Colma Streets Take the Long Road to Performance

High-sloped paver of streets entering Colma’s residential neighborhood have performed well for the past 20 years under vehicular traffic.

Because Colma, California sits on rolling hills just south of San Francisco, it became the unfortunate recipient of the victims of the devastating 1906 earthquake. There are more buried among sprawling, but elegant cemeteries than those alive in this hamlet. In 1998 the streets and sidewalks within the small residential area supporting about 1,600 citizens was adorned with interlocking concrete pavement. With high-slope streets, some as high as 18%, the impressive project was included in an August 2006 article on sloped pavement applications in this magazine.

According to Brad Donohue, Colma’s Director of Public works since the late 1990s, the interlocking concrete pavement application was controversial. Some doubted that the pavers would remain stable under traffic on such high slopes. There is no sign of movement on the sloped streets and traction loss has never been a problem.

Besides consistent widths among the sand-filled joints, surface stability can be likely be attributed to the 45° herringbone pattern. Other factors working in favor of longevity and no surface distresses is mostly car traffic, a mild climate,

The Town of Colma realized lower maintenance costs from interlocking concrete pavement over 20 years compared to asphalt. This will likely result in overall lower life-cycle costs over 30 to 40 years.

and sandy, well-draining soils with R-values between 65 and 70. The longest street, F Street, used a cement stabilized soil subgrade to compensate for a high-water table that sometimes rises during the rainy, winter season and could potentially weaken or alter the pavement subgrade.

Mr. Donohue noted that if the 20-year old streets were paved with asphalt, they would have already applied a slurry or chip seal in the first 8 to 10 years. Within the next 10 years, there would more than likely be asphalt surface milling and fill, or an overlay. This suggests that the life-cycle cost of the Colma’s pavers would likely be lower than asphalt after another 15 or 20 years with another cycle of asphalt milling and overlay. Given the excellent condition of the pavers, this is a reasonable life cycle cost forecast. He noted, regarding costs in long term maintenance and rehabilitation to the residential roadways, Colma is definitely “ahead of the game” on street maintenance costs by using concrete pavers.

Mr. Donahue said that municipalities need to take the long view when it comes to pavement maintenance. Streets will be maintained in perpetuity as they will very likely be around for hundreds of years.

Intersections used light colored pavers to mark crosswalks, thereby eliminating repeated paint applications.

Therefore, selecting long-lasting, 40+ year materials such as interlocking concrete pavement should be considered.

Well over a mile of streets and twice that length of sidewalk pavers (about 200,000 sf) were supplied and manually installed by ICPI members on a 4 inch (100 mm) thick Caltrans Class 2 base over a compacted soil subgrade. All of the pavers were manually placed. Mr. Donahue said that tree roots lifted some of the sidewalk pavers over the years. The pavers were removed, roots addressed, and the pavers reinstated. The streets have seen no lifting from tree roots.

He said that the town’s streets are surveyed every two years and condition data is collected and sometimes used to obtain maintenance grants. He was pleased to learn about a pavement condition survey system

The paver sidewalks made tree root repairs easier than demolishing and replacing cast-in-place concrete slabs.

available as ASTM E2840 Standard Practice for Pavement Condition Index Surveys for Interlocking Concrete Roads and Parking Lots to use for pavement asset management. This standard practice provides a guide for surveying and rating the condition of interlocking concrete pavements. Condition surveys and resulting maintenance budgeting for street maintenance are normal to most municipalities and this ASTM standard enables comparisons on performance among asphalt, concrete and interlocking concrete pavement streets.

For Colma’s paver streets, a very good condition rating would only reinforce the obvious superior performance, plus adding to neighborhood character. This is performance that asphalt and concrete can never achieve.