Axial Behaviour of Slender RC Circular Columns Strengthened with Circular CFST Jackets

'is paper investigates the axial behavior of slender reinforced concrete (RC) columns strengthened with concrete filled steel tube (CFST) jacketing technique. It is realized by pouring self-compacting concrete (SCC) into the gap between inner original slender RC columns and outer steel tubes. Nine specimens were prepared and tested to failure under axial compression: a control specimen without strengthening and eight specimens with heights ranging between 1240 and 2140mm strengthened with CFST jacketing. Experimental variables included four different length-to-diameter (L/D) ratios, three different diameter-to-thickness (D/t) ratios, and three different SCC strengths. 'e experimental results showed that the outer steel tube provided confinement to the SCC and original slender RC columns and thus effectively improved the behavior of slender RC columns. 'e failure mode of slender RC columns was changed from brittle failure (concrete peel-off) into ductile failure (global bending) after strengthening. And, the load-bearing capacity, material utilization, and ductility of slender RC columns were significantly enhanced. 'e strengthening effect of CFST jacketing decreased with the increase of L/D ratio andD/t ratio but showed little variation with higher SCC strength. An existing expression of load-bearing capacity for traditional CFST columns was extended to propose a formula for the load-bearing capacity of CFST jacketed columns, and the predictions showed good agreement with the experimental results.


Introduction
Strengthening of RC structures is critically important for several reasons [1][2][3][4][5][6][7].One is to restore the load-bearing capacity of deteriorated concrete infrastructures because of aging or damage.Another reason is to enhance the serviceability and capacity of structures in response to a load demand increase beyond the original design.A third reason is to improve the load-bearing capacity for deficient members as a result of design or construction errors.
Common strengthening methods such as section enlargement (concrete jacketing) [8], externally bonded steel plates [9,10], and externally bonded fiber-reinforced polymer (FRP) [11,12] have been used for many years to improve structural service performance and ultimate capacity of concrete structures.However, their disadvantages may limit the further application.
e concrete jacketing method obviously enlarges the cross section of concrete members, and the construction of steel cage and formwork costs a lot of labor and time.e steel plate strengthening hardly changes the appearance of concrete structures, but it requires a large amount of steel and antirust work [13][14][15].e FRP strengthening degrades the deformation of concrete structures and the effective utilization of FRP typically ranges from 30 to 35% [16][17][18][19].erefore, new strengthening methods for concrete structures are still causing concern.
Recently, a novel technique, CFST jacketing, has become an option to strengthen concrete columns because of the superior performance of CFST columns (e.g., high loadbearing capacity and good ductility).It has been successfully applied in the strengthening of RC bridge piers [20,21].CFST jacketing is realized by the installation of in-field welded steel tube around the original RC column and pouring concrete into the gap between the inner original column and outer steel tube.e CFST jacketing method has many preferable advantages.In addition to a significant increase of load-bearing capacity and ductility, the CFST jacketing method is quick and easy to apply because the outer steel tube could serve as formwork and steel reinforcement, and thus, it requires less temporary formwork and reduces the usage of steel.Moreover, it could better utilize properties of each material with little change in column section size.
To better understand the performance of RC structures strengthened with CFST jacketing, many studies have been conducted.Priestley et al. [22,23] conducted experiments to verify the effectiveness of CFST jacketing approach for RC columns and concluded that the shear strength and ductility of RC columns were increased significantly.Sezen and Miller [24] compared the performances of bridge piers strengthened with FRP jacketing, concrete jacketing, and CFST jacketing.
e results showed that the CFST jacketing method was much more effective in enhancing the loadbearing capacity and ductility.Wang [25] and He et al. [26] studied the effects of preloading on the compressive strength of CFST jacketed columns.
e results showed that the preloading level had little effect on the load-bearing capacity of specimens.He et al. [27] also studied the axial compressive behavior of CFST jacketed columns with recycled aggregate concrete.e results showed that the influence of the recycled coarse aggregate replacement ratio on the compressive strength might be negligible.Recently, the authors of the present paper have also carried out experimental and theoretical studies on the axial and eccentric performance of CFST jacketed RC stub columns [28][29][30].
e influence of initial eccentricity on the strength of CFST jacketed RC column had been addressed.
Nevertheless, most of the previous studies are limited to stub columns, very limited research has been conducted to verify the effectiveness of CFST jacketing strengthening on slender RC columns. is paper presents an experimental study of slender RC columns strengthened with CFST jacketing under axial compression, and the SCC was used instead of normal concrete.e test program consists of nine specimens, one of which is unstrengthened and serves as control specimen, the remaining eight are strengthened with CFST jacketing.e main parameters in the test are the L/D ratio (5.7, 7.1, 8.4, and 9.8), D/t ratio (56.2, 67.4, and 121.7), and compressive strength of SCC (40,50,and 60 MPa).A modified model is applied to predict the load-bearing capacity of strengthened slender RC columns.

Experimental Programme
2.1.Test Specimens.Nine columns were tested to failure under axial compression, including one control specimen (Ref ) and eight CFST jacketed specimens which are named in the form of tx-Cy-z.e number after "t" represents the nominal thickness of steel tube.e number after "C" donates the design cubic compressive strength of SCC. e last number indicates the slenderness of columns.Table 1 summarizes the details of each specimen.

Preparation of Test Specimens.
All original columns are circular with a diameter of 154 mm and variation of length (1240 mm, 1550 mm, 1850 mm, and 2140 mm).
e reinforcement consists of 6 longitudinal steel rebars (12 mm diameter) and stirrups (6 mm diameter) spacing at 150 mm. e internal reinforcement ratio of the original column is 3.6% which meets the 0.6-5.0%range requirement [31].e original columns are cast using C25-grade normal concrete and cured for 28 days in the laboratory.Afterward, the CFTS jacketing strengthening is followed.
(i) e original column is sandblasted by a hand grinder to remove the irregularities and debris (ii) e outer steel tube is carefully placed on the designed region leaving a uniform gap (iii) e SCC is poured into the gap between original columns and steel tube at three intervals (iv) e strengthened column is then cured for 28 days in the laboratory e reinforcement ratios of CFST jacketed columns range from 5.2% to 9.5%.e ratios are in the range of 3.0-20.0%which are commonly used by others' researches [32].

Material Properties.
e original concrete and SCC are made from 42.5-grade Portland cement, aggregates with a maximum diameter of 20 mm, and river sand.Moreover, the water reducer, expansive agent, and fly ash are added to the SCC.e cubic compressive strength is determined with three 150 × 150 × 150 mm concrete cubes after 28 days of cure.e mix design, cubic strength, and slump of concrete are summarized in Table 2.
e material properties of steel tubes, steel rebar, and stirrup are tested according to the Chinese code GB/T 50081-2002 [33].e results are shown in Table 3.

Test Setup and
Procedure.Eight strain gauges are bonded evenly on the exterior surface of the steel tube to measure the longitudinal and transverse strains at midheight.After the specimen is placed on the hinge supports, two linear variable differential transducers (LVDTs) are used to measure the axial shortening and three other LVDTs are placed to measure the lateral deflection along the specimen's height (0.25 L, 0.50 L, and 0.75 L).
e test is carried out with a universal hydraulic testing machine (capacity of 5000 kN).
e load is applied in increments of one-tenth of the theoretical load-bearing capacity (N u,theo ) before the steel yield and in increments of one-fifteenth of N u,theo afterward.Each load interval is maintained for about 2 minutes as per Chinese code GB/T 50152-2012 [34].
e test setup and instrumentation are shown in Figure 1. . is observation is consistent with the research nding of traditional CFST columns [8,35], where local buckling of steel tubes was also reported.It should be mentioned that our team has also conducted experiments of CFST jacketing strengthening on slender RC square columns  Table 1: Specimen details.Advances in Civil Engineering [36].After the specimens failed, the outer steel tubes were cut o and the crush of SCC at midheight was observed.

Experiment Results and Discussion
When the SCC jackets were removed, obvious de ection of original columns and small cracks near the midheight were observed.It indicated that CFST jacketing could e ectively change the failure mode of slender RC columns from brittle failure to ductile failure.And the failure at the similar location of each part (original column, SCC jacket, and outer steel tube) indicated they worked well together under the con ne of outer CFST jackets.

Axial Load-Lateral De ection
Response.Figure 3 shows the typical curves of axial load and lateral de ection along height for CFST jacketed column.e curves along the height were basically a half-sine shape, and the biggest de ection was obtained at the midheight.e observable lateral de ection at N u and unloading to 85% of N u indicated good ductility of the specimen after CFST jacketing strengthening.
Figure 4 shows the axial load (N) versus the lateral de ection at midheight (Δ) response.
e specimen Ref exhibited an almost linear curve, the load and midheight de ection at limit state were very low (656 kN and 0.47 mm), whereas the CFST jacketed columns exhibited an initial linear elastic phase before the load reached about 80% of N u , followed by a curved ascending phase when outer steel tube began yielding and a smooth descending phase after N u due to the buckling of columns.In general, all the CFST jacketed columns experienced an obvious lateral de ection at much higher load-bearing capacity. Figure 4(a) shows the in uence of L/D ratio on the N-Δ response.
e curves of specimens t3-C50-5.7,t3-C50-7.1,and t3-C50-8.4had a similar linear phase but specimen t3-C50-9.8behaved in a softer manner with a larger de ection at the same load.It means that when the L/D ratio exceeds a certain value, the secondary moment could a ect the behavior of CFST jacketed columns signi cantly.At the limit state, there was a lager de ection at a smaller ultimate strength when the L/D ratio increased from 5.7 to 9.8. Figure 4(b) shows the inuence of D/t ratio on the N-Δ response.e specimen with thicker steel tube attained much higher ultimate strength.Figure 4(c) shows the in uences of SCC strength on the N-Δ response.All the specimens exhibited similar curves.is is because that the exural sti ness correlates with the elastic modulus of materials which varies slightly when the SCC  strength increases from 40 to 60 MPa.However, a slight increase in ultimate strength was also observed.

Axial Load-Strain Response.
Figure 5 shows the axial load (N) versus axial strain (ε v ) of outer steel tube response at midheight.e negative strain indicates compression and the positive strain indicates tension.Figure 5(a) shows the in uence of L/D ratio on the development of axial strain during loading.In the initial loading stage, the maximum axial strain and the minimum axial strain of all the CFST jacketed columns were almost the same, indicating the axial strains were nearly uniform around the cross section.With the increase of the axial load, the L/D ratio a ected the distribution of axial strain signi cantly.For specimens t3-C50-5.7 and t3-C50-7.1,the maximum axial strain grew much faster than the minimum axial strain when the load approached about 90% of N u .While for specimens t3-C50-8.4and t3-C50-9.8,this diversion happened at about 70% and 35% of N u , and the minimum axial strain even changed from negative to positive.is nonuniform distribution of axial strain is because of the development of lateral deection and the consequent secondary moment.Figure 5(b) shows the in uence of D/t ratio on the N-ε v response.After a linear increase of axial strain around the cross section, specimen t2-C50-8.4experienced a curved ascending phase at about 1600 kN while specimens t3-C50-8.4and t3-C50-8.4kept increasing linearly up to about 2500 kN. Figure 5(c) shows the in uence of SCC strength on the N-ε v response.All the specimens exhibited similar curves with a slight variation of axial values at the limit state.e development of axial strain for specimens with di erent SCC strength almost overlapped during the loading, indicating that the SCC strength did not a ect the behavior of the CFST columns signi cantly.
Figure 6 shows the relationship between the axial load (N) and hoop strain (ε h ) of outer steel tube at midheight in terms of L/D ratio, D/t ratio, and SCC strength.In general, the hoop strain near the compression side was much higher than that near the opposite side which even decreased to zero. is is because the dilation of SCC and original concrete was most active in the compression side and thus the most e ective con nement was provided there.As shown in Figure 6(a), the maximum hoop strain of the outer steel tube showed a decreasing trend with the increase of L/D ratio, indicating a gradual decreasing con nement.As shown in Figure 6(b), specimen t2-C50-8.4experienced a nonlinear increased hoop strain much earlier than specimens t3-C50-8.4and t4-C50-8.4.As shown in Figure 6(c), specimens with di erent SCC strength exhibited similar curves, and only little di erences were found after ultimate strength.

Con nement.
e outer steel tube con nes the SCC and original concrete and thus increases their compressive strengths while the inner concrete suppresses the inward bulking of the outer steel tube.
is interaction enhances the load-bearing capacity, material utilization, and ductility of the CFST jacketed columns.e transverse deformation coe cient of the outer steel tube is adopted to evaluate the level of con nement and de ned as ε h /ε v .Figure 7 shows the development of transverse deformation coe cient on the most compressive side (ε hmax /ε vmax ) and the least-compressive side (ε hmin /ε vmin ).In the initial loading stage (before 40% of N u ), the values of ε hmax /ε vmax and ε hmin /ε vmin ranged from 0.25 to 0.35, which were higher than that of concrete (usually 0.17-0.20).It indicated that the con nement was negligible in this stage because the dilation of inner concrete was smaller than that of the steel tube under the same axial deformation.When the axial load increased to 60-90% of N u , the values of ε hmax /ε vmax and ε hmin /ε vmin for several specimens increased signi cantly beyond their initial values, indicating that the inner concrete was subjected to good con nement because the dilation was constrained by the outer steel tube.From Figure 7(a), the values of ε hmax /ε vmax of specimens with L/D ratio from 5.7 to 9.8 were 0.91, 0.86, 0.64, and 0.66 while the values of ε hmin /ε vmin were 0.44, 0.32, 0.28, and 0.49 at the ultimate limit state.
is decreasing trend indicated the con nement was less e ective with the increase of L/D ratio.

Advances in Civil Engineering
It should be mentioned that although the value of ε hmin /ε vmin of specimen t3-C50-9.8was 0.49, it seemed that the connement of the least-compressive side was more e ective than other specimens.In fact, the value of ε hmin /ε vmin for specimen t3-C50-9.8was kept at 0.35 between 80 and 99.5% of N u , indicating a negligible con nement.From Figure 7(b), on the most compressive side, the values of ε hmax /ε vmax for specimens t2-C50-8.4,t3-C50-8.4,and t4-C50-8.4were 0.53, 0.64 and 0.54 at N u , indicating e ective con nement.While on the least-compressive side, the values of ε hmin /ε vmin were 0.39, 0.28, and 0.47.It means that the con nement was negligible for specimens t2-C50-8.4and t3-C50-8.4,but the con nement was e ective for specimen t4-C50-8.4.From Figure 7(c), the curves of specimens with di erent SCC strength agreed quite closely with each other except that stage after 85% of N u , the ε hmax /ε vmax and ε hmin /ε vmin of specimen t3-C60-8.4increased at a faster rate.It may be because that higher strength SCC is prone to forming splitting cracks and therefore, the circumferential stress in the steel tube increased rapidly.

Load-Bearing Capacity.
e load-bearing capacities (N u ) of specimens are shown in Table 1 and compared in Figure 8 in terms of L/D ratio, D/t ratio, and SCC strength.
e N u of t3-C50-8.4was 2751 kN, which was 4.06 times N u,theo (calculated by equations in [31]) and 4.19 times N u of specimen Ref. is signi cant enhancement indicated that the CFST jacketing method was e ective to improve the load-bearing capacity of slender RC columns under axial 6 Advances in Civil Engineering compression.On the other hand, the load-bearing capacity decreased progressively with larger L/D ratio.e N u of specimens t3-C50-7.1,t3-C50-8.4,and t3-C50-9.8were 5.7%, 7.7%, and 9.3% lower than that of specimen t3-C50-5.7,respectively.e load-bearing capacity decreased signi cantly with larger D/t ratio.It means that the loadbearing capacity decreased signi cantly with decreasing of steel CFST jacket thickness with the constant external diameter.e N u of specimen t3-C50-8.4and specimen t2-C50-8.4were 6.2% and 20.9% lower than that of specimen t4-C50-8.4.e load-bearing capacity increased slightly with higher SCC strength.e N u of specimen t3-C50-8.4and specimen t3-C60-8.4were 4.5% and 8.1% higher than that of specimen t3-C40-8.4.e load-bearing capacity increased with higher concrete strength as expected.

Strength Index.
e utilization of the full plastic compressive resistance of a CFST column can be assessed through its strength index (SI) [8,37].Similarly, for CFST jacketed columns, SI is adopted to evaluate the e ectiveness of material utilization and de ned as where f c1 ( 0.80 f cu1 ) and f c2 ( 0.80 f cu2 ) are the compressive strengths of original concrete and SCC; A s1 , A s2 , A c1 , and A c2 are the cross-sectional areas of steel rebar, outer steel tube, original concrete, and SCC, respectively.For specimen Ref, SI can also be calculated using Equation (1) when taking A s2 and A c2 as zero.
e SI of specimen Ref was only 0.89 while that of specimen t3-C50-8.4was 1.24.It indicated that the slender RC column did not take full use of steel rebar and original concrete but the CFST jacketed column exhibited 124% utilization of materials.e 24% increase can be explained as follows: (i) Under axial compression, the original concrete and SCC dilate laterally with the increase of load.However, the original concrete is con ned by the Advances in Civil Engineering SCC jacket and the outer steel tube while the SCC is con ned by the outer steel tube.It means that both the original concrete and SCC are under triaxial compression.e compressive strength of concrete under triaxial stress (f cc ) is higher than concrete compressive strength without con nement (f c ), and it can be written as [31] where f L is the radial stress.
(ii) e existence of original concrete and SCC could avoid the inward local buckling of the outer steel tube and thus the material properties are better exploited.
is behavior of CFST jacketed columns also indicates that the signi cant increase in the load-bearing capacity is not only due to enlargement and additional steel reinforcement in cross section but also because of steel jacket's con nement.
Figure 9 compares the e ects of L/D ratio, D/t ratio, and SCC strength on the SI of specimens.

Ductility.
To qualify the ductility of a column, ductility index (DI) is often adopted by many researchers [8,38].Similarly, for CFST jacketed columns, DI is de ned as where Δ 85% is the midheight de ection when the load drops to 85% of the ultimate load on the unloading branch and Δ u is the midheight de ection at the ultimate load.e DI of specimen Ref was zero because there was no descending branch in the N-Δ curve while that of specimen t3-C50-8.4was 3.48.It indicated that the CFST jacketing method was e ective to improve the ductility of slender RC columns.Figure 10 shows the e ects of L/D ratio, D/t ratio, and SCC strength on the DI of specimens.It should be mentioned that value of DI was not available for specimen t3-C50-7.1 due to the test having not been continued for su cient deformation for the load to reduce to 85% of the ultimate load.Although there was uctuation in the comparison of DI in terms of L/D ratio, the trend of decreasing DI with increasing slenderness might be clearly observed by linear tting the experimental data.On the other hand, it was observed that there was a signi cant reduction in ductility with increasing D/t ratio and that the SCC strength did not have a large in uence on the ductility of specimens.e DI of specimens t3-C50-8.4and t2-C50-8.4were 38.9% and 48.2% lower than that of t4-C50-8.4.e DI of specimens t3-C50-8.4and t3-C60-8.4were 1.8% and 4.4% higher than that of t3-C40-8.4.

Load-Bearing Capacity Prediction.
Because the inner concrete and SCC worked well together under the connement of outer steel tube, the CFST jacketed slender column failed similarly as the traditional CFST slender column.
us the load-bearing capacity prediction model for the traditional slender column may be also applied to CFST jacketed slender column when considering the di erent compressive strengths of original concrete and SCC and the contribution of longitudinal steel rebar.With reference to Chinese code GB 50936-2014 [39], a modi ed formula is proposed to predict the load-bearing capacity for CFST jacketed slender column:

8
Advances in Civil Engineering where φ is the slenderness reduction factor, f sc is the compressive strength of CFST jacketed column, A sc is the sum of the area of original concrete, SCC and outer steel tube.f sc can be written as where ξ is the con nement index and f c is the average concrete strength of original concrete and SCC, which are de ned as φ can be calculated by a modi ed formula for traditional CFST slender column: where λ sc1 is the regular slenderness ratio, which is de ned as λ sc1 0.01λ sc 0.001f s2 + 0.781 , where λ sc is the slenderness ratio and μl and i are the calculating length and radius of gyration of CFST jacketed column.Table 1 compares the predicted strength (N u,theo ) with the experimental results (N u ) and the ratio N u,theo /N u is 0.93 with a coe cient of variation (CV) of 0.09.e good agreement indicates the accuracy of the proposed model in predicting the load-bearing capacity for CFST jacketed slender column.

Conclusions
is paper presents an experiment of slender circular RC columns strengthened with CFST jacketing.Nine slender specimens were tested under axial compression, and the following conclusions can be drawn within the scope of this study: (1) e CFST jacketed columns exhibited good ductile behavior under axial compression, and all experienced ductile failure mode (global bending), while the slender RC columns showed a brittle failure mode (concrete peel-o ).(2) e outer steel tube provided e ective con nement on the SCC and original slender columns, and thus enhanced the load-bearing capacity, material utilization, and ductility of slender RC columns.e L/D ratio and D/t ratio showed obvious in uence on the performance of CFST jacketed columns.e N u , SI, and DI decreased 9.3%, 9.0%, and 51.9% when the L/D ratio increased from 5.7 to 9.8 and dropped 20.9%, 24.1%, and 48.2% when the D/t ratio increased from 56.2 to 121.7.(3) e SCC strength had a slight e ect on the performance of CFST jacketed columns, in which the specimen t3-C60-8.4showed 8.1%, −0.8% and 4.4% increase in the N u , SI, and DI compared to specimen t3-C40-8.4.(4) A modi ed model was proposed to predict the loadbearing capacity for CFST jacketed slender columns based on the model for traditional CFST columns.
Comparison between the prediction and the experimental results showed good agreement.
It is worth mentioning that the CFST jacketed column in practice may not behave similarly to specimens in this study because the external steel jacket may be discontinuous at the column top and bottom.For the CFST jacketed specimen under compression over the existing column and retro t Advances in Civil Engineering area, the elastic stiffness increased more than 100% and strength increased up to 30% compared to the specimen compressed on the existing column only [24].e structural steel collars were placed around the gaps at the top and bottom of the column and tied to the adjacent elements with postinstalled anchors aiming to increase shear strength locally [40].e results showed this technique can transfer the column shears to the footing and adjacent elements and thus enhance the blast resistance to bridge columns seismically retrofitted using steel jackets.erefore, the additional strengthening technique should be applied at the bottom and top of the column, such as structural steel collars, to transfer the whole axial load and shear force over the entire cross section.Otherwise, a relative reduction coefficient should be carefully considered before the application of CFST jacketing.

3. 1 .
Failure Modes of Specimens.Specimen Ref exhibited a brittle failure.Concrete crack occurred near the top at 557 kN (about 85% of N u ).And the concrete cracks widened 2 Advances in Civil Engineering and propagated downwards with the increase of load.At the failure load (656 kN), the concrete on the top began to peel o without obvious axial shortening or lateral de ection, as shown in Figure 2(a).In comparison, all CFST jacketed columns failed in excessive lateral de ection, showing much better ductility.As the load increased, rstly the axial shortening and lateral de ection developed invisibly; secondly obvious axial shortening and lateral de ection were observed after the load reached around 85% of N u ; nally, excessive lateral de ection was obtained with the local buckling of outer steel tube at midheight, as shown in Figure 2(b)

Table 2 :
Mixes and properties of concrete.Concrete Cement Sand Gravel Water Water reducer Expansive agent Fly ash Cubic strength (MPa) Slump (mm)

Table 3 :
Material properties of steel.
SteelDiameter (mm) ickness (mm) Young's modulus (GPa) Yield strength (MPa) Tensile strength (MPa) 1and D 2 are the diameters of unstrengthened and strengthened column; t is the measured thickness of outer steel tube; L is the length of column; f cu1 and f cu2 are the cubic compressive strengths of original concrete and SCC; f s1 and f s2 are the tensile strengths of steel rebar and steel tube; N u and N u,theo are the experimental and theoretical load-bearing capacities of specimens. D