Corrosion of Steel in Concrete Structures

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With isolated exceptions, metals exist in Earth’s crust as minerals such as carbonates, sulfides, sulfates, or oxides formed over billions of years. TheseThe low cost of steel-reinforced concrete and the ready availability of raw materials with which it is formed make it the most widely used structural material available.
Many critical infrastructure facilities are made of steel-reinforced concrete, and since they are under constant degradation from the aggressive environments, they suffer from durability issues, particularly the corrosion of the steel bars within such structures. Corrosion is the main deterioration mechanism of those structures which significantly reduces the service life, reliability, functionality of structures, and safety. This corrosion in turn has created a multi-billion-dollar infrastructure deficit. Given the indisputable societal and economical importance of the impact of corrosion when developing advanced structures, it is quite surprising that the study of corrosion in infrastructure facilities receives little to no attention in most civil engineering curricula at both undergraduate and graduate levels! It is obvious that better education for the nation’s engineers is essential to improving corrosion control and management practices throughout the national infrastructure. However, to become a fully qualified civil or mechanical engineer usually requires at most a one-semester course in materials, one in which the subject of corrosion is rarely mentioned! Civil engineering education and the profession are confronting a challenging crossroads and undergoing

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revolution. Nevertheless, the seeming lack of ability of civil engineers to thoroughly understand corrosion and its impact on the service lives of these structures, and its role in designing materials that are adaptable to today’s challenging world, hinders progress toward addressing 21st-century challenges. Therefore, there is a critical need to deliver such education to future engineers. This book aims to be a handbook for civil engineers who are involved in design and maintenance of steel-reinforced concrete structures as well as a textbook for upper-level undergraduate- and graduate-level students, specifically in the civil engineering discipline. compounds are therefore considered to be in their equilibrium state—that is, their lowest energy state. Extracting the metals from these compounds requires a significant amount of energy in the form of heat or electrical power. Once extracted, the metals and alloys are in a thermodynamically metastable state and depending on the environment, they will always attempt to revert to a lower energy compound, usually by corrosion or oxidation. Since the Earth’s atmosphere contains water and oxygen, and coatings and other barriers are inherently imperfect, it is impossible to completely prevent corrosion. Consequently, all metals and alloys in use today are in a metastable condition. The stability of a metal is usually described in terms of its electrochemical potential, a thermodynamic function that may be defined in the context of corrosion as “the ease of ionizing an atom of the metal.” Potentials are dependent on the pH of the environment and factors such as oxygen availability. Potentials cannot be determined absolutely, and are generally defined as the potential difference between the metal of interest and that of a reference electrode so chosen to have a stable potential (Revie and Uhlig, 2008). The “standard hydrogen electrode” (SHE) is the reference electrode against which all other electrodes are measured and is given the electrochemical potential of 0.00 V. It is designed so that the reaction: 2Hþ þ 2e ¼ H2 is in equilibrium at pH 0 (i.e., 1 M acid solution), 1 atm hydrogen gas, and 25 C. Because the hydrogen electrode is cumbersome to operate and transport, other electrodes consisting of a metal in contact with a saturated solution of its ions, such as Cu/CuSO4, are used. The different common reference electrode potentials relative to that of the SHE are given in Table 1.1, and conversions are illustrated in Figure 1.1 (Roberge). These electrodes are not generally suitable for embedding in concrete because they become unstable over a short period of time at high pH levels. Therefore, an Mn/MnO2 electrode has been developed specifically for use in concrete (Arup et al., 1997).

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