1. Introduction
In multistory RC buildings in Saudi Arabia, RC wall-like columns are primarily used to maximize space efficiency. These columns are frequently in need of retrofitting for several reasons, such as the addition of more stories or an increase in live load due to changes in building use. Traditional methods for upgrading RC members/columns involved wrapping them with RC jackets [
1,
2] or steel jackets [
3,
4,
5,
6,
7,
8], but these approaches are labor-intensive and complex to execute. Consequently, due to its favorable properties [
9,
10,
11,
12,
13], there has been a recent shift towards using FRP composites for upgrading RC wall-like columns, as they offer benefits such as rapid and straightforward installation with minimal alteration to the column’s dimensions.
The majority of research in the field of FRP-strengthened RC rectangular columns subjected to concentric loading has focused on sections with depth-to-width ratios of up to 2. There have been only a limited number of studies [
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25] that specifically addressed the retrofitting of wall-like columns with the help of FRP composites. Tan [
14] conducted experiments to investigate FRP-enhanced columns with an aspect ratio of 3.65. The study compared the peak axial load of the columns with previously proposed prediction models [
26,
27]. In another study by Hosny et al. [
15], the behavior of FRP-enhanced columns with sections with an aspect ratio of 3 was examined. The experimental FRP strains at peak load were significantly less than the failure strains. Tanwongsval et al. [
16] conducted experimental research on wall-like columns subjected to concentric loading, both without strengthening and with strengthening, with sections of the same aspect ratio as mentioned in Ref. [
14]. Two methods were employed for column strengthening, one using conventional externally bonded GFRP sheets, and the other using GFRP wrapping after modifying the section. Columns upgraded with the second technique exhibited superior performance due to enhanced confinement of concrete. Maalej et al. [
17] conducted experiments to investigate the influence of FRP schemes on enhancing the peak load of wall-like columns with sections featuring an aspect ratio of 3.65. In addition to the laboratory tests, they utilized an analytical model previously proposed in Refs. [
28,
29] to calculate the columns’ peak load.
Prota et al. [
18] undertook an experimental campaign to study the effects of applying GFRP sheets for enhancing the axial resistance of wall-like columns characterized by substantial depth-to-width ratios. Their research findings indicated that the GFRP wrapping resulted in enhancements in both the ductility and strength of these columns. Notably, the failure of GFRP-retrofitted specimens was found to be contingent on the shape of the column section, occurring at considerably lower horizontal GFRP strains. De Luca et al. [
19] examined wall-like columns strengthened with GFRP sheets by testing three specimens, including one control and two that were retrofitted. These upgraded specimens utilized two different GFRP confinement ratios. The study’s results demonstrated that, although GFRP confinement did not necessarily enhance the maximum load, it significantly improved the concrete crushing strain.
In their research, Alsayed et al. [
20] investigated the retrofitting of RC wall-like columns with FRP composites under concentric compression. The original rectangular section underwent a transformation into an elliptical shape using cementitious mortar, which was subsequently retrofitted with CFRP sheets. Nonlinear finite element analysis was employed to evaluate the load-displacement characteristics of these columns. The use of CFRP wrapping resulted in an enhancement of both the ductility and strength of the columns.
Triantafillou et al. [
21] experimentally examined the behavior of FRP-strengthened wall-like columns. They conducted tests on a total of forty-five columns, having section aspect ratios of either 3 or 4, subjecting them to axial compression. The study explored different strengthening methods, encompassing anchored and unanchored CFRP wrapping, both without and with the modification of sections. Their findings led to the conclusion that the efficiency of CFRP confinement was nearly doubled when anchors were appropriately distributed
Elsanadedy et al. [
24] conducted a study to come up with an efficient scheme, without section modification, for strengthening existing RC wall-like columns employing a hybrid of NSM and CFRP wrapping systems. Six half-scale specimens were subjected to concentric compression testing. Two of these columns remained unretrofitted, serving as reference, while the remaining four were upgraded using different schemes. Among the four schemes explored, the most efficient approach involved the use of continuous NSM rebars in combination with CFRP wrapping, resulting in an impressive 80% increase in the ultimate load. Additionally, nonlinear FE analysis was employed to assess the response of the tested columns. The test results closely aligned with the FE analysis, confirming the accuracy of the constitutive models employed for different materials. In another companion study, Elsanadedy et al. [
25] studied the axial strengthening of wall-like RC columns using another four different configurations. These schemes were also without shape modification, and they incorporated the use of GFRP wrapping alone, GFRP wrapping combined with bolted steel plates, or GFRP wrapping combined with connected (or disconnected) NSM bars. Nonlinear FEA was also performed to predict the response of columns.
Even though past research was conducted concerning the behavior of control and retrofitted RC wall-like columns, the specimens in these studies were not preloaded prior to their strengthening. This did not reflect the real practice in which unstrengthened columns carry their service load and if strengthening is required when the load is partially or totally released. The objective of this investigation was to explore the response of concentrically preloaded wall-like RC columns after being strengthened using different configurations. In the experimental campaign, four half-scale specimens were tested in the event of concentric compressive load. The first two columns were unstrengthened, loaded to 80% of their axial capacity, and the load was then totally released. After that, these specimens were retrofitted using two different schemes, and hence, they were concentrically loaded until failure. Other than the testing campaign, nonlinear FE analysis was performed to examine the response of tested specimens. The validated models were then employed for conducting parametric studies of practical interest.
5. Parametric Study
The numerical models were then utilized to explore the impact of key parameters such as percent of preloading and amount of load release on the axial behavior of strengthened columns. The FE analysis matrix used for the parametric study is shown in
Table 5. Since the second strengthening scheme had superior performance to the first one, it was employed in this study. As depicted in
Table 5, the analysis matrix included three different percentages of preloading (40%, 80%, and 90% of the peak load of the unstrengthened column). The 40% preloading was selected to represent the service load, and the 90% preloading was nominated to simulate preloading close to the axial load capacity of the unstrengthened specimen. In half of the preloaded specimens, the load was totally released; and in the other half, the load was sustained without releasing. The former case was selected to represent the case of full shoring of the column before strengthening; however, the latter case stands for the worst-case scenario of no shoring prior to column strengthening. In the designation of column specimens listed in
Table 5, the same symbols used previously in
Table 3 were utilized as detailed in
Section 2.1. Also, the symbol “NLR” used in
Table 5 stands for preloaded columns with no load release. It should be reported that specimens CON and ST2 that were previously modeled in Ref. [
25] were included in the analysis matrix (
Table 2). Altogether, the FE matrix used in the parametric study incorporated seven control specimens and seven strengthened columns. In the preloaded control specimens CON-0.4P-TLR, CON-0.8P-TLR, and CON-0.9P-TLR, the load was totally released after reaching its peak value, and the pressure–time history curves used in the analysis are shown in
Figure 13a. Nevertheless, in the preloaded control columns CON-0.4P-NLR, CON-0.8P-NLR, and CON-0.9P-NLR, the load was sustained without release after reaching its peak value, and the pressure–time history plot used in the models is illustrated in
Figure 13b. As identified previously, the FE analysis of the preloaded strengthened specimens was conducted in two stages, as explained earlier in
Figure 14, and the specimens were analyzed under the displacement–time history plot shown in
Figure 15.
A summary of the key FE results for the specimens of the parametric study is reported in
Table 6. These results incorporated the main load-displacement parameters such as yield and maximum loads and their corresponding displacement, ultimate displacement, secant stiffness, and dissipated energy. Also included in
Table 6 are the ultimate concrete strain and strain in main vertical steel rebars at peak load.
Figure 20a,b presents comparisons between the FE load against displacement plots for columns with total load release and columns with no load release, respectively. It is clarified from
Figure 20 and
Table 6 that for the same amount of load release, the parameters of the load-displacement response of the strengthened specimens without preloading (ST1 and ST2) are higher than all their preloaded counterparts. Also, for the preloaded strengthened specimens, the load-displacement response was improved with the reduction of percent of preloading.
Figure 21 shows the effect of the percent of preloading on the percent increase in the response parameters due to strengthening (compared with the unstrengthened specimen CON). These response parameters included peak load, secant stiffness, and dissipated energy. It is noted that for all response parameters, as the percent of preloading increased, the percent increase due to strengthening decreased, and the decrease in the case of no load release was more than the case of total load release.
As seen in
Figure 21a, as the percent of preloading increased from zero (specimen ST2) to 90%, the percent increase in peak load due to strengthening was significantly reduced from 113% to 87% and 37% for the cases of total load and no load releases, respectively. Also,
Figure 21b illustrates that as the percent of preloading increased from zero to 90%, the percent increase in secant stiffness decreased considerably from 57% to 30% and 1% for the cases of total load and no load releases, respectively. Moreover, the increase in the dissipated energy due to strengthening decreased significantly from 551% to 101% and 21% (for total load and no load releases, respectively) as the percent of preloading increased from zero to 90%, as presented in
Figure 21c.
Figure 22 illustrates the impact of preloading on the reduction in peak load, stiffness, and dissipated energy for strengthened specimens. Compared with the strengthened specimen without preloading (ST2), as the percent of preloading increased, the reduction in the response parameters of the retrofitted specimens increased. For the case of total load release, as the percent of preloading increased from 40% to 90%, the reduction in peak load, secant stiffness, and dissipated energy was enhanced from 3% to 12%, 7% to 17%, and 13% to 69%, respectively. However, for the case of no load release, the loss in maximum load, stiffness, and energy dissipated increased from 10% to 36%, 23% to 36%, and 62% to 81%, in turn.
Figure 23 presents the influence of the unrelease of preloading on the percent reduction in response parameters for strengthened specimens. Compared with preloaded strengthened columns with total load release, the unrelease of preloading increased the reduction in peak load from 7% to 17% and 27% as the preloading increased from 40% to 80% and 90%, respectively, as presented in
Figure 23a. Similarly, the unrelease of preloading increased the loss in stiffness from 17% to 22% and 23% when the preloading increased from 40% to 80% and 90%, respectively (see
Figure 23b). The same trend was not found in the dissipated energy since the unrelease of preloading increased its reduction from 56% to 71% as the preloading increased from 40% to 80%. However, as the preloading increased further to 90%, the loss in dissipated energy due to unrelease of preloading decreased to 40%, as depicted in
Figure 23c.
From the parametric study conducted in this research, it is ultimately concluded that even with the worst-case scenario of unstrengthened RC wall-like columns preloaded with 90% of their axial resistance without releasing the load, their strengthening using the second scheme is efficient at enhancing the load-displacement characteristics. Compared with the unstrengthened column, the upgrading scheme in such a case improved the peak load and dissipated energy by 37% and 21%; however, it almost retained the axial secant stiffness of the column.
The novelty of this research is generated from the fact that columns tested and modeled in this study simulated the actual practice in which the unstrengthened columns bear their load, and if retrofitting is necessary, the load is released before the upgrade. However, prior research studies on strengthening RC wall-like columns [
20,
21,
22,
23,
24,
25] overlooked this real-world scenario. For columns tested in Ref. [
20], the studied strengthening scheme enhanced the peak load by 24%. In Ref. [
21], the peak load enhancement in RC wall-like columns owing to strengthening varied from 29% to 119%. For the wall-like columns investigated by Abbas et al. [
22], the peak load enhancement due to strengthening ranged from 30% to 43%. The peak load enhancements for the wall-like columns tested in Refs. [
23,
24] varied from 27% to 80%. Lastly, strengthening schemes explored by Elsanadedy et al. [
25] enhanced the peak load of RC wall-like columns by 34% to 126%. In conclusion, it was proven in previous research on RC wall-like columns without preloading that different strengthening schemes are effective at enhancing the axial capacity by ratios varying from 24% to 126%. The challenge is the success in obtaining similar peak load enhancement in the case of strengthening preloaded RC wall-like columns. This was achieved successfully in this current research. For the worst-case scenario of preloaded columns with no load release, the second scheme investigated in the current research was successful at improving the axial load-bearing capacity of the columns by 93%, 65%, and 37% for 40%, 80%, and 90% of preloading, respectively.
It is worth noting that for practicing structural engineers, the axial capacity of strengthened RC wall-like columns can be reasonably assessed using the simplified analytical approaches discussed in Refs. [
23,
25]. These approaches can be extended to include the effect of preloading after the inclusion of appropriate reduction factors based on the results of this study.