PSI - Issue 10

A. Drakakaki et al. / Procedia Structural Integrity 10 (2018) 59–65 A. Drakakaki et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

From the early 60s and for 30 or more years S400 (St III) single phase grade steel was used as reinforcement in reinforced concrete structures (according to ELOT 959), as it has already been mentioned by Apostolopoulos et al. (2007). Since the early nineties, S400 grade steel was replaced by S500s dual phase grade steel. Most of the structures utilizing these steel grades as reinforcement are still in use. Although in recent years, the problem of the actual residual strength degradation of ageing reinforced concrete structures has attracted considerable attention, it is far from being fully understood and even less resolved. According to Apostolopoulos et al (2007), Papadakis (1999), Capazucca (1995), Diamong (1986) and Alvarez et al (1984), the degradation problem becomes even more complex when there is a combined degradation caused by both corrosion and seismic loads. In the present work, the effects of chloride-induced corrosion on the dual-phase and the single-phase steel bar categories are evaluated, in terms of corrosion resistance and mechanical characteristics, before and after the corrosion process. Comparison of corrosion damage rates between the two types of steel indicates that they demonstrate dif ferent resistance against the ascribed corrosion environments. Two different corrosion procedures were imposed on the steel bar categories, in order to achieve a satisfying cor relation with the environmental conditions. Initially, the simulation of exposure to harsh corrosive environment was achieved with the use of a salt spray fog chamber (Fig.1), under constant spraying with 5 % w.t. NaCl solution. Sec ondarily, a set of impressed current density corrosion tests were organized. The corrosion test was executed in constant wet conditions. Combining the above-mentioned methods, an effort was made to correlate the electrochemical cor rosion system (Fig.2), to the natural environment, by exploiting the results taken from the salt spray fog chamber. At the same time, the differences among the two steel bar categories are analyzed, and an estimation is made concerning their degradation, in reference to their exposure to the environmental conditions. Single-phase steel demonstrates higher corrosion resistance, in comparison to the dual-phase category. Vulnerability of dual-phase steel seems to be attributed to the existence of the martensite layer. Both, mechanical behavior and corrosion resistance of the two categories raise questions concerning the mechanical performance of existing structures as well. The goal of the study was primarily to correlate artificial corrosion to environmental conditions and secondarily to highlight the differences among the two steel categories, as far as their mechanical performance and their resistance to corrosion is concerned, given the fact that they have been both used in existing civil engineering structures. (a) (b)

Fig. 1. Salt spray fog chamber .

Fig. 2. (a) Control System and (b) corrosion cells for the electrochemical corrosion tests.

2. Materials and methods

For the goals of the present study, materials of two different steel reinforcement categories were used. Precisely, S500s dual phase, which is mentioned by Apostolopoulos (2007) and Apostolopoulos et al. (2010), and S400 (StIII) single phase steel bars, which are mentioned by Apostolopoulos et al. (2007), Apostolopoulos et al. (2008) and Apo stolopoulos et al. (2010), were used. Two series of specimens were organized for each steel class (S400 and S500 s). For each steel category, one set of specimens 500 mm long (Fig.3) and one 170 mm long (Fig.4) were prepared. In the case of the “long” specimens, only a length of 250 mm was exposed to the corrosive environment, while in the case of the “short specimens” the exposed length was equal to 20mm. The rest part of the specimens was covered in wax, to prevent corrosion. In Table 1 is given in detail the number of the specimens used in each case: