Archive for Victor Li
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Composite cement successful so far in bridge joint repair
Installation of ECC bridge link slab: (a) location of ECC slab, (b) placement of reinforcing steel within link slab segment, (c) pouring of ECC material and (d) finishing of exterior surface. Credit: Li, J. of Material Structures
I’ve posted before about Victor Li’s work at the University of Michigan using engineered cementitious composites.
A new paper by Li recently became available regarding a demonstration project in which ECC is being tested on a bridge deck within Michigan to replace a conventional joint within the deck.
ECC is in the family of materials known as high performance fiber reinforced cementitious composites. According to Li, ECC has the ability to strain harden under uniaxial tension while forming large numbers of microcracks up to an ultimate strain capacity typically over 4 percent, a level that he says is 400 times that of normal concrete. Under high levels of tensile strain, ECC does not form cracks with large crack width openings (e.g., between 50 µm and 70 µm). Designers can tailor the material on three phases within the composite: the fiber, matrix and fiber/matrix interface.
This particular application for ECC is aimed at coming up with solutions for a growing problem facing highway engineers. In the U.S. bridge deterioration is a big concern, and one problematic area is the
mechanical expansion joints in bridges. These joints are placed between sections of bridge decking, and are necessary because of the changing dimensions of these sections of deck that occur because of thermal expansion or other forms of thermal deformation.
Unfortunately, these joints deteriorate fairly quickly, begin to leak and then bigger problems begin. For example, in colder climates, water containing de-icing salts corrode the ends of the steel girders or penetrate into the rebar in precast concrete deck slabs. At best, costly repairs have to be made. At worst, a catastrophic failure of the bridge can result.
In response, engineers have essentially figured out a way to eliminate these expansion links in new bridges. But, the dilemma is what to do about existing structures.
Li’s idea is to retrofit bridges with ECC ‘‘link slabs’’ by removing the expansion joint and replacing a portion of the two adjacent decks with a section of ECC material overtop the joint. From the exterior, it would appear as a continuous deck surface.
ECC link slab schematic: Credit: Li, J. of Materials and Structures
In a demonstration project sponsored by the Michigan Department of Transportation, completed in 2005, a 225 mm thick, 5.5 m x 20.25 ECC link was added to a demonstration bridge. Thirty cubic meters of the ECC as delivered by standard ready-mix concrete trucks from a batching plant in a mix supervised by Li’s team.
Full scale load tests showed that the ECC link slab did not alter the supported nature of the bridge spans, and that ample strain capacity of the ECC is reserved for temperature-induced straining as designed.
The good news is that, so far, the performance of this link slab remains has stayed constant. More long-term performance monitoring and other demonstrations will be needed, but Li is optimistic that an ECC link slab will provide an excellent expansion joint replacement option for highway engineers.
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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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