Investigating the effect of Interrupted Cathodic Protection on reinforced concrete structures

Loughborough University Institutional Repository Investigating the effect of Interrupted Cathodic Protection on reinforced concrete structures This item was submitted to Loughborough University’s Institutional Repository by the/an author. Citation: CHRISTODOULOU, C. ... et al, 2010. Investigating the effect of Interrupted Cathodic Protection on reinforced concrete structures. Presented at EUROCORR 2010, 13 - 17 September 2010, Moscow, Russia. Additional Information: • This is a conference paper. Metadata Record: https://dspace.lboro.ac.uk/2134/8983 Version: Accepted for publication Publisher: The European Corrosion Congress Please cite the published version. This item was submitted to Loughborough’s Institutional Repository (https://dspace.lboro.ac.uk/) by the author and is made available under the following Creative Commons Licence conditions. For the full text of this licence, please go to: http://creativecommons.org/licenses/by-nc-nd/2.5/ Investigating the effect of Interrupted Cathodic Protection on reinforced concrete structures Christian Christodoulou a, Gareth Glass a, John Webb a, Simon Austin b, Chris Goodier b a: AECOM Europe, 94-96 Newhall Street, Birmingham, B3 1PB, U.K. b: Loughborough University, Department of Civil and Building Engineering, Loughborough, U.K. e-mail to: christian.christodoulou@aecom.com Abstract Impressed Current Cathodic Protection (ICCP) has been one of the major components of the repair and maintenance strategy on many motorway structures in the U.K. It has helped to prolong the life of more than 700 structures, in a significantly sustainable manner, by reducing the need to remove chloride contaminated (but otherwise sound) concrete. This study was initiated after identifying that some of the ICCP systems were reaching the end of their design life and required a significant level of maintenance (including anode replacement) to operate in accordance with the latest Codes of Practice. In addition, there were a number of structures where the application of ICCP has been interrupted due to severe anode deterioration or vandalism. The objective of this work was to collate evidence from structures to support preliminary laboratory results that the application of ICCP to a reinforced concrete structure over a period of time can transform the environment around the reinforcement, even after the protective current has been interrupted. This experimental field study interrupted the current to ten structures which had been protected with ICCP between 5 and 16 years and corrosion rates were monitored to determine when reinforcement corrosion will initiate again. It was found that after five or more years of ICCP, the steel remained passive for at least 30 months after interrupting the protective current, despite the presence of chloride contamination representing a substantial corrosion risk. In some cases, severe anode deterioration meant that the current was interrupted at an unknown point in time prior to the initiation of the scheme. Four main conclusions are drawn regarding this approach: it can give an indication of when repairs to ICCP systems are likely to be critical; provide new evidence for the design lives attributed to systems using lower cost anodes; reduce the requirement to replace systems at the end of their functional lives; and potentially extend the interval 1 between planned maintenance of existing systems with corresponding reduction in monitoring frequency, cost and disruption. 1. Introduction Impressed Current Cathodic Protection (ICCP) is an electrochemical treatment for the arrest and prevention of corrosion. It has been widely and successfully used on reinforced concrete structures since the first application in the 1970s on a bridge deck in the USA [1]. The main protection mechanism of ICCP has been associated with a negative steel polarisation [2]. However, it is widely accepted that the application of ICCP to reinforced concrete structure transforms the environment around the reinforcement over a period of time [3, 4, 5]. The metal surface is polarised negatively, thus repelling the chlorides (Cl-); oxygen (O2) and water (H2O) are consumed and hydroxyls (OH-) are generated at the metal surface. The hydroxyls’ alkalinity will then be responsible for restoring the pH to the metal surface and inducing passivity of the metal. These are the secondary beneficial effects following the application of ICCP. ICCP is a long-term repair option with a life expectancy ranging from 15 years to more than 50 years depending on the type of anode used and the environmental and exposure conditions of the structure [6]. However, failures can occur due to deterioration of the anode, vandalism of the system or even improper material selection. Under such conditions the protective current is no longer applied and the structure might be considered at risk of corrosion. Figure 1 illustrates an example of a deteriorated ICCP system resulting in a loss of the protective current and subsequent corrosion risk. Figure 1: A typical example of a failed ICCP system at potential corrosion risk on the M6 motorway in the UK. Furthermore, a recent study by Presuel-Moreno et. al [4] on the effect of long-term cathodic polarisation in reinforced concrete columns in a marine environment 2 illustrated that corrosion will not initiate immidiately after the protective current was interrupted. The structures tested were partially submerged, with the splash zone exposed to very high chloride contamination levels, in cases up to 4.7% by weight of cement and therefore they had a significant risk of corrosion. The study concluded that given enough time the corrosion could initiate on all the reinforcement again. However, the results show that ICCP has persistent protective benefits and for that reason corrosion did not initiate right away after interruption of the protective current in an aggressive marine environment. The present work aimed to identify the existence of these persistent secondary protective effects afforded by the application of ICCP in a number of field structures. The Midland Links Motorway Viaducts (incorporating parts of the M5, M6 and M42 motorways and associated trunk roads) represent the largest application of ICCP in the U.K., with over 700 reinforced concrete structures being protected by ICCP. Data has been collected from some of these in-service structures and has been compared with published laboratory data. 2. Structures Selection Figure 2 illustrates a typical arrangement of the sub-structure for the Midland Links Motorway Viaducts. Each span of the viaduct is simply supported on a reinforced concrete crossbeam. In total there are approximately 1200 crossbeams in the network and approximately 700 of them have been protected by means of ICCP over the last 20 years. Figure 2: Typical sub-structure arrangement of the UK Midland Links Motorway Viaducts 3 A number of structures were selected for the field study based on the following criteria. i. age of system; ii. residual corrosion risk; iii. accessibility; and iv. deterioration of the ICCP system. This approach aimed to have a selection of structures that would be representative of the varying conditions and systems encountered on the Midland Links, with a total of 10 structures selected. On every structure two locations were identified which would represent the highest corrosion risk based on visual inspection and chloride analysis. The 10 structures selected are shown in Table 1. They were all constructed in the period of 1966 to 1970. Samples for chloride analysis were collected to identify areas of residual risk and all the locations tested were in original un-repaired concrete. All the structures were treated for a period of time with ICCP between 19 and 10 years and the anode system for all the structures comprised an impressed current conductive coating. Table 1: Details of the selected structures Structur e Referenc e Year of Installati on A1 No of test locations Locations with Clgreater than 0.4% Comment s 1991 Location s with Clgreater than 1% 2 4 4 24/7 data logger A2 1995 2 5 3 A3 1995 2 5 5 B1 1996 3 6 4 B2 1998 1 5 4 - B3 1998 2 5 3 - B4 1998 2 5 3 - C1 1999 0 5 2 - C2 2002 0 5 1 - C3 2000 0 5 1 - 4 24/7 data logger - 3. Assessment Methodology The following testing regimes were employed in order to assess residual corrosion activity: a) corrosion potential measurements, undertaken monthly and in some cases continuously; b) polarisation resistance determination of corrosion rates, undertaken monthly to calculate corrosion rates; and c) impedance measurement of corrosion rates initiated after 6 months. Correlating the off steel potentials with a corrosion risk probability is a well established technique [7, 8, 9]. In general, more positive measurements with a flat trend over time indicate that there is small corrosion risk whereas values with a negative rend over time indicate a residual corrosion risk [10]. Corrosion rates are usually expressed as a current density, a rate of weight loss or a rate of section loss. A corrosion rate of 1 mA/m2 when expressed as a current density is approximately equal to a steel weight loss of 10g/m2/year or a steel section loss of 1 m/year. Higher corrosion rates are considered to be significant and in cases where there is easy access of oxygen (i.e. non-saturated with water) then average corrosion rates can reach values up to 100 mA/m2 [11]. The calculation of corrosion rates through the polarisation resistance method is an established technique and its feasibility has been demonstrated in numerous occasions [12, 13, 14]. Impedance is an alternative technique to calculate corrosion rates and was added to the testing regime during this project to provide additional data. Impedance testing differs from polarisation resistance testing in the form of the perturbation applied and the subsequent data analysis. A current pulse delivers a charge to the steel that affects the steel potential and the potential response is recorded and analysed [15]. 4. Testing Arrangement The arrangement used to assess steel passivity is illustrated in Figure 3. The main elements were the existing power supply enclosure located at ground-level, the existing ICCP enclosure at high-level, the anode segment and a new enclosure at high-level to facilitate the new connections to the system. 5 Figure 3: Schematic of the testing arrangement The full details of the testing arrangement are described elsewhere [16]. 5. Results This section describes the findings obtained from the monthly monitoring of the structures over a period of 29 months and discusses in detail the findings. 5.1 Chloride Content Samples for chloride analysis were collected at the start of the project. All the locations were in the parent concrete not previously repaired, in order to identify residual corrosion risk. With reference to Table 1 it can be observed that all the structures under investigation in the present study had high levels of residual chlorides posing a corrosion risk following the interruption of the protective current. 6 5.2 Steel Potentials Figure 4 illustrates the most negative steel potentials for all 10 structures monitored over a period of 29 months. It can be observed that values have generally been stable and at most cases towards positive values. All these observations indicate a low probability of corrosion. Figure 4: Most negative steel potentials from the 10 structures over a period of 29 months 5.3 Polarisation Resistance Testing Manual polarisation resistance testing was also undertaken monthly for every structure. Figure 5 provides a summary of the corrosion rates calculated based on the manual polarisation resistance testing. The level of 2mA/m2 is generally considered the appropriate threshold as at higher values corrosion activity can progress very quickly. It can be observed that over 29 months corrosion rates have remained at most cases considerably lower than the threshold level. This behaviour reinforces the view that impressed current cathodic protection will have persistent protective effects despite of 29 months of no protection delivered to the structures. 7 Corrosion rates summary 2.00 Threshold 1.80 1.60 1.40 ) 2 m / 1.20 A m ( e t 1.00 a r n o i s 0.80 ro r o C 0.60 A1 A2 A3 B1 B2 B3 B4 C1 C2 C3 0.40 0.20 0.00 Oct-07 Jan-08 Apr-08 Jul-08 Oct-08 Jan-09 Apr-09 Jul-09 Oct-09 Jan-10 Apr-10 Time (date) Figure 5: Corrosion rates summary from polarisation resistance testing over a period of 29 months 5.4 Impedance Testing For impedance testing, a short pulse is applied to the structure and the potential decay is then recorded. The potential transient and the pulse can then be transformed into impedance data by means of Laplace transformations [15]. By comparing published data (Figure 6) for non-corroding specimens with the impedance analysis obtained from structure C3 (Figure 7), it can be observed that a similar behaviour has been recorded. Figure 6: Published impedance data illustrating actively passive and actively corroding reinforcement [15] 8 Figure 7: Potential transient for impedance analysis obtained from structure C3 [16] 6. Discussion At the start of the study all the structures were assessed for their corrosion risk. It was found that structures A2 and B1 were the two at most risk due to the impressed current anode deterioration. This meant that the protective current provided by a typical Impressed Current Cathodic Protection system has been interrupted at an unknown point in time, prior to the start of this study. Chloride sampling results showed that these two structures had more than 40% of their test locations with chlorides greater than 1% by weight of cement and about 60% 66% of their test locations with more than 0.4% chlorides by weight of cement at the depth of the steel. With regards to the zone layout of the different ICCP systems it was observed that older systems had one zone covering the entire surface of the structure whereas newer systems had multiple zones. However, no difference in the performance of the ICCP systems was observed due to the difference in the zone layout. At all cases the steel had been rendered passive by the ICCP system. With reference to the steel potentials and the corrosion rates from polarisation resistance testing over a period of 29 months, the data suggests that there is no significant corrosion activity on the structures. More specifically it can be observed that a poorly performing system, as illustrated by Figure 1, it had been capable of inducing and maintaining steel passivity. Chloride attack tends to be localised and the passive oxide film breakdown tends to follow the model of pitting corrosion followed by pit growth [17]. In order to achieve a growth of the corroding pits, pit nucleation must be followed by a fall in the local pH and increase in the chloride content at the pitting site. This reduction in pH will break 9 up the passive oxide film protecting the reinforcement. This together with the presence of chloride ions will promote the dissolution of iron and production of hydrochloric acid [17]. This is also commonly called acidification of the metal– concrete interface. Based on this model, corrosion is dependent on the acidification. This also explains why corrosion activity will initiate at different rates in different environments. As a result, a reservoir of inhibitive hydroxide ions, which may be present in some solids, affects the corrosion process. Solid phase inhibitors release hydroxide ions to inhibit corrosion damage [18]. Other factors such as pore solution, moisture, and temperature and oxygen depletion will affect the mechanism; however the reservoir of hydroxide ions is the dominant factor. The results of the field study presented here confirmed the suggestion that long term-application of ICCP has a persistent protective effect. The protective current has been interrupted for 29 months and the off steel potentials have shifted towards more positive values and have remained passive, despite the fact that some of the beams experienced unplanned interruptions of the ICCP system. Furthermore, all the structures investigated had a substantial corrosion risk as there were several locations with chloride levels higher than 1% by weight of cement. 7. Asset Management The results of this field study can help improve the asset management strategy of Maintenance Agencies. When considering the repair of old ICCP systems the passivation of the reinforcement from the previous system should be taken under consideration. Therefore, the new ICCP system needs only to be designed for corrosion prevention rather than corrosion protection. With this approach, the existing power supply could be utilised if it is still functioning as the power requirements for cathodic prevention are far lower than for cathodic protection [2]. In addition, other forms of corrosion management should be considered, such as monitoring only, concrete repairs, galvanic anodes etc. Alternatively, the failed ICCP systems can just be periodically monitored until corrosion activity becomes significant and the ICCP system can then be renewed. Overall, this approach should result in reduced refurbishment and maintenance costs of ICCP systems. In addition, the results of the study illustrate that improvements can be achieved on the design aspect of ICCP systems. The systems inspected here comprised a conductive coating anode with all of them being able to deliver a current density up to 20 mA/m2 on the concrete surface. These anodes are deemed to offer low current densities when compared with the more powerful MMO/Ti mesh anode systems that are typically used nowadays. With the steel density on the structures inspected varying between 1.4 to 2.2 times the concrete surface area it can be understood that the systems were delivering a low density protective current. However, the results of this study have illustrated that this low protective current has been sufficient to induce and sustain steel passivity. This is despite the fact that protection was interrupted 29 months ago and several structures had a significant corrosion risk with chloride contamination in excess of 10 1% by weight of cement. Furthermore, Polder et al. [19] also illustrated that only a small current will be sufficient to induce and sustain passivity In addition, the results of this study also have an impact on the monitoring needs of a typical ICCP system. It has been shown that corrosion risk will in general be low even if the protective current has been interrupted for 29 months. Therefore, a basis now exists to reduce monitoring intervals to annual. This approach can result in cost benefits for the Maintenance Agencies. Finally, the number of power supplies is a contributing factor to rising costs. Limiting the number used can therefore also contribute towards a more cost-effective ICCP design. The results obtained from this research illustrate that no apparent deference has been observed in the polarisation between single and multi-zone systems. As such the number of zones used for the ICCP system can be reduced and as such limiting the need for numerous power supplies. 8. Conclusions The site data presented here is consistent with the laboratory and other results reported earlier, indicating a persistent protective effect after the interruption of ICCP systems. More specifically we conclude the following: 1) The cathodically protected steel was found to be in a passive state in all ten of the protected structures investigated. Chloride levels never exceeded 2% at the depth of the steel, although it needs to be noted that the structures were previously patch-repaired prior to the application of the ICCP. No apparent difference in the corrosion risk of ICCP systems with different number of ICCP zones was observed. 2) The polarisation resistance, steel potential and impedance data show that ICCP protects reinforced concrete structures not only by shifting potentials to negative values (i.e. pitting potential model) but also by transforming the steelconcrete interface. 3) After 20 months with no ICCP current, all the structures investigated have remained passive including cases where 60% of the test locations exceeded 1% chloride concentration at the depth of steel. This supports previous laboratory evidence suggesting that ICCP does not only arrest ongoing corrosion but it also prevents future corrosion by increasing the chloride threshold of the structure. 4) The absence of corrosion should be taken into account when repairing old CP systems. Replacement anode systems need only to be designed for corrosion prevention rather than corrosion protection. Other forms of risk management include just having corrosion rate monitoring on its own as opposed to repair of the ICCP system. 5) A less conservative design approach could be utilised. The low grade conductive coating anode systems tested in this study have been capable of 11 inducing steel passivity in chloride contaminated concrete and these anodes were never capable of sustaining more than 20 mA/m2 of concrete surface, with steel surface area ranging between 1.4 to 2.2 times the concrete surface area. 6) Monitoring intervals can be safely reduced to annual inspections, resulting into further cost savings for the Maintenance Agencies. Power supplies can also be reduced by decreasing the number of different zones. No difference has been observed in the performance between single and multi-zone ICCP systems. Acknowledgements The authors acknowledge the support and invaluable help throughout all the stages of this work of Chris Spence (Amey), Peter Gilbert (Amey), Dr. Vitalis Ngala (Mouchel) and Sam Beamish (Mouchel). They would also like to thank the Highways Agency, AECOM and EPSRC for supporting the lead author throughout the duration of this project. References [1] Stratful R.F. 1973, Preliminary Investigations of Cathodic Protection of a Bridge Deck, Caltrans, http://www.dot.ca.gov/hq/research/researchreports/1973/bridge_deck.pdf (accessed 24th May 2010). [2] BS EN 12696:2000, Cathodic Protection of Steel in Concrete, British Standards Institution. [3] Glass G. K. and Chadwick J. R. 1994, An investigation into the mechanisms of protection afforded by a cathodic current and the implications for advances in the field of cathodic protection, Corrosion Science, 36, 12, pp. 2193 – 2209. [4] Presuel – Moreno F.J., Sagüés A. A., Kranc S. 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