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Alkali-silica reaction (ASR) is a chemical reaction within concrete which can lead over time to cracking and spalling. Due to the complexity of the problem, it still causes damage to concrete constructions worldwide.
The publication aims to illustrate the interdisciplinary research of the German Federal Institute for Materials Research and Testing (BAM) within the last 20 years, considering all aspects of ASR topics from the macro to the micro level. First, methods for characterization and assessment of ASR risks and reaction products used at BAM are explained and classified in the international context. Subsequently the added value of the research approach by combining different, preferably nondestructive, methods across all scales is explained using specific examples from a variety of research projects. Aspects covered range from the development of new test-setups to assess aggregate reactivity, to analysis of microstructure and reaction products using microscopical, spectroscopical and X-ray methods, to the development of a testing methodology for existing concrete pavements including in-depth analysis of the visual damage indicator and the de-icing salt input using innovative testing techniques. Finally, research regarding a novel avoidance strategy that makes use of internal hydrophobization of the concrete mix is presented.
Ultra high performance concrete (UHPC) is characterised by its high compressive strength of more than 120 MPa and its high durability. With thermal treatment at 90°C, hydration is accelerated and a strength comparable to the 28-day strength can be achieved immediately after the treatment. In applying Optimum process conditions, the strength can be even further increased by up to 30%. Previous Research showed that thermal treatment can lead to inhomogeneities in form of a visible zonation within the cross-section, if the UHPC is not protected sufficiently from desiccation. This led to the question, to what extent the related changes in mechanical properties, microstructure and phase composition are of relevance for the performance of the UHPC. First investigations of small specimens thermally treated without protection exhibited a decrease of compressive strength, indicating that the zonation aspect requires further research.
In this study the zonation of UHPC treated thermally at 90°C is investigated after applying four different procedures of treatment and curing in comparison. The samples are characterized with respect to their chemistry, mineral composition and microstructure to allow an assessment on durability and strength development, with focus on potential depth dependent changes associated with the zonation.
The measurements reveal the formation of a visible zonation in case of unprotected treatment, if followed by immersion in water. The compressive strength is not significantly impaired, but a decrease in bending strength gives reason for concern. Further results allow clear correlation with changes in pore structure, whereas the interpretation of relationships with phase distribution, degree of hydration, microchemistry is more complex and therefore, provides only partial clarification.
Ultra high performance concrete (UHPC) is characterised by its high compressive strength of more than 150 MPa and its high durability. Due to thermal treatment at 90°C a strength comparable to the 28-days-strength can be achieved immediately after the treatment and in some cases can be even further increased up to 30 %. The explanations for the increase in strength are the accelerated hydration of the clinker minerals and the intensified pozzolanic reaction contributing to a denser microstructure and hence, a high performance in compressive strength.
Former research shows that thermal treatment can lead to inhomogeneities in form of a visible zonation within the cross-section. The width of the margin increases with shorter pre-storage time before the thermal treatment and with omitting protective measures against desiccation during the treatment. Specimens exhibiting a zonation typically show a lower compressive strength compared to the undisturbed reference whereas changes in chemistry, mineral content and microstructure were not reported in detail.
In this study the zonation of thermally treated UHPC is investigated with respect to its chemistry, mineral composition and microstructure to allow predictions on durability and strength development. Measurements show a change in pore sizes, minerals phase composition and element distribution leading to the visible zonation and weaker bending tensile strength compared to the reference.
Composite Facade Elements with Self-Cleaning Surface made of Ultra-High-Performance Concrete (UHPC)
(2020)
In the framework of the European project H-House various concrete façade elements were developed with the aim to ensure a long service life by combining a very durable material with self-cleaning properties. The façade elements presented are made of a shell of UHPC filled with blocks of aerated autoclaved concrete as insulating material. Self-cleaning properties were realized amongst others by imprinting a microstructure into the surface during casting. The paper focuses on selected technological aspects of the manufacturing process of prototypes which had to be performed in two concreting sections. Furthermore the challenges faced when upscaling the self-cleaning properties are addressed and the strategy to assess the self-cleaning properties by measuring the contact and the roll-off angel is presented. The results show that a successfull upscalaing process requires detailed planning and that the best results can often be achieved with a moderate work effort or material use.
Composite Facade Elements with Self-Cleaning Surface made of Ultra-High-Performance Concrete (UHPC)
(2020)
In the framework of the European project H-House various concrete façade elements were developed with the aim to ensure a long service life by combining a very durable material with self-cleaning properties. The façade elements presented are made of a shell of UHPC filled with blocks of aerated autoclaved concrete as insulating material. Self-cleaning properties were realized amongst others by imprinting a microstructure into the surface during casting. The paper focuses on selected technological aspects of the manufacturing process of prototypes which had to be performed in two concreting sections. Furthermore the challenges faced when upscaling the self-cleaning properties are addressed and the strategy to assess the self-cleaning properties by measuring the contact and the roll-off angel is presented. The results show that a successfull upscalaing process requires detailed planning and that the best results can often be achieved with a moderate work effort or material use.