Evaluation of mineral building materials: Problems related to resistivity methods

Transcription

1 Materials and Structures/Matériaux et Constructions, Vol. 31, Mars 1998, pp TECHNICAL REPORTS Evaluation of mineral building materials: Problems related to resistivity methods B. Van de Steen, B. Wuytens, A. Vervoort and D. Van Gemert Department of Civil Engineering, Katholieke Universiteit Leuven, Belgium Paper received: December 16, 1996; Paper accepted: January 20, 1997 A B S T R A C T A study of the influence of fracture initiation on the resistivity of mineral building materials was preceded by an investigation into the electrical and electrochemical processes that could interfere with geo-electrical D/C measurements. The phenomena being discussed are the self potentials, the contact potentials, the polarization, diffusion and capillarity effects, the transition problems, and finally the leakage currents. Practical solutions developed for problems related to these phenomena are discussed in this paper. R É S U M É Une étude de l influence de l initiation de la rupture sur la résistivité électrique des matériaux de construction minéraux fut précédée par une investigation sur les processus électriques et électrochimiques qui peuvent interférer avec les mesures géo-électriques en courant continu. Les phénomènes en cours de discussion concernent les potentiels propres, les potentiels de contact, la polarisation, les effets de capillarité et de diffusion, les problèmes de transition et finalement les courants de fuite. Pour des problèmes liés à ces phénomènes, des solutions pratiques ont été développées et sont présentées dans cet article. 1. INTRODUCTION Geo-electrical measuring techniques to evaluate the condition of ancient masonry before, during and after grout injection have been the subject of a number of research projects at the Katholieke Universiteit Leuven. This has resulted in the development of a technique to draw relative resistivity maps, from which the location of voids and cracks can be deduced [1, 2]. The influence on the resistivity measurements of the electrode configuration and of the geometrical boundaries has already been researched extensively [2, 3]. Recent research in the field of geo-electrical surveys for restoration and conservation purposes has been focussed on a better understanding of the relation between resistivity changes and fracture initiation, and on the influence of a changing moisture content on the measured resistivity values, a problem that was identified from the outset [3]. As concrete is one of the most widely applied mineral building materials, concrete cubes of mm 3 were selected as test samples. Before embarking on this test programme, a better insight into the phenomena influencing the direct current (D/C) measurements was required. The electrical and electrochemical processes influencing the resistivity measurements were identified so that the equipment could be modified where necessary and so that the correct measuring practice could be implemented. To avoid leakage currents, the samples are insulated from the earth by placing them on rubber mats, while the D/C source is separated from the mains by means of a transformer. The current electrodes consist of two stainless steel plates, covering the whole area of the side of the sample with which they are in contact. A layer of graphite sprayed onto the sample, together with a sheet of aluminium foil, ensures a good contact. This set-up also has the advantages that a homogeneous current distribution can be realized in the sample, that the contact potential is kept to a minimum, and that no excessive problems are encountered either with polarization effects or with diffusion or capillarity. By making them Editorial note Prof. Dr. D. Van Gemert is a RILEM Senior Member. He is a member of Technical Committee 151-APC on Adhesion Technology in Concrete Engineering - Physical and Chemical Aspects and of TC MMM on Computer Modelling of Mechanical Behaviour of Masonry Structures /98 RILEM 126

2 Van de Steen, Wuytens, Vervoort, Van Gemert small enough, the measuring electrodes, made of cotton wool soaked in a saturated solution of CuSO 4, do not create problems with diffusion or capillarity. The difficulties posed by self potentials and polarization are solved by changing the polarity over the current electrodes at a rate of about 0.1 Hz. The effect of possible contact potentials at the current electrodes is eliminated by the positioning of the four electrodes. Built-in time delays in the measuring programme succeeded in alleviating the problems posed by transition phenomena. 2. THEORETICAL BACKGROUND Among the parameters characterizing a body electrically, only the resistivity ρ or its inverse, the conductivity σ, has to be considered when carrying out D/C measurements [4]. The transport of electrical charges is hereby governed by Ohm s law which states that there is a linear relation between the applied electrical field E and the electric current density J. r r J = E ρ The resistivity ρ, expressed in Ωm, is a material constant and, contrary to the resistance R (Ω), is independent of the geometry of the body. To convert resistances into resistivities, geometry factors, also called K-factors, are used. The geometry factors are a function both of the geometry of the body on which the measurements are carried out and of the electrode configuration. There are three conduction mechanisms to be considered: electronic conduction, ionic conduction, and conduction in solutions. The latter is also referred to as electrolytic conduction, and requires the availability of sufficient moisture. As the minerals and the cement making up rocks and the structural materials under discussion are dielectrics under the prevailing temperature and pressure conditions, only electrolytic conduction can have a contribution of any significance to the current flowing through the materials. A network of pores and cracks filled with water provides the ions from the dissolved salts Fig. 1 Concrete sample, with aluminium foil in between the current electrode and the sample itself. with the current paths along which they can move under the influence of an electrical field. Hence, the electrical conductivity of a rock, concrete or stone sample is related to its porosity and its hydraulic conductivity. The empirical relation between the porosity and the electrical conductivity of rocks is known as Archie s law: σ eff = σ w Φ m where: σ eff : effective conductivity of the sample saturated with water, σ w : conductivity of the pore water, Φ: porosity of the sample; Archie s law is only applicable in a limited porosity interval: 0.05 < Φ < 0.40, and m: a constant, characteristic of the type of rock. 3. EXPERIMENTAL SET-UP Although similar experiments to establish the relation between fracturing and resistivity have been carried out by other researchers [5-7], no standardized measuring procedure is available. An experimental set-up as well as a suitable measuring procedure therefore had to be devised by the authors. The earth, being the medium on which mining, geological and environmental applications are measured, can be modelled as a half-space, and the electrodes can be considered to be point contacts. These assumptions, especially the former, cannot be upheld when measuring in the laboratory. The samples have specific limited dimensions ( mm 3 ), and electrodes with a diameter of 16 mm, such as those used to conduct measurements on brick walls [2], cannot be modelled as point contacts. To avoid influences of the boundaries, difficulties with non-homogeneous current distributions and problems of calculating geometry factors, the current is injected uniformly over the whole cross-section of the sample. This is carried out by taking two stainless steel plates with the same size as the side of the cube as current electrodes, and by placing them against two opposite sides. To ensure a good contact between the stainless steel current electrodes and the sample, a layer of graphite is sprayed onto two opposite sides of the cubes, and before the solvent is able to dry, a sheet of aluminium foil is applied to the two sides treated in this manner (Fig. 1). The graphite ensures an excellent contact between the whole concrete surface and the aluminium foil, and has the advantage of not penetrating into the concrete as the solvent evaporates within a few seconds. If the material to be tested is homogeneous, the described configuration leads to a uniform current distribution, with all equipotential surfaces being planes parallel to the plates. The current through the electrodes is measured by an amperemeter with a measuring range from - 20 ma to + 20 ma. They are connected to the terminals of a P.C.-controlled steerable D.C. source with a maximum output of 380 V and 30 ma, limits set for safety reasons. 127

3 Materials and Structures/Matériaux et Constructions, Vol. 31, Mars 1998 For the potential electrodes, also referred to as measuring electrodes, the concept of point contacts was retained. They consist of two point electrodes made of cotton wool soaked in a saturated CuSO 4 solution, inserted in a plastic tube with an inner diameter of 0.6 mm. The potential electrodes are connected to a digital voltmeter with a measuring range from - 20 V to + 20 V. When taking a reading, they are placed 40 mm apart, in line with the current and in the middle of one of the surfaces in between the plates. The proposed configuration allows relating the resistivity of the sample to the measured current and potential difference: ρ= V A I l where: ρ: resistivity of the sample (Ωm), V: measured potential difference (V), A: area of the side of the cube (m 2 ), I: measured current (A), and l: distance between the potential electrodes (m). Besides steering the D.C. source, the P.C. also takes the current and voltage readings, writes them to a file and calculates the resistivity of the sample being measured. 4. ELECTRICAL AND ELECTROCHEMICAL INTERFERENCE As emphasized above, the measurements are carried out by employing direct current. In actual fact, the current is not controlled, but the potential difference between the current electrodes is kept constant. The current will only remain at its same level provided no electrical or electro-chemical phenomena, such as polarization, diffusion of electrolytes and self potentials, occur. A decrease in the current is observed however. From the time-current diagram in Fig. 2, it can be concluded that the current does not remain constant. A closer investigation into the reasons for this behaviour is therefore warranted. Fig. 2 Behaviour of the current when no measures are taken against the various interferences. A number of those interferences, such as self potentials and polarization, are well known from field practice, not only in measurements on ancient masonry [2], but also from geo-electrical surveys for prospecting or environmental purposes. Procedures and techniques have been developed to eliminate these effects. The question remains if these techniques and procedures are also valid on a laboratory scale. These problems and other interferences that come to light when working in a well-controlled laboratory environment form the subject of the remainder of this paper. 4.1 Self potentials Even when no current is flowing through the sample or, in other words, when there is no potential difference between the current electrodes, the voltmeter measures a potential difference when the measuring electrodes are brought in contact with the sample. In general, these potential differences have a number of causes, that can be subdivided into two groups: background potentials and mineralization potentials. Mineralization potentials occur in the vicinity of sulphides, oxides, and graphite bodies, and are exclusively negative. They can amount to a few hundreds of millivolts [8], but do not occur in our concrete samples. Background potentials on the other hand are caused by: electrolytes of different concentration in contact with each other, electrolytes flowing through capillaries or pores, the so-called flow potentials, contact potentials between solids and electrolytes, electromagnetically-induced earth currents, and human activity, the so-called cultural noise. They can be either positive or negative and generally amount to a few tens of millivolts, and mostly vary in time. Background potentials are also observed in the laboratory. Keeping the electrode distance constant at 40 mm, self potentials with an absolute value ranging between 0 and 80 mv are measured on one and the same concrete sample. As the self potential is superimposed on the potential difference caused by the ohmic resistance of the sample upon an electric current, one has either to know the self potential or to eliminate it. Because we are not interested in the self potential as such, and because measuring it would only involve the procedure, the polarity is changed, thus eliminating the self potential (s.p.) (see Fig. 3). The measured potentials are V 1, V 2, V 3, V 4, V 5,... while one would like to know V = V 1 - s.p., V 2 - s.p., V 3 - s.p., V 4 - s.p.,... For n blocks (i.e. n-1 changes of polarity), and n uneven: V = ( V 1-2 V V 3-2 V V n-1 + V n )/2(n-1) 128

4 Van de Steen, Wuytens, Vervoort, Van Gemert Fig. 3 Superposition of self potential on the actual potential differences at the measuring electrodes. The influence of the self potential can thus be eliminated by changing the polarity of the voltage over the current electrodes. If the self potential is a linear function of the time, as observed experimentally, three measurements suffice. This remedial procedure is identical to the one used to take the self potential into account when measuring on walls and historical buildings [2]. 4.2 Contact potentials The contact potential is the drop in potential experienced at the interface between the electrodes and the sample. Contact potentials occur at both the measuring and the current electrodes. The better the contact between the electrodes and the sample, the lower the contact potentials. The circuit analog of the sample-electrodes system is given in Fig. 4. One could conclude that with the described experimental set-up, the contact potential at the current electrodes is of no importance. This conclusion is only justified if the voltage drop does not prevent sufficient current from flowing through the sample. In order to measure resistivity changes during fracture initiation and propagation, the currents should not drop under 1 ma, so that current changes of 1% still fall within the accuracy of the ampere meter. However, good contact between the current electrodes and the Fig. 4 Circuit analog of the set-up, incorporating the contact potentials caused by the resistances R c. The sample is modelled by the resistance R. sample must also ensure a homogeneous current distribution. Table 1 illustrates the influence of the contact resistance between the current electrodes and the sample for three different configurations. The current through the brick samples increases when improving the contact between the electrodes and the sample by means of cotton wool soaked in saturated CuSO 4. This increase in current is favourable, but not strictly necessary as the current is sufficiently large to determine resistivities. The difference between the two contact methods is much more distinct for the concrete cube; here, the current obtained with direct contact is unacceptably small. For the potential electrodes, fine metal pins were originally tried out to realize a point contact. The unstable and unreproducible readings obtained make such metal electrodes unacceptable. To realize the contact between the sample and the potential electrodes, inspiration was drawn from field measurements where porous pots filled with a saturated solution of CuSO 4 are used [9]. Cotton wool soaked in saturated CuSO 4, and inserted in a fine plastic tube with an inner diameter of 0.6 mm, fulfills the function of the porous pots. Table 1 Influence of contact at the electrodes on the current Sample Brick, contact faces Brick, contact faces not polished polished Concrete cube contact faces not polished Current: Direct contact metal 1.35 ma 1.88 ma 0.08 ma plate electrode-sample Current: Cotton wool soaked in saturated CuSO 4 in 2.34 ma 2.40 ma ma between plate electrode and sample Voltage difference over the current electrodes : 150 V. Voltage difference over the current electrodes : 15 V. 4.3 Polarization The problems of polarization can be traced back to the very essence of the geoelectrical measuring method: working with direct current. When the polarity of the voltage over the current electrodes is not switched, the positive and negative ions move, under the influence of the electrical field, in opposite directions to the cathodic and anionic plate respectively. The effect of the polarization manifests itself as a decrease of both the cur- 129

5 Materials and Structures/Matériaux et Constructions, Vol. 31, Mars 1998 Table 2 Effects due to prolonged polarization: voltages over the faces in contact with anode and cathode Random measurements Potential difference over contact faces V V V V V Fig. 5 Effects of polarization on the current and potential behaviour at the measuring electrodes over successive measurements. rent and the potential difference over a number of successive measurements (Fig. 5). The measurements were taken at time intervals of one minute, while the potential difference over the current electrodes was kept constant at 15V for the duration of the experiment. The electrolytes available to take part in the polarization process however also play an important role. The use of cotton wool or filtering paper soaked in a saturated CuSO 4 solution worsens the polarization problems. Besides a drop in the current and in the measured voltage, a copper deposit is observed at the cathode, while the anodic side colours yellow. After the polarity has remained unaltered during an hour, differences of several volts are measured over the sides of the cube that were in contact with the anode and the cathode (Table 2). The concrete cube has been charged like a battery. It is also noteworthy that the ions seem to be concentrated in pockets, as may be deduced from the strongly varying potential differences in Table 2, and that there is not a uniform concentration of ions within layers parallel to the plates, as the voltages are far in excess of the usual self potentials (Table 3). To remedy the polarization problems, the polarity has to be switched at a frequency of 0.03 to 3 Hz [9]. This will prevent ion layers from building up at the electrodes. To avoid the complication of the polarization problems, one should refrain from using an electrolyte to make the contact with the sample. Hence, use is made of graphite and aluminium foil as described in Section 2. The choice of the switch rate, as well as transition problems due to polarization at the moment of the switch, will be discussed in the section treating transition phenomena. When carrying out experiments in the course of which a number of resistivity measurements have to be taken, the Table 3 Effects due to prolonged polarization: voltages over opposite sides of the cube in planes parallel to the plates Random measurements Potential differences within planes parallel to the plates V V V V V V number of polarity changes per measuring series cannot be chosen arbitrarily: the polarity has to be switched an uneven number of times, so that the number of measuring blocks for both polarities is equal, thereby avoiding a net polarization in one direction or the other. The resistivity curves in Fig. 6 illustrate clearly the effect of both the number of times the polarity is switched and the method used for making contact with the sample. The resistivity of the cube labelled (a) in Fig. 6 increases over time. It was tested by making use of a filter paper soaked in a saturated solution of CuSO 4 together with an even number of polarity changes per measurement. The resistivity of the cube labelled (b), on the other hand, remains approximately constant. Graphite and aluminium foil were used to realize the contact between the electrodes and the sample, and the polarity was changed three times per resistivity measurement. Fig. 6 Resistivity vs. Time curve. (a): Two polarity changes per resistivity value and contact between current electrodes and sample realized by means of CuSO 4. (b): Three polarity changes per resistivity value and contact between current electrodes and sample realized by means of graphite and aluminium foil. 130

6 Van de Steen, Wuytens, Vervoort, Van Gemert 4.4 Diffusion and capillarity In the experiments where a saturated CuSO 4 solution is used, capillary resorption and diffusion of the electrolyte are observed, thereby altering the resistivity of the samples. These effects can be neglected in the case of the graphite, because the solvent with which it is applied evaporates within a few seconds, whereafter the graphite behaves as a solid. The zone influenced by the graphite is therefore limited to the vicinity of the contact faces and has no influence on the resistivity values (see 4.2, Contact potentials). The area in the vicinity of the potential electrodes however will be influenced by the CuSO 4 solution as these electrodes make use of this electrolyte to ensure a good contact. The only question is how important is the influence, and how deep and how much does the CuSO 4 solution penetrate. To answer this question, the copper content of the concrete against which the electrodes are positioned, the copper content of the material in between the electrodes, and the copper content of some concrete close to one of the corners of the cube were tested (Fig. 7). The latter sample is used as a reference: it is situated at a distance of more than 100 times the diameter of the electrodes. The potential or measuring electrodes had been in contact with the concrete cube for an hour. In a first experiment, each sample being tested consisted of 10 g of material; the copper content of the reference sample and of the samples taken at and in between the electrodes was not significantly different. The experiment was therefore repeated, but the material tested was restricted to 2.5 g per sample. These results are summarized in Table 4. It can be concluded that the potential electrodes do have an influence on the conductivity as the Cu content in their vicinity is slightly increased. This influence however is very localized because a sample in between the electrodes is not affected and because the influence can only be detected if the sample is not diluted with concrete taken more than a few millimeters from the contact area. Table 4 Resorption of CuSO 4 at the potential electrodes Samples (2.5 g) Cu ppm First electrode 119 Second electrode 187 In between electrodes 20 Unaffected concrete Transition phenomena Even when there is no potential difference over the current electrodes, a current of a few milliamperes was measured the moment the potential electrodes were brought into contact with the sample. The voltmeter indicated a voltage of a few volts as well. Within three seconds, the ampere meter returned to 0 A, and the voltmeter stabilized around a value of a few tens of millivolts. This peak has to be attributed to phenomena related to the measuring equipment itself, as no mechanisms in the samples could cause such a behaviour. Such peaks do also occur when changing the polarity over the current electrodes, but they do not stand out as much, since they occur in conjunction with transition phenomena due to the polarization. This behaviour is to be attributed to the capacitive and magnetic coupling between the different instruments and between the instruments and the source. For ease of transportation, the rectifier, the source, the voltmeter and the amperemeter are all built-in into one and the same casing. Completely separating the measuring instruments and feeding the D/C source by batteries instead of connecting it to the electric mains would be an excellent alternative. This would however require a new instrument to be built. The polarization itself causes transition problems as well. Polarization will always occur, and can only be kept to a minimum. The redistribution of the ions due Fig. 7 Positions where the concrete was tested on its copper content to determine the influence of CuSO 4 resorptions. to polarization will be slowly undone when switching off the current. The extreme polarizations as observed in the cube which was the subject of Table 2 had returned to a normal state after two days. The breaking down of the ion redistribution however is accelerated by changing the polarity, and reversing the direction of the current therefore results in an increased current until the original polarization has been nullified; the polarization in the opposite direction then gets the upper hand. The transition problems can be overcome by introducing a time delay between the switching of the polarity and the start of the effective current and voltage measurements. After a number of tests with different time delays, a pause of 5 seconds was selected as a compromise between the elimination of all transition phenomena on the one hand and polarization effects on the other. It should be pointed out that the polarity over the electrodes 131

7 Materials and Structures/Matériaux et Constructions, Vol. 31, Mars 1998 is not reversed directly, but in two steps: the voltage over the electrodes is set at 0 V before reversing it. Ten voltage and current readings are recorded before reversing the direction of the current, resulting in a switch rate of approximately 0.1 Hz. The polarity over the current electrodes is changed three times, so that one measurement effectively consists of 40 current and voltage readings. 4.6 Earth leakage A parallel circuit is created with the earth as parallel conductor by grounding the sample and feeding the D/C source straight from the mains. To prevent leakage currents, the sample, or both the P.C. and the D/C source, have to be insulated from the earth. The set-up is such that both the sample and the D/C source are insulated from the earth. The sample is insulated from the earth by placing it on a rubber mat, and the D/C source by placing the casing on a rubber mat and by electrically separating the source itself from the electric mains by means of a transformer. The P.C. remains connected to the ground via the mains. 5. CONCLUSION The investigation of the electrical and electrochemical influence on the resistivity measurements allowed making adjustments to the experimental set-up and the measuring programme. It also provided a better insight into the limits of the method and of the equipment used. The polarization problems are the most difficult to overcome, but the combination of an alternative way of realizing the contact between the current electrodes and the sample, by making use of graphite, and a suitable measuring procedure, whereby the polarity of the current is switched an uneven number of times, not only solves the difficulties caused by the polarization, but also treats the self potentials and most of the diffusion and capillarity problems. Tests over extended time periods still reveal a gradual increase in the resistivity, amounting to an increase of about 2.5% of the initial resistivity value over a time period of half an hour. The reason for this increase has thus far not been established. Notwithstanding the gradual resistivity increase, this provides us with sufficient confidence in the measurements to continue the research into resistivity changes in mineral building materials during crack initiation. 6. ACKNOWLEDGEMENTS The authors wish to thank the geo-electrical survey company Geo-Survey nv, Belgium for sharing their extensive practical experience and technological knowledge. The results presented in this paper are part of the M.Sc. thesis by B. Wuytens to obtain a degree in Mining Engineering at the Katholieke Universiteit Leuven. 7. REFERENCES [1] Van Gemert, D., Janssens, H. and Van Rickstal, F., Evaluation of electrical resistivity maps for ancient masonry, Mater. Struct. 29 (1996) [2] Janssens, H., Geo-electrical control of consolidation injections on masonry structures (in Dutch), Ph.D. thesis, Department of Civil Engineering, Katholieke Universiteit Leuven (1993). [3] Haelterman, K., Lambrechts, A., Janssens, H. and Van Gemert, D., Geo-electrical survey of masonry, Mater. Struct. 26 (1993) [4] Kunetz, G., Principles of Direct Current Prospecting (Gebrüder Bornträger, Berlin, 1966). [5] Marcak, H. and Tomecka-Suchon, S., Model of electric conductivity in rock samples subject to triaxial stresses (in Polish), Publs. Inst. Geophys. Pol. Acad. Sc. M-15 (235) (1991) [6] Tomecka-Suchon, S. and Rummel, F., Fracture-induced electrical resistivity changes in coal, Erdöl und Kohle-Erdgas-Petrochemie vereinigt mit Brennstoff-Chemie Bd. 41 (Heft 4) (1988) [7] Brace, W.F., Orange, A.S. and Madden, T.R., The effect of pressure on the electrical resistivity of water-saturated crystalline rocks, J. Geophys. Res. 70 (22) (1965) [8] Vogelsang, D., Environmental Geophysics (Springer-Verlag, Berlin, 1995). [9] Ward, S.H., Resistivity and induced polarization methods, in Geotechnical and Environmental Geophysics, Investigations in Geophysics no. 5, Volume I (Society of Exploration Geophysicists, Tulsa, 1990)