Using cathodic protection to control corrosion of reinforced concrete structures

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

Concrete electrical resistivity at varied water, chloride contents and porosity – experiment, modelling & application

Understanding and characterizing the relationship between the electrical resistivity and the major influencing factors of the concrete have been all the time a topical research in relation to structural durability. This paper reports an experimental study on the influences of water and chloride contents, and porosity on the electrical resistivity of the Portland cement concrete. The results indicate that the electrical resistivity has a strong correlation with the water and chloride contents in concrete. A new characteristic model has been proposed to represent the correlation. The proposed model has been implemented into a numerical modelling case study of cathodic protection for reinforced concrete structure in saline environment.

An experimental study of concrete resistivity and the effects of electrode configuration and current frequency on measurement

Electrical resistivity, a measurable parameter of the state of concrete, plays an important role in the assessment of reinforced concrete structures. An experimental study using two-electrode method has been conducted to evaluate the resistivity of Portland cement concrete. Internal and external electrodes were varied in order to understand effect of the electrodes configuration, where carbon fibre (CF) sheets were employed as the internal electrodes and CF and copper sheets were used as external electrodes. Furthermore, frequency of applied current was varied from low to high, to identify the most suitable frequency that can be utilized for stable and reliable results. Optimised internal electrodes configuration and the current frequency of 10,000 Hz were used to measure the resistivity on a series of concrete cubes, which were made using three different water to cement ratios and four different chloride contents

Modelling the electrical resistivity of concrete with varied water and chloride contents

The electrical resistivity of concrete is an important property in the assessment of reinforcement corrosion in concrete and an essential parameter in the design and operation of cathodic protection for reinforced-concrete (RC) structures. Water and chloride contents in concrete are highly variable in RC structures in real-world hostile environments, meaning that characterisation of their coupled effects on concrete electrical resistivity is important. The currently available models are investigated in this paper. As all the current models are purely empirical, an improvement with a semi-empirical model is proposed. The improvement highlights the intrinsic linkage between concrete electrical resistivity and water content and the pore size distribution of concrete. The proposed model was tested using two sets of experimental data and was also compared with other two empirical models

The effect of metakaolin and hybrid polymers on the microstructure of concrete

It has been established that metakaolin (MK) can be applied as a supplementary cementitious material and that some polymers can enhance substantial properties of concrete. Previous studies showed that, when used in combination, MK and polymers can complement each other, and enhance further the mechanical and durability properties of concrete, which were not improved by one another separately. To have a deep observation and understanding of the mechanism of concretes modified with various combinations of MK and polymers, this study investigates the changes in the microstructures based on SEM combined with CT scan analysis scan technologies. The findings show that the microstructure of the pore of hardened concrete modified with combined MK and hybrid polymers is significantly improved compared to samples with MK or polymers alone. This indicates that this approach can be effectively used to provide additional proof on the strength and porosity properties of high-performance concrete

The characterization of chloride effect on concrete water sorption and its application in the modelling of concrete conditions in tidal zones

Concrete exposed to cyclic wetting and drying in salty water conditions is thought to be subjected to an accelerated ingress of chloride from the outside environment, and prone to a worsening deterioration process inside. Additionally, there is an osmotic effect on salty water flow in porous concrete. However, so far, a fully profound understanding of the coupled cyclic wetting–drying and osmosis effects on the chloride movement in concrete is still limited. This paper reports on a comprehensive study on the topic. A series of experimental tests was conducted initially for the vapour-water sorption isotherm (VWSI) of normal concrete of different porosity and chloride content. Thereafter, a novel mathematical model was proposed and validated to characterise the effect of chloride salt on the vapour absorption and water retention behaviour of concrete. Finally, the proposed haracteristic model was implemented in a numerical model to simulate chloride ingress in concrete in tidal zones. The vapour-water sorption sotherm model successfully provides an effective tool to quantify the coupled influence of cyclic wetting–drying and osmosis on chloride transportation in concret