Images created using a technology developed at Rice University show concrete fractures that are invisible to the naked eye.
Common Portland cement includes small silicon crystals that exhibit near-infrared fluorescence when irradiated with visible light, according to a collaboration between Rice and the Kuwait Institute for Scientific Research.
That led to two realizations. The first was that the wavelength of the emission might be utilized to determine the type of cement used in construction.
The second, and possibly more crucial, an advantage of near-infrared emission is that it can detect even the tiniest cracks in cement or concrete. The idea is to paint the concrete with a thin coat of opaque paint when it’s fresh. Near-infrared scans demonstrate that undamaged concrete appears black, while blazing light reveals even the tiniest fissures.
Scientific Reports published an open-access study by Rice chemist Bruce Weisman, Rice structural engineer Satish Nagarajaiah, and Kuwait Institute of Scientific Research investigator Jafarali Parol.
Wei Meng, the paper’s first author, found the phenomenon while pursuing the Rice team’s long-standing work 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 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.”
Minerals called silicates are major components of cement, and we hypothesized that during the high-temperature production process, very small amounts decompose to form microscopic silicon crystals. Their emission wavelength tells us that they’re larger than about 10 nanometers, but they can’t be much bigger or people would have noticed them long ago.
Bruce Weisman
The researchers deduced that the source was pure silicon crystals because of the emission’s peculiar spectral signature.
“Minerals called silicates are major components of cement, and we hypothesized 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 nanometers, but they can’t be much bigger or people would have noticed them long ago.”
Meng experimented with little concrete blocks that had been coated black and had holes bored through the center. When the blocks were squeezed, these served as focus places for microcracks to form, which propagated outward and cracked the paint. He discovered that the fluorescent signal could readily be traced with a raster-scanning laser since it passed through the microscopic fractures.
“Concrete structures need monitoring, and this is one way of monitoring them,” said Nagarajaiah, who specializes in infrastructure/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 believes that the advantages of better crack detection could extend beyond bridges and buildings to containment structures at nuclear power plants, ships, even the insides of difficult-to-access wells and pipelines.
The researchers claim that shining light on essential structures and photographing them with a near-infrared camera and narrow-band spectral filter is a realistic strategy.
“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. Sergei Bachilo, a Rice research scientist, is a co-author of the paper.
Nagarajaiah teaches civil and environmental engineering, materials science and nanoengineering, and mechanical engineering at the University of California, Berkeley. Weisman is chemistry, materials science, and nanoengineering professor.
