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.

This paper reports results from experiments aimed at better understanding the influence of fibre dosage and fibre geometry on the AC frequency needed to determine the DC resistivity of cementitious materials containing steel fibres. Impedance spectroscopy and DC galvanodynamic measurements were performed on mortar prisms with varying fibre reinforcement to determine the matrix resistivity (related to ionic current within the pore solution) and composite resistivity (accounting for both ionic current and electronic current through the fibres). The results showed that adding steel fibres did not significantly affect the DC nor the AC matrix resistivity of the mortar prisms. However, the steel fibres yielded a drastic reduction of the frequency associated to the AC matrix resistivity from ~1 kHz in plain mortar to ~1 Hz in steel fibre reinforced mortar. These findings revealed the need to adequately adjust the frequency in AC resistivity measurements of steel fibre reinforced cementitious materials.

There are different views in the literature on the relationship between the location of the carbonation front and the onset of reinforcement corrosion. Theoretically, corrosion starts when the carbonation front reaches the reinforcement, but some authors have observed an apparent earlier start of corrosion. In the present study, mortar samples with and without reinforcement were exposed up to 22 weeks to 20 °C, 60% RH and 1.5% CO2. The state of the reinforcement was monitored by potential measurements of potential. The carbonation of the bulk and the mortar-steel interface was detected by spraying a pH indicator on a freshly split or cut surface.Good agreement was found between low potential values (compared to reinforcement in the passive state) and the carbonation of the mortar-steel interface. A difference in the spatial variation of the carbonation depth was observed between plain and reinforced samples. The differences found in the literature between the location of the carbonation front and the corrosion onset can probably be explained by the spatial variation of the carbonation depth in the vicinity of the reinforcement.

An experimental setup was designed to study the impact of concrete bulk resistivity on the rate of chloride‐induced reinforcement corrosion. Small pieces of mild steel were used to simulate pits (anodes) that form when chlorides come into contact with the reinforcement. The galvanic current was measured between the simulated anodes and a large cathode network. Comparisons were made between the corrosion rates calculated from the galvanic currents and the bulk resistivity. The bulk resistivity was varied using two mortar mixes (made of plain Portland cement and a Portland cement—fly ash blend), which were exposed in different temperature and moisture conditions. Despite a high scatter in the results, it was clear that the relationship between bulk resistivity and corrosion rate depended on the mortars tested. The findings presented in this article and the accompanying work strongly indicate that bulk resistivity alone does not provide sufficient information for assessment of the corrosion rate for chloride‐induced macro‐cell corrosion.

The influence of mortar bulk resistivity on the kinetics of chloride-induced macro-cell corrosion of the reinforcement was experimentally studied. It was found that the corrosion process was limited by a combination of anodic, ohmic and cathodic control for the geometrical conditions tested (small anodes (Ø 6mm) and large cathode-to-anode ratio (685)). The cathodic partial process was independent of the bulk resistivity. Both the anodic and the ohmic partial processes were influenced by local conditions around the anode but were not directly related to the bulk concrete resistivity. The findings indicate that a unique relationship does not exist between bulk concrete resistivity and the corrosion rate for chloride induced macro-cell corrosion

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