Thin pavements—in which new pavements are constructed over an existing base layer—can be an economical option for low- and moderate-volume roads. However, thinner concrete roads are prone to distress caused by weather and traffic loads. The solution, U of M researchers found, may be to add small synthetic fibers to the concrete.
In a new project building on earlier studies, CTS research scholar Manik Barman, an associate professor of civil engineering at the U of M Duluth, worked toward designing and developing pavement-specific synthetic fibers that would enable thin concrete pavements to achieve longer lifespan. The goal of the study—a pool-funded project of the National Road Research Alliance, administered by MnDOT—was to see how these fibers could mitigate one of the primary distresses experienced by thin concrete pavements: transverse joint faulting that can shorten service life.
Concrete pavements have slabs separated by joints to allow for movement, such as expansion or contraction from temperature changes. Transverse joint faulting occurs as a noticeable difference in elevation between two adjacent concrete slabs at a transverse (crosswise) joint, essentially creating a step or "fault" at the joint line. The faulting is caused by poor joint performance or load transfer between thin slabs, Barman says, which “creates a washboard-like effect for drivers on the roadway.”
Joint load transfer is the way in which pressure (for example, from a vehicle driving over the road) moves from one concrete slab across the joint to the next slab, helping to distribute the load over a larger area. In conventional concrete pavements, joint load transfer is achieved in two ways. In the first, dowel bars are placed between slabs. However, dowels can’t be used in pavement that is less than 7 inches thick. In the second way—known as aggregate interlock—chunks of rock embedded in the concrete catch on and connect with one another, creating a stable bond and helping the material resist excessive shifting. Yet again, the nature of the thin structure of the concrete pavement largely prevents this from occurring.
In this project, researchers built a model to determine exactly what both aggregate interlock and structural fibers contribute to the fiber-reinforced thin concrete’s performance. They began by analyzing the data from an earlier research project, in which test sections of fiber-reinforced thin concretes were constructed at the Minnesota Road Research facility, to quantify the load-transfer contributions of the base layer and aggregate interlocking, along with what was needed from the structural fibers.
The new model showed that the thin fiber-reinforced concrete pavement’s performance largely depended on the vertical stiffness of the joints. Vertical joint stiffness was achieved by contributions from the base layer (30 to 40 percent), aggregate interlock (15 to 20 percent), and the structural fibers (the remaining 30 to 40 percent). Additionally, the researchers created a new procedure to characterize the fibers’ contribution to joint-load transfer, which they named the “modulus of fiber support,” and determined a recommended value for its use in fiber-reinforced thin concrete pavements.
In the project’s final phase, researchers tested different types of synthetic fibers to determine which worked best. “We embedded different types of fibers into concrete and then used our equipment to pull out a single fiber, measuring the force it took and observing the effect on the fiber,” Barman says. The team found that fibers with indentations and irregularities had an increased peak load and toughness. The research team is planning to advance its work by developing this type of “bumpy” fiber with the needed modulus of fiber support value. The researchers also hope to develop a simple test to screen fibers specifically for concrete pavement applications, characterizing the fibers’ contribution to mitigate joint faulting.
—Megan Tsai, contributing writer
