Let's Get Technical — October 2025

Assessment of Aggregate-Binder Adhesion Through Field-applicable Modified Sweep, Vialit, and Pull-off Tests:

This Oregon State University study sought to develop and validate practical test methods for evaluating aggregate–binder adhesion and chip retention in emulsion-based chip seal systems. The goal was to improve construction quality and long-term durability in field applications. As budgets tighten and pavement preservation gains emphasis, chip seals remain a leading strategy for cost-effective maintenance; however, issues such as stone loss and raveling can limit their performance in certain conditions.

Methods:

Three complementary tests were evaluated: a Modified Sweep Test (ASTM D7000) using a battery-powered spin scrubber, a Modified Vialit Test (EN 12272-3), and a Pull-Off Test measuring direct tensile bond strength at 5 °C and 25 °C. The study examined two polymer-modified emulsions — CRS-3P (cationic) and HFRS-2P (anionic) — applied to aggregates from Pendleton and Woodburn, Oregon. Both aggregates were predominantly siliceous with minor calcium and iron content; the Woodburn aggregate was slightly smaller and less flaky, though chemically similar to Pendleton.

Results:

All three test methods demonstrated strong correlation (R² > 0.7) and consistent results across curing conditions. Adhesion improved markedly from 3 to 24 hours of curing, with diminishing gains beyond that period. Failure modes varied by temperature — ohesive within the binder at 25 °C and adhesive at the aggregate interface at 5 °C. The HFRS-2P anionic emulsion exhibited better tolerance to dusty aggregates than the cationic CRS-3P system. Slightly damp aggregates (<3 % moisture) provided the best chip retention, especially with cationic emulsions, while dry or dusty aggregates showed the highest chip loss. Smaller, less flaky aggregates also improved retention regardless of chemistry.

Conclusions:

The modified Sweep and Vialit tests offer rapid, inexpensive, and portable methods for assessing chip seal adhesion in the field. The study recommends establishing practical thresholds — such as a bond strength around 100 kPa and chip loss below 8 % — to support construction quality control and acceptance criteria. Future work will expand to include additional aggregate mineralogies, binder chemistries, and long-term durability studies to strengthen the correlation between adhesion performance and field chip loss.

Baran, S., Sukhija, M., & Coleri, E. (2025). Assessment of aggregate-binder adhesion through field-applicable modified sweep, Vialit, and pull-off Tests. Construction and Building Materials, 490, 142495. https://doi.org/10.1016/j.conbuildmat.2025.142495 

Rethinking Demulsibility: Is It Time to Consider the Breaking Index?

In every technical field, specialized terminology often becomes second nature to those within it — yet remains opaque to outsiders. The asphalt emulsion industry is no different. One term frequently referenced but seldom unpacked is demulsibility. So, what does it mean, and why is it important?  In essence, demulsibility is an indirect measure of emulsifier content of an emulsion, most commonly applied to rapid-setting grades like RS-2 or CRS-2. It offers insight into how quickly an emulsion will “break”— that is, separate into asphalt and water — under controlled conditions.The test method varies depending on whether the emulsion is cationic or anionic. A chemical breaking agent — calcium chloride for anionic emulsions or sodium docusate for cationic — is added to a sample to neutralize the emulsifier. Once neutralized, the emulsion destabilizes, and the degree of separation is recorded as its demulsibility. In practice, higher emulsifier content stabilizes the emulsion, resulting in lower demulsibility. This makes demulsibility a useful proxy for set speed, especially in applications like chip sealing, where early aggregate retention is critical.

Limitations of the Demulsibility Test:

Despite its simplicity, the demulsibility test has notable limitations. It is not applicable to slow- or quick-setting emulsions—such as those used in tack coats, fog seals, or micro surfacing—where specifications often lack any measure of set speed. In these cases, the designer relies on performance metrics and experience to guide formulation. This gap in standardized testing has led to discussion on the applicability of the Breaking Index (BI) test, widely used in Europe.

Introducing the Breaking Index Test:

The Breaking Index test offers a more versatile and realistic approach. Instead of relying on chemical agents tailored to emulsion charge, the BI test uses silica flour, a mineral-based material, to break the emulsion. The amount of silica required to induce breaking is recorded as the emulsion’s Breaking Index—with higher BI values indicating greater emulsion stability. Key advantages of the BI test include:

• Real-world relevance: It uses aggregate mineralogy to simulate actual field conditions.
• Charge independence: Unlike demulsibility, which requires different reagents for cationic and anionic emulsions, the BI test applies uniformly across all emulsion types.
• Broad applicability: It provides meaningful data for slow-set (SS), quick-set (QS), medium-set (MS), and rapid-set (RS) emulsions, including their cationic counterparts.

This opens the door to a broader conversation: Could the Breaking Index test offer a more consistent and practical measure of emulsion set behavior across all grades? 

The Breaking Index(BI) test, standard EN-13057-1, uses typical laboratory equipment: stirrer, balance, funnel, constant temperature bath.  A known mass of emulsion is mixed with filler until the emulsion is completely broken.  Results are reported as the amount of filler necessary to break the emulsion.  The standard requires two replicates vs. the three required for AASHTO T59. BI introduces a surface area component to the breaking of the emulsion which is important to field performance. BI Is faster than demulsibility as it does not require hours in the oven to lose all of the water.  

Bottom line: Breaking Index seems to correlate better with field breaking while demulsibility correlates with formulation stability.  Some data below provided by Ingevity, shows the two tests head-to-head.  The BI data is repeatable and shows some snappiness to emulsions that would not test that way per our current standard.  Perhaps it is time to reconsider this meaningful test.  

New Test Methods for Quality Control of Asphalt Emulsions used in Chip Seal Application

This article is by Alvaro Gutiérrez Muñiz, presented at the 8th International Symposium on Asphalt Emulsion Technology (ISAET '24), introduces a set of innovative rheological testing protocols for asphalt emulsions. These methods are designed to better simulate the real-world conditions encountered during chip seal applications, such as spraying, rolling, and exposure to traffic and temperature extremes. The study emphasizes the importance of controlling process variability to improve the quality and performance of chip seal treatments.

What makes this work particularly important is its shift from traditional empirical specifications, like Saybolt Furol viscosity and particle charge tests, to performance-based criteria using Dynamic Shear Rheometer (DSR) technology. By measuring viscosity at different shear rates and temperatures, the new methods provide a more accurate and predictive understanding of how emulsions behave during application and in service. This is crucial for ensuring proper aggregate retention, minimizing bleeding, and preventing cracking, especially under varying climate and traffic conditions.
The novelty of the test lies in its multi-shear-rate approach. For example, low shear rate viscosity (0.1 s⁻¹) is used to assess storage stability and post-application performance, while high shear rate viscosity (1000 s⁻¹) evaluates pumpability and sprayability. Additionally, the study introduces yield energy as a metric for high-temperature flow resistance and flexibility index for low-temperature brittleness which are absent in conventional testing. These parameters are directly linked to field performance, making the tests more relevant and practical.

This article presents a significant advancement in asphalt emulsion quality control by aligning laboratory testing with field performance requirements. The proposed methods offer a more field-driven framework for evaluating emulsions, helping engineers and manufacturers produce materials that are better suited to the demands of chip seal applications. 

Gutiérrez Muñiz, A. (2024). New Test Methods for Quality Control of Asphalt Emulsionsused in Chip Seal Application. Presented at the 8th International Symposium on Asphalt Emulsion Technology (ISAET '24), Washington D.C., November 4–6, 2024. Asphalt Emulsion Manufacturers Association (AEMA) and International Bitumen Emulsion Federation (IBEF). Retrieved from https://www.ibef.net/wp-content/uploads/2025/01/nov._5__session_2__-_p5_-_al.pdf

Characterizing Stability of Asphalt Emulsions Using Electrokinetic Techniques

This study discusses an alternative approach to evaluating the stability of asphalt emulsions using electrokinetic techniques, specifically focusing on electrophoretic mobility and zeta potential. Traditional established methods like demulsibility tests are often qualitative and limited in their ability to predict field performance. The authors propose that electrokinetic measurements can offer a more quantitative and sensitive assessment of emulsion stability by analyzing the behavior of charged asphalt particles in an electric field. These measurements reflect the interaction between asphalt droplets and the aqueous phase, which is critical to understanding how emulsions break and cure.

The researchers conducted experiments on various cationic emulsions, examining how factors like dilution, pH, and ionic concentration affect their electrokinetic properties. They found that emulsions with higher zeta potential values exhibited greater stability, while those with lower values were more prone to breaking. The study also demonstrated that electrophoretic mobility could be used to track changes in emulsion behavior over time, offering insights into how emulsions age and respond to environmental conditions. This method proved to be reproducible and sensitive to formulation changes, making it a promising tool for both research and quality control.

Ultimately, the article concludes that electrokinetic techniques can serve as a valuable complement or alternative to conventional emulsion tests that may not offer the same field performance predictability such as the standard demulsibility tests. By providing a deeper understanding of the physicochemical interactions within emulsions, these methods could help formulators design more robust products and enable agencies to better predict field performance. The authors suggest further research to refine the technique and explore its application across a broader range of emulsion types and field conditions.

Banerjee, A., Bhasin, A., & Prozzi, J. Characterizing Stability of Asphalt Emulsions Using Electrokinetic Techniques. Journal of Materials in Civil Engineering, 25(1), 78–85. https://doi.org/10.1061/(ASCE)MT.1943-5533.0000552

Asphalt Emulsion Specifications & Testing Methods

In North America, asphalt emulsion specifications primarily follow ASTM and AASHTO standards that address both the as-received emulsion and the recovered residue. ASTM D977 (AASHTO M 140) covers anionic emulsions, while ASTM D2397 (AASHTO M 208) covers cationic emulsions. These specifications include grade classifications and reference key testing methods used for quality control, performance, and verification of consistency across production batches (ASTM, 2024a, 2024b).

Routine quality control is based on ASTM D244 (AASHTO T 59), which measures water content, residue by distillation, Saybolt Furol viscosity, particle charge, demulsibility, settlement, and sieve test results. Additional test methods such as ASTM D6930 (storage stability), ASTM D6933 (oversized particles), and rotational viscosity testing (ASTM D7226/D7496) are commonly used by agencies and producers (ASTM, 2023a, 2023b, 2022). Residue testing then characterizes binder properties through penetration, softening point, and rheological analyses.

Ongoing Research: Testing the Emulsion Itself

Traditional specifications focus on the recovered residue, but current research emphasizes characterizing the emulsion itself. Three areas are gaining traction:

1. Particle Size Distribution (PSD): Laser diffraction and dynamic light scattering (DLS) are increasingly used to measure droplet size and distribution. PSD directly affects emulsion stability, pumpability, and breaking behavior. Narrow distributions indicate better stability and uniform field performance. Researchers have proposed draft specifications defining D10/D50/D90 ranges to correlate particle uniformity with storage stability and coating behavior (Diaz Romero, 2022; Kiihnl, 2020; Turben, 2024; Braham, 2024).

2. Zeta Potential: Quantitative charge measurement (zeta potential) helps explain emulsion stability and aggregate adhesion mechanisms. This test goes beyond simple “cationic vs. anionic” classification and offers a measurable indicator of electrostatic stability and coating potential. Recent work has linked zeta potential trends to aggregate type, water chemistry, and polymer modifiers (Miller, Hensley, & Rushing, 2025; Liu & Zhang, 2020; Airey, Rahman, & Collop, 2003).

3. Emulsion Rheology: Rotational viscometry at low shear rates measures flow characteristics and correlates to handling, storage, and mix time performance—offering a more realistic picture than Saybolt Furol viscosity alone. Advanced rheological profiles are being used to relate emulsion flow properties to workability and spray performance across temperature ranges, particularly in polymer-modified systems (Chen, Xiao, & Putman, 2022; FHWA, 2021; TRB, 2023; Braham, 2019).

These emerging testing approaches are helping bridge the gap between laboratory specifications and real-world performance. While ASTM and AASHTO specifications continue to rely on residue-based acceptance, direct emulsion characterization provides valuable insights into formulation stability, compatibility with aggregates, and long-term durability—especially for polymer-modified and specialty emulsions (Braham, 2019, 2024).

References

• Airey, G. D., Rahman, M. T., & Collop, A. C. (2003). Study of the electrical properties of cationic bitumen emulsions. Advances in Colloid and Interface Science, 105(1–3), 83–106. https://doi.org/10.1016/S0001-8686(03)00144-1
• American Society for Testing and Materials. (2024a). ASTM D977–24: Standard specification for emulsified asphalt (anionic). ASTM International.
• American Society for Testing and Materials. (2024b). ASTM D2397–24: Standard specification for cationic emulsified asphalt. ASTM International.
• American Society for Testing and Materials. (2023a). ASTM D244–23: Standard test methods for emulsified asphalts. ASTM International.
• American Society for Testing and Materials. (2023b). ASTM D6930–23: Standard test method for settlement and storage stability of emulsified asphalt. ASTM International.
• American Society for Testing and Materials. (2023c). ASTM D6933–23: Standard test method for oversize particles in emulsified asphalt. ASTM International.
• American Society for Testing and Materials. (2022). ASTM D7226 / D7496–22: Rotational viscosity of emulsified asphalts. ASTM International.
• Braham, A. F. (2019, April 24). Building better micro surfacing and slurry seals [Webinar slides]. Transportation Research Board.
• Braham, A. F. (2024, April 10). Asphalt emulsions in flexible pavement preservation: Part 1 [Workshop presentation]. South-Central Pavement Technology Center, University of Arkansas.
• Chen, Y., Xiao, F., & Putman, B. (2022). Factors influencing the droplet size of asphalt emulsion during fabrication. Coatings, 12(5), 575. https://doi.org/10.3390/coatings12050575
• Diaz Romero, P. L. (2022). Refining particle size specification for asphalt emulsion (Master’s thesis, University of Arkansas). University of Arkansas Institutional Repository. https://scholarworks.uark.edu/etd/6000
• Federal Highway Administration (FHWA). (2021). TechBrief: Advances in asphalt emulsion characterization and performance testing. Publication No. FHWA-HIF-21-028. U.S. Department of Transportation.
• Kiihnl, L. P. (2020). Best practices for asphalt emulsion particle size analyses using a Coulter counter (Master’s thesis, University of Arkansas). University of Arkansas Institutional Repository. https://scholarworks.uark.edu/etd/3657/
• Liu, C., & Zhang, J. (2020). ζ potential as a measure of asphalt emulsion stability. Energy & Fuels, 34(12), 15896–15907. https://doi.org/10.1021/acs.energyfuels.9b03565
• Miller, C., Hensley, M., & Rushing, T. (2025). Probing the stability of emulsified asphalts: A dual analysis of zeta potential. Fuel, 370, 132103. https://doi.org/10.1016/j.fuel.2025.132103
• Transportation Research Board (TRB). (2023). E-Circular 291: Asphalt emulsion technology and field performance update. Transportation Research Board of the National Academies.
  Turben, T. (2024). Impact of equipment type on measured particle size of civil engineering materials and draft procedure for asphalt emulsion PSA (Master’s thesis, University of Arkansas). University of Arkansas Institutional Repository. https://scholarworks.uark.edu/etd/5489/
• Wang, H., Zhang, W., Liu, X., & Li, Z. (2011). Experimental study on the stability of asphalt emulsion for CA mortar by laser diffraction technique. Construction and Building Materials, 25(10), 4152–4159. https://doi.org/10.1016/j.conbuildmat.2011.04.065
• Zhang, X., Hu, W., & Han, R. (2022). Effects of oil/asphalt emulsion formulation on particle size and stability. Transportation Research Record, 2676(5), 512–523. https://doi.org/10.1177/03611981221100512