Materials

New technique illuminates micro-cracks in ordinary concrete

New technique illuminates micr...
Tiny cracks are virtually invisible in a painted block of concrete (left), but show up vividly when crystals within the block are made to fluoresce (right)
Tiny cracks are virtually invisible in a painted block of concrete (left), but show up vividly when crystals within the block are made to fluoresce (right)
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Tiny cracks are virtually invisible in a painted block of concrete (left), but show up vividly when crystals within the block are made to fluoresce (right)
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Tiny cracks are virtually invisible in a painted block of concrete (left), but show up vividly when crystals within the block are made to fluoresce (right)

Although concrete can be altered in order to help tiny cracks show up before they become catastrophic, scientists have now discovered that regular concrete does the job pretty well itself. It just needs a coat of ordinary paint, and a light source.

Working with colleagues from the Kuwait Institute for Scientific Research, researchers at Rice University in Texas were recently conducting a study on the use of carbon nanotubes for optically detecting strain in concrete structures.

When imaging the nanotube-coated concrete, the scientists were surprised to discover that something else within it was fluorescing. Upon further investigation, it was found that microscopic silicon crystals in the concrete's commonly used Portland cement were responsible. Such crystals likely form in the cement during the high-temperature production process, and they fluoresce near-infrared light when exposed to visible light.

The team proceeded to apply opaque black paint to the surface of small concrete blocks, then drill holes in those blocks to serve as propagating points for cracks which would form as the blocks were compressed. Those cracks would form in both the concrete and the paint, the idea being that the paint would shield the concrete from visible light where it was intact, but let the light through wherever cracks formed.

Sure enough, when the stressed blocks were exposed to visible light and then scanned with a near-infrared-sensitive laser, even the tiniest of cracks were found to fluoresce brightly against an otherwise dark background. Going forward, the scientists suggest that a more practical approach would be to initially apply a thin coat of paint to new concrete structures, then periodically inspect those structures by exposing them to a visible light source while photographing them with a near-infrared camera.

"Concrete structures need monitoring, and this is one way of monitoring them," said Rice's Prof. Satish Nagarajaiah. "Getting a clear idea of where cracks are can be quite important in structures, especially in the critical places where we know they’re going to be stressed."

A paper on the research was recently published in the journal Scientific Reports.

Source: Rice University

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