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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.

5Cracks in concrete can self-heal when exposed to prolonged wetting, but this is limited to narrow cracks. In 6 practice, cracks > 0.2mm cause leakage and impair performance of structures. The potential of 7 superabsorbent polymers (SAP) to self-seal such cracks was investigated via transport experiments, 8 microscopy and modelling. Forty samples containing SAP and through-thickness cracks were subjected to 9 0.12wt.%NaCl at 4m/m pressure gradient to simulate groundwater seepage. Results show that SAP can re-10 swell and seal cracks, for example in the case of 0.3mm cracks reducing peak flow rate and total flow by 85% 11 and 98% respectively. Increasing SAP dosage accelerates sealing, but imparts a strength penalty and this 12 limits practical applications. Modelling suggests that the effectiveness of SAP can be enhanced by increasing 13 its re-swelling ratio and particle size, and depressing its initial swelling. These variables increase the SAP 14 exposed in a crack and the gel volume available to seal it. 15

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