Streets, Highways

Phoenix test drives cooler pavement product

By BETH ANNE BRINK-COX | The Municipal

Roads appear to shimmer during the hottest days of summer: Some see it as the road looking wet. It’s not.

Science Focus Magazine says, “As the sunshine beats down on the road, the road surface becomes hot, making the layer of air above it expand and become less dense. This, in turn, changes the way light passes through it from points further down the road. However, the effect is not uniform. The rising air is turbulent, so its density and thus optical properties are constantly fluctuating. This is what creates the shimmering effect we see as we look down the road.”

In cities where the heat is consistently high, the charm of shimmers quickly wears away. Heather Murphy, of the Communications and Public Engagement Department in Phoenix, Ariz., is a fan of something called Cool Pavement, a pilot program that differs from other road seal products.

Murphy said CP has the same basic components as other road sealants, but they’re formulated differently. ABOVE PHOTOS: Pictured is the finished parking lot. (Photo provided by the city of Phoenix, Ariz.)

“The ingredients are mineral fillers; an emulsifying agent, which is actually soap, and they’re bound together; as well as finely crumbled asphalt, water, polymers and recycled fillers. It has the grit and strength you want, and what’s most important to us is that it’s recyclable. It can be ground into asphalt millings.”

Asphalt milling is more durable than conventional gravel. It perfectly seals together when compacted by machines, unlike gravel, and makes a more solid surface.

“When we can do that, it cuts down on the aggregates — inert granular materials such as sand, gravel or crushed stone — that we need to buy,” she explained.

Arizona experiences brutal temperatures during the summer months. Last year was particularly harsh, according to Murphy. “We had a hot run of 110– to 115- degree days. In urban settings, as much as 40% can be pavement, and when you look at all the buildings, driveways and such on the areas that have dark asphalt—it all captures and retains the heat. Then it releases it at night, slowly, and that keeps the urban areas much hotter than rural areas.”

Nighttime temperatures can be as high or higher than daytime temperatures.

“Nighttime temps can be as much as 22 degrees higher, and that’s what we need to fix. You know the way you put zinc oxide on your nose at the beach? It’s like that: reflecting, rather than collecting heat in the hope of lowering those nighttime temps.”

The program was introduced to the prospective neighborhoods before any road work began.

“We were determined right from the start to be totally transparent and able to answer any questions anyone might have,” said Murphy. “We were robust in telling people this was coming; we delivered notices in the neighborhoods. We wanted to build awareness of what we hoped to achieve. This led to enthusiastic overall reception, and we were swamped with calls and requests for speakers at conferences — even in other countries.

“It’s not like this is an unattractive look when it’s finished, but people do say, ‘Hey, why does your street look different?’” Murphy noted. “It’s part and parcel of making sure we’ve done our job in giving them the FAQs to answer those questions.”

The pilot program saw the first installation of CP in June 2020.

“That was the targeted date, even through the pandemic and quarantine, because this is an essential service. Everyone was masked and distanced as best they could and went to work. The city had selected portions in eight different neighborhoods, one in each city council district and part of the parking lot in Esteban Park. We thought we could do an effective job of rehab,” she said.

“Those different areas were exactly that — different. Some had shaded streets; some were near parks; and some were just typical housing. These were areas that already needed repaving, and the CP was applied. All we had to do was sweep the streets clean before beginning. No primer of any kind. That first installation was done with a cart dropping the liquid on the surface, then it was all hand-squeegeed.”

After seeing how labor-intensive it was, Phoenix challenged the contractor to experiment with a spray application. It turned out to be an economical way of laying the liquid down and gave good coverage: two passes of the initial coat, then two passes of the second. It was much more efficient.”

The city partnered with Arizona State University to study if or how the CP was working.

“When we enlisted ASU to help with the scientific study, they embedded I-buttons into the surface. Those sensors were used to measure subsurface temps. Then a special cart was designed with a robot in it that took various measurements from the surface. The subsurface was almost 5 degrees cooler where the CP was installed, a good indicator that it’s not holding onto the heat. And the surface was 10 to 12 degrees cooler during the heat of the day. Even at sunrise, there was a difference of 20 degrees or so.” The work crew prepares for the application of Cool Pavement in the parking lot of Esteban Park, a very popular place that offers lighted baseball, basketball, tennis and sand volleyball courts as well as traditional courts, a playground, picnic area with grills and a rugby field. Unsurprisingly, this parking lot sees a lot of use. (Photo provided by the city of Phoenix, Ariz.)

Using the knowledge gathered in the study, a video program was created. It was presented to city officials and received overwhelming support, including from Mayor Kate Gallego.

“By the end of June, we had installed 33 miles more and we already had 38 miles finished through 2021. We’re not a pilot program anymore,” said Murphy.

Last year the city created its very first Office of Heat Mitigation. Dr. David Hondula, a climate scientist from ASU, leads the team. The office is looking at creating cool corridors, shade and access to more livable conditions that other metropolises can copy.

“Phoenix is not only our capital, it’s the fifth largest city in the United States,” Murphy noted. With more than 1.4 million residents and steadily growing, the city takes up more than 500 square miles, geographically exceeding Los Angeles.

“We are looking at ways to reduce that heat island effect,” continued Murphy. “Neighborhoods are where people are moving most. CP as we know it is developing a product that will be appropriate for major arterial use. The city takes care of the streets, and we are working CP into street rehabilitation.

“We message things like, ‘If it’s too hot for you to walk, it’s too hot for your dog.’ Booties are a big deal around here. We remind people not to leave their children or pets in the car. So, with our next installation of CP, we’re mapping where to put it to provide heat relief where there is little shade. We want to improve the conditions and will plant trees to form more cool corridors and greater comfort.”

CP looks a little different than traditional asphalt, and the second round is a darker gray than the first.

“The formula’s value is its reflective quality—again, that zinc oxide thing,” said Murphy. “It doesn’t cause skidding. One question often asked is, ‘Does it cause glare?’ since it is lighter. Well, freeways are concrete, and that’s lighter still. When we re-striped the parking lot of Esteban Park, the lines showed up just fine.”

Maintenance is pretty low. The seal coat, over time, picks up darker shading from tires, but it usually cleans up during street sweeping or rainstorms. And if the street has to be cut, say, to install a utility line, the city requires that the area be patched when finished according to the usual method and then the CP coat is reapplied.

“Potholes are handled the same way: fill and patch, then we’ll reseal,” said Murphy.

As for how it will hold up during extreme weather, it’s too early to say.

Murphy said, “We do have a monsoon season, and severe thunderstorms will blow through one area of the city while other areas remain untouched. This will be a good indicator of how well it holds up. From everything we’ve seen, it may last longer than traditional seal coats. It does cost more: The street maintenance budget earmarked a certain amount of money for it, initially two and half to three times more, although the cost has come down since then. But if it lasts longer… it’s like sunscreen for the road, not getting sun or heat damage. We can’t really simulate this, but if the early indications are correct, instead of three to five years, we might get five to seven; or instead of five to seven, we might even get seven to nine. Time will tell.”

Murphy said the department has had to disappoint homeowners who want to know if there’s a similar product they could buy for do-it-yourself projects.

“It doesn’t mean there aren’t some good products out there, but this isn’t something they can do themselves, especially since it’s meant for asphalt and most driveways, patios and sidewalks are concrete.” Workers are well underway toward the goal of creating a cooler place to park. (Photo provided by the city of Phoenix, Ariz.)

When asked what other cities need to know in order to begin a similar CP program, Murphy noted that the webpage www.phoenix.gov/streets/ coolpavement is updated regularly, including with questions that have come up.

“We also posted our first and following public meetings, and we have the ASU presentation. Those would be a great place to start,” she said. She also welcomes calls and conversations with city engineers.