Comparative studies of experimental and numerical techniques in measurement of corrosion rate and time-to-corrosion-initiation of rebar in concrete in marine environments
Introduction
Corrosion of embedded steel rebars in concrete structures in chloride-contaminated environments is the most common cause of premature deterioration and failure of concrete leading to the reduction of its service life and thus, tremendous economic losses [1]. Corrosion of rebars in reinforced concrete in aggressive environment is often caused by the penetration of chloride into concrete pores after a prolonged exposure [1], [2], [3], [4]. Several factors are influential on the corrosion process; some of them are related to the properties of the reinforced concrete materials, such as the quality of concrete and the type of rebar (e.g. black steel, stainless steel, and galvanized steel); others are related to environmental factors such as exposure condition, temperature, and humidity. For initiation of the corrosion process, the amount of chloride at the level of rebar in concrete must reach to a certain threshold called critical chloride content [5], [6], [7], [8], [9]. Once chloride reaches the critical amount, steel starts corroding due to the deterioration of the passive oxide layer on steel rebars [10], [11], [12], [13], [14], [15], [16], [17], [18], [19].
There are two different types of chloride-induced corrosion in concrete: microcell corrosion and macrocell corrosion. Macrocell corrosion with a small anode and a large cathode frequently occurs in chloride-induced corrosion of rebars in concrete and is responsible for highly localized corrosion attacks and thus, localized cross-section reduction of rebar. In addition to the macrocell corrosion, there is another corrosion mechanism known as microcell corrosion in which the anode and cathode sites are located close to each other in micro-scale on the surface of steel. These two corrosion mechanisms usually occur on the surface of rebar in concrete at the same time; therefore, the corrosive action of both mechanisms would be superimposed [20].
In spite of numerous investigations carried out on different aspects of corrosion, few studies have been conducted towards the simultaneous occurrence of microcell and macrocell corrosion, especially when the structure is exposed to a splash zone [21], [22], [23]. Due to the higher presence of oxygen in a splash zone, this zone is more aggressive than a tidal zone. As a result, the evaluation of corrosion at a splash zone compared to a tidal zone is more important as the corrosion of rebar in this zone is more severe. Therefore, the corrosion of reinforced concrete elements exposed to the splash zone plays a key role in the overall service life of structures in marine environment.
In this study, reinforced concrete specimens with different water to cement ratios were prepared and exposed separately to tidal and splash zones for 650 days in the Qeshm Island in the Persian Gulf region. The environmental conditions of this zone, as presented in Table 1, clearly show that the relative humidity and average temperature of this region are quite high [24]. The combination of the environmental conditions and high chloride concentration of seawater have made this marine environment one of the most severe geographical regions for reinforced concrete structures in the world [25], [26]. Unlike previous studies [27], [28], in this study the specimens in the splash zone were completely separated from those in the tidal zone so that the splash zone effect can be systematically compared to that of the tidal zone. A number of field techniques such as the macrocell corrosion potential, Galvanostatic pulse, electrical resistivity, and half-cell potential measurement were used to determine the rate of corrosion and time-to-corrosion-initiation. Although these techniques have been used by many researchers to evaluate the corrosion performance of rebar in concrete, few research studies have been conducted to compare the efficiency and accuracy of these techniques in field application. Therefore, a multi-technique approach was implemented in this study to provide some insight into the relative performance of these methods for the assessment of corrosion in concrete structures. In addition, a comparison was made between the experimental results of this work with those obtained from a recently developed model for numerical prediction of corrosion rates in reinforced concrete structures [29], [30].
Section snippets
Materials
Type II Portland cement was used in this experiment. The crushed limestone local source aggregate and river sand were used as coarse and fine aggregates, respectively. The total bulk density of the aggregate was about 1850 kg/m3 with a maximum aggregate size of 19 mm. The chemical composition of the cement, gravel and sand used in this study is presented in Table 2. The polycarboxylate-based superplasticizer was used to achieve desired workability in different mixture proportion designs.
Mixture proportions
Four
Macrocell corrosion test
The average macrocell corrosion current of the four concrete specimens exposed to the tidal and splash zones during the period of 22 months is shown in Fig. 2. In this figure, the time-to-corrosion-initiation of each specimen at both the splash and tidal zones is identified as per ASTM G109. In this standard, the corrosion current of 10 μA is specified as a threshold macrocell current beyond which the corrosion of rebar initiates. The time-to-corrosion-initiation results of the concrete specimens
Conclusions
Comparative studies were conducted on the experimental and numerical data of corrosion rate and time-to-corrosion-initiation of rebar in concrete in a marine environment. It was found that the time-to-corrosion-initiation defined from the three different corrosion measurement techniques was not consistent. The corrosion initiation time is usually defined as a time when the measurements exceed a certain threshold value. The results of this study showed that this methodology was not very accurate
Acknowledgements
This paper is part of a research project under contract with the Iranian Qeshm Free Zone Organization at the Construction Materials Institute (CMI) at the University of Tehran. The authors would like to acknowledge all supports received for this research.
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