New materials and monitoring techniques are
Keeping Our Bridges Safe
In the quest for better bridge materials, graduate student Bhavna Sharma tests the delamination of carbon fiber reinforced polymer that has been bonded to the surface of a concrete specimen
Whether traversing picturesque one-lane stream-crossings along the Hāna Highway or freeway overpasses in Honolulu, most of us trust the bridge we’re on will do its job and support us.
That’s not always the case. In 1989 a 15-second California earthquake caused the Oakland Bay Bridge to collapse. In 2000 more than 50 people were hospitalized after an 80-foot section of bridge connecting a North Carolina NASCAR racetrack to its parking structure snapped, dropping event-goers onto U.S. Highway 29. And on July 2, 2005, a 35-year-old Oregon bridge crumbled, plunging a tow-truck driver 75 feet into the river below.
UH Mānoa Professor of Civil and Environmental Engineering Ian Robertson has been working with Hawaiʻ’s Department of Transportation and the Federal Highway Administration to monitor bridge performance and develop techniques to keep bridges structurally sound.
Bridges face a number of environmental hazards, which Robertson and his colleagues and students address in the department’s structures laboratory.
Don’t look for test tubes. In this lab a cement mixer stands at the ready; ovens bake asphalt, stone, sand and cement; a band saw slices through steel rods, and slabs of concrete endure various ordeals. In one procedure, black mesh made of carbon fiber is glued to concrete beams, which are placed in a machine dubbed "the rack." The device grabs onto both ends of the fiber-bonded concrete and applies up to 55,000 pounds of tension to pull it apart.
"Attaching carbon fiber stirrups increases a concrete beam’s strength and resistance to cracks. We’re testing to determine at what point the fiber detaches from the concrete," explains Robertson. Working with colleagues and the City and County of Honolulu, he plans to use carbon fiber to repair shear cracks in an O’ahu bridge.
Mānoa Professor of Civil and Environmental Engineering Ian Robertson works with Hawaiʻi’s Department of Transportation and the Federal Highway Administration to monitor bridge performance and develop techniques to keep bridges structurally sound.
In the basement, blocks of concrete have been exposed to salt water and drying cycles for six years, waiting for the steel rebar inside to corrode (a factor in the North Carolina bridge collapse and always a possibility in Hawaiʻ’s salt-air environment).
"Now that corrosion has started in the control specimens, we can take core samples and measure the concentration of chlorides, which initiate corrosion," says Robertson. He’ll also check changes in the concrete’s normally high pH level—a factor that protects against corrosion. Other blocks in the test have not yet corroded because they’ve been chemically reinforced with different aggregates or fly ash, a fine dust that fills gaps in concrete.
In 1994 the highway administration and state DOT asked Robertson to monitor H3’s North Hālawa Valley Viaduct to determine its performance over time.
"Concrete moves," notes Robertson. "It shrinks as it dries. Daytime heat and nighttime cooling cause expansion and contraction. When you squeeze it (with weight or pressure from embedded prestressed steel rebar), it shortens, just as a sponge does."
In conjunction with computer modeling, Robertson installed instruments that measured the concrete’s strain and stress, as well as the deflection, or movement, of the bridge. The 10-year study showed that the bridge had been well designed for Hawaiʻ’s environment. "Safety," he emphasizes, "is looking at long-term performance to verify that a structure is not behaving unexpectedly."
On the Big Island’s Hāmākua Coast, Robertson plans to conduct more long-term tests and seismic monitoring. "When the bridges along this coast were built in the early to mid 1900s, designers didn’t accommodate adequately for ground shaking from earthquakes," he says.
In recent years DOT has seismically strengthened or retrofitted most of the bridges. One, the Kealakaha Stream Bridge, is slated for a complete rebuild. To monitor the new structure, Robertson will install electrical-resistance and fiber-optic strain gauges as well as accelerometers to monitor shaking in both the bridge and surrounding soil.
"After an earthquake," he says, "we can analyze all the readings and recreate exactly what happened to the bridge during the earthquake. This will be the first bridge in Hawaiʻ instrumented for earthquake monitoring."
Seismic activity can create some unusual problems. "If you shake soil that is saturated, which is often the case around bridges over rivers, firm soil becomes liquid," explains Robertson. "If soil under a bridge foundation liquefies, you lose support for the bridge."
Associate Professor Peter Nicholson is addressing this problem, mapping the potential for liquefaction under bridges and other island structures.
Associate Professor Michelle Teng employs sonar to chart scour—the amount of riverbed washed away during a flood, which weakens support around bridge piers. Underwater transmitters send signals to the riverbed every 15 minutes and data loggers record the signals’ bounce-back time to monitor changes in depth.
"If scouring becomes too serious, officials can close the bridge for public safety," says Teng. For lack of real data, engineers have had to rely on equations developed from lab experiments under idealized conditions in designing foundations to extend below scour depth, she adds. "We’re collecting that data."
One-third of Hawaiʻ’s 1,099 bridges are functionally obsolete and 14 percent are structurally deficient, according to the U.S. Department of Transportation’s 2004 National Bridge Inventory. The numbers don’t tell the whole story, however. Hawaiʻ’s historic bridges fall into the former category because their narrow lanes and other features don’t meet current highway design standards, but communities insist on keeping historic designs intact.
Are these old bridges safe?
"A number of years ago, with funds from Federal Highways, the state started retrofitting a lot of bridges," Robertson says. "There is continuing repair, too, but Hawaiʻi’s bridges have performed well because the state and counties did a good job of constructing them in the first place."
That’s something Hawaiʻ drivers can rely on.
Bridging the ages: architecture classes document historic structures
Mānoa Professor of Architecture A. Spencer Leineweber and her students ford rivers, climb into gorges and cross every old bridge in their hunt for historic bridges.
"We’ve looked at more than 1,800, all built before 1959—statehood," she says. "We’re doing research for the state Department of Transportation to determine which bridges are important to the history of Hawaiʻi." Some may be eligible for inclusion on the National Register of Historic Places and, consequently, for repair funding.
Director of the school’s Heritage Center, Leineweber looks at criteria developed for the National Register to classify a bridge as historic: was it built by a well-known designer, has its integrity (original materials, design and construction) been maintained, was the construction technique innovative for its time, did the bridge influence development or important changes to the island?
The dozens of bridges holding up the Pali and Likelike Highways may be important, she says, "because those two roads were built at about the same time and were a major factor in the development of the Windward side.
"The importance of a specific item may vary because of the time period," she explains. "In the postwar period, developing ways to build bridges efficiently and effectively in concrete was important, while earlier bridges were steel or wood."
Two Hawaiʻi bridges are already included on the National Register—Oʻahu’s concrete arched Haleʻiwa bridge and Kauaʻi’s Hanalei Bridge, originally constructed in steel (1912) and then modified in steel (1960 and 2003) to preserve original features.
With fieldwork and research completed, Leineweber will meet with DOT’s Historic Bridge Committee to refine the listings and make final recommendations to the Hawai‘i Historic Places Review Board.