged, and helps return the pavement to a suitable condition for traffic with a minimum interruption to traffic flow.
Asphalt Emulsion Quality Control and Assurance: Current Perspectives
Several quality control and quality assurance tests are currently employed to evaluate the quality of asphalt emulsions. Emulsion tests are separated into four categories: classification, handling properties, residue properties, and performance tests. Performance tests are conducted with aggregates incorporated into the asphalt.
Several improvements can be made in asphalt emulsion quality control tests. One such improvement is in relation to surface dressing requirements, taking into account properties such as viscosity, adhesion, breaking, and stability.
Studies have demonstrated an intimate relationship between material parameters such as particle size, asphalt binder type and content, manufacturing temperature, thixotropic behavior, and technical parameters during manufacturing such as rotor speed. These parameters can be controlled during the manufacture of asphalt emulsion.
Study Objectives
The new paper correlates particle size metrics to the rheological behavior of cationic asphalt emulsions. Aggregates are incorporated to improve the performance of the materials. Several studies in the current literature body have revealed the influence of particle size on emulsion rheological behavior. For instance, increasing particle size can have a negative effect on emulsion stability.
The main stated objectives of the study are twofold. Firstly, the paper links emulsion quality control and performance tests to particle size analysis. Secondly, a draft specification for asphalt emulsion particle size analysis is revised. For the second objective, the authors have employed a laser diffraction technique. Three commercially available cationic asphalt emulsions were evaluated in the paper.
Study Findings and Conclusions
The study has highlighted some key conclusions about the relationship between particle size and asphalt emulsion quality. The size of particles in asphalt emulsions is as important as gradation in aggregates. This key material parameter influences rheology and drives stability.
Out of the three commercially available cationic asphalt emulsions (CRS-2, CRS-2L, and CRS-2P) the best correlation between particle size and viscosity was observed in CRS-2, an unmodified emulsion. However, research has indicated that the incorporation of modifiers such as polymers and latex improves the rheological parameters and storage stability of emulsions.
The aggregate retention of CRS-2P was superior to the other two emulsions, and due to the larger particle size and distribution, the authors concluded that smaller particle sizes and narrower distributions do not necessarily mean that the quality of emulsions is better.
There is a disconnect between particle size and oversized particle tests in all three emulsions evaluated in the research. However, the authors have revealed that the optimal mean particle size for CRS-2 is 1.9 µm, CRS-2P is 10.7 µm and for CRS-2L the optimal size is 3.0 µm. Potential difficulties with correlating particle size to real-life applications are due to the specific storage conditions, which may not reflect those in other environments or facilities.
Correlation between particle size, storage time, quality control tests, and performance was amply demonstrated in the research. The authors have identified that this is a potential knowledge gap that could be explored in the future that would better correlate the relevant material parameters and standardized quality control tests.
Finally, research gaps in data acquisition and sample preparation for analysis of particle sizes were identified by the authors. Filling this gap should be a focus of future research in the field of asphalt emulsion manufacture.
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Further Reading
Diaz-Romero, P.L & Braham, A.F. (2022) Refining particle size specification for asphalt emulsion Construction and Building Materials 350 128812 [online] sciencedirect.com. Available at:
https://www.sciencedirect.com/science/article/pii/S0950061822024692
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