Concrete degradation
Structure of “concrete disease” solved
![https://eata2017.empa.ch/documents/56164/217451/Beton-Krankheit.jpg/44f3e8da-7040-4032-9803-7c17cdd87dad?t=1449654673000](https://eata2017.empa.ch/documents/56164/217451/Beton-Krankheit.jpg/44f3e8da-7040-4032-9803-7c17cdd87dad?t=1449654673000)
The so-called “concrete disease”: close-up of cracks in concrete caused by the alkali-aggregate reaction (AAR). Image: Empa |
|||
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 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 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. |
|||
|
|||
|
Light microscope image of a rock grain in an AAR damaged concrete. The AAR has blasted the rock grain. The crack widens gradually and is filled by the reaction product of the AAR. Every time when moisture penetrates, the reaction product swells, exerts pressure and increases the crack again. Image: Empa |
||
|
|||
A new crystal 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.” Images can be downloaded at https://flic.kr/s/aHskoNwE3a |
|||
|
|||
|
|||
|
|||