Deformation von Fassadenplatten aus Marmor

Abstract

For about four decades marble has been used as thin veneer cladding. The durability of such thin slabs (mostly 30-40 mm) is satisfactory at most constructions. However, on numerous buildings all over the world, the long-term deformation (expansion, bowing) of some claddings is associated with structural weakening and strength loss which led to an inevitable restoration of the affected facades and to an image problem of marble used as a structural building stone. Up to now, the existing knowledge about the causes for this special kind of marble weathering is mostly restricted to research on Carrara marble and to the effect of individual parameters. This thesis presents the first study of the combined influence of extrinsic (environmental) and intrinsic (material specific) parameters on marble bowing. The respective impact of these factors on the weathering process is elucidated by damage mapping of four different marble facades. The work focuses on the influence of the microfabric on the damage dynamics of veneer marble and particularly considers moisture as an essential climatic parameter. In order to characterize and understand the weathering process, an array of different marble samples had to be studied with regard to their petrographic, rock physical and microstructural properties. Besides both fresh and artificially weathered marble samples this survey also included demounted facade panels from the investigated buildings with varying degrees of pre-deterioration. All in all 13 different marbles with extremely different fabric properties from Portugal, Switzerland, Sweden and Italy were examined.Generally the weathering process is promoted by thermal-hygric microcrack propagation. In case of temperature variations, as occuring during day-night cycles, the anisotropic strain properties of the main components calcite and/or dolomite generate stress within the interconnection of grains. This finally leads to microcracking along fabric discontinuities such as grain boundaries, cleavage planes or pre-existing cracks. Irreversible residual strain serves as an indicator of the accumulated extension of microcracks. When a sample is exposed to repeating temperature cycles this parameter reaches a constant value under dry conditions, whereas moisture leads to a further increase of residual strain. The difference between frontside and backside residual strain finally causes bowing of a marble slab, so that a correlation of residual strain and bowing can be found. The anisotropy of both residual strain and bowing is affected by the preferred orientation of microcracks and the width anisotropy of such microcracks which again are finally controlled by the texture (lattice preferred orientation) and the grain shape preferred orientation. In a similar way, the preferred orientation of microcracks contributes to the anisotropy of weathering sensitive parameters, i.e. flexural strength and compressional wave velocity (Vp). Depending on the cutting direction, bowing as well as flexural strength can vary up to a factor of three. When assessing the structural integrity of already weathered marbles it has to be considered that Vp can vary up to 30% for different directions.Indicating the degree of deterioration, slab bowing is associated with the frequency of macro-cracks, a change of porosity parameters and a decrease of mechanical and elastic parameters. Aside from the microfabric certain extrinsic factors concerning the exposure of the facade (e.g. height above ground, orientation) and building-specific characteristics are closely connected with the marble bowing, since they are shown to directly influence the intraslab moisture gradient.Exemplary long-term field data from the facade of the University Library building in Goettingen as well as weathering simulations of the concerned veneer marble under laboratory conditions reveal an unbroken progress of slab bowing there. The detailed knowledge of long-term bowing dynamics and its correlation with strength loss of veneer panels can provide the basis for risk assessment and prediction of the service life of damaged marble facades.