Using cathodic protection to control corrosion of reinforced concrete structures

Oleiwi, HM

Abstract

Cathodic protection (CP) has been increasingly used on reinforced concrete structures to protect steel reinforcement from corrosion. However, due to the complexity of environmental conditions, the specifications in national and international standards are still open to discussion in engineering practices for their accurate suitability. To some extent, the design aspects are still based on practical experience. It implies a great deal of estimations and assumptions. The research conducted in the thesis aims to address some of these challenges.
To obtain reliable experimental results, the present study at first investigated the influence of experimental methods on the measurement of concrete electrical resistivity. It studied the effect of alternative current (AC) frequency, electrode materials and electrode configuration. Based on the results, an optimised method was decided for all the series of the experimental tests in this study.


The CP study consists of two major works. The first one was to investigate the chloride contaminated concrete exposed to atmospheric condition. Impressed constant current method was adopted for the operation of CP. A series of electrical and electrochemical measurements were conducted for concrete resistivity, corrosion potential, corrosion rate, degree of polarization, instant-off potential and four-hour potential decay. An evaluation on the current adopted criterion in standards has been carried out on the experimental results.


The second work was to investigate the corrosion of rebar in concrete specimens submerged (fully and partially) in salty water. For such more corrosive environment, a comparison between the impressed CP operation using constant current and that using constant potential has been conducted. The experiments evaluated the effects of the two major environmental factors, i.e. water and chloride contents, on reinforced concrete durability. The work provided a deep understanding on the electrochemical behaviour of the reinforced concrete system and effectiveness of CP implementation under severe conditions. The research work has an important contribution to fundamental science of corrosion and reinforced concrete deterioration, and the technology and practical application of CP for reinforced concrete structures.


The main results of this work indicate the important influence of the frequency and electrode configuration on the electrical resistance measurement. For the reliability of electrical resistivity measurement, a high frequency of 10,000 Hz and an internal carbon fibre electrode method are recommended.


Regarding the CP for the chloride contaminated reinforced concrete exposed to the atmosphere, it is suggested that adopting an instant-off potential of -500 mV with respect to Ag/AgCl/0.5KCl reference electrode can provide sufficient protection for the reinforced concrete of up to 0.59 % total chloride by weight of concrete, or concrete resistivity is greater than 6.7 kΩ.cm. Furthermore, it was found that the 100 mV depolarization criterion for the evaluation of CP performance gives an overestimated protection. A depolarization of 50 mV is therefore proposed.


In terms of the submerged specimens, the results showed that the water content and chloride content should be explicitly related to the corrosion state rather than through a single parameter of the concrete resistivity for the complicated situations because the water content will affect the oxygen transportation in concrete, and the oxygen availability at the rebar surface will play an important role in the corrosion process, and this is unassessable by concrete resistivity. Moreover, 4 or 24 hours for the 100 mV depolarisation criterion in standards is not applicable for CP assessment where concrete structures are fully submerged due to the low availability of oxygen. On the other hand, the depolarization criterion can be used if the specimens are partially submerged, but different parameters affect the depolarization value such as the magnitude of the applied protection current or potential, chloride concentration, oxygen availability and time of depolarization.

Citation

Oleiwi, H. (in press). Using cathodic protection to control corrosion of reinforced concrete structures. (Thesis). University of Salford

• Thesis Type: Thesis
• Acceptance Date: Jul 31, 2018
• Deposit Date: Sep 25, 2018
• Publicly Available Date: Sep 25, 2018
• Additional Information: Funders : Iraqi Ministry of Higher Education and Scientific Research