Читать книгу Monument Future - Siegfried Siegesmund - Страница 70

The combination of the mapping results and the experimental tests allow an evaluation of the rocks. The samples from Mitla are in general very resistant to most forms of weathering. They have low porosities and low w-values with low hydric 58and thermohydric dilatation. Both MG and MR are relatively resistant to salt weathering compared to the other analyzed tuff samples.

Figure 6: Photographic documentation of the salt bursting test.

The pore space has a very important impact on the process of salt weathering. A general dependence between the share of micropores and the number of salt cycles could be observed in the samples. MR and CVO are the strongest samples with high surface hardness, ultrasonic velocities, tensile strength and low porosity with almost no capillary pores present. CVO is not very resistant to salt weathering and shows the highest hydric expansion and strong thermohydric dilatation with non-reversible changes due to the dehydration of clay minerals. This makes CVO less suitable as a building stone. Nevertheless, CVE is even more affected by moisture than CVO. With the highest sorption, high hydric expansion and low tensile strength as well as low ultrasonic velocity CVE is strongly affected by loss of components and fracturing/bursting. CAO and CAE are less affected by component loss and show strong back weathering and fracturing due to a high amount of clay minerals, which makes the yellow tuff varieties rather soft samples. The red tuffs (CRO and CRE) are the only ones which show anisotropic behavior in the weathering features that correlate with relatively high anisotropic properties.

The results suggest, that moisture and thermal changes are the most severe damaging factors for tuffs from Oaxaca. The southern and eastern sides of most buildings are the ones with the highest temperature changes, and therefore are weathered the strongest. The northern side usually experiences a lot of moisture, while the western side is in general the least weathered building side. The tuff varieties from Mitla are very suitable building stones and have remained in the buildings for a long time. The green tuff from Oaxaca (CVO) is a historical building stone of good quality but needs to be protected from moisture. The replacement stone from Etla (CVE), which is used today, is a suitable building rock but is certainly of a lower quality than CVO. Therefore, conservational treatment should be considered for CVE.