This year, my son Mitchell graduated out of Mr. Green’s grade three class.
Like his two older sisters before him, dad was called upon once again to help with Mr. Green’s “special” grade three science project—building a two-foot long bridge using only popsicle sticks, cardboard, glue, and fishing line.
In and of itself, that’s not a problem. But, the bridge also has to stand up under 200 pounds of bricks. That’s the first wrinkle. The second wrinkle is that this “science” project really amounts to a competition between all the grade three dads. There’s a lot on the line here, including bragging rights on the playground. I’m quite proud to say that Mitch and I got an “A.”
When it came to design, one of the crazier ideas we noodled was building the bridge out of solid glue. “That’d really get Mr. Green,” I said to Mitch. “We could make a mold, pour in a bunch of glue, let it dry, and then strip off the forms. A bridge made from solid glue would be pretty strong, would flex, and could easily take 200 pounds.”
“Yeah, that’d be way cool dad,” Mitch replied. “It’d be the only bridge in class made out of glue-crete.”
Glue-what?
Anyway, time constraints kept “glue-crete” on the drawing board. But, the whole business did remind me of a host of innovations that are jumping off the drawing board and revolutionizing the world of cement—one of the world’s oldest construction staples.
“Self-Healing-Crete”
Victor Li is a professor of Civil Engineering and Materials Science at the University of Michigan. For over a decade, Li has been leading a team of researchers on developing a safer, more flexible, and more durable concrete by experimenting with engineered cement composites (ECCs).
Traditional concrete is a ceramic, much like glass. It’s strong, but rigid and brittle. As such, it can suffer catastrophic failure when strained—whether that’s from overloading, long periods of routine use, or an earthquake. When strained, large cracks appear and the concrete crumbles apart. But new developments with engineered cement composites and reinforcing fibres are making possible a flexible concrete that can actually bend under strain without breaking apart.
Traditional concrete has a tensile strength of about 0.01%. This means that stretching a 100-foot piece of concrete more than 1/8 of an inch will compromise the structure. In Li’s lab, some of his cement composites have proved a tensile strength 300 times higher. A 100-foot piece could be stretched up to 36 inches without catastrophically fracturing.
When stressed, traditional concrete will see a few large cracks developing. When the cement composites are stressed, many very small cracks develop. If the cracks can be kept below 50 micrometers—about half the width of a human hair—the new composites are capable of completely healing themselves. The tiny cracks expose small amounts of unhydrated cement to water and carbon dioxide in the air. The result is a chemical reaction that produces a calcium carbonate “scar” that binds and seals the crack. Anywhere between one and five “wet-dry” cycles can produce full “healing.” No human intervention is necessary—just water and carbon dioxide.
“We found, to our happy surprise, that when we load it again after it heals, it behaves just like new, with practically the same stiffness and strength,” Li says. “Self-healing of crack damage recovers any stiffness lost when the material was damaged and returns it to its pristine state.
“It’s like if you get a small cut on your hand, your body can heal itself. But if you have a large wound, your body needs help. You might need stitches. We’ve created a material with such tiny crack widths that it takes care of the healing by itself.”
Today, builders reinforce concrete structures with rebar to keep cracks as small as possible. But they’re just not small enough to heal. Water and salts then penetrate through the concrete right to the steel rebar, causing corrosion that further weakens the material. Li’s “self-healing” concrete composites needs no steel reinforcement to minimize the size of cracks, and eliminates many such corrosion concerns. Click here for more information.
“Bio-Crete”
Dutch researchers are working in a similar vein, but are looking to biology for answers. Researchers at the Delft University of Technology are experimenting with a new type of biological concrete that can also seal its own cracks, preventing water intrusion and the corrosion of steel cement reinforcement.
When the cement is produced, dormant bacteria are dumped into the aggregate, along with packets of chemical food. If the concrete is cracked and oxygen and water are introduced, the bacteria become active and convert the food into calcite which seals the crack. Once sealed, the bacteria become dormant again.
The work is being led by research scientist Henk Jonkers, who has spent four years refining the technique. “It took me over a year to come up with the right combination that would not adversely affect the concrete properties, then another three years of testing,” says Jonkers. “It’s extremely durable. The bacteria are specially adapted to extremely alkaline environments, and can survive dormant inside the concrete for up to 50 years.”
Benefits
To be sure, all of this is more than a little interesting. It’s much like the concept of “artificial” intelligence in computing—infusing a dead object with “life.” The benefits are pretty self-evidence, and include enhanced safety and improved durability. In reversing the typical deterioration process, these new concretes reduce the cost of making new structures and maintaining existing ones.
These developments should be of particular interest to western Canadians. The West has a harsh climate. Periods of wet and dry, freezing and thawing, is naturally hard on concrete. Then, there is the whole matter of snow plows scraping into curbs and the use of road salt, which only hastens the corrosion process.
Cost
These kinds of developments are always great stuff, until a cost-benefit analysis comes along to deflate expectations. The question, of course, is whether a significant extension in the lifespan of a concrete overpass can offset the higher price required to use “self-healing-crete” or “bio-crete.”
According to Jonkers, “bio-crete” would cost twice that of regular concrete, and the earlier versions of Li’s product were about triple. Jonkers has partnered up with industry to try and cut costs and make the product more commercially viable. Despite the cost disadvantage, the University of Michigan is pursuing patent protection for the intellectual property, and is also seeking commercialization partners to help bring the technology to market. The University of Michigan has pursued patent protection and is seeking commercialization partners to help bring the technology to market.
Into the Future
All of this is just the appetizer. There’s more. A lot more. It’s all part of a new and emerging discipline within engineering, complete with its own experts, scientists, and practitioners. The movement also has its own conference series as well—the International Conference on Self-Healing Materials (ICSHM). The first international gathering was sponsored by the Centre for Materials at the University of Technology in Delft, Netherlands in April, 2007. Additional conferences were held in 2009 and 2011 in Chicago and the UK. The fourth event is scheduled for June 16-20, 2013 in Belgium. Victor Li was the keynote speaker at the Chicago conference, and has seen his work published in the Journal of Cement and Concrete Research (Autogenous Healing of Engineering Cementitious Composites, 2011).
What’s on the agenda at these conferences? Well, it’s things like bacteriological concrete, asphalt concrete, porous network concrete, nano-tailored concrete, expansive agents and fibre-cement composites, self-healing polymers, metals, ceramics, and composites, microencapsulated and microvascular systems, bio-inspired materials, self-healing paint and coatings, supramolecular polymers, mechano-chemically active polymers, thermally-activated and thermo-plastic self-healing technology, internal liquid healing agents, and experimental techniques for assessing self-healing products and systems, as well as concepts and limitations of use.
Mr. Green’s Next Bridge
As a basic building material, the use of concrete goes back to ancient Rome. At that time, it was viewed as a new and revolutionary material. Mitchell’s little brother, Spencer, is hitting grade one this year. In two years time, dad will probably be called back in to help the little guy build his grade three bridge. I’m thinking that “glue-crete” might be a real possibility this time around.
Sources:
Lynch, Declan. 2010. “Dutch Develop Self-Healing Bio-Concrete.” New Civil Engineer Magazine. September 9, 2010.
University of Michigan. 2009. “Self-Healing Concrete For Safer, More Durable Infrastructure.” Science Daily. April 22, 2009.
*This article was also featured in the July 17, 2012 Edition of the CCA Weekly
By: Casey Vander Ploeg, Senior Policy Analyst