SDOT’s Microsurfacing Program
Last updated: October 18, 2016
What is Microsurfacing?
Microsurfacing is a protective seal coat which extends the life of pavement. It is a thin, tough layer of asphalt emulsion blended with finely crushed stone for traction.
This is a cost-effective method to renew the road surface and seal minor cracks and other irregularities. This preventive maintenance process protects the pavement from moisture penetration and oxidation.
Similar to painting a house, microsurfacing creates a protective layer which preserves the underlying structure and prevents the need for more expensive repairs in the future.Microsurfacing is seeing growing use by transportation agencies, including SDOT.
An Alternative to Chip Sealing
In the past, 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:
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 in 2013.
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. For more information on what to expect during microsurfacing, please click on any of the neighborhoods above and view the advance notification flier.
If you have questions or comments about the microsurfacing program, please contact David Allen, Pavement Engineering and Management at 206-733-9302 or firstname.lastname@example.org
For Street Maintenance 24-hour emergencies, please call: 206-386-1218.
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