Pavement Research Center Keeps California on the Road

Making What’s Under Your Feet and Wheels Cheaper and More Sustainable

By Andy Fell on October 11, 2018 in Science & Technology

IN THE MISSION IMPOSSIBLE MOVIES, secret agent Ethan Hunt tells people he’s a traffic engineer with the Virginia Department of Transportation, an answer so boring that nobody follows up to ask what he actually does.

Real transportation engineers recognize the stereotype.

“You do tend to tell people you work in infrastructure and leave it at that,” said Professor John Harvey, director of the UC Pavement Research Center.

In fact there are plenty of reasons to care about pavement. Tough commute? Imagine driving it on roads of dirt or gravel. If you are driving on roads that have already missed their freshness date for maintenance or rehabilitation, you can feel it every day.

Rapid, reliable transportation is vital to the economy and to modern life. Keeping California’s tens of thousands of miles of roads, from state highways to suburban streets to bike and walking paths, in good condition is an enormous and expensive task. For most local governments, roads are very likely the most expensive single asset they own.

If you are driving on roads that have already missed their freshness date for maintenance or rehabilitation, you can feel it every day.

There’s also a complicated environmental angle. Rough roads lead to lower fuel efficiency, shorter vehicle life, more freight and crop damage, all of which have environmental impacts in addition to cost increases. Just making asphalt and concrete produces environmental impacts in energy, water use and carbon dioxide emissions. And roads of course alter the environment they pass through.

Spend some time talking to pavement scientists and you can even find yourself turning into a bit of a pavement geek. Who knew there’s so much science in such everyday stuff?

History

Pavement research began at UC Berkeley in 1948 when state legislation established the Institute for Transportation Studies. It was established as a separate program in pavement testing in 1994, at the Richmond Field Station. In 2002, Harvey moved to the UC Davis Department of Civil and Environmental Engineering and in 2008, with support from the California Department of Transportation, established a lab on campus including space for stretches of “test road.”

The center is one of a handful of advanced labs in the world carrying out scientific research on road materials, with the goal of making roads that are cheaper and faster to build and maintain, longer lasting and that reduce environmental impacts throughout their life cycle, for example by using recycled road materials. Most of the center’s work is funded by the California Department of Transportation (in 2017 the department renewed the center’s three-year grant).

Pavement is made from a mixture of hard aggregate in a binder such as asphalt or cement. By better understanding the properties of the raw materials, researchers aim to be able to predict how different pavement mixtures will behave. (Photo by Andy Fell)
Graduate students at the UC Pavement Research Center will make up the next generation of pavement and transportation engineers. (Photo by Andy Fell)

“If it involves pavement, we’re doing research on it,” Harvey said. “If it’s an engineered surface in contact with the ground, it’s pavement, and that’s us.”

Working on a university campus has advantages over being a standalone government lab, Harvey said. There is the rigorous academic culture, a research arc from fundamental science through development to implementation, and easier interactions with potential collaborators in other disciplines.

At the same time, the center is helping to train a new generation of civil engineers. About 12 graduate students at a time are working at or with the center, and there are about a half dozen visiting professors and graduate students from around the world each year.  

The terroir of asphalt and concrete

Your basic road material is a mix of hard aggregate material (crushed rock) glued together with a binder, usually asphalt or cement. Asphalt mixes can be spread hot and harden as they cool in place, providing a smooth yet resilient surface to drive on. Cement is mixed with water creating a chemical reaction that builds a smooth and long-lasting material. People have been paving roads this way since the 19th century.

Yet within that simple summary is a great deal of complexity. What is the aggregate made of? What size are the pieces? What exactly is in the binder?

Asphalt is a naturally occurring crude oil product: its properties vary depending on where it comes from — asphalt from the Bakersfield area, for example, behaves rather differently as a binder to that from coastal California. California refineries also import oil for asphalt from other states and countries, all of it with slightly different properties. To put it in UC Davis terms, you might say that asphalt has terroir.

“You can taste and smell the difference,” Harvey said.

“Asphalt is one of those things where the more you know, the more you realize you don’t know.”

One project at the lab is testing the composition of different asphalts to understand how this affects their quality and properties as binders.

“Asphalt is one of those things where the more you know, the more you realize you don’t know,” said David Jones, associate director of the center.

Engineers try to use asphalt blends suitable for conditions. This mixing used to be done empirically, adjusting the blend until it was right. Center researchers want to put that on a more scientific basis by identifying the critical properties that determine how materials will react to water and traffic. Then they want to move that knowledge into practice in materials design and construction quality control.

Research engineer Julian Brotschi mixing asphalt. (Photo by Gregory Urquiaga)

“The idea is that you can test a sample of asphalt, model its properties and then work out how to blend it and construct it for the optimal result,” Jones said.

Just like wine, the properties of pavement depend on how you treat the raw materials.

“It’s not just the grapes, but how you treat the grapes,” Harvey said. “You can engineer your way out of poor raw materials as long as you understand their properties, measure them and then account for them in your design.”

Just like wine, the properties of pavement depend on how you treat the raw materials.

Concrete materials and pavement structures are designed to meet performance requirements and also the demands of the California traveling public to limit construction windows and the resulting traffic delays, Harvey said. California pioneered the use of concrete that can be placed at night and opened to truck traffic by the next morning, gaining strength within two to four hours. Center researchers are working with Caltrans on ways to place thinner concrete slabs on existing pavement, engineering the interface between the new and old pavement to reduce cost and construction time.

Cement production for roads and construction makes a significant contribution to greenhouse gas emissions worldwide, consuming energy and fresh water. Sabbie Miller, assistant professor in the Department of Civil and Environmental Engineering, is researching ways to reduce cement’s carbon footprint by making it more efficiently, using it more effectively or replacing it all together.

Recycled roads

Once pavement is laid down, it has to withstand years of car and truck traffic as well as the effects of heat, cold, rain and depending on location, snow and ice. Asphalt surfaces become brittle over time and more prone to damage. By better understanding the properties of road materials, engineers could better predict how fast roads will wear and schedule repairs before damage becomes advanced and costly.

Another area of research is to develop new, rapid methods to resurface roads. Replacing worn-out road once involved breaking up the pavement, hauling it away and laying completely new material. New techniques rely instead on grinding up the old road on site and turning it into at least part of the aggregate for the new road, stabilized with a dash of cement sometimes mixed with foamed asphalt (asphalt bubbles). This recycling of asphalt pavement is faster and generates less waste material than older methods of road repair.

A California state law calls for the Department of Transportation to use asphalt containing recycled tire rubber  in a large percentage of its asphalt materials . The Pavement Research Center is working with Caltrans and industry to evaluate the properties of new kinds of mixes using tire rubber crumbs and how best to use them in different applications. Based on this research, they will develop draft specifications for Caltrans to use.

Heavy wear

So how do these new mixes and additives perform in practice? The rubber hits the experimental road outside the center’s lab on the experimental test track.

The center has two Heavy Vehicle Simulators, machines that roll wheels back and forth over pavement, simulating 20 years of truck traffic in a few months. Researchers can lay down a stretch of experimental pavement, embedded with instruments, and observe the results as simulated years of wear rack up.

The center has two Heavy Vehicle Simulators for testing pavement on an outside track. The simulators can reproduce 20 years of truck traffic in a few months. (Photo by Andy Fell)
Putting new pavement to the test. (Photo by Andy Fell)

In 2016, the team got the opportunity for a larger scale test. UC Davis needed to replace aging roads among the agricultural fields on the western side of campus. With support from Caltrans and construction companies, center researchers resurfaced a mile of Brooks Road and Levee Road using in-place recycling of the old road.

The new road is almost entirely composed of recycled material from the old road, with a skin on top to smooth it and keep the water out, Jones said.

You would not know it by driving over it, but the new road includes 36 test sections each with a different construction approach and blend of materials. Instruments are buried inside the road so that researchers can collect data.

“It’s part of a bigger study looking at in-place recycling,” Jones said.

The roads see relatively little traffic, allowing the researchers to look at the effects of age and weathering on the materials. As an added benefit to the campus, the roads likely would not have been repaired anytime soon without the experimental funding.

Old pavement can now be “recycled in place:” ground up and immediately used in new pavement. Here, contractors prepare to recycle an old road surface on the UC Davis campus into new pavement as part of an experimental test. (Joe Proudman)

One goal of the Brooks Road experiment is to look at adding cement as an additive to make stronger, longer lasting roads. Cement is relatively cheap and easy to use, but it is prone to shrinkage cracking. The experiment is looking at strategies to avoid or mitigate that cracking when it occurs.

“We’ve tested it in the lab but we want to test it under road conditions,” Jones said. “We want to understand the cement mechanism with the recycled pavement at the microscopic level.”

Water, breaker of pavement

Water – how to deal with it and how to resist it – is a big problem for road engineers. Whether as rain, snow or ice, water is bad for road surfaces.

“Water breaks pavements,” Jones said.

Traditionally, roads were designed so that water ran off as fast as possible into storm drains. But new approaches are being developed under the pressures of drought awareness and environmental regulation. Cities and counties are under mandates to manage stormwater properly, and road runoff can carry oil and other pollutants. At the same time there’s increasing interest in conserving and making use of stormwater.

Harvey’s group has tested different types of “permeable pavement” that allow water to percolate through rather than flushing it away.

The permeable pavement they tested “held up surprisingly well,” Harvey said, and is ready for use on local roads, although improvements were identified in the testing.

Permeable pavement surfaces can be used for low-traffic, low-speed areas such as shoulders, parking lots and alleyways. In November 2017, the pavement center organized and hosted a workshop in Davis that brought together pavement, stormwater and flood control experts from local, state and federal government, industry, nongovernmental organizations and researchers to identify what needs to be done to fill gaps in awareness about permeable pavement. A road map for moving forward has just been published.

Environmental leadership

Although much of its work is responding to California’s needs, the UC Pavement Research Center has built connections all over the world, Harvey said.

“An area where we’re in the lead globally, is quantifying the environmental impact of pavement,” he said.

The current standard for assessing the costs of different kinds of pavement is Life Cycle Cost Analysis, which takes into account the costs of manufacturing, installing, maintaining and dismantling pavement as well as the cost impacts of things like traffic delays and additional vehicle repairs caused by poor road quality.

We spend much of our time walking, bicycling or riding over pavement without thinking about it – until we hit a rut or pothole.

Harvey, Professor Alissa Kendall of the UC Davis Department of Civil and Environmental Engineering and the pavement team have developed protocols for application of an expanded analysis, Life Cycle Assessment (LCA), to pavement. LCA quantifies the environmental impact of roads — from the energy and emissions of making pavement materials to the effects of road quality on greenhouse gas emissions, to the “end-of-life” scenarios when the life cycle is completed.

“There are always tradeoffs,” Harvey said. “You have to balance two or three different things — cost versus quality, different properties — and find the optimum point, and now we want to bring environmental impact into the equation.”  

If that kind of analysis makes movie characters’ eyes glaze, maybe it shouldn’t. After all, we spend much of our time walking, bicycling or riding over pavement without thinking about it — until we hit a rut or pothole.

Thanks to the work of the UC Pavement Research Center, it will be cheaper, longer-lasting and faster and easier to maintain with a lower environmental impact.

(Getty Images)