Monument Future - Siegfried Siegesmund

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       VARIATIONS OF CHARACTERISTICS OF SANDSTONE SUBJECTED TO WEATHERING AND CONSERVATION INTERVENTIONS

      IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

       – PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

       VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

      1 Institute of Theoretical and Applied Mechanics of the Czech Academy of Sciences, Department of Heritage Science, Prosecká 76, 190 00 Praha 9, Czech Republic, [email protected]

       Abstract

      The paper presents selected results of a comprehensive study of characteristics and behavior of seven typical sandstone types used for historic buildings in the Czech Republic, mostly in Prague. Stone characteristics were studied on materials affected by various historic environmental impacts and conditions generated by previous interventions. From seven types of sandstone were prepared nine series of test specimens which included chemically deteriorated surface layers (crust), cleaned surface layers, and virgin material from the stone core. The above-described sets were manufactured without any consolidation treatment as well as in two further sets consolidated with two agents, namely Funcosil 100 and 300, based on the silicic acid ester. The test specimens were cut from damaged sandstone blocks, which were extracted from a masonry rail of the Charles Bridge in Prague before replacement with new elements. The results supplied data for comparing the efficiency of the consolidation treatment with silicic acid ester products in relation to three pre-treatment stone conditions, as well as to the type of sandstone cementation (mostly a kaolin or silica, rarely goethite cementation). In the paper, the most important results and conclusions taken from the tests and their comparison are discussed.

       Introduction

      Charles bridge in Prague has been subjected to various types of deterioration or damaging actions during its nearly eight hundred years history. As a result, some parts were substantially repaired using different types of sandstone available at the given periods. The stone materials exhibit different characteristics decisive for the application of efficient conservation or maintenance technologies. Therefore, a detailed investigation programme has been launched in order to provide restorers with reliable data on material characteristics as well as the response to a selected pilot consolidation treatment.

       Sandstone types and specimens

      Seven types of sandstone were excavated in the past directly in Prague or in mostly close Bohemian quarries.

      They are denoted by the names of the quarries and include Božanov (arkose sandstone), Žehrovice (arkose), Droba (wacke sandstone, unknown quarry), Hořice (quartz sandstone), Libná (quartz sandstone with glauconite), Praha (quartz sandstone), Petřín (quartz sandstone).

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      Figure 1: Divison of the prismatic samples taken from rail stones that had to be replaced.

      Prismatic samples in dimensions of 50 mm × 50 mm × approx. 200 mm were cut into two test specimens – a weathered part with the degraded surface layer and a part of the unweathered material, Figure 1. The deteriorated stone exhibited a significant variation of its characteristics along the depth profile. Therefore, the stone specimens were prepared first in the form of cubes for non-destructive US tests, Figure 2.

      Then the cubes were cut into thin plates according to the methodology recommended by Drdácký & Slížková (2008). Thin plates enable to design a sequence of tests that provide first data from nondestructive tests, typically volumetric change due to hydric and temperature variations, and then from destructive tests of mechanical characteristics, Figure 3.

      The procedure above was applied to the non-weathered specimens as well as on the weathered stone with both the uncleaned and cleaned deteriorated surfaces. For the cleaning, a sandblasting approach has been adopted.

      Figure 2: Ultrasonic testing of material characteristics in 5 mm equidistant profiles.

      Figure 3: Scheme of the cutting plan for characterization according to the depth profile of the sandstone specimens.

       Sandstone consolidation

      For the pilot consolidation tests, two ethyl silicate-based agents have been selected, namely non-diluted Funcosil® Steinfestiger 100 and Funcosil® Steinfestiger 300. They were applied in amounts of 1l per 1 m2 after one-week conditioning at 20 °C/60 % RH. The agents were applied by syringe extended with the cigarette filter on stone surfaces vertically arranged in order to imitate the expected treatment situation on the rail walls, Figure 4.

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