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Concrete Tells What Lies Beneath

Monday, February 24, 2014

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Some cracks in concrete have been known to heal themselves, but they couldn’t report back on when they did or how the whole operation went.

Now, ultrasound technology may change that.

GT-CEE researchers
Photos unless indicated: Georgia Tech CEE

The research team included (from left) Gun Kim, Katie Matlafk and Chiwon In.

Researchers at Georgia Tech's School of Civil and Environmental Engineering have developed a way to use ultrasonic technology to monitor and measure self-healing of sub-surface concrete cracks in damp and aqueous environments.

Microscopic Self-Healing

Monitoring and evaluation of self-healing in concrete using diffuse ultrasound," a peer-reviewed paper published in Nondestructive Testing and Evlaution (NDT&E) International, explains how ultrasound can measure the progression of autogenous healing (self-healing) below the surface. (The paper was also re-published in a special virtual issue of the journal.)

"Self-healing is an inherent process of concrete crack repair that occurs when there is both moisture present and tensile stress absent from an environment, such as in a marine setting," Georgia Tech reports in a research announcement.

Concrete Piers
Courtesy Flickr user portoftacoma via Georgia Tech CEE

Cracks in concrete may self-heal in marine environments where moisture is present and tensile stress is absent.

"At a microscopic level, self-healing happens due to a combination of mechanical blocking by particles carried into a crack, along with the filling of the crack by calcium carbonate deposits, derived from hydration products within the concrete itself."

This natural process can aid in closing relatively narrow dormant cracks.

But inspectors and maintenance directors would find it more helpful if they knew when it was occurring.

How They Tested

The research was a collaboration by former graduate students Chiwon In and R. Brett Holland (both Ph.D.); professors Kimberly E. Kurtis, Lawrence F. Kahn and Laurence J. Jacobs (all Ph.D.); and civil engineering Senior Research Engineer Jin-Yeon Kim, Ph.D.

Measuring cracks before exposure

Graduate students take crack width measurements before sealing and exposure.

The team performed measurements on cracked and un-cracked concrete specimens of three different mix designs, then exposed the specimens to a simulated marine environment for four months.

Then, the team measured again using two diffuse ultrasonic parameters: their own novel application of the effective diffusivity, as well as an Arrival Time of Maximum Energy (ATME) measure.

What They Found

As self-healing progressed, the ATME of both tensile and flexure cracked specimens decreased, while the diffusivity increased, the university said. The team also recovered the initial values found in the un-cracked control specimens after self-healing.

This suggested not only self-healing of the cracks, but that the researchers' new method predicted self-healing trends more effectively than the ATME method.

The researchers concluded that their method was capable of monitoring and evaluating the self-healing of concrete more reliably than ATME values, which depend on where on the crack face self-healing occurs.
 

   

Tagged categories: Concrete defects; Concrete repair; Cracks; Maintenance programs; Quality control; Research

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