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Island County > Public Works > Roads > Maintenance & Operations > Asphalt Overlays Fact Sheet

​Asphalt Overlays

Asphalt Overlays Fact Sheet

“Asphalt Overlays” is a common pavement maintenance practice that extends pavement life and provides a good driving surface.  Island County is responsible for the maintenance of 583 centerline miles of roadway. Of that there are approximately 233 miles maintained with Asphalt Overlays.

Asphalt surfaces are expected to last between 15 to 20 years, which will vary with the type and amount of daily traffic. The candidate roads are reviewed annually for condition, and added to the pavement preservation program if warranted. We typically work on road’s county wide.

This Fact sheet is prepared to provide the residents of Island County information about the pavement preservation program by answering some of the frequently asked questions.

How is Asphalt Overlays Different from Chip Seals?

The difference is in the construction method.  Asphalt pavement is produced by heating liquid asphalt and mixing it with aggregate, with the mix then spread and compacted to form a durable road structure and riding surface.   Chip Sealing uses the similar ingredients as asphalt paving, but the construction method is different.  With chip seals, a thin film of heated emulsion is sprayed on the road surface, followed by the placement of small aggregates ("chips").  The chips are then “rolled” to orient the chips for maximum adherence to the asphalt.  Excess stones are swept from the surface.

Why Use Asphalt Overlays?

  • Asphalt overlays provide additional structure to the overall pavement section of the road and significantly extending the service life of a road.
  • Asphalt overlays provides for a smoother surface compared to chip seals.

What steps are involved in Asphalt Overlay?

Over the years, vegetation will grow near the edges of the roadway and dirt will accumulate at the base of these vegetation.  The first step in preparing the road for an asphalt overlay is pulling the shoulder material off of the road surface.  This is typically done in early spring.

Next, areas that are demonstrating indications of roadway base failure are repaired with what are termed ‘digout’s.  Digout is actually a misnomer as the process actually grinds and mixes the surface material with the base material and added cement.  This mixture is compacted and left to cure for 2 -3 weeks.  The final step in the repair is to remove the top 2 inches of mixed material and patch the area back with asphalt.

Where needed, the roadway is pre-leveled with asphalt to improve drainage by correcting cross slopes.

Once prep-work is completed, a Contractor averaging 12 employees starts the asphalt application process.  An asphalt distributor truck starts by spraying one lane with hot liquid asphalt to adhere the new layer of asphalt with the existing road.  A paving machine filled with hot asphalt places a specified thickness of asphalt material onto the roadway.  Shortly after placement, steel drum rollers compact the asphalt material to set it in place and to compact the material to proper specifications.  Rolling helps compact the asphalt material and smoothen out the surface.

What does it cost?

A mile of asphalt overlay costs Island County about $86,000, compared to $36,000 per mile of seal coat overlay.  Asphalt overlay is a crucial component of the Island County’s preservation program, especially critical on our high volume and higher speed roads.

Who does the work?

Contractors are hired through a bidding process to perform the asphalt overlay.

When is the work done ?

The asphalt overlay program usually starts after Independence Day and is completed by Labor Day.

Check the County website under Public Works Road Maintenance to see where crews are overlaying each week or to get a list of roads to be overlayed for the year.


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