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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Figure 4: Treated stone specimens with marked points for US velocity measurements and clearly visible depth of penetration.
After the treatment, the specimens were conditioned for one month at 20 °C/60 % RH before starting the testing.
Material testing
During experimental work, the following characteristics were tested: ultrasonic velocity in transmission, micro-drilling resistance, water uptake, porosity, hydric dilation and thermal dilation, bending strength, modulus of elasticity and frost resistance.
Test results
Porosity and mechanical characteristics represent the most interesting data at the consolidation tests. Changes of porosity, as well as in mechanical characteristics, substantially influence the behavior and life cycle of treated historic materials. Naturally, the surface stone deterioration creates very non-homogeneous profiles along the depth in different distances from the surface. Stone material then responses in various ways to consolidation interventions (Sasse & Snethlage 1996). Figure 5 illustrates changes in US velocities in the tested sandstone after consolidation with the Funcosil 300 Steinfestiger.
Figure 5: US velocity changes after consolidation by Funcosil 300 – non-weathered stone (upper) and weathered stone (lower).
The highest increase of ultrasonic velocity was observed in samples from Praha sandstone (light blue lines), which has the highest porosity as well as the largest mean pore size. Other stones appeal improvement in USV only in case of weathered surface treated by the higher concentration of consolidant.
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Figure 6: Three-point bending tests of thin sandstone plates.
Figure 7: Changes in the bending strength of the unweathered sandstone after consolidation.
Similarly, the bending strength tests on thin plates (Figure 6), cut from the unweathered stone samples, exhibited the highest impact of consolidation on the high porosity Praha sandstone, Figure 7.
In most cases, the consolidation by Funcosil 100 caused a higher increase in strength than by Funcosil 300.
Figure 5 above clearly shows significant differences in the material characteristics of the weathered and deteriorated sandstone in the depth profiles. Due to crust formation on some stones, the surface and near-surface layers may have elevated mechanical properties – strength and the modulus of elasticity usually together with a decreased mean pore size. On the other hand, disintegrated sandstone types exhibit lower mechanical properties and higher mean pore size characteristics. As an example of both types, let us present Figure 8 showing a variation of the bending strength in the depth profile and Figure 9 comparing mean pore size variations.
