1. Introduction
Reinforced concrete (RC) structures are one of the most common structural systems in modern society due to their low construction and maintenance costs and high durability compared to other types of structures. However, the structural performance of RC structures deteriorates over time for various reasons, such as defective construction and structural design errors, including changes in environmental conditions, design load, and material properties. Many RC structures are affected by reinforcement corrosion, one of the primary causes of deterioration [
1,
2,
3,
4,
5,
6,
7,
8,
9,
10,
11]. The deterioration of structural performance leads to a shortened service life and reduced structural performance (see
Figure 1).
The main sources of reinforcing rebar corrosion are chloride and carbonation, caused by atmospheric or environmental conditions, for instance, ocean, deicing salt, and pollution.
The corrosion effects of RC columns and beams were vigorously examined by Hansson [
2] and Shamsad [
5], and the deterioration of members was investigated as follows (see
Figure 2):
- (1)
Formation of a white patch: Calcium carbonate is generated by the reaction of carbon dioxide in the atmosphere in cement paste with calcium hydroxide. Calcium carbonate is precipitated on the concrete surface by moisture to form white spots;
- (2)
Brown pieces of steel: When corrosion starts, an iron oxide layer is formed on the upper part of the reinforcement, which is transported to the concrete surface by moisture;
- (3)
Crack formation: The corrosion product takes up more space than the original material, and the pressure applied to the concrete causes the crack. The larger the corrosion, the wider the crack;
- (4)
Concrete spalling: The bonding stress between concrete and reinforcement materials weakens, resulting in a decrease in rebar cross section;
- (5)
Steel rebar snap: Rebar snap occurs when the cross section is reduced;
- (6)
Bar buckling: The detachment or snaps of concrete on the reinforcement contributes to the buckling of the main reinforcement. Concrete bulges and affects the stability and life of RC buildings.
The decrease in yield strength arising from the reduced effective cross section of steel bars lowers the tensile force that the bars can withstand. In addition, the compressive strength is lowered by concrete spalling. The entire structure collapses due to the severe deterioration of structural performance.
The world has experienced frequent earthquakes in recent years. The severe corrosion of RC structures in earthquake-prone areas will inevitably have an impact on their seismic performance and safety. As shown in
Figure 3, the reinforcement corrosion of RC members significantly affects the deterioration of structural performance caused by earthquakes, that is, seismic performance, as demonstrated by past earthquakes such as the 1995 Kobe Earthquake (Japan, M = 7.3), the 1999 Jiji Earthquake (Taiwan, M = 7.3), the 1999 Izmit Earthquake (Turkey, M = 7.4), the 2008 Sichuan Earthquake (China, M = 8.0), the 2010 Chile Earthquake (M = 8.8), the 2012 Great East Japan Earthquake (M = 9.0), the 2013 Lushan Earthquake (China, M = 7.0), and the 2016 Komamoto Earthquake (Japan, M = 7.0).
As shown in
Figure 4, the 2004 Niigtaken Chuetsu Earthquake (Japan, M = 6.8) and the 2017 Pohang Earthquake (Korea, M = 5.4) were of relatively low intensities, but the reinforcement corrosion of RC members caused damage to beams and columns [
1]. Reinforcement corrosion not only reduces the cross-sectional area of steel but also weakens the bonds between steel and concrete over time. As a result, steel and concrete are no longer able to function as expected, and the lateral load resistance capacity drops significantly. As such, it is extremely important to investigate the effect of the seismic performance of corroded members, in terms of strength and deformability, on the seismic performance of the entire building. This will allow a more accurate assessment of the seismic performance of RC structures with corroded members, including beams and columns.
However, current methods of evaluating the seismic performance of RC structures, including FEMA (Federal Emergency Management Agency guidelines) 310 [
12], FEMA 356 [
13], and JBDPA (Japan Building Disaster Prevention Association standard) [
14], fail to fully consider the influence of reinforcement corrosion and other performance deterioration of RC members.
According to the guidelines of FEMA 310 [
12] and 356 [
13], the seismic performance of RC structures must be evaluated in consideration of the extent of damage and impact on structural performance of each deteriorated member. As described above, the deterioration of concrete and steel significantly lowers the lateral load resistance capacity of RC members. Site inspections may be required to evaluate the effects of concrete and steel deterioration. The FEMA 310 guidelines do not provide procedures for quantitative analysis, limiting the evaluation of the seismic performance of RC structures to heuristic judgments.
Meanwhile, Japan’s standard for seismic evaluation [
14] uses the formula I
S = E
O × S
D × T to evaluate the seismic performance of RC structures by story and direction. Here, E
O is the basic structural performance index calculated with respect to the ultimate lateral resistance, deformability, number of stories, and specific story concerned. S
D (irregularity index) is a subindex used to modify E
O according to irregularities in building shape and stiffness distribution relative to building height. T (aging index) is a subindex used to evaluate the time-dependent deterioration of a building.
The aging index (T) is used to evaluate the effects of structural deterioration or aging based on site inspections, which involve an initial inspection, a follow-up inspection, and an additional in-depth inspection. During the initial inspection, T = 0.8 is applied to buildings over 30 years old, while T = 0.9 is used in cases where internal finishing shows significant signs of spalling. The smallest T among T values obtained from site inspections is multiplied by Eo.
In the follow-up and additional in-depth inspection, the overall T is calculated by summing Ti, which is the aging index of each story (i). For example, in the case of beams, Ti = 0.05 is adopted if more than 1/3 of the members inspected in each direction have reinforcement corrosion. Ti = 0.017 is adopted if reinforcement corrosion is between 1/9 to 1/3, Ti = 0.006 if below 1/9, and Ti = 0 if there are no beam members with reinforcement corrosion. In the case of columns, Ti = 0.15 is adopted if more than 1/3 of members inspected in each direction have reinforcement corrosion, Ti = 0.05 if between 1/9 and 1/3, Ti = 0.017 if below 1/9, and Ti = 0 if there is no reinforcement corrosion.
Unlike the guidelines of FEMA 310 and 356, Japan’s standard for seismic evaluation uses T for quantitative analysis of the seismic performance of RC structures with corroded members. However, this approach is not a direct method of evaluation that relies on structural performance reduction factor while taking into account the strength-ductility relationship between EO and T. Instead, it indirectly evaluates the effects of reinforcement corrosion on overall seismic performance. As mentioned above, existing seismic performance guidelines evaluate the strength and deformability of the building structure itself using structural analysis software, which is focused on structural drawings and material strength measurements. In this case, the method of performance evaluation of corrosion-damaged RC members is indirect and qualitative, rather than direct and quantitative.
Most studies on seismic performance evaluation do not consider the effects of deterioration of RC members [
8,
15]. Moreover, the quantitative reduction factor, which looks at the strength-deformability correlation of corrosion-damaged members relative to intact members, has yet to be utilized. According to the necessity of quantitative performance reduction of reinforced concrete buildings due to corrosion, some researchers [
16,
17,
18,
19,
20,
21,
22,
23,
24] have shown that reinforcement corrosion is the main cause of RC member deterioration and that it negatively affects bonding. J. G. Cabrera [
16] and Ballim. Y et al. [
23] reported that the bonding performance acting on the interface between concrete and reinforcement caused by the corrosion degree of the reinforcing bar was reduced, and the deformation occurring in the limit state of the beam member increased due to corrosion. R. Capozucca [
17] and Bhargava, K et al. [
19] presented a theory on the load-bearing capacity of reinforced concrete members reduced by corrosion. Jung, W.Y et al. [
18] and Yang, X et al. [
22] analyzed the performance degradation of RC members reduced by rebar corrosion through FEA.
Table 1 summarizes the major research references related to this study among the previously conducted studies.
The seismic performance of RC structures containing corrosion-damaged members should be directly and quantitatively evaluated using the structural performance reduction factor based on strength-deformability, that is, energy dissipation capacity. This approach will allow more accurate evaluation of the seismic performance of RC structures with corrosion-damaged members, including beams and columns. The main objective of this study is to propose a practical method of evaluating the seismic performance of RC structures with corrosion-damaged members, identifying factors contributing to structural performance deterioration based on strength and deformability for direct, quantitative evaluation of seismic performance.
To achieve the aforementioned objective, the authors examined the effects of reinforcement corrosion on the structural behavior of RC beams and factors contributing to structural performance deterioration and published the results in reference [
1]. The reference examined the effects of reinforcement corrosion on the structural behavior of RC beams and the structural performance reduction factor based on the strength-deformability correlation. For the experiment, eight shear beams and eight flexural beams were designed to evaluate the effects of reinforcement corrosion on shear and flexural failure, respectively. The impressed current technique was used to accelerate reinforcement corrosion.
The corrosion potential of reinforcing bars was quantitatively measured based on the half-cell potential (HCP), and a strong correlation was found between the structural performance reduction factor of corrosion-damaged beams and the volt-based average potential difference. That is, the correlation coefficient (R2) of flexural and shear beams was R2 = 0.78 and R2 = 0.91, respectively. The potential difference measured using the half-cell (HC) method can be used as an indicator of relative deterioration of structural performance if environmental conditions are constant. Additionally, also revealed was the possibility of evaluating energy absorption capacity (structural performance reduction factor) based on the strength-deformability of corrosion-damaged RC members, as well as a correlation between the reduction factor and average potential difference.
However, current research evaluates the correlation between the extent of corrosion and structural performance deterioration of RC beam members, which are not members that resist lateral force. As such, the results cannot be directly applied to the evaluation of seismic performance of RC structures containing corrosion-damaged members. To achieve this study’s main purpose of proposing a practical method of evaluating the seismic performance of the RC structures comprised of corrosion-damaged members, analytical methods including structural experiments should be applied to corrosion-damaged lateral resisting members, namely, column members of the shear failure type with non-seismic details.
As shown in
Figure 5, this study performed cyclic loading tests on columns of the shear failure type having reinforcement corrosion to examine the correlation between HCP before and after corrosion and seismic performance deterioration. At the same time, FEA was carried out in consideration of the weakened bonding between steel and concrete, in order to analyze the correlation between structural performance deterioration before and after corrosion of shear columns. Through a comparison of the experimental findings and FEA results, this study proposed a seismic performance reduction factor in relation to the extent of corrosion of shear columns.
6. Conclusions
It is extremely complex to estimate the effect of the seismic capacity of corroded structures on the seismic capacity of the entire structural system in terms of force and deformation. This will allow a more accurate assessment of the seismic capacity of RC buildings with corroded beams and columns. Existing techniques for estimating the seismic capacity of RC buildings, however, fail to fully consider the influence of reinforcement corrosion and other performance deterioration of RC structural systems, including beams and columns. The essential purpose of this study is to suggest a realistic technique for estimating the seismic capacity of RC structural systems with corrosion-damaged beams and columns, identifying factors contributing to structural performance deterioration based on strength and deformability for direct, quantitative evaluation of seismic performance.
To achieve the aforementioned objective, the authors examined the effects of reinforcement corrosion on the structural behavior of RC beams and performed experiments demonstrating the correlation between the average potential difference of beam members obtained via HCP and the reduction factor based on strength and deformability [
1].
However, current research evaluates the correlation between the extent of corrosion and structural performance deterioration of RC beam members, which are not members that resist lateral force. As such, the results cannot be directly applied to the evaluation of the seismic performance of RC structures containing corrosion-damaged members. To achieve this study’s main purpose of proposing a practical method of evaluating the seismic performance of RC structures comprised of corrosion-damaged members, analytical methods including structural experiments should be applied to corrosion-damaged lateral resisting members, namely, column members of the shear failure type with non-seismic details.
This study performed cyclic loading tests on columns of the shear failure type having reinforcement corrosion to examine the correlation between HCP (mV) before and after corrosion and the seismic performance reduction factor (ϕ). At the same time, FEA was carried out to assess the effects of weakened steel-concrete bonding on seismic performance deterioration based on strength-deformability, and the correlation between bonding factor (β) and seismic performance reduction factor (ϕ) was established. By comparing β–ϕ and mV–ϕ, derived from FEA and structural experiments respectively, this study integrated the correlation of β and mV from the perspective of ϕ, and produced the ϕ–β–mV interaction diagram. The results can be summarized as follows:
- (1)
Structural experiments of shear failure-type column members in relation to reinforcement corrosion showed that the seismic performance reduction factor (ϕ) defined based on dissipation energy before and after corrosion decreases with a smaller average potential difference (mV) measured by HCP. The reduction factor can be approximated using regression analysis as , and the coefficient correlation R2 = 0.93 indicated a strong correlation;
- (2)
The experimental results and FEA results (β = 1) were compared for the control specimen (corrosion-free, SC-C0). The ultimate load in the analysis was 202.9 kN, 105% of the experimental value of 192.4 kN. The ultimate displacement at this point was 12.5 mm in the analysis, about 83% of the experimental value of 15.0 mm. The dissipation energy was found to be 2105.9 kN-m, which is highly similar to the experimental value of 2106.7 kN. The analysis results using VecTor2 were consistent with experimental observations. The proposed FEA model can be seen as effective in examining the effects of weakened bonding on structural performance reduction based on strength-deformability, that is, the correlation between bonding factor (β) and seismic performance reduction factor (ϕ);
- (3)
The bonding factor (β) and structural performance reduction factor (ϕ) can be approximated using , obtained by regression analysis. The correlation coefficient of β-ϕ was R2=0.93, which indicates a very strong correlation. In addition, the β–ϕ correlation and the correlation between potential difference (mV) and structural performance reduction factor (ϕ), that is, , were compared in order to express the β–mV correlation in terms of the seismic performance reduction factor (ϕ) as ;
- (4)
The β–ϕ correlation of each column specimen based on FEA was compared to the mV–ϕ correlation, which is the relationship between average potential difference, quantitatively measured by HCP, and seismic performance reduction factor, derived from structural experiments. The mV–β–ϕ interaction diagram of shear failure-type columns was thus established. Using the mV–β–ϕ interaction diagram, it is possible to evaluate the weakened bonding (β) in relation to the extent of corrosion (mV) and seismic performance reduction factor (ϕ) based on strength-deformability. The results indicate that the proposed method can be utilized for quantitative evaluation of the seismic performance of corrosion-damaged RC members;
- (5)
To develop practical, commercial methods for the evaluation of the seismic performance of corrosion-damaged RC structures, it is necessary to develop techniques for quantitative measurements based on HCP, estimation of seismic performance reduction factor, and nonlinear analysis of the seismic performance of the entire structure in consideration of performance deterioration. As a recommendation for future work, it is recommended to apply multiple NDE (Nondestructive Evaluation) techniques rather than HCP alone to increase the reliability of the interaction diagram proposed in this study.