InTrans / Mar 20, 2024

PROSPER project finds long-term benefits from Otta seal treatments

The recently completed research also recommends specifications for implementation

The first Otta seal, a low-cost bituminous surface treatment (BST) for low-volume roads, was constructed in Iowa in 2017. Since that first 4 mile stretch was built in Cherokee County, more than 50 Otta seal sites have been constructed.

Those sites demonstrate the widespread interest in this relatively new technology imported from Norway and adapted to Iowa’s climate and quarries. They also presented the Program for Sustainable Pavement Engineering and Research (PROSPER) Director Halil Ceylan and his research team the opportunity to evaluate the performance of the existing sites and recommend specifications for Otta seal in Iowa.

“The method developed as part of the Phase II project can help county engineers determine optimal binder and aggregate application rates for local materials and better understand how various parameters—such as aggregate gradation, aggregate and binder type, and aggregate and binder application rate—can affect the performance of Otta seal surfacing,” said Ceylan, who was the principal investigator on the project.

As part of the recently completed Phase II Otta seal project, the researchers also conducted laboratory tests to aid in developing the specifications, evaluated the field performance through additional site construction in Page County, and performed a life-cycle cost analysis using those new test sites.

Photograph showing an Otta seal construction train on a rural road, with a truck applying asphalt binder in front and an aggregate spreader following
Otta seal construction

The latter included four maintenance scenarios and ultimately showed the lowest equivalent uniform annual cost with test sections that utilized a modified McLeod design technique for Otta seals. The modified technique also offered the best performance in terms of aggregate loss during the laboratory investigation, as well as the lowest International Roughness Index (IRI) values at the test sites.

“The lower costs can be attributed to the method’s reduced aggregate and asphalt binder usage, which resulted in cost savings throughout the test section’s life cycle,” said Ceylan.

Additionally, the research found that after several years of service, most of the Otta seal sites in Iowa exhibit excellent performance, and those sites demonstrate that local aggregates and recycled materials can successfully be utilized in Otta seal construction.

Otta seal involves placing a graded aggregate layer on top of a bitumen emulsion layer and then rolling the aggregate into the emulsion to form a seal. In contrast to traditional BSTs such as chip seal, Otta seal supports the use of locally available and marginal aggregates, thereby lowering construction costs.

Additional details, including the recommended specifications in the appendix, about the research are available at the Phase II project page. A short video and an article are also available that summarize the Phase I research, as well as the Phase I project page.