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EVALUATION OF EFFECTIVE COMPRESSION STRENGTH OF NATURAL STONE BY DRILLING RESISTANCE MEASUREMENTS

Stephan Pfefferkorn¹, Christoph Franzen²

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 HTW Dresden, FB Bauingenieurwesen, Friedrich-List-Platz 1, 01069 Dresden, Germany

2 Institut für Diagnostik und Konservierung an Denkmalen in Sachsen und Sachsen-Anhalt e. V. (IDK), Schlossplatz 1, 01067 Dresden, Germany

Abstract

The knowledge of strength parameters for material is necessary for the appraisal of the bearing behaviour of historic buildings. Usually these are determined by suitable check of drilling cores.

The columns under the vault in the ground floor of the historical city hall in Oederan (Germany, Saxony) are strongly loaded according to continuance statics. For the judgement their load-carrying capacity the pressure resistance should be determined. On account of the statics a cross section reduction was not admitted by taking drilling core. Besides, the columns from Hilbersdorfer Porphyrtuff showed considerable decomposition phenomena in its surfaces. The aim of the investigations was to find out the depth of the structure loosening and to estimate the pressure resistance of the uninjured rest cross section.

These questions could be answered with the drilling resistance measurement method. Besides, drillings with small diameter and a steady driving are introduced into the material. The measuring value of the drilling resistance arises from the contact force which is necessary for the realisation of the given driving. It can be given for every tenth millimetre of the drilling depth, so that a depth profile of the drilling resistance arises.

Between compressive strength and the drilling resistance a linear correlation exists. This relation could be determined on the basis of test specimens which were cut of simultaneous removal material. Drilling resistance as well as compressive strength were checked at these cubes and were confronted. Afterwards three drilling opposition measurements were executed per column. From the measuring profiles obtained could be read the depth of the structure decay as well as the drilling resistance of the unweathered core cross section be determined. With these values the compression strength was estimated under use of the before provided correlation function.

Initial situation and objective

Two columns made of Hilbersdorf porphyry tuff carry the vault on the ground floor of the town hall in Oederan, Germany (Figure 1). The volcanic Hilbersdorf porphyry tuff is a regionally used historic building material with a wide range of qualities (SIEDEL 2006, KREISSL 2010, WEDEKIND et al. 2013). The columns showed considerable surface loosening due to the influence of damaging salts leading to significant reduce in the static cross-section. Primary static assessments based of some assumed material parameters determined a high probability of failure. That’s why as an emergency 216measure a massive reinforced concrete jacket was placed around the shafts of the pillars to support the load transfer.

Figure 1: Town hall Oederan (Germany, Saxony, 16. Century A. C.).

To find a more appropriate solution including the preservation of the columns the knowledge of the real material parameters were necessary. Thus the minimum compressive strength of the stone material and the average weathering depth had to be determined in a low destructive measure. The data is needed for the calculation of the effective residual cross-section.

Approach

There is a basic linear relationship between the uniaxial compressive strength and the drilling resistance of natural stones (DELGADO et al. 2000, WENDLER, E., SATTLER, L. 1996). For any material the regression line must be determined in detail. From original Hilbersdorf porphyry tuff construction material from the Oederan Town Hall 10 test dummy cubes were made (Figure 2). On each cube one drilling resistance measurement was proceeded, using DRMS cordless device (SINT Technology, Italy). Afterwards the compressive strengths was determined. The direction of pressure was in the axial direction to the drill channel based on the assumption that borehole does not affect the result of the compressive strength measurement significantly.

Figure 2: Test cube dummies, a) prior to measurements, b) after measurements, note: fracture path by compression test is not correlated to preceeding hole of drilling resistance.

On site, in the townhall of Oederan, openings were cut into the concrete mantles around the columns (Figure 3). Those windows allowed to carry out three drilling resistance measurements in each column. Measurements aimed to estimate the minimum strength of the natural stone material of the column shafts and, moreover, to gather information of the weathering depth.

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Figure 3: Investigation windows in the concrete jacket of the two columns.

Results

The result of drilling resistance measurements carried out with DRMS is a force given in Newton referred to increment of the drilling depth section (Figure 4). With the exception of one measurement (#08_oed8.drm), which was excluded from further evaluation, the profiles were quite balanced. The mean value of the drilling resistance was calculated very precisely. This was compared to the compressive strengths measured according to DIN EN 1926 and the reference line was determined by linear regression (Figure 5).

Figure 4: Drilling resistance on test cubes.

Figure 5: Regression function of drilling resistance and compressive strength.

The drilling resistance profiles determined on the columns have shown some more complex profiles (Figure 6).

Figure 6: Drilling resistance measured on the two original columns.

The mechanical properties of the Hilbersdorf porphyry tuff has been changed to different depths. Most probably those changes are due to salt action leading to loosening, compaction of the surface 218with underlying crumble zones, etc.. The zone of unweathered material is detected at different depths within the profile. That’s why the evaluation to an average value and converted into compressive strengths was done in two parts: the drilling resistances from profile depths of 5 mm and 15 mm were combined and plotted/calculated to the regression value (Figure 7).

Figure 7: Compressive strength deduced from drilling resistance.

Conclusions

On the basis of a linear relationship between compressive strength and drilling resistance, it can be estimated that the strengths essentially lie behind an assumed weathering zone of approx. 15 mm depth in the typical spans of unweathered Hilbersdorf porphyry tuff. According to the available results, compressive strengths from approx. 24 N/ mm^² estimated and basis for further calculations. The value correspond to the literature data for Hilbersdorf porphyry tuff (e. g. Siedel 2006, KREISSL 2010, Wedekind et al. 2013).

However, in one case of the measurements, the depth of softening could not be finally detected. With a drilling depth of 15 mm, there is an estimated compressive strength of only approx. 15 N/ mm^² at this point (column 1 middle hole #07_oed_1).

In order to provide proof of the load-bearing capacity of the columns, this minimum value of the compressive strength should be used as a maximum, additionally reduced by appropriate factors. The statically effective cross-section must be reduced at least by the amount of material loss plus 15 mm weathering depth.

Finally, two uncertainties have to be taken into account regarding the statements made above:

1. The two examination windows give solely partional insight to weathering condition of the column surfaces. Both, LANGE (2011) aswell as DU PUITS (2016) examined the columns before their sheathing and report massive surface losses of up to 5 cm depth.

2. The regression line was determined on the basis of dry-dried material. The literature does not provide information on the actual relationship between compressive strength and drilling resistance under the corresponding moisture conditions. KREISSL (2010) determined significant reduction in compressive strength on fully wet Hilbersdorf porphyry tuff material. In the case of the Hilbersdorf porphyry tuff dealt with here, the information about compressive strength as a funstion of relative humidity as possible in the town hall would have to be elaborated through additional investigations.

The case study of the town hall columns in Oederan compehensibly shows a valuation of the compressive strength of inorganic building material. The in situ determination is based on minimal invasive drilling resistance measurements on site correlated to laboratory parameters.

219References

Delgado-Rodriques, J. & Costa, D 2000: A new method for data correction in drilling resistance. Tests for the effect of drill bit wear. – Int. J. Restoration of Buildings and Monuments, 10: –18, Zürich.

DIN EN 1926 Prüfverfahren für Naturstein – Bestimmung der einachsigen Druckfestigkeit; 2006.

Du Puits V. 2016 Vorgezogener Ausschnitt aus dem Memorandum 1 – Chemnitz 03/2016.

Kreißl, S. (2010) Eigenschaften und Schadensphänomene des Hilbersdorfer Tuffs sowie Möglichkeiten der Steinergänzung mittels Mörtel. Diplomarbeit, 156 S.; http://opus.ba-glauchau.de/opus/volltexte/2010/1357/pdf/Diplomarbeit.pdf

Lange, M. 2011 Befunduntersuchung Rathaus Erdgeschoss – Schloss Kaufungen 06/2011.

Siedel, H. (2006) Sächsische „Porphyrtuffe” aus dem Rotliegend als Baugesteine: Vorkommen und Abbau, Anwendung, Eigenschaften und Verwitterung. Institut für Steinkonservierung e. V. Bericht Nr. 22, Mainz.

Wedekind, W., López-Doncel, R., Dohrmann R., Kocher M.& Siegesmund S., 2013. Weathering of volcanic tuffrocks used as natural building stone caused by moisture expansion. Environmental Earth Science. 69:1203-1224. DOI 10.1007/s12665-012-2158-1.

Wendler, E., Sattler, L. (1996): Bohrwiderstandmessung als zerstörungsarmes Prüfverfahren. – In: Wittmann, F. H. Gerdes, A. (Hrg.): Proc. 4. Intern. Koll. Werkstoffwissenschaften und Bauinstandsetzen (MSR IV’96): 145–159, Esslingen.

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