Contribution of Capacitance Probes for Nondestructive Inspection of External Post-tensioned Ducts

In bridges, external post-tension habitually comes as cables placed in ducts for which the residual internal space is (imperfectly) filled with a fluid cement grout. Detecting the problems of injection is not practicable visually from the outside, and no effective auscultation tools were found yet. A recent laboratory experiment established that capacitance probes can be employed, but the main difficulty is to provide a correct interpretation of the measurements in terms of deterioration of the coating, along with the occurrence of water or grout voids. In order to understand if the presence of the cable itself can disturb the diagnosis in such proportions that any inspection is destined to failure, the subject was tackled here from a numerical point of view. It is shown that the capacitance probe is sensitive to the location of the cable, but that it is still possible to distinguish typical defects present at low depth. This result is confirmed, from a qualitative point of view, by tests performed with an actual probe. Introduction: A lot of bridges and tunnels include “external” post-tension (so called since it is not in the concrete material, hence potentially accessible for measurement), either originally or a after reinforcement of structure. The cables are generally placed in High Density Poly-Ethylene (HDPE) ducts, where the residual space is filled under high pressure with a cement grout intended to prevent corrosion. This protection being imperfect, cable breaking occurs in non-protected zones because of the presence of a “white paste” (not hardened grout with a high water content) or grout voids. The detection of such defects inside an opaque duct is unachievable visually from the outside. Furthermore, already existing procedures (Derobert et al., 2002) often involve the damaging creation of apertures on the duct envelopes to look inside or introduce an endoscope. Another approach employing gamma-rays radiography is cumbersome and expensive, acoustic methods are not precise enough, etc. In these conditions, there are few available modus operandi, and the need for a reliable diagnosis urgently requires the development of effective investigation tools to prevent unnecessary strengthening and repair, or even replacement (Cavell et al, 2001). Electromagnetic methods are suited for many purposes, amid, capacitive measurement is implemented on a regular basis for a quantitative estimation of the water content of soil (Fares and Alva, 2002), cement (Smith et al., 2002), or agricultural products (Nelson, 1992). This technique is also appropriate to assess the porosity of volcanic rocks (Rust et al., 1999), voids fraction in multi-phase flows (Kendoush and Sarkis, 1996), etc. Further, capacitance detectors are employed for the qualitative differentiation of materials like clear wood from knots and distorted grain (Steele and Kumar, 1996), to characterize biologic cells (Asami, 2002), to sound snow packs in avalanche forecast (Louge et al., 1998), and to detect buried plastic landmines (Mamishev, 1999). Capacitor properties provide non destructive tools (Matiss, 1999) at microscale as well, for instance to inspect electronics products (Ruprecht et al., 2003) or control their processing integrity (Jeandupeux et al., 2002). In the same manner, the auscultation of external post-tensioned ducts with a capacitance probe was found to be workable during laboratory experiments (Dupas et al., 2001). The tests utilized long transparent proof bodies, containing a steel cable, and acting as duct pieces. Holes allowed the introduction of water, sand or air, and the emptying. Two rectangular electrodes applied on the outer surface of the samples were moved around and along the proof axis, repeatedly. An alternating current being applied between the plates, the system constituted some sort of capacitor coupled into a high frequency resonant circuit. The resonant frequency shift is indicative of the nature of the materials inside the duct. However, without knowledge of the factors susceptible of influencing the measurements, operators experience and shrewdness are necessary for interpreting the capacitance data. Moreover, previous sampling and calibrating phases are necessary to have a chance for relating measured and searched information. Indeed, the case of post-tensioned ducts is complex, since they appear as an heterogeneous mixing of conductors and dielectrics, and as internal geometry interferes in an undefined manner. Accordingly, it seems judicious to study first the influence of each contribution independently, starting from the most important processes, or at least (those sought to be) governing features. Actually, the first question we have to answer is: does the presence of the steel cable influences the measurement in such proportions that any inspection for detecting inclusions is destined to failure ? The problem was tackled here from an electrostatic point of view, and a model of posttensioned duct was used for a numerical evaluation of its dielectric capacitance by solving the corresponding set of equations with finite elements. After setting up the configuration in the first part of this paper, several situations are studied in a second section for a sensitivity study in typical capacitors where the nature of the constituents as well as the location of the post-tensioned cable both vary. At last, an example of measurement obtained in the laboratory with a capacitance probe is described as well as the corresponding simulation. Setting up: A sketch of external post-tension duct containing a seven-tendons steel cable at the bottom is shown on Figure-1. Here, the volume of the HDPE envelope is imperfectly filled of cement grout, with an air pocket and a water saturated material layer (i.e., not hardened paste), as may occur frequently.