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March 6, 2008

Prof studies pavement under various conditions

Among the most familiar sounds that herald the approach of spring in Pennsylvania is the thump, thump, thump of vehicles rolling over rough and crumbling roadways as pothole season begins in earnest.

As head of Pitt’s pavement mechanics and materials laboratory, civil and environmental engineering professor Julie Vandenbossche aims to make those sounds less common as she searches for factors that will result in better, longer-lasting pavements.

When pavements fail, it’s usually a combination of problems. “You can’t point to any one thing,” she said, noting that the materials, design and construction all need to be considered when trying to determine the cause for crumbling concrete.

Vandenbossche troubleshoots pavement problems for Pennsylvania Department of Transportation project engineers when problems arise in the field: Pitt has a five-year, $25 million intergovernmental agreement that smoothes the way for PennDOT to work with Pitt on a variety of projects. That means she sometimes might be sent pieces of concrete, a binder of photos or emails describing the problem in detail. Or, she could get a request to come see for herself.

She also tackles questions from fellow University researchers or community members in need of help.

“Sometimes it seems people think I have a shovel in the back of my car with asphalt to fill potholes on the spot,” she said with a laugh, admitting that she fields some interesting inquiries.

Among the more unusual requests was one from a dental school researcher about using cement in dental work. Even more unusual was a long-distance caller who mistakenly thought pavement in Pittsburgh was exploding and phoned to ask her about it. Apparently he’d heard reports of concrete “blowup” and misunderstood the lingo. Blowup, Vandenbossche said, is a situation in which slabs of pavement tent upward in response to heat-related expansion. To the consternation of afternoon commuters, it can occur on roadways in the late-day summer heat, but it has nothing to do with actual explosions.

Odd questions notwithstanding, Vandenbossche enjoys her chosen specialty, admitting that as a woman with expertise in concrete research, she herself is a bit of a rarity.

When she arrived at Pitt in 2003, she was the lone female civil engineering faculty member. Two others have joined the department since, she said.

Although she enjoyed building things as a child, she initially pondered a career in business, then physical therapy before finding neither of those fields suited her. She considered civil engineering when a friend became interested in it.

Her future was cemented when she took a summer job on a Michigan transportation department thruway construction project. There, she got to see all aspects of civil engineering and found it “pretty neat” to have a career that offered variety as well as the chance to make an impact by working on projects that people drive by every day.

While her bachelor’s degree emphasized environmental engineering, her master’s is in geotechnical engineering and her PhD is in structural engineering. Specializing in pavement puts a heavy emphasis on materials and allows her to combine a wide range of other aspects of civil engineering ranging from drawing and modeling to structural engineering, as well as dealing with water, soils and what foundation lies beneath the road.

Vandenbossche’s lab in Benedum Hall’s sub-basement contains everything needed to make concrete objects of any size or specialized composition. “We can test pretty much any type of characteristic in concrete you can think of,” she said. The lab’s work isn’t limited strictly to roadway pavements, but also applies to sidewalks, bridges and concrete used in buildings.

In addition to an ordinary cement mixer, the lab houses a restaurant-style mixer and a Kitchen Aid for smaller batches. A crusher enables researchers to make aggregate of any size to add to the mix. One environmental room in the lab is used to cure concrete under the optimal conditions of 70 degrees and 95 percent humidity; another allows researchers to measure drying and shrinkage of concrete under various conditions. Knowing exactly how the concrete responds to changes in moisture and temperature is important for design purposes.

A saw and a drill press allow flat or core samples to be cut for examination — lasers and microscopes are located in a separate lab far from the gritty dust that goes along with the cutting and mixing.

Getting a close look at concrete can give researchers clues into how cracks in the material behave, information that can aid in the development of concrete that cracks less or that cracks in a way that has less of an effect on the material’s surface.

Even more fun than having what it takes to make or test concrete, Vandenbossche said, the lab also contains everything needed to destroy anything made of concrete. One of the lab’s two compression machines can apply loads of up to 400,000 pounds. Another piece of testing equipment can simulate the effect of a vehicle load on a slab of concrete.

Among Vandenbossche’s research interests is the development of “greener” concrete. The material releases carbon dioxide as it dries, but the addition of materials such as recycled industrial byproducts could cut the greenhouse gas emissions, she said.

She also is investigating the use of multi-layered bonded concrete overlays that use less-expensive concrete beneath while reserving stronger, more expensive pavement for the top layer that is in contact with vehicles.

Some of Vandenbossche’s research lies miles away from the Benedum Hall lab. Among her projects is a stretch of 14 slabs of concrete pavement along Route 22 in Murrysville. Since August 2004, 400-plus sensors embedded in this “smart pavement” have collected information on environmental conditions and traffic loads every 15 minutes, creating a vast database to aid PennDOT in developing new highway design standards that take better account of the real-world conditions to which a roadway will be exposed.

The sensors allow researchers to predict the shape of the concrete slab based on the data that’s collected. The slabs curl upward when the surface is colder than the underlying layers. The reverse is true when the surface expands in the heat. The weight of cars and trucks adds stress to the mix.

“It’s not a simple calculation,” she said, noting that a huge database of weather and traffic information is needed.

In a report to PennDOT last month, Vandenbossche recommended the continued collection of data from the Route 22 site for two more years because the moisture in the slabs soon will be reaching equilibrium — a process that takes about five years, she said.

“This is the first time a paving mixture has been characterized to this level of scrutiny,” she said. By measuring the stresses under various loads, temperatures and weather conditions, researchers can better model how paved surfaces will hold up and make recommendations on construction materials and techniques.

PennDOT’s new design standard will take into account the traffic, temperatures and stresses over the 30-year life of a pavement by calculating the stress for all combinations of those factors in two-hour increments over the 30-year span.

“The results of our work will be more cost-efficient concrete pavements,” Vandenbossche said, noting that even a 1 percent improvement in efficiency, given the size of PennDOT’s paving budget, would result in a huge cost savings.

—Kimberly K. Barlow

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