Rehabilitation of Composite Column Subjected to Axial Load

Concrete Filled Double Steel Tube (CFDST) columns are a modern technique of composite structural element that has fire resistance and has been adopted in high-rise building structures. The Concrete Filled Steel Tube (CFST) columns also have high strength and ductility due to composite action. This type of column CFST can sustain a heavy load with high performance and has been adopted in recent years in many countries around the world. The aim of the present work is to study the behavior and strength of rehabilitation of composite columns that are made from concrete core and surrounded steel tubes under the effect of axial compression loads with different height to diameter ratios such as 5.46, 10.91, and 16.37, respectively, by experimental tests. Double skin methodology is adopted to repair the damaged columns that were tested up to 85% of the ultimate load. Strength column capacity of double skin columns, axial and buckling deformations with axial and buckling strains are investigated. Test results showed that the repaired specimens up to 85% of the ultimate load had the same strength carrying capacity as compared with the control specimens, which had the same geometry. The ductility of an 800 mm specimen’s height is greater than the other tested specimens, while the stiffness of short specimens becomes high.


Introduction
The term "composite structure" describes connections of two or more materials that differ in modulus of elasticity and Poisson's ratio to form a structural element such as a composite beam, column, slab, or shear wall. The CFST structural members combine the best mechanical properties of connected materials such as steel and concrete and provide the composite action. The composite action results from the mechanical interlock, friction, and adhesion at the interface of two materials [1][2][3][4][5]. The composite action occurs when the outer surface of concrete touches the surrounding surface of the steel tube, which makes the two materials work as a composite. The composite action relies on the degree of connections between the concrete column and steel tube. When there is full bound between these two materials due to full interaction, which means working as a unity, so that there is no slip at the interface of these two materials. In other cases, when there is partial interaction, that leads to developing slip at the interface. An imperfect connection between concrete column and steel tube at interface bonding due to the initial elastic phase under the effect of uniaxial load will have an unfavorable effect on the composite action that leads to a reduction in the elastic strength and an addition to the reduction in initial stiffness so that the interface bonding becomes more critical as the concrete strength increases [6]. Different methodologies are available to repair the CFST column to prevent the buckled column or a reduction in CFST strength capacity. This type of composite column has good resistance against seismic loading and gives more ductility [1][2][3][4][5]. This type of column has the best mechanical properties of both steel and concrete materials and provides a composite action. The composite action between steel and concrete resulted from the mechanical interlock, friction, and adhesion. There are many methods that can be adopted to strengthen, re-strengthen, or repair composite columns based on the type and level of damage. The conventional repair method is to weld steel plate surrounding the damaged area of the column, but another technique that can be used is to cast large reinforced concrete jackets around the deformed columns [7].
Carbon fiber reinforced polymer CFRP sheet that wraps around the damaged column is another methodology known as structural respire. The compressive behavior of the damaged column by adopting this method was increased by the CFRP jackets. Ductility of the repaired column is enhanced, especially for long columns. Tao & Han (2007) [8]. Repaired of damaged columns by using concrete jackets and steel tubes that were placed surrounding them. This method causes an increase in axial loads. Also, notice that the stiffness and strength capacity of the repaired column increased so that it could be reinstated to its original condition [1]. Confining of damaged column in addition to the used schemes of the FRP family such as GFRP, CFRP, AFRP, and BFRP (Glass, Carbon, Armed, and Basalt fiber reinforced polymer), there are different ways that include internal steel sections, stiffeners, tie bars, and external rings [9].
Among all of the above methods, these methods vary in effectiveness. External rings may be an effective and significant practical solution due to their increasing the strength capacity of the column, ductility, and stiffness. The steel rings are welded externally around the steel tube. Rings can enhance the strength capacity and increase the column stiffness with a decrease in the degradation strength rate. The presence of steel rings creates confinement of the CFST columns. Almasslawi et al. (2020) [10], investigated the behavior of a rapidly buckling CFST column that was subjected to an axial compression load. The column type that was focused on in experimental tests was the concrete-filled circular steel tube (CFCST) that was adopted to repair the buckled CFST columns. The buckled CFST columns were placed centrally inside the CFCST that consisted of a larger diameter steel tube, and concrete was poured into the gap between the deformed CFST column and the larger tube. The main parameters were adopted as tube thicknesses in which all specimens were tested to failure under the effect of axial compression loads. The study included the improvement of strength capacity due to the composite action of the CFST column. The buckled column was, as usual, placed inside a large steel tube and the concrete filled the outer buckled column (inside the large steel tube) in which the study took into account different steel tube thicknesses of large diameter. Test results were compared with control undamaged columns. According to the findings, the adopted methodology for repairing damaged columns restores the strength capacity to 97-100% of the original capacity of the undamaged CFST columns.
Ekmekyapar & Al-Eliwi (2017) [11] investigated stub columns in the case of CFDCST as a concrete field with double circular steel tube through experimental. In this re-strengthening, the deformed column is centered inside a larger column with a diameter calculated to give the same strength as an un-deformed column in which the space between the deformed and larger steel tube is filled with concrete. Different parameters were adopted, such as core compressive strength, shell compressive strength, outer diameter of the steel column, and the yield strength of steel tube. The concrete that was used to repair the deformed columns is classified as normal and high strength concrete. To check out the restrength of deformed columns, the tested columns were up to 85% of the failure load, then the repair procedure was applied and the new column tested. The experimental results showed that the adopted method of double steel tube improved the ductility and stiffness of the composite columns, so that it can be used to repair the deformed column. Based on the test results, a different concrete grade can be used to repair the deformed column. The presence of the outer steel tube gave more confinement to the deformed column and an increase in strength capacity. Ramanagopal (2018) [12] studied the performance of the double tube stub column by an experimental approach. Different parameters were considered, such as the concrete grade of 30, 40, and 50 MPa with a diameter/thickness ratio of 33.3 and a height/diameter ratio of 3. Experimental results showed that the stiffness of the composite column was enhanced because of the extra inner carbon steel tube. Typical failure modes of the outer tubes were local outward buckling and occurred near mid-height for some of the column specimens. Yan & Zhao (2020) [13] investigated the compressive strength of the short double skin of CFST columns. A total of twenty-four specimens were cast and tested under axial compression load. Different parameters were considered, such as the outer steel tube diameter, the thickness of the steel tube and concrete compressive strength of 29.0, 37.5, and 51.0 MPa. Test results showed that the confinement coefficient decreases as the compressive strength of concrete increases; this coefficient also decreases if the diameter/thickness ratio is small. Increases in concrete compressive strength and steel tube thickness significantly improve the CFST strength capacity. Yan et al. (2021) [14] studied the confinement of CFST and CFDST as double skin columns based on the compressive strength model. Various parameters were adopted, such as the outer diameter and thickness of the steel tube and the concrete compressive strength. Test results indicated that the confinement of CFDST was less than CFST. He et al. (2019) [15] studied the confinement effects on CFST columns by using different grades of compressive strength of concrete. A total of six specimens were cast and tested under the influence of axial compression load with different parameters like concrete compressive strength and diameter/thickness ratio of steel tube. The concrete grade ranged between 29.5, 43.5, 58.0, and 81.6 MPa in the first group, and 32.0 and 64.0 MPa in the second. The steel tube diameter and thickness were 165.2 and 3.7 mm for the first group, while 230 mm in diameter and 2.3 mm in thickness for group two. According to experimental investigation, the confinement factor influenced the confinement effect, in which a higher confinement factor gave more confinement effect. Also, the low compressive strength of concrete gave better behavior and more ductility as compared with high grade concrete. Ke et al. (2022) [16], investigated the axial compression performance of CFST columns under the effect of temperature load. Fourteen specimens after exposure to high temperatures. It has been taken into consideration different parameters such as heating temperature, steel tube diameter and concrete cover thickness. Based on test results, it was concluded that the load-displacement curves of the specimens were significantly affected by high temperatures, while the influence of steel tube diameter and concrete cover thickness was relatively weak. Naji et al. (2021) [17] looked at the behavior of reinforced concrete columns that have been rehabilitated with a concrete liner, a confinement FRP fabric, a steel liner, or a metal corner cage. The investigation showed that the usage of fiber reinforcement polymer has no effect on the weight of the structures that gave the best results and behavior. Hadi et al. (2021) [18] studied the influence of CFRP wrap on pre-damaged reinforced concrete columns. According to test results, the mechanical behavior, particularly for ductility, stiffness, and ultimate load capacity, was improved and increased due to the enhancement with CFRP wrap. Jamkhaneh & Kafi (2019) [19] assessment of partially encased composite column behavior under compressive and bending moment loading. Test results showed that the failure mode was similar for all tests the concrete cracking coupled with the steel flange buckling. Al-Adawy et al. (2021) [20] studied the performance of short concrete composite columns under the effects of uniaxial or biaxial eccentric loading. A comparison between the behavior of each tested column before and after being repaired by applying different types of sheets of carbon fiber reinforced polymer layers was made. The comparison showed that wrapping composite steel columns with carbon fiber reinforced polymers is considered a very effective technique for repairing.
The gap that try to fillet by this research is to proposed methodology to respire the damaged composite column by apply double layer skin with same concrete grade of inner concrete.

Experimental Program
The adopted raw materials to cast the concrete and form the steel tube section, in addition to the manufactured composite double layer for repair of tested composite columns, preparations are summarized. Firstly, the steel was formed by steel plate and welded by argon, and then the concrete core was casted inside the steel tube to form a composite column. The single layer composite column specimens were tested for each height up to failure, and some of these specimens were tested up to 85% of the ultimate load. Load control was adopted to apply load to each specimen up to failure. The total height of the adopted columns is 400, 800, and 1200 mm. The inner tube thickness (1.35 mm) and concrete core diameter (76 mm). The specimens were tested under the effect of axial load up to 85% of the ultimate strength capacity of each specimen. The layout of experimental work is shown in Figure 1. The concrete and steel tube mechanical properties are listed in Table 1, in which the concrete mechanical properties tests are based on ASTM C39 [21] for compressive strength. Specification of ASTM C496 [22] for splitting tensile strength. Standard specification ASTM C293 [23] for modulus of rupture. Testing material specifications according to ASTM C597 [24] for modulus of elasticity. The mechanical steel tube was tensile tested based on ASTM A370 [25].    Figure 2 shows the specimens of hollow steel section, Figure 3 shows the double layer composite columns and Figure  4 shows the test setup. Each specimen, a hollow steel tube, was filled with concrete to represent the concrete core and make the top surface smooth so that the applied load is distributed uniformly. The bottoms of the steel tubes were closed by applying silicone and thick steel plates that were removed before the specimen was tested, and then, after that, the concrete was poured into the insides of the steel tubes. The top of each specimen (rounded 5 mm) was left empty to level the concrete with epoxy. Each specimen was placed at the center of the machine test to avoid eccentric loading. Each specimen was setup in the middle of the testing machine, and after that the axial compressive load was applied at a constant rate in which axial, buckling displacement, and strain were recorded for each applied load step up to 85% of the failure load for each specimen. Double layer specimens were tested and an additional to repair of tested specimens up to 85% to check out the rehabilitation method to re-strength the tested composite column specimens. Two different outer diameters are adopted, such as 120 mm with 1.35 mm thickness and 1.35 mm with 0.80 mm thickness, to repair the tested specimens up to 85% of the ultimate load.

Results
Axial and lateral deformation as buckling are recorded during tests for all specimens and an additional to axial and lateral strain for each specimen. Tables 3 and 4 lists the test results as axial displacement and axial strain, buckling displacement and buckling strain based on maximum and failure load in which the specimen CFST100% represent the single layer specimen that tested up to failure (100% ultimate load), CFST85% is the specimen that tested up to 85% from the ultimate load, RD120, RD130 are the specimens that repaired the CFST85% by external diameter 120 and 130 mm respectively, specimens RDR120 and RDR130 represents the control double layer specimens by adopted 120 and 130 mm outer diameters respectively. Figures 5 to 16 shows the axial, buckling deformation, axial and lateral strain for all tested specimens. The specimens behaved as nonlinear after pass the linear zone that end at the point of inflection when the strain become more (or displacement) so that the specimen become weak and reduce in stiffness due to reduce in modulus of elasticity of the specimen. The location of inflection point for each specimen differ than other specimens that relies on many parameters such as slenderness ratio and strength capacity of the specimen. Test results showed that increased in outer tube thickness lead to increase in strength capacity and reduce in deformations such as in control specimens RDR120 when compared with strength capacity of specimens RDR130. The strength capacity of repaired specimens closes with that of control specimens (specimens RDR120 compared with RD120, specimens RDR130 compared with RD130) that is mean the repaired methodology design is suitable and accurate.    Table 5 lists the ductility and stiffness for all tested specimens. Figures 17 to 20 represents the ductility and stiffness comparisons for all specimens for both axial and lateral (buckling) displacements. Ductility of the column as structural element classified as ability to carry considerable deflection prior to failure. The ductility of column as composite indicate and give sign of quality and is there failure that lead to prevent the collapse of the column under large deformations. The presences of steel tube in composite specimens improved the concrete property especially ductility and an additional to make confinement of concrete to increase the compressive strength of concrete core.    The ductility U for all tested specimens calculated by divided the deflection at ultimate load Δu by the deflection at yield Δy as follow: The RD120 and RD130 specimens gave ductility more than the repaired specimens with same diameter and tube thickness such as specimens RDR120 and RDR130. The ductility of specimens RD120 has ductility higher than that of specimens RD130 in case of specimen's height 400 and 800 mm. The higher ductility occurs in case of specimens have height 800 mm while the stiffness as axial and lateral in case of short specimens with height 400 mm. The stiffness of the tested specimens is the resistance of an elastic body to deflection or deformation by an applied force. The stiffness μ is defined as the ratio of the ultimate load to ultimate load deflection Δ as follow: The RD120 and RD130 specimens has stiffness greater than the repaired specimens have same diameter and tube thickness such as specimens RDR120 and RDR130 with height 400 and 800 mm. The stiffness of specimens RD120 is higher as compared with RD130 specimens in case of specimen's height 400 and 800 mm.

Mode of Failure
The failure mode for all specimens are illustrated in Figures 21 to 23. The CFST specimens that tested up to 100% full load, steel surrounded tube having yield and buckling for the global. In case of specimens that repaired by double layer as concrete and outer steel tube RDR120 and RDR130, global buckling occurs for all tested specimens due to progress of axial strain that developed that lead to make fracture in the outer steel tube. In case of double layers as control or repair, the presence of outer steel tube provides more confinement to the concrete core that the specimens behaved as composite action and make the specimens buckled failure mode.

Discussions
The smallest height specimens gave high strength capacity as compared with the same tested group due to the short specimen's low slenderness ratio when compared with the 800 and 1200 mm that led to increasing the buckling load resistance. The strength capacity of control specimens such as RD120 and RD130 compared with the repaired specimens, RDR120 and RDR130, is rounded to the same degree. That means the repaired methodology that was adopted successfully re-strengthened the tested specimens up to 85%. According to investigation results, the strength capacity for each specimen under control and repair is compared in Table 6. The ductility of CFST100% with an 800 mm height greater than other specimens within the CFST100% group shows the maximum displacement value is worthless compared to other specimens. Furthermore, the specimens have a maximum rounded thickness of 800 mm, indicating high ductility. The stiffness increases when the height becomes 400 mm or less. This kind of specimen has a low slenderness ratio that is directly related to more buckling. Axial longitudinal displacement (800 and 1200 mm) increases with height. The axial displacement relies on both (PL/AE) for the same rigidity, with displacement getting more.

Conclusion
Based on the experimental investigation for rehabilitation of tested composite column specimens under axial loadings, test results showed that when applied double layers as concrete core surrounded the inner steel tube of a tested composite single layer column and confinement by external steel tube gave close results as for strength capacity when compared with the control specimens RDR120 and RDR130, respectively. Repaired test specimens up to 85% of the ultimate load gave the same strength carrying capacity as compared with the control specimens that had the same geometry. The ductility of an 800 mm specimen's height is greater than the other tested specimens, while the stiffness of short specimens becomes high. The control specimens (RDR120 and RDR130) and the repaired specimens gave a strength carrying capacity higher than the single layer composite specimen that tested up to ultimate load (specimen CFST100%), whereas the double-layer specimens gave more confinement to the tested specimens up to 85% by the presence of a concrete core layer and an outer steel tube. The outer steel tube gave confinement to the outer and inner concrete cores, which made an increase in concrete compressive strength by applying revised hoop stress. The ductility and stiffness of control and repaired specimens were higher than those of specimens that were tested up to failure load (specimens CFST100%) due to the presence of an outer concrete core and steel tube. Based on the methodology that was adopted for the rehabilitation of damaged columns, it was recommended to apply the same methodology for repairing the damaged composite columns because it gave a strength carrying capacity higher than the original specimens because the present study was limited to the same compressive strength as the inner and outer concrete core diameters.

Author Contributions
A.N.H., and S.R.A. contributed to the design and implementation of the research, to the analysis of the results and to the writing of the manuscript. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement
The data presented in this study are available in the article.