For the protection of the world’s cultural heritage the restoration of listed buildings and monuments is an important task. In many cases the need to repair and reprofile natural stone elements occurs. Repair mortars for such applications have to match historico-cultural interests as well as durability related and constructive interests. Currently a research project dealing with the development and optimisation of such mortars is set up at the German Federal Institute of Materials Research and Testing (BAM). To provide the conservatorwith the best possible options to match the colour, texture and technical requirements of the stone, the repair mortar System will be setup on a modular basis. Different binder Systems are planned to be combined with different aggregate compositions and different pigments. By the modular setup the possibility of independent adjustment of mechanical properties and appearance is aimed. In order to guaranty the reversibility and to ensure that the repair mortar does not induce any damage to the original substance, strength and stiffness of the repair mortar have to be well controlled. The use of polymer dispersions is helpful here, as they allow the production of low stiffness mortars with at the same time relatively high flexural strength values. Also investigations on “alternative” modifications known from historical applications such as the use of casein are carried out.
Despite the urgency of seemingly more important building tasks, there is also a great demand for natural stone repair in cultural heritage on the African continent. There is a wide ränge of different objects reaching from prehistoric monuments up to colonial architecture, all in different säte of preservations and subjected to a variety of individual environmental ant climatic conditions, which demand individual and adjustable restoration techniques and materials.
Trotz der zahlreichen Möglichkeiten zur Beeinflussung der Mörteleigenschaften durch Mischungsmodifikationen ist es nahezu unmöglich, einzelne Eigenschaften unabhängig von anderen zu verändern. Bei einer Mischungsoptimierung muss im Einzelfall immer entschieden werden, welcher der gewünschten Eigenschaften die höchste Priorität zukommt.
Anhand von Parameterstudien wird im vorliegenden Artikel verdeutlicht, wie es für bestimmte Anwendungen sinnvoll sein kann, die Verschlechterung einzelner Eigenschaften zu akzeptieren, wenn dadurch eine für den speziellen Einsatzfall essenzielle Eigenschaft verbessert werden kann. Die komplexen Wechselwirkungen der einzelnen Bestandteile fuhren dazu, dass auch scheinbar widersprüchliche Kombinationen von Zusatzmitteln im Einzelfall erforderlich werden können. Zur Anpassung der meisten Mörteleigenschaften gibt es verschiedene Möglichkeiten der Zielerreichung mit jeweils unterschiedlichen Nebeneffekten. Frisch- und Festmörteleigenschaften sind dabei gleichermaßen zu beachten.
Despite the numerous possibilities to influence the properties of repair mortars by modifications of the mixture proportioning, it is almost impossible to change an individual property without influencing other mortar properties. When optimising a mixture, it has to be decided for each individual case, which of the mortar properties needs to be treated with the highest priority. Most properties can be altered by several different measures, which may vary in effectiveness, side effects and cost efficiency. As the prevention of shrinkage induced constraining cracks is crucial for the durability and Service life of a concrete repair task, in the presented paper special attention is paid to the influences on the mortar’s shrinkage behaviour. On the basis of parameter studies it is shown, how it may be beneficial in certain cases to accept the worsening of some general properties if by this measure another property, which is essential for the individual application task, can be enhanced significantly. In general a tailor-made approach for the mixture proportioning seems beneficial but due to the complexity of the constituents’ interactions in repair mortars, it bears a high risk of unexpected malfunctions.