IMPROVEMENT OF ELECTRICAL RESISTIVITY MEASUREMENT FOR NON DESTRUCTIVE EVALUATION OF CONCRETE STRUCTURES

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1 Life Prediction and Aging Management of Concrete Structures 93 IMPROVEMENT OF ELECTRICAL RESISTIVITY MEASUREMENT FOR NON DESTRUCTIVE EVALUATION OF CONCRETE STRUCTURES LATASTE JF., SIRIEIX C., BREYSSE D., FRAPPA M. CDGA Université Bordeaux I (F)335 Talence Cedex Abstract To propose a new tool to buildings managers in their need for damage assessment of structure, electrical resistivity measurement is developed. The method presents interest (and not yet) exploited sensitivity to damage of concrete. The development of this method, still in progress, is lead to ensure engineers the control of obtained information. Works are then leaded to accurate the technique either by the improvement of on site measurements, or through post processing of measurements. After the presentation of the technique, measurement parameters are listed. Works aiming at the correction of disturbances linked to structural parameters are discussed: boundaries effects and rebar presence. Numerical results are validated by measurements on real structure. And the capacity of electrical resistivity method to be corrected is shown, to finally obtain results depending only on the material condition. Key words: Concrete, Non Destructive Test, electrical resistivity, boundary, rebar. 1. Introduction In Western countries number of concrete buildings are dated after the World War II period, some structures (as dams for instance) being older. The mean age of built heritage is increasing as does its damage degree. In the current context, restoring civil engineering damaged works is often more advantageous than building new ones, particularly from an economical point of view, if works are undertaken soon enough [1][2]. So, building owners want to manage their works in accordance with social and technical parameters. Non Destructive Evaluation (NDE) techniques appear adapted for this purpose: on one hand because of limits of destructive methods based on laboratory studies on samplings (local character of data, high sensitivity of some structures, like nuclear plants, with respect to coring), on the other hand because of the large range of geophysical techniques allowing a large range of study types.

2 94 2nd RILEM Workshop, Paris, 2003 Among various techniques families, electrical measurements appear as allowing an assessment of concrete conditions. From an electrical point of view, the material is a composite whose compounds can be describe as: (a) a solid phase purely resistive (aggregates), (b) a solid phase which participates to conduction through its porous structure (cement matrix) and being the source of ions found in the third phase, (c) the liquid phase, i.e. interstitial solution. Then conduction through concrete is generally identified as an electrolytic phenomenon, linked to the presence and nature (ions) of fluids within matrix, and to their circulation through pore network. Today the technique is used to locate zones presenting corrosion probability (detection of water content and ionic ingress, which are factors conditioning rebar corrosion) [3]. But few works take advantage of the sensitivity of the technique to the volume of porosity and to the porous connectivity degree [3][4]. Furthermore we can consider that classic chemical damages can be perceived as disturbing the material porosity. For instance: concrete carbonation leads to a porosity decrease, and a leaching makes it increase of it. Mechanical and physical (i.e. freeze-thawing) solicitations lead to cracks or micro cracks. All these damages can then be considered as disturbing electrical properties of concrete: respectively modifying electrolytic circulation trough matrix, and/or creating preferential ways for fluids flows. This sensitivity of electrical resistivity measurements appoints the method as adapted to concrete alteration assessment (detection, location, quantification, and evolution). Works are needed to validate and exploit its ability. 2. Electrical resistivity measurement on site 2.1. Presentation of the developed technique The measurement technique consists on a sounding of a concrete facing from the surface. We developed specifically for this application a four probe square device (Figure 1). Two neighbouring electrodes (A & B) inject a known electrical intensity. The potential difference created by the passage of the current in the material is measured between the two remaining electrodes (M & N). Value of the apparent electrical resistivity ( a ) is deduced through a geometrical factor. Sizes of the sets used in our works (that is to say the distance between probes) are 5 or 10 cm. The choice is made to do not be disturbed by aggregate size, and to characterise the upper layer concrete of the structure, location of main alterations (indeed, in first approximation, investigation depth is equivalent to distance between probes). The device, with its square section, is equipped with a switch which enables quick and easy measurements for the two current injection directions (two configurations ), for each location of the set. Then, moving the device, two distinct resistivity maps are obtained for the surface, characterizing electrical properties of the material according to two orthogonal directions.

3 Life Prediction and Aging Management of Concrete Structures 95 Electrical current injected (I) +I -I Switch Resistimeter Switch Resistimeter M A Potential difference measured (V) B N a a I V app Geometrical factor 4-probes device Figure 1: Four-probe square array principle and photo Influence of measurement parameters To use electrical resistivity measurement for NDE concrete damage, engineers need to be sure that values are representative of the researched property, and not of spurious effects. On site measurements depend on various parameters which cannot all be totally controlled. Different ways can be used to minimise their effects, or to correct them at the time of their examination. Let s begin to see which type of disturbance must be considered, then control them. Resistivity measurement influencing parameter, on site, can be summed up by the equation: f ( ext.) g( struct.) am ar n with am the measured apparent resistivity, ar the real apparent resistivity, f(ext.) a function regrouping disturbances linked to external parameters (temperature, moisture, ), g(struct.) a function regrouping disturbances linked to structural parameters (boundaries closeness, rebar presence, ), and n the various noise sources like electronics or coupling problems (i.e. interface probes-concrete). The practical efficiency of electrical measurements will follow a signal to noise ratio improvement. In the previous equation the real apparent resistivity corresponds to the ideal apparent resistivity, that is to say the apparent resistivity obtained if without any disturbance. The value of ar is depending on the material characteristics. Thus, intrinsic properties variations of the sounded concrete can lead to very different measured resistivity. For example, the water to cement mass ratio (W/C), by influencing the final porosity, partly defines the resistivity range of concrete. The aggregate / cement ratio (A/C), the mineralogical aggregate type, the cement type, or the concrete age, are parameters which participate to give concrete its electrical behaviour [5]. The important influences of these factors, and the difficulty to assess them on site, imply that electrical measurement cannot be exploited with absolute value. Generally, results are in term of relative variations, through iso-resistivity profiles or maps. Concrete electrical resistivity is sensitive to ionic concentration of porous fluid. Ions presence can be linked to an external pollution of material. Ions contribute to the electrolytic conduction, then lead to an apparent resistivity decrease, but correction is today impossible, though this effect can be exploited (to assess ionic ingress for example) [3]. Noise sources, disturbing measurement, can be due to various phenomena. Besides electronics (against only the number of measurements multiplication can help to reduce them), they are often linked to coupling problems between the sounded surface and the probes. Here is the contact resistance, resulting from the contrast of resistivity between metal (probes) and concrete, and

4 96 2nd RILEM Workshop, Paris, 2003 function of the surface condition. The disturbance can provoke measurement error, or even prevent any investigation [6]. Despite the use of a contact material (at the interface concrete-probe), moistening of the surface can be needed to create an electrical resistivity grade through the first millimetres then allowing measurement. In that case, a specific study of this operation is needed [7]. External parameters disturbing electrical measurement are mainly temperature, and humidity degree of the investigated surface. The influence of humidity is more detrimental than temperature effect. Moisture can be due to environmental conditions, or linked to the process to low contact resistance (moistening of surface help to improve coupling quality for electrical measurements). Solutions are to define exactly the moistening process, and its associated measurement phase, by the specific previous study (needed for each new case) [7]. Temperature influence is known, and several relations are proposed to take it into account [8]. All these arrangements allow to decrease variation of measurement, for accurate the research of damage. Classically, standard deviation arises around 30% relatively to average resistivity. This value depends on the studied case and its conditions, and varies between % (for compact concrete), to 10% (for saturated or young material). It must be assessed on each new case, such as to define the threshold value above which measurement variations can be considered as representative of variations of material properties. In spite of relatively high value of significant steps, electrical resistivity measurements allow alterations study [9][10], with a comparative approach of results. However, the improvement of results induces a sharper characterisation of damages, through a best exploitation of resistivity variations. To manage it, two ways are possible: either the measurement phase must be improved, but such a technological development of measurement apparatus can lead to better data quality (notably to decrease the influence of contact resistance on measurement), or to define process to clean measurements in order to increase results accuracy relatively to searched variations. The second solution is kept to deal with g(struct.), very important term on reinforced concrete structure. 3. Study of structural effects on resistivity measurements Size of classical structural reinforced concrete elements (some decimetres) generally do obsolete hypothesis of an infinite domain for measurement (with the 5 cm or 10 cm size device). Presence of rebar (conductive element in a relatively insulator material) cannot be neglected for electrical measurement. So assessment of disturbances and arrangements to deal with them are needed to allow a better interpretation of results Boundaries effects Assessment of boundary effects is leaded with three techniques which results are compared: analogical, numerical modelling, and experimental. Whatever the method, work is realised as the same. Due to characteristics of the device, and its way of use, we consider two configurations for the direction of injection for the electrical current: the configuration PL, where current is injected in parallel to the studied boundary, the configuration PP, where electrical current is injected perpendicularly to the boundary.

5 Life Prediction and Aging Management of Concrete Structures 97 The disturbance magnitude is associated to a distance between centre of the device to the boundary (d), reported to the dimensions of the set (a). It is expressed as the ratio d/a. Boundary effects are evaluated as the rate of disturbance (R od ) calculated as: ad at R od 100 with ad the measured apparent resistivity (with disturbance effect), and at the theoretical apparent resistivity (obtain without boundary effect, on a considered infinite field). Three approaches have been used to assess R od = f(d/a) : > The first approach is analogical assessment. Boundary influence on the electrical resistivity measurement is associated to the image theory [11]. According to it, each electrical source close to a boundary implies a virtual image which must be considered during apparent resistivity calculation. Equations allow to obtain an analogical assessment of boundaries effects on electrical resistivity. > The second way is the computation of effects. After the modelling of a boundary with a finite element mesh, conduction equations allow to determine the apparent resistivity measured close to such a feature. The numerical model has been defined with respect to 5% maximal error criterion. > The third methodology is the experimental determination of effects. We realised measurements on fifteen profiles (of three points) on a real concrete structure (with undamaged material). Measurement are realised for various distances (then different d/a ratio). Results drawn on figure 2 and 3, represent the average of the fifteen profiles, and the interval of one standard deviation (A ± 1 ). at Rate of disturbance (%) Analogical Numerical modelling Experimental 0 0,0 0,5 1,0 1,5 2,0 2,5 3,0-20 d/a Figure 2: Rate of disturbance linked to boundaries effects Injection parallel to boundary. Rate of disturbance (%) Analogical Numerical modelling Experimental ,0 0,5 1,0 1,5 2,0 2,5 3,0-20 d/a Figure 3: Rate of disturbance linked to boundaries effects Injection perpendicular to boundary. Curves show that results obtained with the different approaches are in agreement. Theoretical results (analogical and numerical) are confirmed by experimental work (in the interval of standard deviation). We observe that for low d/a ratio, the configuration PL (injection in parallel to boundary), indicates higher disturbances than for the configuration PP. In the first case the rate can reach to 100%, for less than 30 % in the second configuration. We can note that for d/a equal or up to 1, results are comparable for the both cases (around 20%), and negligible beyond d/a = 2.5. For the configuration PL, the maximum discordance between theoretical and empirical approaches (calculated for d/a equal to 0.5) corresponds to the maximum effect (100%). We must note that for the analogical as for numerical works, this case represents the ideal situation with the punctual

6 98 2nd RILEM Workshop, Paris, 2003 injection right to the boundary. This case is not representative of reality, since the injection (through 0.5mm diameter probes) can never be exactly in such a location. Theses results indicate that theoretical approaches are well-suited to describe observed behaviour. Used calculation which can be transposed to various concretes, for each device size (then realised once for all), can help to correct results Rebar effects Today electrical resistivity is used to locate and assess reinforcement corrosion. When it is not exploited, the sensitivity of electrical measurements to rebar disrupts to results interpretation, notably for evaluation of covercrete condition. Several parameters influence electrical behaviour of reinforced concrete with different manners: the rebar diameter (low influence), its cover, its orientation (relatively to the direction of injection), [10][12]. A solution consists in injecting electrical current perpendicularly to rebar (to minimise effect), but it is not always possible, and it prevents the complete investigation of the structure. So, as for boundary effect, works are leaded to assess reinforcement influence on measurement, to finally plan to correct it. The work is leaded relying on of numerical modelling. The mesh represents a concrete with a 12 mm diameter rebar, 35 mm cover, sounded with a 5 cm side square device (parameters chosen to be similar to a real case studied further). The numerical error is checked as being lower than 5%. For various d/a ratio (with the same symbol as before ) and for two configurations (PL corresponding to an electrical current injected in parallel to rebar, PP to an injection perpendicularly to rebar) computations indicate rate of disturbance (R od defined as previous). Rate of disturbance (%) Parallel (PL) Perpendicular (PP) 0 0,5 1 1,5 2 2,5 3 3,5 4 d/a Figure 4: Rate of disturbance linked to rebar presence Numerical modelling result (diam. = 10mm, Cover = 3.5cm) Rate of disturbance (%) PL Experimental PP Experimental PL Numerical modelling PL Numerical modelling rebar d (cm) Figure 5: Rate of disturbance linked to boundaries and rebar effects Experimental evaluation. Results (figure 4) indicate that rebar presence induces a decrease in apparent resistivity. When injection is perpendicular to rebar (PP), the low influence of rebar is observed as expected (variations lower than 5%). We can note that the maximum deviation is observed right to the reinforcement, and for chosen parameter, induces a 30% decrease. Influence of rebar can be neglected for d/a up to 3.5. This work on rebar effects, as for boundary study, implies that corrections of disturbance can be viewed, allowing to accurate exploitation of electrical resistivity for damage assessment Coupling of structural effects Once each effect studied separately, the work consists on the evaluation of their coupling. This step allows to think about an effective correction of structural effects. The task is led through a

7 Life Prediction and Aging Management of Concrete Structures 99 comparison between experimental data obtained on a reinforced concrete beam and the corresponding modelling. Measurements are then realised along fifteen profiles (with three point each) chosen on a reinforced zone (diameter 12mm, cover 35mm), with a boundary. This approach (Figure 5) allows to draw the average apparent resistivity (± one standard deviation), for the two configurations: PL corresponding to an electrical current injected in parallel to rebar, PP to an injection perpendicularly to the rebar. These works lead to observe the combined effect of boundary and of rebar. Results of numerical modelling (exposed independently on figure 2 and 3 for boundary, and 4 for rebar) allow to propose a numerical assessment of disturbance calculate as the sum of each effect. The comparison of numerical and experimental results (figure 5), for the two used configurations of the device (PL and PP) show the correspondence of both approaches (in the accuracy of measurement). The maximum difference between the two ways is observed for the measurement closer to the boundary, with configuration PL. But we noted previously the light imperfection of model for this case (too ideal representation of real geometry). Results confirm the good agreement between numerical modelling and experimental evaluation of structural effects. So it is possible to correct measurement of the disturbance linked to boundaries closeness and/or rebar presence. 4. Exploitation of the improvement on a real damaged structure Presentation of the structure Figure 6: Damaging of a beam under four points bending. The study aims at the characterisation of a reinforced concrete beam subjected to four points bending (Figure 6). The structure, a 3.00 x 0.23 x 0.15 m 3 beam, presents vertical cracks due to mechanical loading. Measurements of electrical resistivity are used to assess damage. Before electrical investigations, electromagnetic measurements allow to characterise reinforcement (location, and cover), as indicated on the lateral face of the beam (figure 7). Only the middle part of the lateral face of the beam is considered (X = [120, 1] cm). Investigation is leaded with a constant step of 5 cm with the four probes device (5 cm side). Thus the studied zone contains 6 vertical profiles (with 5 points on the height). Both device configurations are used (PL when current is injected perpendicularly to longitudinal axis of the beam, and PP when current is injected according the beam height). In this area, the structure presents boundaries (the upper and the lower side of the beam), and the longitudinal reinforcement (crossing all the beam length). Characteristics of reinforcement are comparable to previous study (12 mm diameter, 35 mm cover).

8 100 2nd RILEM Workshop, Paris, 2003 Cracked beam Stress Apparatus Beam Axis Studied zone (f1) (f3) (f4) (f5) (f6) X (cm) cracks Figure 7: Presentation of the damaged studied beam. Rebar position Concerning damage, we note on the considered zone three cracks (f1, f2 and f3 on figure 7). Crack f2 (X 135 cm) looks more active than others: its opening is larger (around 1mm) and its evolution with loading indicates that this crack concentrates deformation Compared results Electrical resistivity variations on the sector are drawn on figure 8. These representations give raw data according to the set configuration. We note the presence of important resistivity gradients close to the upper boundary of the beam. We can observe resistivity variations due to cracks presence. Then isoresistivity maps are corrected according previous results. Respectively, numerical modelling and experimental approaches are used and compared. The corrections are applied using the R od coefficient: ar ac 1 R with ar the raw apparent resistivity, ac the corrected apparent resistivity, and R od the rate of disturbance (linked to boundary, or boundary + rebar). Corrections realised according the two approaches lead to similar results, as expected. Furthermore, it can be noted that this step does not modify results on configuration PP in an important manner. This is due to the fact that for this configuration, disturbances are light and then correction too. However, considering PL configuration, gradient in upper part is deleted. Post processing allows to underlines resistivity variations directly linked to beam damage. od

9 Life Prediction and Aging Management of Concrete Structures 101 Injection Parallel (PL) to boundaries and rebar Injection perpendicular (PP) to boundaries and rebar Raw data Corrected results - Numerical modelling approach Corrected results - Experimental approach Scale of apparent electrical resistivity (.m) Figure 8: Iso-resistivity maps on the studied zone, comparison of results for various correction approaches. The iso-resistivity maps are compatible with assumptions of cracks which can be clearly delineated: resistivity is increased for measurement (1) right on a crack in PL configuration, and (2) between cracks in PP configuration.

10 102 2nd RILEM Workshop, Paris, Conclusion Electrical resistivity measurement appears more and more as an adapted tool for engineers, to assess concrete structure conditions. The method, still in development, confirms its sensitivity to damage of concrete. Furthermore the control of measurement parameters ensures the quality of results. In a first time, we have seen how to take into account on site conditions, by the definition of a measurement process which either controls them (as for temperature effect), or minimises them (contact resistance, or moisture content). In a second time, we have proved the ability to correct results of disturbances linked to structural parameters. Works on boundary effect can be transposed to every case, and abacus calculations for various reinforcement parameters allow to easily correct raw resistivity results from disturbance linked to structural factors. On site investigation, completed by numerical modelling works, allows to obtain information on concrete damage, for finally characterise accurately concrete structure conditions. 6. References 1. Bungey J.H., Millard S.G., 1996, Testing of concrete in structures, Chapman and Hall. 2. Uemoto T., 2000, Maintenance of concrete structures and application of non-destructive inspection in Japan, in NDT in Civil Engineering, ed. Elsevier, pp Andrade C., Alonso C., Arteaga A., Tanner P., 2000, Methodology based on the electrical resistivity for calculation of reinforcement service life, 5th CANMET/ACI International conference, pp Sirieix C., Lataste JF., Breysse D., Frappa M., 2002, Caractérisation de l endommagement du béton par mesures électriques, Matériaux et Construction, vol.35, pp McCarter WJ., Forde MC., Whittington HW., "Resistivity characteristics of concrete", Proceedings of the Institution of Civil Engineers (1981) Part 2 vol.71, pp Ewins AJ., "Resistivity measurement in concrete", British Journal of NDT, 1990, vol.32 n 3, pp Lataste JF., Sirieix C., Breysse D., Frappa M., "Electrical resistivity measurement applied to Non Destructive Testing in civil engineering: influence of the humidity", 7th EEGS, Birmingham September , pp Gowers KR., Millard SG., Bungey JH., "The influence of environmental conditions on the measurement of concrete resistivity for assessment of corrosion durability", International Conference of Non Destructive Testing in Civil Engineering, 1993, (ed. J.H. Bungey, 1993) pp Lataste JF., Sirieix C., Breysse D., Frappa M., 2003 (in press), Electrical resistivity measurement applied to cracking assessment on concrete reinforced structures in civil engineering, Journal of NDT&E International, Elsevier Sciences Ltd. 10. Lataste JF., 2002, Evaluation Non Destructive de l état d endommagement des ouvrages en béton armé par mesure de résistivité électrique, Thèse de l Université Bordeaux I, 13 décembre Telford W.M., Geldart L.P., Sheriff R.E., 1990, Applied Geophysics, Cambridge University Press, Cambridge, 770p, ISBN Millard S.G., Ghassemi M.H., Bungey J.H., 1990, «Assessing the electrical resistivity of concrete structures for corrosion durability studies»3rd Symposium on corrosion of reinforcement in concrete construction, Wishaw, May 1990, Society of Chemical Industry, pp