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Concrete ‘Crystal’ Key to Degradation

Wednesday, November 18, 2015

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Organic materials could provide an answer to degenerative aging concrete structures now that scientists have found a physical structure that appears to be causing much of the damage.

Researchers from the Paul Scherrer Institute (PSI) and the Swiss Materials Science Lab (EMPA) discovered an undocumented sheet-like crystal structure in alkali calcium silicate hydrate that forms during the concrete aging process, also known as alkali-aggregate reaction (AAR).

According to an EMPA statement, the discovery of the crystal-like structure could help researchers learn how to treat “concrete disease” or “concrete cancer” that forms in concrete substrates such as those found on residential and commercial structures.

Photos: EMPA via Flickr

Researchers in Switzerland have discovered a crystal-like structure inside alkali calcium silicate hydrate that forms when an alkali-aggregate reaction (AAR) occurs in aging concrete.

“In principle, it’s possible to add organic materials to the concrete that are able to reduce the build-up of tension,” said materials scientist Andreas Leemann, head of the Concrete Technology Group at EMPA and the researcher who developed the idea for the study.

“Our new results provide a scientific basis for these considerations and could pave the way for the development of new materials,” said Leemann.

AAR Uncovered

With AAR, a material forms within the concrete that takes up more space than the original concrete. Eventually, the concrete cracks.

AAR is a chemical reaction that affects outdoor concrete structures all over the world, researchers said in their statement. When concrete is exposed to water or moisture, the ingredients in the concrete combine to create the problem.

The cement in the concrete contains alkali metals, such as sodium and potassium. When moisture—such as from rainwater—reacts with the metals, an alkaline solution forms, the researchers said.

Concrete’s second main ingredient is a mixture of sand and gravel. Those are made of minerals—such as quartz or feldspar—which are silicates. When the silicates react with the alkaline solution, the outcome is alkali calcium silicate hydrate.

AAR, sometimes known as "concrete disease" or "concrete cancer," happens when water enters concrete and causes a chemical reaction with the metals and silica inside.

But the chemical reaction also is able to absorb water, the researchers said. As more moisture builds up and the mixture expands over decades, the concrete begins to crack. Initially those cracks are not visible to the naked eye, but they can become wide and visible over time.

“Most structures currently suffering from AAR were built between the 1960s and 1980s,” said Erich Wieland, head of the Cement Systems Group at PSI, in the lab’s statement. The research community in Europe only became aware of the AAR problem in the 1970s.

Both the EMPA and PSI statements indicate that more than 20 percent of Switzerland’s dam walls have AAR, as do bridges and similar structures that are routinely exposed to wet conditions around the world.

Finding the Crystal

To find the crystal-like structure inside alkali calcium silicate hydrate, researchers took a sample from a Swiss concrete bridge that had been built in 1969 and was affected heavily with AAR. They then ground down a small piece of that until they were left with a wafer-thin layer that was only 0.02 mm thick.

Researchers found the cyrstal by taking a sample of a bridge with AAR, grinding it down and exposing a wafer-thin sample to an ultra-thin X-ray. They then took measurements.

According to the researchers, they took the sample to Swiss Light Source and exposed it to an X-ray beam 50 times thinner than a human hair. The PSI researchers eventually found the crystal structure after using diffraction measurements and a complex data analysis.

Even with the finding, one of the scientists joked that they might not get the credit some other researchers would.

“Normally, discovering an uncatalogued crystal structure means you get to name it,” said Rainer Dähn, the first author of the study. “But it has to be a crystal found in nature, therefore we didn’t get that honor.”

Researchers published their findings, “Application of micro X-ray diffraction to investigate the reaction products formed by the alkali-silica reaction in concrete structures,” in the Oct. 14 online-first edition of Cement and Concrete Research.

   

Tagged categories: Concrete; Concrete defects; Concrete repair; Corrosion; Research

Comment from Tom Schwerdt, (11/18/2015, 8:29 AM)

We were aware of ASR in the 1940s (note the more commonly used name of Alkali-silica reaction, not AAR) - and you need reactive aggregates and moisture for it to occour, not just time. Sounds like the Swiss need to do a bit more reading of the literature.


Comment from peter gibson, (11/20/2015, 11:19 AM)

The Swiss are way behind on this .They should first see what others are doing before wasting their time.Fools!


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