Evaluation of the role of transverse reinforcement in confining tension lap splices in high strength concrete

Transcription

1 Materials and Sutures/Matériaux et Consutions, Vol. 35, May 2002, pp Evaluation of the role of ansverse reinforement in onfining tension lap splies in high sength onrete B. S. Hamad 1 and S. Najjar 2 (1) Department of Civil and Environmental Engineering, Amerian University of Beirut, Beirut, Leanon. (2) University of Texas at Austin, USA. SCIENTIFIC REPORTS Paper reeived: June 6, 2001; Paper aepted: August 27, 2001 A B S T R A C T Reports on the very rittle and splitting mode of failure of tension lap splies anhored in high sength onrete (HSC) speimens and the lower normalized ond sess ( u/ ) for high sength than for normal sength onrete, instigated several researh projets aiming at reommending mehanisms to provide onfinement and dutility for ars or splies anhored in HSC. The stateof-the-art report of ACI Committee 363, ACI 363R-84, defines high sength onrete as onrete with ompressive sength aove 6,000 psi (41.4 MPa). The main ojetive of the researh reported in this paper was to assess the effet of ansverse reinforement on ond sength and dutility of the mode of failure of tension lap splies anhored in HSC eam speimens with nominal onrete ompressive sength 55 MPa (8,000 psi), and to reommend a ansverse reinforement design requirement related to the ACI ansverse reinforement index K. To meet the ojetive, fifteen full-sale HSC eam speimens were tested. No pozzolani material suh as silia fume or fly ash was added to ahieve high sength to avoid any effet suh material might impose on sutural ehavior. Eah eam was designed with ars splied in a onstant moment region at midspan. The variales used in the investigation were ar size, and the amount of ansverse reinforement plaed in the splie region. R É S U M É Des avaux sur la rupture fragile ausée par la fissuration de la longueur de reouvrement en tension des armatures d'un éton de haute résistane et la fore d'adhérene normalisée ( u/ ) inférieure pour le éton de haute résistane par rapport à elle du éton de résistane normale, ont été à l origine de plusieurs projets de reherhes ayant pour ojetif de reommander des méanismes fournissant le onfinement et d augmenter la dutilité des armatures dans un éton de haute résistane. Un rapport réemment pulié par le omité de l'aci 363, ACI 363R-84, définit e éton omme étant tout éton ayant une résistane à la ompression supérieure à psi (41,4 MPa). L'ojetif prinipal du programme de reherhe ii présenté est d'évaluer l'effet des armatures ansversales sur la fore d'adhérene et la dutilité du type de rupture des longueurs de reouvrement en tension des armatures dans les poues de éton de haute résistane ayant une fore nominale de ompression de 55 MPa (8 000 psi) et de onseiller une reommandation à suivre liée à l'index des armatures ansversales K. Pour ela, quinze spéimens de poues en éton de haute résistane ont été testés. Auun matériau pouzzolanique omme la fumée de silie ou les endres volantes n'a été ajouté au mélange permettant d otenir le éton de haute résistane afin de ne pas affeter le omportement sutural du éton. Les poues ont été étudiées de façon à e que la zone de reouvrement des armatures soit au milieu de la poue, où le moment est onstant. Les variales utilisées dans ette reherhe étaient le diamèe des armatures et la quantité d'armatures ansversales dans la zone de reouvrement. 1. INTRODUCTION Based on the researh reported in the literature on ond performane of reinforing ars in high sength onrete (HSC), two important oservations an e made: 1. The normalized ond sength ( u/ ) of reinforing steel ars is lower in HSC than in normal sength onrete (NSC). 2. HSC speimens with tension lap splies designed ased on the ACI uilding odes, without applying the f limitation ( 100 psi) and without ansverse reinforement in the splie region, failed in a very rittle and undesirale splitting mode in the splie region without exhiiting any dutility. In 1993, Azizinamini et al. presented a ehavioral model in the form of a hypothesis to desrie the ond performane of deformed reinforing ars emedded in high sength onrete (HSC) [1]. They argued that sine /02 RILEM 219

2 Materials and Sutures/Matériaux et Consutions, Vol. 35, May 2002 the earing apaity of onrete is proportional to f whereas the tensile apaity is proportional to, inreasing the ompressive sength of onrete three times for example would result in a earing apaity three times as large while the tensile apaity inreases only y 1.73 times. In the ase of HSC, the higher earing apaity of the onrete, oupled with small onrete over, will prevent rushing of the onrete in the viinity of eah lug to the extent that would otherwise take plae in normal sength onrete. This implies that, at ultimate, all lugs along the reinforing ars in the splie region may not partiipate in resisting the load. The first few lugs on the side of the loading are more ative and would resist more of the applied fores reating higher earing. The higher earing fores in turn result in higher radial tensile fores. Considering the fat that in HSC tensile apaity does not inrease at the same rate as earing apaity, splitting of onrete surrounding the reinforing ars in the viinity of the first few lugs will our. This failure takes plae efore all lugs partiipate in resisting the applied axial fores, produing a nonuniform ond sess disiution at ultimate. This nonuniform sess disiution is hiefly responsile for the lower normalized ond sength ( u/ ) ahieved in the ase of HSC, and ould e more pronouned as the splie length inreases or onrete over dereases. This ehavior suggests that in order to otain higher ond sength from rears emedded in HSC with small onrete over, a mehanism should e provided wherey all lugs over the splied region an e utilized. One suh solution ould onsist in providing some minimum amount of ansverse reinforement over the splied region. This reinforement ould serve as a method of preventing splitting failure over the highly sessed lugs until suh time as most of the lugs over the splied region ear against onrete, resulting in a nearly uniform ond sess disiution at the ultimate stage. In 1999, and ased on the work of Azizinamini et al. [1, 2] on splie sength in high sength onrete aove 96.5 MPa (14,000 psi), Azizinamini et al. [3] suggested adding a new setion to the ACI Building Code, ACI [4]. The ojetive was to provide adequate dutility to the mode of failure. The suggested setion reads as follows [3]: When the value of exeeds 100 psi, l d shall e alulated from either (Setion) or (Setion) with K = 0, and ansverse reinforement with total ross-setional area A sp rossing the potential plane of splitting through the reinforement eing developed shall e provided over the tension development for tensile splie length: A sp = 0.5nA (f / 15000), in 2 n is the numer of splied ars, A is the area of a single splied ar in in 2, and f is onrete sength in psi; 10 3 psi = MPa, and 1 in. = 25.4 mm. The maximum spaing of stirrups in the longitudinal diretion shall not exeed 12 in. (30.5 m). A minimum of three stirrups shall e used, and the minimum stirrup ar size shall e No. 3. (10 mm). 2. RESEARCH SIGNIFICANCE The ansverse reinforement reommendation of Azizinamini et al. [3] was limited to values of f exeeding 69 MPa (10,000 psi). However, tests reported y Hamad et al. [5-7] on tension lap splies anhored in full-sale high sength onrete eam speimens without ansverse reinforement onfining the splie regions, indiated very rittle and noisy mode of failure with onrete sength values ranging from 46.7 to 84.3 MPa (6800 to psi). The state-of-the-art report of ACI Committee 363, ACI 363R-84, defines high sength onrete as onrete with ompressive sength aove 6,000 psi (41.4 MPa). Also, it is argued that to insure dutility, ansverse reinforement should always e reommended for reinforing ars and tension lap splies anhored in NSC or HSC. The main ojetive of the researh program reported in this paper is to assess and revisit the effet of ansverse reinforement on ond sength and the dutility of the mode of failure of tension lap splies anhored in HSC eam speimens with nominal onrete ompressive sength 55 MPa (8,000 psi). It would e signifiant to reommend a ansverse reinforement design requirement related to the ACI ansverse reinforement index K. To meet the ojetive, 15 full-sale high sength onrete eam speimens were tested. No pozzolani material suh as silia fume or fly ash was added to ahieve high sength to avoid any effet suh material might impose on the sutural ehavior of the eam speimens. Eah eam was designed with ars splied in a onstant moment region at midspan. The variales used in the investigation were ar size of the reinforement, and the amount of ansverse reinforement to e plaed in the splie region. 3. EXPERIMENTAL PROGRAM Fifteen eams were tested in positive ending. The loading system was designed to produe a onstant moment region in the middle of the eam speimen. The eams were tested in three series of five eams eah. The differene etween eams in one series was the numer of stirrups plaed within the splie region. Reinforement on the tension side onsisted of deformed ars splied at midspan. The ars were three 20-mm (#6) ars in the five eams of Series One, two 25-mm (#8) ars in the eams of Series Two, and two 32-mm (#10) ars in the eams of Series Three. The variales used in the investigation were ar size (20, 25, or 32 mm), and the numer of ansverse reinforement hoop stirrups (Grade 60) onfining the 305- mm (12 in.) splie region (0, 1, 2, 3, or 4 stirrups). One stirrup implies Φ mm (# 12 in.), two stirrups imply Φ mm (# 6 in.), three stirrups imply Φ mm (# 4 in.), and four stirrups imply Φ mm (# 3 in.). The test speimens are identified in Tale 1. A two-part notation was used to indiate the variales in eah eam. 220

3 Hamad, Najjar Tale 1 - Test parameters and test results Series Speimen Bar /d Numer K + K Conrete Measured Defletion Data normalized to (f ) 1/4 numer notation size of sength ultimate at d stirrups load midspan Ultimate Steel Bond Bond load sess sess ratio d P max δ P max f s u t * (mm) (MPa) (kn) (mm) (kn) (MPa) (MPa) B20S B20S in., 0.46d One B20S in., 0.90d B20S in., 1.36d B20S in., 1.80d B25S B25S in., 0.54d Two B25S in., 1.09d B25S in., 1.63d B25S in., 2.16d B32S B32S in., 0.42d Three B32S in., 0.85d B32S in., 1.27d B32S in., 1.69d * u t = f s d /4l s The first part of the notation indiates the ar sizes: B20 for 20-mm ars, B25 for 25-mm ars, and B32 for 32- mm ars. The seond part indiates the numer of stirrups onfining the 305 mm (12 in.) splie region: no stirrups (S0), one stirrup (S1), two stirrups (S2), three stirrups (S3), or four stirrups (S4). The parameter K, listed in the Tale, is a ansverse reinforement index used in the ACI Building Code (ACI [8]) to indiate the amount of ansverse reinforement rossing the potential splitting plane in the splie region. It arries the unit (in.): K = A f yt (1) 1500 s n A = Total ross-setional area of all ansverse reinforement whih is within the spaing s and whih rosses the potential plane of splitting, (in 2 ) f yt = Speified yield sength of ansverse reinforement, (l/in 2 ) n = Numer of ars eing splied along the plane of splitting s = Center-to-enter spaing of ansverse reinforement within splie length, (in.) The onstant 1500 arries the unit (l/in 2 ) or psi; 1 in. = 25.4 mm, and 10 3 psi = 6.9 MPa. The splie length of the deformed ars was 305 mm (12 in.) in all eam speimens. This value was seleted to develop a steel sess less than yield to ensure splitting mode of failure in all eam speimens. The eam width was 24 m (9.45 in.) in all series. To allow idential onrete onfinement for all splies in one eam, the lear spaing etween the splies was taken twie the ottom or side over. The resulting lear onrete over was 2.0 m in the 20-mm speimens of Series One, 3.5 m in the 25-mm speimens of Series Two, and 2.8 m in the 32-mm speimens of Series Three. The depth was 30.5 m (12 in.) for all eams. Longitudinal and ross-setion details of the eam speimens are shown in Figs. 1, 2 and 3. Transverse reinforement was provided in the shear spans of all eams to avoid shear failure. It onsisted of 10-mm (#3) Grade 60 ars plaed at a spaing of 7.5 m (3 in). Bars used were from the same heat of steel and had the same parallel (amoo) deformation pattern. The ars met ASTM speifiations and were Grade 60. The two top reinforing ars extending outside the load appliation segment were 10-mm (#3) Grade 60 ars. A oupon of eah ar size was tested to onfirm the mill test report otained from the supplier. The average yield sesses were 580 MPa (84 ksi) for the 10-mm (# 3) ars, 639 MPa (93 ksi) for the 20-mm (# 6) ars, 528 MPa (77 ksi) for the 25-mm (# 8) ars, and 433 MPa (63 ksi) for the 32-mm (# 10) ars. Non air-enained onrete mixes were designed to provide a nominal 28-days ompressive sength of 55 MPa (8,000 psi). The water-to-ement ratio, taking into aount the liquid ontent of the superplatisizer, was 0.3. ASTM Type I Portland ement was used. The fine aggregate was natural sand with a ulk speifi gravity of 2.6, an asorption apaity of 2.40 perent, and a fineness modulus of Coarse aggregates with maximum sizes of 10 and 20 mm were used. The oarse aggregates were 221

4 Materials and Sutures/Matériaux et Consutions, Vol. 35, May 2002 Fig. 1 Longitudinal and ross-setion details of eam speimen B20S1 of Series One; d = 20 mm. Fig. 2 Longitudinal and ross-setion details of eam speimen B25S3 of Series Two; d = 25 mm. Fig. 3 Longitudinal and ross-setion details of eam speimen B32S4 of Series Three; d = 32 mm. rushed limestone with a ulk speifi gravity of 2.61 and an asorption apaity of 1.43 perent. The superplastiizer used is produed y the German ompany Fosro. The produt name is Complast 430, and it satisfies ASTM C494 Type F. A dosage of 4 per 100 kg of ementitious material was used. The superplastiizer had 40 perent solids, and had a speifi weight of 1.2 g/m 3. No pozzolani material was added to ahieve high sength to avoid any effet suh material might impose on the sutural ehavior of the eam speimens. The asting proedure was the same for all the eams. The eams were ast using a 5 ui feet apaity onrete mixer. One or two eams were ast at a time. Eah eam was ast in two athes. The first ath was designed to fill aout two-thirds of the eam volume, a ondition set y the apaity of the mixer. From eah onrete ath, several 6 x 12 in. ylinders were ast. The eam and its orresponding ylinders were ured 222

5 Hamad, Najjar for seven days. The eams were tested using MTS (Materials Testing System) losed loop servo-hydrauli mahine with a 1000-kN apaity dynami atuator. Testing was done at least 28 days after asting. Load was applied in inrements of 5 or 10 kn until failure ourred. At eah load stage, defletion readings were taken at the enter of the eam using a dial gauge and flexural raks were marked and measured using a rak-width omparator. 4. MODE OF FAILURE The first flexural raks in all eams ourred randomly in the onstant moment region on the tension side of the eam outside the splie length. As load inreased, raks formed along the entire length of the onstant moment region inluding the splie region. Flexural raks inside and outside the splie region were more numerous in eams with ansverse reinforement. For the eams where there were no hoop stirrups rossing the splitting plane, raks inside the splie region developed mostly after failure. Before failure, and as loading ontinued, the width of the flexural raks in the splie region and their propagation along the height of the eam were notiealy less than the width and propagation of raks outside the splie region. The reason is that at load levels elow failure, the ond sess in the splie is elow apaity and there is effetively twie as muh steel as outside the splie. In eams with no ansverse reinforement in the splie region, failure ourred just after longitudinal splitting raks formed in the ottom over on the tension side diretly elow the splie region and in the side over adjaent to the loation of the ars. The final mode of failure was a fae-and-side split failure. The failure was sudden, very rittle, and noisy. After failure, the eams arried pratially no load. In all other eams with ansverse reinforement in the splie region, although the final mode of failure was a fae-and-side split failure, however, failure developed gradually even with one 10-mm (#3) stirrup at the enter of the 305-mm (12 in.) splie region, orresponding to a K value of around 0.5 d in the three series of eam speimens. Dutility of the mode of failure improved as the numer of hoop stirrups in the splie region inreased. Longitudinal raks started to form in the ottom onrete over right under the lap splies at aout 65 to 90% of the maximum load. The load dropped slightly after reahing a peak, and ontinued to drop gradually with inreasing defletion. The gradual and more dutile mode of failure of eams with stirrups in the splie region and the propagation of the longitudinal raks along the splies efore ultimate imply that the presene of the stirrups allowed most ar lugs in the splie region to oniute to the sess ansfer etween the longitudinal ars and the surrounding onrete. The oserved raking patterns on the ottom tension fae and on the side of all eam speimens with idential amount of ansverse reinforement provided in the splie region were very similar regardless of the ar size used. After the eams reahed ultimate, additional defletions were imposed to inrease the severity of the splitting of the splie region. The added splitting permitted easy removal of the ottom (tension fae) and side onrete overs to reveal the failure plane in the splie region. It was notied that there was etter adhesion etween the splied ars and the surrounding onrete in the viinity of the hoop stirrups. This proves that the stirrups have allowed more lugs to e utilized in the sess ansfer mehanism etween steel and onrete. 5. TEST RESULTS To allow diret omparison of all eam speimens, the orresponding load-defletion data, load-rak width data, and ond sengths, were normalized at a ommon onrete sength of 70 MPa (10,000 psi). The adjustment was made y multiplying the load at eah defletion or rak width y (70/f ) 1/4, where f is the onrete sength in MPa of the eam speimen under onsideration at the day of testing. The mode of failure in all eam speimens was a fae-and-side split failure. The splitting mode of failure indiated that the splie reahed its maximum apaity. Therefore, ond sength ould e determined diretly from the sess developed in the steel. The sess in the steel, f s, was alulated ased on elasti raked setion analysis and was determined from the maximum load otained for eah eam speimen. To evaluate the average ond sess, ut, the total fore developed in the ar, A f s, was divided y the surfae area of the ar over the splie length, πd l s : u t ( Af) s = πdl s fd s ut = 41s Results of the three series of fifteen eam speimens tested in the researh program are presented in Tale 1. The listed data inludes the parameters of eah eam, the onrete sength at the day of testing, the ultimate load at failure (P max ), the defletion at the enter of the eam (δ), and the data normalized at f = 70 MPa (10,000 psi) inluding the ultimate load (P max ), steel sess (f s ), average ond sess (u t ), and the ond ratio. The ond ratio is the ond sess of the eam with ansverse reinforement divided y the ond sess of the ompanion eam with no ansverse reinforement ut with the same ar size. 6. CRACK WIDTH Flexural raks in the onstant moment region outside the splie region were used to study the effet of the variales (ar size and amount of ansverse reinforement) on the width of the flexural raks due to two reasons: 1. Craks outside the region were wider than those (2) 223

6 Materials and Sutures/Matériaux et Consutions, Vol. 35, May 2002 Fig. 4 Variation of steel sess versus average rak width for Series One; d = 20 mm, /d = 1.5. Fig. 5 Load-defletion urves of the eam speimens of Series One; d = 20 mm, /d = 1.5. inside the splie region, whih resulted in etter auray in measuring rak width with the rak width omparator. 2. Usually in a suture, flexural raking outside the splie region is more ritial than flexural raking inside the splie region due to the fat that the steel area in the splie region is greater and the sesses are smaller along the splie than outside. The raks were measured y a rak width omparator, one at eah edge of the splie region, and the other two in the onstant moment region on either side of the splie. The variation of the steel sess versus the average rak width for the five eams of Series One is shown in Fig. 4. In general, the results indiate that the rak width for a given steel sess dereased as the numer of hoop stirrups inreased. This end was valid for the three series. It is interesting to note that although the raks in the eams with ansverse reinforement were generally smaller in size than the ompanion plain eams, however, they were larger in numer. The aility of the eams having ansverse reinforement in the splie region to develop large numer of small-width raks prior to failure is one of the main reasons ehind their dutile ehavior. The presene of the hoops stirrups in the splie region arrested the raks and prevented exessive opening of developed raks at failure and, therefore, maintained the integrity of the eam speimens. 7. LOAD DEFLECTION BEHAVIOR The load-defletion stiffness of eams with different numer of hoop stirrups in the splie region was ompared with the stiffness of the eams with no ansverse reinforement in eah of the three series. The urves are presented in Figs. 5, 6, and 7. In the three eam series, the flexural stiffness of all eams with different amount of ansverse reinforement (0, 1, 2, 3, or 4 stirrups) were almost idential elow and aove the raking load. The raking load was 40 kn for Series One (d = 20 mm, /d = 1.5), 46 kn for Series Two (d = 25 mm, /d = 1.9), and 41 kn for Series Three (d = 32 mm, /d = 1.375). Fig. 6 Load-defletion urves of the eam speimens of Series Two; d = 25 mm, /d = 1.9. Fig. 7 Load-defletion urves of the eam speimens of Series Three; d = 32 mm, /d = The presene of ansverse reinforement did not affet the flexural raking load of the eam speimens. In any series, there was a onsistent inrease in the ultimate load as the numer of hoop stirrups inreased. In Series Two, the inreases were less than in Series One and Series Three. This ould e atiuted to the larger (/d ) ratio, 1.9, as ompared to 1.5 for Series One and for Series Three. After ultimate, the eam with no ansverse reinforement experiened omplete loss of load resistane and arried virtually no load. With inrease in numer of hoop stirrups plaed in the splie 224

7 Hamad, Najjar Fig. 8 Variation of the normamized ond sess with K. Fig. 10 Variation of the normamized ond sess with [( + K )/d ]. Fig. 9 Variation of the ond ratio with K. region, the load-defletion ehavior eyond the ultimate load was more dutile. After reahing ultimate, the load dropped slightly and ontinued to derease gradually with inreasing defletions. Although the numer of splies was different for the three different series (two for the seond and third series, and three for the first series), this fator had no influene on the ends of load-defletion ehavior. 8. BOND STRENGTH The variations of the average ond sength and the ond ratio versus K, the ansverse reinforement index, are plotted in Figs. 8 and 9. In general, there was a onsistent inrease in the average ond sength as the amount of ansverse reinforement in the splie region inreased. The improvement was less reognized in Series Two as ompared to the other two series. This is atiuted to the larger (/d ) ratio in Series Two. However, the values listed in Tale 1 and plotted in Figs. 8 and 9 learly indiate that the improvement of the average ond sength in the three series of eam speimens levelled off for values of K exeeding 1.0d. Based on the results, it ould e reommended to provide ansverse reinforement in the splie regions of high sength onrete speimens equivalent to a K value of at least 1.0d. This will improve the ond sength and was found to provide dutility to the mode of failure of tension lap splies regardless of ar size or the numer of splies. The variations of the average ond sess and the ond ratio versus the term ( + K )/d are plotted in Figs. 10 Fig. 11 Variation of the ond ratio with [( + K )/d ]. and 11. The term ( + K )/d represents the omined onfinement effet of onrete over and ansverse reinforement. Setion of the ACI Building Code (ACI [8]) plaes an upper limit of 2.5 on the term ( + K )/d to safeguard against pullout failures. This implies that for ars and splies anhored in normal-weight onrete speimens, onfinement provided y the omined effet of onrete over and ansverse reinforement improves the tensile splitting ond sength up to a value of 2.5 for the term ( + K )/d. Beyond that value, the mode of failure hanges to pullout rather than onrete splitting in the anhorage or splie region. Test results plotted in Figs. 10 and 11 learly indiate that even in HSC speimens, the improvement in ond sength provided y onrete over and ansverse reinforement prevails up to a value of 2.5 to 3.0 for the term ( + K )/d. It ould e onluded that the reommendation of Setion of the ACI Building Code onerning the term ( + K )/d applies for ars and lap splies anhored in HSC speimens. To assess the positive effet of ansverse reinforement on the average ond sength as funtion of onrete sength, the atual ond sength values were expressed as multiples of with u t in psi and f (the atual onrete ompressive sength) in psi. The multiples are listed in Tale 2 and plotted versus K in Fig. 12 and versus ( + K )/d in Fig. 13. In Series One (d = 20 mm, /d = 1.5), the inreases in ( ut/ ) of eams with stirrups as ompared with the eam with no stirrups slightly exeeded 3 with a K value of 1.8 d. In Series Two 225

8 Materials and Sutures/Matériaux et Consutions, Vol. 35, May 2002 Series Speimen Ultimate f Bond Delta Predited ond sess Bond effiienies notation load sess ut/ u u (Orangun) u (ACI-99) u t /u(orangun) u t /u(aci-99) P max u t/ t (kn) (MPa) (MPa) ** MPa MPa B20S B20S One B20S B20S B20S B25S B25S Two B25S B25S B25S B25S B25S Three B25S B25S B25S * Values of ond sess are not normalized to a ommon f ** u t and f are in psi Tale 2 - Bond sesses and ond effiienies of the eam speimens* (d = 25 mm, /d = 1.9), the maximum improvement in the ond sength was 185. at a K value of 2.16d. In Series Three (d = 32 mm, /d = 1.375), the inrease in ond sength reahed 378. for a K value of 1.69d. It is important to note that aording to the study performed y Orangun, Jirsa, and Breen [9], 3 is the maximum inrease in ond sength of a reinfored onrete eam that ould e ahieved y using ansverse reinforement in the splie region. Fig. 12 Variation of [u t /sqrt (f )] with K. Fig. 13 Variation of [u t /sqrt (f )] with [( + K )/d ]. 9. COMPARISON WITH ORANGUN AND ACI The atual measured average splie ond sength of eah eam was ompared with the value omputed using the empirial equation developed y Orangun, Jirsa, and Breen [9]: u = ( ' d K' ) d l K' s a fyt ' = 25., K' s d d = smaller of lear onrete over or half the lear spaing etween splies a = Area of ansverse reinforement rossing the plane of splitting adjaent to a single anhored reinforement (3) 226

9 Hamad, Najjar Note that the definition of the onfinement parameter (K in the aove equation is different from the parameter K in ACI Comparison was also made with the 1999 ACI Building Code (ACI [8]) ond speifiations using l s = 1.3 l d : l = 1.3 l s l d d 04.; ( + K ) Comining the aove equation with uπd l s = A f y, and setting the appropriate values to the reinforement loation α (1 for all eams), oating fator β (1 for all eams), reinforement size fator γ (0.8 for Series One and 1.0 for Series Two and Three), and lightweight aggregate onrete fator λ (1 for all eams): For Series One : u = 3.2 For Series Two : u = 2.56 (4a) (4) For Series Three : u = (4) d The upper limits in Equation (4) result from the upper limit of 2.5 that the ACI plaes on the [( + K )/d ] ratio. All ars in the study were ottom ast. Therefore, no top ar fator was applied to Equations (3) or (4). The predited ond sesses omputed using Equations (3) and (4) are listed in Tale 2. The measured ond sess for eah speimen ased on the atual onrete sength at day of testing was divided y the predited values to otain the ond effiienies listed in Tale 2. The mean ond effiieny for all ar splies using Equation (3) of Orangun et al., was 0.96 with a standard deviation of Moreover, the mean ond effiieny using Equation (4) of ACI was 1.73 with a standard deviation of The results imply that for the splies tested in the study, Equation (3) of Orangun et al., gave on the average a very good estimate of the ond sength. On the other hand, Equation (4) of the 1999 ACI Code provided a more onservative estimate of the ond sength of all eam speimens. 10. CONCLUSIONS d d = 3 40 fy K = A f yt 1500 s n d αβγ λ ( + K ) Fifteen high sength onrete eam speimens with onrete ompressive sength ranging from 55.4 to 65.3 MPa (8,000 to 9,500 psi) were tested. Eah eam was reinfored with ars splied in a onstant moment region at midspan. Based on the analysis and omparison of modes of failure, ultimate loads, flexural rak widths, load-defletion ehavior, and ond sesses, the following onlusions were made: d d In eams with no ansverse reinforement in the splie region, failure ourred just after longitudinal splitting raks formed in the ottom and side overs adjaent to the loation of the ars. The final mode of failure was a fae-and-side split failure. The failure was sudden, very rittle, and noisy. 2. As the amount of ansverse reinforement in the splie region inreased, the mode of failure was more dutile and more gradual although the final mode of failure was splitting of the onrete over. Dutility was evident even with ansverse reinforement orresponding to a K value of 0.5 d. 3. Although the flexural raks in the eams with ansverse reinforement were generally smaller in size than the ompanion plain eams, they were larger in numer. The aility of the eams having ansverse reinforement in the splie region to develop large numer of smallwidth raks prior to failure is one of the main reasons ehind their dutile ehavior. 4. The load-defletion flexural stiffness of ompanion eams without and with different amounts of ansverse reinforement in the splie region was idential elow and aove the flexural raking load. 5. The improvement in the average ond sength in the three series of eam speimens, aused y inreasing the amount of ansverse reinforement in the splie region, leveled off for values of K exeeding 1.0 d. 6. The improvement in ond sength provided y onrete over and ansverse reinforement prevailed up to a value of 2.5 to 3.0 for the term ( + K )/d. This implies that the upper limit of 2.5 plaed y the ACI Building Code on the term ( + K )/d in omputing development length, applies for ars anhored in NSC and HSC. 7. The Orangun equation gave on the average a very good estimate of the ond sength. The 1999 ACI Code gave a more onservative estimate of the ond sength of all eam speimens. Based on the test results of the researh program reported in this paper, it is reommended to onfine anhored ars and tension lap splies in HSC eam speimens with ansverse reinforement equivalent to a K value of 1.0 d or more. This onfinement will provide dutility to the mode of failure and will improve the average ond sength. ACKNOWLEDGMENT The authors gratefully aknowledge the support of the University Researh Board at the Amerian University of Beirut for supporting this program. Also, the assistane of Mr. Hilmi Khati, Supervisor of the testing laoratories at AUB, is greatly appreiated. REFERENCES [1] Azizinamini, A., Stark, M., Roller, J. J. and Ghosh, S. K., Bond performane of reinforing ars emedded in high sength onrete, ACI Sutural Journal 90 (5) (1993)

10 Materials and Sutures/Matériaux et Consutions, Vol. 35, May 2002 [2] Azizinamini, A., Pavel, R., Hatfield, E. and Ghosh, S. K., Behavior of lap-splied reinforing ars emedded in high sength onrete, ACI Sutural Journal 96 (5) (1999) [3] Azizinamini, A., Darwin, D., Eligehausen, R., Pavel, R. and Ghosh, S. K., Proposed modifiations to ACI Tension Development and Lap Splie for High Sength Conrete, ACI Sutural Journal 96 (6) (1999) [4] ACI Committee 318, Building Code Requirements for Reinfored Conrete and Commentary (ACI /ACI- 318R-95), Amerian Conrete Institute, Farmington Hills, Mihigan, [5] Hamad, B. S. and Itani, M. S., Bond sength of reinforement in high-performane onrete: The role of silia fume, asting position, and superplastiizer dosage, ACI Materials Journal 95 (5) (1998) [6] Hamad, B. S. and Mahaka, M. F., Effet of ansverse reinforement on ond of reinforing ars in silia fume onrete, Mater. Sut. 32 (1999) [7] Hamad, B. S. and Seferian, Z. S., Role of asting position on ond of onfined tension lap splies in silia fume onrete, Mater. Sut. 33 (2000) [8] ACI Committee 318, Building Code Requirements for Reinfored Conrete and Commentary (ACI /ACI- 318R-99), Amerian Conrete Institute, Farmington Hills, Mihigan, [9] Orangun, O. C., Jirsa, J. O. and Breen, J. E., The sength of anhored ars: A reevaluation of test data on development length and splies, ACI Journal (Marh 1977)