The Steel–Concrete Interface

Materials and Structures (2017) 50:143 DOI 10.1617/s11527-017-1010-1 ORIGINAL ARTICLE The steel–concrete interface Ueli M. Angst . Mette R. Geiker . Alexander Michel . Christoph Gehlen . Hong Wong . O. Burkan Isgor . Bernhard Elsener . Carolyn M. Hansson . Raoul François . Karla Hornbostel . Rob Polder . Maria Cruz Alonso . Mercedes Sanchez . Maria Joaõ Correia . Maria Criado . A. Saguë´s . Nick Buenfeld Received: 8 December 2016 / Accepted: 31 January 2017 Ó The Author(s) 2017. This article is published with open access at Springerlink.com Abstract Although the steel–concrete interface and their physical and chemical properties. It was (SCI) is widely recognized to influence the durability found that the SCI exhibits significant spatial inho- of reinforced concrete, a systematic overview and mogeneity along and around as well as perpendicular detailed documentation of the various aspects of the to the reinforcing steel. The SCI can differ strongly SCI are lacking. In this paper, we compiled a between different engineering structures and also comprehensive list of possible local characteristics at between different members within a structure; partic- the SCI and reviewed available information regarding ular differences are expected between structures built their properties as well as their occurrence in engi- before and after the 1970/1980s. A single SCI neering structures and in the laboratory. Given the representing all on-site conditions does not exist. complexity of the SCI, we suggested a systematic Additionally, SCIs in common laboratory-made spec- approach to describe it in terms of local characteristics imens exhibit significant differences compared to U. M. Angst (&) Á B. Elsener O. B. Isgor Institute for Building Materials (IfB), ETH Zurich, School of Civil and Construction Engineering, Oregon Stefano-Franscini-Platz 3, 8093 Zurich, Switzerland State University, Corvallis, OR, USA e-mail: [email protected] C. M. Hansson M. R. Geiker Á K. Hornbostel Department of Mechanical and Mechatronics Department of Structural Engineering, Norwegian Engineering, University of Waterloo, Waterloo, University of Science and Technology (NTNU), ON N2L 3G1, Canada 7491 Trondheim, Norway R. François M. R. Geiker Á A. Michel LMDC, INSA, UPS, Universite´ de Toulouse, Toulouse, Department of Civil Engineering, Technical University of France Denmark (DTU), 2800 Kgs. Lyngby, Denmark R. Polder C. Gehlen TNO Technical Sciences/Structural Reliability, Delft, The Centre for Building Materials (cbm), Technical University Netherlands Munich, Baumbachstr. 7, 81245 Munich, Germany R. Polder H. Wong Á N. Buenfeld Delft University of Technology, Civil Engineering/ Concrete Durability Group, Department of Civil and Materials and Environment, Delft, The Netherlands Environmental Engineering, Imperial College London, London SW7 2AZ, UK 143 Page 2 of 24 Materials and Structures (2017) 50:143 engineering structures. Thus, results from laboratory conditions [9, 11]. There are many other characteris- studies and from practical experience should be tics at the SCI that may, or may not, occur locally and applied to engineering structures with caution. Finally, that may potentially influence the local susceptibility recommendations for further research are made. of the reinforcement to corrosion initiation. A systematic overview and detailed documentation of these, Keywords Steel–concrete interface Á Interfacial however, are lacking. transition zone Á Durability Á Corrosion Á This contribution summarizes the state-of-the-art Inhomogeneity Á Variability concerning the occurrence—both on-site and in laboratory experiments—of different local characteristics at the SCI, and describes their physical and chemical properties. The influence of these local characteristics 1 Introduction on corrosion initiation will be addressed in a separate publication. The steel–concrete interface (SCI) influences the The present paper was prepared by members of structural behavior and durability performance of RILEM TC 262-SCI and is to a large extent based reinforced concrete, and thus, plays an important role on the presentations and discussions during the first in engineering of reinforced concrete structures. The five TC meetings. This paper focuses on conven- SCI has a particular influence on the corrosion tional carbon steel reinforcement in Portland cement behavior in chloride exposure environments and in based concrete, leaving considerations of coated, the case of carbonation. It also influences certain alloyed or high-strength steels as well as other aspects of the bond between steel and concrete such as binder types for future treatment. Additionally, the adhesion between steel and concrete, which is partic- influences of repair techniques (electrochemical ularly important in the case of smooth reinforcing steel remediation, e.g. cathodic prevention and protection) where mechanical interlocking does not exist. and of corrosion inhibitors and other chemical Some of the large uncertainties in durability admixtures on the SCI are excluded in this work. engineering—particularly in explaining and forecast- This paper describes the SCI in the hardened ing initiation of chloride-induced corrosion—can be concrete after initial cement hydration and steel traced to insufficient understanding of the influence of passivation, but before corrosion has initiated, i.e. it local characteristics of the SCI [1, 2]. To name a few considers the SCI of passive steel during the so- prominent examples, pores, voids, and cracks have called initiation stage (Fig. 1). The time scale of this been observed to strongly influence initiation of stage often forms a substantial part of the service chloride-induced corrosion under some conditions life of engineering structures. [3–10], but seem to have no influence under other M. C. Alonso Á M. Sanchez 2 Documented characteristics at the SCI Institute of Construction Science Eduardo Torroja-CSIC, Serrano Galvache 4, 28033 Madrid, Spain 2.1 Terminology and conceptual description of the SCI M. J. Correia Materials Department, National Laboratory for Civil Engineering (LNEC), Av. Do Brasil, 101, Table 1 lists features that are known to occur at the 1700-066 Lisbon, Portugal SCI both in laboratory specimens and in engineering structures. We suggest ‘‘local characteristics’’ as a M. Criado Department of Materials Science and Engineering, The general term to embrace all the individual features and University of Sheffield, Sir Robert Hadfield Building, their local properties. Note that the term ‘‘local Sheffield S1 3JD, UK characteristics’’ may refer to both local differences with increasing distance from the steel surface (per- A. Saguë´s Department of Civil and Environmental Engineering, pendicular to steel surface) and to local differences University of South Florida, 4202 E. Fowler Avenue, across the steel surface, both along the reinforcing Tampa, FL 33620-5350, USA steel bar and along its circumference. Materials and Structures (2017) 50:143 Page 3 of 24 143 Fig. 1 Schematic illustration showing the stage in the service life of a corrosion reinforced concrete initiation stageonset propagation stage structure during which the steel–concrete interface stabilization (SCI) is considered in this stage focus of work for SCI this publication degree of deterioration 0 age of structure Figure 2 shows some selected characteristics of the Europe—reinforcing steel is classified into strength SCI; for reasons of clarity it is not possible to display grades, where for example the value 500 in the class all features listed in Table 1. We found it useful to B500 represents the characteristic yield strength of the divide the local characteristics into those inherent to rebar of 500 MPa [14]. Reinforcing steel is also the reinforcing steel bar (steel part of the SCI) and classified according to its elongation at ultimate those in the concrete adjacent to the steel (concrete strength and depending on the hardening ratio, where part of the SCI). This classification, however, does not three different ductility classes for steel reinforcing mean that we consider these parts independent of each bars are defined. The classes ‘‘A’’, ‘‘B’’ and ‘‘C’’ other. There are obvious interactions when reinforcing belong to Agt higher than 2.5, 5.0 and 7.5% and by steel bars are embedded in concrete as will be hardening ratios higher than 1.05, 1.08 and between discussed in Sect. 3.2. In terms of length scales, we 1.15 and 1.35, respectively. divide the local characteristics into macroscopic and Different methods can be used to satisfy the microscopic features. In agreement with various strength and ductility requirements for construction dictionaries, we define ‘‘macroscopic’’ as something standards, and these have led to differences in steel ‘‘observable by the naked eye’’. The resolution of the microstructure [15–17]. Established processes are human eye in close reading distance is of the order of micro-alloying (MA), cold working (CW), stretching 50–100 micrometers [12]. Thus, ‘‘microscopic’’ refers (STR), and thermomechanical strengthening, e.g. the to dimensions smaller than this. TempCoreÒ process (TEMP). In thermomechanical strengthening, the steel is immediately and rapidly 2.2 Reinforcing steel quenched on the surface by spraying water after the last hot rolling sequence. Following this treatment, the From the beginning of construction with reinforced steel is exposed to air cooling. Within this period the concrete in the late nineteenth century, a wide variety martensitic quenched layer is

Load more