THE EFFECT OF WIDE STIRRUP SPACING ON DIAGONAL COMPRESSIVE CAPACITY OF HIGH STRENGTH CONCRETE BEAMS

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1 - Technical Paper - THE EFFECT OF WIDE STIRRUP SPACING ON DIAGONAL COMPRESSIVE CAPACITY OF HIGH STRENGTH CONCRETE BEAMS Patarapol TANTIPIDOK *1, Koji MATSUMOTO *2 an Junichiro NIWA *3 ABSTRACT To promote the rational eign an wier ue of high trength material, thi paper invetigate the effect of wie tirrup pacing on the iagonal compreive capacity of RC beam an compare the reult with the preiction by the exiting equation. Two 5mm epth I-beam were tete by three-point bening. A a reult, iagonal tre i not concentrate in local area of the pecimen even wie tirrup pacing wa ue becaue of ufficient confinement effect provie by two-legge tirrup. The preictive equation by the author cannot be implemente beyon it applicable range. Keywor: iagonal compreive capacity, high trength concrete, web cruhing, tirrup pacing 1. INTRODUCTION Nowaay, eign of concrete infratructure are require to be more economical an environmental frienly to etablih utainable ociety for future. The utilization of the recently evelope high trength material, e.g. high trength concrete with the compreive trength (f c ) greater than 1 N/mm 2 an high trength reinforcing baith the yiel trength (f y ) greater than 685 N/mm 2 can atify thi eman. Such avance material reult in lower material conumption (maller cro ection, thin web concrete girer) an longer ervice life of infratructure while maintaining remarkable tructural performance. However, in cae of reinforce concrete (RC) beam, the combination of thin web (T- or I-hape cro ection) an high trength hear reinforcing baill lea to an uncommon type of hear failure known a iagonal compreion failure. It i caue by the cruhing of web concrete prior to the yieling of tirrup. The reearch on the mechanim of the iagonal compreion failure wa inufficient ince it wa uually avoie becaue of it brittle phenomenon. The eign equation for the iagonal compreive capacity of RC beam in the current JSCE Stanar Specification for Concrete Structure [1] only conier the effect of f c an limit the applicability to concrete with f c up to 5 N/mm 2. Furthermore, limite tuie on the iagonal compreive capacity of high trength RC beam have been performe, except for previou reearch by the author (Tantipiok et al. [2]). They invetigate the effect of variou parameter on the iagonal compreive capacity of RC beam uing high trength concrete an propoe an accurate an imple preictive equation for the iagonal compreive capacity of RC beam bae on the experimental reult. It wa reporte that the major factor affecting the iagonal compreive capacity were f c an tirrup pacing (). The effect of hear-pan to effective epth ratio (a/), flange with to web with ratio (b f /b w ) an effective epth () wa foun to have le influence on the iagonal compreive capacity in their tuy range. In the cae of tirrup pacing, their propoe equation can be applie in the range from 45 mm to 16 mm. In real tructure, which ize of RC beam can be relatively larger, can excee 16 mm an the equation may not be applicable. Further valiation i require. The purpoe of thi reearch are to invetigate the effect of tirrup pacing larger than 16 mm on the iagonal compreive capacity of relatively large RC beam an valiate the preicting equation by the author for the iagonal compreive capacity (Tantipiok et al. [2]). Finally, the accuracy of the exiting equation i verifie. Thi reearch i a tep forwar towar the evelopment of new eign equation for the iagonal compreive capacity of RC beam applicable to high trength concrete. 2. REVIEW OF THE EXISTING EQUATIONS FOR DIAGONAL COMPRESSIVE CAPACITY OF RC BEAMS 2.1 The equation by Tantipiok et al. The author invetigate the effect of f c,,, a/, b f /b w an on the iagonal compreive capacity an propoe a imple preictive equation bae on the experimental reult a the following [2]: V ( 1.9 ) f bw (1) 19 Tantipiok ' c where; f c an are in N/mm 2 an mm, repectively. The equation wa propoe bae on the reult that the *1 Ph. D. Caniate, Grauate chool of Civil Engineering, Tokyo Intitute of Technology, JCI Member *2 Aitant Prof., Dept. of Civil Engineering, Tokyo Intitute of Technology, Dr. E., JCI Member *3 Prof., Dept. of Civil Engineering, Tokyo Intitute of Technology, Dr. E., JCI Member

2 iagonal compreive capacity linearly ecreae with larger tirrup pacing regarle of it iameter. Thi i becaue inufficient confinement effect provie by tirrup caue the localization of compreive tre in trut. Thu, wa prominent, rather than, for evaluating the iagonal compreive capacity in range of from 45 mm to 16 mm. The effect of pacing became more ignificant with higher concrete trength; hence, the effect of f c an wa interrelate. On the other han, the effect of a/ from 3. to 4.5, b f /b w from 3.75 to 12.5 an from 22 mm to 319 mm ha almot no influence on the iagonal compreive capacity of RC beam. The equation can be implemente to beam with f c : N/mm 2, :.6-4%, : mm, b f /b w : , a/: an : mm. 2.2 The equation by Placa an Regan Placa an Regan propoe an empirical equation for evaluating the iagonal compreive capacity a the following [3]: V Placa ( r ) f ' b (2) Factor involving the iagonal compreive capacity in thi equation are f c (N/mm 2 ) an the ratio of tirrup (%). Although there i no upper limit of f c tate in thi equation, the experimental evience ue to erive thi equation approximate 35 N/mm JSCE Stanar Specification In JSCE tanar pecification [1], only f c (N/mm 2 ) i coniere a the influencing parameter of w c w the iagonal compreive capacity. Becaue thi formula wa originally propoe for application to normal trength concrete, the equation i only vali for concrete with f c not exceeing 5 N/mm 2. V 1.25 f b (3) JSCE ' c 3. EXPERIMENTAL PROGRAM 3.1 Specimen etail The experimental program prepare two RC beam with I-hape cro ection. Three-point bening tet were conucte by a 3kN capacity teting machine. The ummary of experimental cae an etail of pecimen are provie in Table 1 an Fig. 1, repectively. The main parametea tirrup pacing () of 3 an 37 mm which are the maximum allowable tirrup pacing by the eign tanar (.75 or 3 mm) [1]. The contant variable were the following: the web with (b w ) of 8 mm, the effective epth () of 5 mm, hear pan (a) of 15 mm, a/ ratio of 3., longituinal reinforcement ratio of 8.9% an the total length of 36 mm. Auming no effect of bae on the previou reult [2], the ize of the pecimen in thi reearch wa greatly enlarge than in the previou one in orer to provie wier. Tenile reinforcement ha two layehich D22 were ue a top layer (T) while bottom layer (B) wa D25. Compreive reinforcement were D32. All pecimen were eigne to be ymmetric an be able to reit againt the flexure failure an the iagonal tenion failure by uing high trength w Table 1 Lit of experimental cae Specimen f' c b w a a/ p w *1 D *2 *3 f wy *4 *5 * (top) (bottom) *1 longituinal reinforcement ratio (=1A /(b w )), *2 nominal iameter of tenile bar, *3 nominal iameter of tirrup, *4 yiel trength of tirrup, *5 tirrup ratio (=1A w /(b w )), *6 pacing of tirrup B A C L 2D32(f y =421N/mm 2 ) 6@75mm a/2 /2 59 B A D22(f y =124N/mm 2 ) 4D25(f y =1171N/mm 2 ) C L (a) For 3 a/2 4@9mm / : Strain gauge Section A-A B-B Unit: mm Fig. 1 Dimenion an teel layout of pecimen C L (b) For 37 Fig. 2 Location of tri-axi train gauge

3 C L avg n i1 i n B-region β i ci where; n i number of crack Fig. 3 Example of crack pacing an angle meaurement /2 Loa [kn] Deflection Fig. 4 Loa-eflection relationhip Table 2 Experimental reult Specimen f' c a/ f y, max *1 f wy *2 *3 w, max c,avg β avg *4 *5 V exp [egree] [kn] v exp *6 /f' c C [2] C [2] C1.8-1 [2] C2-9 [2] C2-5 [2] C2-16 [2] C3-6 [2] C4-45 [2] C-1L [2] *1 maximum tre in tenile bar, *2 maximum tre in tirrup, *3 average crack pacing in horizontal irection at peak loa, *4 average crack angle in B-region at peak loa, *5 iagonal compreive capacity, *6 v exp =V exp /(b w ) reinforcing bar a both of the tenile an hear reinforcement. In aition, the combination of thin web cro ection with ene tirrup will caue pecimen to exhibit the iagonal compreion failure. In orer to avoi the local failure, the web with outie upport wa increae to that of the bottom flange. Anchor plate an nut were ue to enure the ufficient anchorage of the tenile bar an prevent anchorage failure. 3.2 Intrumentation an tet proceure For all pecimen, applie loa, mi-pan eflection an train of concrete, longituinal bar an tirrup were meaure. Concrete train gauge were attache at the top fiber of the mi pan. Strain gauge were attache at the mi pan to meaure the train of longituinal baherea at the itance of /2 from compreion fiber for all tirrup in the hear pan. Angle of principle train of web concrete wa meaure by tri-axi train gauge. The location of thee train gauge are illutrate in Fig. 2. Beie, urface of all pecimen were painte by white color to eae the rawing an oberving of crack uring the experiment. Picture were taken by two igital ingle-len reflex camera for both hear pan. From the picture taken at the peak loa, the crack pacing in horizontal irection ( ci ) an the crack angle (β i ) were meaure at the mile height of the web. The example of ci an β i meaurement of a crack i preente in Fig. 3. The average of ci of crack in the hear pan ( c,avg ) an the average of β i of crack in B-region (β avg ) will be ue in the later icuion ince it wa oberve that the cruhing area, which i correponing to the failure region, wa motly foun in B-region (the portion outie the itance approximately away from the loaing point an upport). 4. EXPERIMENTAL RESULTS 4.1 Loa-eflection relationhip Loa-eflection relationhip are illutrate in Fig. 4. Firtly, pecimen behave in elatic manner until the firt flexural crack occurre in the bottom flange near the mi pan, which i reflecte in the graph a a rate of inclination ecreae. After the firt flexural crack, the loa-eflection curve remaine to avance almot linearly with the continuou initiation of iagonal crack at the web concrete. In the pre-peak

4 3 Legen: 37 Thicker line = wieith crack Shae area = cruhing area Re line = tirrup Fig. 5 Crack pattern jut before the peak loa v exp /f' c = %, = 5 mm = 1.2-4%, = 22 mm [2] Linear relationhip in range : mm Fig. 6 Effect of Stre i uniform along the beam axi (a) Narrow pace tirrup Stre i concentrate in the pecific part (b) Wie pace tirrup Fig. 7 Effect of pacing of tirrup Normalize crack pacing The author [2] Zakaria et al. [6] Fig. 8 Effect of on iagonal crack pacing region, the eflection increae with a relatively mall increae in applie loa a the web concrete began to cruh. Afterwar, the applie loa reache to the peak. After the peak loa, the applie loa rapily ecreae. The experimental reult are ummarize in Table 2. Data of the tree of longituinal bar an tirrup reveale no yieling at the peak loa. It implie that the failure moe wa neither the flexure failure nor the iagonal tenion failure. The web concrete cruhe at the peak loa an plitting crack along the member axi near tenile bar i not initiate at that time; hence, the caue of failure wa not by anchorage failure of both tenile bar an tirrup. It can be conclue by coniering thee obervation that the failure moe of all pecimen wa eignate a the iagonal compreion failure. The iagonal compreion failure in which the web concrete cruhe before the yieling of tirrup exhibite the brittle moe. Crack pattern jut before the peak loa are emontrate in Fig. 5. The thicker line an the hae area repreent the wier with crack an the cruhing area, repectively. 4.2 Effect of wie tirrup pacing The author [2] aopte a metho to eliminate the variation of f c by normalizing the obtaine hear capacitie (v exp =V exp /(b w )) by f c which i ue in the eign equation of JSCE [1] an the preictive equation by Placa et al. [3]. Thi metho i alo applie in thi tuy. The relationhip between an v exp /f c incluing the pecimen in the author previou experiment [2] are emontrate in Fig. 6. Thee pecimen ha f c approximately 1 N/mm 2. The pecimen in the previou reearch [2] ha in the range from mm an of 22 mm while beam in thi tuy ha = 3 mm an 37 mm an = 5 mm. Figure 6 how that the iagonal compreive capacity ha a linear relationhip with when 45 mm 16 mm regarle of it iameter a reporte by the author [2]. They explaine that thi reult came from two mechanim caue by the confinement effect by tirrup. One i maller iagonal crack with (w) caue by proviing cloer hear reinforcement; therefore, the critical average tre in web concrete (σ 2max ) woul be greater becaue w affect the iagonal compreive capacity a reporte by Schäfer et al. [4] an Reineck [5]. The other i the localization of compreive trut. Figure 7 explain the moel of compreive trut formation uner ifferent tirrup

5 =13.1 mm =9.53 mm [2] =12.7 mm [2] =7.1 mm [2] 37 3 v exp /f' c Fig. 9 Effect of tirrup ratio ( ) pacing. Figure 7(a) emontrate that the iagonal tre generate uniformly along the beam axi with cloe-pacing tirrup. In contrary, a hown in Fig. 7(b), the iagonal tre i concentrate in a local portion of the beam with wie-pacing tirrup. Thi tre concentration caue early cruhing in the web concrete; hence the iagonal compreive capacity ecreae. The localization of compreive trut i inuce by a lack of the confinement effect. It i becaue the preence of confinement effect by tirrup can prevent the exceive crack opening o that the tre can itribute along the beam axi. In the previou experiment [2], it wa inicate that in cae of the cloer-pacing pecimen, crack itribute more finely an the cruhing area at web itribute more wiely than that of the wier-pacing pecimen. A well, c,avg how a ecreaing tren with cloer tirrup pacing an it wa implie that the failure localization occurre when wier c,avg were oberve [2]. Zakaria et al. [6] reporte that larger beam caue greater iagonal crack pacing. The relationhip between crack pacing normalize by crack pacing of the mallet pecimen an effective epth i illutrate in Fig. 8. It can be oberve that the normalize crack pacing increae proportionally with the increae in the ize of pecimen. The experimental reult obtaine in thi tuy o not correpon to the previou experiment a can be een from Fig. 6. Even though wa greatly increae, the iagonal compreive capacity increae. Crack itribute finely an the cruhing area at web itribute wiely a can be oberve from Fig. 5. Coniere from the previou reult that c,avg houl be proportional with the increae of an, c,avg of both pecimen were relatively narrow. From thee evience, it implie that the confinement effect by tirrup wa ufficient in thi cae an the iagonal tre i not concentrate in a local area of the pecimen even wie tirrup pacing wa ue. It i becaue the uage of two-legge tirrup of =13.1 mm in thi tuy reulte in higher effective area in which the confinement effect by the tirrup can control Angle of principle train [egree] Ditance from the mi pan Fig. 1 Ditribution of angle of principle train exceive crack opening while the uage of ingle-legge tirrup in the previou experiment, which the effective area wa comparatively maller, inuce the localization of compreive trut. When the iagonal tre i uniform, tirrup woul only affect the iagonal compreive capacity by the former mechanim, which i influence by tirrup ratio. Therefore, the ame et of experimental reult i plotte againt in Fig 9. It can be notice that can repreent the iagonal compreive capacity in the range of from 3-37 mm, although it wa icovere by the author [2] that i the governing factor rather than in the range of = mm. 4.3 Angle of principle train an iagonal crack The reult of meaure angle of principle train at the peak loa are exhibite in Fig. 1. The location of the meaurement wa hown in Fig. 2. Some gauge were broken uring the experiment. The angle of principle train varie from 24.3 to 36.6 egree. The average angle of principle train of 3 an 37 are 29.6 an 31.2 egree, repectively. A for the angle of iagonal crack, the meaurement wa focue in B-region ince it wa oberve that the cruhing area, which are correponing to the failure region, were motly foun in that region. In the previou reearch [2], a clear tenency of β avg an each poible factor influencing the crack angle cannot be foun. β avg wa varie from 27 to 47 egree an the average value β avg for all pecimen wa 36.6 egree. From Table 2, β avg of 3 an 37 are 36. an 33.7 egree, repectively. Thee value correpon to thoe oberve in the previou experiment. 5. COMPARISON WITH THE EQUATIONS Table 3 preent ratio between the iagonal compreive capacitie from the experiment incluing by the author previou experiment [2] an reult obtaine by the equation reviewe in chapter 2. The average of thee ratio (avg.) with a coefficient of

6 Table 3 Comparion between the experiment an calculate reult Specimen f' c b w a/ V exp / V Tantipiok V exp / V Placa V exp / V JSCE * * C [2] C [2] C1.8-1 [2] C2-9 [2] C2-5 [2] C2-16 [2] C3-6 [2] C4-45 [2] C-1L [2] *: Out of the application range avg C.V % 18.8% variation (C.V.) i alo provie in Table 3. The average of V exp /V Placa equal to.95 with a C.V. of 13.8 %. It implie that Placa equation (Eq. 2) can evaluate an average value of the iagonal compreive capacity even f c approximate 1 N/mm 2. On the contrary, thi equation exhibit large variation a C.V. equal to 13.8 %. JSCE tanar pecification (Eq. 3) emontrate the average of V exp /V JSCE = The pecification may be conervative becaue of afety reaon. The reult calculate by Eq. 3 give larger variation than Eq. 2 a C.V. = 18.8%. In the cae of 3 an 37, the equation by Placa et al. (Eq. 2) an JSCE equation (Eq. 3) are accurate while the equation by the author (Eq. 1) cannot preict the iagonal compreive capacity accurately. Thi i an expecte outcome ince Eq. 1 i an empirical equation which it may not be applicable outie of it application range. Within it range of application, the author equation reult precie preiction (avg. = 1.6, C.V. = 6.%). In aition, the fact that Placa equation i accurate againt 3 an 37 implie that i an appropriate inicator for the iagonal compreive capacity in thi cae. From thee reaon, further invetigation i till require. 6. CONCLUSIONS The experiment of practical ize an thin web reinforce concrete beam with wie tirrup pacing wa carrie out. The reearch contribute to more general ue of high trength material an further evelopment of rational eign approach for iagonal compreive capacity. A a reult, even though wie tirrup pacing wa provie, the iagonal crack itribute finely, the cruhing area at web itribute wiely an the average crack pacing in horizontal irection of both pecimen wa relatively narrow compare to the previou experiment. It implie that the iagonal tre i not concentrate in a local area of the pecimen. It i becaue ufficient confinement effect provie by two-legge tirrup can prevent the localization of compreive tre in trut. Although it wa reporte by the author that the tirrup pacing i important for evaluating the iagonal compreive capacity when uing tirrup pacing from 45 mm to 16 mm; on the other han, the tirrup ratio wa a better repreentative for preicting the iagonal compreive capacity in the range of from 3-37 mm. REFERENCES [1] Japan Society of Civil Engineer (JSCE), Stanar Specification for Concrete Structure-27, Deign, 27 [2] Tantipiok, P. et al., Propoe Preictive Equation for Diagonal Compreive Capacity of Reinforce Concrete Beam, Journal of Japan Society of Civil Engineer, Ser. E2 (Material an Concrete Structure), Vol. 67, No. 4, 211, pp [3] Placa, A. an Regan, P. E., Shear Failure of Reinforce Concrete Beam, ACI Journal, Vol. 68, No. 1, 1971, pp [4] Schäfer, K., Schelling, G. an Kuchler, T., Compreion an Tranvere Tenion in Reinforce Concrete Element, Deutcher Auchu für Stahlbeton No. 48, Beuth Verlag, GmbH, Berlin, 199, pp [5] Reineck, K. H., Theoretical Conieration an Experimental Evience on Web Compreion Failure of High Strength Concrete Beam, CEB Bulletin D Information No. 193, Dec. 1989, pp [6] Zakaria, M. et al., Experimental Invetigation on Shear Cracking Behavior in Reinforce Concrete Beam with Shear Reinforcement, Journal of Avance Concrete Technology, Vol. 7, No. 1, Feb. 29, pp