When it comes to building a skyscraper, the main ingredient has remained the same for nearly 50 years: reinforced concrete. A water, cement, and aggregate mixture poured over steel reinforcement bars, the composite material creates a sturdy, robust foundation for structures.
But tell that to an office worker on the 50th floor when the wind is gusting.
“We’re talking buildings that are thousands of feet tall. And when they’re that tall, they actually move a substantial amount,” says David Corr, the Charles Deering McCormick University Distinguished Clinical Professor of Civil and Environmental Engineering. “It could be up to a foot in high winds. Sometimes a person on a high floor can even become seasick.”
Combating that sway — which can range from the motion sickness–inducing variety to that caused by earthquakes and other environmental hazards — has been the focus of research being conducted by Corr and Gianluca Cusatis, civil and environmental engineering. Both researchers are looking to improve on two key characteristics of building materials: strength and ductility.
Finding the 'Magic Recipe'
Strength, Cusatis says, refers to the amount of weight a structure can bear. Ductility measures a solid material’s ability to stretch, which helps dissipate the energy of, say, earthquakes. Cusatis is developing an ultra-high-performance concrete that requires less water and material.
But while the new mixture is typically three to five times stronger than normal concrete, it lacks ductility, Cusatis says. “These very-high-strength materials tend to be more brittle, like glass.”
The goal now is to add in enough fiber reinforcement — perhaps steel or polyvinyl alcohol, a high-strength plastic — to make the concrete ductile without a loss of strength.
“If I add the plastic fibers, I do have quite a bit of increase in ductility, but then the strength might decrease,” he says. “It really is about optimization. You want to find that magic recipe so that you can have the best possible properties all together.”
Thinking Small to Build Big
Corr is developing a concrete mixture that includes carbon nanotubes, cylindrical molecules that are incredibly strong. The nanotubes stiffen the concrete mixture, increasing the rigidity of a building’s structural frame at a relatively low cost.
“That improvement to concrete will reduce the motion in tall buildings and make the occupants more comfortable,” says Corr. But first Corr has to scale the development to a level that is applicable for building purposes while also keeping the nanotube molecules from clumping together. “When materials are that small, they tend to stick together,” he says. “You have to separate them before you can use them.”
The nanotubes are dispersed in a solution, which is then mixed with chemicals called superplasticizers to keep the molecules from sticking together again. Ultrasonic mixers are then used to disperse the nanotubes into the concrete mixture.
Once this process has been perfected, the technology will move toward commercial application.
“We’re making our case that these materials do what we say they do and that the dispersion technology is within the grasp of an industrial partner,” Corr says.