Person Responsible:

Gyde Hartmut, M.Sc., Insitute

Supervisors:

Prof. Dr.-Ing. Frank Schmidt-Döhl, Prof. Dr.-Ing. Irina Smirnova

Project topic

The alkali-silica reaction (ASR) occurs in the pore solution in concrete structures and can damage them. During an ASR, various forms of silica react with alkalis to form a gel, which can increase in volume by absorbing calcium and water. The gel is initially absorbed into the pores of the concrete. When it expands beyond this, it exerts pressure on the concrete structure and cracks may develop in the concrete structure. Characteristic damages of an ASR are net-like cracks (map cracking) and gel leaking from the surface, which, however, is water-soluble and, accordingly, rarely observed. The reactants of an ASR: water, alkalis and various forms of silica are present in the main constituents of concrete. Enough liquid for the reaction is already provided by the mixing water of the concrete. The alkalis are bound as alkali sulfates in the cement or contained in the clinker phases and are released during hydration of the cement. The different forms of silica result from the aggregate. Amorphous, poorly crystalline, cryptocrystalline silicate rock or stress quartz can be dissolved by the high pH in the pore solution. Various forms of silica result from this process.
ASR that has already started is difficult to impossible to stop and only at great expense. The concrete structure often must be completely renewed. Therefore, it makes most sense to avoid ASR preventively. This is most effectively achieved by avoiding alkali-reactive aggregate.
The test methods for avoiding a damaging ASR are often based on measurements of reaction induced expansion of test specimens made of concrete or mortar. The test specimens contain the aggregate to be tested and are made from a standard recipe. The disadvantage of these methods is the long test duration of two weeks to several months. In addition, the aggregate is examined petrographically or in dissolution tests. These methods have the disadvantage that they require sufficient experience of the inspector or that they cannot detect every type of reactive aggregate.
The aim of this project is to develop a new evaluation method for the alkali reactivity of common aggregates. This procedure should allow a short-term assessment of the reactivity of an aggregate. For this new fast test, specimens made of concrete or mortar are avoided. The procedure is similar to conventional dissolution tests, but with a different assessment of reactivity. Physical and chemical parameters are recorded during the dissolution of the aggregate. The assessment of reactivity is based on the kinetic evaluation of the recorded parameters. For the acceleration of the solution, the aggregate is ground in advance, the experimental temperature is 60°C and solution takes place in 1 molar potassium hydroxide solution.

Methods:

•    Monitoring and evaluation of pH, electrical conductivity, and redoxpotential during the reaction of aggregates and solution
•    Determination of particle size and shape and their changes (laser-granulometry, microscopy)
•    Determination of specific surface (BET)
•    Element analysis of solids and solutions (XRF)
•    Phase-analysis of the solids (XRD, ATR-IR)

Modeling:

•    Chemical kinetics
•    Statistical evaluation

Literature:

Hartmut, G.; Schmidt-Döhl, F.: Detection of ASR aggregates by dissolution experiments: a new approach. In: Júlio, E.; Valenca, J.; Louro, A. S. (Ed:): Concrete Structure: New Trends for Eco-Efficiency and Performance, Proceedings of the 2021 fib Symposum, Lisbon, Portugal, June 14-16, 2021. p. 344-351.

Hartmut, G.; Schmidt-Döhl, F.: Reaction kinetics in solution experiments to avoid the alkali-silica reaction. 3rd. International Conference on the Cemistry of Construction Materials (ICCCM), Karlsruhe, 15.-17.3.2021.

Hartmut, G.; Schmidt-Döhl, F.: Investigations on reaction kinetics in solution experiments to prevent the alkali-silica reaction. 16th International Conference on Alkali Aggregate Reaction in Concrete (ICAAR 2020-2022), Lisbon, 31.5.- 2.6.2022, First Book of Proceedings, April 2021.

Hartmut G. und Schmidt-Döhl F. (2022): Detection of ASR aggregates by the evaluation of chemicalparameters of dissolution experiments. In: Book of Proceedings fib International Congress 2022, Oslo, Norway, June 12-16, 2022.

Hartmut, G. und Schmidt-Döhl F. (2022): Investigations for the development of a new test method to prevent the alkali-silica reaction. At: PintPFS Conference (Graduiertenkolleg, Hamburg, Germany, 26.–27.09.2022

Hartmut,G. und Schmidt-Döhl F. (2021): Avoiding alkali-silica reaction (ASR) by observing kinetic effects in particle-fluid systems (PFS). At: VII International Conference on Particle-Based Methods (PARTICLES 2021), Hamburg, Germany

Osterhus, L.: Solubility testing of rock powders in KOH solution at variable measuring conditions with respect to a rapid ASR estimation. Symposium DFG FOR 1498: Alkali-silica reactions in concrete structures - cyclic loading, fluid transport and modeling. Bochum 5.-6.10.2016

Osterhus, L.; Dombrowski, C.; Schmidt-Döhl, F.: Löslichkeitsuntersuchungen zur schnellen Beurteilung der Alkalireaktivität von Gesteinskörnungen 19. Int. Baustofftagung ibausil, Weimar, 16.-18.9.2015, Vol. 1, p. 1405-1412, 2015

Osterhus, L.; Schmidt-Döhl, F.: Verbessertes Prüfverfahren zur Beurteilung der Alkalireaktivität von Gesteinskörnungen basierend auf Lösungsversuchen. GDCH Tagung Bauchemie, 6.-8.10.2014, Kassel. GDCH-Monographie, Vol. 48, p. 41-44, 2014, ISBN 978-3-936028-86-7

Osterhus, L.; Dombrowski, C.; Schmidt-Döhl, F.: Studies of aggregate solubility for rapid assessment of their reaction capacity. Cement and its Applications, No. 2, 2017, p. 89-94 (in Russian)