EXPERIMENTAL STUDY ON THE FRACTURE OF LIGHTLY REINFORCED CONCRETE ELEMENTS SUBJECTED TO ECCENTRIC COMPRESSION

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1 VIII Interntionl Conference on Frcture Mechnics of Concrete nd Concrete tructures FrMCo-8 J.G.M. Vn Mier, G. Ruiz, C. Andrde, R.C. Yu nd X.X. Zhng (Eds) EXPERIMENTA TUDY ON THE FRACTURE OF IGHTY REINFORCED CONCRETE EEMENT UBJECTED TO ECCENTRIC COMPREION R. PORRA, J. R. CARMONA, R. C. YU AND G. RUIZ Universidd de Cstill- Mnch (UCM) Avd. Cmilo José Cel s/n, 1371 Ciudd Rel, pin E-mil: Key words: Cohesive Frcture, ightly Reinforced Concrete, Brittle-ductile trnsition Abstrct. The behvior of lightly reinforced concrete elements under eccentric compression is complex phenomenon due to mteril nd geometric nonlinerities. This is field where complete experimentl results re lcking. Thus need ws felt to crry out n experimentl cmpign to investigte the sensitivity of the relted prmeters on the filure of these type of elements. In this current work, micro-concrete specimens were tested to study the influence of reinforcement mount, slenderness rtio nd lod eccentricity. The pplied lod nd the displcement t midspn of the specimen (dditionl eccentricity) were mesured during the entire loding process, including the post-pek stge. At the sme time, independent tests were crried out for mteril chrcteriztion. Bsed on this study, brittle-ductile clssifiction ccording to the slenderness rtio nd the eccentricity is performed. uch clssifiction is prticulrly useful for the element design to void brittle filure. 1 Introduction The buckling filure produced in reinforced concrete (RC) slender elements, such s columns or pnels, is complex phenomenon. This is due to mteril nd geometricl nonlinerities [1], the bond deteriortion between the concrete bulk nd the steel rebr [], the effect of creep [3,, ] s well s the crcking nd compressive dmge in concrete [6]. All of them led to stiffness reduction of the column, which in turn exerts gret influence on its buckling filure due to second-order effects [1, 7, 8]. In 199, Bžnt nd Kwon studied the size effect in columns [9]. hortly fter tht, in 1997, Kim nd Yng [1] tested 3 columns mde of reinforced high strength concrete (HC). By chnging the slenderness rtio, the concrete strength nd the reinforcement rtio, they got the min conclusion tht the use of HC improves the pek lod for more stocky columns but hs lmost no influence on tht of the slender ones. In the sme yer, Foster nd Attrd [11] observed tht the ductility of the column is relted to the trnsverse reinforcement, nd tht the effectiveness of this reinforcement is hevily influenced by the concrete strength. All these studies hve helped us to understnd the phenomenon of buckling in slender RC elements nd some vribles involved in it. However, given the difficulty of testing elements with lrge slenderness rtio, these studies did not tke into ccount ll the prmeters involved in the frcture process nd relevnt to the nlysis. Thus need ws felt for complete experimentl progrm in which ll mteril chrcteristics, including those relted to the steel-concrete interction, 1

2 R. Porrs, J.R. Crmon, R.C. Yu nd G. Ruiz should be mesured by direct testing. In this work, we present the results on the tests crried out on pnels of reduced size to fcilitte mteril control nd specimen hndling, s well s to minimize the dt dispersion [1]. All the specimens re wekly reinforced, this entitles us to employ the frmework bsed on frcture mechnics of concrete [13, 1] for result interprettion. The rest of the pper is structured s follows: brief overview of the experimentl progrm is given in ection. The mterils nd specimens re described in ection 3. ection summrizes the experimentl procedures. The experimentl results re presented nd discussed in ection. Finlly, in ection 6 some conclusions re extrcted. Overview of the experimentl progrm The experimentl progrm ws designed to study the frcture behvior of lightly RC slender elements. In prticulr, we wnt to determine whether this behvior is dependent on the specimen slenderness rtio, the initil lod eccentricity nd/or the reinforcement mount. In ddition, the progrm hd to provide n exhustive mteril chrcteriztion to llow complete interprettion of the test results. As mentioned before, we designed tests on pnels of reduced size. This is bsed on the size effect lw described through the Hillerborg s brittleness number β H = H l ch ; l ch = EG F f t (1) where H is the specimen height, l ch is the chrcteristic length; E, G F nd f t re respectively the elstic modulus, the frcture energy nd the tensile strength of the concrete. According to the size effect lw, two geometriclly similr structures will disply similr frcture behvior if their brittleness numbers re comprble [1, 16]. For instnce, the behvior of specimens of 3, 6 nd 1 mm in height, mde from micro-concrete with l ch of 9 mm will represent tht of specimens of 1, nd m mde from n ordinry concrete with l ch of 3 mm. Therefore tests on fifty-four micro-concrete specimens were crried out. All the specimens were geometriclly similr with the sme rectngulr cross-section of 7 mm (B D), see Fig. 1. Three different heights of 3, 6 nd 1 cm were chosen to give slendernesses rtio λ of 1, nd 83 respectively, nd λ is clculted s follows λ = p Iy /A ; I y = 1 1 DB3, A = BD () where p is the effective length, which is dependent on boundry conditions, in this cse, it is equl to the height H. Cross-section Reinforcement (3, nd 1 brs) od eccentricity (, b nd c) Height nd slendernesses z e initil eccentricity e dditionl =1m b=m c=m mm 7mm M eccentricity H= 3 cm H= 6 cm H= 1cm (, M nd ) M λ= 1 M λ= λ= 83 3cm 6cm y B=mm 1cm D=7mm y P ρ=.% ρ=.8% ρ=.1% Figure 1: Geometry of the specimens. The specimens were longitudinlly reinforced with one, two or three ribbed steel brs of. mm in dimeter (which resulted z x

3 R. Porrs, J.R. Crmon, R.C. Yu nd G. Ruiz reinforcement rtio of.1%,.8% nd.% respectively), three settings of the initil lod eccentricity, 1, nd mm (which we denominted s setting type, b nd c), were pplied, see Fig. 1. tndrd chrcteriztion nd control tests were performed to determine the compressive strength, elstic modulus, tensile strength nd frcture energy of the concrete. teel properties such s the yield strength nd the modulus of the steel were obtined from tensile tests. And the steel-to-concrete bond chrcteristics were determined from pull-out tests in which both the pulling force nd the steel slip were mesured. The results of ll these tests re shown in the next section. 3 Mteril chrcteriztion 3.1 Micro-concrete A single micro-concrete mix ws employed throughout the experiments. The mximum grin size of the siliceous snd ws mm nd the portlnd cement ws of ATM type II. All of the cement used ws tken from the sme cement continer nd dry stored until use. The mixing proportion by weight were.7:3.:1 (wther:ggregte:cement). The grnulometric curve of the ggregte follows UNE Nine btches were necessry to cst ll the pnel specimens plus the necessry chrcteriztion specimens. The Abrms cone slump, mesured immeditely before csting, ws of 1.8 cm in verge. All the specimens were cst in steel molds, vibrted during 1 seconds while fstened to vibrting tble, wrp-cured for hours, demolded, nd right fter removl from the moist room, were stored in moist chmber t ± C until the test (bout 1 month). Mteril chrcteriztion ws crried out on specimens of the sme ge s the pnel specimens. For ech btch, four compressive tests were conducted ccording to ATM C39 on cylinders of 7 mm in dimeter nd 1 mm in height. Four Brzilin tests were performed on cylinders of the sme dimension, following the procedures recommended by ATM C96. The specific frcture energy G F ws mesured through three-point bending bems of 7 mm in depth, mm in width nd 3 mm in spn. The testing procedures followed were ccording to RIEM TC- [17] nd with the improvements on the til prt proposed by Elices, Guine nd Plns [18, 19, ]. uch tests were performed in position control on bem rested on nti-torsion supports. Three liner rmps t different displcements rtes were devised: 1 µm/min for the first minutes, µm/min for the following 1 minutes nd µm/min until the end of the test. Tble 1: Micro-concrete properties f c E c f t G F [MP] [GP] [MP] [N/m] Men D The mechnicl nd frcture properties of the micro-concrete mesured re given in Tb. 1. The chrcteristic length ccording to Eq. 1 is 93 mm. It needs to be remrked tht, the men vlues in Tb. 1 re the test results of 36 mesurements (four in ech btch). The reltively low level of stndrd devitions mong specimens from nine different btches is the outcome of the strict control during the specimen mking process. 3. teel ince the tests were crried out in reduced scle, in order to chieve the desired reinforcement rtios, the dimeter of the steel brs hd to be smller thn tht of stndrd rebrs. In this cse, commercil ribbed wires with nominl dimeter of. mm were used. The mesured chrcteristics of such wires, such s the nominl elstic modulus E s, the.% offset yield strength σ., the ultimte strength σ u, nd the ultimte strin ɛ u re given in Tb.. 3

4 R. Porrs, J.R. Crmon, R.C. Yu nd G. Ruiz Tble : Reinforcement steel properties E s σ. σ u ɛ u [GP] [MP] [MP] % Men D teel-concrete interfce The stress trnsfer between steel nd concrete ws mesured through pull-out tests. Thirty-six pull-out tests were performed, four for ech btch. Pull-out specimens consist of prisms mm with wire embedded long their longitudinl xis. The bonded length ws mm to llow constnt sher stress t the interfce [1, ], see Fig. for the experimentl setup. The concrete specimen ws held by the stiff frme fstened to the mchine ctutor while the wire ws pulled out through hole in the upper steel plte. The reltive slip between the wire nd the concrete surfce ws mesured t the bottom end to eliminte the effect of the elstic deformtion in the wire. The tests were crried out t constnt displcement of µm/s. The bond strength τ c, 3.8 MP in verge, is clculted s the mesured pek force divided by the contct re. Experimentl procedure for eccentric compression tests As mentioned before, fifty-four micro concrete specimens of three sizes (smll, medium nd lrge), three reinforcement rtios (one, two nd three rebrs) nd three setting of initil eccentricities (, b nd c for 1, nd mm respectively), two for ech type, were tested. We nme ech specimen ccording to this order, for exmple, b-1 represents lrge size specimen with two rebrs, loded with type b setting for the initil eccentricity, nd it is the first of the two columns tested. Figure : Experimentl setup for pullout tests. Figure 3: ) pecimen during the test. b-c) Top loding device. d-e) Antitorsion support. f) teel block bolted to the specimen for eccentric loding.

5 R. Porrs, J.R. Crmon, R.C. Yu nd G. Ruiz All specimens were tested with position control in verticl position, see the experimentl setup in Fig. 3, where the detils of the nti-torsion support nd the steel block bolted to the specimen for eccentric loding re lso depicted. The mesure prmeters include the pplied lod P, the loding point displcement d nd the dditionl eccentricity t the midspn of the specimen e, see Fig.. A servo-controlled testing mchine, with lod cpcity of kn, ws employed for the lod ppliction. An inductive trnsducer, with mm of stroke nd ± percent error of its mximum cpcity, ws used to mesure the dditionl eccentricity. Figure : chemtic digrm of the specimens nd the mesured prmeters (pplied lod P, lodingpoint displcement d nd the dditionl eccentricity e ). Experimentl results nd discussion The experimentl lod versus dditionl eccentricity curves for the RC slender elements re shown in Fig., where the curves re gthered so tht the lod eccentricity is kept constnt for verticl columns nd the slenderness rtio for horizontl rows. Ech subfigure depicts the curves for 6 specimens of the sme size, sme type of setting for the lod eccentricity nd for ll three reinforcement rtios, except two results for the smller specimens were not recorded due to the bolt filure of the loding plte. In ddition, the exct vlues of the initil lod eccentricity mesured before ech test, re lso shown below ech subfigure..1 Brittle-ductile clssifiction A typicl lod-versus-dditionl-eccentricity curve in Fig. strts with liner rmp-up. Then loss of linerity is observed before reching the pek lod, which indictes the initition of the frcture process. In ddition, the pek lod decreses with the increse of either the slenderness rtio or the initil lod eccentricity. The effect of the two prmeters is clrified in Fig. 6, where we put together the curves for smll nd lrge specimens, with different settings of initil eccentricity, but ll reinforced with one rebr. It cn be observed tht, for the sme size (or slenderness rtio), the increse of initil eccentricity improves the ductility; for the sme initil eccentricity, the increse of size lso enhnces the ductility. Therefore we hve identified two types of behvior in the specimens tested: ductile nd brittle. For this purpose, we employ two vlues of the dditionl eccentricity, e l nd e, which correspond to the linerity limit nd the pek lod respectively, see Fig. 7. The structurl behvior is considered s brittle if e is less thn five times e l, ductile if e is more thn ten times e l, ductile-brittle if otherwise. The lod pek is more pronounced for brittle type of behvior, wheres the re below the lod-eccentricity curve is more extensive for ductile type of behvior. It needs to be pointed out tht, the reinforcement rtio plys n importnt role in the bove ductile-brittle clssifiction, this is due to the fct tht the rebrs hve yielded t post pek stge. Depending the ultimte lod for rebr yielding is lrger or not thn the pek lod, the behvior cn sty ductile or chnge to brittle, see the second row of Fig. 7.

6 R. Porrs, J.R. Crmon, R.C. Yu nd G. Ruiz e e e mm 6.7 mm. mm mm. mm 3. mm e 1 1 1b-1 1b- b-1 b- 3b b-1 1b- b mm b- 3.3 mm 3b-1. mm 3b- 6. mm 6.1 mm --- mm e c-1 1c- c-1 c- 3c c-1 1c- c-1 1. mm c-. mm 3c-1.8 mm 3c- 1 M1-1 M1b-1 Mb- M1c-1 Mc- M1- M M1b- M3b-1 1 M1c- M3c-1 M-1 M M Mb-1 M3b- Mc-1 M3c- M- 8 M3-1 1 M M1-1 M1- M-1. mm 6.9 mm 1.9 mm M- M3-1 M3-. mm 7.1 mm 6. mm mm.9 mm. mm mm 6. mm 8. mm e e M1b-1 M1b- Mb mm Mb- 6.9 mm M3b-1 8. mm M3b- 1b-1 1b- b-1 b- 3b-1 3b- 9. mm 6. mm 7. mm b-1 1b- b-1. mm b-. mm 3b mm 3b- 3. mm 9. mm 9.7 mm e e 9.8 mm 6.1 mm 7. mm M1c-1 M1c- Mc mm Mc- 7. mm M3c-1.3 mm M3c- 1c-1 1c- c-1 c- 3c-1 3c- 1. mm.1 mm. mm c-1 1c- c mm c- 1. mm 3c-1 9. mm 3c- 7. mm 7. mm 7.7 mm Figure : od versus dditionl eccentricity for smll (first row), middle (second row) nd lrge (third row) size specimens loded with eccentricity of 1 mm (left column), mm (centrl column) nd mm (right column). pecific vlues of the initil eccentricity for ech specimen is given below ech subfigure. 6

7 R. Porrs, J.R. Crmon, R.C. Yu nd G. Ruiz ) 8 b) c e c) λ e λ d) c 1 1 c c Figure 6: Four typicl behvior t post pek, s function of the initil lod eccentricity nd the slenderness rtio. The rows re of different size, wheres the columns re of different settings of initil eccentricity. BEHAVIOR P BRITTE M P DUCTIE-BRITTE Frágil-Dúctil P DUCTIE Dúctil e l e e l e e < e l e = e l -1e l e l e e>1 e l Ductile behvior ccording to the shpe of the P-e curve P Dúctil P Ultimte Crking od ρ=.% od Dúctil: Dúctil Ductile filure e l e Dúctil: Frágil ρ=.1% Brittle filure e>1 e l e Figure 7: (Top row) Clssifiction criterion for brittle, ductile-brittle nd ductile behvior ccording to reltive vlues of the dditionl eccentricity t the linerity limit nd t the pek lod. (Bottom row) Influence of the reinforcement rtio on the filure type (ductile or brittle). 7

8 R. Porrs, J.R. Crmon, R.C. Yu nd G. Ruiz We summrize the previous ductile-brittle clssifiction in Fig. 8, where the initil lod eccentricity hs been normlized by the specimen thickness, ech point represents one specimen tested in the lb. The significnce of Fig. 8 lies in the fct tht, given slender reinforced element cn be clssified s brittle or ductile structure ccording to its slenderness rtio nd initil lod eccentricity. b c M b c b c DUCTIE e /B.6.. BRITTE DUCTI-BRITTE λ Figure 8: Brittle-ductile trnsition with respect to the slenderness rtio nd the initil lod eccentricity normlized by the thickness of the element, note tht the ductile region is dependent on the reinforcement rtio.. Crck ptterns In Fig. 9 we show the typicl crck ptterns obtined for ll the lod eccentricityslenderness rtio combintions in spited of the number of rebrs. For the specimens, which structurl behviors hve been clssified s brittle, crcks prllel or inclined to the loding direction re observed; wheres for those specimens, which structurl behviors hve been clssified s ductile, crcks re perpendiculr to the longitudinl xis nd concentrted round the mid spn. In either cse, the reinforcement rtio does not ply significnt role on the observed crck ptterns. Figure 9: Typicl crcking mps for smll, medium nd lrge size (, M nd ), nd for ech three setting of initil lod eccentricities (, b nd c.) 6 ummry nd conclusions We hve presented experimentl results on lightly reinforced concrete slender elements under eccentric compressive lod. Geometriclly similr specimens nd chrcterizing coupons were cst from single micro-concrete. Tests were crried out for specimens of three sizes, with three reinforcement rtios nd three initil lod eccentricity. Mteril (concrete nd steel rebrs) nd interfce properties were mesured through independent tests. Concrete-mking nd testing procedures were strictly controlled to ensure to reduce experimentl sctter. Through nlyzing the lod-versusdditionl-eccentricity curves, we hve clssified the structurl behvior of the tested elements ccording to their slenderness rtio nd initil lod eccentricity. It hs been observed tht the reinforcement rtio plys on n importnt role on the ductility or brittleness of the structure, but it brely influences the observed crck ptterns. The clssifiction chrt obtined provides guideline to predict the structurl behvior for lightly reinforced slender elements. 8

9 R. Porrs, J.R. Crmon, R.C. Yu nd G. Ruiz References [1] Z. P. Bžnt nd. Cedolin. tbility of tructures. Dover, Mineol, New York, 3. [] C. Ylcin nd M. tcioglu. Inelstic nlysis of reinforced concrete columns. Computers nd tructures, 77():39, 199. [3] H. G. Kwk nd J. K Kim. Ultimte resisting cpcity of slender rc column. Computers nd tructures, 8:91 91,. [] H. G. Kwk nd J. K Kim. Nonliner behviour of slender RC columns(1). Numericl formultion. Construction nd Building Mterils, :7 37, 6. [] H. G. Kwk nd J. K Kim. Nonliner behviour of slender RC columns(). Introduction of desing formul. Construction nd Building Mterils, :38 3, 6. [6] T. Mjewski, J. Bobinski, nd J. Tejchmn. Fe nlysis of filure behviour of reinforced concrete columns under eccentric compression. Engineering tructures, 3():3 317, 8. [7] J.G. McGregor, J.E. Breen, nd E.O. Pfrng. Design of slender concrete columns. ACI Journl, 67(1):6 8, 197. [8] R.W. Furlong. lenderness of columns in brced frmes. Journl of tructurl Div. ACE, 119(11):3 31, [9] Z. P. Bžnt nd Y. W. Kwon. Filure of slender nd stocky reinforced-concrete columns. test of size effect. Mterils nd tructures, 7(166):79 9, 199. [1] K.D. Kim nd J.K. Yng. Buckling behviour of slender high-strength concrete colums. Engineering tructures, 17:39 1, [11].J. Foster nd M.M. Attrd. Experimentl test on eccentriclly loded high-strength concrete colums. ACI tructurl Journl, 9:9 33, [1] G. Ruiz, M. Elices, nd J. Plns. Experimentl study of frcture of lightly reinforced concrete bems. Mterils nd tructures, 31: , [13] Z.P. Bžnt nd J. Plns. Frcture size effect in concrete nd other qusibrittle mterils. CRC Press, [1] J.R. del Viso J.R. Crmon, G. Ruiz. Mixed-mode crck propgtion through reinforced concrete. Engineering Frcture Mechnics, 7:788 89, 7. [1]. Elfgren nd.p. hh, editors. Anlysis of Concrete tructures by Frcture Mechnics: Proceedings of the Interntionl RIEM Workshop dedicted to Professor Arne Hillerborg. RIEM Committee on Frctures Mechnics of Concrete, June [16] P. E. Petersson. Crck growth nd development of frcture zone in plin concrete nd similr mterils. Report No. TVBM-16, Division of Building Mterils, und Institute of Technology, und, weden, [17] RIEM. Determintion of the frcture energy of mortr nd concrete by mens of the three-point bend tests on notched bems. Mterils nd tructures, 18:8 9, 198. [18] G.V. Guine, J. Plns, nd M. Elices. Mesurement of the frcture energy using three point bend test. 1. influence of experimentl procedures. Mterils nd tructures, (11-18), 199. [19] J. Plns, M. Elices, nd G.V. Guine. Mesurement of the frcture energy using three point bend test.. influence of 9

10 R. Porrs, J.R. Crmon, R.C. Yu nd G. Ruiz bulk energy dissiption. Mterils nd tructures, (3-31), 199. [] M. Elices, G.V. Guine, nd J. Plns. Mesurement of the frcture energy using three point bend test. 3. influence of cutting the p-δ til. Mterils nd tructures, (37-33), 199. [1] A. osberg nd P. A. Olsson. Bond filure of deformed reinforcing brs bsed on the longitudinl splitting effect of the brs. ACI Journl, 76(1): 17, [] RIEM/CEB/FIP. Test nd specifictions of reinforcement for reinforced nd prestressed concrete: Four recomendtions of the RIEM/CEB/FIB,: Pullout test. Mterils nd tructures, 3(1):17 178,