An experimental study of cathodic protection for chloride contaminated reinforced concrete
- 406 Downloads
Cathodic protection (CP) is being increasingly used on reinforced concrete structures to protect steel reinforcing bars from corrosion in aggressive conditions. Due to the complexity of environmental conditions, the design specifications in national and international standards are still open to discussion to achieve both sufficient and efficient protection for reinforced concrete structures in engineering practices. This paper reports an experimental research to investigate the influence of chloride content on concrete resistivity, rebar corrosion rate and the performance of CP operation using different current densities. It aims to understand the correlation between the chloride content and concrete resistivity together with the CP current requirement, and to investigate the precision of the CP design criteria in standards.
KeywordsCathodic protection Concrete resistivity Reinforcement corrosion CP design criteria
The corrosion of steel reinforcements has been recognised as the major cause for the premature deterioration of reinforced concrete structures worldwide . Extensive researches on the deterioration mechanisms have concluded that the combined presence of chloride and the decrease in pH due to carbonation plays the most significant role in the corrosion of concrete reinforcements [2, 3]. So far, many technologies using chemical, mechanical, and electrochemical methods have been developed to address the problem [4, 5]. Among those, cathodic protection (CP), has been widely recognised and become the most popular technique implemented in civil engineering practices for its reliable long term protection [6, 7, 8].
Adequate protection provided by CP for the steel reinforcement in concrete depends on many factors. In addition to the steel composition and the nature of concrete components, the physical conditions, such as concrete porosity, degree of carbonation, water and chloride contents, and environmental temperature, play the important roles affecting the effectiveness of CP operation. CP arrangement and the applied current densities are all related to the above conditions [9, 10]. Additionally, the service life of the anode is another factor to be taken in consideration [11, 12]. Traditionally, titanium mesh sheet with noble metal oxides coating, such as iridium, ruthenium and cobalt, have been the most common type of anodes . Other materials, offering ease of installation and cost efficiency, have also been employed . In recent years, due to its good chemical stability, carbon fibre has been successfully used as anode material in CP implementation for concrete structures [13, 14, 15].
In general, there are two acceptable criteria in CP performance appraisal. One relates to the instant-off potential (the potential measured immediately when the CP system is switched off) of the reinforcement. The other one relates to the potential decay (depolarization) of the reinforcement [16, 17]. The specifications in national and international standards for the criteria were principally established on the empirical evaluation of the data obtained from successfully operated CP cases . For example, Takewaka  suggested that the corrosion of reinforcement in concrete structures could be stopped when the potential of the rebars was less than − 600 mV with respect to Ag/AgCl/0.5KCl reference electrode. For chloride-contaminated concrete, more negative potentials in the range of − 645 to − 705 mV with respect to Ag/AgCl/0.5KCl were reported by Shi et al. . British standard 12696:2012  specifies that the instant-off potential should be more negative than − 720 mV with respect to Ag/AgCl/0.5KCl for any concrete structures. For the depolarization criterion, the widely adopted specification is that the reinforcement potential should decay (i.e. become less negative) by at least 100 mV over a period of 4–24 h starting from an ‘instant-off’ potential [18, 21, 22].
Applying an adequate current density to ensure sufficient current across the critical areas of the protected reinforcement  but at a cost efficient energy consumption and without overprotection is vital to avoide unnecessary expenses and the potential negative effect of the hydrogen production due to the activated cathodic reactions at the rebar and concrete interface. No CP implementation can achieve an effective protection using a specified constant current density throughout the life span of concrete structures . A previous work suggested that, for newly built concrete structures, a current density in the range of 1–2 mA/m2 on the rebars is sufficient for protection, while for the structures that have already suffered from reinforcement corrosion, a current density in the range of 5–20 mA/m2 is recommended . Higher practical CP current densities in the range of 30–50 mA/m2 were also suggested when reinforcements are exposed to severe environmental conditions .
Based on the discussion above, it is noted that some uncertainty still exist on the topic of defining the current specification for CP design for reinforced concrete structures for varied and complex application conditions. As an effort to obtain more detailed specific information for the CP design for chloride contaminated reinforced concrete structures, this paper reports an experimental study on the effect of concrete chloride contamination degree on the corrosion evaluation parameters that are employed for reinforcement cathodic protection assessment. Specifically, this work investigates the correlation between the chloride content and concrete resistivity, and the relationship of these two parameters with the rebar corrosion rate. These studies enable identification of more precise characteristic relationships between concrete chloride content, the applied current density and the instant-off potential. Thus, the experimental results provide a direct guidance for the specification of the CP current density requirements for atmospherically exposed concrete structure at different levels of chloride contamination.
2 Specimens preparation
Concrete specimens used in this study were prepared following the method recommended by the British Building Research Establishment (BRE)  to give a 28 days compressive strength of 38 N/mm2. Locally produced limestone Portland cement (CEM II/A-LL in British standard BS EN 197-1: 2011) was used at 390 kg/m3. Natural sands of the maximum size of 4.75 mm and a specific gravity of 2.47 were used for the fine aggregates at 1125 kg/m3. The coarse aggregates were limestone of maximum size of 10 mm and a specific gravity of 2.49, and were used at 580 kg/m3 in proportion. Pure NaCl (0, 1, 2, 3.5, and 5% of the cement weight) was added as contaminant into the mix water, to prepare specimens with different chloride contamination contents. The concrete mixes had a water to cement ratio of 0.4.
All the prepared concrete specimens were placed in water with the same chloride concentration as that of the mix water used and cured for 28 days. Such method aims to ensure an even chloride distribution. Thereafter all concrete specimens were taken out and exposed to an atmosphere of a relative humidity of 50 ± 5% and a temperature of 20 ± 3 °C for 5 weeks, i.e. until they attained a stable weight before conducting all the experiments. To obtain the accurate total chloride contents in the specimens, another ten concrete specimens of all the same mixtures (two specimens for each designed chloride content) with the size of 100 × 100 × 100 mm3 and cured in the same conditions were analysed using potentiometric titration method described in ASTM C1152/C1152M-12 .
3 Experimental methods
3.1 Corrosion rate and concrete electrical resistivity
The corrosion rates of rebars in the reinforced concrete specimens were assessed before the implementation of CP using the linear polarization method described by Stern and Geary . The potential of the three reinforcing bars were acquired together through the soldered wires. The corrosion rate of the three rebars was measured in terms of the average current density, icorr. A small potential shift, ΔE, was applied on rebars of an open circuit potential, Ecorr. The potential shift varied from − 20 to + 20 mV [28, 29] at a scan rate of 0.125 mV/s using a computer controlled Gamry potentiostat (Model 1000E). The IR drop was automatically compensated by the programmed potentiostat.
3.2 Cathodic protection
Each test had a certain CP current density applied for 24 h and afterwards switched off for more than one day (24 h) to ensure a sufficient depolarization of the rebars. In the time, the potential of rebars was continuously recorded from the start and until 4 h after the interruption of the CP current in the time of depolarization. Based on the recorded data, the instant-off potential, and 4-h potential decay can be obtained.
4 Results and discussion
4.1 Chloride contents, corrosion rate and concrete resistivity
Chloride content, reinforcement corrosion rate and concrete resistivity
Added NaCl % cement weight
Measured total Cl− % cement weight
Concrete resistivity kΩ cm
4.2 The effect of CP operation time on instant-off potential
4.3 Effects of CP current density and chloride content on instant-off potential
4.4 4-h potential decay
The initial formation of passive layer was not taken into account, as chloride was added into the mix water to accelerate corrosion. Without considering the initial passivation, the measurement obtained in this work can only provide the guidance for the reinforcements that have already been experiencing active corrosion, such as, for example, those in chloride contaminated concrete with a low pH pore solution.
All measurements are based on the hypothesis that corrosion on the rebars is homogeneously distributed, i.e. even if the corrosion might be localized on the microscopic scale, on a macroscopic scale there are no regions where corrosion is substantially more severe compared to others. In order to work under this assumption, in preparing specimens, efforts were made to enhance even chloride distribution in concrete by (1) adding Cl into mixing water as the most conventional measure [42, 43, 44, 45, 46] but also (2) curing samples in water containing the same amount of chlorides. The examination of the rebars after CP measurement had confirmed the corrosion took place on the whole exposed rebar surfaces.
A total chloride content of 0.31% by weight of cement or 17 kΩ cm concrete electrical resisitivity may be set as a threshold for CP implementation to protect the reinforcements in Portland concrete from corrosion.
An instant-off potential of − 500 mV with respect to Ag/AgCl/0.5KCl electrode can provide adequate protection, in relation to the 100 mV depolarization criterion, for the reinforced concrete of up to 3.4% chloride contamination by weight of cement, or concrete resistivity is no less than 6.7 kΩ cm.
A clear correlation between CP current requirement and chloride content and concrete resistivity were obtained, and characterisation modelling has been suggested.
This study was funded by the Iraqi Ministry of Higher Education and Scientific Research Scholarship Program.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- 2.Kendell K (1995) A five year review of the application of cathodic protection to various industrial concrete structures in the Arabian Gulf. In: Second regional concrete durability in the Arabian Gulf, Bahrain, pp 265–280Google Scholar
- 4.Verma SK, Bhadauria SS, Akhtar S (2014) Monitoring corrosion of steel bars in reinforced concrete structures. Sci World J 2014:957904Google Scholar
- 5.Popoola A, Olorunniwo O, Ige O (2014) Corrosion resistance through the application of anti-corrosion coatings. In: Aliofkhazraei M (ed) Developments in corrosion protection. InTech, pp 241–270. https://doi.org/10.5772/57420
- 9.Aperador W, Bautista-Ruiz J, Chunga K (2015) Determination of the efficiency of cathodic protection applied to alternative concrete subjected to carbonation and chloride attack. Int J Electrochem Sci 10:7073–7082Google Scholar
- 11.Kepler JL, Darwin D, Locke CE (2000) Evaluation of corrosion protection methods for reinforced concrete highway structures, Kansas department of transportation K-tran project No. Ku-99-6, University of Kansas centre for research, Inc. Lawrence. http://www2.ku.edu/~iri/projects/corrosion/SM58.PDF
- 18.NACE SP0290 (2007) Impressed current cathodic protection of reinforcing steel in atmospherically exposed concrete structures. In: NACE International, Houston, TX, USAGoogle Scholar
- 20.Shi X, Cross JD, Ewan L, Liu Y, Fortune K (2011) Replacing thermal sprayed zinc anodes on cathodically protected steel reinforced concrete bridges. Oregon Department of Transportation Research Section and Federal Highway Administration, Washington. http://www.trb.org/BridgesOtherStructures/Blurbs/166080.aspx
- 21.BS EN ISO 12696 (2012) Cathodic protection of steel in concrete. In: British Standards InstitutionGoogle Scholar
- 22.Beamish S, El-Belbol S, Ngala V (2016) Maintenance of structural integrity using cathodic protection. Proc Inst Civ Eng Forensic Eng 169:72–80Google Scholar
- 24.Teychenné DC, Franklin RE, Erntroy HC (1997) Design of normal concrete mixes, 2nd edn. Construction Research Communications Ltd, UKGoogle Scholar
- 25.Oleiwi H, Wang Y, Xiang N, Curioni M, Augusthus-Nelson L, Chen X, Shabalin I (2018) Electrical resistivity at varied water, chloride contents and porosity—an experimental study. Construction and Building Materials, submittedGoogle Scholar
- 26.ASTM C1152/C1152M (2012) Standard test method for acid-soluble chloride in mortar and concrete. In: ASTM International, West Conshohocken, PA, USAGoogle Scholar
- 32.Zafeiropoulou T, Rakanta E, Batis G (2013) Carbonation resistance and anticorrosive properties of organic coatings for concrete structures. J Surf Eng Mater Adv Technol 3:67Google Scholar
- 34.Broomfield JP (2007) Corrosion of steel in concrete: understanding, investigation and repair, 2nd edn. Taylor & Francis, LondonGoogle Scholar
- 37.Cavalier P, Vassie P (1981) Investigation and repair of reinforcement corrosion in a bridge deck. In: Institution of civil engineers, proceedings, Pt 1Google Scholar
- 38.Langford P, Broomfield J (1987) Monitoring the corrosion of reinforcing steel. Constr Repair 1:32–36Google Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.