Concrete Solutions

Researcher looks to nature to create more sustainable material

Oct. 26, 2016 - Civil engineer Mo Li grew up in a coastal town in China best known for producing Tsingtao, a popular beer. She came to the U.S. to earn two master’s degrees and a doctorate at the University of Michigan. After serving a short stint as an assistant professor at University of Houston, Li settled in another coastal area, at UCI, where she is brewing up a new mix for concrete.

The most heavily used manmade material in the world, concrete plays an integral role in the development of civilization, dating back to ancient Roman times. Even the Egyptians used an early form of concrete over 5,000 years ago to build pyramids.

Today, the manufacturing of Portland cement – the key ingredient in concrete – accounts for 5 percent of worldwide industrial energy consumption and is responsible for 5-8 percent of human-generated global CO2 emissions, roughly, the CO2 equivalent of 305 million automobiles.

As the world population grows and the economy expands, annual consumption of concrete increases and so, too, does its environmental footprint. It’s no wonder scientists around the world are trying to make a better, greener, more sustainable concrete.

Li, an assistant professor of civil engineering, explores the chemistry and physics of concrete at the nano-, micro- and macroscales, aiming to create the next generation of the widely used material. And while some researchers are looking for ways to green the production of cement, Li is focused on its life cycle to improve sustainability and resilience.

Her research shows progress with three new enhanced capabilities: repeatable self-healing, extreme strength and toughness, and an encoded “smart” sensibility.

These new concrete materials can have many applications: from long-lasting and low-maintenance bridges and pavements to coastal infrastructure better equipped to withstand natural hazards; from safer and longer-term storage of nuclear wastes to more effective carbon sequestration.

“Concrete is inherently brittle and thus cracks easily,” explains Li. “Cracking can be caused by a variety of loads and environmental conditions. It can open channels for chemical attack, water permeation, chloride penetration and corrosion of steel reinforcements. The conventional approach to designing stronger, higher-strength concrete does not necessarily lead to more a durable concrete. Just like steel, increased material strength is often at the cost of increased brittleness.”

For a blueprint, Li looks to nature − seashells or injured skin – where biological materials naturally possess damage tolerance and self-healing capacity optimized through millennia of evolution.

She wondered if she could design these beneficial features into concrete. To find out, her group investigates the complex kinetics, chemistry, structure and properties of self-healing products formed within cracks (e.g. calcium silicate hydrate, C-S-H, and calcite, CaCO3), linking nanoscale healing phenomena to macroscale recovery of material properties. By controlling the formation and submicron properties of C-S-H within cracks, coupled with modifying the damage behavior of the material under loads, Li’s new concrete can manage and heal its own damage.

“Our work is a unique integration of cement chemistry, fracture mechanics and multiscale materials science,” says Li, whose new material goes beyond sealing the cracks; it actually heals the cracks to regain mechanical properties.”

“The repeatable self-healing concrete that Mo Li is developing with her group is quite innovative,” says A. Erik Schlangen, a civil engineer and pioneer of selfhealing materials at Delft University of Technology in the Netherlands. “This is important because concrete structures are constantly under load. Cracked and healed material will again be stretched and new cracks will develop or the healed cracks will open again. Selfhealing concretes make structures more durable and ensure longer service life with continued functionality without having to perform maintenance.”

Li has two other ideas to improve the life cycle of concrete, both funded by the U.S. Department of Energy. One is to design an ultrastrong and super-tough material, which could be used by the DOE to build small modular reactors. Her other project is to encode concrete with a “smart” self-sensing functionality, so that a structure would be able to sense distributed damage, strain and corrosion wherever it is located and deliver a more accurate early warning for maintenance crews.

Li believes California’s diverse geography, with its mountains, desert and coastal areas, wet and dry regions and various temperatures, aids her research goals. “I can investigate new concrete materials in different natural environments. Southern California provides various conditions for field tests in addition to laboratory studies.”

Materials research intrigues Li because it allows her the opportunity to do something that’s never been done. “Materials are everywhere. They are how we interact with the world; they are how we live in the world,” she says. “If you can fundamentally improve or change the material, you might change the world.”

-Lori Brandt