Slurry Seal
Updated: March 2024
What We Do
Slurry seal 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, slurry seal creates a protective layer which preserves the underlying structure and prevents the need for more expensive repairs in the future.
Background
The Slurry Sealing Process
The process of slurry sealing is very similar to microsealing. A mixture of asphalt emulsion and aggregate (crushed stone, gravel and sand) is applied to the road. As shown in this short video, as the slurry seal 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.
After the 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.
Factoids
- Slurry sealing is an 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 slurry seal project. A contractor will then come in to complete the actual slurry seal process.
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 to keep these streets in serviceable condition. SDOT typically sealed the streets on a 10-year cycle. For many years these streets were sealed using a process called chip seal. In 2013, SDOT tested microsurfacing in place of chip sealing, and then used microsurfacing through 2017. In 2018, SDOT began using slurry seal. Each method has its advantages, but they all seal the surface and extend the life of the pavement.
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. Such blocks resurfaced in a slurry seal project are selected based on the age of the current pavement and an on-site inspection by SDOT staff.
A second category of streets which the City has identified for preventative maintenance are streets which are part of a neighborhood greenway.
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 slurry seal process. SDOT crews will begin to patch deteriorated areas of the selected roads well in advance of the slurry seal operation. In addition, low hanging branches and overgrowth may need to be trimmed to allow for the slurry seal equipment to navigate the roads. On the day of slurry sealing, crews will sweep the street before applying the slurry seal material.