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Research Article Axial Behaviour of Slender RC Circular Columns Strengthened with Circular CFST Jackets Yiyan Lu , 1 Tao Zhu , 1 Shan Li , 1 Weijie Li, 1 and Na Li 2 1 School of Civil Engineering, Wuhan University, Wuhan 430072, China 2 School of Civil Engineering and Architecture, Wuhan University of Technology, Wuhan 430070, China Correspondence should be addressed to Shan Li; [email protected] Received 2 July 2018; Accepted 23 August 2018; Published 12 September 2018 Academic Editor: Eric Lui Copyright © 2018 Yiyan Lu et al. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 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 CFSTjacketing decreased with the increase of L/D ratio and D/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. 1. Introduction Strengthening of RC structures is critically important for several reasons [1–7]. One is to restore the load-bearing capacity of deteriorated concrete infrastructures because of aging or damage. Another reason is to enhance the ser- viceability 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 en- largement (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 ca- pacity 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 [13–15]. e FRP strengthening degrades the deformation of con- crete structures and the effective utilization of FRP typically ranges from 30 to 35% [16–19]. erefore, new strength- ening 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 load- bearing 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 Hindawi Advances in Civil Engineering Volume 2018, Article ID 7923575, 11 pages https://doi.org/10.1155/2018/7923575

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  • Research ArticleAxial Behaviour of Slender RC Circular ColumnsStrengthened with Circular CFST Jackets

    Yiyan Lu ,1 Tao Zhu ,1 Shan Li ,1 Weijie Li,1 and Na Li2

    1School of Civil Engineering, Wuhan University, Wuhan 430072, China2School of Civil Engineering and Architecture, Wuhan University of Technology, Wuhan 430070, China

    Correspondence should be addressed to Shan Li; [email protected]

    Received 2 July 2018; Accepted 23 August 2018; Published 12 September 2018

    Academic Editor: Eric Lui

    Copyright © 2018 Yiyan Lu et al. 'is is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

    '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 slenderRC columns and outer steel tubes. Nine specimens were prepared and tested to failure under axial compression: a controlspecimen without strengthening and eight specimens with heights ranging between 1240 and 2140mm strengthened with CFSTjacketing. 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 tothe SCC and original slender RC columns and thus effectively improved the behavior of slender RC columns. 'e failure mode ofslender 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. 'estrengthening effect of CFST jacketing decreased with the increase of L/D ratio andD/t ratio but showed little variation with higherSCC strength. An existing expression of load-bearing capacity for traditional CFST columns was extended to propose a formulafor the load-bearing capacity of CFST jacketed columns, and the predictions showed good agreement with theexperimental results.

    1. Introduction

    Strengthening of RC structures is critically important forseveral reasons [1–7]. One is to restore the load-bearingcapacity of deteriorated concrete infrastructures because ofaging or damage. Another reason is to enhance the ser-viceability and capacity of structures in response to a loaddemand increase beyond the original design. A third reasonis to improve the load-bearing capacity for deficientmembers as a result of design or construction errors.

    Common strengthening methods such as section en-largement (concrete jacketing) [8], externally bonded steelplates [9, 10], and externally bonded fiber-reinforcedpolymer (FRP) [11, 12] have been used for many years toimprove structural service performance and ultimate ca-pacity of concrete structures. However, their disadvantagesmay limit the further application. 'e concrete jacketing

    method obviously enlarges the cross section of concretemembers, and the construction of steel cage and formworkcosts a lot of labor and time. 'e steel plate strengtheninghardly changes the appearance of concrete structures, but itrequires a large amount of steel and antirust work [13–15].'e FRP strengthening degrades the deformation of con-crete structures and the effective utilization of FRP typicallyranges from 30 to 35% [16–19]. 'erefore, new strength-ening methods for concrete structures are still causingconcern.

    Recently, a novel technique, CFST jacketing, has becomean option to strengthen concrete columns because of thesuperior performance of CFST columns (e.g., high load-bearing capacity and good ductility). It has been successfullyapplied in the strengthening of RC bridge piers [20, 21].CFST jacketing is realized by the installation of in-fieldwelded steel tube around the original RC column and

    HindawiAdvances in Civil EngineeringVolume 2018, Article ID 7923575, 11 pageshttps://doi.org/10.1155/2018/7923575

    mailto:[email protected]://orcid.org/0000-0001-7999-2484http://orcid.org/0000-0001-5124-2059http://orcid.org/0000-0002-9515-578Xhttps://doi.org/10.1155/2018/7923575

  • pouring concrete into the gap between the inner originalcolumn and outer steel tube.'e CFST jacketing method hasmany preferable advantages. In addition to a significantincrease of load-bearing capacity and ductility, the CFSTjacketing method is quick and easy to apply because theouter steel tube could serve as formwork and steel re-inforcement, and thus, it requires less temporary formworkand reduces the usage of steel. Moreover, it could betterutilize properties of each material with little change incolumn section size.

    To better understand the performance of RC structuresstrengthened with CFST jacketing, many studies have beenconducted. Priestley et al. [22, 23] conducted experiments toverify the effectiveness of CFST jacketing approach for RCcolumns and concluded that the shear strength and ductilityof RC columns were increased significantly. Sezen andMiller[24] compared the performances of bridge piers strength-ened with FRP jacketing, concrete jacketing, and CFSTjacketing. 'e results showed that the CFST jacketingmethod was much more effective in enhancing the load-bearing capacity and ductility. Wang [25] and He et al. [26]studied the effects of preloading on the compressive strengthof CFST jacketed columns. 'e results showed that thepreloading level had little effect on the load-bearing capacityof specimens. He et al. [27] also studied the axial com-pressive behavior of CFST jacketed columns with recycledaggregate concrete. 'e results showed that the influence ofthe recycled coarse aggregate replacement ratio on thecompressive strength might be negligible. Recently, theauthors of the present paper have also carried out experi-mental and theoretical studies on the axial and eccentricperformance of CFST jacketed RC stub columns [28–30].'e influence of initial eccentricity on the strength of CFSTjacketed RC column had been addressed.

    Nevertheless, most of the previous studies are limited tostub columns, very limited research has been conducted toverify the effectiveness of CFST jacketing strengthening onslender RC columns. 'is paper presents an experimentalstudy of slender RC columns strengthened with CFSTjacketing under axial compression, and the SCC was usedinstead of normal concrete.'e test program consists of ninespecimens, one of which is unstrengthened and serves ascontrol specimen, the remaining eight are strengthened withCFST jacketing. 'e main parameters in the test are the L/Dratio (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 60MPa). Amodified model is applied to predict the load-bearing ca-pacity of strengthened slender RC columns.

    2. Experimental Programme

    2.1. Test Specimens. Nine columns were tested to failureunder axial compression, including one control specimen(Ref) and eight CFST jacketed specimens which are namedin the form of tx-Cy-z. 'e number after “t” represents thenominal thickness of steel tube. 'e number after “C” do-nates the design cubic compressive strength of SCC.'e lastnumber indicates the slenderness of columns. Table 1summarizes the details of each specimen.

    2.2. Preparation of Test Specimens. All original columns arecircular with a diameter of 154mm and variation of length(1240mm, 1550mm, 1850mm, and 2140mm). 'e re-inforcement consists of 6 longitudinal steel rebars (12mmdiameter) and stirrups (6mm diameter) spacing at 150mm.'e internal reinforcement ratio of the original column is3.6% which meets the 0.6–5.0% range requirement [31]. 'eoriginal columns are cast using C25-grade normal concreteand cured for 28 days in the laboratory. Afterward, the CFTSjacketing strengthening is followed.

    (i) 'e original column is sandblasted by a handgrinder to remove the irregularities and debris

    (ii) 'e outer steel tube is carefully placed on thedesigned region leaving a uniform gap

    (iii) 'e SCC is poured into the gap between originalcolumns and steel tube at three intervals

    (iv) 'e strengthened column is then cured for 28 daysin the laboratory

    'e reinforcement ratios of CFST jacketed columnsrange from 5.2% to 9.5%. 'e ratios are in the range of3.0–20.0% which are commonly used by others’ researches[32].

    2.3. Material Properties. 'e original concrete and SCC aremade from 42.5-grade Portland cement, aggregates witha maximum diameter of 20mm, and river sand. Moreover,the water reducer, expansive agent, and fly ash are added tothe SCC. 'e cubic compressive strength is determined withthree 150×150×150mm concrete cubes after 28 days ofcure. 'e mix design, cubic strength, and slump of concreteare summarized in Table 2.

    'e material properties of steel tubes, steel rebar, andstirrup are tested according to the Chinese code GB/T50081-2002 [33]. 'e results are shown in Table 3.

    2.4.Test SetupandProcedure. Eight strain gauges are bondedevenly on the exterior surface of the steel tube to measure thelongitudinal and transverse strains at midheight. After thespecimen is placed on the hinge supports, two linear variabledifferential transducers (LVDTs) are used to measure theaxial shortening and three other LVDTs are placed tomeasure the lateral deflection along the specimen’s height(0.25 L, 0.50 L, and 0.75 L). 'e test is carried out witha universal hydraulic testing machine (capacity of 5000 kN).'e load is applied in increments of one-tenth of the the-oretical load-bearing capacity (Nu,theo) before the steel yieldand in increments of one-fifteenth of Nu,theo afterward. Eachload interval is maintained for about 2minutes as perChinese code GB/T 50152-2012 [34]. 'e test setup andinstrumentation are shown in Figure 1.

    3. Experiment Results and Discussion

    3.1. Failure Modes of Specimens. Specimen Ref exhibiteda brittle failure. Concrete crack occurred near the top at557 kN (about 85% of Nu). And the concrete cracks widened

    2 Advances in Civil Engineering

  • and propagated downwards with the increase of load. At thefailure load (656 kN), the concrete on the top began to peelo� without obvious axial shortening or lateral de�ection, asshown in Figure 2(a). In comparison, all CFST jacketedcolumns failed in excessive lateral de�ection, showing muchbetter ductility. As the load increased, rstly the axialshortening and lateral de�ection developed invisibly; sec-ondly obvious axial shortening and lateral de�ection were

    observed after the load reached around 85% of Nu; nally,excessive lateral de�ection was obtained with the localbuckling of outer steel tube at midheight, as shown inFigure 2(b). is observation is consistent with the researchnding of traditional CFST columns [8, 35], where localbuckling of steel tubes was also reported. It should bementioned that our team has also conducted experiments ofCFST jacketing strengthening on slender RC square columns

    Table 2: Mixes and properties of concrete.

    Concrete Cement Sand Gravel Water Water reducer Expansive agent Fly ash Cubic strength (MPa) Slump (mm)RC C25 1.000 2.103 4.082 0.635 0.000 0.000 0.000 32.83 N/ASCC C40 1.000 2.239 3.075 0.522 0.014 0.143 0.284 43.01 670SCC C50 1.000 1.892 2.838 0.459 0.017 0.143 0.286 52.58 675SCC C60 1.000 1.667 3.000 0.423 0.022 0.146 0.282 57.29 672

    Table 3: Material properties of steel.

    Steel Diameter (mm) ickness (mm) Young’s modulus (GPa) Yield strength (MPa) Tensile strength (MPa)Steel tube 219 1.80 191 390 587Steel tube 219 3.25 211 352 425Steel tube 219 3.90 234 342 522Steel rebar 12 — 190 365 527Stirrup 6 — 203 214 278

    LVDT

    Load cell

    Axial strain gauges

    Hoop strain gauges

    Strain gauges

    Figure 1: Test setup and instrumentation.

    Table 1: Specimen details.

    Specimen D2(D1)× t× L (mm) L/D1 L/D2 D2/t fcu1 (MPa) fcu2 (MPa) fsl (MPa) fs2 (MPa) Nu (kN) Nu,theo (kN) Nu,theo/NuRef 154× 0×1850 12.0 — — 32.83 — 365 — 656 677 1.03t3-C50-5.7 219× 3.25×1240 8.1 5.7 67.4 32.83 52.58 365 352 2980 2622 0.88t3-C50-7.1 219× 3.25×1550 10.1 7.1 67.4 32.83 52.58 365 352 2810 2574 0.92t3-C50-8.4 219× 3.25×1850 12.0 8.4 67.4 32.83 52.58 365 352 2751 2525 0.92t3-C50-9.8 219× 3.25× 2140 13.9 9.8 67.4 32.83 52.58 365 352 2703 2474 0.92t2-C50-8.4 219×1.80×1850 12.0 8.4 121.7 32.83 52.58 365 390 2319 2621 1.13t4-C50-8.4 219× 3.90×1850 12.0 8.4 56.2 32.83 52.58 365 342 2932 2500 0.85t3-C40-8.4 219× 3.25×1850 12.0 8.4 67.4 32.83 43.01 365 352 2633 2373 0.90t3-C60-8.4 219× 3.25×1850 12.0 8.4 67.4 32.83 57.29 365 352 2845 2600 0.91D1 andD2 are the diameters of unstrengthened and strengthened column; t is the measured thickness of outer steel tube; L is the length of column; fcu1 and fcu2are the cubic compressive strengths of original concrete and SCC; fs1 and fs2 are the tensile strengths of steel rebar and steel tube; Nu and Nu,theo are theexperimental and theoretical load-bearing capacities of specimens.

    Advances in Civil Engineering 3

  • [36]. After the specimens failed, the outer steel tubes werecut o� and the crush of SCC at midheight was observed.When the SCC jackets were removed, obvious de�ection oforiginal columns and small cracks near the midheight wereobserved. It indicated that CFST jacketing could e�ectivelychange the failure mode of slender RC columns from brittlefailure to ductile failure. And the failure at the similar lo-cation of each part (original column, SCC jacket, and outersteel tube) indicated they worked well together under theconne of outer CFST jackets.

    3.2. Axial Load-Lateral Deection Response. Figure 3 showsthe typical curves of axial load and lateral de�ection alongheight for CFST jacketed column. e curves along theheight were basically a half-sine shape, and the biggestde�ection was obtained at the midheight. e observablelateral de�ection atNu and unloading to 85% ofNu indicatedgood ductility of the specimen after CFST jacketingstrengthening.

    Figure 4 shows the axial load (N) versus the lateralde�ection at midheight (Δ) response. e specimen Refexhibited an almost linear curve, the load and midheightde�ection at limit state were very low (656 kN and 0.47mm),whereas the CFST jacketed columns exhibited an initiallinear elastic phase before the load reached about 80% of Nu,followed by a curved ascending phase when outer steel tubebegan yielding and a smooth descending phase after Nu dueto the buckling of columns. In general, all the CFST jacketedcolumns experienced an obvious lateral de�ection at muchhigher load-bearing capacity. Figure 4(a) shows the in�u-ence of L/D ratio on the N-Δ response. e curves ofspecimens t3-C50-5.7, t3-C50-7.1, and t3-C50-8.4 hada similar linear phase but specimen t3-C50-9.8 behaved in

    a softer manner with a larger de�ection at the same load. Itmeans that when the L/D ratio exceeds a certain value, thesecondary moment could a�ect the behavior of CFSTjacketed columns signicantly. At the limit state, there wasa lager de�ection at a smaller ultimate strength when the L/Dratio increased from 5.7 to 9.8. Figure 4(b) shows the in-�uence of D/t ratio on the N-Δ response. e specimen withthicker steel tube attained much higher ultimate strength.Figure 4(c) shows the in�uences of SCC strength on the N-Δresponse. All the specimens exhibited similar curves. is isbecause that the �exural sti�ness correlates with the elasticmodulus of materials which varies slightly when the SCC

    Concrete peel-off

    (a)

    Local buckling

    (b)

    Figure 2: Typical failure modes of specimens. (a) Specimen Ref. (b) Specimen t3-C50-9.8.

    0.00

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    Rela

    tive h

    eigh

    t

    10 20 30 40 500Lateral deflection (mm)

    0.74Nu1.00NuUnloading to 0.93Nu

    Unloading to 0.85NuHalf sine waves

    Figure 3: Typical load-lateral de�ection curves (specimen t3-C50-8.4).

    4 Advances in Civil Engineering

  • strength increases from 40 to 60MPa. However, a slightincrease in ultimate strength was also observed.

    3.3. Axial Load-Strain Response. Figure 5 shows the axialload (N) versus axial strain (εv) of outer steel tube response atmidheight. e negative strain indicates compression andthe positive strain indicates tension. Figure 5(a) shows thein�uence of L/D ratio on the development of axial strainduring loading. In the initial loading stage, the maximumaxial strain and the minimum axial strain of all the CFSTjacketed columns were almost the same, indicating the axialstrains were nearly uniform around the cross section. Withthe increase of the axial load, the L/D ratio a�ected thedistribution of axial strain signicantly. For specimens t3-C50-5.7 and t3-C50-7.1, the maximum axial strain grewmuch faster than the minimum axial strain when the loadapproached about 90% of Nu. While for specimens t3-C50-8.4 and t3-C50-9.8, this diversion happened at about 70%and 35% of Nu, and the minimum axial strain even changedfrom negative to positive. is nonuniform distribution ofaxial strain is because of the development of lateral de-�ection and the consequent secondary moment. Figure 5(b)shows the in�uence of D/t ratio on the N-εv response. Aftera linear increase of axial strain around the cross section,specimen t2-C50-8.4 experienced a curved ascending phaseat about 1600 kN while specimens t3-C50-8.4 and t3-C50-8.4 kept increasing linearly up to about 2500 kN. Figure 5(c)shows the in�uence of SCC strength on the N-εv response.All the specimens exhibited similar curves with a slightvariation of axial values at the limit state. e developmentof axial strain for specimens with di�erent SCC strengthalmost overlapped during the loading, indicating that theSCC strength did not a�ect the behavior of the CFST col-umns signicantly.

    Figure 6 shows the relationship between the axial load(N) and hoop strain (εh) of outer steel tube at midheight interms of L/D ratio, D/t ratio, and SCC strength. In general,the hoop strain near the compression side was much higherthan that near the opposite side which even decreasedto zero. is is because the dilation of SCC and original

    concrete was most active in the compression side and thusthe most e�ective connement was provided there. Asshown in Figure 6(a), the maximum hoop strain of the outersteel tube showed a decreasing trend with the increase of L/Dratio, indicating a gradual decreasing connement. Asshown in Figure 6(b), specimen t2-C50-8.4 experienceda nonlinear increased hoop strain much earlier than spec-imens t3-C50-8.4 and t4-C50-8.4. As shown in Figure 6(c),specimens with di�erent SCC strength exhibited similarcurves, and only little di�erences were found after ultimatestrength.

    4. Discussion

    4.1. Con�nement. e outer steel tube connes the SCC andoriginal concrete and thus increases their compressivestrengths while the inner concrete suppresses the inwardbulking of the outer steel tube.

    is interaction enhances the load-bearing capacity,material utilization, and ductility of the CFST jacketedcolumns. e transverse deformation coe¤cient of the outersteel tube is adopted to evaluate the level of connement anddened as εh/εv. Figure 7 shows the development of trans-verse deformation coe¤cient on the most compressive side(εhmax/εvmax) and the least-compressive side (εhmin/εvmin). Inthe initial loading stage (before 40% of Nu), the values ofεhmax/εvmax and εhmin/εvmin ranged from 0.25 to 0.35, whichwere higher than that of concrete (usually 0.17–0.20). Itindicated that the connement was negligible in this stagebecause the dilation of inner concrete was smaller than thatof the steel tube under the same axial deformation.When theaxial load increased to 60–90% of Nu, the values ofεhmax/εvmax and εhmin/εvmin for several specimens increasedsignicantly beyond their initial values, indicating that theinner concrete was subjected to good connement becausethe dilation was constrained by the outer steel tube. FromFigure 7(a), the values of εhmax/εvmax of specimens with L/Dratio from 5.7 to 9.8 were 0.91, 0.86, 0.64, and 0.66 while thevalues of εhmin/εvmin were 0.44, 0.32, 0.28, and 0.49 at theultimate limit state. is decreasing trend indicated theconnement was less e�ective with the increase of L/D ratio.

    5 10 15 20 25 30 350∆ (mm)

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    )Nu

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    Reft3-C50-5.7t3-C50-7.1

    t3-C50-8.4t3-C50-9.8

    (a)

    Reft2-C50-8.4

    t3-C50-8.4t4-C50-8.4

    Nu85%Nu

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    Figure 4: Axial load-lateral de�ection response. (a) L/D ratio. (b) D/t ratio. (c) SCC strength.

    Advances in Civil Engineering 5

  • It should be mentioned that although the value of εhmin/εvminof specimen t3-C50-9.8 was 0.49, it seemed that the con-nement of the least-compressive side was more e�ectivethan other specimens. In fact, the value of εhmin/εvmin forspecimen t3-C50-9.8 was kept at 0.35 between 80 and 99.5%ofNu, indicating a negligible connement. From Figure 7(b),on the most compressive side, the values of εhmax/εvmax forspecimens t2-C50-8.4, t3-C50-8.4, and t4-C50-8.4 were 0.53,0.64 and 0.54 at Nu, indicating e�ective connement. Whileon the least-compressive side, the values of εhmin/εvmin were0.39, 0.28, and 0.47. It means that the connement wasnegligible for specimens t2-C50-8.4 and t3-C50-8.4, but theconnement was e�ective for specimen t4-C50-8.4. FromFigure 7(c), the curves of specimens with di�erent SCCstrength agreed quite closely with each other except that

    stage after 85% of Nu, the εhmax/εvmax and εhmin/εvmin ofspecimen t3-C60-8.4 increased at a faster rate. It may bebecause that higher strength SCC is prone to formingsplitting cracks and therefore, the circumferential stress inthe steel tube increased rapidly.

    4.2. Load-Bearing Capacity. e load-bearing capacities(Nu) of specimens are shown in Table 1 and compared inFigure 8 in terms of L/D ratio, D/t ratio, and SCC strength.

    e Nu of t3-C50-8.4 was 2751 kN, which was 4.06 timesNu,theo (calculated by equations in [31]) and 4.19 times Nu ofspecimen Ref. is signicant enhancement indicated thatthe CFST jacketing method was e�ective to improve theload-bearing capacity of slender RC columns under axial

    0500

    10001500200025003000

    N (k

    N)

    0 2000 4000 6000

    t3-C50-5.7 Hoop maxt3-C50-7.1 Hoop maxt3-C50-8.4 Hoop maxt3-C50-9.8 Hoop maxt3-C50-5.7 Hoop mint3-C50-7.1 Hoop mint3-C50-8.4 Hoop mint3-C50-9.8 Hoop min

    8000 10000–2000εh(με)

    (a)

    t2-C50-8.4 Hoop maxt3-C50-8.4 Hoop maxt4-C50-8.4 Hoop maxt2-C50-8.4 Hoop mint3-C50-8.4 Hoop mint4-C50-8.4 Hoop min

    0 2000 4000 6000 8000 10000–2000εh(με)

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    t3-C40-8.4 Hoop maxt3-C50-8.4 Hoop maxt3-C60-8.4 Hoop maxt3-C40-8.4 Hoop mint3-C50-8.4 Hoop mint3-C60-8.4 Hoop min

    0 2000 4000 6000 8000 10000–2000εh(με)

    0500

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    N (k

    N)

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    Figure 6: Axial load-hoop strain of outer steel tube response. (a) L/D ratio. (b) D/t ratio. (c) SCC strength.

    –10000

    t3-C50-5.7 Axial maxt3-C50-7.1 Axial maxt3-C50-8.4 Axial maxt3-C50-9.8 Axial maxt3-C50-5.7 Axial mint3-C50-7.1 Axial mint3-C50-8.4 Axial mint3-C50-9.8 Axial min

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    t2-C50-8.4 Axial maxt3-C50-8.4 Axial maxt4-C50-8.4 Axial maxt2-C50-8.4 Axial mint3-C50-8.4 Axial mint4-C50-8.4 Axial min

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    Figure 5: Axial load-axial strain of outer steel tube response. (a) L/D ratio. (b) D/t ratio. (c) SCC strength.

    6 Advances in Civil Engineering

  • compression. On the other hand, the load-bearing capacitydecreased progressively with larger L/D ratio. e Nu ofspecimens t3-C50-7.1, t3-C50-8.4, and t3-C50-9.8 were5.7%, 7.7%, and 9.3% lower than that of specimen t3-C50-5.7, respectively. e load-bearing capacity decreased sig-nicantly with larger D/t ratio. It means that the load-bearing capacity decreased signicantly with decreasing ofsteel CFST jacket thickness with the constant external di-ameter. e Nu of specimen t3-C50-8.4 and specimen t2-C50-8.4 were 6.2% and 20.9% lower than that of speciment4-C50-8.4.e load-bearing capacity increased slightly withhigher SCC strength. e Nu of specimen t3-C50-8.4 andspecimen t3-C60-8.4 were 4.5% and 8.1% higher than that ofspecimen t3-C40-8.4. e load-bearing capacity increasedwith higher concrete strength as expected.

    4.3. Strength Index. e utilization of the full plastic com-pressive resistance of a CFST column can be assessedthrough its strength index (SI) [8, 37]. Similarly, for CFST

    jacketed columns, SI is adopted to evaluate the e�ectivenessof material utilization and dened as

    SI �Nu

    fs1As1 + fs2As2 + fc1Ac1 + fc2Ac2, (1)

    where fc1 (� 0.80 fcu1) and fc2 (� 0.80 fcu2) are the compressivestrengths of original concrete and SCC;As1,As2,Ac1, andAc2are the cross-sectional areas of steel rebar, outer steel tube,original concrete, and SCC, respectively.

    For specimen Ref, SI can also be calculated usingEquation (1) when taking As2 and Ac2 as zero.

    e SI of specimen Ref was only 0.89 while that ofspecimen t3-C50-8.4 was 1.24. It indicated that the slenderRC column did not take full use of steel rebar and originalconcrete but the CFST jacketed column exhibited 124%utilization of materials.e 24% increase can be explained asfollows:

    (i) Under axial compression, the original concrete andSCC dilate laterally with the increase of load.However, the original concrete is conned by the

    2980 2810 2751 2703

    0

    1000

    2000

    3000

    4000

    Nu (

    kN)

    6 7 8 9 10 115L/D

    (a)

    2932 27512391

    50 75 100 125 15025D/t

    0

    1000

    2000

    3000

    4000

    Nu (

    kN)

    (b)

    2633 2751 2845

    40 45 50 55 60 6535SCC strength (MPa)

    0

    1000

    2000

    3000

    4000

    Nu (

    kN)

    (c)

    Figure 8: Load-bearing capacity of specimens. (a) L/D ratio. (b) D/t ratio. (c) SCC strength.

    0.2 0.4 0.6 0.8 1.0

    t3-C50-5.7 most-compressed sidet3-C50-7.1 most-compressed sidet3-C50-8.4 most-compressed sidet3-C50-9.8 most-compressed sidet3-C50-5.7 least-compressed sidet3-C50-7.1 least-compressed sidet3-C50-8.4 least-compressed sidet3-C50-9.8 least-compressed side

    1.2 1.40.0εh/εv

    0.0

    0.2

    0.4

    0.6

    0.8

    1.0

    N/N

    u

    (a)

    t2-C50-8.4 most-compressive sidet3-C50-8.4 most-compressive sidet4-C50-8.4 most-compressive sidet2-C50-8.4 least-compressive sidet3-C50-8.4 least-compressive sidet4-C50-8.4 least-compressive side

    0.2 0.4 0.6 0.8 1.0 1.2 1.40.0εh/εv

    0.0

    0.2

    0.4

    0.6

    0.8

    1.0

    N/N

    u

    (b)

    t3-C40-8.4 most-compressive sidet3-C50-8.4 most-compressive sidet3-C60-8.4 most-compressive sidet3-C40-8.4 least-compressive sidet3-C50-8.4 least-compressive sidet3-C60-8.4 least-compressive side

    0.2 0.4 0.6 0.8 1.0 1.2 1.40.0εh/εv

    0.0

    0.2

    0.4

    0.6

    0.8

    1.0

    N/N

    u

    (c)

    Figure 7: Normalised load-transverse deformation coe¤cient response. (a) L/D ratio. (b) D/t ratio. (c) SCC strength.

    Advances in Civil Engineering 7

  • SCC jacket and the outer steel tube while the SCC isconned by the outer steel tube. It means that boththe original concrete and SCC are under triaxialcompression. e compressive strength of concreteunder triaxial stress (fcc) is higher than concretecompressive strength without connement (fc), andit can be written as [31]

    fcc � fc +(4.5 ∼ 7.0)fL, (2)

    where fL is the radial stress.

    (ii) e existence of original concrete and SCC couldavoid the inward local buckling of the outer steeltube and thus the material properties are betterexploited.

    is behavior of CFST jacketed columns also indicatesthat the signicant increase in the load-bearing capacity isnot only due to enlargement and additional steel re-inforcement in cross section but also because of steel jacket’sconnement.

    Figure 9 compares the e�ects of L/D ratio, D/t ratio, andSCC strength on the SI of specimens. e SI decreasedgradually with larger L/D ratio. e SI of specimens t3-C50-7.1, t3-C50-8.4, and t3-C50-9.8 were 5.2%, 7.5%, and 9.0%lower than that of t3-C50-5.7, respectively. e SI decreasedsignicantly with larger D/t ratio. e SI of specimens t3-C50-8.4 and t2-C50-8.4 were 6.8% and 24.1% lower than thatof t4-C50-8.4. But the SCC strength showed little in�uenceon the SI of specimens. e SI of specimens t3-C50-8.4 andt3-C60-8.4 were only 1.6% and 0.8% lower than that of t3-C40-8.4.

    4.4. Ductility. To qualify the ductility of a column, ductilityindex (DI) is often adopted by many researchers [8, 38].Similarly, for CFST jacketed columns, DI is dened as

    DI �Δ85%Δu

    , (3)

    where Δ85% is the midheight de�ection when the load dropsto 85% of the ultimate load on the unloading branch and Δuis the midheight de�ection at the ultimate load.

    e DI of specimen Ref was zero because there was nodescending branch in the N-Δ curve while that of speciment3-C50-8.4 was 3.48. It indicated that the CFST jacketingmethod was e�ective to improve the ductility of slender RCcolumns. Figure 10 shows the e�ects of L/D ratio, D/t ratio,and SCC strength on the DI of specimens. It should bementioned that value of DI was not available for speciment3-C50-7.1 due to the test having not been continued forsu¤cient deformation for the load to reduce to 85% of theultimate load. Although there was �uctuation in the com-parison of DI in terms of L/D ratio, the trend of decreasingDI with increasing slenderness might be clearly observed bylinear tting the experimental data. On the other hand, it wasobserved that there was a signicant reduction in ductilitywith increasing D/t ratio and that the SCC strength did not

    have a large in�uence on the ductility of specimens. e DIof specimens t3-C50-8.4 and t2-C50-8.4 were 38.9% and48.2% lower than that of t4-C50-8.4.e DI of specimens t3-C50-8.4 and t3-C60-8.4 were 1.8% and 4.4% higher than thatof t3-C40-8.4.

    4.5. Load-Bearing Capacity Prediction. Because the innerconcrete and SCC worked well together under the con-nement of outer steel tube, the CFST jacketed slendercolumn failed similarly as the traditional CFST slendercolumn.

    us the load-bearing capacity prediction model for thetraditional slender column may be also applied to CFSTjacketed slender column when considering the di�erentcompressive strengths of original concrete and SCC and thecontribution of longitudinal steel rebar. With reference toChinese code GB 50936-2014 [39], a modied formula isproposed to predict the load-bearing capacity for CFSTjacketed slender column:

    1.34 1.27 1.24 1.22

    6 7 8 9 10 115L/D

    0.00

    0.25

    0.50

    0.75

    1.00

    1.25

    1.50

    SI

    (a)

    1.331.24

    1.01

    50 75 100 125 15025D/t

    0.00

    0.25

    0.50

    0.75

    1.00

    1.25

    1.50

    SI

    (b)

    1.26 1.24 1.25

    40 45 50 55 60 6535SCC strength (MPa)

    0.00

    0.25

    0.50

    0.75

    1.00

    1.25

    1.50

    SI

    (c)

    Figure 9: SI of specimens. (a) L/D ratio. (b) D/t ratio. (c) SCCstrength.

    8 Advances in Civil Engineering

  • Nu,theo � φ fscAsc + fs1As1( ), (4)

    where φ is the slenderness reduction factor, fsc is thecompressive strength of CFST jacketed column, Asc is thesum of the area of original concrete, SCC and outer steeltube.

    fsc can be written as

    fsc � 1.212 +0.176fs2

    213 + 0.974( )ξ +−0.104fc

    14.4 + 0.031( )ξ2[ ]fc,

    (5)

    where ξ is the connement index and fc is the averageconcrete strength of original concrete and SCC, which aredened as

    ξ �As2fs2

    Ac1 + Ac2( )fc,

    fc �Ac1fc1 + Ac2fc2Ac1 + Ac2

    .

    (6)

    φ can be calculated by a modied formula for traditionalCFST slender column:

    φ �1

    2λ2sc1λ2sc1 + 0.95 + 0.5λsc1( )

    ·��������������������������λ2sc1 + 0.95 + 0.5λsc1( )( )

    2− 4λ2sc1

    √,

    (7)

    where λsc1 is the regular slenderness ratio, which is dened as

    λsc1 � 0.01λsc 0.001fs2 + 0.781( ),

    λsc �μli,

    i �

    ���IscAsc

    �D24,

    (8)

    where λsc is the slenderness ratio and μl and i are the cal-culating length and radius of gyration of CFST jacketedcolumn.

    Table 1 compares the predicted strength (Nu,theo) withthe experimental results (Nu) and the ratio Nu,theo/Nu is 0.93

    with a coe¤cient of variation (CV) of 0.09. e goodagreement indicates the accuracy of the proposed model inpredicting the load-bearing capacity for CFST jacketedslender column.

    5. Conclusions

    is paper presents an experiment of slender circular RCcolumns strengthened with CFST jacketing. Nine slenderspecimens were tested under axial compression, and thefollowing conclusions can be drawn within the scope of thisstudy:

    (1) e CFST jacketed columns exhibited good ductilebehavior under axial compression, and all experi-enced ductile failure mode (global bending), whilethe slender RC columns showed a brittle failuremode (concrete peel-o�).

    (2) e outer steel tube provided e�ective connementon the SCC and original slender columns, and thusenhanced the load-bearing capacity, material utili-zation, and ductility of slender RC columns. e L/Dratio and D/t ratio showed obvious in�uence on theperformance of CFST jacketed columns. e Nu, SI,and DI decreased 9.3%, 9.0%, and 51.9% when theL/D ratio increased from 5.7 to 9.8 and dropped20.9%, 24.1%, and 48.2% when the D/t ratio in-creased from 56.2 to 121.7.

    (3) e SCC strength had a slight e�ect on the per-formance of CFST jacketed columns, in which thespecimen t3-C60-8.4 showed 8.1%, −0.8% and 4.4%increase in the Nu, SI, and DI compared to speciment3-C40-8.4.

    (4) A modied model was proposed to predict the load-bearing capacity for CFST jacketed slender columnsbased on the model for traditional CFST columns.Comparison between the prediction and the ex-perimental results showed good agreement.

    It is worth mentioning that the CFST jacketed column inpractice may not behave similarly to specimens in this studybecause the external steel jacket may be discontinuous at thecolumn top and bottom. For the CFST jacketed specimenunder compression over the existing column and retrot

    8.77

    4.22

    3.48

    6 7 8 9 10 115L/D

    0

    2

    4

    6

    8

    10D

    I

    (a)

    5.70

    3.48

    2.95

    50 75 100 125 15025D/t

    0

    2

    4

    6

    8

    10

    DI

    (b)

    3.42 3.48 3.57

    40 45 50 55 60 6535SCC strength (MPa)

    0

    2

    4

    6

    8

    10

    DI

    (c)

    Figure 10: DI of specimens. (a) L/D ratio. (b) D/t ratio. (c) SCC strength.

    Advances in Civil Engineering 9

  • area, the elastic stiffness increased more than 100% andstrength increased up to 30% compared to the specimencompressed on the existing column only [24]. 'e structuralsteel collars were placed around the gaps at the top andbottom of the column and tied to the adjacent elements withpostinstalled anchors aiming to increase shear strength lo-cally [40]. 'e results showed this technique can transfer thecolumn shears to the footing and adjacent elements and thusenhance the blast resistance to bridge columns seismicallyretrofitted using steel jackets. 'erefore, the additionalstrengthening technique should be applied at the bottomand top of the column, such as structural steel collars, totransfer the whole axial load and shear force over the entirecross section. Otherwise, a relative reduction coefficientshould be carefully considered before the application ofCFST jacketing.

    Conflicts of Interest

    'e authors declare that they have no conflicts of interest.

    Acknowledgments

    'e authors are grateful for the financial supports providedby the Natural Science Foundation of China (51678456), theFundamental Research Funds for the Central Universities ofChina (2015210020201), and the Hubei Provincial NaturalScience Foundation of China (2016CFB271).

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