PSI - Issue 64

Fritz Binder et al. / Procedia Structural Integrity 64 (2024) 175–182 Fritz Binder/ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Reinforced concrete structures are omnipresent in every developed country, which fulfill various purposes from roads and bridges to airports and shopping centers. However, they are particularly vulnerable to corrosion induced by chlorides, especially in areas near bridges where exposure to de-icing salt is common. Reinforced concrete comprises steel bars embedded within a matrix of cement, aggregate, and water, forming a porous yet durable material. Initially, the steel bars are immune from corrosion by a protective passive layer, facilitated by the high alkalinity of the concrete (Schueremans et al., 2017). Due to environmental influences this passive layer on the steel is destroyed over time. Foundations are typically affected by the chloride-induced corrosion where the chlorides are originating from the de-icing salt which comes within the spray water (Eichinger-Vill et al., 2010). When the front of chlorides exceeds a critical concentration at the reinforcement depth, the steel bars locally lose their passive layer (Breit et al., 2011). This area now offers points of attack for corrosion (Glass et al., 1997), (Alonso et al., 2000). The result is a cross section loss of the reinforcing steel and, consequently, a reduction of load bearing capacity of the steel and the reinforced concrete. The metal dissolution by electrochemical corrosion is increased by a high electrolytic conductivity of the concrete covering the reinforcement. A characteristic feature of this process is the flow of galvanic currents. To effectively detect and monitor corrosion in structures, the implementation of an accurate and reliable monitoring regime needs to be available. 2. Monitoring Continuous monitoring of corrosion-related parameters significantly enhances understanding of their interplay, enabling more precise condition assessments and better predictions of future developments. Moreover, it increases the likelihood of achieving durable repairs. Nowadays, many techniques for assessment of steel corrosion in concrete are available. Among these methods, simple approaches involve measurements of electrochemical rebar potential, concrete resistivity, chloride ion content, and carbonation depth. However, it's important to note that these methods are indirect in nature, offering insights into corrosion presence rather than directly quantifying corrosion rates. Additionally, the thresholds defining onset of corrosion may vary significantly. However, there are techniques specifically designed for direct measurement of steel corrosion rates. To measure the corrosion potential, a reference electrode is installed near the steel reinforcement. The electrode is placed in a pre-made slot approximately 1 cm away from the reinforcement depth, electrically connected to the reinforcement, and then covered with mortar (see Fig. 1a). By connecting a voltmeter, the electrical potential of the reinforcement can be continuously measured. The free corrosion potential is determined by the voltage between the embedded metal (the reinforcement), and the adjacent reference electrode. Potential differences of several hundred millivolts can be anticipated (Elsener et al., 2003). Out of the many electrochemical techniques proposed for monitoring the corrosion rate of concrete reinforcement in practice, the most popular are the linear polarization resistance (LPR) and the use of galvanostatic pulse or transient analysis methods (Andrade et al., 2004). The measurement principle is graphically described in Fig. 1(b). LPR monitoring is an effective electrochemical method of measuring corrosion rate. Monitoring the relationship between electrochemical potential and current generated between electrically charged electrodes in a process stream allows the calculation of the corrosion rate. The LPR method is most effective in aqueous solutions and has proven to be a rapid response technique.