Paving the Way to Better Streets, Bridges
A century ago, North Carolina became known as "The Good Roads State" because of its extensive system of state-maintained highways. Now, as increasing traffic volume strains the capacity and durability of state roads, NC State researchers are developing ways to make asphalt pavement last longer, predict maintenance needs on bridges, and redesign congested intersections to improve safety and efficiency. The efforts are done with an eye toward extending North Carolina's reputation for quality transportation infrastructure well into the future.
North Carolina Department of Transportation (NCDOT) road crews are probably the only people who have seen steam rising off asphalt more than Dr. Richard Kim. A professor in the Department of Civil, Construction and Environmental Engineering (CCEE), he has pioneered research on how to make pavement last longer, testing asphalt mixes from across the U.S., as well as Canada, China, and his native South Korea. In a basement lab in Mann Hall, Kim simulates traffic volume by repeatedly running tires over pavement samples and exposes the samples to extremes of hot and cold. As part of a Federal Highway Administration project, he is applying models to the test results to develop performance-related specifications for pavement. State highway agencies have traditionally used "recipes" for asphalt based on what worked elsewhere, because predicting pavement performance for a range of conditions was difficult. "We have developed models that reflect the mechanical behavior of the asphalt," Kim says. "This will give contractors and highway agencies more confidence in using performance-based specifications."
Kim also is helping NCDOT improve the pavement used on secondary roads statewide. "Chip-seal" involves spreading a layer of aggregate atop a thin layer of asphalt and compacting the layers. "There's been no science behind it until recently," Kim says, noting road crews often tested the quality of their work by kicking at the edges of new pavement. His research demonstrates lightweight aggregate and asphalt that contains a polymer emulsion to improve adhesion on the road surface, decreasing the likelihood that bits of aggregate will loosen and go flying as cars drive by. "We can now quantify the impact of different materials and find the best construction methods so chip-seal becomes more durable and can be used on roads with higher traffic volumes."
Models that reflect the mechanical behavior of asphalt should give contractors and highway agencies more confidence in using performance-based specifications.
At the Constructed Facilities Laboratory (CFL) on NC State's Centennial Campus, Dr. Sami Rizkalla, Distinguished Professor of Civil Engineering and Construction, is likewise adding quantifiable measures to the field of bridge maintenance. For years, Rizkalla has studied how to use optical sensors embedded in bridges to monitor their structural integrity. Following the fatal 2007 collapse of an interstate highway bridge in Minneapolis, he created a statistical model to predict when bridges might experience problems and to estimate their functional lifespan. "People think an annual bridge inspection is sufficient," he says, "but a visual inspection cannot tell you everything you need to know."
Because the uniquely equipped, 20,000-square-foot CFL allows full-scale testing of structures like bridges, Rizkalla's research team can use impact hammers and hydraulic jacks to vibrate and place loads on steel girders and concrete bridge decks. Sensors attached to test bridges each provide 10,000 readings per second to a computer model that scans the data for anomalies that might indicate damage, and provides an analysis of other problems that might result from such damage. A small crack in an I-beam, for example, could cause a chain-reaction that ends in a catastrophic failure elsewhere on the bridge.
"People think an annual bridge inspection is sufficient, but a visual inspection cannot tell you everything you need to know."
The primary factor limiting the use of Rizkalla's model is the paucity of bridges with fiber-optic sensors. Rizkalla says sensors are an easy item for states to delete from construction projects to save money, regardless of the potential for future savings in maintenance spending. Still, he says, it's only a matter of time for them to become a common feature. "Civil engineering is slow to change," he says. "Century-old bridges are exactly the same as those built today."
Dr. Joe Hummer, a CCEE professor, is on a similar quest to convince traffic engineers to design so-called "superstreets"
at complex intersections to improve safety and relieve congestion. Superstreets use a median to divide traffic on the main road and block through-traffic from a side street. Drivers who want to turn left onto the main road must turn right and then make a U-turn across the median. Hummer says superstreets can cut the number of points in an intersection at which vehicles cross paths — creating the potential for collisions or delays — by up to 75 percent. "There are too many things going on in the middle of a conventional intersection," he says. "With superstreets, drivers have only one thing to worry about at a time."
The concept was developed in Alabama more than two decades ago, and Hummer became enamored with it as a variation of the specialized intersections he grew up using in Michigan. His research has optimized the superstreet design, testing factors like median width, signal timing, and accommodations for pedestrians and bicyclists. He also evaluates potential superstreet locations for NCDOT, which has installed 20 such intersections statewide — the most in any state. Hummer's studies show that fewer collisions occur on superstreets and that traffic flows better because there are no green turn-arrow signals to wait through. "Engineers usually have to trade efficiency for safety, but this is the rare case where we can improve both," he says. Still, Hummer says, many drivers dislike making multiple turns to get where they want to go, even if it's a safer route. "Safety is a hard sell in our profession," he says. "Nobody thinks he's going to be the next statistic."