ELECTROCHEMICAL AND MICROSTRUCTURAL INVESTIGATION OF REINFORCED CONCRETE WITHIN CORROSION & CATHODIC PROTECTION: APPLICATION OF DERIVED PARAMETERS IN MODELLING TIME TO CONCRETE COVER CRACKING D.A. Koleva (1), K. van Breugel (1), J.M.C. Mol (2) and J.H.W. de Wit (2) (1) Delft University of Technology, Faculty of Civil Engineering and Geosciences, Materials and Environment, Stevinweg 1, 2628 CN Delft, The Netherlands (2) Delft University of Technology, Faculty 3mE, Surfaces and Interfaces, Mekelweg 2, 2628 CD, Delft, The Netherlands Abstract Various models for assessment and prediction of mechanical properties or materials behaviour of reinforced concrete, subjected to chloride induced corrosion, are present and available. These models provide promising results. However, most of them are based on theoretical considerations and simplified assumptions (e.g. certain type of corrosion products, say magnetite, deposit uniformly on the steel surface). This modelling strategy ignores the complex character of the corrosion products and the effect of their transformations on the bulk matrix microstructure. Furthermore, the existing models do not consider a thorough investigation of the reinforced concrete system as a global (integrating electrochemistry and concrete material science) i.e. the importance of correlating electrochemical and microstructural parameters is not considered. This paper briefly presents the application of experimentally derived parameters as an input in otherwise reported modelling approach for estimation of time to concrete cover cracking. Consideration is hereby given to the properties of the system “reinforced concrete” on macro and micro-level after corrosion and cathodic protection, taking into account electrochemical response of the embedded steel, surface layers properties and bulk matrix characteristics. 1. INTRODUCTION Numerical modelling of chloride-induced corrosion-related issues in reinforced concrete (including modelling of water/ion transport through a bulk concrete matrix) also covers modelling approaches that focus on the time to crack initiation at the steel/cement paste interface; crack propagation into the bulk matrix; concrete-cover cracking etc. These latter approaches are inevitably related to considering the properties of the steel reinforcement, the interface steel/cement paste and the concrete bulk matrix i.e. an integration of 2nd International Symposium on Service Life Design for Infrastructure 4-6 October 2010, Delft, The Netherlands 139
Transcript
ELECTROCHEMICAL AND MICROSTRUCTURAL INVESTIGATION OF REINFORCED CONCRETE WITHIN CORROSION & CATHODIC PROTECTION: APPLICATION OF DERIVED PARAMETERS IN MODELLING TIME TO CONCRETE COVER CRACKING
D.A. Koleva (1), K. van Breugel (1), J.M.C. Mol (2) and J.H.W. de Wit (2)
(1) Delft University of Technology, Faculty of Civil Engineering and Geosciences, Materials and Environment, Stevinweg 1, 2628 CN Delft, The Netherlands
(2) Delft University of Technology, Faculty 3mE, Surfaces and Interfaces, Mekelweg 2, 2628 CD, Delft, The Netherlands
Abstract
Various models for assessment and prediction of mechanical properties or materials behaviour of reinforced concrete, subjected to chloride induced corrosion, are present and available. These models provide promising results. However, most of them are based on theoretical considerations and simplified assumptions (e.g. certain type of corrosion products, say magnetite, deposit uniformly on the steel surface). This modelling strategy ignores the complex character of the corrosion products and the effect of their transformations on the bulk matrix microstructure. Furthermore, the existing models do not consider a thorough investigation of the reinforced concrete system as a global (integrating electrochemistry and concrete material science) i.e. the importance of correlating electrochemical and microstructural parameters is not considered. This paper briefly presents the application of experimentally derived parameters as an input in otherwise reported modelling approach for estimation of time to concrete cover cracking. Consideration is hereby given to the properties of the system “reinforced concrete” on macro and micro-level after corrosion and cathodic protection, taking into account electrochemical response of the embedded steel, surface layers properties and bulk matrix characteristics.
1. INTRODUCTION Numerical modelling of chloride-induced corrosion-related issues in reinforced concrete
(including modelling of water/ion transport through a bulk concrete matrix) also covers modelling approaches that focus on the time to crack initiation at the steel/cement paste interface; crack propagation into the bulk matrix; concrete-cover cracking etc. These latter approaches are inevitably related to considering the properties of the steel reinforcement, the interface steel/cement paste and the concrete bulk matrix i.e. an integration of
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electrochemistry and concrete material science should be considered. Various experimental investigations have been carried out on corrosion-induced cracking [1-4]; corrosion-induced degradation has been approached by computer simulations as well [5-9]. These models provide promising results; however, most of them are based on simplified assumptions: e.g. a certain type of corrosion products (say, magnetite) deposit uniformly on the steel surface. This modelling strategy ignores the complex character of corrosion products and/or the particularities of the bulk matrix and the steel/cement paste interface. Further, if a performance prediction for reinforced concrete, subjected to remediation, maintenance, repair or corrosion protection is to be performed, the complexity of a modelling approach increases considerably; the higher complexity is due to the fact that the fundamental assessment of already altered material properties needs a sound interpretation, which is in addition to the (necessary in these cases) relatively long-term monitoring for deriving reliable parameters.
Cathodic protection (CP), for example, is an electrochemical technique (with high complexity of involved phenomena, similar to corrosion), that has been widely used in concrete technology to inhibit, or stop, chloride-induced corrosion of the steel reinforcement. Fundamental principles and aspects are not subject to this work (more details in [10,11]). What needs to be mentioned only, is that the electrical current flow, involved in CP applications, affects not only the embedded steel, but the interfacial zones (ITZs) and the bulk matrix as well [12]. It has been demonstrated that CP leads to favourable conditions in terms of type and morphology of corrosion products i.e. lower crystallinity, lower salinity, protective, stable and well adhered layers on the steel surface [13]. However, CP also induces bulk matrix alterations i.e. bulk matrix densification, enlarging of ITZs [14]. Therefore, if a modelling approach has to predict the performance of a reinforced concrete structure after steel corrosion, and further after applying electrochemical protection techniques (e.g. CP in this case), simplifying the complexity of interacting interfaces, corrosion/hydration products and electrochemical phenomena will result in misleading results and interpretation.
To this end, this works aims to give an example for the application of experimentally derived parameters as input in reported modelling approaches (using the Petre-Lazar model [6]). Along with the results and discussion on deriving the necessary input parameters, the emphasis hereby is on illustrating the difference in time scales for concrete cover cracking (resulting from chloride-induced corrosion) when the model uses theoretical considerations or real experimental data. Further, in order to illustrate the higher complexity in application of modelling strategies to previously corroding and later on protected reinforced concrete, an example of predicting concrete-cover cracking, when CP was involved, is also briefly discussed.
2. EXPERIMENTAL MATERIALS AND METHODS
Materials: Reinforced concrete cylinders were cast from Ordinary Portland cement (OPC CEM I 32.5 R), using water-to-cement ratio 0.6, with dimensions: H = 250 mm, D = 120 mm, embedding low-carbon construction steel (type FeB500HKN ribbed bars, d=12mm), used “as received” (no pre-treatment, no apparent “mill scale”) for all technical conditions. Conditioning regimes (including CP and pulse CP regimes) are hereby briefly introduced: all specimens (6 replicates per type) were cured for 28 days in fog room (98% RH and 20°C). Further, aiming at initiation of corrosion before applying CP, the specimens were maintained in a salt spray chamber (SSC, 5% NaCl) for 460 days (control ones were maintained in
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water). The CP regimes used DC current of 5 mA/m2 steel surface; mixed metal oxide (MMO) Ti mesh, embedded in the concrete cover served as anode. The protection was applied after corrosion was initiated at certain intervals (60 days relevant to this study). The pulse CP used the same current density, but applied as a block pulse signal (square wave), tested frequency 100 Hz to 100 KHz (rise time of the pulse shaped current larger than 100 A/m2s), duty cycle of 1% to 50 %, preferably less than 25 %, at certain frequencies [15] i.e. the effective CP current density in case of pulse CP was always 50% of the CP current density in the relevant conventional CP regime. Four main groups of specimens were investigated: a freely corroding group (specimens C), a non-corroding group (specimens R), and groups CP and pCP (conventional and pulse CP). Electrochemical methods involved were electrochemical impedance spectroscopy (EIS) and potentio-dynamic polarization (PDP). The measurements were performed at open circuit potential (OCP) for all cells. PDP was in the range of -0.15 V to +0.75 V vs OCP at scan rate 0.5 mV/s; EIS was carried out in the frequency range of 50 KHz to 10 mHz by superimposing an AC voltage of 10 mV. The used equipment was a High performance Autolab PGSTAT302N, combined with FRA2 module, using GPES and FRA interface. Scanning electron microscopy (ESEM Philips XL30), has been employed for visualization and microstructural investigations directly on the steel surface and on cross sections of the steel/cement paste interface. Image analysis was performed using OPTIMAS software package. On the basis of mathematical morphology transformations, porosity and pore size distribution was obtained by using a sequence of similarly shaped structuring elements of increasing size [16]. The results from image analysis are an average of 35 locations per sample of 20 × 20 mm for the bulk matrix and 90 locations (for a bar diameter 12 mm) at the steel/cement paste interface in each specimen. The physical size of the reference region is 226 μm in length and 154 μm in width, with resolution of 0.317 μm/pixel, corresponding to a magnification of 500x (sample preparation and imaging procedures are as reported in [12,16]. XRD measurements were performed by scanning representative areas of about 1 cm2 (directly on the surface) of the test specimens, using D8 Advance Diffractometer, “Bruker AXS”. A VANTEC position sensitive detector (window 6 degr.) was used for detection. Energy source was CoKα (1.789Å) and the tube settings were 45kV and 35mA.
3. THE PETRE-LAZAR MODEL AND THE NEEDED EXPERIMENTAL DATA The expected time to corrosion induced concrete cover cracking depends on various
factors, including the aggressiveness of the environmental medium (e.g. chloride concentration). For significantly lower than the hereby reported (5% NaCl) aggressiveness of the environment (or for natural conditions), 3.2 to 4.4 years are numerically predicted [9]. Figure 1 presents the derived time to concrete-cover cracking (tr) in reinforced concrete, calculated based on the existing model of Petre-Lazar [6]. The equation on which the model and calculations are based is incorporated in Figure 1 (left), where: Z – valence of iron ions; ρ -iron density (7.8 g/cm3); F – Faraday const = 96500 A.s; Φ- concrete porosity; tint – thickness of steel-paste interface (µm); M – iron atomic mass (56 g); Vox- volume of corrosion products compared to iron; Icorr – corrosion current density (µA/cm²). The estimation as plotted is derived considering only the corrosion current density (Icorr) from real experiments, whereas the rest of the parameters are based on theoretical assumptions. In order to compare the predictions from this model as reported, with predictions of time to concrete cover cracking,
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when experimental data are involved, the following (previously defined) parameters are needed: Φ; tint; Vox (i.e. type of corrosion products needs to be defined) and Icorr. In what follows, the determination of the latter parameters is discussed. Further, an illustration is given for their application in the Petre-Lazar model.
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z F ttM V I
ρ φ+= ⋅
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Figure 1: The Petre-Lazar model without correction in the model prediction of time to concrete-cover cracking.
4. RESULTS AND DISCUSSION
4.1. Corrosion current density (Icorr) Electrochemical parameters (in particular Icorr as a hereby relevant parameter) can be
derived via Impedance Spectroscopy (EIS), Linear polarization resistance (LPR), Potentio-dynamic polarization (PDP), etc. EIS for example is a valuable tool, providing information for bulk matrix properties (e.g. electrical resistivity, pore network capacitance, etc.) as well.
Polarization resistance Rp and corrosion current density Icorr, derived from EIS and LPR measurements
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E (V
olts
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Figure 2: PDP curves (left) and polarization resistance (Rp) and Icorr values, derived from
PDP and EIS (right)
A detailed discussion on EIS response and/or steel behavior with external polarization (PDP) is not subject to this work. Hereby relevant is only the presentation of derived Icorr values. For clarity, however, the difference from expected and actually obtained current densities for corroding and protected specimens is briefly discussed (for substantial discussion on electrochemical response, pls see [13, 17]). Figure 2 (left) presents PDP curves for all specimens after 460 days of conditioning. The measurements were performed after 24 h depolarization (awaiting for the establishment of a stable OCP, for the protected CP and pCP
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groups, 24h before and in the time of all electrochemical measurements there was no CP current flow through the cells). Figure 2(right) presents the calculated corrosion current density Icorr (after IR drop correction, using Stern-Geary equation [18] and Tafel and Butler-Volmer fits of the GPES software), along with the Rp values (derived from EIS) for all specimens. The Icorr values for the reference group R are lowest, for the rest of the specimens Icorr is increasing in the order of: C, CP and pCP. Although it is generally expected that the protected specimens will exhibit lower Icorr values, compared to corroding ones (group C), apparently this is not the case. Based on fundamentals, the measured corrosion current density will be controlled by the kinetics of the electrochemical reactions during the measurement and the diffusion of reactants both towards and away from the electrode. Hence, higher diffusion limitations, as in specimens C, denoted to the formation of substantial amounts of corrosion products on the steel surface [14], will result in effectively higher Rp values and lower Icorr values respectively. These observations can be explained on the basis of several mechanisms: maintaining the steel in CP specimens cathodic during the time of the experiment (with interruptions of the protection current only in the time of depolarization and electrochemical measurements), suggests a relatively “clean” steel surface in the protected cells, compared to the corroding cells. Thus, any layers formed prior to CP application (which was 60 days, thus CP running 400 days at the investigation stage of 460 days) would only bear conversions but further oxidation of the steel will not occur in the time of protection. In the time of interrupting the CP, and after depolarization for 24 h, allowing establishment of a steady OCP, the steel gradually slips to a more “active” state (as result of shifting the steel potential from strongly cathodic to natural for the relevant conditions potential). Considering the cement chemistry around the re-bars, the chloride concentration (1.2 – 1.8 wt.% chloride per cement weight, [12,13]) being above the thresholds of 0.15 w.% per dry cement weight [19], there might be conditions for steel activity on isolated locations in the period of electrochemical measurements; the recorded averaged corrosion current density for the protected specimens might end up higher, compared to corroding and non-corroding conditions. Further, the nature of the product layer, formed on the steel surface (for each technical condition), is significantly contributing to the performance of the systems and influencing the electrochemical response [13, 17]. To this end, Icorr values for protected cells need to be recorded on intervals and within longer time scales, in order to elucidate the secondary effects of CP in terms of the so called “open-circuit steel passivity”, being the evidence for decreased current densities with altered cement chemistry within a working CP. Summarized trends of Rp values, calculated on basis of PDP measurements with time are presented in Figure 3.
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Figure 3: Summarized Rp values (PDP) at time intervals 210, 310 and 460 days
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As seen from the plot, the polarization resistance values for corroding specimen C were higher and still maintain highest at 460 days of age, but exhibit a decreasing trend with time. In contrast, the Rp values for the reference R and the protected specimens CP and pCP increase with time, suggesting increased protective properties of the product layers on the steel surface within longer periods of CP operation.
4.2 Microstructure (bulk matrix and steel/cement paste interface) An example of derived porosity values (460 days of age, bulk matrix and interface) are
given in Figure 4, depicting the significantly different values, resulting from different conditioning regimes. Microstructural analysis was performed as previously described. Figure 4 (left) shows that the specimen R (reference) has highest bulk porosity (~8% at 460 days of age), while the corroding specimen C and specimen CP exhibit lowest bulk porosity (~3 to 4%). In the interfacial region (steel/cement paste) specimen C exhibits highest porosity as contributing factor is the corrosion induced cracking (~7%); specimen CP has lower interfacial porosity (~3.3%) but still higher than specimens R and pCP (~2.7%). This proves the minor influence of pulse CP, which maintains the material properties close to reference conditions. Supporting evidence is the derived critical pore size distribution, the specimens R and pCP having critical pore size of 0.951 µm while specimens C and CP – 0.634 µm (the CP current flow, in addition to corrosion induced changes in the bulk matrix (as CP was applied at 60 days of age), is affecting the material properties to a higher extent). The current flow causes densification of the bulk matrix in both regimes (CP and pCP), compared to rest conditions, while the interface region is maintained slightly coarser, compared to rest conditions and denser, compared to specimens C, the latter influenced by corrosion induced cracking. The coarser pore structure in the interface region for specimens CP and pCP, compared to specimen R, is denoted to de-bonding mechanisms as result of current flow (for more details on the relevant techniques and procedures, please see [12]).
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Figure 4: Porosity (pore size): bulk matrix and steel/paste interface for all investigated
specimens and conditions.
4.3 Morphology and chemical composition Except electrochemical parameters (Icorr) and structural parameters (porosity, pore size
distribution, etc.), the input parameters for modelling approaches include variables, based on the type, composition and morphology of product layers formed on the steel surface. Figure 5 presents the XRD diffractograms for corroding specimen C (top) and protected specimen (CP)
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bottom, along with the typical morphology of pronounced products on the steel surface in corroding specimens (akaganeite) and protected specimens (magnetite).
Corroding specimen
Protected specimen
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30 40 50 60 70 80
HG
MG
W
A+L
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MG
MGMG
W MG
W
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H G
HG
MG Ar
Ar P
P
MG
MG
Q
Q
C MG
FeFe
FeFe
H
Figure 5: Products morphology/composition on the steel surface in specimen C (top) and
specimen pCP (bottom)
The presence of well crystallized akaganeite (Figure 5, top) denotes for high salinity in the corroding specimens, whereas the product layers in protected specimens, consists mainly of well adhered and protective magnetite (Figure 5, bottom). Based on surface analysis, (a detailed presentation of XRD, coupled with XPS, EDX and SEM is reported in [14, 17]) the ratios and volume of corrosion products (Vox) were calculated (summarized data are given in Table 1). Table 1 Correlation of α and α1 for the relevant products, as detected on the steel bars.
FeO Fe3O4 Fe2O3 Fe(OH)2 Fe(OH)3 Fe(OH)3.3H2O βFeOOH.Cl2 α 0.777 0.724 0.699 0.622 0.523 0.347 0.559 α1 1.8 2 2.2 3.75 4.2 6.4 4.6 α – ratio of MFe to Mcorr..products ; α1 – ratio Vexpansive corr.. prod to VFe consumed during corrosion
4.4. Illustration of the Petre-Lazar model, using the above derived experimental data As previously discussed (Figure 1), calculation of the time to concrete-cover cracking,
based on only Icorr as an experimentally derived parameter gives misleading results: the estimated time for concrete cover cracking for control conditions is approx. 10 years, while for the corroding case is approx. 0.5 years (Figure 1, Figure 6, yellow bars). Further, if real experimental data for the type and volume of corrosion products (Vox), concrete porosity (Φ) and properties of the steel/cement paste interface are incorporated in the modelling approach (adding each parameter and all of these together represented by different colours in Figure 6), the estimated concrete cover cracking is considerably different i.e. approx. 65 years for the control case and 0.2 years for the corroding case (Figure 6). In other words, without
(CoKα radiation): C - calcite, G - goethite, MG - magnetite, L -lepidocrocite, Q - quartz, W - wustite, Ar - aragonite, P - portlandite, A - akaganeite, H - hematite, Q – quartz.
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considering reliable (experimental) data, the result is underestimation for control conditions and overestimation for conditions of chloride-induced corrosion.
CP pulse CP Corroding Figure 6: The Petre-Lazar model without (yellow bars) and with correction in the model
prediction of time to concrete-cover cracking, by using real experimental data.
As seen form Figure 6, this model is not applicable for CP cases: there is a discrepancy in obtained results and the logically expected ones for the protected specimens: the protected steel (since there is no corrosion occurring in conditions of CP) will be expected to approach the reference situation. Obviously, these modelling approaches are applicable for corroding and non-corroding cases, but are not suitable for assessment of protected specimens. The following remarks deserve attention here: The Petre-Lazar model (as well as all models, predicting mechanical behaviour) is based on calculations, incorporating Icorr as main variable, which determines complexity of modelling in terms of electrochemical condition of the steel surface. The fundamental method for deriving Icorr values is not straight forward for reinforcement under CP [13, 20]. After a prolonged CP, the corrosion rates for protected steel (being considered as bare steel) are higher (corresponding to lower Rp values) than the original passive corrosion rate. Another consideration, with respect to CP and modelling of crack initiation and propagation, is related to the influence of CP current on bond strength in reinforced concrete. CP current is known to bring about loss of concrete adhesion, as reported elsewhere [21, 22] and also investigated by the present authors [12]. To this end, the time for crack initiation and propagation are obviously underestimated, since along the peculiarities for Icorr determination, the calculation in the available models can not distinguish microstructural changes from corrosion induced cracking or CP induced alterations.
5. CONCLUSIONS This paper briefly presented the combination of electrochemistry and concrete material
science for deriving main parameters for the reinforced concrete system as a global. Further, an illustration was given for using these real experimental data (compared to theoretical estimation) in modelling the time to concrete cover cracking, induced by steel corrosion in reinforced concrete. A significant contribution and close dependence was found to exist for all macro- and micro-level interfaces, and their properties, affecting the global performance of the materials under study.
When the input parameters are only partly based on experimental data and largely on theoretical considerations, the modelled prediction gives overestimation for corroding cases and underestimation for control cases. The presented example clearly shows the importance of
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using real experimental data as input parameters and gives satisfactory and logically expected results. The hereby discussed model is not applicable for modelling material performance when CP is involved, due to the complexity in fundamental assessment and recording of needed parameters, therefore alternative approaches (and significantly longer monitoring periods) are necessary for these cases.
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