paper tecn mat.pdf

11
Structural behaviour of geopolymeric recycled concrete filled steel tubular columns under axial loading Xiao-Shuang Shi a,b,, Qing-Yuan Wang b,c , Xiao-Ling Zhao d , Frank G. Collins d a College of Architecture and Environment, Sichuan University, Chengdu 610065, PR China b Key Laboratory of Energy Engineering Safety and Disaster Mechanics, Ministry of Education, Sichuan University, Chengdu 610065, PR China c Advanced Research Institute, Chengdu University, Chengdu 610106, PR China d Department of Civil Engineering, Monash University, Clayton, VIC 3800, Australia highlights The geopolymeric recycled concrete (GRC) is a new constructional material. The structural properties of GRCFST columns were firstly tested and analysed. The influence of RCA replacement ratio to GRCFST and RACFST columns were discussed. A theoretical simulation model of GRCFST columns under axial loading was proposed. article info Article history: Received 1 August 2014 Received in revised form 18 January 2015 Accepted 18 February 2015 Available online 2 March 2015 Keywords: Geopolymer concrete Recycled aggregate Steel tubular column Load capacity Ductility Load-deformation relationship abstract Geopolymeric recycled concrete (GRC) is a new construction material which takes environmental sus- tainability into account. In this paper, an experimental study was carried on 12 concrete filled steel tubu- lar columns under axial loading, in order to fill a knowledge gap on the engineering and structural properties of GRC filled steel tube (GRCFST). Two section sizes of square hollow sections filled with GRC and recycled aggregate concrete (RAC) respectively, with different recycled aggregate (RA) replace- ment ratios of 0%, 50% and 100%, were used in the experiments. The test results indicated that the ulti- mate strength was reduced when adding more RAs in the columns, while the peak strain increased. The ductility of the columns was improved by increasing the RA replacement ratio. Overall, the influence of RA on the strength and ductility of GRCFST columns is greater than that of RAC filled steel tubular (RACFST) columns. The assumed theoretical model for predicting load versus deformation relation of GRCFST columns under axial loading was examined, and a revised theoretical model proposed. The results of the new model show good correlation with the experimental results. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Over the past few decades, human awareness has been strengthened by the global climate changes resulting from the rapid expansion of industry and infrastructure, solid waste dispos- al and greenhouse gas emission, etc. To reuse solid waste and reduce greenhouse gas emissions, mostly carbon dioxide (CO 2 ), has become a major target for all the human activities and a key feature of the sustainable development. Therefore, in infrastruc- ture construction, besides smart structural styles and intelligent technology, the ‘‘low carbon’’ concept should also be taken into account. Customarily, concrete is produced by using Ordinary Port- land Cement (OPC) as the binder. However, the manufacturing of OPC is a highly energy consuming process, resulting in cement pro- duction releasing nearly 10% of the total worldwide anthropogenic CO 2 emissions [1]. Geopolymer concrete is considered to have a great potential in the production of ‘‘green’’ concrete with a lower carbon footprint [2–4]. It has better mechanical and chemical properties than OPC with high compressive strength, low creep, good bonding with reinforced steel, as well as good resistance to acid sulphate and fire [5–9]. The application of recycled aggregate concrete (RAC) has been widely studied and approved, however, geopolymeric recy- cled concrete (GRC) is less study and in the focus of our study. In such material, cement is substituted totally by alkali solution and fly ash, and the natural coarse aggregate (NA) is replaced by http://dx.doi.org/10.1016/j.conbuildmat.2015.02.035 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved. Corresponding author at: College of Architecture and Environment, Sichuan University, Chengdu 610065, PR China. Tel.: +86 13982287467; fax: +86 28 86264996. E-mail address: [email protected] (X.-S. Shi). Construction and Building Materials 81 (2015) 187–197 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Transcript of paper tecn mat.pdf

Page 1: paper tecn mat.pdf

Construction and Building Materials 81 (2015) 187–197

Contents lists available at ScienceDirect

Construction and Building Materials

journal homepage: www.elsevier .com/locate /conbui ldmat

Structural behaviour of geopolymeric recycled concrete filled steeltubular columns under axial loading

http://dx.doi.org/10.1016/j.conbuildmat.2015.02.0350950-0618/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: College of Architecture and Environment, SichuanUniversity, Chengdu 610065, PR China. Tel.: +86 13982287467; fax: +86 2886264996.

E-mail address: [email protected] (X.-S. Shi).

Xiao-Shuang Shi a,b,⇑, Qing-Yuan Wang b,c, Xiao-Ling Zhao d, Frank G. Collins d

a College of Architecture and Environment, Sichuan University, Chengdu 610065, PR Chinab Key Laboratory of Energy Engineering Safety and Disaster Mechanics, Ministry of Education, Sichuan University, Chengdu 610065, PR Chinac Advanced Research Institute, Chengdu University, Chengdu 610106, PR Chinad Department of Civil Engineering, Monash University, Clayton, VIC 3800, Australia

h i g h l i g h t s

� The geopolymeric recycled concrete (GRC) is a new constructional material.� The structural properties of GRCFST columns were firstly tested and analysed.� The influence of RCA replacement ratio to GRCFST and RACFST columns were discussed.� A theoretical simulation model of GRCFST columns under axial loading was proposed.

a r t i c l e i n f o

Article history:Received 1 August 2014Received in revised form 18 January 2015Accepted 18 February 2015Available online 2 March 2015

Keywords:Geopolymer concreteRecycled aggregateSteel tubular columnLoad capacityDuctilityLoad-deformation relationship

a b s t r a c t

Geopolymeric recycled concrete (GRC) is a new construction material which takes environmental sus-tainability into account. In this paper, an experimental study was carried on 12 concrete filled steel tubu-lar columns under axial loading, in order to fill a knowledge gap on the engineering and structuralproperties of GRC filled steel tube (GRCFST). Two section sizes of square hollow sections filled withGRC and recycled aggregate concrete (RAC) respectively, with different recycled aggregate (RA) replace-ment ratios of 0%, 50% and 100%, were used in the experiments. The test results indicated that the ulti-mate strength was reduced when adding more RAs in the columns, while the peak strain increased. Theductility of the columns was improved by increasing the RA replacement ratio. Overall, the influence ofRA on the strength and ductility of GRCFST columns is greater than that of RAC filled steel tubular(RACFST) columns. The assumed theoretical model for predicting load versus deformation relation ofGRCFST columns under axial loading was examined, and a revised theoretical model proposed. Theresults of the new model show good correlation with the experimental results.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Over the past few decades, human awareness has beenstrengthened by the global climate changes resulting from therapid expansion of industry and infrastructure, solid waste dispos-al and greenhouse gas emission, etc. To reuse solid waste andreduce greenhouse gas emissions, mostly carbon dioxide (CO2),has become a major target for all the human activities and a keyfeature of the sustainable development. Therefore, in infrastruc-ture construction, besides smart structural styles and intelligenttechnology, the ‘‘low carbon’’ concept should also be taken into

account. Customarily, concrete is produced by using Ordinary Port-land Cement (OPC) as the binder. However, the manufacturing ofOPC is a highly energy consuming process, resulting in cement pro-duction releasing nearly 10% of the total worldwide anthropogenicCO2 emissions [1].

Geopolymer concrete is considered to have a great potential inthe production of ‘‘green’’ concrete with a lower carbon footprint[2–4]. It has better mechanical and chemical properties than OPCwith high compressive strength, low creep, good bonding withreinforced steel, as well as good resistance to acid sulphate and fire[5–9]. The application of recycled aggregate concrete (RAC) hasbeen widely studied and approved, however, geopolymeric recy-cled concrete (GRC) is less study and in the focus of our study.In such material, cement is substituted totally by alkali solutionand fly ash, and the natural coarse aggregate (NA) is replaced by

Page 2: paper tecn mat.pdf

188 X.-S. Shi et al. / Construction and Building Materials 81 (2015) 187–197

recycled coarse aggregate (RA) partially or totally. Therefore, due toeliminating cement as well as the CO2 absorbing effect of RCA [10],the CO2 emission problem would be further improved.

Concrete filled steel tubular (CFT) columns are well recognisedand widely applied for intelligent composite action, with theadvantages of both steel tube and in-filled materials. In the pastfew decades, many types of materials were encased in the steeltubes, considering environmentally friendly materials in order tominimize pollution and energy consumption, For example, steeltubes filled with RAC [11–15] involving the reuse of waste con-crete, self-consolidating concrete without vibration benefits forconstruction and energy saving [16,17], and polymers or poly-mer-based materials as positively considered to enhance the struc-tural behaviour of columns with higher tensile and adhesioncapacity, lower weight and shrinkage, as well as high ductility[18–20]. So far, research shows that some mechanical propertiesand structural behaviour of RAC filled steel tubular (RACFST) col-umns can be improved with better ductility compare with OPCfilled steel tubes [11,12,21–23]. A few studies on the theory ofRACFST under axial loading have been undertaken [12,24], but lit-tle theoretical research on geopolymer concrete filled steel tubularcolumns has been reported.

In our previous study [25] on the microstructure of GRC, it wasshown that, the mechanical properties of GRC is stronger overalldue to the different formation processes that result in much denserand stronger reaction products compared with OPC or RAC. In thispresent study, axial compression experiments were carried out on12 concrete filled steel tubular columns, including 6 RACFST col-umns and 6 GRC filled steel tubular (GRCFST) columns with differ-ent recycled aggregate replacement ratios of 0%, 50% and 100%. Theload capacity, structural behaviour and failure mode were testedand analysed. The theoretical analysis method based on existingmodels to simulate the load versus deformation relation of GRCFSTis discussed and compared with experimental results. Further-more, an improved model is proposed according to the experimen-tal results and the GRC characteristics.

2. Experimental program

2.1. Material properties

2.1.1. Properties of steelCold-formed square hollow sections (SHS) of size 200 mm � 6 mm and

150 mm � 5 mm manufactured to AS 1163 [26] were used in the test program.The mechanical properties of steel were obtained by tensile coupon tests accordingto Australian Standard AS1391 [27]. The coupons were cut from the flat face of theSHS along the longitudinal direction of the section. The yield stress and ultimatetensile strength are listed in Table 1.

2.1.2. AggregatesThe nominal size of the RA and NA were 20 mm and 14 mm, respectively. The

test results shown in Table 2 indicate that the RA are lower than those of NA byabout 15%, 18% and 9% for the apparent density, dry density and SSD density,

Table 1Mechanical properties of steel.

B � t (mm) Elastic modulus/std E (GPa)

Yield stress/std fy (MPa)

Ultimate tensilestrength/std fu (MPa)

150 � 150 � 5 197/1.53 486/2.83 558/2.83200 � 200 � 6 199/1.50 467/4.55 544/5.56

Table 2Physical properties of NA and RA.

Aggregatetype

Apparentdensity/std (kg/m3)

Dry density/std (kg/m3)

SSD density/std (kg/m3)

Waterabsorption/std (%)

NA 2850/5.66 2819/5.37 2908/6.44 1.08/0.01RA 2433/2.77 2304/2.58 2645/6.87 5.60/0.11

respectively. However, the water absorption of RA is about as 5 times as that ofNA. This is due to the existence of porous and less dense residual mortar lumpsadhering to the RA, as well as much more micro-cracks being produced duringthe crushing process in RA production.

2.1.3. Fly ashFly ash (ASTM Class F) with 2.8% of CaO was used as the main aluminium and

silicate source for synthesizing the geopolymeric binder, mainly consisting of glasswith some crystalline inclusions of mullite, hematite and quartz.

2.1.4. Alkali solutionSodium silicate solution (Na2SiO3) with specific gravity of 1.53 and sodium

hydroxide (NaOH) flakes of 98% purity were supplied by PQ Australia. Sodiumhydroxide was dissolved using distilled water to provide 8 molarity alkaline solu-tions. Na2SiO3 and NaOH solutions were prepared one day prior to usage.

2.2. Specimen preparation

2.2.1. Mixture designDue to the high water absorption of RA, the water determined according to the

effective absorption of RA was used to pre-soak the RAs before mixing, and theaggregates were introduced to the mixture in a saturated surface dry (SSD) condi-tion. In order to compare the influence of the RA content on the concrete filled col-umns, six mixtures were designed with different RA replacement ratios for the RACand GRC concrete. The concrete mixture proportions are summarized in Table 3.The numbers ‘‘0’’, ‘‘50’’ and ‘‘100’’ after the concrete name refer to the RA replace-ment ratio of 0%, 50% and 100%, respectively. W/G is the ratio of total water togeopolymeric binder solids, including fly ash and solids in the alkali solution. Themixing of the concrete was undertaken in a mechanical mixer according to the pro-cedure in AS 1012.2 [28].

2.2.2. Manufacturing processThe steel tubes were cut into 750 mm lengths. A 20 mm thick steel plate was

welded on one end of the tubes to ensure the flatness of the base, as well as actingas the mould for the concrete. Following concrete placement in the tubes, the con-crete was compacted by an electric poker vibrator. The columns were covered withpolyethylene sheets for curing. The mixture was then poured into the steel cylindermoulds of 100 mm diameter � 200 mm length for compressive strength tests byapplying 20 manual strokes per layer in three equal layers on a vibration table.For each type of tubular column, at least 3 cylinders were prepared. When the col-umns were ready for the test, the top surface of the columns were ground and pol-ished by a diamond cutter to ensure flatness so that the loading was applied on thecore concrete and steel tube simultaneously.

In total, 12 steel tubular columns were manufactured. The geometric dimen-sions and experimental results are summarized in Table 4. The square hollow sec-tions of 200 mm � 200 mm � 6 mm and 150 mm � 150 mm � 5 mm specimenswere labelled as ‘‘S1’’ and ‘‘S2’’, respectively.

2.2.3. Curing conditionThe RACFST columns and RAC cylinder specimens were cured under polyethy-

lene sheets in the laboratory ambient environment. After 24 h, the RAC cylinderswere demoulded and transferred into a tank with saturated limewater at 23 ± 2 �C.

The GRCFST column and GRC cylinder specimens were sealed by polyethylenesheets to prevent excessive evaporation during curing at 80 �C for 24 h in an oven,and then moved out into the laboratory ambient environment.

2.2.4. Test set up and proceduresThe experimental study aimed at investigating not only the maximum load

capacity of the columns, but also the deformation and failure mode beyond the ulti-mate load. All the specimens were tested on AMSLER5000 in the Civil EngineeringLaboratory at Monash University. A longitudinal and a transverse strain gage wereattached to the middle of the column on each side to record the deformation of thesteel tubular columns. Two linear variable displacement transducers (LVDTs) withstring pots were set up between loading and bottom plates diagonally to measurethe vertical displacement of the columns. The test set up is shown in Fig. 1. Duringthe experiment, the axial compression and the longitudinal deformation, and thevertical and lateral strains were automatically collected by the data logger everysecond. The loading process was manually controlled by hydraulic valve.

3. Results and discussions

According to the experimental results, the load capacity, loadversus deformation relation and the ductility, as well as the failuremodes of all the concrete filled steel tubular columns are presentedand discussed in the following sections. The influence of the RAreplacement ratio on the properties of the different columns arealso discussed and compared.

Page 3: paper tecn mat.pdf

Table 3Summary of concrete mixture proportions (kg/m3).

Mixture RA ratio (%) RA/NA Cement Sand Fly ash NaOH solution Na2SiO3 solution Added water W/G ratio

RAC0 0 0/1294 364 554 – – 182 0.50RAC50 50 647/647 364 554 – – 182 0.50RAC100 100 1294/0 364 554 – – 182 0.50GRC0 0 0/1294 – 554 368 53 131 0 0.50GRC50 50 647/647 – 554 368 53 131 0 0.50GRC100 100 1294/0 – 554 368 53 131 0 0.50

Table 4Summary of the specimens and the corresponding test results.

Specimens B (mm) t (mm) B/t (mm) As/Ac fc/std (MPa) fy (MPa) n Nue (kN) DI SI (%)

S1RAC0 200 6 33.3 0.132 68/0.36 467 1.08 3561 2.70 –S1RAC50 200 6 33.3 0.132 57/1.35 467 1.30 3466 3.00 2.7S1RAC100 200 6 33.3 0.132 50/0.18 467 1.49 3297 3.04 7.4S1GRC0 200 6 33.3 0.132 69/2.35 467 1.07 4497 1.89 –S1GRC50 200 6 33.3 0.132 56/4.55 467 1.32 3380 3.39 24.8S1GRC100 200 6 33.3 0.132 44/2.78 467 1.68 3376 2.74 24.9S2RAC0 150 5 30.0 0.148 68/0.36 486 1.26 2184 2.83 –S2RAC50 150 5 30.0 0.148 57/1.35 486 1.52 2100 3.36 3.9S2RAC100 150 5 30.0 0.148 50/0.18 486 1.75 1947 4.83 10.9S2GRC0 150 5 30.0 0.148 69/2.35 486 1.25 2676 1.64 –S2GRC50 150 5 30.0 0.148 56/4.55 486 1.54 2100 2.87 21.5S2GRC100 150 5 30.0 0.148 44/2.78 486 1.97 2057 3.07 23.1

X.-S. Shi et al. / Construction and Building Materials 81 (2015) 187–197 189

3.1. Load capacity

The experimental ultimate loads (Nue) of the specimens underaxial compression loading are listed in Table 4. The compressivestrength of the inner concrete for the corresponding columns ontesting day is recorded as fc. The confinement factor which canindicate the constraining effect of the outer steel tube to the innerconcrete is defined as n = Asfy/Acfck, where fy is the yield strength ofthe steel, and fck is the characteristic concrete strength (fck = fc� 0.67/0.8). Fig. 2 shows that load capacity of the columns withtwo different sections varies with different RA replacement ratios.Overall, the load capacity of the columns decreased with increaseof the RA in both RAC and GRC. This is attributed to the lower com-pressive strength of the in-filled concrete with more RA [29]. Asimilar phenomenon on RACFSTs was also observed by Yang andHan [11]. For the convenience of comparison of the load capacityof the columns filled with RAC or GRC and normal concrete, thestrength index SI is defined as [30]:

SI ¼ Ne0 � Ne;r

Ne0ð1Þ

where Ne0 represents the experimental results of concrete filledsteel tubular columns with 0% RA replacement ratio (normal con-crete); and Ne,r refers to the experimental results of concrete filledsteel tubular columns with r% RA replacement ratio. The resultssummarized in Table 4 clearly show that the influence of RAreplacement ratio is greater for GRCFSTs than for RACFSTs. Overall,the load capacity was reduced more than 20% between GRCFSTsfilled with 50% and 100% RAs, but was only within 10% for RACFSTs.Moreover, considering each 50% RA increment to the in-filled con-crete, the load capacity of columns with RAC decreased gradually,i.e. 2.7% and 7.4% between S1RAC50 and S1RAC100 compared withS1RAC0; 3.9% and 10.9% between S2RAC50 and S2RAC100 com-pared with S2RAC0, respectively. While for the GRCFST columns,the load capacity decreased sharply between the GRC50 and GRC0filled columns, but very slightly between columns filled withGRC50 and GRC100. Especially, the difference in load capacitybetween S1GRC100 and S1GRC50 is only 0.1%, which can be ignoredfor deviation. Therefore, it can be inferred that, the load capacity ofGRCFST columns is only slightly influenced when the RAs content is

over 50% in the geopolymer concrete. Contrarily, for the RACFST col-umns, when the RA replacement ratio greater than 50%, the differ-ence is very small, but with more than 50% RAs, the difference in theload capacity is somewhat lower than that of normal concrete [11].

3.2. Strain response and ductility

The strain response with the load (N–e) curves, indicating therelationship between bearing load and strain of the steel tubularcolumns, present the structural behaviour of the columns duringthe loading process. The tested axial load versus longitudinal strain(N–eL) curves are shown in Fig. 3. It can be seen that all the col-umns performed in a similar way with four typical stages: elastic,elastic–plastic, strain hardening, and failure stage, from beginningto the end [16]. Due to the confinement factor (n) being smallerthan 4.5, which is the characteristic value of the confinement factor(n0) for a square section [31], the curve after the peak decreases.According to the N–eL curves, with increasing of RA replacementratio, the ultimate strength of the columns decreased, while thepeak strain increased slightly for all the columns. For the curvesof the RACFST columns shown in Fig. 3(a) and (c), the reductionof the strength and the increase of the peak strain changed almosthomogeneously with different RA replacement ratios. However,the curves of the GRCFST columns depicted in Fig. 3(b) and (d) indi-cate that, with 50% RAs in the core geopolymer concrete, the ulti-mate strength decreased significantly. However, with more RAcontents, the changes trailed off, i.e. very close between S1GRC50and S1GRC100, as well as for S2GRC50 and S2GRC100.

The decreasing part of the N–eL curves after the peak reducedslightly with more RAs for both RACFST and GRCFST columns. Thisis evident in the decrease of the slope after the peak. Evidently, thedescending part is distinct for the columns filled with GRC0, butmuch more slight for the columns filled with GRC50 andGRC100, indicating better ductility. In order to identify the duc-tility quantitatively, the ductility index (DI) was adopted accordingto the method for ordinary concrete filled steel tubular columns,defined as [32,33]:

DI ¼ e85%

eyð2Þ

Page 4: paper tecn mat.pdf

N

N

Specimen

Strain gauges

(a) Schematic of specimens

(b) A GRCFST column to be tested

Fig. 1. Experimental set-up and instrumentation.

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 50 100

Load

cap

acit

y (k

N)

RA replacement ra�o (%)

S1RACFST S1GRCFST

S2RACFST S2GRCFST

Fig. 2. Load capacity of columns versus different RA replacement ratios.

190 X.-S. Shi et al. / Construction and Building Materials 81 (2015) 187–197

where e85% is the longitudinal strain when the load falls to 85% ofthe ultimate load; ey ¼ e75%=0:75; e75% is the axial strain when theload attains of 75% the ultimate load in the pre-peak stage. The DIresults for each column are listed in Table 4. With a higher DI value,the ductility is better. It can be seen that the ductility of RACFST col-umns is improved with more RAs in the concrete for all the speci-mens. The ductility of the GRCFST columns is basically increasedwith a larger RA replacement ratio, except in the case of theS1GRC50 column which is better than that of S1GRC100. Overall,the ductility of the columns are improved by 12%, 45%, 70% and87% for S1RACFST, S1GRCFST, S2RACFST and S2GRCFST between100% and 0% RA replacement ratios, respectively. This indicates thatthe influence of the RA content on ductility for the columns filled

with GRC is greater than those filled with RAC. Furthermore, asthe confinement factors (n) of the S2 columns are better than thoseof the corresponding S1 columns, the DI values are accordinglyhigher. Therefore, it can be inferred that with a better confinementeffect, the ductility of the columns under compression load could begreatly improved. This is agreed with other researchers’ study onordinary concrete filled tubes [32].

3.3. Failure mechanism

During the loading process, there is no obvious deformation atthe beginning of the loading for all the columns. When the loadwas applied near the ultimate load, cracking sounds were audible,and local buckling on the columns appeared. The configurations ofthe columns after the testing are shown in Fig. 4. For the S1 col-umns shown in Fig. 4(a), the local buckling firstly appeared nearthe middle of the columns. While the load increased, the bulgesalso occurred on the top part of the column. The bulges on the eachside of the column gradually connected together to form the typi-cal ‘‘roof’’ type failure mode. For the S2 columns shown in Fig. 4(b),the local buckling mostly started from the top or the bottom of thecolumn and appeared on the diagonally sides as the load increased.At the end of the loading, the bulges were measured as about 2–3 cm above the steel tube surface. Overall, the failure mechanismof the columns was similar to the local (outward folding) failure,and this is accordance with other researchers [31,32]. Moreover,during the deformation process, it was found that with more RAsand larger confinement factors, the failure mode has a trend ofchanging from a shear type to a drum type. Compared with thedeformation shown in Fig. 4, the bulges on the columns filled withRAC0 and GRC0 have greater angle on the crossing. However, thebulges on the columns filled with RAC100 and GRC100 almostappeared on the same level around the column. This is the sameas that observed by other researchers [32], i.e. with a larger con-finement factor, the failure generally occurs with a drum failuremode; while, with a smaller confinement factor, the failure isalways by the shear break mode.

4. Theoretical analysis

Over the past decades, with the fast development of steel andconcrete composite structures, there have been many studies onthe theory and calculation methods for concrete filled steel tubularcolumns [34–37]. So far, the primary calculation methods includethe fibre element method, the finite element method, the finitestrip method, the synthesis method and so on. According to thefibre element method for calculations on ordinary concrete filledsteel tubes, a theoretical approach on load versus deformation rela-

Page 5: paper tecn mat.pdf

0 5000 10000 15000 20000 25000 30000 350000

500100015002000250030003500400045005000

S1RAC0 S1RAC50 S1RAC100

N (K

N)

εL(με)(a) RACFST columns of S1

0 5000 10000 15000 20000 25000 30000 350000

500100015002000250030003500400045005000

N (k

N)

εL (με)

S1GRC0 S1GRC50 S1GRC100

(b) GRCFST columns of S1

0 5000 10000 15000 20000 25000 30000 350000

500100015002000250030003500400045005000

N (K

N)

εL (με)

S2RAC0 S2RAC50 S2RAC100

(c) RACFST columns of S2

0 5000 10000 15000 20000 25000 30000 350000

500100015002000250030003500400045005000

N (K

N)

εL (με)

S2GRC0 S2GRC50 S2GRC100

(d) GRCSFT columns of S2

Fig. 3. Load versus longitudinal strain (N–eL) curves of the columns.

X.-S. Shi et al. / Construction and Building Materials 81 (2015) 187–197 191

tion of RACFST columns under axial loading was proposed, whichwas a simplified numerical analysis method [12] that agreed wellwith experimental results. Therefore, the simplified method wasadopted herein to develop an improved model simulating the loadversus deformation relation of GRCFST columns under axialloading.

4.1. Constitutive relationships

4.1.1. SteelFor cold formed steel, the ideal constitutive relationship can be

described as an elastic–plastic strain–stress relationship based onmultilinear isotropic strain hardening [38]. It is given as:

rs ¼

Ee e 6 e1

f P þ ET1ðe� e1Þ e1 < e 6 e2

f ym þ ET2ðe� e1Þ e2 < e 6 e3

f y þ ET3ðe� e1Þ e3 < e

8>>><>>>:

ð3Þ

where, fy is the yield strength of the steel; E = 2.00 � 105 MPa;ET1 = 0.5E, ET2 = 0.1E, ET3 = 0.05E; f p ¼ 0:75f y, f ym ¼ 0:875f y;e1 ¼ 0:75f y=E, e2 ¼ e1 þ 0:125f y=ET1, e3 ¼ e2 þ 0:125f y=ET2.

4.1.2. Core concreteIn the concrete filled steel tubular (CFST) structure, the stress

state of the core concrete is different from the concrete under uni-axial load due to the confinement effect of the outer tube. Accord-ingly, Han [39] proposed the constitutive relationship for the coreconcrete. Based on Han’s study and the RAC properties, Yang [12]revised this mode for the in-filled RAC’s strain–stress relation. Con-sidering the influence of the RA replacement ratio on the RAC’scompressive strength and peak strain [24], for square sectional

RACFST columns, the constitutive relationship of the core RACcan be described as:

y ¼2x� x2 ðx 6 1Þ

xbðx�1Þgþx ðx > 1Þ

(ð4Þ

where, r0 is the maximum stress of the concrete; e0 is the peakstrain of the concrete; x ¼ e

e0; y ¼ r

r0;

r0 ¼ 1þ ð�0:0135n2 þ 0:1nÞ 24f c

� �0:45" #

f cð1� 0:3r þ 0:13r2Þ ð5Þ

e0 ¼ ecc þ 1330þ 760 f c24� 1� �� �

n0:2� �

� 1þ rh

� ðleÞ ð6Þ

h ¼ 65:715r2 � 109:43r þ 48:989; ð7Þ

ecc ¼ 1300þ 12:5f c; ð8Þ

g ¼ 1:6þ 1:5=x; ð9Þ

b ¼f 0:1

c1:35

ffiffiffiffiffiffi1þnp ðe 6 3:0Þ

f 0:1c

1:35ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þnðn�2Þ2p ðe > 3:0Þ

8><>: ð10Þ

Based on regression analysis on the test data, the compressivestrength of GRC with different RA replacement ratios can be calcu-lated by the following formula:

FrG ¼ Fn

Gð1� 0:26r � 0:1r2Þ ð11Þ

where FnG is the compressive strength of normal geopolymer con-

crete; FrG is the compressive strength of geopolymer concrete with

r% RA replacement ratio; r is the RA replacement ratio (%).

Page 6: paper tecn mat.pdf

(b) S2 columns

(a) S1 columns

S1RAC0 S1RAC50 S1RAC100 S1GRC0 S1GRC50 S1GRC100

S2RAC0 S2RAC50 S2RAC100 S2GRC0 S2GRC50 S2GRC100

Fig. 4. Failure modes of all the columns.

192 X.-S. Shi et al. / Construction and Building Materials 81 (2015) 187–197

So, the maximum stress of the core GRC can be expressed as:

r0¼ 1þð�0:0135n2þ0:1nÞ 24f c

� �0:45" #

f cð1�0:26r�0:1r2Þ ð12Þ

4.2. Simulation model of load-deformation relation

Based on the definition of the materials constitutive relation-ship, the load versus deformation relation of RACFST and GRCFSTcolumns under axial loading can be investigated by setting upequilibrium and deformation consistency conditions, as well asthe load-deformation curves of steel and core concrete being iden-tified. According to the simplified model introduced by Han [16],the simulated load versus deformation curves can be calculated.In order to compare the bearing load proportion, the N/Nmax–eLcurve is used herein to describe the load-deformation relation.Figs. 5 and 6 show the typical N/Nmax–eL curves for RACFST andGRCFST columns, respectively.

The curves in Fig. 6 demonstrate a similar configuration indicat-ing similar structural behaviour. It was found that as the RA con-tents increase, the calculated results are much closer to theexperimental results. During the loading process, when the loadwas applied up to about 70–80% of the ultimate load, the columnswent into the plastic-elastic stage from the initial elastic stage,which can be proven by the reduction of the slope. During thisstage, in-filled concrete is confined by the steel tube due to

Poisson’s ratio of the concrete being larger than that of steel. Theconfinement effect increases as the longitudinal deformationincreases. The other two dash curves in the figures show thebehaviour of the steel and concrete respectively, and indicate that,in the elastic stage, the stiffness of the steel is greater than that ofthe core concrete, as well as the stiffness of the concrete decreasingwith higher RA content. In addition, with the increasing RAreplacement ratio, the outer steel tube carries much more load,i.e. when the load reaches the peak, the load carried by steel andcore concrete shown in Fig. 5(a) and (b) are about equally shares;while they are about 60% and 40% in Fig. 5(e) and (f).

The load versus deformation relations shown in Fig. 6 are basi-cally similar to those shown in Fig. 5. However, the calculated peakstrains of the columns are significant larger than the experimentalresults (Fig. 6(a) and (b)). This is because the predicted peak strainmodel of the GRC is not close to the actual situation due to the dif-ference between the constitutive relationships of GRC and RAC,especially for the post-peak period. For example, in Fig. 6(a) and(b), the curves decreased very sharply indicating very poor duc-tility, and are quite different from the RACFST columns seen inFig. 5(a) and (b). With more RAs in the columns, the peak strainis larger and the ductility of the columns performs much better.This is evident by a decrease in the slope of the post-peak descend-ing branch. Therefore, the simulation model according to the exist-ing model should be revised and improved, as going to bediscussed in the following section.

Page 7: paper tecn mat.pdf

(a) S1RAC0 ξ=1.08

0 5000 10000 15000 20000 250000.0

0.2

0.4

0.6

0.8

1.0N/N max

ε (με)

test calculated

steelconcrete

(b) S2RAC0 ξ=1.26

0 5000 10000 15000 20000 250000.0

0.2

0.4

0.6

0.8

1.0

N/N max

ε (με)

test calculated steelconcrete

(c) S1RAC50 ξ=1.30

0 5000 10000 15000 20000 250000.0

0.2

0.4

0.6

0.8

1.0

N/N max

ε (με)

test calculated steelconcrete

(d) S2RAC50 ξ=1.52

0 5000 10000 15000 20000 250000.0

0.2

0.4

0.6

0.8

1.0

N/N max

ε (με)

testcalculated steelconcrete

(e) S1RAC100 ξ=1.49

0 5000 10000 15000 20000 250000.0

0.2

0.4

0.6

0.8

1.0

N/N max

ε (με)

test calculated steel concrete

(f) S2RAC100 ξ=1.75

0 5000 10000 15000 20000 250000.0

0.2

0.4

0.6

0.8

1.0

N/N max

ε (με)

test calculatedsteelconcrete

Fig. 5. Load (N/Nmax) versus longitudinal strain (e) relation of RACFST columns.

X.-S. Shi et al. / Construction and Building Materials 81 (2015) 187–197 193

4.3. Improved model of load-deformation relation for GRCFST

Experimental data on the stress–strain curves of geopolymerconcrete are very limited in the literature. Hardjito et al. [40]reported a calculation based on normal high strength concretewhich agreed well with experimental results. Sarker [41] also

reported the equation from Hardjito with a minor revision thatpresented a good correlation with the experimental stress–straincurves. The equations are expressed as:

rc ¼ f cmec

ecm

nn� 1þ ðec=ecmÞnk ð13Þ

Page 8: paper tecn mat.pdf

Fig. 6. Load (N/Nmax) versus longitudinal strain (e) relation of GRCFST columns.

194 X.-S. Shi et al. / Construction and Building Materials 81 (2015) 187–197

where, f cm is the peak stress, MPa; ecm is the peak strain;

n ¼ 0:8þ ðf cm=17Þ ð14Þ

k ¼ 0:67þ ðf cm=62Þ; ec=ecm > 1; ð15Þ

k ¼ 1:0; ec=ecm 6 1: ð16Þ

Here, assume x ¼ ec=ecm; y ¼ rc=f cm; then we can get:

y ¼x n

n�1þxn ð0 6 x 6 1Þx n

n�1þxnk ðx > 1Þ

(ð17Þ

In this study, considering the confinement effect to the coregeopolymer concrete, as well as the typical characteristics ofgeopolymer concrete, the peak strain is given as:

Page 9: paper tecn mat.pdf

Fig. 7. Load (N/Nmax) versus longitudinal strain (e) relation of GRCFST columns from revised model.

X.-S. Shi et al. / Construction and Building Materials 81 (2015) 187–197 195

ecm ¼ 2400þ 720� f ck13:5� 1� �0:7

� n0:2 ðleÞ ð18Þ

Considering the strength of the inner geopolymer concrete has agreat effect on the configuration of descending branch of the load-deformation curves, the constitutive relationship of the core GRC

confined by square section steel tubes can be revised from Eq. (4),and is given as:

y ¼2x� x2 ðx 6 1Þ

xbðx�1Þgþxðf c=36Þ ðx > 1Þ

(ð19Þ

Page 10: paper tecn mat.pdf

196 X.-S. Shi et al. / Construction and Building Materials 81 (2015) 187–197

where r0 is given by Eq. (12); e0 is given by Eq. (18); other symbolsare as the same as presented in Section 4.1.2.

The calculation method is as the same as introduced in Sec-tion 4.2. So the load versus deformation relation of GRCFST col-umns can be obtained according to the aforementionedimproved model. The revised N/Nmax–eL curves of the GRCFSTcolumns are shown in Fig. 7. It can be seen clearly that therevised curves have a better correlation with the test results.Compared with the results calculated from the previous model,the peak strain shown in Fig. 7 are much closer to theexperimental peak strain, as well as the descending branch ofthe curves. It is found that the RAs content has a great influ-ence on the load-deformation relationship of GRCFST columns,indicating the descending part of the curves in GRC50 andGRC100 filled steel columns have less effect than the columnsfilled with GRC0. Meanwhile, the peak strains in Fig. 7(a) and(b) are smaller, indicating the columns with more RAs in thecore concrete reached the ultimate strength with larger defor-mation which is beneficial for energy absorption in earthquakes.It is inferred that the ductility of geopolymer concrete filledsteel tubular columns can be improved by using RAs. Further-more, with increasing RA replacement ratio, the steel tube car-ries more and more bearing load. In S1GRC100 (Fig. 7(e)) andS2GRC100 (Fig. 7(f)), the load carried by the steel tube takesup 70% of the total load, and only 30% is carried by the coreconcrete. Overall, the loads carried by the steel and core con-crete are distributed in a similar manner to those in RACFSTcolumns.

5. Conclusions

The present study is an attempt to investigate the structuralbehaviour of geopolymeric recycled concrete filled steel tubularcolumns under axial loading. Experiments were conducted onGRCFST columns and RACFST columns with different RA replace-ment ratios. The load-deformation relations of all the columnswere investigated by means of a simulated numerical method. Fur-thermore, a revised simulated model is discussed and proposed forGRCFST columns. The following conclusions can be drawn withinthe scope of this study:

(1) The load capacities of GRCFST columns decrease withincreasing RA replacement ratios, so do RACFST columns.Comparing the strength index SI, the ultimate strength ofGRC100 filled steel tubular columns were reduced by about25% compared with the columns filled with GRC0, and iswithin 10% for RACFST columns. Therefore, the amount ofRAs plays a greater influence on the load capacity of GRCFSTcolumns than in RACFST columns.

(2) The load versus deformation relation shows that, withincreasing RA replacement ratio, the peak strains increaseand the descending branches fall down more slightly. Theresults calculated by the ductility index DI indicate thatthe ductility of the columns under axial loading is improvedby RAs and the GRCFST columns are more sensitive to RAcontent in the same condition. The failure mechanisms aresimilar for both RACFST and GRCFST columns.

(3) The theoretical model was analysed by the load-deforma-tion curves of RACFST and GRACFST columns. The resultsof the RACFST columns show comparative agreement withthe experimental results, but were not so good for theGRCFST columns. Furthermore, the improved model indi-cates a better correlation with test results. Overall, thetheoretical model and improved model can demonstratethe load versus deformation relation of the columns underaxial loading.

(4) In order to get a more accurate and more universal theoreti-cal model of GRCFST columns, more work should be under-taken on the constitutive relations of geopolymer concretein the future.

Acknowledgements

The authors gratefully acknowledge the support from theNational Natural Science Foundation of China (No. 51208325),Sichuan Province Science and Technology support program(2015GZ0245) and the Program for Changjiang Scholars and Inno-vative Research Team (IRT 1027). The first author has been sup-ported by CSC and the Monash-Sichuan University Strategic Fundfor this research opportunity, which is greatly appreciated. Theexperimental testing was conducted in the Civil EngineeringLaboratory at Monash University. The support and assistance withthe laboratory work provided by Long Kim Goh, Jeff Doddrell, KevinNievart, Mark Taylor are also gratefully acknowledged.

References

[1] Habert G, D’Espinose De Lacaillerie JB, Roussel N. An environmental evaluationof geopolymer based concrete production: reviewing current research trends.Journal of Cleaner Production 2011;19(Compendex):1229–38.

[2] Duxson P, Provis JL, Lukey GC, Van Deventer JSJ. The role of inorganic polymertechnology in the development of ‘green concrete’. Cem Concr Res2007;37:1590–7.

[3] Meyer C. The greening of the concrete industry. Cement Concr Compos2009;31(8):601–5.

[4] Yang K-H, Song J-K, Song K-I. Assessment of CO2 reduction of alkali-activatedconcrete. J Cleaner Prod 2013;39:265–72.

[5] Swanepoel JC, Strydom CA. Utilisation of fly ash in a geopolymeric material.Appl Geochem 2002;17(8):1143–8.

[6] Hardjito D, Wallah SE, Sumajouw DMJ, Rangan BV. On the development of flyash-based geopolymer concrete. ACI Mater J 2004;101(6):467–72.

[7] Duxson P, Fernandez-Jimenez A, Provis J, Lukey G, Palomo A, van DeventerJ. Geopolymer technology: the current state of the art. J Mater Sci2007:2917–33.

[8] Sumajouw DMJ, Hardjito D, Wallah SE, Rangan BV. Fly ash-based geopolymerconcrete: study of slender reinforced columns. J Mater Sci2007;42(Compendex):3124–30.

[9] Sofi M, van Deventer JSJ, Mendis PA. Engineering properties of inorganicpolymer concretes. Cem Concr Res 2007;37(2):251–7.

[10] Collins F. Inclusion of carbonation during the life cycle of built and recycledconcrete: influence on their carbon footprint. Int J Life Cycle Assess2010;15(6):549–56.

[11] Yang YF, Han LH. Experimental behaviour of recycled aggregate concrete filledsteel tubular columns. J Constr Steel Res 2006;62:1310–24.

[12] Yang YF. Behaviour of recycled aggregate concrete-filled steel tubular columnsunder long-term sustained loads. Adv Struct Eng 2011;14(2):189–206.

[13] Mohanraj EK, Kandasamy S, Malathy R. Behaviour of steel tubular stub andslender columns filled with concrete using recycled aggregates. J S Afr Inst CivEng 2011;53(2):31–8.

[14] Shi XS, Wang QY, Qiu CC, Zhao XL. Mechanical properties of recycled concretefilled steel tubes and double skin tubes. In: Xiao JZ, Zhang Y, Cheung MS, ChuRPK, editors. 2nd international conference on waste engineering management,Icwem 2010; 2010. p. 559–67.

[15] Shi XS, Wang QY, Zhao XL, Collins FG. Strength and ductility of recycledaggregate concrete filled composite tubular stub columns. In: 21st Australianconference on the mechanics of structures and materials. Melbourne; 2010. p.83–9.

[16] Han L-H, Yao G-H, Zhao X-L. Tests and calculations for hollow structural steel(HSS) stub columns filled with self-consolidating concrete (SCC). J Constr SteelRes 2005;61(9):1241–69.

[17] Lu H, Zhao X-L, Han L-H. Testing of self-consolidating concrete-filled doubleskin tubular stub columns exposed to fire. J Constr Steel Res 2010;66(8–9):1069–80.

[18] Oyawa WO, Sugiura K, Watanabe E. Polymer concrete-filled steel tubes underaxial compression. Constr Build Mater 2001;15(4):187–97.

[19] Oyawa WO. Steel encased polymer concrete under axial compressive loading:analytical formulations. Constr Build Mater 2007;21(1):57–65.

[20] Rebeiz KS, Craft AP. Polymer concrete using coal fly ash. J Energy Eng ASCE2002;128(3):62–73.

[21] Konno K, Sto Y, Kakuta Y. The property of recycled concrete column encased bysteel tube subjected to axial compression. Trans Jpn Concr Inst1997;19:351–8.

[22] Chen Z-P, Chen X-H, Ke X-J, Xue J-Y. Experimental study on the mechanicalbehavior of recycled aggregate coarse concrete-filled square steel tube

Page 11: paper tecn mat.pdf

X.-S. Shi et al. / Construction and Building Materials 81 (2015) 187–197 197

column. In: 2010 international conference on mechanic automation andcontrol engineering, MACE 2010, June 26, 2010–June 28, 2010. Wuhan,China: IEEE Computer Society; 2010. p. 1313–6.

[23] Yang Y-F, Man L-H. Compressive and flexural behaviour of recycled aggregateconcrete filled steel tubes (RACFST) under short-term loadings. Steel CompStruct 2006;6(Compendex):257–84.

[24] Xiao JZ, Li JB, Zhang C. Mechanical properties of recycled aggregate concreteunder uniaxial loading. Cem Concr Res 2005;35:1187–94.

[25] Shi XS, Collins FG, Zhao XL, Wang QY. Mechanical properties andmicrostructure analysis of fly ash geopolymeric recycled concrete. J HazardMater 2012;237–238:20–9.

[26] 1163 AN. Cold-formed structural steel hollow sections. AS/NZS 1163. Sydney,Australia: Standards Australia/Standards New Zealand; 2009.

[27] 1391 A. Metallic materials-Tensile testing at ambient temperature. AS 1391.Sydney, Australia: Standards Australia; 1997.

[28] 1012.2 A. Methods of testing concrete method 2: preparation of concretemixes in the laboratory. AS 10122. Sydney, Australia: Standards Australia;1994.

[29] Shi X-S, Wang Q-Y, Qiu C-C, Zhao X-L. Experimental study on the properties ofrecycled aggregate concrete with different replacement ratios fromearthquake-stricken area. Sichuan Da Xue Xue Bao (Gongcheng Kexue Ban)/JSichuan Univ (Eng Sci Ed) 2010;42(Compendex):170–6.

[30] Yang YF, Han LH. Experimental performance of recycled aggregate concrete-filled circular steel tubular columns subjected to cyclic flexural loadings. AdvStruct Eng 2009;12(2):183–94.

[31] Tao Z, Han LH, Zhao XL. Behaviour of square concrete filled steel tubessubjected to axial compression. In: The fifth international conference onstructural engineering for young experts. Shengyang, P.R.China; 1998. p. 61–7.

[32] Han LH. Tests on stub columns of concrete-filled RHS sections. J Constr SteelRes 2002;58:353–72.

[33] Ge HB, Usami T. Strength analysis of concrete-filled thin-walled steel boxcolumns. J Constr Steel Res 1994;30(3):259–81.

[34] Han LH. Concrete-filled steel tubular structures-theory and practise. 2nded. Peking: China Science Press; 2007.

[35] Cai CS. Modern steel and concrete composite structure. reviseded. Peking: People’s Communication Press; 2007.

[36] Zhong ST. Unified theory of concrete filled steel tube – research andapplication. Peking: Tsinghua University Press; 2006.

[37] Ding Fa-xing, Yu Zhi-wu, Bai Yu, Gong Yong-zhi. Elasto-plastic analysis ofcircular concrete-filled steel tube stub columns. J Constr Steel Res2011;67(10):1567–77.

[38] Chen WF, Han DJ. Plasticity for structural engineers. New York: Springer-Verlag New York, Inc.; 1988.

[39] Han L-H, Yao G-H, Tao Z. Performance of concrete-filled thin-walled steeltubes under pure torsion. Thin-Walled Struct 2007;45(1):24–36.

[40] Hardjito D, Rangan BV. Development and properties of low calcium fly ash-based geopolymer concrete. Research report GC1. Western Australia: Facultyof Engineering, Curtin University of Technology; 2005.

[41] Sarker PK. Analysis of geopolymer concrete columns. Mater Struct/MaterConstr 2009;42(Compendex):715–24.