Читать книгу Monument Future - Siegfried Siegesmund - Страница 187

The durability of the Hungarian rhyolitic least porous welded tuffs may reach 90 freeze-thaw cycles and 20 salt crystallization cycles with minor or no decay in structure and technical properties. However, the most porous softest varieties, which have found a larger application in the historical built heritage, can withstand only few weathering cycles, before disintegration, erosion, and cracking. The magnitude of the differences in porosity, and consequently in the absorption of water and salt solutions, controls primarily the diverse durability.

135Secondary factors are pore-size distribution especially in the range 0.1–10 µm – critical for ice and salt crystallization damage. Considering the textural features, the most significant discriminating factor is the proportions between crystals and the weaker pumice and groundmass.

Table 1: Physical properties of studied tuffs (data of Hungarian rhyolitic tuffs is from Germinario and Török 2019, Hungarian andesite tuff is from Török 2007, data of the German and Armenian tuffs are from Pötzl et al. 2018a and Pötzl et al. 2018b, data marked by * is from López-Doncel et al. 2016; and marked by + is from Wedekind et al. 2013).

The Hilbersdorf tuffs withstand 11 to 21 cycles before complete disintegration due to its high porosity, bimodal pore radii distribution and high amounts of micropores.. The hydric and thermal expansion of these tuffs is greatly influenced by their clay mineral content and ratio of micropores. Maximum hydric expansion can reach values of 7.5 mm/m (Pötzl et al. 2018a).

Armenian tuffs also show great variation in their durability and weathering behaviour, dependent on their petrophysical properties. The resistance against salt attack varies from 16 to way over 150 cycles. Even considerably soft tuffs, like the Hoktemberyan varieties, are not strongly attacked by the salt crystallization and withstand more than 150 cycles. The Golden Armenia, on the other hand, disintegrates after 34 cycles. The main difference between both tuffs is the clay mineral content as well as the much higher ratio of micropores in Golden Armenia.

Regarding the durability of the Mexican tuffs, the conducted salt bursting tests show that after 14 cycles the most porous sample was completely destroyed and two samples, both with a bimodal pore distribution, have resisted at the end of the test 47 and 48 cycles, respectively. All SLP samples 136were tested for hydric and thermal expansion and all of them did not show any expansion, and one sample showed even hydric contraction with values around 0.03 mm/m.