The method, which potentially provides a new way of monitoring cement and concrete structures such as bridges or nuclear power plants, was discovered by chance at Rice University in Houston, Texas.
A collaboration between research groups at Rice and the Kuwait Institute for Scientific Research found that common Portland cement contains microscopic crystals of silicon, which emit near-infrared fluorescence when illuminated with visible light.
The finding led to two realisations: that the exact wavelength can be used to identify the type of cement in a structure; and, perhaps more importantly, that the near-infrared emission can reveal tiny ‘microcracks’ that are invisible to the naked eye.
The process is enabled by applying a thin coat of opaque paint to the concrete when it is new. In near-infrared scans, intact concrete appears black and glowing light reveals the tiny fractures.
First author of the new research, Wei Meng, found the phenomenon while working on optical strain sensing with carbon nanotubes.
“This arose from a project in which we were trying to apply our strain measurement technique to cement and concrete, but we ran into an unexpected problem when we illuminated a specimen coated with a nanotube film,” said fellow researcher Bruce Weisman, a pioneer in nanotube spectroscopy. “We found that one of the peaks in our film spectrum was obscured by much stronger emission coming from somewhere. We never expected it would be from the cement itself.”
He said he was not aware of any other lab reporting the phenomenon. “Eventually, we were able to mask off the specimen so the emission didn’t interfere with our strain measurement,” he said. “But we kept in the backs of our minds that maybe this could be interesting on its own.”
The emission’s unusual spectral signature led to the realisation that the source was pure silicon crystals.
“Silicates are major components of cement, and we hypothesised that during the high-temperature production process very small amounts decompose to form microscopic silicon crystals,” Weisman said. “Their emission wavelength tells us that they’re larger than about 10 nanometres, but they can’t be much bigger or people would have noticed them long ago.”
Meng experimented on small concrete blocks painted black, with holes drilled in the middle. The holes served as focal points for the formation of microcracks, which would propagate outwards when the blocks were compressed, also cracking the paint. Meng found the fluorescent signal came through the tiny cracks and could easily be mapped with a raster-scanning laser.
“Concrete structures need monitoring, and this is one way of monitoring them,” said Satish Nagarajaiah, who specialises in structural monitoring, system identification, damage detection, and adaptive stiffness structure systems to withstand seismic events. “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.”
He said the benefits of better crack detection could extend beyond bridges and buildings to containment structures at nuclear power plants, or on ships or the insides of wells and pipelines that are difficult to access.
The researchers said a practical approach is to shine light on critical structures and photograph them using a near-infrared camera and narrow-band spectral filter.
“Cement cracking can be an early symptom of failure, so people who are concerned with the structural integrity and safety of concrete structures want to detect microcracks before they grow,” Weisman said.
The research was published in Scientific Reports.
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