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Analysis of geopolymer concrete columns

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Analysis of geopolymer concrete columns
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  See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/226045215 Analysis of geopolymer concrete columns  Article   in  Materials and Structures · July 2008 DOI: 10.1617/s11527-008-9415-5 CITATIONS 29 READS 318 1 author: Prabir Kumar SarkerCurtin University 50   PUBLICATIONS   595   CITATIONS   SEE PROFILE All content following this page was uploaded by Prabir Kumar Sarker on 13 February 2015. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the srcinal documentand are linked to publications on ResearchGate, letting you access and read them immediately.  ORIGINAL ARTICLE Analysis of geopolymer concrete columns Prabir Kumar Sarker Received: 7 February 2008/Accepted: 28 July 2008/Published online: 6 August 2008   RILEM 2008 Abstract  Ordinary portland cement (OPC) hasbeen traditionally used as the binding agent inconcrete. However, it is also necessary to search foralternative low-emission binding agents for concreteto reduce the environmental impact caused bymanufacturing of cement. Geopolymer, also knownas inorganic polymer, is one such material that usesby-product material such as fly ash instead of cement. Recent research has shown that fly ash-based geopolymer concrete has suitable propertiesfor its use as a construction material. Since thestrength development mechanism of geopolymer isdifferent from that of OPC binder, it is necessary toobtain a suitable constitutive model for geopolymerconcrete to predict the load–deflection behaviour andstrength of geopolymer concrete structural members.This article has investigated the suitability of usingan existing stress–strain model srcinally proposedby Popovics for OPC concrete. It is found that theequation of Popovics can be used for geopolymerconcrete with minor modification to the expressionfor the curve fitting factor, to better fit with the post-peak parts of the experimental stress–strain curves.The slightly modified set of stress–strain equationswas then used in a non-linear analysis for reinforcedconcrete columns. A good correlation is achievedbetween the predicted and measured ultimate loads,load–deflection curves and deflected shapes for 12slender test columns. Keywords  Column    Geopolymer concrete   Fly ash    Stress–strain relationship 1 Introduction Concrete is the most widely used constructionmaterial in the world. Ordinary portland cement(OPC) has been traditionally used as the bindingagent for concrete. The worldwide consumption of concrete is estimated to increase due to the increaseof infrastructure especially in countries such as Indiaand China [1]. The amount of carbon dioxide releasedduring the manufacturing process of OPC is in theorder of 1 ton for every ton of OPC produced.Globally, the OPC production contributes about 7%of the world’s carbon dioxide. This is adding about1.6 billion tons of carbon dioxide into the atmosphere[1]. Since it has now become a priority to control thetrend of global warming by reducing the carbondioxide emission, it is appropriate to search foralternative low-emission binding agents for concrete.Geopolymer, also known as inorganic polymer, is onesuch alternative material that acts as the bindingagent in concrete. The geopolymer binder uses P. K. Sarker ( & )Department of Civil Engineering, Curtin Universityof Technology, Perth, WA, Australiae-mail: p.sarker@curtin.edu.auMaterials and Structures (2009) 42:715–724DOI 10.1617/s11527-008-9415-5  by-product materials instead of cement, and thus itsuse by the construction industry will reduce thecarbon dioxide emission and the environmentalimpact of the manufacturing of cement.Geopolymer is a type of alumino-silicate productobtained from the geochemistry process [2]. Thegeopolymer binders show good bonding propertiesand utilize a material such as fly ash or metakaolin asthe source of silicon and aluminium for reaction byan alkali. In fly ash-based geopolymer binder, fly ashis reacted with an alkaline solution to create analumino-silicate binder. Geopolymer binders are usedtogether with aggregates to produce geopolymerconcrete. Fly ash-based geopolymer concrete is arecently developed concrete in which no portlandcement is used and the geopolymer paste acts as theonly binder. The basic ingredients of fly ash-basedgeopolymer concrete are fly ash, sodium hydroxide,sodium silicate, fine aggregates and coarse aggre-gates. However, water and super plasticizer can beadded to improve workability of the concretemixture.Recent research works [3–9] have studied the properties of heat cured fly ash-based geopolymerconcrete. The results of these studies have shownpotential use of geopolymer concrete as a construc-tion material. The studies have shown thatgeopolymer concrete has the properties of highcompressive strength, very little drying shrinkage,low creep, good bond with reinforcing steel, and goodresistance to acid, sulphate and fire. It was also foundfrom the experimental and analytical works that theperformance of geopolymer concrete structural mem-bers such as beams and columns was similar to that of OPC concrete members. Other recent studies [10–12] have also reported similar engineering properties of geopolymer concrete which are favourable for its useas a construction material.Computations of the load–deflection behaviourand the ultimate load capacity of reinforced concretemembers need the stress–strain relationship of con-crete. Past research works [10, 11, 13, 14] have determined the experimental values of modulus of elasticity of geopolymer concrete. The experimentalresults of complete stress–strain behaviour of geo-polymer concrete were reported by Hardjito et al.[14]. While the strength development in OPCconcrete is because of the hydration reaction of cement with water, the strength development ingeopolymer concrete is because of the geopolymer-isation reaction between the source of silicon andaluminium with the alkaline liquids. Because thestrength development mechanism of geopolymerconcrete is very different from that of OPC concrete,it is necessary to obtain a suitable expression for thestress–strain relationship of geopolymer concrete. Itis also necessary to evaluate the application of theconventional methods of analysis used for OPCconcrete structures to geopolymer concrete structuralmembers. This article has evaluated the suitability of using an existing stress–strain model originallyproposed by Popovics [15] for OPC concrete to geopolymer concrete. The slightly modified set of stress–strain equation is used in a non-linear analysisof reinforced concrete columns [16] to analyse the 12 geopolymer concrete columns tested by Sumajouwet al. [9]. The calculated ultimate axial loads, load– deflection curves and the deflected shapes of thecolumns are compared with the corresponding exper-imental results. 2 Material properties 2.1 Modulus of elasticity of geopolymer concreteThe modulus of elasticity ( E  c ) of geopolymerconcrete was determined by testing cylinder speci-mens and reported in literature by Fernandez-Jimenezet al. [10], Sofi et al. [11] and Hardjito et al. [14]. These test results are shown in Fig. 1 and comparedwith the predictions by different empirical equations.There were some variations in these reported testresults in terms of the ingredients of the testspecimens and the test methods used. The test resultsof Fernandez-Jimenez et al. [10] were measured in accordance with the Spanish Standard UNE 83316.These specimens were made using low-calcium flyash, 12.5 molar NaOH, Na 2 SiO 3  with SiO 2  to Na 2 Oratio of 3.4, and coarse and fine aggregates. The testdata by Sofi et al. [11] and Hardjito et al. [14] were measured in accordance with the Australian Standard1012.17 [17]. The test specimens of Sofi et al. [11] were made by using low-calcium fly ash from threedifferent sources, slag containing 40% CaO by massand a combination of NaOH or KOH and Na 2 SiO 3  asthe alkaline liquid. The specimens did not have anycoarse aggregates except the one corresponding to 716 Materials and Structures (2009) 42:715–724  compressive strength of 39 MPa. The test specimensby Hardjito et al. [14] used low-calcium fly ash, 14molar NaOH, Na 2 SiO 3  with SiO 2  to Na 2 O ratio of 2,and coarse and fine aggregates. The type of coarseaggregates used in these specimens was granite.It is known that the mechanical properties of geoplymer vary with the chemical composition of theproduct obtained after the reaction. It was found inthe previous studies [10, 11] that geopolymer showed different mechanical properties depending on thetype of fly ash, and the type and concentration of the alkali used. Usually a higher concentration of thealkali dissolves a higher proportion of the fly ashparticles. Thus, a higher degree of geopolymerisationoccurs and a denser microstructure is achieved in thegeopolymer matrix. The denser microstructure of thematrix provides better mechanical properties togeopolymer concrete. Also, the mechanical propertiesof geopolymer is found to improve with the increasein the ratio of Si to Al of the reaction product. It canbe seen that the ingredients and the mixture propor-tions varied in the test specimens. Because of thevariation in the ingredients and their mixture propor-tions, scatter is observed in the test data presented inFig. 1.While the modulus of elasticity of concrete variesdepending on the paste and the type of aggregates,simplified empirical equations in terms of concretecompressive strength (  f  0 c ) and concrete density ( q ) areoften used for normal-weight concretes. The valuesof the modulus of elasticity calculated by theempirical equations are compared with the test resultsof geopolymer concrete. Some empirical equationsproposed for OPC concrete (Eqs. 1–4) and forgeopolymer concrete (Eq. 5) are given below.American Concrete Institute, ACI 363 [18]: E  c  ¼ 3320  ffiffiffiffi   f  0 c p   þ 6900  ð 1 Þ Australian Standard, AS 3600 [19], within  ± 20%: E  c  ¼ 0 : 043 q 1 : 5  ffiffiffiffiffiffi   f  cm p   ð 2 Þ Carrasquillo et al. [20]: E  c  ¼  3320  ffiffiffiffi   f  0 c p   þ 6900   ð q = 2320 Þ 1 : 5 ð 3 Þ Ahmad and Shah [21]: E  c  ¼ 3 : 38 q 2 : 5  ffiffiffiffi   f  0 c p   0 : 65  10  5 ð 4 Þ Hardjito et al. [14]: E  c  ¼ 2707  ffiffiffiffi   f  0 c p   þ 5300  ð 5 Þ The prediction equations for the modulus of elasticity of OPC concrete recommended by theAustralian Standard AS 3600 [19], Carrasquillo et al.[20] and Ahmad and Shah [21] are functions of the density of concrete and the concrete compressivestrength. The equation proposed by Hardjito et al.[14] for geopolymer concrete is similar to that given Fig. 1  Modulus of elasticity of geopolymerconcreteMaterials and Structures (2009) 42:715–724 717  by the ACI 363 [18] with different values of theconstants. These equations are relatively simple touse since they are expressed as function of concretecompressive strength only. The trend lines throughthe predicted values of the test results by the fiveequations (Eqs. 1–5) are shown in Fig. 1. It can beseen that the equations of the ACI 363 [18], AS 3600[19], Carrasquillo et al. [20] and Ahmad and Shah [21] overestimate most of the test results of geo- polymer concrete. The prediction of the modulus of elasticity by Eq. 5 is close to the test results and isconsidered reasonable taking the variations of testspecimens into consideration. Therefore, this equa-tion is used to calculate the modulus of elasticityrequired for the stress–strain relationship of geo-polymer concrete, presented in the next section.2.2 Stress–strain relationship of geopolymerconcreteExperimental data on the stress–strain curves of geopolymer concrete are very limited in literature.Hardjito et al. [14] reported the experimental stress–strain curves of three different mixes of heat cured flyash-based geopolymer concretes using granite aggre-gates. These results are shown in Figs. 2–4. The expression for the complete stress–strain response of conventional OPC concrete cylinders proposed byPopovics [15] was subsequently modified by Thorenfeldt et al. [22] by introducing a factor  k   inthe equation to ensure a steeper descending part of the curve for high-strength concrete. This expressionof Thorenfeldt et al. is selected here to evaluate thesuitability of its use for geopolymer concrete bycomparing with the experimental stress–strain curves.The stress–strain relationship of Popovics, modi-fied by Thorenfeldt et al. is given by the followingexpression:  f  c  f  0 c ¼  e c e 0 c :  nn  1 þ  e c e 0 c   nk   ð 6 Þ where  f  c  =  concrete compressive stress,  e c  =  strainin concrete,  f  0 c  =  maximum compressive stress inconcrete,  e 0 c  =  strain when  f  c  reaches  f  0 c  and n  =  curve fitting factor. The factor  k   equals 1 when e c  /  e 0 c  is less than 1. Collins and Mitchell [23] suggested that  k   is given by Eq. 7 for  e c  /  e 0 c  is greater Fig. 2  Stress–strain curve of geopolymer concrete (  f  0 c  = 41 MPa) Fig. 3  Stress–strain curve of geopolymer concrete (  f  0 c  = 61 MPa) Fig. 4  Stress–strain curve of geopolymer concrete (  f  0 c  = 64 MPa)718 Materials and Structures (2009) 42:715–724
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