PSI - Issue 17

S. Tsouli et al. / Procedia Structural Integrity 17 (2019) 268–275 S. Tsouli, A.G. Lekatou, C. Nikolaidis, S. Kleftakis/ Structural Integrity Procedia 00 (2019) 000 – 000

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One of the most critical mechanisms affecting the overall performance of reinforced concrete as a composite material and consequently the load-bearing capability, the serviceability and the seismic resistance of reinforced concrete structures is the bond strength, i.e. the interaction between the steel reinforcement and the surrounding concrete (Xu et al. (2018)). The bond strength is affected by the chemical adhesion between the steel reinforcement and the concrete, the friction resistance at their interface or the mechanical interlock provided by the bearing of the ribs of deformed bars against the concrete surface (Dewi et al. (2017), Xu et al. (2018)). The most important factors influencing the bond strength include the corrosion level of the steel reinforcement, the compressive strength and the age of the concrete, the embedment length, the concrete cover to bar depth, the concrete cover to diameter ratio (c/d), the crack width and the surface characteristics of the rebar (friction coefficient and deformation geometry) (Yalciner et al. (2012)). The mechanical strength and durability are prerequisites for a long-term service life and sustainability of the reinforced concrete infrastructure (Shi et al. (2012)). Apostolopoulos et al. described durability as the ability of a reinforced concrete structure to withstand aggressive environmental conditions, namely its performance should not deteriorate below a minimum limit (Apostolopoulos et al. (2008)). Corrosion of steel reinforcement, an increasing problem during the last decades, is the most significant factor responsible for the premature deterioration of the construction service life, the durability and the seismic resistance of reinforced concrete structures. The three major deterioration mechanisms for reinforced concrete due to environmental pollution are: a) Chloride-induced corrosion, due to the Cl - ions penetration into concrete through its porous structure, (marine environments and deicing solutions), b) concrete carbonation, when the atmospheric CO 2 reacts with the Ca(OH) 2 of concrete (more often in urban areas), and c) acid rain (AR) attack, due to the intensive urban and industrial activity. All mechanisms neutralize the alkaline pore solution leading to depassivation and consequent localized corrosion of the reinforcement (Hansson et al. (2012)). Corrosion affects the bond strength of reinforced concrete structures, in the following manner: The formation of corrosion (oxidation) products with a volume several times greater than the original steel that gets consumed by corrosion, leads to cracking and spalling of the concrete cover. Consequently, more aggressive species, oxygen and moisture diffuse into the concrete increasing the corrosion rate and, thus, the bond strength. The bond strength increases at the onset of corrosion, however, after the cracking of concrete, it rapidly decreases (Ayop et al. (2017), Hansson et al. (2012)). At the same time, the mechanical properties of the steel bar deteriorate, due to the decrease in the bar diameter and the embrittlement caused by the corrosion; the latter induces local acidification at the steel/concrete interface (Apostolopoulos et al. (2008), Yalsiner et al. (2012)). In the last decades, the structural performance, service lifetime, aesthetical value and maintenance costs of the concrete infrastructure and monuments have dramatically been affected by acid rain (AR) (Camuffo (2014)). AR attack takes place especially in urban areas, due to the high concentrations of sulfur oxides and nitrogen oxides in the polluted atmosphere. These oxides react with atmospheric water and form sulfurous/sulfuric and nitrous/nitric acids (Aperador et al. (2012)). However, the mechanism of concrete degradation by AR is much more complex than the mechanism of pure acid attack, as AR contains not only H + , but also NH 4 + , Mg 2+ , SO 4 2- , NO 3 - , Cl - etc. (Chen et al. (2013)). H 2 SO 4 reacts with the Ca(OH) 2 of concrete and gypsum (CaSO 4 ·2H 2 O) is formed of almost double volume. Gypsum reacts with calcium aluminate hydrate (C 3 A) to form ettringite (C 3 A(CS) 3 H 32 ) with nearly 7 times volume expansion, leading to internal stress exertion and concrete cracking. At the same time, the main acidic constituents of AR (H + & SO 4 2- ) react with Ca(OH) 2 to form hydrated salts soluble in rainwater, which penetrate concrete through its pores causing more cracking and spallation (Chen et al. (2013), Girardi et al. (2010)). Ordinary Portland Cement (OPC), which is almost exclusively used in restoration works of monuments, has minor resistance to AR (Barbhuiya et al. (2017)). When dealing with cultural heritage in environmentally polluted regions, an excellent localized corrosion performance of the steel reinforcement is a prerequisite. The employment of AISI 316L stainless steel reinforcement of architectural members (e.g. the ancient theater of Dodona, Epirus, Greece) has been a common practice in the last decades. Its high corrosion resistance is owning to the Cr 2 O 3 -based passive surface film, the passivity of which is assisted by the presence of Ni and the Mo-due resistance to localized corrosion (Mendez (2014)). However, the need for relatively inexpensive combined with earthquake-resistant solutions is a critical factor to consider. Hence, the replacement of the 316L steel reinforcement with the less costly 304L steel in combination with low-cost corrosion inhibitors could be a profitable alternative, since 316L is highly resistant to seismic activity. Among the various methods applied to enhance the corrosion resistance of reinforced concrete, the partial replacement of OPC with mineral admixtures, such as fly ash (FA), is a low-cost and ecological method (Chousidis et