The steel–concrete interface

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

Non destructive assessment of in situ concrete strength: comparison of approaches through an international benchmark

Strength assessment of concrete in existing structures is a key issue. Many non destructive techniques (NDT) are available which can provide information about the material condition and several approaches can be used to derive strength estimates from NDT test results. However, common practice suffers from many drawbacks: (a) it is difficult to ascertain the level of accuracy/confidence of concrete strength estimates, (b) one lacks established guidelines for estimating the concrete strength variability, (c) the best way to combine NDT methods and coring remains an open issue. The RILEM TC 249 "Non destructive in situ strength assessment of concrete'' is addressing these problems. A benchmark was carried out in order to compare (a) how experts define and can carry out an NDT investigation program and (b) how experts derive strength values from the NDT measurements. The benchmark was based on synthetic simulations which reproduced a synthetic data set corresponding to a grid of twenty 3 m-high columns defining a single-storey building structure. The comparison of the various strategies and the analysis of results enabled identification of the most influential parameters that define an investigation approach and influence its efficiency and accuracy. A special emphasis was placed on the magnitude of the measurement error. A model of the investigation strategy is proposed, that will be detailed in future guidelines. The reader is invited to refer to a companion paper (Alwash et al. in Mater Struct, 1. doi: 10.1617/s11527-016-0962-x), based on more extensive Monte-Carlo simulations of the same case-study, which analyzes and quantifies the efficiency of the investigation strategies with respect to both average strength and strength variability assessment, in order to draw conclusions based on rational analyses rather than on the element of chance