This Highway Will Crack
CTR Researchers Induce Cracks in Concrete for Your Benefit!
A section of new pavement currently under construction on Texas Highway Loop 330 just southeast of Houston is going to crack, and it will do so because Jeong-Hee Nam wants it to.
Nam, a third-year Ph.D. student at the Center for Transportation Research, is studying the mechanical behavior of continuously reinforced concrete pavement (CRCP), the predominant pavement employed for heavy-use highways in Texas.
Together with Dr. B. Frank McCullough, Dr. Seong-Min Kim, and Terry Dossey, Nam has devised a method to induce a crack in a real-world highway—one that is actively in use by the public—and measure the mechanical behavior of the pavement before, during, and after cracking. Jeong Hee Nam testing the Campbell Scientific CR10X Data Logger used to capture crack data. While the crack will only be only a hairline fracture 0.02 inches wide, unnoticeable to drivers at highway speeds, it will, however, yield important information about how pavement cracks, leading to design recommendations to improve future construction. Laboratory testing is sometimes useful in this area, but scaled-down lab models can't compete with real-world conditions.
"We believe our field tests will produce better data than is currently available," Nam explains.
How does one persuade a slab of heavily reinforced concrete to crack on cue, and then document the event? First, special sensors—vibrating wire gauge, steel strain gauge, and “I-button” sensors—are placed around the area of the intended crack and concrete is poured over them. When the crack occurs, they’ll be in place to record what happens. The crack itself is produced by an innocuous looking device called—what else?—a “crack inducer.” The inducer is a thin wood or metal strip pressed into the surface of still-wet concrete. When the inducer is removed, a narrow groove (of proprietary dimensions) remains in the surface, which leads the concrete to crack over time with the daily expansion and contraction caused by temperature changes in the environment and the curing process. Calibrating sensors embedded in concrete sample prior to field study.
"It's a bit like cutting glass," Dossey explains. "A carefully placed groove in the surface of the concrete can produce a crack straight through the entire depth of the pavement."
As that crack develops, the research team's sensors will be there to witness it. A standard Campbell Scientific CR10X Data Logger will collect data from the sensors at periodic intervals over the course of a year. That data will capture temperature, moisture, expansion, and contraction changes in the concrete and its steel reinforcements to paint a detailed picture of the dyanamics of CRCP over time. Curiously, the goal of this research is not to eliminate cracking.
"Cracks are natural," Nam explains "Concrete is always in motion, even after it has hardened, due to environmental changes in temperature and the loads placed upon it."
A hardened, reinforced structure such as a highway that is miles long will inevitably crack from the tensions that develop in the material. Years ago, highway engineers sought to eliminate cracking by building pavement in sections separated by cushioned gaps that would absorb the expansion and shrinkage that is natural over the course of a highway's life. However, such "jointed pavements" often became rough and uneven over time as some sections of the pavement sank, and others rose.
Sometime in the late 1930s the Indiana Highway Department was attempting to create just such a jointed pavement, but inadvertantly created the first continuously reinforced concrete pavement (CRCP), instead. Two decades later in the late 1950's, while working at the Texas State Department of Highways and Public Transportation (now TxDOT), Dr. McCullough expanded the CRCP concept and implemented it in Texas. This was the first time CRCP was used in Texas and it became the foundation for CRCP work around the world.
Placement of Flexi-Probe in still-wet concrete.
The key to CRCP is that concrete is poured in a continuous ribbon over networks of reinforcing steel. Instead of inserting joints in the pavement, engineers design the structure to crack on its own at strategic intervals so as to absorb the inevitable shrinkage and expansion that pavements experience. This method not only reduces the cost of construction, but improves road surface smoothness, as well, and has become standard procedure throughout the U.S.
Not all cracks are created equal, however. If a crack is too narrow, it won't relieve enough stress and anchor cracks will form; too wide, and moisture will penetrate to rust the steel. If cracks are too close together, a "punchout" may result—the concrete pavement version of the familiar pothole. Intentional cracking of CRCP can be managed by modifying certain variables such as the percentage of steel to concrete, the thickness of the pavement, and by designing the mix of materials that make up the pavement.
"We want to be able to control these variables under various conditions to optimize the placement and size of pavement cracks," says Dossey.
While few drivers develop a fondness of cracked concrete, Texas drivers at least can feel reassured that some hairline fractures are helping researchers create better roads for the future.