Microsurfacing

An Alternative to Chip Sealing

Before 2013, SDOT’s primary method of preventive maintenance has been chip sealing. While chip sealing is an effective preventive maintenance measure, it leaves the roads very rough and there is an extended period of loose rock on the street. Microsurfacing and chip sealing are both seal coats that extend the life of the pavement for about the same amount of time (5 to 10 years).

However, microsurfacing has two key advantages over chip sealing:

• No loose rock chips are involved, so there is no need to sweep loose rocks weeks after the project is complete.
• The final product provides a finished surface which is black in color and looks similar to a conventional asphalt surface.

Chip sealing has the advantage that you can drive on the street immediately, but with microsurfacing you must wait 2 hours or more for the surface to cure before driving or walking on it.

How does SDOT select the streets for treatment?

In the 1950's and 1960's, the City of Seattle annexed several parts of King County. Most of these streets had a dirt or gravel surface. The City paved these streets with a minimal amount of asphalt and began a regular preventive maintenance cycle, typically chip sealing the streets on a 10-year cycle.

Because these streets are mostly low volume non-arterial streets and have received preventive maintenance in the past, they continue to be the best candidates to receive preventive maintenance.

All the blocks resurfaced in a microsurfacing project are selected based on the age of the current pavement and an on-site inspection by SDOT staff.

Our street looks fine. Why resurface it?

Flexible pavements (typically asphalt) need periodic resealing to protect them from the deteriorating effects of water (rain) and sun. If left untreated, the surface becomes brittle and may crack and ravel. Periodic resealing prevents more extensive and costly repairs. Regular preventive maintenance is the most cost-effective way to maintain streets.

Preparing the street: fixing potholes, cracks, etc.

It is essential that structural damage is repaired prior to the microsurfacing process. SDOT crews will begin to patch deteriorated areas of the selected roads well in advance of the microsurfacing operation.

In addition, low hanging branches and overgrowth may need to be trimmed to allow for the microsurfacing equipment to navigate the roads.

On the day of microsurfacing, crews will sweep the street before applying the microsurfacing material.

The Microsurfacing Process

Microsurfacing is a process similar to a slurry seal. A mixture of asphalt emulsion and aggregate (crushed stone, gravel and sand) is applied to the road. However, unlike a slurry seal, microsurfacing uses emulsion that is modified with polymers and other ingredients so it cures more quickly.

As shown in this short video, as the microsurfacing equipment moves along the street, the mixture is fed into a spreader box. The material is spread across the full width of a traffic lane and then smoothed by a squeegee. The equipment also feathers the edges for a smooth transition. All this happens in one step. Here’s a very short video showing microsurfacing on a Seattle street.

After the microsurfacing seal coat cures (hardens), the street can be reopened and used normally. However, there may be parking restrictions and road closures for the entire day the work is scheduled. In some locations, new curb ramps are installed as part of a microsurfacing project to comply with Americans with Disabilities Act (ADA) standards and improve safety and accessibility for all sidewalk users.

Factoids

• Microsurfacing is a cost-efficient surface treatment particularly where traffic volumes are low and the street is not regularly used by heavy trucks or buses.
• City crews will complete all of the preparatory work for the microsurfacing project. A contractor will then come in to complete the actual microsurfacing. Past Projects In 2013, SDOT switched from chip sealing to microsurfacing.

Since 2013, SDOT microsurfaced streets in Wedgwood, Arbor Heights, North Rainier Valley / North Beacon Hill, SODO/Georgetown, North Maple Leaf, Madison Valley/Madrona/Leschi, Matthews Beach, Meadowbrook, Montlake, North Green Lake, South Beacon Hill/New Holly, South Wallingford, and Southeast Ballard.