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VARIATIONS OF CHARACTERISTICS OF SANDSTONE SUBJECTED TO WEATHERING AND CONSERVATION INTERVENTIONS

Miloš Drdácký¹, Dita Frankeová², Zuzana Slížková²

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, drdacky@itam.cas.cz

2 Institute of Theoretical and Applied Mechanics of the Czech Academy of Sciences, Department of Material Research, Prosecká 76, 190 00 Praha 9, Czech Republic

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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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.

Figure 8: Examples of the bending strength variations according to the dept profile of the weathered and “virgin” sandstone samples.

Figure 9: Comparison of mean pore size before and after consolidation.

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Figure 10: Consolidation impact on the stone porosity.

Figure 11: Hydric dilatation after Funcosil 300 treatment on the first two plates under the treated surface – αH in μm/m.

It is seen in Figure 8 that the bending strength of the inner layers of the weathered stone is higher than that of the virgin material. Here must be taken into account that the weathered layers might have some consolidation history, which is not exactly known but could increase the strength of the original material in the near-surface layers.

The weathering with subsurface deposits, as well as the consolidation, decreases the volume of pores especially by filling the small pores which reflect in an increase of the mean pore size value. The value of porosity changes can be studied on Figure 10.

From the other test results, the hydric dilation changes are interesting. Figure 11 shows a series of results of hydric dilation measurements on the surface and the first subsurface layers of the weathered stones. At the same time, the effect of sandblasting cleaning has been investigated.

In Figure 11 the dark blue denotes the weathered uncleaned material, the light blue the material which was sandblasted.

It is apparent that the cleaning of the stone surface significantly reduced hydric dilatation up to 5 mm depth. Probably some effect of packing during blasting may be the reason.

In depths from 5–10 mm (second plate), the hydric dilatation is more affected by a consolidation agent.

Conclusion

The tests were required by restorers before planning a rather massive conservation campaign on the Charles bridge in Prague – one of the most important stones Gothic structure. The results achieved helped to make an appropriate choice of consolidation agent, to decide about necessity and type of surface cleaning, to be prepared for a selection of an appropriate stone in cases of replacement needs and to assess intervention impacts. It enhanced the overall design of restoration interventions.

Acknowledgements

The paper is based on the results of research supported by the institutional project RVO 68378297. The authors acknowledge experimental support of E. Čechová, A. Zeman, J. Valach and professional advice of J. Novotný.

88References

Drdácký, M. F., Slížková, Z. Performance of glauconitic sandstone treated with ethylsilicate consolidation agents, In Proc. of the 11th Int. Congr. on Stone, Vol. 2. Toruń; 2008. pp.1205–1212.

Sasse, H. R., Snethlage, R. Evaluation of stone consolidation treatments. Science and Technology for Cultural Heritage. 1996;5(1):85–92.

Table 1: Annex. Sorption characteristics of the tested sandstones.