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Concrete Mix Adds Flexibility, Durability

High-performance, fiber-reinforced concrete that is superhydrophobic and is also more flexible and more durable than typical concrete
Researchers at the University of Wisconsin-Milwaukee have developed a type of high-performance, fiber-reinforced concrete that is superhydrophobic and is also more flexible and more durable than typical concrete. The material should be especially useful in constructing longer-lasting bridge abutments. Courtesy of Professor Sobolev’s research group at UWM

Researchers at the University of Wisconsin have developed a hydrophobic concrete that is more flexible and durable than typical concrete.

April 22, 2014—Bridge approach slabs experience higher dynamic loads than either the roadway pavements or the bridge decks to which they connect. This is because the differential settlement of the three structures creates uneven surfaces that cause vehicles to “bump” as they travel from one to the other. This bump causes sudden—and repetitive—loads to be applied to the approach slab, which in turn causes a faster rate of deterioration of the concrete used to build it—especially if that concrete is exposed to such harsh conditions as freeze-thaw cycles and road salts.

To combat this problem, researchers at the University of Wisconsin-Milwaukee have developed a type of high-performance, fiber-reinforced concrete that is at once superhydrophobic, flexible, and more durable than typical concrete. “An approach slab is a very delicate structure that needs a smart design,” says Konstantin Sobolev, Ph.D., an associate professor in the Department of Civil Engineering and the Department of Materials Science and Engineering at the University of Wisconsin-Milwaukee. Sobolev heads the team that is developing and testing the concrete. “Usually bridges are heavily reinforced structures, and the pavement usually has zero reinforcement,” he says. “When you have these structures connected [with approach slabs], an effective connection is very difficult to achieve.”

Sobolev’s solution is called superhydrophobic engineered cementitious composites (SECC), and the material is expected to have a life span of 120 years, compared with the 4- to 5-year lifespan of more typical concrete. While the material may be used in any number of applications, its best fit is in approach spans, Sobolev says.

Due to the use of chemical admixtures and the control of microlevel surface roughness, the angle at which water will contact the concrete is increased to 150 degrees, making it superhydrophobic. Concrete, by its nature, absorbs water, Sobolev says. By tailoring the surface properties of the concrete at microscopic and nanoscale levels, the team is able to control the water-repelling nature of the concrete, creating a scenario in which the water beads and flows off the concrete at the slightest inclination, limiting the concrete’s saturation and ultimately its exposure to, and damage from, freeze-thaw cycles.

Additionally, the inclusion of unwoven polyvinyl-alcohol fibers that measure the width of a human hair as well as carefully sized air bubbles enables the concrete to experience microcracking that leads to a superior level of strain hardening, Sobolev says. The strain hardening allows the concrete to exhibit a level of flexibility that is similar to metal, he says. That flexibility stands in stark contrast to the brittle nature of most concretes, he adds. The fibers and air bubbles also enable the concrete to withstand 4 times the compression and 200 times the ductility of normal concrete, according to Sobolev. The air bubbles alone improve the fiber-reinforced concrete’s ductility by up to 30 percent, Sobolev says.

“The air voids themselves act as almost artificial flaws within the material to incite cracking in flexure or tension and allow for more ductile behavior,” explains Scott Muzenski, a doctoral candidate and member of Sobolev’s team who is conducting research into the optimal mix for bridge approach spans.

“When you’re looking for flexural behavior or tensile behavior, you don’t want one single crack, you want multiple small cracks, and that’s what these voids are helping us to accomplish,” Muzenski explains. “Once a small crack occurs, the fibers will be able to bridge that crack and transfer the stresses to another location where another crack can form, and [thus] we see a lot of multicracking behavior.”

For his master’s degree, Muzenski created and tested a variety of mixes to create an “even stronger, even more ductile, even more durable” material, Sobolev says. “The goal is a material with ultimate performance and ultimate durability,” he says. Muzenski’s doctoral work will further expand on this by developing ultrahigh-strength cementitious matrices with nanofibers; he will then model the composite’s behavior.

Creating the optimal mix for bridge approaches takes a bit of fine-tuning, Muzenski says. “There is a bit of trade-off between strength and ductility because the increase in strength can make the material more brittle,” Muzenski says. “One way to increase the ductility without the loss in strength is to add small air bubbles to promote multiple cracking.”

Last August, Muzenski and Ismael Flores-Vivian, Ph.D., a research associate at the university, created two prototypes using the mix Muzenski developed for his master’s thesis. The first is a real-world pavement-connection slab placed in a ramp on the university’s campus. The second is located in the controlled environment of the lab and is being used for additional research. While both have the same thickness and section, the lab prototype has a smaller footprint to make testing easier.

The real-world prototype is a 4 by 15 ft slab that replaced a deteriorated portion of a slab in the ramp to a parking structure. Measuring 8 in. thick, the slab makes use of a sandwich construction method. The lower 2/3 of the slab contains the hydrophobic, fiber-reinforced mix created by Muzenski and contains a series of parallel hollow core sections. This layer is topped with a 1 in. thick layer of concrete containing carbon nanofibers. These two layers together are sealed with a 1 in. top layer of Muzenski’s mix.

Muzenski says that the middle layer containing the carbon nanofibers was created by Josuhua Hoheneder, A.M.ASCE, also a master’s student. The carbon nanofibers make the middle layer of the slab electrically conductive so that it can be used to sense any cracking and loading that occurs within the slab. “His material is very similar to mine [because] it still has the fiber reinforcement [and] it still has a similar composition and water-cement ratio,” Muzenski says. “But he added the carbon nanofibers to make that layer electrically conductive.” The similarities in composition between the two types of layers mean that they behave in similar manners and work well together, he says.

The hollow-core sections were added to create weaker areas that will guide the development of the cracks to locations just below the electrically conductive layer, Muzenski says.

While testing of the lab model has been ongoing, the real-world prototype’s carbon nanofibers will be electrified within the next few weeks, once the winter’s snow has cleared and the system can be hooked up, according to Muzenski. The team will then be able to monitor the development of cracks and assess the concrete’s performance.

Software is currently being developed so that if the real-world prototype experiences excessive loading or if a crack develops, the system will either send an email or text notifying the team so that they will know to examine the slab, Muzenski says.

The information gathered from the real-world test will help the team’s work as they further their research into the practical applications of this type of concrete.

The results of Muzenski and other team members’ work can be read in reports issued to the agency that funded the research, the National Center for Freight & Infrastructure Research & Education. The prototypes of the SECC material were created in collaboration with Habib Tabatabai, P.E., S.E., M.ASCE, and Jian Zhao, M.ASCE, both associate professors of civil engineering at the university, with funding from the university’s Research Growth Initiative. Sobolev’s ongoing work on the SECC material, which is highlighted on his website, has also included Michael Oliva, Ph.D., a professor of civil and environmental engineering, and Tom Krupenkin, Ph.D., an associate professor of mechanical engineering, both at the University of Wisconsin-Madison.



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