Let's Get Technical — August 2025

Throwback Tech: Revisiting the Roads Less Traveled

In this edition of Let’s Get Technical, we decided to take it back in time.   Please enjoy these blasts from the past that in some cases remain relevant today.  If you make it to the last review, inquiring minds would like to know if there is any update from Oregon’s stance on rubber modified chip seals.

A General Method of Design for Seal Coats and Surface Treatments:

Norman McLeod’s A General Method of Design for Seal Coats and Surface Treatments laid the groundwork for how engineers design and apply protective layers to roads. These treatments extend pavement life, improve safety, and reduce maintenance costs. McLeod’s method introduced a structured way to calculate how much stone and asphalt binder should be used based on factors like traffic volume, aggregate shape, and surface texture. His approach was one of the first to combine practical field experience with mathematical design.

One of the most important contributions of the paper is its focus on aggregate quality and shape. McLeod emphasized that the size and geometry of the stone used in surface treatments directly affect how well the treatment performs. He introduced the concept of the Average Least Dimension (ALD) to help engineers select aggregates that would embed properly in the binder and resist dislodging under traffic. This was a major step forward in improving the durability and effectiveness of surface treatments.

McLeod also developed a formula for determining the correct amount of asphalt binder, needed to hold the aggregate in place. His equation accounts for traffic conditions, surface roughness, and the absorption characteristics of the pavement and aggregate. This allowed engineers to tailor treatments to specific environments, reducing waste and improving performance. He also provided practical advice on construction techniques, such as rolling procedures and traffic control, which are still relevant today.

For roads requiring multiple layers of treatment, McLeod also explored multi-layer designs, adjusting binder and aggregate quantities for each layer. This approach is especially useful for high-traffic roads or those in harsh climates. His recommendations for seasonal timing and binder selection based on temperature remain valuable for modern engineers working in diverse conditions.

Even decades later, McLeod’s paper is relevant in pavement engineering. Its blend of theory and field-tested practice continues to influence modern design standards and specifications. As infrastructure ages and budgets tighten, the need for cost-effective, long-lasting surface treatments is more critical than ever, making his work important and relevant today.

McLeod, Norman W. "A General Method of Design for Seal Coats and Surface Treatments." Proceedings of the Association of Asphalt Paving Technologists, vol.38, 1969, pp. 537–628.

Report on Laboratory Generation and Evaluation of Paving Asphalt Fumes (Kriech, Wissel, et al., 1999):

The 1999 study by Kriech, Wissel, and colleagues, published in the Transportation Research Record, set out to answer an important question: could asphalt paving fumes be recreated in the laboratory in a way that truly represented what workers experience in the field. By creating a controlled fume generation system, the researchers were able to compare lab-generated samples with emissions collected during real paving projects.

Their results showed that the laboratory method closely matched the chemical makeup of field fumes, especially when looking at polycyclic aromatic compounds and how vapors and particulates were distributed. This confirmed that the lab setup could be trusted as a stand-in for field studies, an important step in assessing worker exposure to asphalt fumes. Another key finding was that the concentrations of harmful compounds, such as carcinogenic PACs, were relatively low compared with other industrial emissions. This helped form a scientific basis for later occupational health guidelines and exposure limits.

Since its publication, the paper has been cited in occupational hygiene studies, NIOSH evaluations, and toxicological reviews. It provided the groundwork for later research into worker exposure, especially in comparing field and laboratory conditions. Even today, it remains relevant as new paving technologies are introduced. Warm-mix asphalt and asphalt emulsions operate at lower temperatures, producing significantly fewer fumes, and this paper is often referenced to highlight the advantages of these methods over traditional hot-mix paving.

In short, the work of Kriech and Wissel continues to stand as a cornerstone in the field. It demonstrated that laboratory studies could meaningfully reflect real-world paving conditions and gave researchers and regulators a dependable framework for evaluating asphalt fume exposure—knowledge that is still applied in today’s discussions of worker safety and the benefits of emulsion-based paving.

Citation: Kurek, J. T., Kriech, A. J., Wissel, H. L., Osborn, L. V., & Blackburn, G. R. (1999). Laboratory Generation and Evaluation of Paving Asphalt Fumes. Transportation Research Record: Journal of the Transportation Research Board, 1661, 35–40. https://doi.org/10.3141/1661-06

Summary of Evaluation of Rubber-Asphalt Chip Seals in Oregon (1982, ODOT):

• Key Takeaway: Rubber-asphalt chip seals in Oregon (1977–1981) did not justify their higher cost, offering little to no long-term performance advantage over conventional chip seals.
• Purpose: ODOT tested rubber-asphalt chip seals (made by blending ground scrap tire rubber with hot asphalt binder) on several highway sections in Oregon between 1977–1981. The goal was to compare cost, performance, and constructability with conventional chip seals.

Methods:

• Rubber-asphalt binders were mixed with 18–22% ground tire rubber.
• Test sections: U.S. 97 (Klamath Falls–Bend), U.S. 20 (Burns), OR 19, and a few others.
• Compared rubberized chip seals with adjacent standard ODOT chip seals.
• Evaluated performance based on bleeding, raveling, chip retention, and overall surface condition over 3–4 years.

Performance:

• Early years (1–2 yrs): Rubber chip seals showed good chip retention and flexibility, sometimes better than conventional.
• Later years (3–4 yrs): Performance differences disappeared; both types showed similar levels of cracking, raveling, and wear.
• Bleeding issues were more frequent with rubber-asphalt, especially in hot weather.
• No clear evidence of significantly longer life compared to conventional chip seals.

Cost:

• Rubber-asphalt chip seals were ~1.5 to 2 times more expensive than conventional chip seals.
• High costs came from: Special mixing requirements, Transport of rubber additive, Slower construction (temperature-sensitive).

Constructability:

• More difficult to handle: Rubberized binder required higher temperatures (375–400°F), Shorter workable time window, Specialized handling equipment
  

Conclusions:

• Rubber-asphalt chip seals were not cost-effective compared to ODOT’s standard chip seals. Performance was comparable, not superior, despite higher costs and added construction completion time. Do not adopt rubber-asphalt chip seals for routine use in Oregon. Suggested continued observation in warmer states (Arizona, California) where climate might favor performance.

Beecroft, D. W., Hardy, R. L., & Steyskal, R. A. (1982). Evaluation of rubber-asphalt chip seals in Oregon (Final Report, Research Project No. 75-02). Oregon Department of Transportation, Research Unit.