Although the steel-concrete interface (SCI) is widely recognized to influence the durability of reinforced concrete, a systematic overview and detailed documentation of the various aspects of the SCI are lacking. In this paper, we compiled a comprehensive list of possible local characteristics at the SCI and reviewed available information regarding their properties as well as their occurrence in engineering structures and in the laboratory. Given the complexity of the SCI, we suggested a systematic approach to describe it in terms of local characteristics and their physical and chemical properties. It was found that the SCI exhibits significant spatial inhomogeneity along and around as well as perpendicular to the reinforcing steel. The SCI can differ strongly between different engineering structures and also between different members within a structure; particular differences are expected between structures built before and after the 1970/1980s. A single SCI representing all on-site conditions does not exist. Additionally, SCIs in common laboratory-made specimens exhibit significant differences compared to engineering structures. Thus, results from laboratory studies and from practical experience should be applied to engineering structures with caution. Finally, recommendations for further research are made. This report was prepared by the working group within RILEM TC 262-SCI, and further reviewed and approved by all members of the RILEM TC 262-SCI.

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This article has been prepared within a framework of RILEM TC 262-SCI. The article has been reviewed and approved by all TC members.

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The impedance behavior of nanotubular self-organized porous TiO 2 ͑PTO͒ films with thickness of Ͼ2 m, 100 nm pore diameter, and 150 nm average spacing formed on titanium by anodization in 1 M Na 2 SO 4 solution +0.5 wt % NaF was investigated and compared to the behavior of compact TiO 2 layers ͑CTO͒. At potentials close to the flatband potential ͑E FB ͒, PTO had an apparent interfacial capacitance at 30 Hz that was greater than the capacitance of CTO. This effect, however, was not only related to the larger effective area of PTO. Differentiation between both surface conditions vanished both at potentials significantly higher than E FB and with increasing test frequency to 1 kHz. These findings together with observations of appreciable frequency dispersion suggest that the pore walls are rich in deep-lying localized states, which become evident only at lower test frequencies and at potentials negative enough that depletion zones do not merge at pore walls. PTO did not show a transmission line effect at frequencies as high as 100 kHz. Model calculations based on pore dimensions and electrolyte conductivity predicted that those effects could only take place at much higher test frequencies, but the slow response of deeper states might impair observation anyway.

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