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Transcript of !!!blabla! - Ghent University...!!!blabla! III Toelating tot bruikleen “De auteur geeft de...

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  • III

    Toelating tot bruikleen

    “De auteur geeft de toelating deze masterproef voor consultatie beschikbaar te stellen en delen van de masterproef te kopiëren voor persoonlijk gebruik. Elk ander gebruik valt onder de beperkingen van het auteursrecht, in het bijzonder met betrekking tot de verplichting de bron uitdrukkelijk te vermelden bij het aanhalen van resultaten uit deze masterproef.”

    Permission for usage

    “The author gives permission to make this master’s dissertation available for consultation and to copy parts of this master dissertation for personal use. In the case of any other use, the limitations of the copyright have to be respected, in particular with regard to the obligation to state expressly the source when quoting results from this master's dissertation.”

    Ghent, June 2017

    Jim Declerck

  • IV

    Acknowledgements It’s almost over, after a year of hard work on this master dissertation, I am arrived at the final and most important task, the acknowledgements. I would like to thank the following people for all their effort, motivation and support during this study.

    First of all, I would like express my gratitude to Prof. dr. ing. Kim Van Tittelboom, Dr. ir.-arch. Philip Van den Heede and Ir. Bjorn Van Belleghem for the guidance, the knowledge and feedback. I would like to thank Bjorn for the aiding during the corrosion monitoring and Philip for answering my questions and guiding me to develop a new corrosion model. Secondly my sincere acknowledgements to Prof. dr. ir. Nele De Belie for giving me the opportunity to work with self-healing concrete.

    I am also very grateful for the assistance of the technical staff from the Magnel Laboratory. They helped me with the concrete casting and the experimental set-up, everyone was very kind to me and there was always time for a laugh during my struggle.

    I would like to thank my best friend Niels from Mariaburg, for supporting me and being always ready for me at any time. Working on my thesis had been much less fun without my great friends of Coffee and Concrete, it was fun working with you guys in the library. I will miss the coffee breaks, the laughs and the hearts hunting during lunch. I would like to thank the coffee machine of the Magnel Laboratory for providing me enough coffee during my master dissertation. Of course my girlfriend Astrid deserves a thousand times thank you. Thanks for standing by me during this period and supporting me whenever you could, it was always a lovely pleasure coming home to you after a day in the lab.

    The support I experienced during my entire academic career from my brothers and sister, my aunt Kris and uncle Eddy was really appreciated, thank you very much.

    The final and most important gratitude is for my parents. Mom and dad, thank you for encouraging me, for giving me the opportunity to study Civil Engineering and for always welcoming me with open arms when I was back home. Words are not enough to thank you both for the support you have given me during this 5 year (sometimes difficult) journey.

  • V

    Overview Title: Corrosion monitoring of manually and autonomously healed

    concrete to enable service life assessment in marine environments

    Author: Jim Declerck

    Supervisor: Prof. dr. ir. Nele De Belie, Prof. dr. ing. Kim Van Tittelboom

    Counsellors: Dr. ir-arch. Philip Van den Heede, Ir. Bjorn Van Belleghem

    Master’s dissertation submitted in order to obtain the academic degree of Master of Science in Civil Engineering

    Department of Structural Engineering Chairman: Prof. dr. ir. Luc Taerwe Faculty of Engineering and Architecture Academic year 2016-2017 Summary: Cracks in reinforced concrete create preferential ingress paths for chlorides in marine environments. These chlorides can cause steel reinforcement corrosion and concrete degradation. This impairs the durability and shortens the service lifetime of concrete structures. More information on the implementation of healing agents and the extensions of the concrete structure’s lifetime is of important value. In literature, the focus is mainly on the corrosion initiation period which only quantifies the time to depassivation of the reinforcing steel. The total lifetime is found by adding the propagation period which starts when the initiation periods ends. In this thesis the focus is on estimation of the propagation period and the influence of healing mechanisms on this propagation period. Corrosion monitoring, by means of electrochemical measurements, was performed on concrete beams with different healing mechanisms. Further bulk resistivity measurements were performed on concrete cylinders to obtain the influence of NaCl wet/dry cycles on the aging factor of concrete. Finally the obtained results from the corrosion and resistivity monitoring were used to model the propagation period in the probabilistic software Comrel. The DuraCrete model (based uncracked concrete) was used and updated to implement more influencing factors of the corrosion process and the effect of a crack. It was found that implementation of a water repellent agent and a low viscosity polyurethane lead to an improvement of the propagation period while no improvements were noticed for the concrete beams encapsulated with a high viscosity polyurethane or beams with a coated (epoxy) anodic rebar. Keywords: self-healing concrete, chloride-induced corrosion, polyurethane, water repellent agent, service life assessment, propagation period

  • Corrosion monitoring of manually and autonomously healed concrete to enable

    service life assessment in marine environments Jim Declerck

    Supervisors: Prof. dr. ir. Nele De Belie, Prof. dr. ing. Kim Van Tittelboom Counsellors: Dr. ir-arch. Philip Van den Heede, Ir. Bjorn Van Belleghem

    Abstract: Cracks in reinforced concrete create preferential ingress paths for chlorides in marine environments. These chlorides can attack the reinforcement and initiate corrosion. More information on the implementation of healing agents and the improvements on the concrete structure’s lifetime is of important value. Corrosion monitoring was performed on concrete beams subjected to NaCl wet/dry cycles, to determine the effect of healing mechanisms on the service lifetime of concrete structures. Keywords: self-healing concrete, chloride-induced corrosion, polyurethane, water repellent agent, service life assessment, propagation period

    I. INTRODUCTION Cracks in reinforced concrete structures are preferential ingress paths for aggressive substances such as chlorides in a marine environment. As soon as a critical amount of chlorides has reached the location of the rebar chloride-induced corrosion occurs [1]. Many concrete structures suffer from reinforcement corrosion leading to high costs for maintenance and/or repair. Therefore the efficiency of healing agents and the estimation of the lifetime of a concrete structure using predictive models attracts considerable interest. The service life consists of an initiation period, describing the chloride ingress and a propagation period which starts after depassivation of the reinforcement and terminates after unacceptable deterioration of the structure. The currently available prediction models for chloride-induced corrosion are only valid for uncracked concrete, the outcome of such a prediction does not reflect the actual performance of ordinary concrete observed in practice. In this master dissertation, the influence of healing agents on the propagation period of cracked concrete structures is investigated. Corrosion monitoring was performed on cracked concrete beams and resistivity

    monitoring on concrete cylinders for 12 weeks. The results of these measurements were used as input values for different limit state models in the probabilistic software Comrel. A first model used is the available DuraCrete model, mainly based on the electrical resistivity of uncracked concrete [2]. Based on the DuraCrete model, another model is developed with implementations of additional factors influencing reinforcement corrosion and the effect of a crack.

    II. MATERIALS AND METHODS A. Materials

    1) Concrete beams

    The concrete mixture used for the concrete beams is depicted in Table 1. The reinforcement configuration of the concrete beams is shown in Figure 1. A central rebar acted as anode while sufficient cathodic area is provided by two reinforcement cages at the sides. Only the middle 50 mm of the anodic rebar was exposed to the concrete, the sides were coated with two layers of cement paste (W/C = 0.4) and two layers of epoxy resin. The cathodic cage consisted of four longitudinal steel bars with five stirrups. Cathode and anode were only connected through copper wires at the exterior.

    Table 1: Overview concrete mix

    Sand 0/4 [kg/m³] 696 Gravel 2/8 [kg/m³] 502

    Gravel 8/16 [kg/m³] 654 CEM I 52.5 N [kg/m³] 317.6

    Water [kg/m³] 153 Fly ash [kg/m³] 56

    Superplasticizer [ml/kg binder] 4

  • Figure 1: Reinforcement configuration

    Glass fibre reinforcement was used as longitudinal reinforcement to be able to create a crack without brittle failure of the beam. An internal silver/silver chloride reference electrode (CP20) was located as close as possible to the anode to avoid high ohmic drops. A central bending crack, with a target crack width of 300 µm, was created, providing a preferential ingress path for the chlorides to penetrate into the concrete.

    Different healing agents were used. The healing agents were encapsulated (two layers of six capsules) or manually injected to test their influence on corrosion resistance. Borosilicate glass capsules with an inner diameter of 3 mm and a length of 50 mm were used. An overview of the used samples is listed in Table 2. Two types of polyurethane (PU) were used, one with a high viscosity (6700 mPas) and one with a low viscosity (200 mPas). A water repellent agent (Sikagard 705 L) was encapsulated in three specimens and the last set consisted of anode bar with an epoxy coating (Beckopox).

    Table 2: Overview concrete beams

    Type Abbreviation Uncracked plain concrete UNCR

    Cracked plain concrete CR Plain concrete external ref. CR (ext.) Capsules PU high viscosity PU HV caps Capsules PU low viscosity PU LV caps Manual PU low viscosity PU LV man

    Water repellent agent WRA Coated bars COAT

    2) Concrete cylinders

    Additional concrete cylinders were manufactured for bulk resistivity measurements. The 36 concrete cylinders had a height of 120 mm and a diameter of 100 mm. The same concrete composition was used as for the concrete beams.

    B. Methods 1) Corrosion monitoring

    A bending crack of 300 µm was formed. The concrete beams were locked in a frame (Figure 2) to maintain the initial crack width.

    To test the resistance against chlorides, the beams were subjected to wet/dry cycles of 1/6 days. The concrete beam’s top surface was divided into three compartments (Figure 2). The side compartments were filled with a Ca(OH)2 solution while the middle compartment was filled with a NaCl solution of 33 g/l. The specimens were stored at 20°C and a relative humidity of 60%.

    Figure 2: Subdivisions compartments on top surface

    A continuous current monitoring was performed during 12 weeks of testing. Weekly the corrosion potential and ohmic drop was measured after the wetting period. Further, open circuit potential measurements were performed for the anode and cathode, electrochemical impedance spectroscopy (EIS) measurements (to estimate the concrete resistance) and also the anodic and cathodic polarisation resistance was measured. The measurements were performed relative to the internal Ag/AgCl reference electrode and afterwards converted to an external saturated calomel electrode (SCE).

    2) Bulk resistivity

    The cylinders were divided in three different groups. One group was kept constantly dry; one group was subjected to wet/dry cycles of distilled water while the final group was subjected to wet/dry cycles of a NaCl solution 33 g/l. Within these group, half of the cylinders were split in two, to test the influence of a crack to the resistivity of concrete.

    Weekly the two-electrode method was used to determine the bulk resistivity of the concrete cylinders at 20°C and 60% RH. During the wetting period, the cylinders subjected to wet/dry cycles

  • were stored in a plastic container on supports. The depth of immersion of the specimens was 5 mm.

    3) Probabilistic modelling propagation period

    The results from the corrosion and resistivity monitoring were used as input values for modelling the propagation period of the considered beams. In the probabilistic software Comrel, a limit state function g(x,t) was investigated and the probability of failure as a function of time was assessed.

    The DuraCrete model BE95-134/R9 was tested with the obtained experimental results.

    An improved model based on the DuraCrete model was made. In this adapted model, a more accurate expression for the corrosion rate was used (Model 2). An additional effect of a cracked state was implemented in Model 1. For both models the aging factor and wetness period factor were determined especially for this experimental set-up.

    III. RESULTS AND DISCUSSION 1) Corrosion monitoring

    The macro-cell corrosion current represents the flow of electrons between the anode and cathode and gave a direct assessment of the corrosion activity of the samples. Higher values were measured during the wetting periods and negligible values were found for the UNCR specimens. Consistency was found between the macro-cell corrosion current and the other measurements. An increase in corrosion current, corresponded with an increase in corrosion potential, an increase in driving potential ΔE and a drop in anodic polarisation resistance Rp,A. It was clearly visible to notice the period of severe corrosion of the specimens by comparing the different measurements per specimens. The driving potential of all beams is illustrated in Figure 3.

    Figure 3: Driving potential concrete beams

    Higher values for ΔE (around 400 mV and 500 mV) were obtained for the set CR, PU HV caps, COAT and PU LV man A which was the only sample of the PU LV set that showed a clear indication of a more active corrosion state. Values between 100 mV and 300 mV were found for PU LV caps, PU LV man (B,C), WRA and UNCR. The UNCR beams were the only one with a steady development of driving potential, all the other specimens experienced an increase where after ΔE converged to a more constant value.

    Figure 4: Anodic polarisation resistance CR samples

    The cathodic polarisation resistance (Rp,C) and the concrete resistance (Rconc) increased linearly during the total experiment for every specimen. Before the wet/dry cycle, the most contributing factor to the overall corrosion resistance was Rp,A which was one order of a magnitude larger than the Rconc and Rp,C. This can change during the lifetime of the specimens because of the possible drop of Rp,A when the beam corrodes and the increasing trend of Rconc and Rp,C.

    2) Bulk resistivity monitoring

    An increase in resistivity was noticed for every concrete cylinder, this can be expressed with an aging factor. The samples subjected to wet/dry cycles experienced a lower resistivity than the dry samples due to the ionic and moisture conductivity of the applied current through concrete. The cracked samples experienced a lower resistivity than the uncracked samples when subjected to wet/dry cycles. The aging factor was lowest for the cracked cylinders subjected to NaCl wet/dry cycles, while the highest aging factor was noticed for the cracked dry cylinders.

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  • 3) Probabilistic modelling

    The obtained propagation periods tprop of the DuraCrete model were misleading. The COAT samples approached the tprop of the UNCR samples despite the noticeable worse corrosion resistance during the corrosion monitoring. The shortcomings of the DuraCrete model are that the life expectancy is determined from the initial values of resistivity and that the only other variable parameter is the splitting tensile strength of the concrete. Possessing a high initial resistivity does not directly mean that the concrete will have high resisting properties against corrosion during the concrete’s total lifetime.

    For Model 1 and 2, the results are depicted in Figure 5. In Model 1 and 2, the severe corroded sets are clearly visible. General for Model 1 (except for UNCR), lower propagation periods were found, this is due to the implementation of the effect of a crack (which is absent for UNCR). In Model 2, the crack width was reduced to zero for the healed specimens, the actual propagation period will be in between the 𝑡𝑝𝑟𝑜𝑝 found for Model 1 and 2.

    Figure 5: Propagation period concrete beams according to Model 1 and 2

    The self-healing efficiency was calculated according to Van den Heede et al. and listed in Table 3 [3].

    Table 3: Self-healing efficiency according to tprop

    Group 𝑆𝐻𝐸1 [%] 𝑆𝐻𝐸2 [%] PU LV man A 1.9 6.0

    PU LV man (B,C) 15.1 26.3 PU LV caps 16.4 23.7 PU HV caps -1.1 0.9

    WRA 34.5 53.2 COAT -0.4 -0.4

    A reason for the better corrosion resistance of the PU LV samples than CR is the blocking effect of the healing agent. When the PU LV was in contact with moisture it hardened into a flexible foam inside the crack. This blocked the ingress of chlorides leading to less attack of the passive layer and less corrosion. PU LV man A corroded actively after week 5, a possible explanation of this behaviour was the bad injection of the cracks next to the middle crack. Water repellent agents limited the ingress of the solution by forming a hydrophobic surface layer. Chlorides diffusion is reduced when the concrete is dried out leading to a better resistance against chloride-induced corrosion.

    A possible explanation of the less corrosion resistant behaviour of PU HV can be found in the composition of PU HV, especially the NCO/OH ratio of the methylene diphenyl diisocyanate part of this PU. The corrosion resistance of a PU decreases when this ratio is low which is possible for PU HV due to his high viscosity [4].

    For the COAT specimens, it is possible that the solution penetrated the coating locally to form a new liquid/metal interface under the coating, this is not beneficial for the corrosion resistance of the rebar.

    IV. CONCLUSION

    Less corroding samples were characterised with low steady corrosion currents, corrosion potentials, driving potentials and with high anodic polarisation resistances. This behaviour was noticed for UNCR, PU LV caps/man and WRA. For the samples that showed major corrosion activity, a sudden increase in current, corrosion potential, driving potentials and a drop in anodic polarisation resistance indicated severe corrosion. This behaviour was noticed for CR, PU HV caps and COAT samples. The concrete resistance and cathodic polarisation resistance increased for every specimen.

    Wet/dry cycles influenced the bulk resistivity of the concrete cylinders, lowest values were found for the cylinders subjected to NaCl solution wet/dry cycles.

    The implementation of low viscosity PU encapsulated or manual injected resulted in an improvement in propagation period with respect to the CR samples. A similar conclusion is found for the encapsulated WRA samples with an SHE of

    UNCR CR PU HVcapsPU LV

    capsPU LVman A

    PU LVman (B,C) WRA COAT

    Model 1 4.84 0.20 0.15 0.96 0.29 0.90 1.80 0.18Model 2 4.84 0.20 0.24 1.30 0.48 1.42 2.67 0.18

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  • 34.5% - 53.2%. High viscosity PU and coated anodic bars were not efficient.

    ACKNOWLEDGEMENTS

    The author would like to acknowledge Prof. dr. ir. Nele De Belie, Prof. dr. ing. Kim Van Tittelboom, Dr. ir-arch. Philip Van den Heede and Ir. Bjorn Van Belleghem for their contributions to this research.

    REFERENCES

    [1] Bertolini, L., Elsener, B., Pedeferri, P., Redaelli, E., and Polder, R. (2013). Corrosion of Steel in Concrete: Prevention, Diagnosis, Repair. Wiley VCH.

    [2] Visser, J., Mijnsbergen, J.: Statistical Quantification of the Variables in the Limit State Functions. Brite-EuRam project BE95-1347 DuraCrete, report BE95-1347/R9, January 2000, 130 pp. ISBN 90 376 0374 2.

    [3] Van den Heede, P.; Van Belleghem, B.; De Keersmaecker, M.; Adriaens, A.; De Belie, N. Sustainability effects of including concrete cracking and healing in service life prediction for marine environments. In Proceedings of the 4th International Conference on Sustainable Construction Materials & Technologies, Las Vegas, NV, USA, 7–11 August 2016.

    [4] Oliveira, M. C. L., Antunes, R. A., & Costa, I. (2013). Effect of the NCO/OH molar ratio on the physical aging and on the electrochemical behavior of polyurethane-urea hybrid coatings. Int. J. Electrochem. Sci, 8, 4679-4689.

  • XIII

    Contents 1 Introduction ......................................................................................................................... 1

    1.1 Problem statement ....................................................................................................... 1

    1.2 Outline of the thesis ..................................................................................................... 2

    I State of the Art 3

    2 Corrosion of reinforcement steel ........................................................................................ 5

    2.1 Introduction ................................................................................................................. 5

    2.2 Carbonation-induced corrosion ................................................................................... 6

    2.3 Chloride-induced corrosion ......................................................................................... 7

    2.4 Initiation-propagation .................................................................................................. 9

    2.5 Relationship between resistivity and corrosion rate .................................................. 11

    2.6 Effect of cracks .......................................................................................................... 13

    3 Self-healing concrete ........................................................................................................ 15

    3.1 Overview ................................................................................................................... 15

    3.1.1 Intrinsic self-healing ........................................................................................... 15

    3.1.2 Capsule based healing ........................................................................................ 16

    3.2 Healing agents ........................................................................................................... 16

    3.3 Self-healing concrete based on polyurethane ............................................................ 17

    4 Corrosion monitoring ........................................................................................................ 19

    4.1 Corrosion potential .................................................................................................... 19

    4.2 Corrosion current ....................................................................................................... 21

    4.3 Linear polarisation resistance .................................................................................... 23

    4.4 Electrochemical impedance spectroscopy ................................................................. 24

    4.5 Resistivity monitoring ............................................................................................... 25

    4.5.1 Multi-ring resistivity cell .................................................................................... 26

    4.5.2 Multi-ring electrode ............................................................................................ 26

    4.5.3 Multi-electrode resistivity probe ........................................................................ 27

  • XIV

    4.5.4 Two electrode method (TEM) ............................................................................ 28

    4.6 Critical remarks ......................................................................................................... 29

    5 Modelling of results .......................................................................................................... 30

    5.1 Introduction ............................................................................................................... 30

    5.2 Probabilistic model .................................................................................................... 30

    5.2.1 Limit state function ............................................................................................ 31

    5.2.2 Attack penetration function ................................................................................ 32

    II Materials and Methods 35

    6 Materials ........................................................................................................................... 37

    6.1 Healing agents ........................................................................................................... 37

    6.2 Capsules ..................................................................................................................... 37

    6.3 Concrete ..................................................................................................................... 38

    6.4 Concrete beams .......................................................................................................... 39

    6.4.1 Reinforcement configuration .............................................................................. 39

    6.4.2 Reinforcement cages (cathode) .......................................................................... 40

    6.4.3 Central reinforcement bar (anode) ..................................................................... 41

    6.4.4 Capsule location ................................................................................................. 41

    6.4.5 Internal reference electrode ................................................................................ 42

    6.4.6 Concrete beam configuration ............................................................................. 42

    6.5 Concrete cylinders specimens ................................................................................... 45

    7 Methods ............................................................................................................................. 46

    7.1 Crack formation of concrete beams ........................................................................... 46

    7.2 Crack width measurements ........................................................................................ 47

    7.3 Corrosion measurements ........................................................................................... 48

    7.3.1 Current monitoring ............................................................................................. 49

    7.3.2 Corrosion potential measurements ..................................................................... 49

    7.3.3 Open circuit potential measurements ................................................................. 49

    7.3.4 IR drop ................................................................................................................ 50

    7.3.5 Linear polarisation resistance ............................................................................. 50

    7.3.6 Electrochemical impedance spectroscopy measurements .................................. 52

  • XV

    7.4 Resistivity measurements .......................................................................................... 53

    7.4.1 Bulk resistivity measurements ........................................................................... 54

    7.5 Probabilistic modelling propagation period .............................................................. 55

    7.5.1 Introduction ........................................................................................................ 55

    7.5.2 DuraCrete model ................................................................................................ 56

    7.5.3 Improved model ................................................................................................. 60

    7.5.4 Estimation propagation period ........................................................................... 62

    III Results 63

    8 Corrosion monitoring ........................................................................................................ 65

    8.1 Crack width................................................................................................................ 65

    8.2 IR drop ....................................................................................................................... 66

    8.3 Uncracked specimens (UNCR) ................................................................................. 66

    8.4 Cracked specimens (CR) ........................................................................................... 68

    8.5 Healed specimens (PU HV caps and PU LV caps/man) ........................................... 70

    8.6 Specimens with encapsulated water repellent agent (WRA) ..................................... 72

    8.7 Specimens with a coated rebar (COAT) .................................................................... 74

    8.8 Cathodic polarisation resistance ................................................................................ 75

    8.9 Electrochemical impedance spectroscopy ................................................................. 77

    8.10 Measured current vs. current from corrosion monitoring .......................................... 78

    8.11 Overall conclusion ..................................................................................................... 80

    9 Bulk resistivity .................................................................................................................. 83

    9.1 Influence of the wet/dry cycle ................................................................................... 83

    9.2 Influence of the crack ................................................................................................ 83

    9.3 Aging factor ............................................................................................................... 84

    10 Estimation propagation period ...................................................................................... 86

    10.1 DuraCrete model ........................................................................................................ 86

    10.2 Improved DuraCrete model ....................................................................................... 88

    10.2.1 Implementing the crack width ............................................................................ 88

    10.2.2 Aging factor ........................................................................................................ 89

    10.2.3 Wetness period ................................................................................................... 89

  • XVI

    10.2.4 Propagation period ............................................................................................. 89

    10.2.5 Testing the results of De Maesschalck (2016) ................................................... 93

    10.3 Comparison DuraCrete model and model 1 and 2 .................................................... 94

    10.4 Conclusions ............................................................................................................... 96

    IV Conclusions 97

    11 Conclusions and future research ................................................................................... 99

    11.1 Conclusions ............................................................................................................... 99

    11.2 Future research ........................................................................................................ 100

  • XVII

    List of Figures Figure 2-1: Schematic illustration of steel reinforcement corrosion in concrete (Ahmad, 2003)

    ................................................................................................................................ 6

    Figure 2-2: Three different forms of chlorides in concrete (Tuutti, 1982) ................................ 8

    Figure 2-3: Damage-time diagram for structures suffering from reinforcement corrosion (Osterminski and Schießl, 2012) .......................................................................... 10

    Figure 2-4: Preferential path for aggressive substances (Bertolini et al., 2013) ...................... 13

    Figure 4-1: Principle corrosion potential measurements (Elsener et al., 2003) ....................... 20

    Figure 4-2: Macro-cell current between non-corroding and corroding zones and micro-cell current within corroding zone (Andrade et al., 2008) .......................................... 21

    Figure 4-3: Electrical equivalent circuit diagram for macro-cell corrosion model (Beck et al, 2012) ..................................................................................................................... 22

    Figure 4-4: Nyquist diagram and its equivalent circuit (Ribeiro et al., 2015) ......................... 24

    Figure 4-5: Multi-ring resistivity cell and its potential difference measurements (Du Plooy 2013) ..................................................................................................................... 26

    Figure 4-6: Schematic presentation of the MRE (Schießl and Breit, 1995) ............................ 27

    Figure 4-7: Wenner configuration for four-point surface resistivity measurements (Du Plooy 2013) ..................................................................................................................... 28

    Figure 4-8: Principle of the two-electrode method (Gehlen et al., 1998) ................................ 29

    Figure 5-1: Residual reinforcement bar section (Vidal et al., 2004) ........................................ 33

    Figure 6-1: Injected and sealed borosilicate glass capsules ..................................................... 38

    Figure 6-2: Glass fibre reinforcement in the compressive zone ............................................... 39

    Figure 6-3: Technical drawings of the reinforcement scheme (redrafted after De Maesschalck (2016)) .................................................................................................................. 40

    Figure 6-4: Electrical connection between both reinforcement cages ..................................... 41

    Figure 6-5: Location two layers of capsules (dimensions in mm) ........................................... 42

    Figure 6-6: Location and connection internal reference electrode ........................................... 42

    Figure 6-7: Concrete beam configuration just before casting .................................................. 43

    Figure 6-8: External reference electrode used for monitoring extra beams ............................. 44

    Figure 6-9: Epoxy coating on anodic rebars ............................................................................ 44

  • XVIII

    Figure 6-10: Used concrete cylinders ....................................................................................... 45

    Figure 7-1: Three-point bending test frame ............................................................................. 46

    Figure 7-2: Additional cracks taped with aluminium butyltape ............................................... 47

    Figure 7-3: Leica S8 APO microscope with DF 295 camera ................................................... 47

    Figure 7-4: Three compartments used for wet/dry cycles ........................................................ 48

    Figure 7-5: Schematic representation of the connections during corrosion potential measurements ....................................................................................................... 49

    Figure 7-6: Corrosion potential curve with IR drop ................................................................. 50

    Figure 7-7: Schematic representation of the connections during LPR measurements ............. 51

    Figure 7-8: Counter electrode cathodic linear polarisation measurements .............................. 51

    Figure 7-9: LPR measurement: curve with tangent at zero current density ............................. 52

    Figure 7-10: EIS measurement: Nyquist plot ........................................................................... 53

    Figure 7-11: 6 concrete cylinders subjected to wet/dry cycles ................................................ 54

    Figure 7-12: TEM resistivity measurement set-up ................................................................... 55

    Figure 8-1: Crack width at the start of the experiment ............................................................ 66

    Figure 8-2: Corrosion monitoring for the UNCR specimens ................................................... 67

    Figure 8-3: Corrosion monitoring for the CR specimens ......................................................... 69

    Figure 8-4: Corrosion monitoring for the healed specimens .................................................... 71

    Figure 8-5: Corrosion monitoring for the WRA specimens ..................................................... 73

    Figure 8-6: Corrosion monitoring for the COAT specimens ................................................... 75

    Figure 8-7: Cathodic polarisation resistance of the COAT and WRA specimens ................... 76

    Figure 8-8: Cathodic polarisation resistance of the CR specimens .......................................... 77

    Figure 8-9: Concrete resistance development during experiment ............................................ 78

    Figure 8-10: Concrete resistance development COAT specimens during experiment ............ 78

    Figure 8-11: Corrosion current comparison of CR, PU HV and COAT .................................. 79

    Figure 8-12: Corrosion current comparison of UNCR, PU LV and WRA .............................. 79

    Figure 9-1: Bulk resistivity; top left: NaCl, top right: water, bottom left: dry ......................... 84

    Figure 9-2: Outline of the increasing trend in concrete resistivity ........................................... 85

    Figure 10-1: Reliability index for the WRA samples according to DuraCrete model ............. 87

    Figure 10-2: Probability of failure for the WRA samples according to DuraCrete model ...... 87

    Figure 10-3: Comparison between the propagation period [yr] of Model 1 and 2 .................. 92

  • XIX

    List of Tables Table 2-1: Corrosion risk from resistivity (Song and Saraswathy, 2007) ................................ 13

    Table 4-1: Corrosion condition related with Ecorr values (Song and Saraswathy, 2007) ...... 20

    Table 5-1: Corrosion rate and wetness period for different exposure classes (Andrade and Arteaga, 1998) ...................................................................................................... 33

    Table 6-1: Concrete mix ........................................................................................................... 38

    Table 6-2: Overview of concrete beams used in corrosion tests .............................................. 44

    Table 7-1: Input parameters LPR measurements ..................................................................... 52

    Table 7-2: Input parameters EIS measurements ...................................................................... 53

    Table 7-3: Independent parameters with distribution .............................................................. 59

    Table 7-4: Dependent parameters with distribution ................................................................. 59

    Table 7-5: Approximations of parameters in Comrel .............................................................. 60

    Table 8-1: Hardened concrete properties at 28 days ................................................................ 65

    Table 8-2: Self-healing efficiency concrete beams according to corrosion monitoring .......... 81

    Table 8-3: Self-healing efficiency concrete beams according to current monitoring .............. 81

    Table 9-1: Aging factor of the different specimens ................................................................. 85

    Table 10-1: Initial resistivity value .......................................................................................... 86

    Table 10-2: Propagation period DuraCrete model ................................................................... 87

    Table 10-3: Initial crack width used for the improved DuraCrete model ................................ 89

    Table 10-4: Period of no measurable corrosion ....................................................................... 90

    Table 10-5: Other variable parameters improved DuraCrete model ........................................ 91

    Table 10-6: Propagation period model 1a and 1b .................................................................... 91

    Table 10-7: Self-healing efficiency concrete beams according to propagation period ............ 93

    Table 10-8: Initial resistivity value De Maesschalck (2016) ................................................... 93

    Table 10-9: Other variable parameters De Maesschalck (2016) .............................................. 94

    Table 10-10: Propagation period specimens De Maesschalck (2016) ..................................... 94

    Table 10-11: Self-healing efficiency beams De Maesschalck (2016) according to propagation period .................................................................................................................... 94

    Table 10-12: Comparison corrosion rate DuraCrete vs model 1 and 2 .................................... 95

  • XX

    List of abbreviations Symbol Description Unit 𝛼 Pitting factor [−] 𝑎 Wenner probe spacing [𝑐𝑚] 𝑎 Atomic weight for mole iron [𝑔] 𝑎1, 𝑎2, 𝑎3 Regression parameters [−] A Relation constant between corrosion rate and resistivity [−] 𝐴𝐴 Anodic area [𝑚²] 𝐴𝐶 Cathodic area [𝑚²] 𝐴𝑔 Silver 𝐴𝑔𝐶𝑙 Silver chloride 𝑏 Parameter depending on rebar position, Chapter 5 [−] 𝐶𝑠 Total chloride content at the surface [% 𝑏𝑦 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑒𝑚𝑒𝑛𝑡] 𝐶𝑥 Total chloride content at a certain depth x [% 𝑏𝑦 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑒𝑚𝑒𝑛𝑡] 𝐶𝐴 cyanoacrylate 𝐶𝑎2+ Calcium ion 𝐶𝑎𝐶𝑙2 Calcium chloride 𝐶𝑎(𝑁𝑂2)2 Calcium nitrite 𝐶𝑎𝐶𝑂3 Calcium carbonate 𝐶𝑎𝑂 Calcium oxide 𝐶𝑎(𝑂𝐻)2 Calcium hydroxide 𝐶𝑐𝑟𝑖𝑡 Critical chloride content or threshold value [𝑚% 𝑏𝑦 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑒𝑚𝑒𝑛𝑡] 𝐶𝐸 Counter electrode 𝐶𝑙− Chloride ion 𝐶𝑂2 Carbon dioxide 𝐶𝑆𝐻 Calcium silicate hydrate 𝐶𝑢𝑆𝑂4 Copper sulphate 𝑑 Diameter of the rebar [𝑚𝑚] 𝐷𝑎𝑝𝑝 Apparent diffusion coefficient for chloride [𝑚²/𝑠²] Δ𝐸 Driven voltage of a corrosion cell [𝑚𝑉] 𝑒− Electron 𝐸𝑐𝑜𝑟𝑟 Corrosion potential 𝐸𝐼𝑆 Electrochemical impedance spectroscopy 𝐸0,𝐴 Resting potential of anode [𝑚𝑉] 𝐸0,𝐶 Resting potential of cathode [𝑚𝑉] 𝑒𝑟𝑓(. ) Error function 𝐹 Faraday’s constant [𝐶/𝑚𝑜𝑙𝑒] 𝑓𝑐,𝑠𝑝 Splitting tensile strength [𝑀𝑃𝑎] 𝐹𝐶𝑙 Chloride corrosion rate factor [−] 𝐹𝑔𝑎𝑙𝑣 Galvanic effect factor [−] 𝐹02 Oxygen availability factor [−] 𝐹𝑜𝑥𝑖 Oxide factor [−] 𝐹𝑒2+ Ferrous ion 𝐹𝑒2𝑂3 Iron oxide 𝐹𝑒(𝑂𝐻)2 Ferrous hydroxide 𝐺 Geometrical factor [−]

  • XXI

    𝑔(. ) Limit state function 𝐻2𝐶𝑂3 Bicarbonate 𝐻2𝑂 Water 𝐻𝑉 High viscosity 𝐼 Current [𝐴] 𝑖𝑐𝑜𝑟𝑟 Corrosion current density [µ𝐴] 𝐼𝑐𝑜𝑟𝑟 Corrosion current [µ𝐴/𝑐𝑚²] 𝐼𝑚𝑎𝑐𝑟𝑜 Macro-cell corrosion current [µ𝐴/𝑐𝑚²] 𝐼𝑚𝑖𝑐𝑟𝑜 Micro-cell corrosion current [µ𝐴/𝑐𝑚²] 𝐾 Temperature influence factor on conductivity [1/°𝐶] 𝑘𝑐 Curing factor [−] 𝑘𝑒 Difference lab and field measurements factor, chapter 2 [−] 𝑘𝑒 Geometry constant of macro-cell [𝑚−1] 𝑘𝑒𝑛𝑣 Environmental factor [−] 𝑘𝑅,𝐶𝑙 Chlorides factor [−] 𝑘𝑅,𝑅𝐻 Relative humidity factor [−] 𝑘𝑅,𝑇 Temperature factor [−] 𝑘𝑡 Evolution in time factor [−] 𝐿𝑃𝑅 Linear polarisation resistance 𝐿𝑉 Low viscosity 𝑚0 Constant for corrosion rate and resistivity [−] 𝑀𝑀𝐴 methylmethacrylate 𝑀𝑅𝐸 Multi-ring electrode 𝑛 Aging factor, Chapter 1 [−] 𝑛 Number of exchanged electron, Chapter 4 [−] 𝑁𝑎2𝐹𝑃𝑂3 Sodium monofluorophosphate 𝑁𝑎2𝑆𝑖𝑂3 Sodium silicate 𝑂2 Oxygen 𝑂𝐶𝑃 Open circuit potential 𝑂𝐻− Hydroxyl ion 𝑃(𝑥) Attack penetration function [𝑚𝑚] 𝑃(𝑥0) Attack penetration function to induce a crack width of w0 [𝑚𝑚] 𝑝𝐻 Acidity scale [−] 𝑃𝑀𝑀𝐴 Polymethylmethacrylate PU Polyurethane 𝜙 Residual bar diameter [𝑚𝑚] 𝜙0 Initial bar diameter [𝑚𝑚] 𝜌 Resistivity [Ω𝑚] 𝜌𝑒 Electrolytic concrete resistivity [Ω𝑚] 𝜌𝑠𝑡𝑒𝑒𝑙 Density of steel [𝑔/𝑐𝑚³] 𝜌0 Resistivity at 𝑡0 [Ω𝑚] 𝑅 Resistance [Ω] 𝑅(. ) Resistance variable 𝑅𝑐𝑜𝑛𝑐 Concrete resistance [Ω] 𝑅𝑒𝑙 Electrolytic resistance [Ω] 𝑅Ω Ohmic resistance [Ω] 𝑅𝑝 Polarisation resistance [Ω] 𝑅𝑙𝑜𝑤𝑒𝑟 𝑓𝑜𝑎𝑚 Measured resistivity lower foam [𝑘Ω𝑚]

  • XXII

    𝑅𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 Measured resistivity specimen [𝑘Ω𝑚] 𝑅𝑠𝑎𝑚𝑝𝑙𝑒 Sample resistivity [𝑘Ω𝑚] 𝑅𝑠𝑎𝑚𝑝𝑙𝑒,𝑐𝑜𝑟𝑟 Corrected sample resistivity [𝑘Ω𝑚] 𝑅𝑢𝑝𝑝𝑒𝑟 𝑓𝑜𝑎𝑚 Measured resistivity upper foam [𝑘Ω𝑚] 𝑟𝑝,𝐴 Specific integral polarisation resistance of anode [Ω𝑚²] 𝑟𝑝,𝐶 Specific integral polarisation resistance of cathode [Ω𝑚²] 𝑆(. ) Load variable 𝑆𝐴𝑃 Super absorbent polymers 𝑆𝐻𝐸 Self-healing efficiency [%] 𝑆𝑖𝑂2 Silicon dioxide 𝑉𝑐𝑜𝑟𝑟 Corrosion rate [𝑚𝑚/𝑦𝑟] 𝑉𝑐𝑜𝑟𝑟,𝑎 Mean corrosion rate during active corrosion [𝑚𝑚/𝑦𝑟] 𝑤 Crack width [𝑚𝑚] 𝑤𝑐𝑟 Critical crack width [𝑚𝑚] 𝑤0 Initial crack width [𝑚𝑚] 𝑤𝑡 Wetness period [−] 𝑊 𝐵⁄ Water to binder ratio 𝑊𝐸 Working electrode 𝑊𝑅𝐴 Water repellent agent x Depth from the concrete surface, chapter 2 [𝑚] 𝑥 Cover thickness, chapter 5.2.1 [𝑚𝑚] 𝑥 Pit penetration, chapter 5.2.2 [µ𝑚] 𝑋 Penetration depth of carbonation front [𝑚𝑚] 𝑘 General parameter in the diffusion formula [−] 𝑡 Time [𝑠] 𝑡ℎ𝑦𝑑𝑟 Time for hydration [𝑦𝑟] 𝑡𝑠𝑒𝑟𝑣𝑖𝑐𝑒 𝑙𝑖𝑓𝑒 Service life time of a structure [𝑦𝑟] 𝑡𝑖𝑛𝑖𝑡𝑖𝑎𝑡𝑖𝑜𝑛 Initiation period of concrete [𝑦𝑟] 𝑡𝑝𝑟𝑜𝑝𝑎𝑔𝑎𝑡𝑖𝑜𝑛 Propagation period of concrete [𝑦𝑟] 𝑇𝐸𝑀 Two electrode method 𝑍 Complex impedance

  • 1

    1 Introduction

    1.1 Problem statement

    The formation of cracks during the lifetime of a reinforced concrete structure is inevitable. These cracks are preferential paths for aggressive substances such as chlorides in a marine environment. As soon as a critical amount of chlorides has reached the location of the rebar corrosion may occur. This is the start of the propagation period; corrosion products may form on the reinforcement inducing stresses and causing concrete cracking. Many concrete structures suffer from reinforcement corrosion leading to high costs for maintenance and/or repair. Therefore, the efficiency of healing agents and the estimation of the lifetime of a concrete structure using predictive models attracts considerable interest.

    The corrosion resistance of reinforced concrete structures can be enhanced by implementing healing mechanisms, this can be done manually of encapsulated. Since the implementation of these products is applied after crack formation, chlorides may already have penetrated the concrete through these cracks. The effect of the healing agents on the regain in corrosion resistance after cracking which can cause an extended service lifetime, is of important value. Prediction of the propagation period can be performed using the available DuraCrete model. However, this model has some shortcomings, firstly it is only valid for uncracked concrete which is rarely observed in practice. Secondly, the tabulated parameters in the DuraCrete guidelines are limited and are not applicable in every specific case. Finally, the corrosion process is simplified to a relationship between the steel corrosion rate and the concrete resistivity, while the concrete resistivity is not the only contributing factor to the corrosion resistance of concrete.

    In this master dissertation, the influence of healing agents on the propagation period of concrete structures was investigated. This investigation started with corrosion monitoring on cracked concrete beams with different healing mechanisms subjected to wet/dry cycles of 1/6 days to approach the effect a marine environment. Secondly bulk resistivity monitoring on concrete cylinders was performed. The results of these measurements were used as input values for different limit state models in the probabilistic software Comrel to predict the propagation period of the concrete beams.

    The aim of this master dissertation was to create an updated model based on the DuraCrete model for healed cracked concrete. Therefore, a model was created where the effect of healing was implemented in a more complex expression for the corrosion rate. The effect of a crack

  • 1: Introduction

    2

    was implemented and specific parameter values were used instead of the prescribed parameters in the DuraCrete guidelines. The output of this improved model is useful for the prediction of the total lifetime of the structure and on the other hand useful for the prove of the efficiency of different healing mechanisms in comparison with plain concrete.

    1.2 Outline of the thesis

    This master dissertation is divided in four main parts. Part I: State of the art provides the necessary information found in literature. Information about the corrosion process supported with experimental investigations is provided in Chapter 2 while Chapter 3 focusses on the self-healing of concrete found in literature. Possible experimental testing for corrosion and resistivity monitoring are summarised in Chapter 4 and available probabilistic prediction models are provided in Chapter 5.

    Part II consists of chapter 6 and 7. In these chapters, the used materials for the experiments and the experiments itself are described. Section 7.5 focusses on the new developed probabilistic models.

    In Part III: the obtained results are discussed. In Chapter 8, the corrosion monitoring of the concrete beams is discussed followed by the results of the resistivity monitoring on the concrete cylinders in Chapter 9. Chapter 10 includes the output of the probabilistic modelling together with a comparison between the DuraCrete model and the updated model.

    Finally, in Part IV, Chapter 11 is divided in an overall conclusion of this master thesis, followed by suggestions for future research.

  • 3

    Part I State of the Art

  • 4

  • 5

    2 Corrosion of reinforcement steel

    2.1 Introduction Reinforced concrete has some great advantages which makes it one of the most used building materials. The main advantages are the relatively low cost and the high compressive strength. Sometimes structures constructed in reinforced concrete must endure severe environments and their durability is put to the test when cracking occurs. Corrosion of the steel rebars leads to deterioration of the reinforced concrete.

    Corrosion occurs after chemical reaction of the steel with its environment. With the electrochemical process of corrosion, half-cell reactions take place. An oxidation reaction at the anode:

    𝐹𝑒 → 𝐹𝑒2+ + 2𝑒−

    (2.1)

    and, in case of steel in concrete, a reduction reaction at the cathode. With this reaction, oxygen is reduced and the electrons of the previous process are consumed to form 𝑂𝐻− ions:

    𝑂2 + 2𝐻2𝑂 + 4𝑒− → 4𝑂𝐻−

    (2.2)

    This process is schematically illustrated in Figure 2-1. The electrons are transferred trough the steel bar to the cathodic zone, this flow induces a nominal electrical current. The supply of oxygen and water (moisture content) is needed for the cathodic reaction to occur. Other reduction reactions are possible for corrosion of steel but expression (2.2) is the reduction reaction specific for corrosion in reinforced concrete. The 𝑂𝐻−-ions are transferred through the electrolyte which is the pore solution and can then form with the 𝐹𝑒2+-ions, ferrous hydroxide 𝐹𝑒(𝑂𝐻)2 (Ahmad, 2003).

    𝐹𝑒2+ + 2𝑂𝐻− → 𝐹𝑒(𝑂𝐻)2

    (2.3)

    The availability of oxygen and water does not guarantee that corrosion will occur. There are still two barriers. The first barrier is the concrete cover, which is the shortest distance between the reinforcement surface and the exterior. The other barrier is the passive layer. This is a layer of iron oxide (Fe2O3) formed due to high concentrations of calcium, sodium and potassium oxides in the pore structure of the cement matrix (El-Reedy, 2007). When high pH-values in

  • 2: Corrosion of reinforcement steel

    6

    concrete of above 11.5 are maintained, this layer will automatically form and leads to an initial passive state of the steel (Tian, 2013). This layer prevents the chemical reaction (2.1).

    Figure 2-1: Schematic illustration of steel reinforcement corrosion in concrete (Ahmad, 2003)

    According to Ahmad (2003), corrosion is influenced by the following factors:

    Availability of oxygen and moisture content Temperature and relative humidity Availability or ingress of aggressive substances (CO2 or Cl-) Concrete and steel quality parameters

    2.2 Carbonation-induced corrosion

    Carbonation is a process that can lead to depassivation of the reinforcing steel inside the concrete. The process starts by the ingress of carbon dioxide from the environment through the concrete pores. The diffusion of carbon dioxide in the concrete can cause carbonation reactions. A reaction can occur between dissolved CO2, which leads to 𝐻2𝐶𝑂3, and calcium hydroxide 𝐶𝑎(𝑂𝐻)2 (Thiery et al., 2007):

    𝐶𝑂2 + 𝐻2𝑂 → 𝐻2𝐶𝑂3

    (2.4)

    𝐶𝑎(𝑂𝐻)2 + 𝐻2𝐶𝑂3 → 𝐶𝑎𝐶𝑂3 + 2 ∙ 𝐻2𝑂

    (2.5)

    Another carbonation reaction is the reaction of carbon dioxide with 𝐶𝑎2+-ions in calcium silicate hydrates (C-S-H). This process is shown in equation (2.6) (Gruyaert, 2011).

    (𝐶𝑎𝑂)𝑥 ∙ (𝑆𝑖𝑂2)𝑦 ∙ (𝐻2𝑂)𝑧 + 𝑥 ∙ 𝐶𝑂2

    → 𝑥 ∙ 𝐶𝑎𝐶𝑂3 + (𝑆𝑖𝑂2)𝑦 ∙ (𝐻2𝑂)𝑧−𝑤 + 𝑤 ∙ 𝐻2𝑂 (2.6)

  • 2: Corrosion of reinforcement steel

    7

    Other differences between both carbonation reactions are not further discussed in this report.

    A carbonation process leads to a neutralisation of the alkalinity of concrete and a drop in pH-value under 9. As mentioned before, the high pH environment manages to maintain a stable passive layer, a drop in pH consequently results in a loss of stability of the passive layer (Bertolini et al., 2016). This depassivation occurs over the complete steel surface, carbonation leads to uniform corrosion.

    The carbonation process can be described with equation (2.7). This equation is a simplification by expressing the CO2-flux according to Fick’s law and assuming a linear decrease of the concentration of carbon dioxide over the carbonation depth (Visser 2012).

    𝑋 = 𝑘√𝑡

    (2.7)

    With X the penetration depth of the carbonation front in function of a constant k and the exposure time t. The factor k depends on the diffusivity, concentration and the quantity of CO2 (Tuutti, 1982).

    The carbonation rate depends on:

    Pore structure Exposure environment Humidity inside concrete

    Before the process expressed in equation (2.4) can occur, the moisture content needs to be dissolved. But diffusion occurs slower in water-filled pores than in air-filled pores. So, the humidity has a positive and negative influence on the carbonation rate (Bertolini et al., 2016). A humidity in the range of 50-80% is considered optimal for carbonation to occur. (Bertos et al., 2004)

    2.3 Chloride-induced corrosion

    Another form of corrosion is chloride-induced corrosion whereby chlorides penetrate the concrete; these chlorides are present in seawater or in de-icing salts. With chloride-induced corrosion or pitting corrosion, more localised corrosion occurs. Chlorides will attack the passive layer of the steel reinforcement leading to local damage of this film. This happens when a threshold value or critical chloride content (Ccrit) is achieved at the rebar position (Bertolini et al., 2016).

    Diffusion, capillary suction, permeation or migration are the main mechanisms of chloride penetration. For the diffusion reaction to occur, a concentration gradient needs to be present.

  • 2: Corrosion of reinforcement steel

    8

    The gradient can be expressed using Fick’s second law and the resulting equation (2.8) is often used to model chloride ingress at a time t in seconds and at a depth s in metres (Collepardi et al., 1972).

    𝐶𝑥(𝑥, 𝑡) = 𝐶𝑠 (1 − erf (𝑥

    2√𝐷𝑎𝑝𝑝 ∙ 𝑡))

    (2.8)

    With:

    - 𝐶𝑥 total chloride content [% by mass of cement or concrete] - 𝐶𝑠 chloride content at the surface - 𝐷𝑎𝑝𝑝 apparent diffusion coefficient [m²/s]

    The apparent diffusion coefficient is defined as the gradient averaged out over the whole cross section area (Lizarazo-Marriaga and Claisse, 2009).

    Capillary suction on the other hand, is the main mechanism when dried concrete encounters water. Chlorides are transported in the concrete due to the surface tension in capillary pores. This is the case in wet/dry cycle structures e.g. concrete roads with de-icing salts. Capillary suction and diffusion are the main transport mechanisms in practice and can also occur simultaneously. With permeation and migration, a hydraulic pressure and an electrical field causes the transport of chloride ions respectively.

    The chloride binding in the concrete is another important aspect to discuss, chlorides can be free, chemically bound or physically absorbed to the pore wall. This is depicted in Figure 2-2. But the corrosion process is only influenced by the free chlorides. There is an equilibrium between the different forms. When one phases increases, the other phases will increase as well (Tuutti, 1982).

    Figure 2-2: Three different forms of chlorides in concrete (Tuutti, 1982)

  • 2: Corrosion of reinforcement steel

    9

    The chloride threshold value Ccrit has two definitions. The chloride content needed for the depassivation and the content which results in an acceptable deterioration of the rebar. Both definitions correspond to different Ccrit values. The first definition (the electrochemical approach) is related to the initiation stage (see also section 2.4) and the second definition (the engineering approach) considers initiation and propagation stage. Ccrit can be achieved when chlorides penetrate the concrete or when there are chlorides already present in the concrete mix. For avoiding the latter, restrictions are made to the addition of CaCl2 in the mix design. Initially adding calcium chloride enhances the acceleration of the binding process, a critical limit for the initiation of the corrosion process lies around 2-3% CaCl2 per cement quantity (Tuutti, 1982). But mostly CaCl2 is not used anymore.

    Angst et al. (2009) made a review of experiments estimating 𝐶𝑐𝑟𝑖𝑡. Over the experiments a wide range for Ccrit are found for different test setups. For outdoor exposure conditions: 0.1 – 1.96% total 𝐶𝑙− by weight of cement, for steel embedded in a cement based material: 0.04 – 8.34% total 𝐶𝑙− by weight of cement (Angst et al., 2009).

    In the Eurocode, a value of 0.2 – 0.4 𝐶𝑙− by weight of cement for reinforced concrete and 0.1 – 0.2% 𝐶𝑙− by weight of cement for prestressed concrete are defined as chloride limits for fresh concrete (Cement, 2000).

    When comparing chloride-induced corrosion with carbonation-induced corrosion, chloride-induced corrosion is more severe and leads to higher corrosion rates. The rebar’s cross-section can be significantly reduced which influences the load-bearing capacity of the structure (Bertolini et al., 2016).

    2.4 Initiation-propagation

    The process of reinforcement corrosion starts when aggressive substances (CO2 or Cl-) penetrates the concrete. When the Ccrit, explained in section 2.3, is reached or when the carbonation front reaches the rebar, the initiation period ends and the propagation period starts (this is depassivation of the reinforcement). Some failure events corresponding with too high deterioration of the rebar are listed by Beck et al. (2012) in Figure 2-3. When one of these events occurs, the propagation period ends.

  • 2: Corrosion of reinforcement steel

    10

    Figure 2-3: Damage-time diagram for structures suffering from reinforcement corrosion (Osterminski and Schießl, 2012)

    Using Figure 2-3, Tian (2013) stated that the lifetime of a structure is defined as follows:

    𝑡𝑠𝑒𝑟𝑣𝑖𝑐𝑒 𝑙𝑖𝑓𝑒 = 𝑡𝑖𝑛𝑖𝑡𝑖𝑎𝑡𝑖𝑜𝑛 + 𝑡𝑝𝑟𝑜𝑝𝑎𝑔𝑎𝑡𝑖𝑜𝑛

    (2.9)

    Herein is tinitiation the time needed for the depassivation of the steel reinforcement while tpropagation defines the time needed for unacceptable deterioration of the rebar. This deterioration can be the loss of steel cross sectional area, cracking or spalling of concrete and the loss of bond strength between the rebar and the concrete due to corrosion or spalling.

    During the initiation phase, the corrosion rate is low but not equal to zero while the corrosion rate is not constant during the propagation phase. The propagation time is highly dependent on the corrosion rate which is influenced by following factors (Tuutti, 1982).

    The moisture content of the concrete The temperature around the steel reinforcement The pore solution around the steel reinforcement The porosity of the concrete The concrete cover thickness Environmental factors

    For service life prediction of reinforced concrete structures, most of the time the initiation period has been investigated but when neglecting the propagation period, the service life will be underestimated. The underestimation is also dependent on type or corrosion that occurs. For

  • 2: Corrosion of reinforcement steel

    11

    carbonation-induced corrosion a higher underestimation will be found than for chloride-induced corrosion (Van den Heede et al., 2015 and Callens, 2015).

    2.5 Relationship between resistivity and corrosion rate

    Several studies reveal that there is a relation between the resistivity of concrete and the corrosion rate. A long period study about this relationship was done by TNO (‘Netherlands Organisation for Applied Scientific Research’).

    Bertolini and Polder (1997) investigated resistivity measured with the two and four-point Wenner probe method, and concluded that it is inversely proportional with the corrosion rate.

    Both are influenced by:

    Temperature Carbonation Chloride concentration inside the concrete Moisture content

    Valente et al. (1991) found an expression for the resistivity of concrete specimens:

    𝜌𝑐𝑜𝑛𝑐𝑟𝑒𝑡𝑒 = 2𝜋 ∙ 𝑎 ∙ 0.8 ∙ 𝑅

    (2.10)

    With R, the measured resistance obtained from the Wenner method and ‘a’ the electrode spacing. 0.8 is a correction coefficient. They also found a relationship between corrosion rate (Vcorr) and resistivity (𝜌).

    𝑉𝑐𝑜𝑟𝑟 =𝐴𝜌𝑏

    (2.11)

    Herein is A the constant for the geometrical and electrochemical parameters and the power ‘b’ assumed to be equal to 2. If the logarithm of the left and right hand side is applied, a straight line shows the relationship between resistivity and Vcorr. In their test, following expression was found for concrete with ordinary Portland cement:

    𝑉𝑐𝑜𝑟𝑟 = −1.94𝜌 + 5.78

    (2.12)

    Other relationships between both were found by Andrade and Arteaga, (1998), they came up with following empirical expression:

  • 2: Corrosion of reinforcement steel

    12

    𝑉𝑐𝑜𝑟𝑟 =𝑚0

    (𝜌0 ∙ 𝑘𝑡 ∙ 𝑘𝑒𝑛𝑣 ∙ 𝑘𝑒)∙ 𝐹𝐶𝑙 ∙ 𝐹𝑔𝑎𝑙𝑣 ∙ 𝐹𝑜𝑥𝑖 ∙ 𝐹𝑜2

    (2.13)

    With:

    - m0 Constant for Vcorr and 𝜌 - 𝐹𝐶𝑙 Chloride corrosion rate factor - 𝐹𝑔𝑎𝑙𝑣 galvanic effect factor - 𝐹𝑜𝑥𝑖 oxide factor - 𝐹𝑜2 oxygen availability factor - 𝜌0 measured resistivity - 𝑘𝑡 evolution in time factor - 𝑘𝑒𝑛𝑣 environmental influences factor - 𝑘𝑒 difference between laboratory and field measurements factor

    Gehlen and Nilsson defined and alternative expression for the corrosion rate (Visser and Mijnsbergen, 2000):

    𝑉𝑐𝑜𝑟𝑟 =𝑚0𝜌

    ∙ 𝐹𝐶𝑙 ∙ 𝐹𝑔𝑎𝑙𝑣 ∙ 𝐹𝑜2

    (2.14)

    Herein another definition for the resistivity is used to include the effect of the concrete properties.

    𝜌 = 𝜌0 ∙ (𝑡ℎ𝑦𝑑𝑟

    𝑡0)

    𝑛∙ 𝑘𝑡 ∙ 𝑘𝑐 ∙ 𝑘𝑅,𝑇 ∙ 𝑘𝑅,𝑅𝐻 ∙ 𝑘𝑅,𝐶𝑙

    (2.15)

    The electrolytic resistivity is corrected with factors for aging ((𝑡ℎ𝑦𝑑𝑟𝑡0

    )𝑛

    ), test method (𝑘𝑡),

    curing (𝑘𝑐) and environmental conditions (𝑘𝑅,𝑇, 𝑘𝑅,𝑅𝐻, 𝑘𝑅,𝐶𝑙). The included environmental conditions are the temperature T, the relative humidity RH and chlorides Cl. In both expressions for the corrosion rate, all the main influencing factors are implemented (Visser and Mijnsbergen, 2000).

    In Table 2-1 some values are shown for the resistivity with their corresponding corrosion risk.

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    13

    Table 2-1: Corrosion risk from resistivity (Song and Saraswathy, 2007)

    Resistivity [𝛺𝑐𝑚] Corrosion risk Greater than 20,000 Negligible

    10,000 to 20,000 Low 5,000 to 10,000 High Less than 5,000 Very high

    2.6 Effect of cracks

    When no cracks are present in concrete structures, the initiation and propagation phase are influenced by the corrosion resistance of the rebars, the concrete cover and the permeability of the concrete surface. When cracks are present in concrete, preferential paths for corrosion inducing substances (water, oxygen, carbon dioxide and chlorides) are created to penetrate the concrete (Figure 2-4). The contribution of the concrete resistivity to the corrosion resistance will decrease and the corrosion resistance of the rebars will have more influence on the initiation and propagation time (Otieno et al., 2010).

    Figure 2-4: Preferential path for aggressive substances (Bertolini et al., 2013)

    Macrocracks in concrete structures occur due to the low tensile strength of concrete. Also, microcracks are present due to bleeding, shrinkage or temperature gradients in the concrete (Lim et al., 2000). The direction of different cracks can also be different, bending cracks are mostly parallel to the rebar while cracks due to shrinkage can run parallel to the rebars (Bertolini et al., 2013). When performing chloride tests of cracked concrete specimens, high ion concentrations are found in the so-called influence zone of a crack. Suzuki et al. (1990) did experiments by manually placing cracks in the concrete specimens. From the results, it was stated that corrosion at the rebar occurs only at the location where they manually placed cracks. It is also known that the penetration of chlorides through cracks is faster than carbonation (Tuutti, 1982).

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    14

    Another possible mechanism after the initiation period is repassivation of the steel bars. An oxide layer is again formed on the rebar. This process is possible when the crack is sealed due to formation of corrosion products or by adding healing agents. Also, the pH can increase due to alkalinity migration of surrounding concrete which will re-stabilise the passive layer (Bertolini et al., 2013).

    Thus, cracks lead to durability problems. In many experiments the same results were found that corrosion initiation occurs faster when cracks are present. Audenaert et al (2009) investigated crack widths between 0.02 mm and 0.2 mm and pointed out that the maximum penetration depth increases for increasing width. For smaller cracks the influence zone is less than for the largest cracks of 0.2 mm. Schießl and Raupach (1997) observed in specimens with a crack width of 0.1 – 0.5 mm that the corrosion initiation was earlier in wider cracks but reinforcement corrosion in the long-term was independent of crack width. Results of Jacobsen et al. (1998) confirm that the corrosion process is independent of the crack width on long-term situation. So, this concludes that cracks may decrease the initiation time but has less effect on the propagation time. More important influencing factors for the latter are the concrete cover and concrete quality.

    The effects of concrete quality were checked by Otieno et al. (2010). Their study started with the confirmation that the corrosion rate increases with increasing crack width (no cracks, incipient cracks and cracks of 0.4 and 0.7 mm were tested). The mean corrosion rate exceeded 0.1 µA/cm² for the specimens with crack width of 0.4 and 0.7 mm. A corrosion rate of 0.1 µA/cm² is often defined in literature as the threshold value for active corrosion (Alonso et al., 1999). Otieno et al. (2010) illustrated that the increase in corrosion rate with increasing crack width was smaller (40%) in Corex slag specimens than in OPC specimens (210%). For both binder type, the corrosion rate decreased with decreasing water to binder ratio. To conclude, corrosion in cracked structures can be controlled by changing the concrete quality.

    Many service life models for concrete structures have been created over the years but most of them are for uncracked structures (e.g. DuraCrete models) which is rarely observed in practice. For cracked structure, the initiation can be almost immediately so predicting the service life only in function of the corrosion rate for cracked structures is not reliable. A better approach is to implement factors which take into account the influence of cracking.

  • 15

    3 Self-healing concrete Corrosion of concrete structures as described in section 2 leads to deterioration and decreases the lifetime of the structure. A preferential pathway for aggressive substances is created when cracking occurs in concrete structures. This accelerates the corrosion process. Repair is an expensive process and is difficult when cracks are not visible or accessible. Therefore, self-healing concrete is an interesting alternative to extend the service life of concrete structures.

    3.1 Overview Van Tittelboom and De Belie (2013) divided self-healing concrete into three groups:

    Intrinsic self-healing Capsule based healing Vascular healing

    The intrinsic self-healing mechanisms occurs in all concrete structures. Capsule based healing is the technique that will be used in this master dissertation so both mechanisms will be explained in detail in section 3.1.1 and 3.1.2. Vascular healing is a system where healing agents are implemented in hollow tubes in the concrete, this mechanism is not explained in detail.

    3.1.1 Intrinsic self-healing With intrinsic self-healing, the composition of the cementitious matrix gives the self-healing properties to the concrete. This is also called autogenous healing.

    Autogenous healing is based on four mechanisms. Hydration of unreacted cement particles is the major mechanism in young concrete. In a later phase, dissolution and carbonation of Ca(OH)2 becomes the main mechanism. The other two mechanisms are related to blocking of the crack. This is done by swelling of the matrix or due to impurities in the water or loose concrete particles. Autogenous healing can be improved by adding water, by restricting the crack width or by improving the possibility of ongoing hydration and crystallisation (Van Tittelboom and De Belie and Wu et al., 2013, 2012).

    The supply of water can be enhanced by mixing in super absorbent polymers (SAP’s) into the concrete. These polymers can absorb a lot of water and swell at the same time. This water is used to enhance the hydration and carbonation reaction to heal and close the crack (Snoeck et al., 2014). The improvement of autogenous healing by restricting the crack width can be done by adding fibres to reinforce the concrete (Snoeck et al., 2014). Replacing part of the cement by fly ash or blast furnace will improve the ongoing hydration since a lot of these binders remain

  • 3: Self-healing concrete

    16

    unhydrated. Other possibilities are adding expansive additives, chemical agents or bacteria and nutrients to enhance the precipitation of CaCO3 crystals at the crack (Van Tittelboom and De Belie, 2013).

    3.1.2 Capsule based healing With capsule based healing, capsules with healing agents are placed in the concrete. The capsules can be made of glass, polypropylene or a gelatine shell can be used to implement healing agents in the concrete. The principle is that the healing agent is exposed after crack and fracture of the capsules. With this release, the agent reacts and create a barrier for further ingress of aggressive substances. This reaction can be due to heating or by contacting; water (mostly moisture in concrete itself), air, cementitious matrix or another component of the healing agent (Van Tittelboom and De Belie, 2013).

    Capsules should only release the healing agent when cracks occur in the concrete structure. Therefore, capsules should not break during mixing. An alternative way is to place brittle capsules beforehand into the mould which also results in an additional cost for the capsules and the additional handling (Van Belleghem et al., 2016). By placing them manually, the capsules can be implemented at the locations where cracks are expected which cannot be done by mixing capsules into the concrete. They can also be placed in a certain direction, so that they are perpendicular to the crack and can break easily.

    3.2 Healing agents The main purpose of healing agents used in self-healing concrete is to close the crack and create a barrier for further ingress of aggressive substances. Mostly used healing agents are listed here (Van Tittelboom and De Belie, 2013):

    Cyanoacrylate (CA) Epoxy Methylmethacrylate (MMA) Silicone Foam Polyurethane (PU) Polyacrylate Tung oil Alkali silica solution Solution of Ca(OH)2, Na2SiO3, Na2FPO3 or Ca(NO2)2 Bacterial solution

    The viscosity is an important parameter; a low viscosity may lead to leakage but an agent with a high viscosity may not be able to entirely fill the crack. Also, when a high viscosity agent is

  • 3: Self-healing concrete

    17

    used, it is possible that not everything flows out of the capsule. Curing time is another important parameter as well as the sealing ability, the mechanical properties and the stability over time. The curing time of an agent should not be too short as the purpose is to fill the entire crack. The curing time should also not be too long to minimise leakage of low viscosity agents (Van Tittelboom and De Belie, 2013).

    MMA and CA have a very low viscosity but CA cures faster than MMA. Epoxy resins are too viscous to fill all cracks. PU comes in a very wide range of viscosities; the viscosity can be high or low.

    Because PU agents are available in a wide range of viscosity and has decent properties of filling and sealing cracks by expansion (Van Tittelboom et al., 2011), these will be used in this master dissertation. The self-healing properties of the concrete will be provided with an encapsulated one component PU precursor. A PU precursor starts to react when it is exposed to water. The moisture content of the concrete can serve as water source (Van Belleghem et al., 2015 and Van den Heede et al., 2016).

    3.3 Self-healing concrete based on polyurethane In this section a summary is given about experimental tests found in literature to determine the self-healing efficiency of PU.

    Van Belleghem et al. (2016) did some testing on concrete cylinders with borosilicate glass capsules filled with a one component PU precursor with a viscosity of 6700 mPas. Standardized artificial and realistic cracks were introduced in the specimens and the efficiency of the PU was tested after 7 weeks of chloride exposure. The healed specimens had a decrease of chloride concentration of 10-30% in the top layers (0-6 mm) and a decrease up to 74% at a depth of 14-20 mm.

    Manually injected PU was tested by Maes et al. (2014). Almost full healing and sealing of the crack was observed by 83% and 67% of the specimens with a crack width of 100 µm and 300 µm respectively. Encapsulated PU was also tested with the conclusions that in 33% of the specimens no traces of PU were found in the crack after the test. But in the other healed specimens, almost no chlorides penetrated the concrete. The PU used was a two component commercially available PU healing agent, MEYCO MP 355 1K. One component had a viscosity of 600 mPas while the other had a viscosity of 70 mPas.

    Promising results were found by Van Tittelboom et al. (2011). They concluded that the expansion ability of PU, the same PU as with the experiments of Maes et al. (2014), is a development of self-healing concrete. With this advantage, healing and sealing of wider cracks is possible, the degradation rate will decrease and the service life of the concrete will increase.

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    18

    A low viscosity (200 mPas) PU was tested by Van Belleghem et al. (2016) on cylindrical concrete samples. From a depth, higher than 6 mm, a decrease of chloride concentration was noticed of 75% or higher.

  • 19

    4 Corrosion monitoring Deterioration of concrete structures due to corrosion leads to an enormous cost of restoration or reconstruction. By monitoring corrosion in an early stage, money can be saved and more information can be obtained about the changing condition of the concrete structure. For monitoring existing structures, non-destructive techniques are most desirable but non-destructive monitoring is also interesting for lab experiments.

    In this report, linear polarisation resistance, electrochemical impedance spectroscopy and open circuit potential measurements are discussed. Also, test methods for measuring the resistivity which is related to the corrosion rate are implemented in this study. Important to notice is that quantifying corrosion using one technique is not accurate, always a combination of different techniques is necessary. Combining the results from different measurement techniques can lead to a more accurate conclusion of the corrosion state of the reinforcement.

    4.1 Corrosion potential

    Corrosion potential measurements can be used to locate corroding rebars or evaluate corrosion rate after reparations, but does not provide quantitative information of the actual corrosion rate (Elsener et al., 2003). With these tests, the corrosion potential Ecorr is measured and compared against the potential of a reference electrode. Following reference electrodes are widely used:

    Copper sulphate electrode (CuSO4) Saturated calomel electrode (SCE) Silver-silver chloride electrode (Ag-AgCl)

    Due to the concrete cover, it is not possible to measure the Ecorr directly at the interface concrete/steel. Therefore, the measurements are influenced by the ohmic drop (IR-drop) in the concrete cover and the macro-cell current (Angst, 2011). The IR-drop can be reduced by using internal reference electrodes instead of external as shown in Figure 4-1.

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    Figure 4-1: Principle corrosion potential measurements (Elsener et al., 2003)

    The test set-up is illustrated in Figure 4-1, the reference electrode must be in electrolytic contact to the concrete surface. Often a wet sponge is used in between to lower the surface resistance or an internal reference electrode can be used.

    The actual potential difference between the steel and the reference electrode depends on the type of reference electrode and the corrosion condition of the steel. In Table 4-1 some values are shown for the corrosion potential with respect to a reference electrode (the saturated cupper and calomel electrode). The Ecorr value is the equilibrium potential of the metal. The cathodic reaction tends to decrease the potential with respect to the anodic reaction that tends to increase the potential. When a stable Ecorr is measured, the system has reached the steady state condition. The anodic or cathodic current when the steady state is reached is defined as the corrosion current Icorr (Instruments, 2014).

    Table 4-1: Corrosion condition related with Ecorr values (Song and Saraswathy, 2007)

    Corrosion potential values Corrosion condition (mV vs. SCE) mV vs. CSE

    < -426 < -500 Severe corrosion < -276 < -350 High ( -125 > -200 Low (10% risk of corrosion)

    The corrosion condition depicted in Table 4-1 is a rough approximation. Information only from corrosion potential measurements does not give a proper assessment of the real corrosion state of the rebars. A combination between other experiments e.g.; corrosion current measurements (section 4.2), linear polarisation resistance measurements (section 4.3) and electrochemical impedance spectroscopy (section 4.4) is needed to get a clear view about the corrosion state of the rebars.

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    21

    To conclude it can be stated that the corrosion potential method is easy in use but does not provide information about the kinetics of the corrosion process and the method cannot be used to determine the corrosion rate (Morris et al., 2002).

    4.2 Corrosion current

    The corrosion current measured between anode and cathode of the reinforcement, can also give an indication of the corrosion state of the reinforcement. Directly current monitoring is possible but an alternative is to make use of the knowledge of the macro-cell corrosion model. Figure 4-2 illustrates briefly what happens when localized corrosion occurs on a rebar in the concrete.

    With macro-cell corrosion, consumed electrons in the oxidation reaction (equation (2.1)) flow through the steel to the cathodic site. But when corrosion occurs, a pit can be formed (especially for chloride-induced corrosion). This pit can operate as a small corrosion system; this is called micro-cell corrosion. So instead of acting as a pure anode, the anodic zone on the rebar can hold cathodic zones. Thus, there exists a macro-cell current, or galvanic current, with surrounding cathodic zones and micro-cell current within the corroded zone (Andrade et al., 2008). This is illustrated in equation (4.1).

    𝐼𝑐𝑜𝑟𝑟 = 𝐼𝑚𝑖𝑐𝑟𝑜 + 𝐼𝑚𝑎𝑐𝑟𝑜

    (4.1)

    Figure 4-2: Macro-cell current between non-corroding and corroding zones and micro-cell current within corroding zone (Andrade et al., 2008)

    To get a clearer view of the different processes, Figure 4-3 is a more detailed illustration than Figure 4-2. In Figure 4-3, the concrete sample is represented as an equivalent electrical circuit with the micro-cell corrosion as a system parameter.

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    Figure 4-3: Electrical equivalent circuit diagram for macro-cell corrosion model (Beck et al, 2012)

    A driving potential ΔE is applied on the system with resistances of all the components; the cathodic resistance, the anodic resistance, the concrete resistance and the steel resistance. The resistant of steel is neglected because it is negligible relative to the other resistances. Iself is the micro-cell corrosion. Beck et al (2012) defined a formula corresponding to this circuit (equation (4.2)).

    Following parameters have a significant influence on the micro-cell corrosion:

    Type of cement W/C factor Chloride content Temperature Moisture content

    𝑖𝑐𝑜𝑟𝑟 =1

    𝐴𝐴∙ [

    𝐸0,𝐶 − 𝐸0,𝐴𝑟𝑝,𝐴𝐴𝐴

    +𝑟𝑝,𝐶𝐴𝐶

    + 𝑘𝑒 ∙ 𝜌𝑒+ 𝐼𝑚𝑖𝑐𝑟𝑜]

    (4.2)

    With:

    - 𝑖𝑐𝑜𝑟𝑟 corrosion current density [A/m²] - AA anodic area [m²] - AC cathodic area [m²] - E0,C resting potential of cathode [mV] - E0,A resting potential of anode [mV] - rp,C specific integral polarisation resistance of cathode [Ωm²] - rp,A specific integral polarisation resistance of anode [Ωm²] - ke geometry constant of macro-cell [𝑚−1] - 𝜌𝑒 resistivity of concrete [Ωm] - 𝐼𝑚𝑖𝑐𝑟�