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Chair of Ceramics

Main Research

09/07/2011

Akali-aggregate-reaction

Among the key activities of the Chair of Ceramics is research in the field of alkali-aggregate reaction. Alkali-aggregate reaction is a bursting reaction caused by certain reactive aggregates. Under unfavourable conditions AAR may lead to a total disintegration of a concrete structure. Harmful aggregates always contain reactive silica (SiO2) which is characterized by a high disorder in the crystal lattice. Dependent on the degree of disorder amorphous, crypto- to microcrystalline and stressed silica – with contortions of the SiO2 network due to tectonic loads – can be distinguished. Reactivity thereby is a function of disorder. There are rapid reacting aggregates (e.g. opal sandstone and flint) and slow/late reacting aggregate (e.g. stressed quartz).

Damages may occur after 2 – 5 years for rapid reacting aggregates, but may also take up to 10-15 or more years for slow/late reacting aggregates to appear. Besides reactive aggregates a high humidity in the concrete and sufficient levels of alkalis are necessary for AAR to take place.AAR constitutes a huge problem in numerous countries around the world – among them e.g. Canada, Denmark, Iceland, China and also Germany. In these countries AAR has therefore become topic of many research works. In Austria AAR has rather been in the background – up until 2003 only two damage cases have been reported of.

Aim of the work at the Chair of Ceramics is the built-up of know-how in Austria. Thereby an emphasis is put upon the identification and assessment of Austrian AAR damage cases. International accepted diagnose methods are evaluated for their applicability and adapted to Austrian needs if necessary. Damage rating indices which allow the quantification of damage with simple petrographic methods are of special interest.

Ziel der Arbeiten am Lehrstuhl für Gesteinshüttenkunde ist ein Know-How-Aufbau hinsichtlich der Alkali-Gesteinskörnungsreaktion in Österreich. Ein Schwerpunkt ist dabei die Identifikation und Beurteilung der AGR an österreichischen Schadensfällen. Dabei werden international anerkannte Diagnosemethoden auf deren Eignung geprüft und gegebenenfalls adaptiert. Besonderes Interesse gilt sogenannten Schadenskennzahlen, die eine Quantifizierung des Schadens mit einfachen petrografischen Untersuchungen ermöglichen.

Another emphasis is the testing of damaged concretes with non-destructive testing techniques. With in the laboratory produced and damaged concretes the effect of AAR on the mechanical properties of the concrete is investigated.

 

Fig. 1: Cracks of a AAR-damaged concrete carriage-way.


Fig. 2: Transmitted light microscopical picture of the structure of a damaged concrete. Crack and air space on the right side of the picture are lined with alcali-silica-gel.

References:

Harmuth, H., Fischböck, E.: Identifikation und Beurteilung der Alkali-Zuschlag-Reaktion. Bundesministerium für Verkehr, Innovation und Technologie, Straßenforschung Heft 560, 2006.

Fischboeck, E.K., Harmuth, H.: An Austrian experience with identification and assessment of alkali-aggregate reaction in motorways. Proc. of the 2nd International Conference of Concrete Repair, Rehabilitation and Retrofitting. Cape Town, Südafrika, November 2008.