Advances in Structural Engineering

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1 Bond Characteristics and Shear Behavior of Concrete Beams Reinforced with High-Strength Steel Reinforcement by Tarek K. Hassan, Ahmed Mantawy, Judy Soliman, Ali Sherif and Sami H. Rizkalla Reprinted from Advances in Structural Engineering Volume 15 No MULTI-SCIENCE PUBLISHING CO. LTD. 5 Wates Way, Brentwood, Essex CM15 9TB, United Kingdom

2 Bond Characteristics and Shear Behavior of Concrete Beams Reinforced with High-Strength Steel Reinforcement Tarek K. Hassan 1, *, Ahmed Mantawy 1, Judy Soliman 1, Ali Sherif 1 and Sami H. Rizkalla 2 1 Department of Structural Engineering, Faculty of Engineering, Ain Shams University, Cairo, Egypt 2 Department of Civil, Construction and Environmental Engineering, NCSU, Raleigh, NC 27695, USA (Received: 18 January 211; Received revised form: 22 June 211; Accepted: 27 June 211) Abstract: This paper evaluates the bond behavior of high strength (HS), steel reinforcing bars and highlights the effect of various key parameters believed to affect the bond characteristics. Nine reinforced concrete spliced beams were constructed and tested. The beams had different splice lengths and levels of confinements. The applicability of different hypotheses for development of conventional steel bars was examined for the HS bars. The study is extended to examine the behavior of the reinforcing bars as shear reinforcement for concrete beams by testing twelve concrete beams reinforced with HS steel stirrups under static loading conditions. The main variables in the study included steel type, concrete compressive strength, web reinforcement ratio and shear span-to-depth ratio. The applicability of various building codes and standards for concrete beams with HS shear reinforcement was also evaluated. Key words: beams, bond, concrete, development length, high strength, shear, stirrups. 1. INTRODUCTION Bond between concrete and reinforcing steel is required to transfer the forces between the two materials and therefore, it significantly influences the behavior of reinforced concrete structures (ACI 48 3). Previous research (Orangin et al. 1977; Darwin et al. 1996; Esfahani and Rangan 1998; Zuo and Darwin ) provided basic understanding of the bond characteristics between concrete and the conventional steel. Advancement in material science has led to the production of Micro Composite Multi-Structural Formable Steel (MMFX steel), which is innovative new high-strength steel reinforcement. The reinforcement is characterized by higher tensile strength compared to conventional mild steel reinforcement. The stress-strain behavior of the material has no a well-defined yield plateau (EL-Hacha and Rizkalla 2). Comparing the behavior of these bars to those of conventional steel Grade 6, a significant improvement in strength and corrosion resistance can be clearly demonstrated. Using these bars in concrete structures and bridges allows reduction of the reinforcement requirements and hence leads to a more economical design for any particular project. Due to the difference in the mechanical properties of these bars to the conventional steel, the bond behavior between the high strength steel and the concrete must be investigated. Since the applicability of the bond equations proposed by various researchers (Orangin et al. 1977; Darwin et al. 1996; Esfahani and Rangan 1998; Zuo and Darwin ) including the current ACI Committee *Corresponding author. address: tarek.hassan@dargroup.com; Tel: Associate Editor: J.G. Dai. Advances in Structural Engineering Vol. 15 No

3 Bond Characteristics and Shear Behavior of Concrete Beams Reinforced with High-Strength Steel Reinforcement Report for Bond 1 are limited to conventional steel, there is a need to examine the bond characteristics of the new high strength steel reinforcement. In most applications, the HS steel bars have been used by direct replacement of the amount required for conventional steel (Grade 6) and, thus, neglecting the benefits of the high yield strength of the new material. Lack of information regarding the behavior of concrete members reinforced with this type of material prevents design engineers from utilizing the full strength of the material. The first phase of the experimental program in this paper presents test results of 9 reinforced concrete spliced beams. The beams had different splice lengths, levels of confinements and were tested under four point bending setup to provide a constant moment region over the splice zone. The second phase of the experimental program presents test results of 12 medium-scale reinforced concrete beams tested up to failure to investigate the contribution of the HS steel stirrups in the shear resisting mechanism compared to conventional steel stirrups. The key parameters considered were the steel type, amount of shear reinforcement, concrete strength and shear span-todepth ratio (a/d). 2. BOND BEHAVIOR OF CONCRETE SPLICED BEAMS The first phase of the experimental program consisted of nine reinforced concrete spliced beams divided into three main groups according to the diameter of the longitudinal reinforcing bars and the splice length as given in Table 1. The main parameters in this study included the bar size, splice length and the confinement level. The specimens were designed using cracked section analysis. The splice length was selected to ensure failure due to bond for all the tested specimens. All tested beams were 4 mm long and have cross-sectional dimensions of 25 mm wide by 4 mm deep. The side and bottom concrete Stirrups 1 mm mm Splice length Stirrups 1 mm distances according to beam type Figure 1. Typical elevation of bond specimens Unconfined beams c c Dimensions are given in meters c.4 Confined beams covers were 4 mm. HS steel bars were used as the main tensile reinforcement while conventional steel reinforcing bars were used in the compression zone to provide a mechanism to hold the beam together after rupture of the splice. Each of the three main groups consists of three beams with three different confinement levels along the splice length. Within each group, the first beam was tested without transverse reinforcement and used as a control specimen. The transverse reinforcement of the second and the third beams within each group consisted of 1 mm diameter conventional steel stirrups spaced at mm and 1 mm center-to-center, respectively. Stirrups were also added outside the splice zone at spacing of mm to prevent premature shear failure. Figures 1 and 2 show elevation and cross sections of a typical test beam. The measured cylindrical concrete compressive strength at 28 days was 65 MPa..25 Stirrups 1 mm distances according to beam type Figure 2. Typical Cross-section of bond specimens at the splice zone Table 1. Bond specimens Splice bar Splice Confinement within splice zone diameter length Spacing No. of stirrups within Failure Group Beam ID (mm) (mm) between stirrups the splice zone load (kn) B1 36 N/A 128 Group 1 B2 13 (3φ) mm 2 15 B3 1 mm B4 22 N/A 18 Group 2 B5 13 (18φ) mm B6 1 mm B7 57 N/A 226 Group 3 B8 19 (3φ) mm B9 1 mm Advances in Structural Engineering Vol. 15 No

4 Tarek K. Hassan, Ahmed Mantawy, Judy Soliman, Ali Sherif and Sami H. Rizkalla 2.1. Material Properties High strength reinforcing steel The high-strength steel used in the current study was provided by MMFX Technologies Corp., CA, USA. The mechanical characteristics of the material compared to conventional steel bars are shown in Figure 3 based on test results conducted at NC State University (Hassan et al. 8). The material exhibits an initial linear elastic portion and there is no observation of yielding plateau or strain hardening. The yield strength corresponds to.2% strain offset is 83 MPa. The following equations are proposed for the stress-strain behaviour of high strength steel based on extensive testing conducted at North Carolina State University (Hosny 7). f s = 122 (1 e 185ε MMFX) MPa for 13 mm (1) diameter rebars f s = 196 (1 e 248ε MMFX) MPa for 19 mm (2) diameter rebars 2.2. Test Setup All beams were tested using four point loading configuration to develop a constant moment region of 16 mm for the spliced bars location. The beams were supported on a roller support at one end and a hinge support at the other. A hydraulic jack of 4 kn capacity was used to apply the load on the top of a rigid steel beam that equally distributes the load at both load points. A total of four electrical resistance strain gages and four LVDTs were used to monitor the strains and the deflections of the beams during testing. The electrical resistance strain gages were attached to the longitudinal reinforcing bars immediately outside Stress (MPa) MPa High-strength steel Conventional steel Strain (mm/mm), (in./in.) Figure 3. Typical stress-strain behavior for conventional and HS steel reinforcement (Hassan et al. 8) Stress (ksi) Support reaction Applied load Test zone Hydraulic jack Applied load Figure 4. Test setup for bond specimens Rigid beam Support reaction the splice length to measure the maximum strains in the spliced bars. The mid-span deflection was monitored using two LVDTs as shown in Figure Crack Pattern Crack width was measured at different load levels using crack comparators. It was observed that the initiation of the first flexural cracks occur at the two ends of the splice zone and near the location of the applied load for all tested beams where the maximum moment and shear are combined. Flexural cracks propagated upwards and increased in number associated with an increase in the crack width as the load was increased. Further increase in the load led to the formation of splitting cracks that were parallel to the longitudinal spliced bars initially on the bottom surface of the beams followed by splitting cracks on the side of the beam close to failure. The beams without confining transverse reinforcement failed immediately after initiation of the splitting cracks while for the beams with confining transverse reinforcement, propagation of the splitting cracks was observed over the splice zone before failure occurred Load-Deflection Behavior The load-deflection behavior of the test specimens is shown in Figure 5 for the three groups. Test results indicated that the pre-and-post-cracking stiffness were relatively similar for each group of beams regardless of the confinement level. The beams without confining transverse reinforcement failed due to splitting and loss of the concrete cover over the length of the splice shortly after the initiation of the splitting cracks. When confining transverse reinforcement was added within the splice zone, the beams were capable of carrying more loads and the failure was more ductile and associated with high mid-span deflections prior to failure. For the first group of beams, the maximum Advances in Structural Engineering Vol. 15 No

5 Bond Characteristics and Shear Behavior of Concrete Beams Reinforced with High-Strength Steel Reinforcement Load (kn) Load (kn) Load (kn) Group Mid-span deflection (mm) 5 Group Mid-span deflection (mm) B7 B8 B9 B1 B2 B3 B4 B5 B6 Group 3 higher than that measured for B1, respectively. This behavior was typical for the second and third group of beams Mode of Failure Failure of the spliced beams was almost identical. All beams without confining transverse reinforcement along the splice length failed suddenly after the initiation of the splitting cracks without warning or propagation of the cracks accompanied by loss of the concrete cover. Failure of the beams with confinement reinforcement was also due to splitting of the concrete cover over the splice length. The splitting failure of these beams was more ductile and allowed propagation of the splitting cracks prior to failure as shown in Figure Stresses in the Spliced Bars The stresses in the spliced bars were evaluated based on the measured strains from the strain gages attached to the longitudinal bars located immediately outside the splice zone. The measured strains were used to calculate the stresses in the spliced bars based on the given stress-strain relationships in Eqns 1 and 2. Furthermore, using crack section analysis, the stresses in the MMFX corresponding to the measured ultimate load were determined for each tested beam. The stresses in the MMFX steel before failure given in Table 2, indicate that adding confining transverse reinforcement increases the measured stresses in the longitudinal bars and thus increasing the ultimate load carrying capacity Analysis of Test Results Test results of the concrete beams without confining transverse reinforcement were used to examine the relationships proposed by previous researchers Mid-span deflection (mm) Figure 5. Load-deflection behaviour of bond specimens Splice length measured steel stress at splitting failure for B1 (without transverse reinforcement at the splice zone) was 855 MPa. Using confining transverse reinforcement for B2 and B3, the measured stresses at failure were 27% and 42% B7 Figure 6. Typical splitting failure for bond specimens 36 Advances in Structural Engineering Vol. 15 No

6 Tarek K. Hassan, Ahmed Mantawy, Judy Soliman, Ali Sherif and Sami H. Rizkalla Table 2. Measured strains and the corresponding stresses in the spliced bars Stresses in spliced bars Measured Strain gages Cracked section d b l s /d b Confinement strain readings analysis Group Beam ID (mm) ratio level (%) (MPa) (MPa) B1 C Group 1 B C B3 C B4 C Group 2 B C B6 C B7 C Group 3 B C B9 C (Orangin et al. 1977; Darwin et al. 1996; Esfahani and Rangan 1998; Zuo and Darwin ) for conventional steel. It should be noted that these empirical models developed by various researchers (Orangin et al. 1977; Darwin et al. 1996; Esfahani and Rangan 1998; Zuo and Darwin ) were originally developed for conventional steel reinforcement. The dimensions and material characteristics of the beams tested at Ain Shams University and at N.C. State University (Hosny 7) used in the current study are given in Table 3. For a detailed list of symbols used, refer to the ACI 48R-3 (3). The predicted stresses for different beams using various descriptive equations proposed by researchers for conventional steel are given in Table 4. It should be noted that the current expression proposed by the ACI Committee 48 (3) provided an acceptable level of accuracy when applied to high strength steel reinforcement. The analysis showed that the proposed model by Esfahani and Rangan (1998) did not account for the confinement effect provided by the transverse reinforcement and hence yielded the highest standard deviation for the ratio of the measured to predicted stress at the onset of splitting failure. Furthermore, the expression proposed by Orangun et al. (1977) and that proposed by Zuo and Darwin (), overestimates the stresses in the HS longitudinal bars. 3. SHEAR BEHAVIOR OF CONCRETE BEAMS WITH TRANSVERSE HS STEEL REINFORCEMENT The second phase of the current study included testing twelve medium scale concrete beams at the Structural Laboratory of Ain Shams University. Eight beams were reinforced with HS steel stirrups; three beams had no stirrups and a control beam with conventional steel stirrups. Beams without shear reinforcement were used to evaluate the concrete contributions at different shear span-to-depth (a/d) ratios. All beams were T-section with a total height of 5 mm, a flange of 6 mm wide and 8 mm thick. The thickness of the web was mm. The length of all beams is 3. m. The span of the beam was divided into two simply supported spans of 1.35 m each except for B7, B8 and B9, which had a 2.7 m span as shown in Figure 7. All beams had identical bottom longitudinal reinforcement consisted of six 25 mm diameter conventional bars. The beams were designed to achieve a brittle shear failure prior to reaching the ultimate flexural capacity. Anchorage of the flexural reinforcement was provided using U-shaped bars to prevent any possible slippage. Details of the tested specimens as well as the concrete compressive strength of different specimens are given in Table 5. To monitor the behavior of the tested beams under the applied loading, the instrumentation included LVDTs for deflections, electrical resistance strain gauges for steel strains, and displacement gauges (Demic points) for concrete strains on the concrete surface. Demicpoints were mounted on the web surface of each beam in three directions (rosette shape) and at different locations to evaluate the shear deformations in terms of the shear crack width and the concrete strain. Each rosette consisted of three demic lines of a mm gauge length. One demic line was placed horizontal. The other two demic lines were placed vertically and diagonally at a 45 angle, respectively as shown in Figure Test Setup All beams were tested under three point bending configuration according to its (a/d) ratio. For beams of (a/d) ratio of 1. and 1.5, the span of the beam was 1.35 m and the remaining portion of the beam was cantilevered, and therefore, unstressed so that the beam can be tested again at the far end. The load was applied using 15 kn capacity hydraulic jack. The Advances in Structural Engineering Vol. 15 No

7 Bond Characteristics and Shear Behavior of Concrete Beams Reinforced with High-Strength Steel Reinforcement Table 3. Bond specimens properties f c c c min c max c med d b A b l d Darwin Zuo & A tr f yt s Mode of Group ID (MPa) (mm) (mm) (mm) (mm) (mm) (mm 2 ) (mm) t r et al.1996 Darwin (mm 2 ) (MPa) (mm) n N M failure t d Beams Tested at Ain Shams University 1 B1 N/A N/A B B B4 N/A N/A B Splitting B B7 N/A N/A B B9 1 6 Beams Tested At North Carolina State university 1 BN1 N/A N/A BN Splitting BN BN4 N/A N/A BN Flexure BN BN7 N/A N/A BN BN9 N/A N/A BN (Continued) 38 Advances in Structural Engineering Vol. 15 No

8 Tarek K. Hassan, Ahmed Mantawy, Judy Soliman, Ali Sherif and Sami H. Rizkalla 5 BN11 N/A N/A BN Splitting BN BN14 N/A N/A 6 BN BN BN17 N/A N/A 7 BN BN Flexure BN2 N/A N/A Splitting 8 BN BN Flexure beams were placed on two steel rails, one at a distance of 15 mm from the right end of the beam, while the other was positioned at a specific distance from the left end according to the span of the beam. The beams were monotonically loaded up to failure using displacement control. A summary of the test results is presented in Table 5 including shear cracking load, shear capacity, angle of shear crack and mode of failure. All the tested beams failed in shear before flexural capacity is reached. No slip of the flexural reinforcement was observed during any of the beam tests Deflections The load-deflection plots for all beams are shown in Figure 8 according to its a/d ratio and concrete compressive strength. For beams B1, B2 and B3 with a/d = 1., no significant increase in the shear capacity was observed for beams with shear reinforcement (B2 and B3) compared to the beam without shear reinforcement B1. This is attributed to the small a/d ratio of those beams, which makes the arch action behavior dominant. A significant increase in the shear capacity for beams with HS shear reinforcement (B5 and B6) compared to the beam without shear reinforcement (B1) and the control beam (B). The same behavior was also observed for beams with a/d ratio of For beams B1 and B11 having a/d = 1.5 but with f c = 45 MPa, the plots are very close up to failure, where B11 provides higher shear capacity than B1. It is clear that for different (a/d) ratios, adding web reinforcement to the concrete beams increased both stiffness and shear capacity significantly except for beams with a/d = 1. where the increase in the shear capacity is not significant because the shear resisting mechanism is a pure arch action, which is almost independent on the presence of shear reinforcement. Also, increasing the shear reinforcement ratio slightly increases the shear capacity and decreases the deflection at failure without significant increase in the stiffness. The control beam B shows that using HS steel stirrups for beams improved the behavior and provided significant increase in both shear capacity and stiffness than conventional steel stirrups Crack Pattern As shown in Figure 9, it was observed that shear cracks initiated with a steep angle at the tension side of the beam and approached the compression flange at a relatively flat angle. The angle of shear crack was determined as the angle at which the shear crack intersects the mid-height of the web. The angle of the Advances in Structural Engineering Vol. 15 No

9 Bond Characteristics and Shear Behavior of Concrete Beams Reinforced with High-Strength Steel Reinforcement Table 4. Predicted stresses by various researchers Measured Orangun Darwin Zuo and Esfahani and ACI Committee Stresses et al. 2 et al. 3 Darwin 5 Rangan f sm f sp f sm /f sp f sp f sm /f sp f sp f sm /f sp f sp f sm /fs P fs P.92 x f sp f sm /f sp Group ID ratio (MPa) ratio (MPa) ratio (MPa) ratio (MPa) (MPa) (MPa) (MPa) ratio Beams Tested at Ain Shams University B B B B B B B B B Beams Tested At North Carolina State university BN BN BN3* BN BN5* BN6* BN BN BN BN BN BN BN (Continued) 31 Advances in Structural Engineering Vol. 15 No

10 Tarek K. Hassan, Ahmed Mantawy, Judy Soliman, Ali Sherif and Sami H. Rizkalla BN BN BN BN BN BN19* BN BN BN22* Average ** Standard Deviation** Coef. of Variation** Maximum** Minimum** * Beams failed in flexural mode ** Calculations do not include the beams that failed due to flexure. shear cracks in the beams without shear reinforcement ranged between 39 and 41 degrees. This relatively low range for shear crack angles is due to the absence of web reinforcement where only longitudinal reinforcement exists. The angle of the shear cracks for beams with shear reinforcement ranged between 43 and 47 degrees which is typical for reinforced concrete beams. It was also observed that, by increasing the shear reinforcement ratio, the crack angle increases. For all beams, shear cracking occurred before or at the same time as the flexural cracking except for beams of a/d = Beams B1, B2 and B3 with a/d = 1., almost have the same crack pattern at shear cracking load. The crack pattern consisted of one major shear crack from the support to the load, followed by another shear crack appeared nearly parallel to the major one with other minor flexural cracks. These two shear cracks can be considered to identify the concrete strut going diagonally from the support to the load and the shear resisting mechanism in this case is Arch Action. Also, for beam B4, which had no shear reinforcement, crack pattern is similar to beams with a/d = 1.. For beams B5 and B6, one major shear crack developed from the support to the load. Increasing the applied shear resulted in development of small flexural cracks near the loading area at the bottom of each beam. The shear transfer mechanism for both beams can be identified as a combination between the arch action and the contribution of shear reinforcement in a beam action. For beam B7, which has no shear reinforcement, only one major shear crack appears. Increasing the applied shear, the crack widens rapidly causing failure of the beam. For beams B8 and B9, one major shear crack appeared at the middle of the tested shear span with some flexural cracks. Some of the existing flexural cracks changed its direction to become flexural-shear cracks. The crack patterns for both beams became more irregular at higher load levels. The shear transfer mechanism for both beams can be identified as a typical beam action. The crack patterns of B1 and B11 are somehow similar to the crack patterns for B2 and B3. The shear transfer mechanism for both B1 and B11 also can be identified as a combination between the arch action and beam action but with more participation of the arch action than that for beams B5 and B6 having lower concrete strength. This may be attributed to the high concrete compressive strength of these beams, which increased both strength and stiffness of the diagonal concrete strut in the arch action and lead the shear mechanism to approach towards the pure arch action. Advances in Structural Engineering Vol. 15 No

11 Bond Characteristics and Shear Behavior of Concrete Beams Reinforced with High-Strength Steel Reinforcement a P 8@spacing (s) d = 5 1@spacing (s) 6 25 a P S Cantilever end T1 L LVDT(1) LVDT(2) Span LVDT(3) Figure 7. Typical beam details and instrumentation Table 5. Test matrix and results for concrete beam specimens tested in shear Shear Shear at Angle of Stirrups' cracking flexural major Failure Shear Mode steel f c Span Spacing ρ vf v capacity cracking crack Ioad P test capacity of Specimen type (MPa) (m) a/d (mm) (MPa) V er (kn) V m (kn) (deg) (kn) V test (kn) failure B Conventional SY B DC B2 High-Strength DC & Corrosive- B3 Resistant DC B SC 25 B5 High-Strength SR & Corrosive- B6 Resistant SC B ST B SC B9 High-Strength SC & Corrosive- B1 Resistant SR B SR DC: Shear failure initiated by crushing of diagonal concrete strut SC: Shear compression failure SR: Shear failure initiated by rupture of steel stirrups ST: Shear failure initiated by diagonal tension of concrete SY: Shear failure initiated by yielding of steel stirrups 3.4. Crack Width Total deformations within the strain rosette configuration were used to determine the width of the shear cracks. For a typical strain rosette at a specific location and considering diagonal and vertical measurements, the summation of shear crack widths can be determined. Each beam had one or two strain rosettes in the shear span according to its a/d ratio. The width of the shear crack was determined based on the observed number of shear cracks passing within each rosette. The 312 Advances in Structural Engineering Vol. 15 No

12 Tarek K. Hassan, Ahmed Mantawy, Judy Soliman, Ali Sherif and Sami H. Rizkalla (a) Shear (kn) No stirrups a/d = 1. fc = 25 MPa B1 B2 B3 (b) Shear (kn) Control No stirrups a/d = 1.5 fc = 25 MPa B B4 B5 B Deflection (mm) Deflection (mm) (c) 7 (d) a/d = 2.25 fc = 25 MPa a/d = 1.5 fc = 45 MPa Shear (kn) B7 B8 B9 Shear (kn) B1 B Deflection (mm) Deflection (mm) Figure 8. Shear deflection relationships for test specimens applied shear force versus the width of shear crack for all specimens is shown in Figure 1. It is clear that increasing the amount of shear reinforcement, reduces the shear crack width considerably. Narrower cracks were observed using high strength stirrups compared to conventional stirrups at the same load level. This may be attributed to the better bond characteristics of the high-strength steel than that for the conventional steel Failure Modes All the tested beams failed in shear. In general, the observed mode of failure was either shear-tension or shear-compression failure. Control beam (B) failed in a shear-tension mode due to the yielding of the stirrups that led to the development of a major shear crack. Beams B1, B2 and B3 having a/d = 1., failed in a shear-compression mode due to the crushing of the diagonal concrete strut in the arch action mechanism. Beams B4, B5 and B6 having a/d = 1.5, failed in a shear-compression mode due to the crushing of the concrete compressive zone under the applied load, except beam B5 where the mode of failure was a shear-tension failure due to the rupture of the HS steel stirrups. Beams B7, B8 and B9 having a/d = 2.25, failed in a shear-compression mode due to the crushing of the concrete compressive zone under the applied load, except beam B7 that has no shear reinforcement, which failed in direct diagonal tension of the concrete web. Shear tension failure was also observed for beams B1 and B11 having a/d = 1.5 but with f c = 45 MPa. The shear-tension failure due to the rupture of the HS steel stirrups is attributed to the limited ductility of the HS steel relative to conventional steel. For higher concrete compressive strength, this type of failure is attributed to the redistribution of a portion of the high shear carried by the arch action mechanism when the concrete strut softens near failure causing higher stresses in the stirrups that lead to rupture. Great attention should be paid to the determination of the minimum amount of shear reinforcement for the HS steel to account for its ductility as well as the effect of high concrete compressive strengths to avoid this brittle type of failure. Advances in Structural Engineering Vol. 15 No

13 Bond Characteristics and Shear Behavior of Concrete Beams Reinforced with High-Strength Steel Reinforcement B1 2/12/6 B2 6/12/6 B3 3/12/6 B4 2/12/6 B5 6/12/6 B6 3/12/6 B8 23/11/6 B7 25/11/6 B9 26/11/6 B1 4/12/6 B11 4/12/6 B 5/12/6 Figure 9. Crack patterns at failure 3.6. Stirrup Contribution The stirrup contribution, V s, can be determined based on the measured stirrup strain and the mechanical and geometric properties of the stirrups using the concept of smeared reinforcement in concrete beams as follows: Av Vs = ε v Eb s w d b s w (3) where A v is the area of one leg of stirrups, b w is the web width, s is the spacing between stirrups, ε v is the measured strain in stirrups crossing the shear crack, E s is the modulus of elasticity of the shear reinforcement and d is the distance from extreme compression fiber to centroid of longitudinal tension reinforcement. The relationships between the applied shear and the components of the shear resisting mechanism V c and V s are presented in Figure 11. It can be seen that the concrete contribution component, V c, at any load level was almost the same as the shear force at the initiation of the first shear crack, V cr, except for beams with a/d = 1. where the concrete contribution increased by increasing the applied shear. This is attributed to the pure arch action mechanism for those beams where the diagonal concrete strut is the main component of the mechanism with the longitudinal reinforcement acting as a tie. In addition, the concrete contribution component, V c, at failure decreased due to softening of concrete strut. It can also be seen that the contribution of shear reinforcement is higher for beams with a/d = 1.5 and 2.25 than that for beams of with a/d = 1.. Therefore, it can be concluded that stirrups in concrete beams contribute to the shear carrying capacity in two different ways: (1) Creating the stirrup contribution component, V s ; (2) Maintaining the concrete contribution component, V c, almost constant up to failure by 314 Advances in Structural Engineering Vol. 15 No

14 Tarek K. Hassan, Ahmed Mantawy, Judy Soliman, Ali Sherif and Sami H. Rizkalla (a) 7 (b) 7 Shear (kn) No a/d = 1. fc = 25 MPa B1 B2 B3 Shear (kn) No stirrups a/d = 1. fc = 25 MPa Control B B4 B5 B Crack width (mm) Crack width (mm) (c) 7 (d) a/d = 2.25 fc = 25 MPa a/d = 1.5 fc = 45 MPa Shear (kn) Shear (kn) No stirrups B7 B8 B9 1 B1 B Crack width (mm) Crack width (mm) Figure 1. Applied shear versus crack width for different specimens controlling the shear cracks and thereby improving the shear resisted by aggregate interlock Effect of Steel Type The influence of using HS steel stirrups can be evaluated by comparing test results of the control beam B to beam B6. The only difference between these beams was the type of the steel type used for stirrups. Using HS stirrups enhanced the shear capacity by 31%. After isolating the concrete contribution from the shear capacity of each beam, B6 had an increase in the steel contribution by 57% compared to that for B. The use of HS steel instead of conventional steel for stirrups changed the mode of failure from a shear-tension failure mode for B to a shear-compression failure mode for B Effect of Shear Reinforcement Ratio The influence of the shear reinforcement ratio was examined by changing the stirrup spacing from 25 to mm in addition to one beam without shear reinforcement for each a/d ratio. From Table 5, it is clear that the shear cracking load wasn t affected by the shear reinforcement ratio. However, the shear capacity increased by increasing the shear reinforcement ratio. After isolating the concrete contribution from the shear capacity, beams with stirrup spacing of mm experienced an increase in the steel contribution by 1 to 14% compared to other beams having stirrup spacing of 25 mm. The width of shear crack was also reduced by increasing the shear reinforcement ratio. In all beams reinforced by HS steel stirrups, the concrete contribution was almost the same as its value at the initiation of the first shear crack Effect of a/d Ratio From Table 5, it is clear that increasing a/d ratio, the shear capacity decreases. For the same shear reinforcement ratio, beams with a/d = 1., the steel contribution is insignificant compared to that predicted for other beams with larger a/d ratios. The crack width for beams dominated by the arch action mechanism is relatively smaller than those for beams dominated by the beam action mechanism. For all beams, the Advances in Structural Engineering Vol. 15 No

15 Bond Characteristics and Shear Behavior of Concrete Beams Reinforced with High-Strength Steel Reinforcement (a) Shear compontent (kn) B8 B9 Shear V s V c V s V c a/d = (b) Shear compontent (kn) B8 B9 Shear a/d = 1. Applied shear V c Applied shear (kn) Applied shear (kn) 7 (c) Shear compontent (kn) B B5 B6 Shear a/d = 1.5 Applied shear V c Control (B) Applied shear (kn) 7 Figure 11. Applied shear vs. shear components concrete contribution was almost the same as the shear value at the initiation of the first shear crack. Nevertheless, for beams with a/d = 1., the concrete contribution continued to increase after the initiation of the first shear crack then it begins to decrease near failure due to softening of the concrete in the diagonal concrete strut Effect of Concrete Strength From Table 5, it can be seen that beams with f c = 45 MPa had an increase in its shear capacity ranging from 24 to 28% compared to other identical beams with f c = 25 MPa. It should be noted that those beams with higher concrete compressive strength demonstrated higher stiffness compared to other beams with lower concrete strength at the same load level. Increasing the concrete compressive strength resulted in an increase in the stiffness of the diagonal concrete strut and enhanced the contribution of the arch action in the shear resisting mechanism. Furthermore, using higher concrete strength altered the mode of failure to shear-tension failure governed by rupture of the stirrups Code Predictions Predictions based on the current shear design code provisions were carried out to evaluate the applicability of these approaches in the design of concrete beams reinforced with HS steel stirrups. The codes investigated in the current study included ACI (8), AASHTO LRFD (5), CSA A (4), EC-2 (3), DIN (1) and BS 811 (1997). Most of the mentioned code provisions include additional considerations for the relatively deep concrete members that have shear span-to-depth ratios (a/d) less than 2.. These additions are taken into consideration in predicting the shear capacity of the tested beams with a/d ratio less than 2.. Predictions of the shear capacity of the tested beams were carried without using any material safety factor for both concrete and steel contributions. The results of code application are summarized in Table 6 with idealized yield strength of 83 MPa for HS steel. Most of the design codes included in the current study conservatively predict the shear capacity for HS steel stirrups with an associated yield stress of 83 MPa. 316 Advances in Structural Engineering Vol. 15 No

16 Tarek K. Hassan, Ahmed Mantawy, Judy Soliman, Ali Sherif and Sami H. Rizkalla Table 6. Code predictions Experimental shear capacity Test/ACI Test/AASHTO Test/CSA Test/EC-2 Test/DIN Test/BS- Specimen (kn) (%) (%) (%) (%) (%) 811(%) B B B B B B B B B B B B Average % The analysis shows that the high yield strength cannot be fully utilized under the umbrella of the current design codes. 4. CONCLUSIONS Based on the findings of the current study, the following conclusions can be drawn: (1) Members reinforced with high strength steel without confining transverse reinforcement exhibit brittle failure. Using transverse reinforcement, in the form of stirrups, increases the flexural carrying capacity and ductility of flexural members. (2) Test results indicate that using a splice length of 3d b, stresses up to 85 MPa and 63 MPa can be achieved for bar sizes 13 mm. and 19 mm without confining transverse reinforcement, respectively. Using transverse reinforcement at the splice region allowed the high strength rebars to develop its yield strength. (3) The current expression by the ACI 48R-3 can be used to estimate the stresses in the longitudinal high strength bars using a reduction factor of.92. (4) The use of HS steel as shear reinforcement for concrete beams did not affect the shear cracking capacity (initiation of the first diagonal crack). The angle of shear cracks in concrete beams reinforced with HS steel stirrups varied between 44 to 47 degrees, which are typical values for concrete beams reinforced with conventional stirrups. (5) Direct replacement of the conventional steel stirrups by a similar amount of HS steel stirrups increases the shear capacity of concrete beams, increases their stiffness and reduces the shear crack width due to the better bond characteristics of the HS steel. (6) HS steel stirrups allowed the concrete contribution to remain unchanged at its initial value at the initiation of the first shear crack, up to failure. (7) Increasing the shear reinforcement ratio of HS steel stirrups increases the shear capacity of concrete beams and enhances their stiffness. (8) The use of HS steel stirrups as a shear reinforcement for concrete beams becomes more effective for shear span-to-depth ratio greater than 1.5. (9) The use of high concrete compressive strength for beams reinforced with HS steel stirrups increases the shear capacity of the beams, improves their stiffness, and decreases the shear crack width. It also changed the mode of failure for the beams from shear-compression failure mode to shear-tension-failure mode. (1) Most of the design codes included in the current study conservatively predict the shear capacity for HS steel stirrups. The analysis shows that the high yield strength cannot be fully utilized under the umbrella of the current design codes. ACKNOWLEDGMENTS The authors would like to thank MMFX Technologies Corporation, CA, USA for the donation of the materials used in this study. The authors are grateful to NSF for its financial support for this research project. Special thanks are extended to the technicians at the Structural Laboratory at the Faculty of Engineering at Ain Shams University. Advances in Structural Engineering Vol. 15 No

17 Bond Characteristics and Shear Behavior of Concrete Beams Reinforced with High-Strength Steel Reinforcement REFERENCES ACI Committee 48 (3). Bond and Development of Straight Reinforcing Bars in Tension (ACI48R-3), American Concrete Institute, Farmington Hills, Michigan, USA. Orangin, C.O., Jirsa, J.O. and Breen, J.E. (1977). A reevaluation of test data on development length and splices, ACI Journal Proceedings, Vol. 74, No. 3, pp Darwin, D., Tholen, M.L., Idun, E.K. and Zuo, J. (1996). Splice strength of high relative rib area reinforcing bars, ACI Structural Journal, Vol. 93, No. 1, pp Esfahani, M.R. and Rangan, B.V. (1998). Local bond strength of reinforcing bars in normal strength and high-strength concrete (HSC), ACI Structural Journal, Vol. 95, No. 2, pp Zuo, J. and Darwin, D. (). Splice strength of conventional and high relative rib area bars in normal and high-strength concrete, ACI Structural Journal, Vol. 97, No. 4, pp EL-Hacha, R. and Rizkalla, S.H. (2). Fundamental Material Properties of MMFX Steel Rebars, Report No. 2 4, Constructed Facilities Laboratory (NCL), North Carolina State University (NCSU), USA. Hassan, T., Seliem, H., Dwairi, H., Rizkalla, S. and Zia, P. (8). Shear behavior of large concrete beams reinforced with highstrength steel, ACI Structural Journal, Vol. 15, No. 2, pp Hosny, A. (7). Bond Behavior of High Performance Reinforcing Bars for Concrete Structures, Master Thesis, North Carolina State University, Raleigh, USA. ACI Committee 318 (8). Building Code Requirements for Structural Concrete (ACI 318-8) and Commentary (318R-8), American Concrete Institute, Farmington Hills, MI, USA. AASHTO LRFD (5). Bridge Design Specifications and Commentary, 3 rd Edition, American Association of State and Highway Transportation Officials, Washington, DC, USA. CSA Committee A23.3 (4). Design of Concrete Structures, Canadian Standards Association, Rexdale, Ontario, Canada. Eurocode 2 (3). Design of Concrete Structures, Part 1: General Rules and Rules for Buildings, European Committee for Standardization, Brussels, Belgium. DIN (1). Deutsche Norm: Tragwerke aus Beton, Stahlbeton und Spannbeton Teil 1: Bemessung und Konstruktion. S. (Concrete, reinforced and prestressed concrete structures Part 1: Design), Normenausschuss Bauwesen (NABau) im DIN Deutsches Institut für Normung e.v. Beuth Verl, Berlin, German. BS 811 (1997). Structural Use of Concrete, Part 1, British Standard Institute, London, UK. NOTATION a shear span A b area of bar being developed or spliced A tr area of each stirrup or tie crossing the potential plane of splitting adjacent to the reinforcement being developed, spliced, or anchored. A v area of one leg of stirrups crossing the shear crack b w width of the web of a beam c cover dimension = c min + d b /2 c b bottom concrete cover for reinforcing bar being developed or spliced c max maximum (c b, c s ) c med median (c so, c b, c si + d b /2) c min minimum (c so, c b, c si + d b /2) c s minimum [c so, c si mm] c si 1/2 of the bar clear spacing c so side concrete cover for reinforcing bar d distance from extreme compression fiber to centroid of longitudinal tension reinforcement d b diameter of bar E s modulus of elasticity of steel reinforcement f_ c specified compressive strength of concrete f sm measured stress in reinforcing bar f sp predicted stress in reinforcing bar f y yield strength of steel being developed or spliced f yt yield strength of transverse reinforcement l d development or splice length M constant used in expressions for the bond strength of bars not confined by transverse reinforcement M cosh (.22ld ) (Esfahani and Rangan 1998) n number of bars being developed or spliced N the number of transverse stirrups, or ties, within the development or splice length R r relative rib area of the reinforcement s spacing of transverse reinforcement t d.28db +.28 (Darwin et al.1996) t d.3db +.22 (Zuo and Darwin ) t r 9.6R r +.28 (Darwin et al. 1996; Zuo and Darwin ) V c nominal shear strength provided by concrete V cr shear force at the initiation of the first shear crack V s nominal shear strength provided by shear reinforcement ε v measured strain in stirrups crossing the shear crack shear reinforcement ratio ρ v 318 Advances in Structural Engineering Vol. 15 No