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Monday, October 11, 2010

The Era of Flexible Concrete?

 PDF version here
 Looks like the brittle concrete has been tamed at last!
A team of researchers at the University of Michigan has developed a concrete material that bends like rubber, cracks very little, heals itself with no manual intervention, and is almost as good as new concrete upon recovery, with its stiffness and strength intact.
The research team led by Professor Victor C. Li more here , Professor of Civil and Environmental Engineering
at the University of Michigan, has achieved this by designing the new material with tiny crack widths. This ensures that any damage caused due to overloading and subsequent tensile strain manifests itself as small cracks that are autogenously healed.
Here’s how the self-healing mechanism works. The extra dry cement that is exposed on the surface of the crack reacts with water and carbon dioxide to form calcium carbonate, a strong and resilient compound that brings back the material to its original state. But this works only if the crack width is tiny, a factor that is taken care of by the nature of the material itself. The new material is an improvement over the bendable engineering cement composite (ECC) that Li and his team have been developing for the past decade and a half. The research team discovered that the brittleness of concrete could be altered by limiting the crack width to 150 microns, preferably 50 microns to enable full healing. The average crack width in the ECC was found to be 60 microns, half the width of human hair.
While traditional concrete is brittle and rigid, prone to failure and breakage under strain, the flexible ECC is held together with reinforcing fibers. So while traditional concrete fractures under a tensile strain of 0.1%, experiments revealed that the ECC is able to withstand a tensile strain of up to 5%. That makes it an astounding 500 times more durable than concrete.
The flexible ECC has several obvious advantages over traditional concrete as a construction material.
Stronger Structures
Presently, concrete structures are reinforced with steel reinforcement (aka “rebar”) to minimize concrete cracking, as well as provide tensile strength for bending moments for structural beams and columns. While ECC cannot replace rebar for structural tensile strength, it can reduce the need for rebar to limit concrete cracking. In addition, ECC has the added benefit of self-healing these cracks, thereby reducing the risk of water and de-icing salts penetrating into the structure, causing corrosion of reinforcement steel that might be present.
Decreased Costs
While ECC is three times as expensive as traditional concrete, these costs are outweighed in the long run since the structure would not require extensive repair and maintenance. Li claims that ECC could help do away with repair and rebuilding processes for about an additional five to ten years. It could also eliminate the need to monitor seismic stresses on structures.
Reduced Environmental Impacts
Use of the ECC is also expected to reduce the energy and carbon footprints of infrastructure, thereby reducing the detrimental effects of construction on the natural environment.
Quieter Structures
In 2006, a bridge over Interstate 94 in Michigan was built with a similar self-healing concrete, which was reinforced with toothed metal slats that allowed concrete to expand and contract without bending. However, this structure turned out to be a noisy affair as vehicles rattled over the metal slats. In contrast, ECC is a silent material.
The research certainly bodes well for the construction industry. In addition to the obvious applications in buildings and infrastructure, self-healing concrete could also very well be the solution to potholes and cracks on roads and bridges, and leaky walls. Flexible ECC is also being considered for use in irrigation channels in Montana.
However, the one crucial factor that could put a wrench in the works is that the self-healing process is almost entirely dependent on the availability of water. Under laboratory conditions, ECC was found to require about one to five cycles of wetting and drying in order to self-heal. Extrapolating this finding to large structures such as bridges, it can be concluded that the ability for the material to self-heal is likely to be seasonal in nature. This leads one to question whether the new material would be suitable for commercial use in dry arid lands, and under all climatic conditions. And, would the alternate freeze-thaw cycles during our cold winters, complicated by use of de-icing salts, affect ECC’s material properties? These are some of the questions that should be addressed.
All said and done, should ECC prove to be a success in terms of industrial and commercial use, we are likely to see safer, smarter and more durable structures being erected.


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  4. Civil engineering has achieved a lot with in the past few years. The strategy has been developed and the result reflects in the taller buildings all around the city. Flexible concrete is something that is adding more advantage to the civil engineering creations. For any creation, foundation plays the most vital role and it bears the whole structure. I'm having lots of interest to know about the engineering behind the mega structures and I found your article really interesting, presenting the flexible concrete. Thanks a lot for sharing. foundation underpinning Woodland Hills, CA.

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