Some elements for improving interpretation of concrete electrical resistivity

Transcription

1 Some elements for improving interpretation of concrete electrical resistivity Jean-François LATASTE 1, Stéphane LAURENS 2, Andrzej MOCZKO 3 1 Université Bordeaux 1, Talence, France, jf.lataste@ghymac.u-bordeaux1.fr 2 Université de Toulouse ; UPS, INSA ; LMDC, Toulouse, France. 3 Wroclaw University of Technology, Wrocław, Poland. Abstract In the frame of the French ANR project SENSO, several non destructive techniques are tested in the laboratory on numerous concrete slabs. All the concrete slabs have been conditioned with a saturation degree close to natural condition (about 50%). Electrical resistivity measurements have been systematically carried out using different devices adapted to on site works, each of them composed of four probes. The first device involves 4 probes in Wenner linear configuration; the second involves 4 probes in a square configuration. In the first part, the analysis of electrical resistivity method is discussed considering the reliability between the two devices. In the second part, non destructive tests are compared to semi-destructive and destructive evaluation. Based on capo-tests and rebound hammer tests performed on a reduced set of the same samples, the link between electrical results and mechanical tests is discussed. Particular attention is focussed on actual quality of concrete cover layer defined by means of electrical resistivity measurements and their influence on associated mechanical properties of concrete structure. Interest of electrical resistivity measurement (totally non destructive) is studied, highlighted by mechanical information on concrete (used for structural re-assessment of structures). Résumé Dans le cadre du projet français ANR SENSO, plusieurs techniques non destructives sont testées en laboratoire sur dalle en béton de différentes compositions. Toutes les formulations ont été conditionnées avec un degré de saturation proche d un état naturel (de l ordre de 50%). Des mesures de résistivité électrique ont été systématiquement réalisées en utilisant plusieurs dispositifs quadripolaires de mesure, adaptées aux investigations sur site. Le premier dispositif présente une configuration en ligne de type Wenner, le second présente une configuration carrée. Dans une première partie, l analyse des résultats électriques est menée, considérant les corrélations et différences entre dispositif de mesure. Dans une seconde partie, les mesures non destructives sont comparées à des résultats d essais semi- destructifs et destructifs. Des essais au capo-test et au scléromètre sont réalisés sur un jeu réduit d éprouvettes. Les liens entre résultats électriques et tests mécaniques sont discutés. L attention est particulièrement portée sur la qualité de la couche de surface du matériau, évaluée par les méthodes électriques, et sur leur influence sur les essais semi destructifs de surface. L intérêt des mesures non destructives électriques est évalué relativement aux essais semi destructifs aujourd hui utilisés dans les opérations de réévaluation des structures en béton armé. Keywords Electrical resistivity measurements, concrete, non destructive evaluation, semi destructive tests, mechanical properties.

2 1 Introduction The SENSO ANR French project aims at the improvement of some non destructive testing techniques, today available. It is to improve the answer to engineers by non destructive tests. The work is based on the construction of a large data base of measures from several techniques, through tests performed on samples (25x50x12 cm 3 ), and on site. The laboratory program (Table 1) presents a first set of 5 concretes (G1, G2, G3 & G3a, G7 and G8) very similar in their formulation and differing mainly by their water/cement ratio (W/C). This influences final porosity (porosities from 12.5 to 18.1 %). A second set of three other concretes is also available (G4, G5, and G6), varying by their aggregates characteristics (dimensions, crushed or rounded, and mineralogy) with respect to the first set. All the concrete slabs have been conditioned from dry to saturated states, with intermediate steps [1]. We consider in this presentation, only results of measurement on partially saturated concrete (about 50% for saturation rate). Table 1. ANR SENSO program samples composition [1] Code G1 G2 Aggregates G3 G3a Round siliceous 0 12,5 mm G7 G8 G4 G5 G6 Round siliceous 0-20 mm Crushed siliceous 0-12,5 mm Crushed calcareous 0-12,5 mm W/C (Porosity) 0,30 0,45 0,55 0,65 0,80 0,55 0,55 0,55 Porosity (%) Number of batches Number of samples In this work, the link between mechanical properties, assessed on site, and electrical resistivity is studied, since electrical properties of concrete as well as mechanical strength are depending on concrete porosity. The work is based on resistivity measurements with several devices, and some mechanical techniques for in situ assessment (rebound hammer, capo-test and mechanical strength tests on samples). 2 Electrical measurements 2.1 Electrical resistivity of concrete Electrical conduction through concrete is essentially an electrolytic phenomenon. As in porous material, the fluid flowing within connected porosity supports the conduction, the solid part (relatively more insulating in concrete) being neglected. This phenomenon has been described in geology by an empirical law called Archie s law [2]. The law links the measured resistivity to interstitial fluids quantity and nature, but also on porosity, with a power function. 2.2 Resistivity measurements Measurements are done with three different four-probe devices. The first is a classical Wenner type set (named W), where probes are on line, and equally spaced with 4 cm (Figure 1a) [3]. The second and third have square sections, with probes at each corner of the square (Figure 1b) [4]. Probes are spaced respectively with 5 cm (named Q5) and 10 cm (named Q10). The square device being less extended (for a same distance between probes), measured variations or more reliable to the volume close to electrodes than linear device (which could be influenced by their length): one spokes about device anisotropy lower with the square

3 configuration. Furthermore this reduce extension allows limitation of influence of edges for investigation on reduced section elements. Concerning depth of investigation, the larger the distance between probe is, the deeper the measurement; considering that the resistivity measured integrates all the volume from the surface (one speaks about apparent resistivity, to differentiate from true resistivity). Wenner and larger square device (that is to say W and Q10) have comparable depths of investigation. (a) Wenner configuration device (b) Square configuration devices on the same set Figure 1. Devices for electrical resistivity measurements 2.3 Electrical results and discussion Resistivity considered are values measured on partially saturated slabs. The saturation degree is measured about 50% for all the 9 mixes (average = 51.5%, standard deviation = 1.9%, in the range of [47.8; 53.0]). This value can be considered as possible natural range for concrete in natural atmosphere. A minimum of three slabs per mix, and between 3 to 10 points per slabs allow assessment of average apparent resistivity and its variability for a concrete. Measures are corrected of bias (edge effect f.i.). Apparent resistivities with the three devices are compared (Figure 2). Apparent resistivity (Ohm.m) Apparent resistivity with Q10 (Ohm.m) Device Q5 Device W Figure 2. Electrical resistivity with the different devices (average values +/- 1. s.d.) It can be seen the link between resistivities with different devices. That shows the stability of resistivity measurement and the preservation of information in terms of relative variations. Differences and shifts between values with different devices should be linked to light variations in the investigated volume. The calculation of the ratio Q5/Q10 allows to reach to a constant value equal to 2 (average = 1.94, s.d. = 0.35, in the range [1.87; 2.09]). This traduces partly the effect of contact resistance (appearing at the interface between the conducting probes and the resistive concrete), which is constant whatever the device, so leads to a shift of apparent resistivity in function of distance between probes. This traduces also partly the probable gradient in resistivity (due to variation of porosity and/or saturation) seating from the surface to in depth. Later, Q10 is considered as reference resistivity value, because of its

4 weaker sensitivity to bias and the large investigated volume (more representative of concrete properties). 3 Mechanical tests 3.1 Assessment of mechanical properties As engineers are interested in concrete strength assessment on site, several techniques can be found to characterize more or less directly material mechanical properties. Among them, rebound hammer as well as Capo-tests, are today very current whereas strength estimates can be derived from these using empirical relations provided by the reference manual for each test [5]. For Rebound hammer the calculation is provided by the curve given by manufacturer, of expected mechanical strength (in MPa, on 150 mm cubes) can be done with the relation (1), where R is the rebound number. f c, cub (150 mm) = ( R R 13.51) ( 1 ) For Capo-tests, there are two cases depending of the strength of concrete (in MPa), but offering mechanical strength expected also on 150 mm cubes. Equations (2) and (3) are proposed by manufacturer, where F the pull out force (in kn): for concretes of compressive strength under the 50 MPa (f c, cub 50 MPa) f c, cub (150 mm) = 1.41 F 2.82 ( 2 ) for concretes of compressive strength over the 50 MPa (f c, cub > 50 MPa) f c, cub (150 mm) = 1.59 F 9.52 ( 3 ) On each slab are done rebound hammer tests on 6 zones (3 on each larger opposites faces). Each test is composed by 10 elementary measurements to consider variability of this measure. Capo tests are distributed on the same faces but, according to measurement conditions and ability to prepare the slab 3 to 5 tests (varying between slabs) are done. So finally : 1080 rough rebound numbers, corresponding to 10 measurements on each of 6 zones distributed on each of the 18 slabs studied (among the 9 mixes considered); and 37 Capo-Tests, corresponding to between 3 and 5 tests for each mix, are done. 1 core on 1 slab from each of the 9 concrete mixes is also done. The data from these measurements are compared with direct measure of compressive strength, done on samples cored from slabs having undergone mechanical tests previously (Figure 3). Core extracted are 10 cm in diameter and 10 cm high. Figure 3. View of a slab after mechanical tests, and coring 3.2 Mechanical tests results analysis The relation between rebound hammer calculations (non destructive technique), Capo-tests results (medium destructive technique) and strength assessment on cores (laboratory

5 technique) can be drawn on these tests (Figure 4). The equality line (slope 1:1) is also drawn to see reliability between direct and indirect assessment of mechanical strength. Rebound hammer and Capo-tests show the both good relations with the direct assessment of mechanical strength. It can be seen results from capo-test lead to under estimation of the mechanical strength for all studied samples. This is particularly viewable for the slab (G1) the only one with fc > 50 MPa (98.3MPa) Strength from rebound number (Mpa) MPa Rebound-test Capo-test Mechanical strength on core (MPa) Figure 4. Mechanical tests results (average value +/- 1 s.d.) The correspondence between direct and indirect assessment of mechanical strength with rebound hammer, seems good particularly for concrete with fc < 50MPa. As for electrical results the two indirect methods seem to correlate in term of relative variations, or grading between concretes. In fact, one can observe that results are particularly influenced by the weakest and the higher value for mechanical strengths, corresponding to G8 and G1 (respectively highest and lowest W/C ratios with 0.8 and 0.3). For other concretes (in the range of W/C = [0.45; 0.65]), regarding the variability of measurements, these techniques seem not to discriminate concretes, as direct assessments on samples. 4 Discussion Mechanical properties as well as electrical resistivities depend on porosity: The first, schematically by the fact that the more the concrete is porous, the less there is solid skeleton to undergo mechanical stress, so the lower the mechanical strength. The second, by the fact that also schematically, the more material is porous, the more electrolytic conduction can proceed, so the less the concrete is resistive. In other words, one can expect that a resistive concrete have also a high mechanical strength. In this study we compare mechanical and electrical properties assessed by non (or medium) destructive techniques (Figure 5). The link between electrical and mechanical properties appears clearly if one considers large range of properties (here f.i. there is a clear distinction between concretes with mechanical strength up to 80 MPa and others in the range of MPa). But for material more similar mechanically or electrically, one cannot clearly discriminate them. Considering all mixes used for the study we can define a global variability built as the coefficient of variation calculated on the average values (resistivity or mechanical) on each mixes and the variations between mixes. The global variability, or variability between mixes, is comparable whatever the techniques considered, for a given property: about for mechanical and for electrical (Table 2); then ratio between mechanical and electrical about 3:1. Even if mechanical discrimination is better than for electrical properties (0.442 > 0.130), resistive methods are very fast and allow investigations on large surfaces. In other words both families of techniques appear complementary Strenght from Capo-Test (MPa)

6 00 Apparent resistivity with Q10 (Ohm.m) Mechanical strength from Capo Test (MPa) Figure 5. Electrical resistivity (Q10) in function of mechanical strength (rebound hammer), average value +/- 1 s.d. Table 2. Global variability by technique Parameter Mechanical strength (MPa) Log 10 of Apparent resistivity (Ohm.m) From From Direct test Square 5cm Square 10cm Technique Rebound Wenner (W) Capo-test on core (Q5) (Q10) hammer Global variability 5 Conclusion and prospects This study has shown the reliability of electrical resistivity measurement to mechanical properties of concrete. Devices considered have been all developed for on site measurements. It has been proven that whatever the device used, relative variations of apparent resistivity could be assess. Use of three methods for mechanical assessment of concrete (whom two are adapted for on site measurement) shows also that although quantification are not exactly in accordance, grading between concrete is possible. The study highlights the problem of discrimination between concrete when they have close properties. This is in agreement with the two families of techniques considered. So, the next step should be to improve methodology of interpretation, considering combination of results from several fields, as some works have been proposed with acoustic techniques (SonReb method [6]). Senso Project deals notably with this point, and works are in progress. Finally, the identification of biases and noises should be done for each technique to solve them, as for instance the influence of carbonation, influencing surface properties, which should be assessed considering several sizes for resistivity device, then to sound several volumes. Aknowledgements Authors want to acknowledge the French National Agency for Research to support these works, as well as all teams who have contributed to the SENSO project and the constitution of the database. References [1] Rapport d avancement du projet ANR SENSO, juillet 2007, 88p. [2] ARCHIE G.E., The electrical resistivity log as an aid in determining some reservoir characteristics, Trans. Amer. Inst. Mining and Metallurgical Eng., 146, 54-62, 1942 [3] Millard S.G., 1991, Reinforced concrete resistivity measurement techniques, Proceedings of the Institution of Civil Engineers, Part 2, vol. 91, pp [4] LATASTE J.-F.,2002, Évaluation Non Destructive de l état d endommagement des ouvrages en béton armé par mesures de résistivités électriques, Thèse de doctorat, Université Bordeaux I, 295 p.

7 [5]Moczko, A.: Comparison between compressive strength tests from cores, CAPO-TEST and Schmidt Hammer, Wroclaw Technical University, Poland, 2002 [6] M. S. Akman, A. Gfiner, 1984, The applicability of sonreb method on damaged concrete, Materials and Structures, 17(3), pp