City of Newcastle
Pavement Management Program – Slurry Seal

What is a Slurry Seal?
Slurry sealing is a thin seal coat made up of aggregates, asphalt emulsion, and mineral filler that fills in small cracks and surface imperfections to give the roadway a uniform color and texture. It's one of many effective tools in the City’s preventative maintenance program and is typically applied to residential streets that are in fair-to-good condition. Slurry seal application can extend a street’s good conditions for five to ten years by protecting the underlying surface from the effects of wear and weather. Many cities locally and nationally use this cost-effective strategy to renew the road surface and to seal minor cracks and other irregularities. This preventive maintenance process protects the pavement from moisture penetration and oxidation.

Like painting a house, slurry seal creates a protective layer that preserves the underlying structure and prevents the need for more expensive repairs in the future. This short YouTube video from the City of Los Angeles explains how slurry seal treatment helps protect and preserve local streets today and for the future.

Why Does the City Perform Slurry Seals?
Slurry seal work is selected because it's a proven and cost-effective technique to maintain existing pavement. The cost to perform the 2022 slurry seal work was $8.08 per square yard. For comparison, the average cost of the 2021 pavement overlay work was $41.40 per square yard, over five times more expensive. Lower cost isn't the only reason that a slurry seal was chosen: The slurry seal will defer the need for a costly pavement overlay, thereby lengthening the lifespan of City streets while reducing the cost to maintain them, and use of slurry seal will improve the citywide pavement condition at a lower overall cost. The use of slurry seal on local roads was a recommendation included in the 2019 and 2021 Pavement Management System Update Reports (link).

Why Was My Street Chosen for Slurry Seal?
Streets considered for a slurry seal project are determined based on the current pavement conditions, pavement age, and locations with recent or concurrent crack-sealing work. 

Dissatisfaction of the Road Surface – Slurry seal is quite a bit different than an asphalt overly. Slurry seal is a pavement-maintenance technique, whereas an asphalt overlay is considered rehabilitation work. With a slurry seal, the new surface is rougher and sheds some of the aggregate, creating gravel deposits on top of the new slurry seal. The contractor’s crews sweep the streets, typically one week after application and then again three weeks after application. As time progresses, traffic smooths the surface by dislodging the larger aggregate and pushing smaller aggregate into the underlying pavement. Sweepings and rain also help. Below are photos from the City of Kirkland that show how the roadway surface smooths out over time:

Much of the feedback about the appearance and roughness of the slurry seal are the result of an unfamiliarity with this type of maintenance work; for additional information on slurry seals, focusing on “What Can I expect From the Slurry Seal” (link) describes the difference between the slurry seal work and asphalt overlay. Going forward, city staff will consider additional outreach efforts to educate and prepare residents for planned roadway resurfacing work.

"Not an Appropriate Technique for Urban Roads" - Slurry seal is commonly used by surrounding cities on local urban streets. Recently completed slurry seal projects are listed below.

- Redmond (2022)

- Seattle (2022)

- Kirkland (2021)

- Bothell (2021)

- Kenmore (2020)

- Shoreline (2020)

- Burien (2018)

- Mukilteo (2018)

In contrast, chip seal is a maintenance technique commonly used on rural roads. Chip seal is different from slurry seal in that the aggregate size in chip seal is larger, resulting in a considerable rougher surface. A photo from a recently completed chip seal project in Kittitas County is shown below.