INVESTIGATIONS ON THE INCREASE IN THE RATE OF WEATHERING OF CARRARA MARBLE IN CENTRAL EUROPE

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1 167 INVESTIGATIONS ON THE INCREASE IN THE RATE OF WEATHERING OF CARRARA MARBLE IN CENTRAL EUROPE KOHLER,W. Labor tor Bauwerksdiagnose, Archaometrie und Geophysik, Bergholz-RehbrUcke/Potsdam SUMMARY In the course of the EUREKA 496 projekt EUROCARE-EUROMARBLE, ultrasonic measurements were carried out on more than 1 free-standing sculptures, gravestones and architectural stone elements composed of Carrara marble. A attempt was made to ascertain the role played by specific local conditions, regional climates and the avarage rate of weathering of Carrara marble on the overall weathering rate of these monuments. The evaluation of a number of free-standing monuments of similar construction and appearance from widely differing climatic regions showed that, although the influence of the original climate may be detected, it is not the decisive factor in the avarage rate of weathering of Carrara marble. Since the 19th century the rate of weathering of Carrara marble has increased in all regions of Central Europe and is continuing to increase. By correlating the ultrasonic velocities with the technical parameters of stone such as the compositive strength, tensile-bend strength, dynamic elasticity modulus and porosity it is possible to trace the temporal phases of the weathering processes and the corrosponding development of the specific deterioration phenomena. From this a prognosis of future weathering can be made. These investigations constitute an essential pre-condition for the formation of conservation concepts for this increasingly endangered group of monuments in Central Europe. 1. INTRODUCTION 1.1 The material In Europe, fine-crystalline Carrara marble has been used in sculpture and construction since almost 2 years. In Central Europe it is for about 3 years that this material has been worked on. Today, it is exported all over the world. In the past 3 years an increase of the weathering rate of Carrara marble objects is observed. In paticular cases this has lead to total collapse (1) Aim of the study and methods During the past 1 O years, ultrasonic transmission velocity proved to be an appropriate parameter to assess properties and condition of marble. Hence, an approach was made to correlate the most important properties of Carrara marble with the velocity of the p-wave ( v P) obtained from non-destructive measurements. Total porosity (P) was assessed for samples showing different grades of weathering by measurements of the water vapour sorption isotherm, accompanied by ultrasonic velocity measurements (2). Such tests run for several years. Further calculations based on the velocities measured for the range of 7% rh. This corresponds to the most frequent ambient conditions on site. Measurements of the compressive strength (D) were performed by Fitzner (3) in dependance on total porosity, and measurements of the tensile-bend strength (BZ) by Winkler in dependance on moisture absorption. Both relationships were correlated with the relationship between porosity and ultrasonic velocity. The dynamic elasticity modulus(e) and the Poisson's constant (IJ) were assessed for differently weathered samples of Carrara marble by means of the resonance frequency. Evaluation of the decrease of ultrasonic velocity (dvp) as a measure of weathering velocity was based on 21 sculptures and free-standing construction members, which were chosen out

2 168 of over 1 objects measured within the last 1 O years. All of them can be considered freely exposed to weathering. On each of them 2 to 6 single measurements of ultrasonic velocity had been performed. Mean values calculated from these readings were used for furhter calculations. 2. RESULTS AND DISCUSSION 2.1. Porosity The relation between porosity (P) and ultrasonic velocity (vp) can be expressed by the following equation: This relationship is thermodynamically suitable. However, the numerical constant can not yet be analysed. 1, 8,.. -1e. > 6, 4, e c:i.. 2, o Carrara marble \ \ o o p v..., -e Figure 1: Correlation between porosity and ultrasonic velocity of Carrara marble In Figure 1 it can be seen, that unweathered marble from Carrara has a very low porosity. Once the ultrasonic velocity decreases below about 3 km/s, porosity increases progressively Compressive strength and tensile-bend-strength Correlation of porosity values with the readings of the compressive strength made by Fitzner (3) is expressed as follows: D = * v - 2 ' ' p

3 169 Carrara marue ] 14 z 12.c 1 8 "'.. > 6... "' c. 4 s Q 2.. T L z I Figure 2: Correlation of compressive strength and ultrasonic velocity of Carrara marble Starting from the unweathered state, it can be seen that the compressive strength of marble from Carrara remains at relatively high levels down to an ultrasonic velocity of less to 2 km/s. Only below readings of 1 km/s a total collapse of the cristalline structure can be observed. The statue having totally collapsed showed velocity readings below 1 km/s. Correlation of porosity with tensile-bend strength readings obtained by Winkler (4) shows the following relationship between tensile-bend strength and ultrasonic velocity: BZ = 594 *v 2 ' 67 ' p Tensile-bend strength starts to decrease markedly already at rather high ultrasonic velocities. The rate of decrease is lowered towards lower velocities. Already at readings below 3 km/s, the load bearing capacity of the marble is substantially reduced. When the ultrasonic velocity drops below 2 km/s, the marble has practically lost ist capacity to support flexural stresses. In fact show sculptures with lost parts, as e.g. on stumps of arms, frequently ultrasonic velocities around 2 km/s. This points to corrosion referring to a major cause of material loss. Objects of Carrara marble with partial values around 2 km/s cannot be transported without risk.

4 17 I Carrara marlie l r r---, r-----r--;-iz r--t--lv-r t-,[/7" -----?''-----1' t 4 +--t t--t-7l----r----ir-- "Cl.i [ j;/" --L"-j------; =:=;;:..._ t Figure 3: Correlation between tensile-bend strength and ultrasonic velocity of Carrara marble 2.3. Dynamik elasticity modulus The dynamic elasticity modulus is related to the ultrasonic velocity as follows: E = 1 75*v 2 ' p Carrara marlie I 6 "SI 'g s c c :.. "' " 8 4.s 2 l "Cl Figure 4: Correlation between dynamic elasticity modulus and ultrasonic velocity of Carrara marble

5 171 In the above equation, the numerical factor of 1,75 describes the correlation between the density D and the Poisson's constantµ. For a density of =2,71 g/cm 3 the Poisson's constant is µ=,341 The measured value averages at µ=, Decrease of ultrasonic velocity In order to calculate the yearly rate of decrease in ultrasonic velocity of Carrara marble objects, the age of each single sculpture was correlated with the respective average decrease of the velocity. Thus, in a first approach it was assumed, that each object underwent a linear ageing process during the whole time of its existence. PLACE SCULPTURE DATE OF AGE US- DECREASE ORIGIN MEASURM. VELOC. US-VELOC. in years in km/s in km/s/ year a dvp 1 Stockholm (S) Germanikus ,85,167 2 Stockholm (S) Herkules ,18,153 3 Florenz (I) Dom/Kampanile ,56,52 4 Florenz (I) Dom/Kampanile ,41,35 5 Wien/Gloggnitz (A) Proserpina ,5,4 6 Munchen (D) Wag muller ,8,173 7 Munchen (D) Kybele ,61,89 8 Berlin (D) Schulze-Delitzsch ,2,49 9 Hannover (D) Sophie ,74,269 1 Potsdam (D) Diana ,34, Leipzig (D) Schiller ,, Neustrelitz (D) Carl ,99, Neustrelitz (D) Brunnen ,13, Dresden (D) Corradinivase ,64, Potsdam (D) Merkur ,82,2 16 Potsdam (D) Venus ,16, Potsdam (D) Lenne ,24, Leipzig (D) Koch ,4, Neuhardenberg(D) Friedrich ,44,114 2 Cottbus (D) Vase ,27, Schwerin (D) Alexandrine ,95,446 Table 1: meassured Carrara marble objects The dependance of the yearly rate of decrease in ultrasonic velocity (dvp) on the absolute age (a) of the object is expressed by the following equitation, which can serve as a preliminary base for calculations: dv = 4 46 *a- I p '

6 172 Carrara Marble y = 4,456x 1 64 R 2 =,9144,5 -r , ,---..,.-----,----r--1---, f o ' ' &:' T'.J---+--t---t----t----- t o\.!i! O,D l t----r---1 f l\ ooo l :, ,& t----r i g t, l-- -,+!-' r i r---+-, ,.., age in years Figure 5: Correlation between the yearly rate of decrease in ultrasonic velocity and absolute age of objects made of Carrara marble The above correlation seems to be surprisingly quite simple. It must be proved in the future whether the numerical amount of both factor and exponent of the equation will hold for further measurements. These. two numbers can be assumed to be either time- or exposure-dependant. However, it is beyond doubt that, the younger an object is, the higher is its average rate of weathering. There are two possible explanations: 1. older Carrara marble objects are made of a higher quality material, which is of a better resistance to weathering, or 2. the weathering rate of all objects made of Carrara marble has considerably increased in the last years. No other explanation for the above observation could be found so far. Thus, further measurements within the next two years shall prove, weather such a rapid increase of the weathering rate of marble from Carrara placed in Central Europe actually exists or not (financial support by the.deutsche Bundesstiftung Umwelt").

7 3. CONCLUSIONS 173 By means of measuring the ultrasonic transmission velocity of objects made of Carrara marble, all important parameter like porosity, compressive strength, tensile-bend strength and the dynamic elasticity modulus, can be assessed in a non-destructive way on site. Thus, decisions on the preservation of such objects can now be put on a mathematical base. It is very likely, that we deal with a substantial increase of the yearly rate of decrease in the ultrasonic velocity since the 19th century, and hence with marked increase in the rate of weathering of free-standing objects of Carrara marble. Further investigations will prove whether this hypothesis is true or not, and which are the causes of such a phenomenon. ACKNOWLEDGEMENTS The present study was facilitated by the members of the project EU-496 EUROCARE EUROMARBLE. The author is also grateful for discussion and collaboration to: K.Bayer, R.Bohm, I.Kohler, M.Krempler, S.Simon, R.Snethlage, J.Weber and E.Winkler. REFERENCES 1. KOHLER, W. (1988): Preservation problems of Carrara marble sculptures, Potsdam-Sanssouci (.Radical structural destruction of Carrara marble"). Vith Int Congr on Deterioration and conservation of stone. Proc. Torun, Sept, 1988, pp KOHLER, W. (199): Structure changes of weathered Carrara marble sculptures as a function of their destructioned position. ICOM Committee for Consevation, 9th Triennal Meeting, Proc. Dresden, Aug, 199, pp FITZNER, B (1988): Untersuchungen der Zusammenhange zwischen dem Hohlraumgefi.ige von Natursteinen und physikalischen Verwitterungsvorgangen. Mitt lngenieurgeol Hydrogeol 29: WINKLER, EM. (1986): A durability index for stone. Bull Assoc Engin Geol XXll(3):