FRACTURE TOUGHNESS AND MICROSTRUCTURE OF CONCRETE AT EARLY-AGES

Transcription

1 Advane in Civil Engineering Material The 50-year Teahing and Reearh Anniverary of Prof. Sun Wei, 15 Otober 008, Nanjing, China FRACTURE TOUGHNESS AND MICROSTRUCTURE OF CONCRETE AT EARLY-AGES Zongjin Li (1), Xianyu Jin () and Chen Lin () (1) Dept. of Civil Engineering, Zhejiang Univerity, Hangzhou, China () Dept. of Civil and Environmental Engineering, Hong Kong Univerity of Si. and Teh., Hong Kong, China Abtrat Thi paper report the reult of a tudy on frature behavior of onrete at early age. The two-parameter frature model ha been ued to determine the frature toughne and ritial rak tip opening diplaement ( CTOD ). Meanwhile, mirotruture parameter uh a pore ize ditribution have been tudied by merury intruion poroimetry (MIP). The parameter were obtained for three type of onrete at different age from 18 hour to 8 day. It wa found that inreae with hydration time, but poroity hange in the K Ι oppoite way. The lower water-ement ratio led to a more rapid development of K Ι K Ι at early age. There exited a loe relationhip between and poroity. In addition, the interfae between oare aggregate and ement matrix were tudied by anning eletron miroopy (SEM) at the age of 8 day to interpret it influene to mehanial propertie. Keyword: ompreive trength; frature toughne; onrete, poroity; ITZ. 1. INTRODUCTION Sine linear elati frature mehani (LEFM) wa firt applied to onrete by Kaplan in 1961 [1], many invetigation have been onduted to meaure the apparent value of frature toughne K Ι for ement pate, mortar, and onrete peimen. A great deal of tet reult howed that K Ι wa dependent of peimen geometry and ize. It implied that the frature toughne K Ι baed on LEFM and brittle frature onept ould not be diretly applied to material uh a onrete []. With the development of nonlinear frature mehani (NLFM) of onrete, whih took frature proe zone (FPZ) of onrete into onideration, RILEM had tentatively reommended three equivalent effetive rak model Jenq and Shah two-parameter model (TPM), Bazant ize effet law (SEL), and effetive rak model (ECM) by Karihaloo and Nallathambi [3,4], for determining the frature parameter of onrete, whih were material ontant, and independent of geometry and ize. K Ι 139

2 Advane in Civil Engineering Material The 50-year Teahing and Reearh Anniverary of Prof. Sun Wei, 15 Otober 008, Nanjing, China Mot of the related literature foued on the frature propertie of mature onrete. However, it wa known that the propertie at early age (generally referring to age le than 7 day) had great influene the long-term behavior and wa ritial for ontrution quality ontrol. Reently, Zolinger et al. [5] meaured two frature parameter, the frature b toughne K Ι and the ritial effetive rak length of the FPZ f, for onrete at 0.5, 1, 7, 1, b and 8 day, and onluded that K Ι inreaed with hydration proe. In urrent tudy, experimental invetigation have been onduted on onrete at early age uing beam peimen with three different mix proportion. The frature parameter have been obtained with two parameter model at 18 hour, 1,, 7 and 8 day. From a material iene point of view, propertie at one level ould be explained by the haraterization of lower level. Unlike ompreive trength, there were not too many literature dealing with the relationhip between mirotruture and frature harateriti of onrete. Although with ome limitation, in all the method applied to explore the pore truture, MIP beome the mot extenively ued, for it impliity and large range of pore diameter aeed (about µm) [6]. In thi paper, pore truture wa tudied by MIP uing the ample awed from the ruptured beam. The reult howed that K Ι inreaed with hydration time while the poroity value dereaed with hydration proe. There exited a negative relationhip between thee two quantitie.. EXPERIMENTAL PROGRAM.1 Material Cement: ASTM Type I Portland ement, with peifi gravity of 3.15 and peifi urfae area of 385 m /kg. The average partile ize of ement i 19.98μm. Fine aggregate: Natural river and with a finene modulu of.3, and peifi gravity of.66. Coare aggregate: Cruhed aggregate with maximum ize of 10mm and peifi gravity of.57. Silia fume: Compatible ilia fume with average partile ize 0.1μm. Water: Tap water. Superplatiizer: KFDN (produed by Zhaohen Building Chemial (Shenzhen) Co. Ltd).. Mix proportion Three different mix proportion of onrete are ummarized in Table 1. Table1: Conrete mix proportion Cement Binder Silia fume Fine aggregate Coare aggregate Water Superplatiizer* W/B % % 0.30 * Perentage of by weight of binder 140

3 Advane in Civil Engineering Material The 50-year Teahing and Reearh Anniverary of Prof. Sun Wei, 15 Otober 008, Nanjing, China.3 Speimen preparation At leat four beam with a preat noth were prepared for eah bath of onrete. The peimen dimenion wre ummarized a follow: 75mm in width(b), 150mm in depth (D), and 750 mm in length (L), The depth of noth (initial rak length, a 0 ) wa 50mm, and the width 3mm. More detail ould be found in Fig. 1. Detail of bathing, ating, onolidating, and uring wre ummarized herein. A pan mixer wa ued for mixing. The beam peimen were at in reuable mould and onolidated with vibrator. After fully onolidation, all peimen were troweled and ured under polyethylene membrane in a laboratory environment for 4 hour exept the peimen teted at 18hour and 1 day. Speimen were then removed from the mould and tranferred to a uring room with the temperature of 3± and relative humidity of 100% until about 4 hour before tet. All the operation onformed to ASTM C19/C 19M 98. Two ube with 13±1mm per ide were ut from the ruptured beam peimen with a water-ooled, diamond-bladed aw a oon a the frature tet wa over. Then the ample were dried in an oven at a temperature of for 4 hour or more to a ontant weight. After drying, the ample were kept in deiator till tet..4 Intrumentation All beam were loaded uing MTS 810 in three-point bending, a 50-kN hydrauli loed-loop mehani tet ytem. Crak mouth opening diplaement (CMOD) wa ued a feedbak ignal to ahieve a table failure. A high-peed data aquiition ytem wa ued to ollet the neeary data. For MIP, Miromeriti Autopore IV 9500 with apaity of 07 MPa (30,000 pia) wa ued. The range of diameter aeed wa from 0.006µm to 360µm. JEOL-6300 SEM (Model JSM 6300) wa ued in SEM tudy. 3. TWO-PARAMETER MODEL Aording to the propoal of RILEM, The modulu of elatiity ould be obtained a follow: E = 6Sa V ( α) /( C D ) (1) 0 1 i B Where S, B, D and a 0 were pan, thikne, height and initial noth depth repetively and Ci wa the initial ompliane a illutrated in Fig. 1. V 1 wa a geometrial funtion of α : 3 1 ( α) = α α.04α /(1 α) V () in whih, α = ( a 0 + H 0 ) /( D + H 0 ), H 0 wa the height of lip gauge holder. The frature toughne K Ι wa given by: K I 3( Pmax + 0.5WS / L) S = πa F( α) (3) e D B where W and L were the elf-weight and length of beam repetively. Pmax wa the maximum load. And a wa the ritial rak length, whih i determined by iteration from: e 141

4 Advane in Civil Engineering Material The 50-year Teahing and Reearh Anniverary of Prof. Sun Wei, 15 Otober 008, Nanjing, China ae = ECu D B / 6SV1 ( α) (4) in whih, α = a e + H ) /( D + ),the geometrial funtion F wa given by: ( 0 H 0 F( α) = [1.99 α(1 α)( α +.7α )] [ π (1+ α )(1 α) ] And Cu wa the unloading ompliane. Finally, the ritial rak tip opening diplaement CTOD in whih, 6( Pmax + 0.5WS / L) Sae = V 1( α ){(1 β ) D BE α = a e / D, β = a 0 / ae. 3 CTOD wa given by: + ( 1.149α )( β β )} 1 (5) (6) ae a0 LOAD,P (N) 1 Ci Cu 1 C i, a 0 C u, E E a e CM OD Figure1: Typial load-cmod plot 4. EXPERIMENTAL RESULTS AND DISCUSSIONS Tet reult and analye were hown in Table and Table 3. The meaning of the notation wa ummarized a follow: f - uniaxial ompreive trength; E - modulu of elatiity in ompreion; f - plitting tenile trength; K Ι - frature toughne; CTOD - ritial rak tip t opening diplaement; G Ι - ritial train energy releae rate, GΙ = K Ι / E ; lh - harateriti length defined in Hillerborg Fititiou Crak Model [1], l h = ( E GF / f t ),aording to Plana and Elie [13], G = α G, α wa a ontant and α >. F Ι 14

5 Advane in Civil Engineering Material The 50-year Teahing and Reearh Anniverary of Prof. Sun Wei, 15 Otober 008, Nanjing, China Table : Compreive trength, Splitting tenile trength and modulu of elatiity at different age (a) Age f ( MPa ) E ( GPa ) f t ( MPa ) f / ft (Day) (b) Age (Day) f ( MPa ) E ( GPa ) f t ( MPa ) f / ft () Age f ( MPa ) E ( GPa ) f t ( MPa ) f / ft (Day) Table 3: Frature parameter and material brittlene of onrete at different age Age K (Day) Ι ( MPa m ) CTOD ( mm ) G Ι ( N / m ) Q (mm) Table 4: Frature parameter and material brittlene at 8 day for different onrete Age (Day) K Ι ( MPa m ) CTOD ( mm ) G Ι ( N / m ) Q (mm) 143

6 Advane in Civil Engineering Material The 50-year Teahing and Reearh Anniverary of Prof. Sun Wei, 15 Otober 008, Nanjing, China 4.1 Compreive Strength and Tenile Strength A we expeted, the ompreive trength, plitting tenile trength and modulu of elatiity inreaed with the hydration proe (ee Fig. 3). But the developing rate for the three quantitie were different. Inreae of the ratio of f t to f with age wa found in three onrete, whih indiated the lower developing rate of tenile trength. Thi wa true for E. The lower the w/ ratio, the higher the value of ompreive trength, plitting tenile trength and modulu of elatiity at the ame time. However, the ratio of f t to f dereaed with the dereae of w/ ratio. Thee value were about 1/10, 1/14 and 1/15 for, and repetively for example Compreive trength (MPa) Age (day) Figure : Development of ompreive trength with hydration age 4. Frature Toughne and Material Brittlene A hown in Fig. 4, frature toughne inreaed with the hydration proe. On the ontrary, the CTOD value jut had little development with the inreae of hydration age, epeially in and. At the ame time, the developing rate of K Ι value were ditint with varied w/ ratio and admixture doage. The lower w/ ratio, the more rapidly K Ι inreae at early age. For intane, the ratio of K Ι at 7 day to that at 8 day are 77.3%, 77.8% and 84.8% for, and repetively. Alo, the K Ι value inreaed in the equene of, and at the ame age. K Ι and CTOD value from different peimen were in good agreement, whih implie that K Ι and CTOD determined by TPM ould be regarded a material ontant independent of geometry and ize. Baed on Fig. 3, it eemed that the relationhip between frature toughne K Ι and ompreive trength f wa bi-linear, independent of w/ ratio and hydration age. Although K Ι inreaed with dereaed w/ ratio, frature behavior of the peimen with lower w/ ratio wa more brittle. Therefore K Ι wa not an appropriate parameter for haraterizing the frature behavior of quai brittle material uh a onrete. Shah had propoed the material length, Q, to determine the brittlene of onrete in hi fititiou rak 144

7 Advane in Civil Engineering Material The 50-year Teahing and Reearh Anniverary of Prof. Sun Wei, 15 Otober 008, Nanjing, China model. The lower the Q value, the more brittle the material. Generally peaking, the Q dereaed with the hydration age, whih howed early-age onrete i more dutile than mature one a hown in Table 3. The Q value at 8 day for three onrete were ummarized in Table 4. We have known that high trength onrete behave more brittle than normal trength onrete. Thi an be roughly jutified from the tet reult. Frature toughne(mpa*m 1/ ) Age(day) Figure 3: Development of frature toughne with hydration age Frature toughne(mpa*m 1/ ) Compreive trength(mpa) Figure 4: Relationhip between frature toughne and ompreive trength 4.3 Pore Struture It generally aepted that bulk mehanial propertie are diretly related to the void ontent of material, not only onrete [7]. We try to obtain ome link between the poroity and trength of onrete in thi etion. The MIP reult are ummarized in Table 5 and Fig. 5. The total poroity dereae with hydration age and w/ ratio. At the ame time, the proportion of finer pore inreae, whih mean the improvement of mehanial propertie, aording to Minde [8]. The tet reult verify thi point. Fig. 5 and Fig. 6 how the relationhip between the poroity P and ompreive trength f and. The lower the poroity, the higher f value. Thi i true for P and K Ι K Ι. There exit a linear relationhip with negative lope 145

8 Advane in Civil Engineering Material The 50-year Teahing and Reearh Anniverary of Prof. Sun Wei, 15 Otober 008, Nanjing, China oeffiient between K Ι and poroity, viz. K Ι = ap + b, a i a negative number. Although further tudie need to arry out, it lear that, like ompreive trength, the frature toughne depend diretly on poroity too. Table 5: Poroity (per ent) of three onrete at different age Age 18h d 7d 8d Cumulative intruded volume(ml/g) h d 7d 8d Pore diameter(nm) (a) Cumulative intruded volume(ml/g) Pore diameter(nm) Figure 5: Pore ize ditribution urve of (a) at different age, (b) of three onrete at 8 day. (b) Compreive trength(mpa) Poroity(per ent) Figure 6: Relationhip between ompreive trength and poroity 146

9 Advane in Civil Engineering Material The 50-year Teahing and Reearh Anniverary of Prof. Sun Wei, 15 Otober 008, Nanjing, China 1.8 K I (MPa*m1/ ) Poroity(per ent) Figure 7: Relationhip between frature toughne and poroity 4.4 Aggregate-Matrix Interfae On aount of ome reaon, uh a wall-effet and flow of water, the interfaial tranition zone (ITZ) between aggregate and ement pate ha a water film formed during mixing proe. Thi reult in an inreae of poroity, whih make ITZ to be a weak part of onrete. Additionally, miro-rak exit beaue of the mimath between aggregate and ement matrix. Fig. 8 how the image of the interfae in three onrete at 8 day. The miro-rak an be oberved obviouly. We found that the failure ourred around oare aggregate in and, whih mean that ITZ i the weaket link in thee two onrete. The miro-rak beome maller and the bond in ITZ i enhaned with dereae of w/ ratio. Thi make have better mehanial propertie (ompreive trength, frature toughne, et.) than. But for, the ae i not true. The reaon may be twofold. On one hand, the w/ ratio dereae. On the other hand, thi maybe attribute to ue of ilia fume. Firt of all, there i the fine partile effet. Silia fume will redue the pore-pae, whih ha a poitive effet on trength. Seond, the ilia fume will reat with the alium hydroxide in ITZ, o-alled pozzolani effet, whih alo redue the poroity of the interfae. [9-11] In a word, the dereae of w/ ratio and the poitive effet of ilia fume lead to the inreae of bond trength of ITZ, whih make differene in mehanial parameter of the three onrete 147

10 Advane in Civil Engineering Material The 50-year Teahing and Reearh Anniverary of Prof. Sun Wei, 15 Otober 008, Nanjing, China. A C A C (a) (b) C A-Coare Aggregate A C-Cement Pate () Figure 8: SEM Image at 8 day (a), (b), () 5. CONCLUSIONS In light of the experimental meaurement of frature propertie, the invetigation of pore truture and the aggregate-matrix interfae; the following onluion an be drawn. The value of f, E, f t and K Ι inreae with the hydration age from 18 hour to 8 day. The lower w/ ratio, the more rapidly the four quantitie inreae at early age. Inreaing K Ι indiate the reitane to raking inreae with the age for three onrete. Inreae of the harateriti length with the hydration age implie that early-age onrete i more brittle than mature onrete. The lower w/, the more brittle the onrete at 8 day, aording to the Q value. The relationhip between frature toughne K Ι and ompreive trength f eem to be bilinear, regardle of w/ ratio. The higher the poroity, the lower ompreive trength and frature toughne. The relationhip between thee two quantitie i linear, viz. K Ι = ap + b, a i a negative number. 148

11 Advane in Civil Engineering Material The 50-year Teahing and Reearh Anniverary of Prof. Sun Wei, 15 Otober 008, Nanjing, China The aggregate-matrix interfaial zone beome dener with lower w/ ratio and the ue of ilia fume. It an eential way to get onrete with high frature propertie to add mineral admixture uh a ilia fume. ACKNOWLEDGMENTS The author gratefully aknowledge the finanial upport of key projet from National Natural Siene Foundation of China ( ) and the Grant from Reearh Grant Counil (616008). REFERENCES [1] Kaplan, M.F., Crak Propagation and the Frature of Conrete, ACI Journal, Proeeding V.58, No. 11, Nov. 1961, pp [] Bazant, Z. P. and J. Plana, Frature and Size Effet in Conrete and Other Quaibrittle Material, CRC Pre, New York, [3] Shah, S. P. and Carpinteri, A., Frature Mehani Tet Method for Conrete, RILEM Report 5, Chapman and Hall, London, 1991, pp [4] Jenq, Y. S. and S. P. Shah, Two Parameter Frature Model for Conrete, Journal of Engineering Mehani, V.111, No. 10, Ot. 1985, pp [5] Zolinger D a n G., Tang Tianxi, and Yoo Rae H., Frature Toughne of Conrete at Early Age, ACI Material Journal, V. 90, No. 5, Sep.-Ot. 1993, pp [6] Kumar Rakeh, Bhattaharjee B., Study on ome fator affeting the reult in the ue of MIP method in onrete reearh, Cement and Conrete Reearh, V. 33, Iue 3, 003, pp [7] Neville A. M., Propertie of Conrete, Fourth and Final Edition, J. Wiley & Son, New York, pp [8] Minde, S. Relationhip between trength and mirotruture for ement-baed material: an overview, Very High Strength Cement-Baed Material (J. F. Young Editor), Sympoia Proeeding, Material Reearh Soiety, Vol. 4, 1984, pp [9] Jan G. M. van Mier, Frature Proee of Conrete, CRC Pre, New York, 1997, pp [10] Mao J. C. (Editor), Interfaial Tranition Zone in Conrete, RILEM Report 11, E &FN SPON, London, [11] Ji Yajun, Jong Herman Cahyadi, Effet of denified ilia fume on mirotruture and ompreive trength of blended ement pate, Cement and Conrete Reearh, V. 33, Iue 10, 003, pp