# Bond Strength Assessment of Concrete-Corroded
## 1 Introduction  
Rebar corrosion is a serious durability problem for reinforced concrete (RC) structures, which has been widely studied for decades globally [1; 2; 3; 4; 5; 6; 7]. In harsh environments [8; 9], such as in coastal areas and areas exposed to chloride ions, corrosion of rebars could occur when the concentration of chloride ions on the surface of rebar reaches a critical value [10; 11]. The rebar corrosion can cause cracking of concrete, and it can result in the concrete cover debonding or spalling [12; 13]. Further, as the corrosion level increases, the bond properties between the rebar and the concrete can degrade and finally threaten the safety of RC structures [14] because of the disintegration and drop in the bearing capacity. Therefore, to avoid the collapse of structures and sustain serviceability, deteriorated RC structures need rehabilitation when they are severely damaged. The bond properties between rebar and concrete should be assessed prior to taking appropriate maintenance measures [15]. To evaluate bond strength loss, it is of considerable importance to understand the cause of rebar corrosion and the mechanism of the bond strength loss. From the existing studies, they are basically revealed. Owing to chloride penetration or neutralization such as carbonation in concrete [16; 17; 18], the alkali environment in concrete is spoiled, the protective layer of ferric oxide at the rebar surface can be decomposed and rebar corrosion can then be initiated. When the rebar is corroded, corrosion products can induce radial expansive stress at the interface between the concrete and the rebar, and the concrete will then crack when the expansive stress exceeds the tensile strength of concrete [19; 20]. The bond between the concrete and the rebar is mainly contributed by the friction and mechanical interlocking force. Owing to the increase of friction contributed by a rougher surface after corrosion, a slight corrosion level may enhance bond behavior [1]. However, as the corrosion level increases, internal cracks develop and the mechanical interlocking force reduces gradually, which leads to the crack propagation towards the concrete surface. The crack width on the surface of the concrete increases with the severity of deterioration and the reduction of bond strength [21]. In this case, crack width or corrosion level is considered as the index to evaluate bond property degradation [22; 23; 24; 25]. In addition to the corrosion level, bond behavior is affected by other factors such as concrete strength and cover-to-diameter ratio because they can contribute to the confinement that restricts the development of cracking in concrete. For example, when the concrete strength is higher, with the same corrosion level, the crack width on the surface is considerably smaller [26; 27]. When considering the relationship between the corrosion level and the crack width developed based on a lower concrete strength, the calculated results will lead to underestimating the corrosion damage because a smaller crack width is observed. It is not appropriate to assess the bond strength by simply fitting the data with regard to a single factor. Thus, to accurately assess the bond strength of corroded RC member, more information is required for the calculation [28]. Although most existing studies considered specimens without stirrups, in engineering practice, stirrups confinement is commonly used for better structural performance [29; 30]. When stirrups are utilized, the diameter and spacing of the stirrups are important to the mechanical performance of the RC structures, especially to the residual bond properties after rebar corrosion because of its confinement effect [31]. There is a distinctive degradation tendency between cases with and without stirrups, and, therefore, the two cases should be considered separately [28]. For bond problems, both analytical and experimental methods have been conducted to evaluate the residual bond performance [32; 33; 34; 35; 36; 37; 38].