5 th International Conference on Durability of Concrete Structures Jun 30 Jul 1, 2016 Shenzhen University, Shenzhen, Guangdong Province, P.R.

Transcription

1 5 th Internatonal Conference on Durablty of Concrete Structures Jun 30 Jul 1, 2016 Shenzhen Unversty, Shenzhen, Guangdong Provnce, P.R.Chna Mcromechancal Modelng for Materal Desgn of Durable Infrastructural Materals: The Influence of Aggregate and Matrx Modfcaton on Elastc Behavor of Mortars Sumanta Das, Amt Marol, and Narayanan Nethalath School of Sustanable Engneerng and the Bult Envronment, Arzona State Unversty, Tempe, AZ, USA ABSTRACT Ths paper reports the fundamental dfference n mcrostress dstrbutons n tradtonal hardened cement paste wth quartz nclusons and a cement paste wth lghtweght aggregate (LWA) nclusons usng mcrostructurebased numercal smulaton nvolvng fnte element method wth perodc boundary condtons. Varaton of relatve stress dstrbutons under varyng component materal propertes and varyng mcrostructural features n both the systems s elucdated for a comprehensve understandng. The presented mcrostructure-based numercal technque accurately captures the stress concentratons nsde mcroheterogeneous systems, whch s otherwse not detectble usng analytcal homogenzaton schemes. Numercal smulatons reveal that strengthenng and stffenng of matrx and nterfacal transton zone (ITZ) wth slca fume ncorporaton as cement replacement n LWA ncorporated cementtous systems has a detrmental effect n terms of overall strength of the materal, contrary to tradtonal quartz-based cement mortar system. Proper selecton of stffness and strength of LWA nclusons s crtcal towards performance of such materals. Ths paper lnks the mcrostructure wth mechancal behavor of two dfferent mcroheterogeneous materals and provdes valuable nput towards materal desgn of such non-tradtonal cementtous systems wth dfferent nclusons. 1. INTRODUCTION The lnk between the mcrostructure and mechancal property provdes valuable nformaton towards desgn and development of compostes for engneerng applcatons. Cementatons materals are the most wdely used composte materal n ths world and ther performance s crtcal n structural nstallatons such as hgh rse buldngs, brdges, and nuclear plants. In the recent years, tradtonal concrete s beng modfed by usng supplementary cementatons materals as cement replacement or completely alternatve cement-free bnders are proposed for reducton n carbon footprnt (Das, Soulman, Stone, & Nethalath, 2014; Lothenbach, Scrvener, & Hooton, 2011; Ravkumar, Peethamparan, & Nethalath, 2010). Besdes, the tradtonal quartz nclusons are beng replaced by lghtweght aggregates (Al-Jabr, Hago, Al-Nuam, & Al-Sady, 2005; Cusson & Hoogeveen, 2008; Km, Jeon, & Lee, 2012; Nguyen, Beaucour, Ortola, & Noumowé, 2014) for varous benefts such as reducton of dead load, and thermal and acoustc nsulaton. Modfcaton n the matrx/ncluson s lkely to have a sgnfcant nfluence on the stress dstrbuton n the mcrostructure and thus on the falure path/mechansm of the materal. Therefore, a comprehensve understandng of the nfluence of such modfcaton on the mcrostress dstrbuton n the mcrostructure s necessary n order to desgn such materals for optmal mechancal performance. The current study focuses on understandng the fundamental dfference n stress dstrbuton n tradtonal cement mortar and lghtweght aggregate mortar systems. The dfference n stffness between harder quartz partcles and softer lght weght aggregate nclusons s lkely to mpart a sgnfcant nfluence on the fundamental stress dstrbuton characterstcs n the mortar system, whch has been studed n detal n ths paper through mcromechancal modellng. Researchers have studed the falure mechansms of tradtonal concrete and lghtweght aggregate concrete (LWAC) by employng experment-based emprcal formulatons (Ke, Ortola, Beaucour, & Dumontet, 2014) and through mcromechancal modellng nvolvng analytcal technques such as Mor Tanaka approach (Nlsen, Montero, & Gjørv, 1995; Yang, 1997; Yang & Huang, 1998), double ncluson model (Stora, He, & Bary, 2006; Yang & Huang, 1996) and teratve homogenzaton technque (Das et al., 2015; Dunant et al., 2013). Analytcal homogenzaton technques have been shown to provde good estmates. However, applcaton of these technques has been shown to be lmted f the phase stffness contrast s too hgh or the morphology s very complcated. Besdes, these analytcal and sem-analytcal homogenzaton technques do not accurately predct local stress concentratons around nclusons, whch nfluence macroscopc behavor especally for mcroheterogeneous materals. Thus, numercal modellng usng mcrostructural 155

2 156 5TH INTERNATIONAL CONFERENCE ON THE DURABILITY OF CONCRETE STRUCTURES representatve volume element (RVE) s a better approach under such scenaros. In ths paper, 2D vrtual perodc mcrostructures for quartz mortar and LWA mortar systems wth sphercal nclusons ncludng nterfacal transton zone (ITZ) around nclusons are generated vrtually and the representatve element areas (REA), thus obtaned, are numercally analyzed usng fnte element smulaton nvolvng perodc boundary condtons (L, 2008; Slus, Schreurs, Brekelmans, & Mejer, 2000; Xa, Zhou, Yong, & Wang, 2006). Stresses, thus obtaned n REA and ndvdual mcrostructural components, are evaluated to brng out fundamental dfference n stress dstrbutons mparted by LWA nclusons over tradtonal quartz partcle nclusons. In addton, the nfluence of matrx stffenng n both the systems s elucdated. Such a comprehensve numercal evaluaton of fundamental dfference n mechancal behavor or tradtonal quartz mortar and LWA mortar system has not been reported n the lterature to the best of our knowledge. Such quanttatve nformaton on the fundamental dfference n stress dstrbutons and local stress concentratons wthn two fundamentally dfferent mcroheterogeneous systems provdes valuable ndcaton on relatve damage and debondng of the nclusons from the matrx, whch s crtcal towards approprate desgn and mprovement of such materals. Two-dmensonal plan stran fnte element models are employed here n order to characterze the nfluence of ncluson and matrx modfcatons on the bulk elastc behavor of the bnder system. A suffcently large (4.15 mm 4.15 mm) representatve element area (REA) has been consdered for analyss. The vrtual random perodc mcrostructure s generated usng a mcrostructural stochastc packng algorthm (Kumar et al., 2013; Meer, Kuhl, & Stenmann, 2008; Torquato, 2013). Ths algorthm requres the partcle sze dstrbuton (PSD) and the volume fracton of partcles as nputs and t packs the crcular nclusons wth an nterface layer of predefned thckness around them nsde a REA of 4.15 mm 4.15 mm. Generaton of partcles and ther packng n the REA s allowed f the mnmum dstance between the centrods of neghborng partcles s always greater than the sum of ther rad,.e., the nterfacal zones are allowed to overlap n ths packng scheme. Generaton and random spatal placement of nclusons goes on smultaneously untl the target volume fracton of nclusons s reached. The algorthm s descrbed n detal n Meer et al. (2008). Fgure 1(a) shows the generated perodc mcrostructure. 2. MATERIAL PROPERTIES AND MICROSTRUCTURAL MODELS The consttutve relatonshps for all the components: cement paste, hard (quartz aggregates), and soft (lghtweght aggregate LWA) nclusons, and the paste ncluson nterfaces are consdered n ther respectve lnear elastc regmes only. The default elastc propertes of the components, extracted from avalable lterature (Grondn & Matallah, 2014; Hashn & Montero, 2002; Ke, Beaucour, Ortola, Dumontet, & Cabrllac, 2009; Lutz, Montero, & Zmmerman, 1997; Nlsen et al., 1995; Yang, 1998; Zanjan & Bobko, 2014), are presented n Table 1. However, for parametrc studes dscussed later n the paper, a range of values are consdered, whch are ndcated n the respectve sectons. Fgure 1. (a) FE model showng the nclusons wth the nterfacal zones around them. The model contans 50% of nclusons by volume (or area); (b) effect of random dstrbuton and perodc boundary condtons on the stress dstrbuton under an mposed stran of 0.12%. The average REA stress s MPa. Table 1. Elastc propertes of the components of the mortar for FE smulaton. Elastc property Young s modulus (GPa) Hardened cement paste Quartz Quartzcement paste nterface LWA LWAcement paste nterface Posson s rato In order to elmnate the boundary effects, perodc boundary condtons (L, 2008; Slus et al., 2000; Xa et al., 2006) are employed n the 2-D REA, as shown n Fgure 2. Fgure 2(a) shows schematc perodc arrays of repettve unt cells, and Fgure 2(b) shows the perodc boundary condtons appled on one of such schematc representatve elements for llustraton. Perodc boundary condton ensures two contnuty crtera at the boundares of neghborng unt cells n order to ensure

3 Mcromechancal Modelng for MATERIAL Desgn of Durable Infrastructural MATERIALS 157 assembly of ndvdual unt cells as a physcal contnuous body (Suquet, 1987): () dsplacement contnuty,.e., neghborng unt cells cannot be separated or they cannot penetrate each other, and () tracton contnuty at the boundary of neghborng unt cells. The dsplacement feld n any perodc mcrostructure s gven as: 0 * v ( x, x ) = ε x + v ( x, x ) 1 2 j j * here, ε s the appled stran tensor and v j s a perodc functon representng the modfcaton of lnear dsplacement feld due to the heterogeneous mcrostructure. For, the unt cell shown n Fgure 2(b), the dsplacements on a par of parallel opposte boundary edges are gven as: s v + s = ε x + + v 0 * j j s s v = ε x + v 0 * j j (1) (2a) (2b) here, s + and s - are sth par of two opposte parallel boundary surfaces of the unt cell, as shown n Fgure 2(b). The perodc functon n* s the same at both the parallel opposte edges due to perodcty. The dfference between the dsplacement felds of the two opposte parallel boundary edges s gven as: s+ s 0 s+ s 0 v v = ε ( x x ) = ε x j j j j s j (3) s For a par of opposte parallel boundary edges, x j s 0 constant for a specfed ε j. Such equatons are appled as nodal dsplacement constrants n the fnte element (FE) mcrostructural analyss. Fgure 2. Schematc representatve element area (REA) under 0 appled stran ( ε 22 ) wth perodc boundary condtons. Perodc boundary condton s mplemented on the REA as nodal dsplacement constrants through a Python language program appended to the orphan mesh fle contanng the perodc mcrostructure nformaton. The REA s meshed usng a Python scrpt (Van Rossum, 2007) through ABAQUS. A specfc stran s appled on the REA, and the analyss s mplemented through ABAQUS solver. Thus, ths approach smulates a stran (or dsplacement)-controlled test scenaro. To effcently handle post-processng of the smulated ndvdual element stresses, a homogenzaton module s developed to obtan effectve area-averaged REA stresses/strans (Sun & Vadya, 1996) and the effectve ndvdual phase stresses/strans. Fgure 1(b) shows the stress dstrbuton obtaned after analyss under the applcaton of an mposed stran of 0.12% (whch s well wthn the lnear elastc range of cementtous systems). 3. NUMERICAL RESULTS AND DISCUSSION Ths secton evaluates senstvty of averaged stress responses wth varyng materal propertes and geometrc parameters n the mcrostructure and brngs out the fundamental dfference n elastc response between mortars prepared wth quartz partcles and lghtweght aggregates. The vrtual mcrostructures wth random spatal placement of crcular nclusons under perodc boundary condtons are used for numercal llustraton. The sze of nclusons s consdered to be dentcal (600 mm) for smplcty for both quartz and lghtweght aggregate. However, the modellng technque and dscussons presented heren are not restrcted to such smplfcaton and can tackle any realstc ncluson szes. The effect of rregular ncluson sze on the effectve stresses s also demonstrated later n ths paper. The thckness of ncluson matrx nterface has been kept constant at 30 mm. The default volume fracton of ncluson s 50%. The default materal propertes of dfferent components are reported n Table Influence of ncluson stffness Fgures 3(a) and (b) show the domnant prncpal stress (s 22 ) dstrbuton consderng the materal propertes shown n Table 1 for both the quartz and LWA mortar systems, respectvely, when a stran of 0.12% s appled to the REA. The LWA nclusons exhbt sgnfcantly hgher deformaton as compared to the quartz nclusons as can be seen from these fgures, whch s expected. Whle the quartz partcles are hghly stressed n the quartz mortar system, n the LWA mortar, t s the ITZ that bears the hghest stress. Ths s expected consderng the sgnfcantly hgher stffness of quartz partcles as compared to that of LWA, as shown n Table 1. Another dstnct observaton n Fgure 3 s that the magntude of stress nsde the quartz partcles ncreases when the partcles are very close to each other, attrbutable to the sgnfcant stffness msmatch between the nclusons and the matrx. On the contrary, LWA mortar does not exhbt an ncrease n stress nsde LWA. Instead, the stress concentraton n the ITZ s hgher f the neghborng partcles are close to each other. Thus, the relatve stffness of the nclusons wth respect to the matrx results n dstnctvely dfferent stress dstrbutons, and thus dfferng propenstes of

4 158 5TH INTERNATIONAL CONFERENCE ON THE DURABILITY OF CONCRETE STRUCTURES falure n the mcrostructure. Note that the magntude of the maxmum domnant prncpal stresses (s 22 ), whch occur n the nclusons when the ncluson s stffer and the ncluson paste stffness msmatch s hgher, and n the ITZ when ncluson paste stffness msmatch s lower, are rather comparable. The quantfed averaged REA stress and the stresses n the other mcrostructural phases are plotted n Fgure 4 as a functon of the ncluson stffness. Fgure 3. Domnant prncpal stress (s 22 ) (MPa) dstrbutons n (a) quartz mortar system and (b) lghtweght aggregate mortar system. Magnfed representaton of stress dstrbutons n both mortar systems contanng the zones around the nclusons are shown for clarty. The REA s subjected to a stran of 0.12%. Fgure 4. Effectve REA and ndvdual component stresses (s 22 ) as a functon of ncluson stffness for (a) quartz mortar system and (b) LWA mortar system; and (c) lnear consttutve relatonshp for quartz and LWA mortar systems for defaults values of materal parameters (shown n Table 1) and mcrostructural features. Fgure 4(a) shows the area-averaged domnant prncpal stresses n the REA and those n the ndvdual mcrostructural phases for a mortar system contanng quartz partcles as a functon of the Young s modulus of quartz (ranged from 50 to 100 GPa). Wth ncreasng E of quartz (at the same ncluson volume fracton, and leavng the E of the paste and the nterface unchanged), the average stresses n all the phases n the mcrostructure ncrease lnearly. The quartz nclusons experence the hghest average stress amongst all the phases because of the sgnfcantly hgher E values compared to the paste or the ITZ, n lne wth Fgure 3(a). The ITZ and the paste matrx components show smlar averaged stresses, attrbutable to the fact that the stffness dfference between these phases s neglgble when compared to the dfference n stffness between quartz and these phases. Fgure 4(b) shows the averaged REA and other component stresses n the LWA mortar system for varyng stffness of LWA nclusons [between 6 and 21 GPa (Ke et al., 2009)]. Here, the hghest stress s observed n the ITZ as shown n Fgure 4(b) because of the fact that ts stffness s the hghest among all the phases n ths mcrostructure. Whle the average stress ncreases n all the phases when stffer LWA s used, the rate of ncrease n stress s hgher n the LWA nclusons compared to the other phases or the REA. The average stress n the cement paste matrx and ITZ of the LWA mortar system lnearly ncreases wth LWA stffness, whereas the stress ncrease n the LWA nclusons s found to be nonlnear, prmarly attrbuted to the deformatonal effects of the LWA and the consderaton of perfect bondng between the phases n the model. The rate of stress ncreases n the LWA, the paste decreases wth ncreasng LWA stffness, and the stresses n these phases are almost equal when the LWA and the paste stffness are smlar, as expected. Fgures 4(a) and (b) also show that the averaged stresses n all the components are lower n the LWA system as compared to those n quartz mortar system. Ths can be attrbuted to the stresses concentrated over large areas n quartz partcles that ncrease the average ncluson stresses and the other phase stresses n quartz mortar system due to assumpton of perfect bondng between phases. On the contrary, LWA system shows lower stresses due to lower stffness of LWA nclusons and lower effectve stffness of REA. Fgure 4(c) shows the consttutve response of the quartz and LWA mortars contanng 50% of nclusons by volume, extracted from numercal smulatons. The domnant prncpal stresses (s 22 ) and prncpal strans (e 22 ) n the lnear elastc range of these systems are used. Thus, the approach presented here can be used to determne the composte elastc modul of systems

5 Mcromechancal Modelng for MATERIAL Desgn of Durable Infrastructural MATERIALS 159 contanng multple ncluson types. In addton to the E value, as descrbed earler, ths methodology also provdes estmates of the mcrostresses n the dfferent phases under mposed strans (n the lnear elastc regme) and facltates the development of consttutve relatonshps for composte materals, whch otherwse would be expermentally tedous. Comparson of Young s modulus values obtaned from numercal smulaton (FEA) to those calculated usng analytcal/sem-analytcal approaches and expermental valdaton of the adopted numercal technque s detaled n a later secton of ths paper. 3.2 Influence of varyng volume fracton of ncluson Fgures 5(a) and (b) show the effect of volume fracton on the averaged domnant prncpal stress n quartz and LWA mortar systems, respectvely. an am of understandng the relatve nfluences of matrx modfcaton. The Young s modulus of slca fume modfed cement paste s taken as 25 GPa (Hu & L, 2015) as opposed to 20 GPa for the unmodfed systems. The stffness of the ITZ was also ncreased proportonally (Young s modulus of ITZ s taken as and 37.5 GPa n quartz and LWA mortars, respectvely) snce slca fume ncorporaton s known to result n nterface densfcaton and stffenng (Duan, Shu, Chen, & Shen, 2013; Hu & L, 2015). Fgures 6(a) and (b) show average stresses n the REA and n the ndvdual mcrostructural phases correspondng to an appled stran of 0.12% for the quartz and LWA mortars. Fgure 6. Effect of slca fume ncorporaton on the average REA and phase stresses for (a) mortar contanng quartz ncluson and (b) mortar wth LWA nclusons. The modfed matrces contan 10% slca fume by mass as a cement replacement materal. Fgure 5. Effectve REA and ndvdual phase stresses as a functon of ncluson volume fracton for (a) quartz mortar and (b) LWA mortar. Wth ncreasng volume fracton of ncluson n the REA systems, the average stresses n all the components n both the model ncrease lnearly. For the quartz model [Fgure 5(a)], the rate of ncrease n component stresses wth respect to ncrease n volume fracton s hgher as compared to that n LWA mortar system [Fgure 5(b)]. Ths s attrbuted to the hgher stffness of nclusons n quartz mortar systems as compared to LWA system as explaned elsewhere n ths paper. Whle ncrease n ITZ stress at hgher volume fracton of quartz s lkely to be responsble for nterface falure (ITZ s the weakest component n quartz mortar system) and thus materal falure under smaller appled strans n quartz mortar system, ncrease n stress nsde weaker LWA (weakest component) nclusons at hgher volume fractons of LWA s expected to be the cause of falure of LWA mortar system under smaller external appled strans. 3.3 Influence of matrx and ITZ stffenng wth slca fume addton Ths secton reports the nfluence of matrx and ITZ strengthenng/stffenng (ncludng those accomplshed through the use of addtves such as slca fume as a partal replacement of cement) n mortars contanng quartz or LWA as nclusons wth Stresses ncrease n all the phases for both the mortar types when the paste phase contans slca fume. In the quartz mortar, the average stress n the ITZ ncreased by about 15% when 10% slca fume was ncorporated. However, the strength enhancement of both the ITZ and the paste phase wll lkely be larger than the stress ncrease, thereby renderng mproved mechancal performance to the quartz mortar when slca fume s ncorporated n the paste phase. Ths has been demonstrated through expermental studes (Hu & L, 2015; Shannag, 2000). The stress ncrease nsde the quartz nclusons has an nsgnfcant nfluence on materal falure because of the hgher strength of quartz partcles (Axelson & Edgar, 1950). On the contrary, the nclusons n the LWA mortar system are relatvely weak and even a margnal ncrease n ncluson stress s lkely to result n materal falure at even lower appled strans as compared to that n LWA mortar systems wthout slca fume ncorporaton. Thus, the combned stffenng of ITZ and the matrx n LWA mortar system has a detrmental effect on the strength, provded t s the lghtweght ncluson that s weaker and fals frst. Ths pont to the fact that matrx strengthenng methods such as the use of slca fume mght not be benefcal from a vewpont of mechancal response n these systems, unless the LWA s stronger. However, the densfcaton of the matrx and the ITZ wll stll lead to better durablty propertes n such concretes.

6 160 5TH INTERNATIONAL CONFERENCE ON THE DURABILITY OF CONCRETE STRUCTURES 3.4 Influence of partcle sze dstrbutons Whle all the prevous parametrc studes show response of sngle-szed nclusons, the current secton shows the effect of rregular ncluson szes on the stress response. Here, the ITZs are homogenzed wth the nclusons usng analytcal Mor Tanaka approach (Mor & Tanaka, 1973) as a compromse between computatonal effcency and demand snce generaton of mcrostructure for a gven partcle sze dstrbuton wth ITZ layer around nclusons s computatonally expensve. Default values of materal/ geometrcal propertes of nclusons and ITZ (Table 1) are used n homogenzaton process. Fgures 7(a) and (b) show the domnant prncpal stress dstrbuton n sngle szed and rregular ncluson szed LWA mortar system (mean 0.6 mm and standard devaton of 0.24 mm wth unform dstrbuton), respectvely. Fgure 7(a) shows stress concentratons n between closely spaced sngle-szed LWA nclusons, especally f they are placed n the drecton perpendcular to the drecton of applcaton of stran. On the other hand, Fgure 7(b) shows a slght stress relaxaton trend n the LWA mortar system wth non-unform ncluson szes compared to the sngle-szed case. Here also, the smlar szed nclusons when placed n the drecton perpendcular to the drecton of applcaton of stran show maxmum stress concentratons. But, such hgher stress concentratons are absent when the szes of such nclusons are sgnfcantly dfferent. These are reflected n the averaged domnant prncpal stresses n REA for varous partcle sze dstrbutons and ndvdual components reported n Fgure 8. In order to elucdate the effect of partcle sze dstrbuton (PSD) on the quantfed averaged stress responses, three dfferent PSDs wth same mean sze (0.6 mm) and varyng standard devaton wth unform dstrbuton s consdered. Fgures 8(a) and (b) show averaged component and REA stress responses, respectvely, for varous partcle sze ranges of LWA nclusons. (a) Fgure 8. Effect of LWA ncluson sze dstrbuton on (a) the matrx and ncluson stresses and (b) effectve REA stresses (mean partcle sze s 0.6 mm). The averaged stresses are hghest n the sngle-szed LWA model due to hgher stress concentratons n the nter-ncluson areas as explaner earler. The stresses reduced consderably (by about 20%) when nonunform partcle szes are consdered snce nteractons between neghborng smaller and larger partcles show lower value of concentrated stresses, contrary to the hgher stress concentratons encountered between two closely spaced smlar-szed nclusons. For the same reason, the averaged stress n all components reduces as the standard devaton of partcle sze dstrbuton s ncreased keepng the mean partcle sze same as reflected n Fgures 8(a) and (b). (b) 4. CONCLUSION Fgure 7. Influence of partcle szes on the domnant prncpal stresses (MPa) n the REA for (a) sngle-szed LWA nclusons and (b) multple-szed (mean = 0.6 mm and standard devaton = 0.24 mm) LWA nclusons embedded n a cement paste. Magnfed representaton of stress dstrbutons n the vcnty of the partcles (smlar and dssmlar szes) are shown for clarty. Ths paper employs a mcromechancal model nvolvng fnte element analyss wth perodc boundary condtons and reports the fundamental dfference n stress dstrbutons n tradtonal cement paste wth quartz partcle nclusons and lghtweght aggregate mortar system. 2D perodc representatve element areas (vrtual mcrostructure) have been generated usng a partcle-packng based algorthm on known mcrostructural features. Perodc boundary condtons are appled on the 2-D REAs, and the mcrostructural stress analyss s performed usng fnte element method. The presented mcrostructurebased numercal technque accurately captures the stress concentratons nsde mcroheterogeneous systems, whch s otherwse not detectble usng analytcal homogenzaton schemes. Mcrostructural stress dstrbuton and averaged REA stress as well as ndvdual component stresses are evaluated under varous parametrc varatons (materal propertes

7 Mcromechancal Modelng for MATERIAL Desgn of Durable Infrastructural MATERIALS 161 and mcrostructural features) n both the systems for a comprehensve understandng on ther relatve performance. Numercal smulatons reveal that tradtonal quartz mortar shows stress concentratons nsde quartz partcles, whereas LWA mortar system exhbts stress concentraton at the LWA cement paste nterfaces due to sgnfcant dfference n relatve ITZ-ncluson stffness n the two systems. Whle the falure of tradtonal quartz mortar system s not dctated by stress nsde quartz partcles due to ther hgher strength, the stress nsde weaker LWA nclusons n LWA system becomes crtcal towards materal falure due to lower strength of LWA nclusons. Hence, strengthenng of LWA aggregates s a key to mprove performance of such materals. But, ncrease n strength and thus stffness of LWA nclusons results n ncrease n stress nsde LWA nclusons. Hence, an optmal stffness of LWA nclusons s desred wth a perfect balance between ncrease n strength of LWA nclusons and assocated stress ncrement. Increase n stffness of ITZ shows sgnfcant ncrease n all component stresses n quartz mortar system. On the contrary, such ncrease n ITZ stffness results n stress ncrement only n ITZ n LWA system. The matrx/ncluson stresses n LWA system reman unaltered wthout any stress relaxaton nsde LWA nclusons. Combned stffenng of matrx and ITZ ncreases n stresses, wth technques such as slca fume ncorporaton, as cement-replacement yelds n all the components n both the systems. Increase n the ITZ stress (weakest component n quartz mortar system) can be balanced aganst enhancement of strength of ITZ and thus superor performance can be obtaned through slca fume ncorporaton n tradtonal quartz mortar. But, same s not true for LWA system snce the LWA stress ncreases wth stffness enhancement of ITZ and matrx whle the strength of LWA remans unaltered under such scenaro. Hence, strengthenng and stffenng of matrx and ITZ are not a vable technque of qualty mprovement n LWA system, contrary to tradtonal quartz mortar system. Ths study lnks the mcrostructure wth mechancal behavor of two dfferent mcroheterogeneous materals to brng out ther fundamental dfference n mcrostress dstrbuton and provdes some valuable nput towards materal desgn of such non-tradtonal cementtous systems wth dfferent nclusons of varyng stffness. REFERENCES Al-Jabr, K. S., Hago, A. W., Al-Nuam, A. S., & Al-Sady, A. H. (2005). Concrete blocks for thermal nsulaton n hot clmate. Cement and Concrete Research, 35(8), Axelson, J. W., & Edgar, L. P. (1950). Crushng of sngle partcles of crystallne quartz Applcaton of slow compresson. Industral & Engneerng Chemstry, 42(4), Cusson, D., & Hoogeveen, T. (2008). Internal curng of hgh-performance concrete wth pre-soaked fne lghtweght aggregate for preventon of autogenous shrnkage crackng. Cement and Concrete Research, 38(6), Das, S., Soulman, B., Stone, D., & Nethalath, N. (2014). Synthess and propertes of a novel structural bnder utlzng the chemstry of ron carbonaton. ACS Appled Materals & Interfaces, 6(11), Das, S., Yang, P., Sngh, S. S., Mertens, J. C. E., Xao, X., Chawla, N., & Nethalath, N. (2015). Effectve propertes of a fly ash geopolymer: Synergstc applcaton of X-ray synchrotron tomography, nanondentaton, and homogenzaton models. Cement and Concrete Research, 78, Duan, P., Shu, Z., Chen, W., & Shen, C. (2013). Effects of metakaoln, slca fume and slag on pore structure, nterfacal transton zone and compressve strength of concrete. Constructon and Buldng Materals, 44, 1 6. Dunant, C. F., Bary, B., Gorla, A. B., Pénguel, C., Sanahuja, J., Toulemonde, J. C., Yvonnet, J. (2013). A crtcal comparson of several numercal methods for computng effectve propertes of hghly heterogeneous materals. Advances n Engneerng Software, 58, Grondn, F., & Matallah, M. (2014). How to consder the nterfacal transton zones n the fnte element modellng of concrete? Cement and Concrete Research, 58, Hashn, Z., & Montero, P. J. M. (2002). An nverse method to determne the elastc propertes of the nterphase between the aggregate and the cement paste. Cement and Concrete Research, 32(8), Hu, C., & L, Z. (2015). Property nvestgaton of ndvdual phases n cementtous compostes contanng slca fume and fly ash. Cement and Concrete Compostes, 57, Ke, Y., Beaucour, A. L., Ortola, S., Dumontet, H., & Cabrllac, R. (2009). Influence of volume fracton and characterstcs of lghtweght aggregates on the mechancal propertes of concrete. Constructon and Buldng Materals, 23(8), Ke, Y., Ortola, S., Beaucour, A. L., & Dumontet, H. (2014). Mcro-stress analyss and dentfcaton of lghtweght aggregate s falure strength by mcromechancal modelng. Mechancs of Materals, 68, Km, H. K., Jeon, J. H., & Lee, H. K. (2012). Workablty, and mechancal, acoustc and thermal

8 162 5TH INTERNATIONAL CONFERENCE ON THE DURABILITY OF CONCRETE STRUCTURES propertes of lghtweght aggregate concrete wth a hgh volume of entraned ar. Constructon and Buldng Materals, 29, Kumar, A., Oey, T., Km, S., Thomas, D., Badran, S., L, J., Sant, G. (2013). Smple methods to estmate the nfluence of lmestone fllers on reacton and property evoluton n cementtous materals. Cement and Concrete Compostes, 42, L, S. (2008). Boundary condtons for unt cells from perodc mcrostructures and ther mplcatons. Compostes Scence and Technology, 68(9), Lothenbach, B., Scrvener, K., & Hooton, R. D. (2011). Supplementary cementtous materals. Cement and Concrete Research, 41(12), Conferences specal: Cement hydraton knetcs and modelng, Quebec Cty, 2009 & CONMOD10, Lausanne, Swtzerland, Lutz, M. P., Montero, P. J. M., & Zmmerman, R. W. (1997). Inhomogeneous nterfacal transton zone model for the bulk modulus of mortar. Cement and Concrete Research, 27(7), Meer, H. A., Kuhl, E., & Stenmann, P. (2008). A note on the generaton of perodc granular mcrostructures based on gran sze dstrbutons. Internatonal Journal for Numercal and Analytcal Methods n Geomechancs, 32(5), 509. Mor, T., & Tanaka, K. (1973). Average stress n matrx and average elastc energy of materals wth msfttng nclusons. Acta Metallurgca, 21(5), Nguyen, L. H., Beaucour, A. L., Ortola, S., & Noumowé, A. (2014). Influence of the volume fracton and the nature of fne lghtweght aggregates on the thermal and mechancal propertes of structural concrete. Constructon and Buldng Materals, 51, Nlsen, A. U., Montero, P. J. M., & Gjørv, O. E. (1995). Estmaton of the elastc modul of lghtweght aggregate. Cement and Concrete Research, 25(2), Ravkumar, D., Peethamparan, S., & Nethalath, N. (2010). Structure and strength of NaOH actvated concretes contanng fly ash or GGBFS as the sole bnder. Cement and Concrete Compostes, 32(6), Shannag, M. J. (2000). Hgh strength concrete contanng natural pozzolan and slca fume. Cement and Concrete Compostes, 22(6), Slus, V. O., Schreurs, P. J. G., Brekelmans, W. A. M., & Mejer, H. E. H. (2000). Overall behavour of heterogeneous elastovscoplastc materals: Effect of mcrostructural modellng. Mechancs of Materals, 32(8), Stora, E., He, Q. C., & Bary, B. (2006). Influence of ncluson shapes on the effectve lnear elastc propertes of hardened cement pastes. Cement and Concrete Research, 36(7), Sun, C. T., & Vadya, R. S. (1996). Predcton of composte propertes from a representatve volume element. Compostes Scence and Technology, 56(2), Suquet, P. (1987). Elements of homogenzaton for nelastc sold mechancs. Torquato, S. (2013). Random heterogeneous materals: Mcrostructure and macroscopc propertes. Sprnger Scence & Busness Meda. Van Rossum, G. (2007). Python programmng language. USENIX annual techncal conference (Vol. 41). Xa, Z., Zhou, C., Yong, Q., & Wang, X. (2006). On selecton of repeated unt cell model and applcaton of unfed perodc boundary condtons n mcro-mechancal analyss of compostes. Internatonal Journal of Solds and Structures, 43(2), Yang, C. C. (1997). Approxmate elastc modul of lghtweght aggregate. Cement and Concrete Research, 27(7), Yang, C. C. (1998). Effect of the transton zone on the elastc modul of mortar. Cement and Concrete Research, 28(5), Yang, C. C., & Huang, R. (1996). Double ncluson model for approxmate elastc modul of concrete materal. Cement and Concrete Research, 26(1), Yang, C. C., & Huang, R. (1998). Approxmate strength of lghtweght aggregate usng mcromechancs method. Advanced Cement Based Materals, 7(3 4), Zanjan, Z. V., & Bobko, C. P. (2014). Nanomechancal characterstcs of lghtweght aggregate concrete contanng supplementary cementtous materals exposed to elevated temperature. Constructon and Buldng Materals, 51,