On-site transient analysis for the corrosion assessment of reinforced concrete

12 A range of methods exist to assess the condition of steel reinforcement in concrete. The analysis of 13 the transient response to a small perturbation has been employed successfully in laboratories to 14 assess corrosion. This work examines a simplified method for the application of transient analysis to 15 in-situ reinforced concrete structures. The complex analysis has been simplified and undertaken with 16 the use of common spreadsheet packages. The results illustrate that transient response analysis is a 17 viable technique for use on site and appears to provide a more accurate representation of steel 18 corrosion current densities at very low values than polarisation resistance.

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Introduction 24
The study outlines a trial of transient response analysis on full-scale motorway bridge structures to 25 obtain information concerning the steel-concrete interface and is part of a larger study to assess the 26 long-term sustained benefits offered by Impressed Current Cathodic Protection (ICCP) after the 27 interruption of the protective current [1]. These structures had previously been protected for 5 to 16 28 years by an ICCP system prior to the start of the study. The protective current was interrupted, in 29 order to assess the long-term benefits provided by ICCP after it has been turned off. This paper 30 develops and examines a simplified approach for the on-site use of transient response analysis and discusses the potential advantages of the technique as a tool for the assessment of the corrosion 32 condition of steel in reinforced concrete structures. Impedance has been used previously to obtain corrosion information regarding the steel-concrete 36 interface [2][3]. To obtain this information, data is required at very low frequencies (mHz -μHz) [2][3][4][5].

37
The conventional method of obtaining impedance is to subject the specimen to a cyclic perturbation at 38 the frequency of interest and analyse the response [2,6]. However, at very low frequencies it is 39 preferable to subject the specimen to a perturbation and analyse its response resulting from the 40 perturbation [7-10].

42
The steel-concrete system can be described in the form of an electrical circuit. A common and simple 43 approach is the use of the Randle's circuit (Figure 1a). This analysis characterises the steel-concrete 44 interface with a polarisation resistance (R p ), interfacial capacitance (C) and electrolyte resistance (R e ).

45
R p can be directly associated with the steel corrosion current density (I corr ) [11][12]. The validity of the 46 simple Randle's circuit to adequately represent the steel-concrete interface is still subject to debate.

47
Impedance data may appear to produce a distorted or flattened semi-circle and at high frequencies a and is particularly suitable for use on full-scale site structures due to its simplicity. Feliu et al. [18] also 66 support the use of a simplified abstract representation of the system in order to interpret its 67 fundamental properties as opposed to a more accurate but significantly more complex circuit model.
where ω' is the highest frequency of interest, T is the period of the pulse and Q is the charge.

4
Under these conditions the Laplace transformation of the current perturbation will be the charge. iii. Figure 2c illustrates a typical example of the contents of the imaginary integral of Equation
The impedance value of equation (6) is given by the area under the curve of Figure 2a for the potential 119 transient divided by the charge (DC resistance). The low-frequency real axis intercept (highest x-axis 120 value of the semi-circle) represents the polarisation resistance (R p ) and electrolyte resistance (R e ).

121
The high-frequency real axis intercept (lowest x-axis value of the semi-circle) represents the 122 electrolyte resistance (R e ). Applying a short pulse and measuring the potential decay after the pulse 123 has been applied eliminates the effect of R e in the measurement process. Care must be taken however, as for some models the above assumptions will not be true. In these 136 exceptions, transients should be measured for a period which is longer than the period which is longer 137 than the lowest period of the frequency of interest. (since October 2007) in order to evaluate these long-term benefits [1]. A total of 10 structures were 151 selected based on the age of the installed ICCP system, accessibility and chloride levels. All 152 structures selected for this study were protected by ICCP and the anode system comprised a 153 conductive anode coating. The anode systems were from varying anode suppliers and this helped to 154 also assess their relative performance and durability.