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Study: Concrete 'Crystal' Key to Aging

Tuesday, November 17, 2015

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Researchers believe they may be closer to finding a solution for a common concrete bridge and dam aging problem.

Scientists 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 structures that get wet.

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. 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 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 crystal 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 then took that 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, 2015, online-first edition of Cement and Concrete Research.

   

Tagged categories: Asia Pacific; Concrete; Concrete defects; Concrete repair; Corrosion; EMEA (Europe, Middle East and Africa); Environmental Controls; Latin America; North America; Research

Comment from Tom Schwerdt, (11/17/2015, 8:34 AM)

Okay, this sounds just like what is more commonly called Alkalai Silicate Reaction or ASR in concrete - the first publication discussing it that I am aware of is from 1940. https://en.wikipedia.org/wiki/Alkali%E2%80%93silica_reaction


Comment from Brandon Lecrone, (11/17/2015, 9:21 AM)

That was my initial thought as well Tom. I thought such reactions only occurred in the presence of certain types of aggregate. This article makes it sound like it occurs in all concrete.


Comment from Barry Marcks, (11/17/2015, 11:48 AM)

Wouldn't teatment with an alkyalkoxy silane be effective at minimizing any damage


Comment from Michael Quaranta, (11/17/2015, 7:42 PM)

Now all we need is that guy, who when you meet him he askes if you read his book... LOL On the serious side there is a compete lack of atomic particle physics to support this "crystal" finding. Then why is that these research reports deal mainly with the surface of concrete and not the separate ingredients with a thorough quantum physics report? There is a perfect coating to fend off the "AAR" on the surface that removes the threat caused by the alkali-silica side of the mixture. Then please do not allow this report to limit the problem to bridges where it is a pure lack of regular maintenance that leads to most of the problems. I'm going to read the link that Tom provided above >>> https://en.wikipedia.org/wiki/Alkali%E2%80%93silica_reaction


Comment from Tom Schwerdt, (11/18/2015, 8:22 AM)

Barry - since moisture is required for the ASR to damage the concrete, silanes can be used in many cases to slow the damage.


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