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Video of the week II - Self-healing concrete
Following on the heels of our first Video of the week, this is look at Victor Li’s latest innovation: self-healing concrete.
Cement and Concrete Research is soon to publish a paper by Li, who is a professor in material science and engineering at the University of Michigan, that describes a type of concrete that forms many tiny cracks when overloaded instead of a few large ones, leading to a process in which the concrete effectively “heals” itself.
Even after a 3 percent tensile strain, Li’s samples recovered nearly all of its original strength. “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 said. “Self-healing of crack damage recovers any stiffness lost when the material was damaged and returns it to its pristine state. The material can be damaged and still remain safe to load.”
Li and his research group have spent more than a decade developing what he calls engineered cement composites. An early version of this ECC is what made the bendable concrete possible. The current version of ECC keeps cracks under 60 micrometers. The cracks, though small, expose small amounts of unhydrated cement in the concrete. When the concrete is subjected to water and carbon dioxide, it forms a tiny calcium carbonate “scar.” Li found in his lab that between one and five wet-dry cycles are needed to reach final level of healing.
This kind of thing is always great stuff, but cost-benefit analyses often deflate some great innovation. Here, the question is whether a signficant extension in the lifespan of something like a concrete highway can offset the premium paid for ECC-enhanced materials that at one point were looking like they would cost three-times as much as traditional concrete. Nevertheless, U-M says it is is pursuing patent protection for the intellectual property, and is seeking commercialization partners to help bring the technology to market.
Li is supposed to be delivering a keynote address on self-healing concrete at the International Conference on Self-Healing Materials in Chicago in June 2009.
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Video of the week I - Bendable concrete
In mid-2005, University of Michigan professor Victor Li unveiled a fiber-reinforced bendable concrete. The concrete is made of what Li’s group calls Engineered Cement Composites. The concrete certainly looks like regular concrete, and Li says it is 500 times more resistant to cracking and 40 percent lighter in weight.
In an interview given at that time, Li told the University of Michigan Daily that ECC-built roads might be able to last for 10 years. That kind of life span would be desirable given that Li estimated that ECC concrete might be triple that of traditional concrete.
The secret to ECC is the use of stretchable fibers that are embedded in the concrete. Traditional concrete has tremendous compressive strength but doesn’t do well under tension. In the past, builders have tried to get around this problem using rebar and mesh.
The use of fibers within concrete goes back at least 30 years. Li’s breakthrough is because of the type of polymers his group used.
Later today I will be posting another video and update on this story featuring Li’s latest improvement: Self-healing concrete.
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