International Symposium (NDTCE 2003) NonDestructive Testing in Civil Engineering 2003  
Start > Contributions >Lectures > Electrical Methods: 
Sensitivity to crack parameters of two non destructive techniques: Electrical resistivity measurements and acoustic methodsLataste JF., Abraham O., Breysse D., Sirieix C.AbstractNon Destructive Testing of concrete structure to assess their condition is a tool particularly adapted to on site investigations. Unfortunately, methods are sometimes limited by their sensitivity to several parameters which cannot be separately controlled. Coupling of techniques might help solving this problem. Electrical resistivity measurement and acoustic methods are both sensitive to cracks parameters. Electrical measurement enables to accede to depth and equivalent resistivity of the crack (linked to opening and moisture condition of the defect). Acoustic methods are sensitive to the crack depth and to its eventual partial closure. We propose to use both methods to reach a better characterisation of crack. A rough draft of abacus is built to deduce parameters from various datasets. Works are still in progress but the tests carried out on an experimental slab will drive further works on this way, to propose an original response to engineers. 1 IntroductionIn civil engineering, importance of structural management is increasing according to economical pressure. New needs of damage characterisation are appearing in order to forecast and plan rehabilitation works. In this frame, Non Destructive Techniques are appearing as efficient tools for on site study. They are more and more included in the assessment strategy of structure owners. Today, new methods are under development in view of more complete structure characterisations than before. Cracks are defects whose description can provide engineers a better assessment of structural condition. To date the usual characterisation performed on a structure are: the description of the crack profiles on the surface (eventually their quantification through image analysis), and the follow up of the crack opening in time. This information is not sufficient to a complete assessment of structural condition. Electrical resistivity and acoustic measurements devoted to crack characterization are two NDT still in progress. They show a promising sensitivity to concrete damage, but their dependence to a lot of uncontrolled parameters prevents a deeper data analysis. Combination of measurements seems to be a solution to exploit more completely information drawn from electrical and acoustic techniques. Works presented in this paper show the ability of both techniques, and the interest of a coupled approach for further works. 2 Electrical resistivity measurement for cracks characterisation2.1 Electrical resistivity technique In that aim, we have developed a device (figure 1) which enables to characterise the upper layer concrete of the structure (location of main alterations), by on site resistivity measurement. The fourprobe square device injects electrical current between two lateral probes, and measures the potential difference between the two others. The apparent resistivity is deduced reporting the potential on the intensity, in function of the set geometrical characteristics. Measurements are performed for two investigation orthogonal directions, at each sounded point. Resistivity variations are linked to cracks presence and dependant on their properties. Results are exploited by drawing isoresistivity maps or profiles. The calculation of material electrical anisotropy (defined as the ratio between resistivity obtained for the two injection orientations), is also an indicator of local behaviour of the material.
Electrical characteristics of concrete obtained depend on material properties around probes and is named apparent resistivity. The crack presence leads to various signal disturbances according its characteristics. 2.2 Sensitivity of electrical resistivity to equivalent resistivity of cracks Numerical works have been made with a 3D finite element software (CESAR LCPC), to study the influence of the equivalent resistivity of crack. Computations are made for various resistivities of crack, and considering two orientations for electrical current injection directions (indicated with the double arrows on figures). Figures 2 and 3 show results for conductive and resistive cracks (relatively to the concrete) and for the electrical injection in parallel with the crack (PL) and perpendicularly to it (PP); the device being located right to the crack.
In terms of electrical resistivity variations, graphs show that for an injection in parallel with defect (configuration PL) is leading to qualitatively the same result for resistive and conductive crack. For an injection with the configuration PP, crack increases the apparent resistivity only for the resistive case. Thus, measurements combined in two orthogonal axis enable to characterise the equivalent resistivity of crack whatever it is dry or wet. 2.3 Sensitivity of electrical resistivity to crack depth
2.4 Expression of damage with the electrical anisotropy.
Numerical computations help to understand that influences of equivalent resistivity of crack, and crack depth, independently leads to same electrical apparent resistivity disturbances (qualitatively). So, at this stage, we cannot make the part between effects of these two parameters. From an electrical point of view, we treat about a crack intensity corresponding indistinctively to influence of equivalent resistivity and depth of the crack. 3 Acoustic methodsIn order to determine the crack depth of an opening crack at least two acoustic methods exists. The first one relies on compression wave the second one on surface waves. 3.1 Compression wave technique
3.2 Surface waves technique
3.3 Sensitivity to crack depth The interaction of surface waves with a crack either fully open or partially open has been investigated numerically with an Indirect Boundary Element Method (IBEM) [PED94]. In the case of a crack in a semiinfinite medium the average spectral ratio of the transmitted signal to the incident signal is shown figure 10. The cutoff frequency f_{c} is determined by derivation of these curves. f_{c} has been empirically set to the lowest frequency at which the slope of the curve shown figure 10 is equal to 0.075. The relation between fc and the crack depth h is given by:
where V_{R} is the Rayleigh wave velocity (homogeneous isotropic half space). It should be noted that this result is only valid if the concrete medium can be considered as non dispersive with regards to surface waves propagation (i.e. surface waves velocity do not depend on frequency). It has been verified that the presence of water does not change this result. 3.4 Sensitivity to partial closure 4 Coupling of techniques4.1 Synthesis of theoretical ability of techniques Electrical resistivity measurements are sensitive to various crack parameters. Previous works prove the link between two main characteristics called as the depth, and the equivalent resistivity of crack (which combines effects of crack opening, resistivity of the filling and crack bridging). Theses two factors lead to anisotropy variations, but at this stage no distinction can be made between their effects, without additional assumptions. It is thus expected that the determination of the crack depth with the acoustic method will constrain the interpretation of the electrical resistivity measurements. 4.2 Coupling of techniques We can propose to assess the crack depth by acoustic measurements, and then use electrical measurements to obtain the intensity of crack (linked to depth and equivalent resistivity of crack). Combination of results leads to distinguish effects of depth and of equivalent resistivity on electrical measurements, then to characterise geometrical profile of the damage. 5 Experimental test on a damage slab5.1 Presentation of the experimental study
Electrical and acoustic measurements are realised along three profiles, perpendicularly to each crack. The electrical measurement sessions are led with two square devices: 5 and 10 cm size, with the both orientations of the set (injection in parallel or perpendicularly to the crack  that is to say respectively PL or PP configuration). We consider in succession three crack conditions  crack filled in by air, by water, and by wet sand  to compare results for various equivalent resistivities for the nine (3 x 3) depth / fillin combinations. The acoustic setup consist is 12 sensors located on both side of the cracks. The distance between sensors is 2cm. The sources are steel balls located at the far ends of the slab (in order to be in the far field as in the numerical modelling study). The surface waves velocity is equal to V_{R}=2400±220m/s. It has been check that it is independent on the frequency for frequency larger than 8 kHz. 5.2 Results
The spectral amplitude ratio for the surface waves method are given on figure 14. The dark zones on the left of each graph correspond to wavelengths that are influenced by the thickness of the slab and thus must not be taken into account. It can be seen that as the crack depth increases the low frequencies are more and more filtered. The experimental curve and corresponding numerical one are matching well. Estimations of the crack depth for the two deeper cracks (9cm and 16cm) are easily obtained with the slope of the curve procedure described above. It is not the case for the shallowest crack even though the filtering effect is still visible and concerned higher frequencies than for the 9cm crack. This might come from the fact that for small wavelengths the concrete cannot be considered as a homogenous material (typically wavelength shorter than 7.5 times the nominal size of the aggregates [CHA02]). 5.3 Coupling Based on the study of the slope of the curves given Figure 14, the surface wave method give the following depth estimation for the two deeper cracks : 9,7±1,9cm, 16,0 ± 3,4 cm.
For example, for a blind study of the crack with 9 cm in depth and filled with sand, the assessment by acoustic method of its depth, with its uncertainty, leads to estimate the ratio d/a (depth / size of the set) to be equal to 0,97 ± 0,19. The measurement of electrical anisotropy right to the defect (here Log_{10}An = 1, see on figure 13), should lead to the determination of the equivalent resistivity of crack, as the result of data issued from both techniques (grey arrows on figure 15).
At this stage of progress we cannot estimate the accuracy on equivalent resistivity of crack determination. Experimental works have been realised only on three different cracks. We have not enough values with electrical method to be able to choose the corresponding equivalent resistivity of the crack. Works are in progress to complete the abacus (based on numerical modelling). Even if, results are not complete enough to use the coupling of theses techniques (acoustic and electrical) to assess crack internal characteristics, the proposed approach proves its potential concerning the described problematic. 6 ConclusionThe interest of combining electrical and acoustic methods to characterise cracks has been tested. These two Non Destructive Techniques give complementary information on the defect, since they are based on different physical phenomena. Their coupling leads to clear and to enrich measurements. As information is obtained separately, they are not redundant and they give solutions to exploit more completely measurements. Though each technique is still in progress, this way is innovating, by the approach, and by the type of results. A lot of parameters are still to study to understand all their influence (bridges, opening ...) but the present results are encouraging and further works are planned. Besides solutions proposed by each technique independently, the coupling of methods opens on new perspectives for the characterisation of concrete cracks on site. Bibliographie

