Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Structure of "concrete disease" solved

06.11.2015

When bridges, dam walls and other structures made of concrete are streaked with dark cracks after a few decades, the culprit is AAR: the alkali-aggregate reaction. Also called the "concrete disease" or even "concrete cancer", it is a chemical reaction between substances contained in the material and moisture seeping in from outside. Researchers from the Paul Scherrer Institute PSI and Empa have now solved the structure of the material produced in the course of AAR at atomic level – and have thereby discovered a previously unknown crystalline arrangement of the atoms.

Researchers from the Paul Scherrer Institute PSI teamed up with colleagues from the Swiss Materials Science Lab Empa to study a degenerative sign of ageing in concrete: the so-called alkali-aggregate reaction (AAR). In the course of AAR, a material forms that takes up more space than the original concrete and thus gradually cracks the concrete from within as the decades go by.


The so-called "concrete disease": close-up of cracks in concrete caused by the alkali-aggregate reaction (AAR).

Photo: Empa/Andreas Leemann

The researchers have now explored the exact structure of this material. They managed to demonstrate that its atoms are arranged extremely regularly, making it a crystal. They also showed that the structure of this crystal is a so-called sheet-silicate structure. This specific structure had never been observed before. The researchers made their discovery thanks to measurements at the Swiss Light Source SLS at PSI. The research results could help towards the development of more durable concrete in future.


A global problem

AAR is a chemical reaction that affects outdoor concrete structures all over the world. It happens when concrete is exposed to water or moisture. For instance, numerous bridges and up to twenty per cent of the dam walls in Switzerland are affected by AAR.

With AAR, the basic ingredients in the concrete are actually the problem: cement – the main component of concrete – contains alkali metals such as sodium and potassium. Any moisture infiltrating the concrete – stemming for example from rainwater – reacts with these alkali metals, leading to an alkaline solution.

The second main ingredient in concrete is sand and gravel, which in turn are composed of minerals, such as quartz or feldspar. Chemically speaking, these minerals are so-called silicates.

The alkaline water reacts with these silicates and forms a so-called alkali calcium silicate hydrate. This is itself able to absorb more moisture, which causes it to expand and gradually crack the concrete from within. This entire process is referred to as AAR.

AAR takes place extremely slowly, so that the cracks are initially only tiny and invisible to the naked eye. Over the course of three or four decades, however, the cracks widen significantly and eventually jeopardise the durability of the entire concrete structure.

"Most structures currently suffering from AAR were built between the 1960s and 1980s," explains Erich Wieland, Head of the Cement Systems Group at PSI. "The research community in Europe only became aware of the AAR problem in the 1970s."

A new crystal

Even if the chemical processes involved in AAR have long been known, nobody had identified the physical structure of the alkali calcium silicate hydrate formed in the course of AAR. The researchers at PSI and Empa have now managed to fill this knowledge gap.

They studied the substance of a Swiss bridge constructed in 1969, which has been affected heavily by AAR. Researchers from Empa cut out a material sample from the bridge and ground down a small piece of it until they were left with a wafer-thin sample that was merely 0.02 millimetres thick. The sample was then taken to the Swiss Light Source SLS and irradiated with an extremely narrow X-ray beam, fifty times thinner than a human hair. Performing so-called diffraction measurements and a complex data analysis, the PSI researchers were eventually able to determine the crystal structure of the material with pinpoint precision.

They found that the alkali calcium silicate hydrate has a previously undocumented sheet-silicate crystal structure. "Normally, discovering an uncatalogued crystal structure means you get to name it," explains Rainer Dähn, the first author of the study. "But it has to be a crystal found in nature, therefore we didn’t get that honour," says the researcher with a smile.

Andreas Leemann, Head of the Concrete Technology Group at Empa, had the idea for the current study. The researchers from PSI then brought their knowledge of the X-ray beam method to the table.

"In principle, it’s possible to add organic materials to the concrete that are able to reduce the build-up of tension," explains materials scientist Leemann. "Our new results provide a scientific basis for these considerations and could pave the way for the development of new materials."

Text: Paul Scherrer Institute/Laura Hennemann


About PSI

The Paul Scherrer Institute PSI develops, builds and operates large, complex research facilities and makes them available to the national and international research community. The institute's own key research priorities are in the fields of matter and materials, energy and environment and human health. PSI is committed to the training of future generations. Therefore about one quarter of our staff are post-docs, post-graduates or apprentices. Altogether PSI employs 1900 people, thus being the largest research institute in Switzerland. The annual budget amounts to approximately CHF 380 million.


Contact

Dr. Rainer Dähn, Clay Sorption Mechanisms Research Group, Paul Scherrer Institute,
telephone: +41 56 310 21 75, e-mail: rainer.daehn@psi.ch [German, English]

Dr. Erich Wieland, Cement Systems Research Group, Paul Scherrer Institute,
telephone: +41 56 310 22 91, e-mail: erich.wieland@psi.ch [German, English]

Dr. Andreas Leemann, Concrete/Construction Chemistry Laboratory, Empa,
telephone: +41 58 765 44 89, e-mail: andreas.leemann@empa.ch [German, English]

Original Publication

Application of micro X-ray diffraction to investigate the reaction products formed by the alkali-silica reaction in concrete structures
R. Dähn, A. Arakcheeva, Ph. Schaub, P. Pattison, G. Chapuis, D. Grolimund, E. Wieland and A. Leemann
Cement and Concrete Research 14 October 2015 (online)
DOI: 10.1016/j.cemconres.2015.07.012 http://www.sciencedirect.com/science/article/pii/S0008884615002094

Weitere Informationen:

http://www.psi.ch/media/structure-of-concrete-disease-solved Press release on PSI website with additional figures

Laura Hennemann | idw - Informationsdienst Wissenschaft

More articles from Architecture and Construction:

nachricht Smart buildings through innovative membrane roofs and façades
31.08.2017 | Fraunhofer-Institut für Organische Elektronik, Elektronenstrahl- und Plasmatechnik FEP

nachricht Concrete from wood
05.07.2017 | Schweizerischer Nationalfonds SNF

All articles from Architecture and Construction >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Scientists channel graphene to understand filtration and ion transport into cells

Tiny pores at a cell's entryway act as miniature bouncers, letting in some electrically charged atoms--ions--but blocking others. Operating as exquisitely sensitive filters, these "ion channels" play a critical role in biological functions such as muscle contraction and the firing of brain cells.

To rapidly transport the right ions through the cell membrane, the tiny channels rely on a complex interplay between the ions and surrounding molecules,...

Im Focus: Towards data storage at the single molecule level

The miniaturization of the current technology of storage media is hindered by fundamental limits of quantum mechanics. A new approach consists in using so-called spin-crossover molecules as the smallest possible storage unit. Similar to normal hard drives, these special molecules can save information via their magnetic state. A research team from Kiel University has now managed to successfully place a new class of spin-crossover molecules onto a surface and to improve the molecule’s storage capacity. The storage density of conventional hard drives could therefore theoretically be increased by more than one hundred fold. The study has been published in the scientific journal Nano Letters.

Over the past few years, the building blocks of storage media have gotten ever smaller. But further miniaturization of the current technology is hindered by...

Im Focus: Successful Mechanical Testing of Nanowires

With innovative experiments, researchers at the Helmholtz-Zentrums Geesthacht and the Technical University Hamburg unravel why tiny metallic structures are extremely strong

Light-weight and simultaneously strong – porous metallic nanomaterials promise interesting applications as, for instance, for future aeroplanes with enhanced...

Im Focus: Virtual Reality for Bacteria

An interdisciplinary group of researchers interfaced individual bacteria with a computer to build a hybrid bio-digital circuit - Study published in Nature Communications

Scientists at the Institute of Science and Technology Austria (IST Austria) have managed to control the behavior of individual bacteria by connecting them to a...

Im Focus: A space-time sensor for light-matter interactions

Physicists in the Laboratory for Attosecond Physics (run jointly by LMU Munich and the Max Planck Institute for Quantum Optics) have developed an attosecond electron microscope that allows them to visualize the dispersion of light in time and space, and observe the motions of electrons in atoms.

The most basic of all physical interactions in nature is that between light and matter. This interaction takes place in attosecond times (i.e. billionths of a...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Innovative strategies to tackle parasitic worms

08.12.2017 | Event News

AKL’18: The opportunities and challenges of digitalization in the laser industry

07.12.2017 | Event News

Blockchain is becoming more important in the energy market

05.12.2017 | Event News

 
Latest News

New research identifies how 3-D printed metals can be both strong and ductile

11.12.2017 | Physics and Astronomy

Scientists channel graphene to understand filtration and ion transport into cells

11.12.2017 | Materials Sciences

What makes corals sick?

11.12.2017 | Earth Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>