His Flexible Concrete Bends But Doesn’t Break
If you ask Victor Li about concrete, he will sing its praises. “Concrete is a great construction material, one of the most successful ones man has ever made. That explains its popularity. It is a very good material for constructing bridges, roads, and buildings and many of the things we depend on in our daily life.”
Why, then, has he embarked on a mission to replace the everyday concrete we know? “The main shortcoming of the material is that concrete is brittle, meaning that it cracks, and as a result of cracking, it brings deterioration and damages that require repeated maintenance,” he explains. And he adds, “In situations where a structure experiences severe loading like earthquakes, there can be collapses.”
An engineering professor at the University of Michigan (UM), Li has developed a new type of flexible concrete known as an engineered cement composite (ECC). He hopes it will find widespread use across the country. “We’re trying to create a new generation of concrete that if you put it under excessive bending loads, it bends but doesn’t fracture into pieces like glass.”
Conventional concrete is made by mixing sand, cement, and aggregates such as gravel and then activating it by adding water. It typically has steel or fiberglass reinforcing bars – known in industry slang as rebar – running through it for added tensile strength and to reduce cracking. This results in a material strong in compression but weak in tension or bending.
ECC resembles regular concrete but can weigh up to 40 percent less, consisting mostly of the same ingredients except for the coarse aggregates. It has small polyvinyl alcohol fibers embedded within it, 8-12 millimeters long and about 40 microns in diameter, about half the thickness of a human hair. They have a nanometer-thick surface coating that allows them to slip rather than break under heavy loads. In place of coarse aggregates, it relies on fine sand, as aggregates disturb placement of the fibers and destroy the ductility. In some applications, rebar can be eliminated.
The material has a compressive strength similar to that of regular concrete. But while normal concrete has a strain capacity of .01 percent, ECC has a tensile strength capacity of 3 to 5 percent, or about 300 to 500 times as much, making it far more ductile.
In addition, Li says, “Even when ECC gets damaged by excessive loading, the micro cracks are self-controlled, and the crack widths are limited to less than 50 microns. In structures like a bridge deck, we don’t want water or deicing salts to get through the cracks and attack the steel. This kind of deteriorating mechanism is greatly delayed or eliminated.” The net result: “From a long-term standpoint, to improve durability means less maintenance requirements, and that means lower lifecycle costs, particularly for infrastructure like bridges and roadways, where a lot of maintenance is required.”
Li, 55, grew up in Hong Kong and came to the United States to go to college. “The U.S. has strong engineering programs in college. I was hoping to participate in that process of being innovative and creative,” he recalls. First came a B.S. in Engineering Science from Brown University, followed by an M.S. in Mechanical Engineering and a Ph.D.
Following that, Li became a professor of civil engineering at MIT, where he stayed nine years. Then in 1990, he became a professor of civil and environmental engineering at the UM, and in 2004, he added the title of professor of materials science and engineering.
About 15 years ago, Li began developing ECC technology at UM. In recalling those early days, he reveals, “A lot of people had the wish for a concrete that doesn’t crack. To make something that actually behaves so requires extensive engineering work. In fact, we spent quite a few years to understand what really makes concrete brittle.”
Working with his colleagues, Li formulated a theory for a new concrete when he taught at MIT and implemented that after he moved to Michigan, extensively testing the material for about six years. Since then, he recalls, “We’ve done a lot of field testing the last 6 to 7 years both in the United States and overseas, particularly in Japan.”
In explaining his motivations for developing bendable concrete, Li reveals, “It was a response to many of the major concerns we face every day in society, things like climate change; our infrastructure is experiencing more and more loads from extreme weather conditions. The concerns about environmental sustainability relating to the high energy and carbon dioxide emissions of producing concrete. The infrastructure in the United States not being in great shape. And of course, the economic crisis we are facing now. States are short of budgets for maintaining their infrastructure. All these are crying out for better materials for construction, concretes that are more damage-tolerant with less burden on the environment and greater durability requiring less repair.”
Actually, the original application of ECC was for seismic structures, particularly in Japan, which lies directly on seismic faults. “The material can deform during the earthquake and absorb the energy without fracturing,” Li reports. “In fact, in Japan, it has gotten beyond field testing and into some full-scale structures.” That work was led by one of his former graduate students. The most recent building to use the technology, a 60-story residential tower was completed about nine months ago in the city of Osaka.
ECC can be used any place where regular concrete is used, though the material is still more expensive. It costs about three times as much, but Li foresees the price dropping in the future. Because they have a longer life than regular concrete, engineered cement composites are expected to cost less in the long run, especially when experience is gained in large-scale production.
But Li points out a number of applications, including the buildings in Japan, where the initial cost of ECC is already low. In such buildings, by using this material in the core of the building, they eliminated other seismic resistance devices used to maintain safety of the building in earthquakes, and those can be extremely expensive.
Another area where ECC can result in savings is on bridge decks, as major problems occur when expansion joints between deck sections jam frequently. In a demonstration project Li’s charges did in Ypsilanti for the Michigan Department of Transportation, the bridge had regular expansion joints replaced by a slab of ECC material 17 feet wide crossing four lanes of traffic. Known as link-slab in this application, the ECC actually expands and contracts as the deck moves with temperature fluctuations. It eliminates many of the common problems associated with conventional expansion joints like joint jamming and leakage, which results in water and deicing salts passing through the joints and corroding the steel supporting the structure. Lifecycle calculations indicate as much as a 40-percent savings in carbon and energy footprints through the use of link-slab on this deck.
ECC can be mixed and placed by the same equipment used for traditional concrete. Li is working with suppliers and Michigan DOT to develop procedures so regular ready-mix trucks can deploy the material. Placement of ECC is actually easier because it is self-consolidating and needs no vibration.
Looking back on his career, Li reveals, “This is exactly what I was hoping to do. This type of work is very challenging, but it leads to solutions that help society. It’s very satisfying. By contributing to solving these problems, it makes our work meaningful, problems like climate change and our infrastructure.” He also gets satisfaction from passing on new knowledge to students. “They should be even more clever and innovative than we are. Our students are future innovators.”
In the near future, Li says, “We are conducting research to further improve the material by adding smart functionalities. We’re working on two projects. One is to make the material smarter to know when it has been damaged. Then, it can heal itself when it cracks.” In Li’s lab, self-healed specimens recovered most if not all of their original strength after researchers subjected them to a 3-percent tensile strain, the equivalent of stretching a 100-foot piece three feet. To accomplish this, dry cement in the concrete exposed on the crack surfaces reacts with water and carbon dioxide to heal and form a thin white scar of calcium carbonate. Calcium carbonate is a strong compound found naturally in seashells.
Working with Jerry Lynch, a young colleague at UM, Li is developing a self-sensing version of ECC, meaning if it gets damaged, it knows how much damage it has experienced. It could be used in applications like warning motorists of damage to a bridge structure before they come too close. Or it can help bridge inspectors monitor the condition of bridges and determine when and where to conduct repairs.
“These things are down the road,” Li says. “Our civil infrastructure can be much smarter.” He also thinks ECC “opens the door to potential applications where concrete currently cannot be used.” The future looks bright for concrete as a building material and an agent for improving our infrastructure. Years from now, we’ll probably hear Li extolling its virtues even louder than today.
For more information on bendable concrete, visit www.engin.umich.edu/acemrl/