by Helen Tunnicliffe
www.tcetoday.com/tcetoday/NewsDetail.aspx?nid=11674 27/4/2009 Self-healed specimens just as strong as before
SCIENTISTS working at the University of Michigan, US, have developed a type of concrete which can heal its own cracks. The team, led by civil engineering and materials science professor Victor Li, have been developing engineered concrete composite (ECC) for the past 15 years. ECC contains extra-dry cement, which reacts with carbon dioxide and water to form calcium carbonate, filling in the cracks. In the laboratory, complete healing required 1-5 wetting and drying cycles. Standard concrete is classed as a ceramic. It is very brittle and cracks easily and requires steel reinforcement to keep cracks to a minimum, however, any cracks allow in water and air, corroding the steel and further weakening the structure. ECC acts more like a metal and bends rather than breaking. It contains coated reinforcing fibres and therefore does not require steel reinforcement. To test ECC, the team exposed it to a tensile strain of around 3%, equivalent to stretching a 100 feet (30.48m) section an extra 3 feet, which would destroy standard concrete, which cannot carry a load at tensile strains of 0.01%. The ECC was examined before and after the tests using resonant frequency measurements to detect changes in structure. It was found that the concrete recovers 76-100% of its initial resonant frequency value following self-healing. For a crack to heal fully, it must be below 150 Ám in length. ECC's average crack width is 60 Ám. The team says the concrete remains safe to use at a tensile strain of 5%. Li believes ECC could be used for rebuilding and repairing roads, bridges and other civil infrastructure, something for which the US government has set aside $100m. "Our hope is that when we rebuild our roads and bridges, we do it right, so that this transportation infrastructure does not have to undergo the expensive repair and rebuilding process again in another five to ten years," says Li. The research was published in Cement and Concrete Research and DOI: 10.1016/j.cemconres.2009.01.013. See also "Self Sealing Cement".
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