Asphalt Rubber Chip Seal
What is an asphalt rubber chip seal?
An asphalt rubber chip seal is a surface treatment used on pavement to extend the life of the street. It consists of an asphalt rubber ‘binder’ covered by rock aggregate.
How is an asphalt rubber chip seal installed?
First the street is swept clean, then the asphalt rubber chip seal is applied in three layers. The bottom layer is a thick oil and rubber mixture that is sprayed onto the street. Next, small rock ‘chips’ are placed on top and pressed by rollers into the oil and rubber mix. Then an oil ‘fog seal’ is sprayed on top of the rock. The street is then swept regularly for a few weeks to remove loose rock. An asphalt rubber chip seal typically provides 10 to 15 years of pavement life for about $100,000 per mile.
How do asphalt rubber chip seals fit within Tigard’s overall Pavement Management Program? How are streets evaluated to determine what kind of treatment/repair they receive? In what situations is an asphalt rubber chip seal used? How and why are these streets selected?
Tigard’s Pavement Management Program has typically used two types of pavement treatments.
1) Slurry Seal is a surface treatment consisting of a liquid mixture of oil and sand applied to the surface of a street. Slurry seals are typically used on streets with lower traffic volumes and fairly good pavement condition – where the pavement surface has become rougher due to weathering, but does not have significant cracking. Slurry seals typically provide 5 to 10 years of pavement life at a cost about $50,000 per mile.
2) Pavement Overlays build new pavement on the street, and are typically used on streets with high traffic volumes and/or very poor pavement condition. This work typically starts by retrofitting curb ramps as required by the Americans with Disabilities Act. Pavement grinders then remove areas of deteriorated pavement and often grind along the curb so the new pavement will match driveways, etc. A new layer of pavement (typically a couple of inches thick) is then placed on top of the road. Utility valves and signal loops are then adjusted or replaced to match the new street level. Pavement overlays typically provide about 12 to 25 years of pavement life at a cost of about $500,000 per mile.
The Pavement Management Program’s goal is to give taxpayers the best value by selecting pavement treatments is to provide the most people-miles of good pavement for the dollar. Pavement treatments are selected to try to minimize the long-term street maintenance life-cycle cost and maximize the pavement quality of our streets. As slurry seals provide several years of pavement life at a fairly low cost, we have slurry sealed all of the streets in Tigard that are good candidates for slurry seal, and are now returning to slurry seal streets done nine years ago. Pavement overlays are constructed on streets with high traffic volumes when those streets have deteriorated enough to need paving.
There is currently a backlog of about 22 miles of streets in Tigard that are in poor condition and need to be paved, but have not been paved due to lack of funding. We are trying to pave as many of these streets as possible and are generally paving the busiest streets first. With the funding currently available, it will be decades before we will be able to pave all of the backlogged streets, so we are looking for cost-effective ways to address these backlogged streets more quickly. Asphalt rubber chip seals have been successfully used in other areas on streets in poor condition and they have kept the underlying pavement cracks from coming up to the surface, resulting in streets that agencies can keep in good condition for many years. It is our hope that asphalt rubber chip seals will allow us to address backlog streets much sooner that we could with just pavement overlays.
What streets were selected for the 2017 asphalt rubber chip seal, and why?
In selecting streets for the 2017 asphalt rubber chip seal project, we were looking for streets with:
a) Significant surface cracking, making them not a good candidate for slurry seal;
b) Relatively low traffic volumes, meaning it would be many years before they would reach the top of the paving list;
c) Roadway structure (sub-base, base gravel, and asphalt) that is largely intact except for surface cracking; and
d) A number of such streets fairly close together for an efficient project.
How is the Pavement Management Program funded?
Funding for the Pavement Management Program comes from the Street Maintenance Fee paid by Tigard residents and businesses each month on their water/sewer bill. As the program has dedicated funding, any project savings remains in the program and would allow us to get more streets done.
What other agencies are using asphalt rubber chip seals?
Vancouver, WA and Clark County, WA have been using asphalt rubber chip seals for several years and continue to do an asphalt rubber chip seal project each year. Asphalt rubber chip seals are heavily used throughout California, Nevada, and Arizona in urban residential, commercial, suburban, and rural areas.
How and when will the city evaluate its effectiveness?
City staff are evaluating the project on an ongoing basis – checking issues such as surface roughness, binder stability, watching for issues such as excessive raveling (rocks becoming loose), surface cracking, and monitoring overall pavement structure.
What if the city finds it not effective? What solutions could be implemented?
A variety of solutions could be implemented depending on the issues observed. A future slurry seal could help smooth the surface. Other solutions would be considered for crack propagation, or other issues with pavement structure, such as crack sealing and/or additional surface treatments. Removal and replacement of the existing pavement is an option but a very expensive one, meaning many other streets would not get done if this option is chosen.
What other options are available to address paving backlog streets?
Staff continues to look for other options to address backlog streets for less than the cost of a full paving project. The asphalt rubber chip seal appears to be the best option available at this time.
The city is interested to hear feedback from area residents about the performance of the chip seal over the next year. The information we gather will help us decide if this is another tool we can use to restore streets in bad condition and address the 22-mile backlog of streets needing attention. This feedback can be shared with Mike McCarthy at 503-718-2462 or firstname.lastname@example.org.