Monument Future. Siegfried Siegesmund

      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.

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       THERMAL BEHAVIOR OF BUILDING SANDSTONE: LABORATORY HEATING EXPERIMENTS VS. REAL FIRE EXPOSURE

Nadine Freudenberg¹, Thomas Frühwirt¹, Klaus-Jürgen Kohl², Monika Kutz², Heiner Siedel³, Jörn Wichert³

      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