Corrosion that proceeds along the reinforcing element corrupts the entire structural integrity mainly by reducing cross-sectional area of the remaining metal. This is because steel is consumed through corrosion process. The bond with the surrounding concrete weakens as the diameter and mass of the steel decreases (Kwon et al, 2011). This process can lead to major damages in PS concrete where the interface with the steel is important. Furthermore, the formed rust causes more stress on surrounding concrete due to the rust's 2 to 4 times greater volume than the initial steel amount (NRMCA, 1995). Hence, delamination and spall of cover concrete occur and exposes the steel to a hazardous environment. It will eventually accelerate the corrosion process and in serious case, early failure of entire structures can occur.
Although steel corrosion in the RC structure leads to critical functional degradation, corrosion state condition is difficult to diagnose and predict it's degradation until major damage is apparently observed. This is mainly because steel is designed to be encased under concrete cover. Due to this unseen location, a large amount of assets has been invested to rapidly detect and solve corrosion problems in the U.S. According to a study from Federal Highway Administration (FHWA) in the year of 2002 (Gerhardus et al, 2002), total corrosion costs were $276 billion per year which is 3.14% of U.S. GDP in the year of 1998. Especially, for highway bridges under infrastructure category ($22.6 billion-16.4% of total), the annual direct cost estimates to a total of $8.3 billion (37% of infrastructure), that includes replacement cost for deficient bridges ($3.8 billion), concrete bridge deck maintenance ($2 billion), and other types of maintenances ($2.5 billion) for concrete substructures and steel bridge repainting. Moreover, indirect costs such as traffic delay and productivity loss that were induced by maintenance and rehabilitation process would be much larger (up to 10 times) than that of direct corrosion costs (Gerhardus et al, 2002). Figures 1.1 shows corrosion cost diagrams especially in highway section.
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As a mean of alleviating the financial burden that is caused by steel corrosion on the existing infrastructures made of RC or PS, early detection is the best possible solution. Early corrosion detection carries a benefit of diagnosing the severity of corrosion; therefore, it allows to understand in the damage progression and to find ways to reduce or interfere with the corrosion process. Moreover, it is able to elongate the operation cycle of existing RC structures. As a result, the direct cost that comes with repairing unplanned outages, emergency damages and its corresponding indirect costs as previously mentioned can be significantly lowered (Gerhardus et al, 2002).
Hence, a number of corrosion detection techniques that can estimate corrosion condition before critical signs appear have been studied and applied by many civil engineers not only for protecting people, but also for reducing economic burdens from controllable catastrophes. Some pre-installed electrochemical sensors can provide reasonable indication of steel corrosion (Fuhr et al, 1998; Andrade et al, 1978). But most of them are not applicable to existing RC structures. Many approaches utilizing current and electrical potential measurement of RC such as Half-cell and linear polarization resistance techniques have been actively studied in past few decades (Andrade et al, 1978; ASTM, 1998; Erdogdu, 2004; Choi et al, 2005; Asrar et al, 1999). However, partial breaking of cover concrete is unavoidable for steel exposure (Baek et al, 2012). In addition to direct corrosion detection, indirect corrosion detection techniques are also developed such as 'cover-meter' or 'profometer' by measuring damping of a parallel resonant circuit. Although they are able to identify the approximate corrosion condition throughout re-bar location and rust volume variations, the results are greatly affected by concrete composition. Moreover, this approach is only applicable when concrete cover depth is below 40 mm (Rajagopalan et al, 1978).
It is important to mention the approaches discussed above are still controversial in civil engineering field due to uncertainty of the results and difficulties in real application. Therefore, it is urged to discover novel corrosion detection techniques that are accurate and practical without any harmful effects given to the RC structure.
This subchapter contains fundamental study of corrosion in terms of its procedures, causes and effects. Corrosion is a chemical reaction that deteriorates the material properties by involving moisture, and it usually occurs when the material is located in harsh environment (ASTM G15-08, 2008).
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Throughout chemical reaction involved with Ferrous Hydroxide (Fe(OH)2) and Ferric Hydroxide (Fe(OH)3), Ferric Oxide (Fe2O3) and water are manufactured as the byproducts. The chemical reaction of steel corrosion is denoted from Eq. (1.1) to Eq. (1.5) (Arndt et al, 2009).
Once corrosion occurs, the chemical property changes in both concrete and re-bar take place in the steel that have been embedded over a number of years. As chemical reaction proceeds, the steel deteriorates as rust is produced. This is because the bonds formed between the steel and surrounding concrete are weakened and creates byproducts (rust) that causes stress to the concrete's environment (Arndt et al, 2009).
Deterioration in reinforced concrete due to steel corrosion was studied by many engineers in the past decades. Melchers et al. and Weyers classified corrosion phase with 5 levels (Melchers et al, 2006; Weyers et al, 1994), Chen and Mehadraven categorized 3 levels with its corresponding effect (Chen et al, 2008). Details are summarized in Table 1.1~1.2 and Figure 1.2 below.
Steel corrosion in RC is mainly caused by two different reasons. One is a direct contact to harsh environment of embedded steel and the other is a carbonation of concrete which is surrounding structural steel.
When structures are located in harsh environment such as high humidity and frequent raining zone, probability of corrosion occurrence is relatively higher than dry zone (Fuhr et al, 1998).
Due to heavy load and long-term usage of RC structures, small cracks are formed on the concrete surface. Those generated cracks tend to stretch into the embedded re-bar due to weaker load-endurance. When the re-bar is exposed to environment consisted with harmful factors such as moisture and chloride ions, the corrosion process initiates from the cracked spots (Hillemeier, 1984).
Another type of corrosion in RC is indirect contact to the environment which is due to carbonation of concrete. Carbonation is caused by many factors such as chloride penetration, short cover depth and low cement ratio (Glass et al, 1991). Once the carbonation proceeds, calcium carbonate and moisture pores are formed and changes the homogenous property of concrete. Originally, soon after 28 days curing process, concrete will have the pH value of 13 which indicates strong alkalinity and ideally free from any chemical reactions. However, the formed calcium carbonate lowers the alkalinity of concrete and drives to a more acidic condition. This in turns creates an adequate environment for re-bar to ionize with holding moisture in concrete pores. Hence, it allows ion flow through the concrete, and electron flow through the re-bar (Arndt et al, 2009).
The Figure 1.3 shows the concept of carbonation of concrete and chemical equations from Eq.(1.6) to Eq.(1.8) are the carbonation process in concrete.