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Chip Seal​

Chip Seal Fact Sheet

“Chip Sealing” 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 345 miles maintained with BST-Bituminous Surface Treatment (Commonly referred to as chip seal).

The chip seal will last an average of seven years, which will vary with the type and amount of daily traffic. The candidate roads are reviewed annually for condition, and added to the chip seal program if warranted.

This fact sheet is prepared to provide the residents of Island County information about the chip seal program by answering some of the frequently asked questions.

How is Chip Seals Different from Asphalt Overlays?

The difference is in the construction method.  Hot Mix Asphalt pavement is produced by heating liquid asphalt and mixing it with aggregate in a batch plant, with the mix then spread and compacted to form a durable road structure and riding surface.  Chip Sealing uses the same basic ingredients as asphalt concrete paving, but the construction method is different.  With chip seals, a thin film of heated asphalt liquid 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, and excess stone is swept from the surface.  The basic ingredients of hot mix asphalt and chip seals are the same; only the construction methods are different.

Why Use Chip Seals?

  • Chip seals provide Island County Public Works (ICPW) with the opportunity to maintain the roads for very low cost.
  • A chip seal is less than half the cost of a conventional maintenance asphalt overlay.
  • By extending the time between asphalt overlays, chip seals result in lower costs over the long term.
  • By placing a chip seal sooner than when asphalt overlay is required to be placed, the traveling public benefits from roads maintained in better condition.
  • Chip seals enhance safety by providing good skid resistance.
  • Chip seals provide an effective moisture barrier for the underlying pavement against water intrusion by sealing cracks in the pavement.
  • Chip seals prevent deterioration of the asphalt surface from the effects of aging and oxidation due to water and sun.

What steps are involved in Chip Seals?

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

Next areas that are demonstrating indications of 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 concrete.  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, county crews averaging 19 employees start application. An asphalt distributor truck starts by spraying one lane with hot liquid asphalt to adhere the new layer of chip seal with the existing road.  The asphalt used is applied at a temperature of approximately 190 degrees Fahrenheit.  A chip spreader follows as rapidly as possible with a rock application, preferably within one minute.  The asphalt must be fluid enough to allow the rocks to be embedded by the displacement of the asphalt.  The rocks are an aggregate crushed to a special specification for size and cleanliness.  Next, pneumatic rollers set the rock into the liquid asphalt.  Rolling orients the flat sides of the rock down and produces a tighter chip seal.  It takes several passes of the rollers to set the rock to its optimum orientation. 

Within three days after the application, the road is watered, excess rock is swept from the surface and warning signs are removed. Permanent road striping is redone before the end of the season.

What issues affect a quality chip seal application?

Since a roadway is rarely closed to traffic for the chip seal application, a smooth flow of traffic through the work area is required.  Driving slowly, not turning sharply, or braking quickly or spinning tires is crucial for the fresh chip seal to properly set and cure.

Until a road is broomed, the lower, 25 mph speed limit needs to be observed so chip rock isn’t dislodged, and passing vehicles aren’t damaged by “flying” chip rock. During the chip seal process, from the time the gravel is placed on the road to when the excess is swept away.  By driving 25 mph, you protect your vehicle from unnecessary damage that can be caused by the sprayed asphalt and loose gravel. The slower speed also decreases the chance of damage from rocks which may be thrown up from other vehicles.

Traffic moving at higher speeds can create dust, limit visibility, and cause an inconvenience to local residents.  Increased speeds can also cause gravel to break loose from a fresh chip seal creating the risk of flying rock. Rocks thrown from your tires may crack or break a nearby vehicle’s windshield.  Flying rocks might also injure pedestrians and bicycle riders.

Please be patient. Traveling at the posted construction speed limit in construction zones will ensure your safety and the safety of others including the county road crews.  It's also the law.  Traffic fines are doubled in construction zones. Obeying warning signs and flagging personnel instructions benefits all those who share the roadway with you, too.

What does it cost?

A mile of chip seal costs Island County about $36,000, compared to $86,000 + per mile for asphalt overlay, it is very a cost effective alternative to preserving your County Road.

Who does the work?

The chip seal program is done by Island County Road Operation & Maintenance employees.

When is the work done?

The chip seal program usually starts after Memorial Day and is completed by Labor Day.

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


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