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An Indirect Impedance Technique to Determine Reinforcing Steel Properties

^{1,2,3,4 }This technique can be applied to the study of reinforcing steel in concrete, and in this particular paper, to the study of post-tensioning steel in grouted cylinder tendons used in bridge construction. With the use of Electrochemical Impedance Spectroscopy, a ﬁnite-element model has been developed to solve for the frequency-dependent potential distribution through a cylindrical grout specimen containing one axially-located steel strand. Impedance simulations were performed at diﬀerent steel corrosion states while incorporating the kinetics at the current-injecting electrodes.

Following the procedure of Huang et al.^{5}, the electrode kinetics are expressed as an oscillating current density as the summation of a charging and faradaic current. The grout is modeled as a uniform conductivity media and Laplace’s equation is used to solve for the potential distribution. The impedance is simulated over a range of frequencies as the potential difference between two reference electrodes divided by the current of the working electrode. The results are compared to experimental measurements throughout the hydration process.

The effectiveness of this method relies on the use of accurate representations of the steel surface impedance. Passive film models developed from rotating disk experiments in simulated pore solution are used to describe the steel boundary condition in the finite element model. The simulated results incorporating these models show a similar impedance response as the measured impedance of an aged cylinder put through a wetting and drying cycle.

Barring an accurate model, this technique has the potential to effectively determine the interface properties of the embedded steel from the overall response of the system.

**References**

J. Zhang, P. J. Monteiro, F.H. Morrison, “Noninvasive surface measurement of corrosion of reinforcing bar in concrete: Part I Experimental results”,

*ACI Materials Journal,***98**(2001) pg. 116-125.M Keddam, X. R. Novoa, V. Vivier, “The concept of floating electrode for contact-less electrochemical measurements: Application to reinforcing steel-bar corrosion in concrete,”

*Corrosion Science,***51**(2009) pg. 1795-1801.C. Andrade, I. Martnez, “Metal corrosion rate determination of diﬀerent solutions and reinforced concrete specimens by means of a noncontacting corrosion method,”

*Corrosion***66**(2010) pg. 056001-056001.C. Andrade, J. Sanchez, I. Martinez, N. Rebolledo, Analogue circuit of the inductive polarization resistance,”

*Electrochimica Acta***56**(2011) pg. 1874-1880.V. M. W. Huang, V. Vivier, M. E. Orazem, N. Pbre, B. Tribollet, “The apparent constant-phase-element behavior of a disk electrode with faradaic reactions a global and local impedance analysis,”

*Journal of the Electrochemical Society***154**(2007) C99-C107.