On-site evaluation of the mechanical properties of Maastricht limestone and their relationship with the physical characteristics

Maastricht limestone is a soft bioclastic calcarenite of the Upper Cretaceous period cropping out in southern Limburg between Belgium and The Netherlands. This material was widely used from the Middle Ages to the Renaissance. Four different varieties can be distinguished according to fossil content and petrographic characteristics, which determine slight differences in compressive strength. Despite its poor mechanical characteristics, the material is very durable with remarkable frost resistance. This is mainly due to the pore dimensions (the most frequent pore radius class is 16-64 mu m) but also to the particular kind of weathering that causes the formation of a protective 'skin' through a process of dissolution of unstable aragonite from serpulids and calcite precipitation in the pores of the external layer. The physical characteristics and the mechanical properties (using the drilling resistance measurement system (DRMS) method) of the hard layer that developed on the surface of Tongeren Cathedral, constructed using the Sibbe variety of Maastricht limestone, were investigated and compared with those of the quarry material. This comparison made it possible to emphasize the particular hardness of this surface in contrast to the outer layer of the quarry material. Moreover, it was possible to determine its thickness and to infer that this hard layer was formed after only 15 years of exposure.