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Journal of Constructional Steel Research 62 (2006) 1310–1324www.elsevier.com/locate/jcsr

Experimental behaviour of recycled aggregate concrete filled steel tubularcolumns

You-Fu Yanga, Lin-Hai Hanb,c,∗a College of Civil Engineering, Fuzhou University, Gongye Road 523, Fuzhou, Fujian Province 350002, People’s Republic of China

b Department of Civil Engineering, Tsinghua University, Beijing 100084, People’s Republic of Chinac Key Laboratory of Structural Engineering and Vibration of China Education Ministry, Tsinghua University, Beijing 100084, People’s Republic of China

Received 19 September 2005; accepted 23 February 2006

Abstract

This paper describes a series of tests on steel tubular columns of circular and square section filled with normal concrete and recycled aggregateconcrete. Thirty specimens, including 24 recycled aggregate concrete filled steel tubular (RACFST) columns and 6 normal concrete filled steeltubular (CFST) columns, were tested to investigate the influence of variations in the tube shape, circular or square, concrete type, normal concreteand recycled aggregate concrete, and load eccentricity ratio, from 0 to 0.53 on the performance of such composite columns. The test results showthat both types of filled columns failed due to overall buckling. Comparisons are made with predicted ultimate strengths of RACFST columns usingthe existing codes, such as ACI 318-1999, AIJ-1997, AISC-LRFD-1999, BS5400-1979, DBJ13-51-2003 and EC4-1994. A theoretical model fornormal CFST columns is adopted in this paper for RACFST columns. The predicted load versus deformation relationships are in good agreementwith test results.c© 2006 Elsevier Ltd. All rights reserved.

Keywords: Recycled aggregate concrete filled steel tube (RACFST); Recycled aggregate concrete (RAC); Composite columns; Composite action; Ultimate strength;Design codes

1. Introduction

Hollow structural steel (HSS) sections are often filled withconcrete to form a composite column. Such kinds of compositecolumns have been the interest of structural engineers fortheir high load bearing capacity, saving formwork, smallcross section over reinforced concrete structures, and high fireresistance over the steel structures (ASCCS [6]).

Recycled aggregate concrete (RAC) can be recognized as anew kind of concrete construction, in which broken pieces ofwaste concrete are used as aggregate. Due to the low strengthand elastic modulus, bad workability, high water infiltration andhigh shrinkage and creep of RAC, they are only used as non-structural concrete mostly (Ajdukiewicz and Kliszczewicz [5],Sagoe-Crentsil et al. [18]). However, RAC is well recognizedin view of its low thermal conductivity, low brittleness as

∗ Corresponding author at: Department of Civil Engineering, TsinghuaUniversity, Beijing 100084, People’s Republic of China. Tel.: +86 1062797067; fax: +86 10 62781488.

E-mail address: [email protected] (L.-H. Han).

0143-974X/$ - see front matter c© 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.jcsr.2006.02.010

well as the low specific gravity that reduces the self-weightof the structures. Most importantly, the use of RAC can savenatural resources and protect our living environment (Katz[15]; Topcu and Sengel [20]). When the recycled aggregateconcrete is deployed in a new construction, the consequencesof its weakness need to be reduced or avoided. Recycledaggregate concrete filled steel tube (RACFST), which placesthe recycled aggregate concrete in the state of confinement andprotection of an outer steel tube, is less likely to be affected byharmful environment factors, i.e. water, temperature and winds,as experienced in the case of reinforced concrete structures.

In the past, there are a large number of studies on normalconcrete filled steel tubular (CFST) columns (Schneider [19]);however, little research has been conducted on RACFSTcolumns. Konno et al. [16] studied the behaviours ofconfined recycled aggregate concrete columns subjected toaxial compression. It was found that the new composite column,where the progress of fractures is faster than in the confinednormal concrete column, has enough capacity to be utilizedthough its stiffness and ultimate strength are smaller than those

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Y.-F. Yang, L.-H. Han / Journal of Constructional Steel Research 62 (2006) 1310–1324 1311

Nomenclature

CFST Concrete filled steel tubeCHS Circular hollow sectionD Sectional dimension, in mme Eccentricity of load, in mme/ro Load eccentricity ratio, ro = D/2Ec Concrete modulus of elasticity, in MPaEs Steel modulus of elasticity, in MPafcu Characteristic 28-day concrete cube strength, in

MPafsy Yielding strength of steel, in MPaH Distance away from the bottom support, in mmHSS Hollow structural steelL Effective buckling length of column in the plane

of bending, in mmN Axial load, in kNNCA Natural coarse aggregateNuc Predicted ultimate strength, in kNNue Experimental ultimate strength, in kNRAC Recycled aggregate concreteRACFST Recycled aggregate concrete filled steel tubeRCA Recycled coarse aggregateSHS Square hollow sectionSI Strength indext Wall thickness of steel tube, in mmum Mid-height lateral deflection of the column, in

mmδ Axial shortening, in mmε Strainσ Stress

(a) Circular section. (b) Square section.

Fig. 1. Cross-sectional dimension of the test specimens.

of the confined normal concrete columns. Konno et al. [17]performed studies on the strength and the deformational abilityof RACFST, and the conclusions were that the deformationalbehaviour of RACFST was similar to that of CFST and thestiffness of RACFST could be predicted approximately withconsideration of the Young’s modulus of RAC, which waslower than that of normal concrete.

This paper studies the behaviour of RACFST columns withthe cold-formed HSS tube being used. A series of tests werecarried out on 30 composite columns. The main parametersvaried in the tests are: (1) tube shape, circular or square, (2)concrete type, normal concrete or recycled aggregate concrete,and (3) load eccentricity ratio (e/ro, ro is given by D/2),

Fig. 2. Layout of the column test.

(a) Circular specimen. (b) Square specimen.

Fig. 3. A general view of the specimen after test.

Table 1Steel properties

Steelsection

Dimensionof sectionD × t (mm)

Yieldingstrengthfsy (MPa)

Tensilestrengthfu (MPa)

YieldingratioY ( fsy/ fu)

Modulus ofelasticity Es(MPa)

CHS ©-165×2.57 343.1 423.6 0.81 1.79 × 105

SHS �-150×2.94 344.4 450.5 0.76 2.07 × 105

from 0 to 0.53. Comparisons are made with predicted ultimatestrengths of RACFST columns using the existing codes, such asACI 318-99 [2], AIJ [3], AISC-LRFD [4], BS5400 [7], DBJ13-51-2003 [8] and EC4 [9]. A theoretical model for normal CFSTcolumns is adopted in this paper for RACFST columns. Thepredicted load versus deformation relationships are in goodagreement with the test results.

2. Experimental investigation

2.1. Material properties

Standard tensile coupon tests were conducted to measurematerial properties of the steel tubes. Three coupons were takenfrom each kind of steel tube, and for the square steel tube, the

1312 Y.-F. Yang, L.-H. Han / Journal of Constructional Steel Research 62 (2006) 1310–1324

Table 2The mix proportions and properties of the new concrete

Type ofconcrete

Cement(kg/m3)

Sand(kg/m3)

NCA(kg/m3)

RCA(kg/m3)

Water(kg/m3)

W/C 28-day cubestrength, fcu (MPa)

Test-day cubestrength, f ′

cu (MPa)Modulus ofelasticity, Ec (MPa)

Slump(mm)

Normalconcrete

414 630 1170 – 207 0.5 42.7 50.8 2.75 × 104 40

Recycledaggregateconcrete

414 630 878 292(25%)

207 0.5 41.8 46.7 2.61 × 104 35

414 630 585 585(50%)

207 0.5 36.6 44.1 2.46 × 104 33

Table 3Specimen labels and member capacities

Section types No. Specimen labels D × t (mm) L (mm) e (mm) e/ro Nue (kN) SLI (%)Measured value Average value

Circular

1 CA0 ©-165 × 2.57 1650 0 0 1217 1217 –

2 CA1-1 ©-165 × 2.57 1650 0 0 11581158 4.8

3 CA1-2 ©-165 × 2.57 1650 0 0 1158

4 CA2-1 ©-165 × 2.57 1650 0 0 10901106.5 9.1

5 CA2-2 ©-165 × 2.57 1650 0 0 1123

6 CB0 ©-165 × 2.57 1650 20 0.24 877 877 –

7 CB1-1 ©-165 × 2.57 1650 20 0.24 817836 4.7

8 CB1-2 ©-165 × 2.57 1650 20 0.24 855

9 CB2-1 ©-165 × 2.57 1650 20 0.24 795800 8.8

10 CB2-2 ©-165 × 2.57 1650 20 0.24 805

11 CC0 ©-165 × 2.57 1650 40 0.48 615 615 –

12 CC1-1 ©-165 × 2.57 1650 40 0.48 602604.5 1.7

13 CC1-2 ©-165 × 2.57 1650 40 0.48 607

14 CC2-1 ©-165 × 2.57 1650 40 0.48 600601 2.3

15 CC2-2 ©-165 × 2.57 1650 40 0.48 602

Square

1 SA0 �-150 × 2.94 1732 0 0 1285 1285 –

2 SA1-1 �-150 × 2.94 1732 0 0 12601266.5 1.4

3 SA1-2 �-150 × 2.94 1732 0 0 1273

4 SA2-1 �-150 × 2.94 1732 0 0 12521248.5 2.8

5 SA2-2 �-150 × 2.94 1732 0 0 1245

6 SB0 �-150 × 2.94 1732 20 0.27 910 910 –

7 SB1-1 �-150 × 2.94 1732 20 0.27 842858.5 5.7

8 SB1-2 �-150 × 2.94 1732 20 0.27 875

9 SB2-1 �-150 × 2.94 1732 20 0.27 825830 8.8

10 SB2-2 �-150 × 2.94 1732 20 0.27 835

11 SC0 �-150 × 2.94 1732 40 0.53 740 740 –

12 SC1-1 �-150 × 2.94 1732 40 0.53 686659 10.9

13 SC1-2 �-150 × 2.94 1732 40 0.53 632

14 SC2-1 �-150 × 2.94 1732 40 0.53 625640 13.5

15 SC2-2 �-150 × 2.94 1732 40 0.53 655

coupons were cut from the flat part of the tube. The 0.2% proofstress was adopted as the yielding strength. From these tests,the average yielding strength ( fsy), tensile strength ( fu), andmodulus of elasticity (Es) of the steel tubes are listed in Table 1.

Three types of concrete mixes were prepared. The mix wasdesigned for compressive cube strength ( fcu) at 28 days ofapproximately 40 MPa. In producing RAC, in place of naturalcoarse aggregate (NCA), portions of 25% and 50% recycledcoarse aggregate (RCA) were added as coarse aggregate. RCAwere obtained by crushing waste concrete, which was taken

from failure CFST specimens, and sieving with a mesh squareof 26.5 mm. The compressive cube strength of the wasteconcrete was about 50 MPa. The unit weight of RCA was2470 kg/m3, bulk density 1260 kg/m3, crushing value 19.7%and water absorption 8.43%. As for NCA, the unit weight was2600 kg/m3, bulk density 1420 kg/m3, crushing value 15.3%and water absorption 0.42%. All specimens were cast from onebatch of concrete. Several 150 mm cubes and 150 mm×300 mmprisms were also cast from the concrete and cured in conditionssimilar to the corresponding composite columns.


wei

(a) CA series.

(b) CB series.

(c) CC series.

Fig. 4. Failure modes of tested specimens (circular specimens).

The mix proportions and properties of the new concreteere summarized in Table 2. The workability of fresh concrete,

xpressed in terms of slump values, was also summarizedn Table 2. It can be seen that a reduction of slump values

(a) SA series.

(b) SB series.

(c) SC series.

Fig. 5. Failure modes of tested specimens (square specimens).

was produced in RAC by raising the extent of RCA in themixture. Although the slump value of RAC is lower than that ofnormal concrete, there was no difficulty in achieving the desireduniformity and subsequent compactness of RAC.


(a) CB1-1.

(b) SB1-1.

Fig. 6. Lateral deflection curves of test specimens.

In all the concrete mixes, the fine aggregate used wassiliceous sand, and the NCA was carbonate stone.

2.2. Specimen preparations

A total of 30 composite columns, including 24 RACFSTcolumns and 6 corresponding normal CFST columns weretested. A summary of all specimens is presented in Table 3,where the section sizes and load eccentricity ratios (e/ro)

are given. The main experimental parameters are listed below,along with the labels used to characterize each specimen:

• Section shape (C = circular, S = square);• Eccentricity of load (A: e = 0 mm, B: e = 20 mm, C:

e = 40 mm);• Filled concrete type (0 = normal concrete, 1 = recycled

aggregate concrete containing 25% RCA, 2 = recycledaggregate concrete containing 50% RCA).

For example, the specimen beginning with the label “CB1-1”would be the first circular composite column filled withrecycled aggregate concrete containing 25% RCA, and its loadeccentricity is 20 mm.

Fig. 1 shows the cross section of the test specimens, whereD is the diameter or the width of the steel tube with circular or

(a) CA series.

(b) CB series.

(c) CC series.

Fig. 7. Axial load (N)–axial shortening (δ) responses (circular specimens).

square sections respectively; t is the wall thickness of steel tube.The tubes were all manufactured from long cold-formed HSScolumns, and the ends of the steel tubes were cut and machinedto the required length. The insides of the tubes were wirebrushed to remove any rust and loose debris present. Depositsof grease and oil, if any, were cleaned away.


(a) SA series.

(b) SB series.

(c) SC series.

Fig. 8. Axial load (N)–axial shortening (δ) responses (square specimens).

The concrete was filled in layers and was vibrated by a pokervibrator. The specimens were placed upright to air-cure at roomtemperature until testing. During curing, a very small amountof longitudinal shrinkage of 0.8 mm or so occurred at the topof the columns. A high-strength epoxy was used to fill thislongitudinal gap so that the concrete surface was flush with thesteel tube at the top.

Prior to testing, the top surfaces of the specimens wereground smooth and flat using a grinding wheel with diamondcutters. A horizontal ruler was used to check for the flatness.This was to ensure that the load was applied evenly across thecross section and simultaneously to the steel and concrete.

2.3. Test procedures

The experimental study was to determine not only themaximum load capacity of the specimens, but also toinvestigate the failure pattern up to and beyond the ultimateload. All the tests were performed on a 5000 kN capacityuniversal testing machine, and the test data were collected byan IMP data acquisition system.

The columns were tested as pin-ended supported (Han andYao [10]). The desired eccentricity was achieved by accuratelymachining grooves 6 mm deep into the stiff end plate thatwas welded to the steel tubes. For the pure axial compressioncolumn, the groove was in the middle of the plate. The endplatewas very stiff with a thickness of 16 mm. The axial load wasapplied through a very stiff top platen with an offset trianglehinge, which also allowed specimen rotation to simulate pin-ended supports.

Both the endplate and the top platen were made of very hardand very high strength steel.

Eight strain gauges were used for each specimen to measurethe longitudinal and transverse strains at the mid-height. Threelinear voltage displacement transducers (LVDTs) were usedalong the specimen height to monitor the lateral deflections.Two displacement transducers, at the end of the specimen, wereused to monitor the axial shortening, shown as in Fig. 2.

A load interval of less than one tenth of the estimated loadcapacity was used. Each load interval was maintained for about2–3 min. At each load increment the strain readings and thedeflection measurements were recorded. All specimens wereloaded to failure. Each test took approximately 30 min to reachthe maximum load and 1.5 h to complete. All the tests werecompleted within 3 days.

2.4. Test results

All the test specimens behaved in a relatively ductile mannerand testing proceeded in a smooth and controlled fashion.Typical failure modes of HSS columns filled with normalconcrete and recycled aggregate concrete were all overallbuckling. Fig. 3 gives a general view of the specimens after test.When the load was small, the lateral deflection of the specimenat middle height was small and approximately proportional tothe applied load. When the load reached about 60%–70% of themaximum load, the lateral deflection at middle height startedto increase significantly. Figs. 4 and 5 show the failure modesof all tested specimens. It can be seen that, generally, for thecircular specimens and square specimens filled with normalconcrete and recycled aggregate concrete containing 25% RCA,the buckle is formed near the center without reference to loadeccentricity ratio. However, for square specimens filled withrecycled aggregate concrete containing 50% RCA, buckling


Fig. 9. Load (N) versus mid-height lateral deflection (um) curves (circular specimens).

is formed near the top or the bottom of the columns. Thismay be explained by the compactness of recycled aggregateconcrete containing 25% RCA is similar to that of normalconcrete; nevertheless, the compactness of recycled aggregateconcrete containing 50% RCA is somewhat lower than thenormal concrete. And the constraining effect of a circular steeltube is better than a square steel tube (ASCCS [6]).

During the test, the deflection curve was approximately inthe shape of a half sine wave. Specimens CB1-1 and SB1-1 areselected to illustrate the lateral deflection development of thecomposite columns with different axial load level (n) beforeand after peak load, as shown in Fig. 6, where n is given byN/Nue. The sinusoids with the same values in the middle heightare also shown in Fig. 6 using dashed lines. This indicatesthe validity of the assumption of ‘the deflection curve of themember is a half sine wave’, which is adopted in the theoretical

model of this paper and the literature (Han and Yao [11]; Hanet al. [14]).

Typical axial load (N) versus axial shortening (δ) responsesof the composite columns are shown in Figs. 7 and 8. It canbe seen that the ultimate strength of the columns filled withrecycled aggregate concrete is lower than the correspondingnormal CFST columns, and generally RACFST columnsundergo higher deformation than normal CFST columns.

The load (N) versus lateral deflection (um) responses ofnormal CFST and RACFST columns are presented in Figs. 9and 10, where um is the lateral deflection at mid-height of thecolumn. It can be found that the type of in-fill concrete almosthas no influence on the shape of N–um curves of the compositecolumns. The ultimate loads (Nue) of RACFST columnsare lower than the corresponding normal CFST specimens,and the value of Nue decreases with the increasing of the


Fig. 9. (continued)

load eccentricity ratio. No local buckling was found in thecompression zone of a steel tube before achieving the ultimatestrength. The ultimate loads (Nue) obtained in the test aresummarized in Table 3. The axial load (N) versus extremelongitudinal fibre tensile and compressive strain relationshipsare shown in Figs. 11 and 12. It can be seen that bothnormal CFST and RACFST columns show similar behaviour.In general, the strain corresponding to the ultimate strengthincreases with the increase of the load eccentricity ratio. Forcircular specimens, the compressive strain corresponding to theultimate strength exceeds the steel yielding strain. However, forsquare specimens, the strain is close to the steel yielding strain.This indicates that the specimens with larger load eccentricityratio show more ductility. The composite action between steeltube and core concrete can improve the performance of suchcomposite columns and the confinement of circular steel tubeto core concrete is better than that of a square steel tube.

During the test, the cross sections remain plane. SpecimensCB1-1 and SB1-1 are also selected to illustrate the compressivestrain, tensile strain and centroid axes strain development ofthe composite columns with different axial load level (n)

before and after peak load, as shown in Fig. 13, where n isgiven by N/Nue, and x represents the position of longitudinalstrain gauges at the mid-height section of the specimens. Thisindicates that, generally, the assumption of “cross-sectionsremain plane” adopted in the theoretical model of this paper andthe literature (Han and Yao [11]; Han et al. [14]) is reasonable.

Figs. 14 and 15 show the differences in the ultimate strength(Nue) of the columns with normal concrete and recycled

aggregate concrete containing different amounts of RCA, wherespecimens with normal concrete were expressed as “NC”,while “25% RCA” and “50% RCA” were used to indicatethe RACFST columns containing 25% RCA and 50% RCArespectively.

3. Analysis of test results and discussions

For convenience of comparisons of the ultimate strengthof the composite columns with normal concrete and recycledaggregate concrete, the strength index (SI) is defined as follows:

SI = Nue0 − Nue1(or Nue2)

Nue0(1)

where, Nue0 are member capacities of the specimens withnormal concrete; Nue1and Nue2 are member capacities of thespecimens with recycled aggregate concrete containing 25%RCA and 50% RCA respectively.

The strength index (SI) so determined is listed in Table 3, inthe calculations, Nue1 and Nue2 are taken as the average valueof member capacities of the tested specimens.

The results summarized in Table 3 clearly show that,generally, specimens with normal concrete result in higherultimate strengths. It was found that the ultimate strengths(Nue) of the members with normal concrete were 1.7% to9.1%, and 1.4% to 13.5% higher than those of circular andsquare RACFST columns containing 25% RCA and 50% RCArespectively. Simultaneously, on the test day, the cube strengthof normal concrete was 8.1% and 13.2% higher than that of


Fig. 10. Load (N) versus mid-height lateral deflection (um) curves (square specimens).

the recycled aggregate concrete containing 25% RCA and 50%RCA respectively. The lowering in bearing capacity of recycledaggregate concrete in-fill columns can be attributed to the lowerstrength of recycled aggregate concrete as compared to normalconcrete.

The member capacities of RACFST columns predicted usingthe following six design methods are compared with the columntest results obtained in the current tests, i.e.

• ACI318-99 [2]• AIJ [3]• AISC-LRFD [4]• BS5400 [7]• DBJ13-51-2003 [8] (The equations were listed in detail in

Han et al. [12,13])• EC4 [9].

In all design calculations, the material partial safety factorswere set to unity.

Predicted member capacities (Nuc) using the differentmethods are compared with test results (Nue) in Tables 4 and5 for specimens with circular and square sections respectively.For the eccentrically loaded RACFST columns, Nuc waspredicted by replacing the end moment with Nuc · e in theinteraction flexure and compression equations of the above sixdesign codes.

Results in Table 4 show that ACI 318-99, AIJ, AISC-LRFD, BS5400 and DBJ13-51-2003 are conservative forpredicting the member capacities of the circular specimens withdifferent RCA contents and load eccentricity ratio. Overall,AISC-LRFD, ACI 318-99 and DBJ13-51-2003 give a membercapacity about 25%, 17% and 11% lower than the resultsobtained in the tests. However, EC4 gives a member capacity


Fig. 10. (continued)

Table 4Comparison between predicted member capacities and test results (circular specimens)

No. Specimen Nue(kN)

ACI 318-99(1999)

AIJ (1997) AISC-LRFD(1999)

BS5400 (1979) DBJ13-51-2003(2003)

EC4 (1994)

Nuc(kN)

NucNue Nuc

(kN)

NucNue Nuc

(kN)

NucNue Nuc

(kN)

NucNue Nuc

(kN)

NucNue Nuc

(kN)

NucNue

1 CA1-1 1158 757 0.654 1092 0.943 1022 0.883 1188 1.026 1067 0.921 1132 0.9782 CA1-2 1158 757 0.654 1092 0.943 1022 0.883 1188 1.026 1067 0.921 1132 0.9783 CA2-1 1090 736 0.675 1055 0.968 988 0.906 1155 1.06 1039 0.953 1091 1.0014 CA2-2 1123 736 0.655 1055 0.939 988 0.88 1155 1.028 1039 0.925 1091 0.9725 CB1-1 817 757 0.927 733 0.897 574 0.703 715 0.875 710 0.869 889 1.0886 CB1-2 855 757 0.885 733 0.857 574 0.671 715 0.836 710 0.83 889 1.047 CB2-1 795 736 0.926 708 0.891 563 0.708 698 0.878 693 0.872 860 1.0828 CB2-2 805 736 0.914 708 0.88 563 0.699 698 0.867 693 0.861 860 1.0689 CC1-1 602 562 0.934 580 0.963 399 0.663 512 0.85 535 0.889 673 1.118

10 CC1-2 607 562 0.926 580 0.956 399 0.657 512 0.843 535 0.881 673 1.10911 CC2-1 600 549 0.915 562 0.937 394 0.657 500 0.833 522 0.87 658 1.09712 CC2-2 602 549 0.912 562 0.934 394 0.654 500 0.831 522 0.867 658 1.093

Mean value 0.831 0.926 0.747 0.913 0.888 1.052

COV(Coefficient of variation)

0.128 0.036 0.106 0.092 0.035 0.055

about 5% higher than these of the measured ultimate strength,and is an unsafe predictor. The design methods proposed byBS5400 predicted a slightly lower capacity than the test results.Overall, the proposed method by AIJ gives a mean of 0.926 anda COV of 0.036, and is the best predictor to predict the ultimatecapacity of circular HSS columns filled with recycled aggregateconcrete.

Results in Table 5 clearly show that ACI 318-99, AIJ,AISC-LRFD, BS5400 and DBJ13-51-2003 are conservative forpredicting the member capacities of the square specimens withdifferent RCA contents and load eccentricity ratio. Overall,AISC-LRFD and BS5400 give a member capacity about 14%and 10% lower than the results obtained in the tests. However,EC4 gives a member capacity about 12% higher than these


(a) e/ro = 0. (b) e/ro = 0.24.

(c) e/ro = 0.48.

Fig. 11. Axial load versus extreme fibre strains at mid-height of test specimens (circular specimens).

Table 5Comparison between predicted member capacities and test results (square specimens)

No. Specimen Nue(kN)

ACI 318-99(1999)

AIJ (1997) AISC-LRFD(1999)

BS5400 (1979) DBJ13-51-2003(2003)

EC4 (1994)

Nuc(kN)

NucNue Nuc

(kN)

NucNue Nuc

(kN)

NucNue Nuc

(kN)

NucNue Nuc

(kN)

NucNue Nuc

(kN)

NucNue

1 SA1-1 1260 906 0.719 1227 0.974 1180 0.937 1165 0.925 1178 0.935 1303 1.0342 SA1-2 1273 906 0.712 1227 0.964 1180 0.927 1165 0.915 1178 0.925 1303 1.0243 SA2-1 1252 883 0.705 1189 0.95 1145 0.915 1131 0.903 1147 0.916 1262 1.0084 SA2-2 1245 883 0.709 1189 0.955 1145 0.92 1131 0.908 1147 0.921 1262 1.0145 SB1-1 842 900 1.069 863 1.025 720 0.855 770 0.914 821 0.975 993 1.1796 SB1-2 875 900 1.029 863 0.986 720 0.823 770 0.88 821 0.938 993 1.1357 SB2-1 825 877 1.063 838 1.016 707 0.857 751 0.91 801 0.971 962 1.1668 SB2-2 835 877 1.05 838 1.004 707 0.847 751 0.899 801 0.959 962 1.1529 SC1-1 686 685 0.999 679 0.99 518 0.755 575 0.838 632 0.921 780 1.137

10 SC1-2 632 685 1.084 679 1.074 518 0.82 575 0.91 632 1.00 780 1.23411 SC2-1 625 670 1.072 660 1.056 511 0.818 563 0.901 618 0.989 762 1.21912 SC2-2 655 670 1.023 660 1.008 511 0.780 563 0.86 618 0.944 762 1.163

Mean value 0.936 1.000 0.855 0.897 0.95 1.122

COV(Coefficient of variation)

0.168 0.039 0.06 0.025 0.029 0.081

of the measured result, and gives an unsafe prediction. The

design methods proposed by ACI 318-99 and DBJ13-51-2003

predict a slightly lower capacity than the test results. Overall,

the proposed method by AIJ gives a mean of 1.0 and a COV of


(a) e/ro = 0. (b) e/ro = 0.27.

(c) e/ro = 0.53.

Fig. 12. Axial load versus extreme fibre strains at mid-height of test specimens (square specimens).

(a) CB1-1. (b) SB1-1.

Fig. 13. Distribution of the strain across the section of the mid-height of test specimens.

0.039, and is the best predictor to predict the ultimate strengthof square HSS columns filled with recycled aggregate concrete.

For comparison purposes, the load versus lateral deflectioncurves at mid-height (plotted as dashed lines) predicted usinga mechanics model, which has been described in detail by Hanand Yao [11] and Han et al. [14] for normal CFST columns, arecompared with the test results of RACFST columns obtainedin current tests in Figs. 9 and 10. Due to page limitations,

only two figures for the comparisons between the predicted andmeasured axial load versus extreme fibre strains are given inthis paper, as shown in Fig. 16.

In the calculations, the idealized elastoplastic stress–strainmodel, based on the test results for both the flat zone and thecorner zone of cold-formed steel section, proposed by Abdel-Rahman and Sivakumaran [1], has been used for the squaresteel tube, as shown in Fig. 17, where fsy is the yielding


(a) e/ro = 0. (b) e/ro = 0.24.

(c) e/ro = 0.48.

Fig. 14. Influences of concrete type on the member capacities (circular specimens).

strength of steel and fsp(=0.75 fsy) is the proportional limitstress. In this model, the strain hardening and the corner effectsof a cold-formed steel tube are taken into account. The curveconsists of three stages: the elastic stage (from point o to pointa), the elastoplastic stage (from point a to point c) and the strainhardening stage (from point c to point d). The elastoplasticstage is idealized using a bilinear representation between fsp

and fsy with an intermediate stress fsm (corresponding to pointb) being the half value between fsp and fsy. The modulus inthe strain hardening stage (E1) is taken as 0.0075Es, whereEs denotes the modulus of elasticity of steel, and is taken as206,000 MPa in this paper.

The equation for the increase in the yielding strength ofthe corner zone is as follows (Abdel-Rahman and Sivakumaran[1]):

Δ fsy(corner zone) = 0.6[Bc · (r/t)−m − 1] · fsy (2)

where, Bc and m is the factor related to the yielding strength andthe tensile strength of the steel, r is the inside bending radius ofthe corner, and t is the wall thickness of a square steel tube.

It can be found that, generally, good agreement is obtainedbetween the predicted and tested results. The mechanics modeldeveloped for normal CFST columns is thus acceptable for

the analysis of HSS columns filled with recycled aggregateconcrete.

4. Conclusions

The present study is an attempt to study the possibilityof using hollow structural steel columns filled with recycledaggregate concrete in practice. Based on the results of thisstudy, the following conclusions can be drawn within the scopeof these tests:

(1) The typical failure modes of RACFST columns are similarto those of the normal CFST columns. They were all overallbuckling failure. The ultimate capacities of such compositecolumns decreased with the increase in load eccentricityratio.

(2) The recycled aggregate concrete in-fill columns haveslightly lower but comparable ultimate capacities comparedwith the specimens filled with normal concrete. It wasfound that, in general, the ultimate capacities of the mem-bers with normal concrete were 1.7%–9.1% higher thanthose of circular columns with recycled aggregate concretecontaining 25% recycled coarse aggregate and 50% recy-cled coarse aggregate, and for square specimens, the rangesare 1.4%–13.5%. The lowering in capacities of RACFST


(a) e/ro = 0. (b) e/ro = 0.27.

(c) e/ro = 0.53.

Fig. 15. Influences of concrete type on the member capacities (square specimens).

Fig. 16. Comparison of relationships of axial load versus extreme fibre strains at mid-height between calculated results and tested ones.

columns can be attributed to the lower strength of recycledaggregate concrete as compared to the normal concrete.

(3) Generally, both ACI 318-99, AIJ, AISC-LRFD, BS5400and DBJ13-51-2003 methods are conservative for predict-ing the strengths of circular and square composite columnsfilled with recycled aggregate concrete. However, EC4

gives a member capacity about 5% and 12% higher thanthe experimental result for circular and square RACFSTcolumns respectively, and gives an unsafe prediction.

(4) It was found that, in general, the mechanics model devel-oped for normal CFST columns is acceptable for the calcu-lations of RACFST columns.


Fig. 17. Idealized stress–strain model for square steel tube.

Acknowledgements

The tests reported herein were made possible by the NationalNatural Science Foundation of China (No. 50425823), theStart-Up Fund for Outstanding Incoming Researchers Projectof Tsinghua University, the Education Bureau fund of FujianProvince (JB05060), and the Science and Technology Fundof Fuzhou University (2004-XQ-19). The financial support ishighly appreciated. The authors also wish to thank Mr. BoZhang, Mr. Xin Ye and Miss Feng-Ying Wu for their assistancein the experiments.

References

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