## 1 Introduction  
The increasing amount of deteriorating infrastructure demands for effective methods to obtain an enhanced and early assessment of the damage level to reliably decide on maintenance and repair interventions. Therefore, by combining certain non-destructive techniques (NDT) and structural health monitoring (SHM), the structural integrity of different parts of the structure can be assessed non-invasively. Some techniques would even allow the detection of damage that is not yet visible for the human eye. Specifically for damage mechanisms that start internally, such as corrosion in reinforced concrete (RC), such techniques are particularly interesting. Corrosion of the reinforcement is generally found to be the most common deterioration problem in existing RC structures. Due to the high alkaline environment of the surrounding concrete, the reinforcement is normally protected from corrosion. However, the protective oxide layer is easily destroyed after which a complex electrochemical process takes place. At anodic sites, steel dissolves which causes a reduction of the cross section of the rebar. At cathodic sites, iron ions combine with water and oxygen forming corrosion products or rust. The corrosion process will influence the behavior of the steel, the concrete, and the interface. First, as previously mentioned, corrosion will lead to a reduction of the cross section of the rebar. Contradicting findings are reported in the literature on the effect of corrosion on the yield stress and tensile strength [1; 2; 3; 4]. However, authors have reached the consensus that corrosion causes a drastic reduction of the ductility of the steel [3; 4; 5; 6]. The rebar will undergo less plastic deformation before rupture. Second, as the formed corrosion products have a lower density, and thus occupy a greater volume than the original steel volume, they exert an internal pressure on the surrounding concrete. Eventually, the tensile strength of concrete is reached due to this pressure build-up which will result in concrete cover cracking, delamination, and even spalling of the concrete cover. Third, cracking and steel section loss will have an influence on the bond between steel and concrete. A small amount of corrosion (0-4% mass loss) is found to have a positive effect on the bond strength as corrosion products fill the pores closest to the rebar leading to more confinement [7; 8]. For higher corrosion levels, the bond strength decreases. This is mainly caused by the formation of cracks, but also the reduction of the mechanical interlocking plays an important role [9]. These three mechanisms affect the eventual load-bearing capacity of RC components and structures. A reduction in bending capacity as well as changes in failure mechanism from flexural to bond are reported [10; 11]. Rupture of the rebar is possible at high corrosion levels due to the formation of deep pits. Corrosion in RC is typically a slow process. Therefore, it can take years before the first signs of concrete degradation can visually be observed, leading to large financial costs for maintenance and repair. An accurate assessment of the actual damage accumulation would allow a more accurate estimation of the residual capacity. In addition, early detection of damage progress with NDT can assist in preventive maintenance schemes which can possibly enable prolonging the service life of the structure. Presently, existing RC structures are typically assessed by visual inspection, core drilling, and electrochemical techniques [12]. These inspection techniques are valuable; however, they have several disadvantages such as being inspector-dependent, local, or destructive. Furthermore, they do not allow the continuous monitoring of the structure. Therefore, only sporadic inspections can be performed. The acoustic emission (AE) technique may serve as an important addition as it has four important advantages. First, it enables the continuous detection of damage in a non-invasive way by placing sensors on the surface of the structure.