Putting Research Into Practice: Fog Sealing Chip Sealed Roads Can Improve Performance

Applying a protective surface—or chip seal—to existing asphalt on relatively low-traffic roads is a pavement preservation method used by many transportation agencies. Yet despite its popularity, chip seals can cause various issues. Some agencies have found that applying a fog seal following a chip seal is a cost-effective strategy for improving asphalt performance and public satisfaction.

During the chip seal process, a thin film of heated asphalt liquid is sprayed on the road’s surface, and small aggregate, or chips, are spread and compacted. Costing a fraction of a new asphalt overlay, chip seals fill cracks, which keeps water from penetrating the pavement and can reduce skidding and night glare on wet roads.

While the treatment can extend the life of roadways and provide a temporary solution for roads needing more significant repair or replacement, chip seal applications can be problematic. Loose aggregate can damage vehicles, and public complaints of cracked windshields or rocks tracked from roadways to driveways, yards and homes often follow a chip seal application. There are also concerns that chip sealing can result in some roads experiencing stripping, in which the lower layers of asphalt start to disintegrate and damage the upper layers.

“For agencies using a chip seal to maintain their roads, this research clearly showed that adding a fog seal to the chip sealing process results in quantifiable and nonquantifiable benefits that are worth the cost,” said Steven Bot, public works director, City of St. Michael.

To alleviate these effects, some agencies use a fog seal after chip sealing a road. A diluted asphalt emulsion acts as a sealant when sprayed onto the road surface in a mist or fog, improving the aggregate retention and protecting the road from abrasive damages caused by snowplows. The Local Road Research Board (LRRB) was interested in learning more about fog seal performance and best practices for using it.

What Was Our Goal?

The goal of this LRRB project was to explore the cost-effectiveness of fog sealing over chip sealed roads, the ability of fog sealing to improve chip retention and fog sealing application methods.

What Did We Do?

City and county transportation agencies throughout Minnesota were surveyed regarding their experiences with fog seal. Of 52 responding agencies, 13 were identified as having the most relevant experience and participated in follow-up interviews. Detailed information was gathered regarding the types and condition of roads on which fog seal was applied, costs and application methods. Additionally, the agencies were asked about changes in road performance, public and maintenance crew feedback, and lessons learned.

A distributor truck spraying emulsified asphalt onto a road. — For best results, a fog seal should be applied within 24 to 48 hours following chip sealing.

Responses from both the surveys and interviews on application and maintenance costs of chip sealing alone and combined with fog sealing provided a basis for analyzing the cost-effectiveness of fog seal. Both overall project costs and itemized costs were analyzed, and a cost per square yard of fog sealing was calculated.

To quantify the benefits of fog sealing, researchers analyzed differences in pavement performance to estimate an annual value of a chip seal alone versus chip seal followed by fog seal. Assumptions regarding pavement life expectancy were based on previous research.

What Was the Result?

Survey respondents reported numerous benefits, some unexpected, of fog seal applications. Chip retention improved in roads with a fog seal application, reducing or eliminating public complaints of loose rock damage and nuisance. Fewer stripping issues were also reported, though investigators did not definitively conclude fog seal eliminated them. Additional benefits included reduced damage from snowplowing and an extended pavement life with fog sealing.

“This project showed that a fog seal, in addition to being a cost-effective preservation tool, has numerous unexpected benefits. There are many factors, however, that agencies should consider in deciding whether and how to use a fog seal,” said Mike Rief, senior vice president of construction services, WSB & Associates, Inc.

While a fog seal requires additional curing time, disruption to the traveling public is lessened overall due to reduced maintenance needs. Less street sweeping is needed due to better chip retention. Respondents also reported receiving positive comments from members of the public about pavement aesthetics, with many indicating the road surface looked like new pavement without the expense of repaving.

Investigators calculated the cost of chip sealing at 22 cents per square yard per year; adding a fog seal increased the cost to 29 cents. While the information gathered did not provide long-term data indicating how long a fog seal may extend pavement life, respondents indicated the application results in a more durable and protected road surface. If pavement life is extended by just one year, then a fog seal provides 1.3 times the value of a chip seal alone, leading researchers to conclude that the benefits of fog sealing justify its cost.

Conclusions and findings were summarized into a best practices manual for agencies and contractors, including considerations to determine if fog sealing is an appropriate choice for a specific road and if it is cost-effective. Other topics include: