Concrete Band-Aids for Buildings
by David Pescovitz
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Professor Paulo Monteiro of the Department of Civil and Environmental Engineering(Peg Skorpinski photo)
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UC Berkeley civil engineers are developing a new concrete Band-Aid for bridges and buildings. Surprisingly, their key research tool is borrowed from biology. Professor Paulo Monteiro of the Department of Civil and Environmental Engineering and his colleagues are using a state-of-the-art soft x-ray microscope to study the chemistry of concrete more closely than previously possible. What they observe may help them produce better patching materials for structures ravaged by old age and circumstance.
"Concrete is a wonderful material but it gets distressed over time," Monteiro says. "The entire infrastructure in the United States is decaying very quickly. It could cost 120 billion to repair just the bridges."
The reaction of alkali and water in cement maxtrix with amorphous silica found in some aggregates can result in the formation of a gel that causes the concrete to expand and crack over time. The researchers use the soft x-ray microscope to study accelerated alkali-silica reactions and possible methods to prevent the formation or expansion of the gel. (courtesy the researchers)
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The average lifespan of a bridge is approximately fifty years, he explains, and most of the infrastructure in this country was built during the economic boom after World War II. Already, sixty-percent of the half-million bridges in the United States are showing distress.
While cement repair materials are commercially available, it's extremely difficult to forge a long-lasting bond with the old concrete. Quite often, Monteiro says, "the patches just pop off." The Berkeley researchers are developing a polymer-based admixture for concrete that acts a glue, tightly bonding the repair material to the original structure.
To perfect the formula for their novel admixture, the researchers turn to the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. The ALS produces x-rays approximately a hundred million times brighter than the beam from a dentist's machine. With wavelengths nearly the size of atoms, the ALS's soft x-rays enable biologists to study the inner workings of cells. A decade ago, Monteiro realized that the same instrument could enable researchers to see concrete in a whole new light.
Traditionally, concrete is studied at molecular scale using devices like scanning-electron and transmission-electron microscopes. The problem though is that these tools can't accommodate water, a key ingredient in the chemistry of concrete. On the other hand, soft x-ray microscopes penetrate water, rendering it transparent for biologists as they peek inside cells.
"Water is vital in our work too," Monteiro says. "So we tag along with the biologists and sometimes they learn from us as well."
In this soft x-ray microscopy image, the two bright spots are cement grains. When combined with water, hydration products form in the pore space and the mixture hardens into concrete. (courtesy the researchers)
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Creating concrete band-aids is only the latest stage in Monteiro's x-ray microscopy research. Previously, he and his collaborators used the ALS to examine the reactions that occur when concrete is spiked with other admixtures. For example, construction crews often add a bit of sugar to their concrete mix to prevent it from hardening prematurely on hot days. Other admixtures are specifically formulated to speed hardening or produce a certain color. The researchers are also examining the science behind "self-compacting" concrete, a new material developed in Japan that doesn't require machine-aided vibration to ensure that the end product is densely packed.
In recent days, Monteiro and his colleagues have trained the microscope on bits of aged concrete. By simulating various reactions known to cause concrete to crack, the researchers may eventually learn how to prevent the damage. A picture is indeed worth a thousand words.
"Using the x-ray microscope, we're able for the first time ever to watch these reactions as they happen," Monteiro says. "Every time I see the images, I'm amazed."
Paulo Monteiro's home page
Advanced Light Source at Lawrence Berkeley National Laboratory
Center for X-Ray Optics at Lawrence Berkeley National Laboratory
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