Contour Scaling on Building Facades - Dependence on Stone Type and Environmental Conditions
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CONTOUR SCALING ON BUILDING FACADES - DEPENDENCE ON STONE TYPE AND ENVIRONMENTAL CONDITIONS EBERHARD WENDLER*, DIETRICH D. KLEMM* AND ROLF SNETHLAGE** *Geological Institute of the Ludwig-Maximilians-University, Luisenstraje 37, D-8000 Munich 2, Federal Republic of Germany **Bavarian State Conservation Office, Central Laboratory, Luisenstraje 37, D-8000 Munich 2, Federal Republic of Germany ABSTRACT Contour scaling is often observed on building facades, especially built from clay-rich sandstones. The paper shows that a soluble salt enriched zone and a decrease of the flexural strength below the stone surface can be detected even if a damage is not yet visible. A computer simulated wetting-drying process demonstrates that the mean humidity maximum is situated below the surface. Its depth coincides with the depth of the salt maximum and strenth minimum. INTRODUCTION The formation of contour scales is mostly observed on sculptures and buildings that are made of clay rich sandstones. However, they also occur on clay containing arkose sandstones, on limestones and on granites. The favourite environment for contour scales formation are sun-, wind- and driving rain exposed parts of a building which are characterized by rapid wetting and drying cycles. Contour scales never are found in sheltered parts. Fig. 1 shows a typical contour scaling profile: scale (A) weak middle zone (B) - "unaffected" stone (C). A phenomenological analysis of the damage types already gives an evidence that the damage type of the weak middle zone should be characterized by high permanent moisture and salt content.
Fig.l: Contour scale. St.Vitus church, Iphofen/Bavaria.
Schilfsandstone Photo: H. Leisen
Mat. Res. Soc. Symp. Proc. Vol. 185., 1991 Materials Research Society
266
The following paper describes the results obtained by three different analytical methods - strength profile, chemical profile, moisture distribution - which undependently from each other lead to an identical explanation for the formation of contour scales. STRENGTH PROFILE ANALYSIS Three drillcores were taken from an ashlar of the St.Vitus church of Iphofen/Bavaria, which showed an intensive contour scaling in its peripheral area. Sample Sli, which showed no visible damage, was located in the central region of the stone block. Sample S12 had a scale in the state of detachment. At sample S13, taken from the peripheral part, the scale was completely loose. The method to determine the strength profile by biaxial flexural strength with ring loading is discribed in detail by Sattler & Snethlage [1]. The results obtained are presented in fig. 2. It is evident that strength profiles 12 and 13 show a decrease of the strength at the depth of the middle zone. Because the thickness of the contour scale of core 13 was too small, its strength could not be measured. Therefore it is marked with the value "zero". However, core 11, which was taken in a sound part of the ashlar, also shows a decrease of the strength in that depth where the detachment of the scale will occur in the future. T
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