bond characteristics and shear behavior of concrete beams

Bond Characteristics and Shear Behavior ofConcrete Beams Reinforced with High-Strength

Steel Reinforcement

by

Tarek K. Hassan, Ahmed Mantawy, Judy Soliman, Ali Sherifand Sami H. Rizkalla

Reprinted from

Advances in Structural EngineeringVolume 15 No. 2 2012

MULTI-SCIENCE PUBLISHING CO. LTD.5 Wates Way, Brentwood, Essex CM15 9TB, United Kingdom

1. INTRODUCTIONBond between concrete and reinforcing steel is requiredto transfer the forces between the two materials andtherefore, it significantly influences the behavior ofreinforced concrete structures (ACI 408 2003). Previousresearch (Orangin et al. 1977; Darwin et al. 1996;Esfahani and Rangan 1998; Zuo and Darwin 2000)provided basic understanding of the bond characteristicsbetween concrete and the conventional steel.Advancement in material science has led to theproduction of Micro Composite Multi-StructuralFormable Steel (MMFX steel), which is innovative newhigh-strength steel reinforcement. The reinforcement ischaracterized by higher tensile strength compared toconventional mild steel reinforcement. The stress-strain

Advances in Structural Engineering Vol. 15 No. 2 2012 303

Bond Characteristics and Shear Behavior of

Concrete Beams Reinforced with High-Strength

Steel Reinforcement

Tarek K. Hassan1,*, Ahmed Mantawy1, Judy Soliman1, Ali Sherif1

and Sami H. Rizkalla2

1Department of Structural Engineering, Faculty of Engineering, Ain Shams University, Cairo, Egypt2Department of Civil, Construction and Environmental Engineering, NCSU, Raleigh, NC 27695, USA

(Received: 18 January 2011; Received revised form: 22 June 2011; Accepted: 27 June 2011)

Abstract: This paper evaluates the bond behavior of high strength (HS), steelreinforcing bars and highlights the effect of various key parameters believed to affectthe bond characteristics. Nine reinforced concrete spliced beams were constructed andtested. The beams had different splice lengths and levels of confinements. Theapplicability of different hypotheses for development of conventional steel bars wasexamined for the HS bars. The study is extended to examine the behavior of thereinforcing bars as shear reinforcement for concrete beams by testing twelve concretebeams reinforced with HS steel stirrups under static loading conditions. The mainvariables in the study included steel type, concrete compressive strength, webreinforcement ratio and shear span-to-depth ratio. The applicability of various buildingcodes and standards for concrete beams with HS shear reinforcement was alsoevaluated.

Key words: beams, bond, concrete, development length, high strength, shear, stirrups.

behavior of the material has no a well-defined yieldplateau (EL-Hacha and Rizkalla 2002). Comparing thebehavior of these bars to those of conventional steelGrade 60, a significant improvement in strength andcorrosion resistance can be clearly demonstrated. Usingthese bars in concrete structures and bridges allowsreduction of the reinforcement requirements and henceleads to a more economical design for any particularproject. Due to the difference in the mechanical propertiesof these bars to the conventional steel, the bond behaviorbetween the high strength steel and the concrete must beinvestigated. Since the applicability of the bond equationsproposed by various researchers (Orangin et al. 1977;Darwin et al. 1996; Esfahani and Rangan 1998; Zuo andDarwin 2000) including the current ACI Committee

*Corresponding author. Email address: [email protected]; Tel: +2010-2188-906.Associate Editor: J.G. Dai.

Report for Bond1 are limited to conventional steel, thereis a need to examine the bond characteristics of the newhigh strength steel reinforcement. In most applications,the HS steel bars have been used by direct replacement ofthe amount required for conventional steel (Grade 60)and, thus, neglecting the benefits of the high yieldstrength of the new material. Lack of informationregarding the behavior of concrete members reinforcedwith this type of material prevents design engineers fromutilizing the full strength of the material.

The first phase of the experimental program in thispaper presents test results of 9 reinforced concretespliced beams. The beams had different splice lengths,levels of confinements and were tested under four pointbending setup to provide a constant moment region overthe splice zone. The second phase of the experimentalprogram presents test results of 12 medium-scalereinforced concrete beams tested up to failure toinvestigate the contribution of the HS steel stirrups inthe shear resisting mechanism compared toconventional steel stirrups. The key parametersconsidered were the steel type, amount of shearreinforcement, concrete strength and shear span-to-depth ratio (a/d).

2. BOND BEHAVIOR OF CONCRETESPLICED BEAMS

The first phase of the experimental program consisted ofnine reinforced concrete spliced beams divided into threemain groups according to the diameter of the longitudinalreinforcing 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 specimenswere designed using cracked section analysis. The splicelength was selected to ensure failure due to bond for allthe tested specimens. All tested beams were 4000 mmlong and have cross-sectional dimensions of 250 mmwide by 400 mm deep. The side and bottom concrete

covers were 40 mm. HS steel bars were used as the maintensile reinforcement while conventional steel reinforcingbars were used in the compression zone to provide amechanism to hold the beam together after rupture of thesplice. Each of the three main groups consists of threebeams with three different confinement levels along thesplice length. Within each group, the first beam wastested without transverse reinforcement and used as acontrol specimen. The transverse reinforcement of thesecond and the third beams within each group consistedof 10 mm diameter conventional steel stirrups spaced at200 mm and 100 mm center-to-center, respectively.Stirrups were also added outside the splice zone atspacing of 200 mm to prevent premature shear failure.Figures 1 and 2 show elevation and cross sections of atypical test beam. The measured cylindrical concretecompressive strength at 28 days was 65 MPa.

304 Advances in Structural Engineering Vol. 15 No. 2 2012

Bond Characteristics and Shear Behavior of Concrete Beams Reinforced with High-Strength Steel Reinforcement

3.700.15 0.15

4.00

Stirrups 10 mm diameter@200 mm

Stirrups 10 mm diameter@variable distances

according to beam type

Splice length

Figure 1. Typical elevation of bond specimens

c

c

c

Dimensions are given in meters

Unconfined beams Confined beams

0.40

0.25

Stirrups 10 mm diameter@variable 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 360 N/A 0 128Group 1 B2 13 (30φ) 200 mm 2 150

B3 100 mm 4 169B4 220 N/A 0 108

Group 2 B5 13 (18φ) 200 mm 1 118B6 100 mm 2 152B7 570 N/A 0 226

Group 3 B8 19 (30φ) 200 mm 3 275B9 100 mm 6 324

2.1. Material Properties

2.1.1. High strength reinforcing steel

The high-strength steel used in the current study wasprovided by MMFX Technologies Corp., CA, USA.The mechanical characteristics of the material comparedto conventional steel bars are shown in Figure 3 basedon test results conducted at NC State University (Hassanet al. 2008). The material exhibits an initial linear elasticportion and there is no observation of yielding plateau orstrain hardening. The yield strength corresponds to0.2% strain offset is 830 MPa. The following equationsare proposed for the stress-strain behaviour of highstrength steel based on extensive testing conducted atNorth Carolina State University (Hosny 2007).

fs = 1220 (1−e–185εMMFX) MPa (1)

fs = 1096 (1−e–248εMMFX) MPa (2)

2.2. Test Setup

All beams were tested using four point loadingconfiguration to develop a constant moment region of1600 mm for the spliced bars location. The beams weresupported on a roller support at one end and a hingesupport at the other. A hydraulic jack of 400 kNcapacity was used to apply the load on the top of a rigidsteel beam that equally distributes the load at both loadpoints. A total of four electrical resistance strain gagesand four LVDTs were used to monitor the strains andthe deflections of the beams during testing. Theelectrical resistance strain gages were attached to the longitudinal reinforcing bars immediately outside

for 19 mm

diameter rebars

for 13 mm

diameter rebarsthe splice length to measure the maximum strains in the spliced bars. The mid-span deflection was monitoredusing two LVDTs as shown in Figure 4.

2.3. Crack Pattern

Crack width was measured at different load levels usingcrack comparators. It was observed that the initiation ofthe first flexural cracks occur at the two ends of thesplice zone and near the location of the applied load forall tested beams where the maximum moment and shearare combined. Flexural cracks propagated upwards andincreased in number associated with an increase in thecrack width as the load was increased. Further increasein the load led to the formation of splitting cracks thatwere parallel to the longitudinal spliced bars initially onthe bottom surface of the beams followed by splittingcracks on the side of the beam close to failure. Thebeams without confining transverse reinforcementfailed immediately after initiation of the splitting crackswhile for the beams with confining transversereinforcement, propagation of the splitting cracks wasobserved over the splice zone before failure occurred.

2.4. Load-Deflection Behavior

The load-deflection behavior of the test specimens isshown in Figure 5 for the three groups. Test resultsindicated that the pre-and-post-cracking stiffness wererelatively similar for each group of beams regardless ofthe confinement level. The beams without confiningtransverse reinforcement failed due to splitting and lossof the concrete cover over the length of the spliceshortly after the initiation of the splitting cracks. Whenconfining transverse reinforcement was added withinthe splice zone, the beams were capable of carryingmore loads and the failure was more ductile andassociated with high mid-span deflections prior tofailure. For the first group of beams, the maximum


Tarek K. Hassan, Ahmed Mantawy, Judy Soliman, Ali Sherif and Sami H. Rizkalla

0

200

400

600

800

1000

1200

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18Strain (mm/mm), (in./in.)

0

29

58

87

116

145

174

Str

ess

(ksi

)Conventional steel

High-strength steel

830 MPa

0.002

Str

ess

(MP

a)

Figure 3. Typical stress-strain behavior for conventional and HS

steel reinforcement (Hassan et al. 2008)

Applied load

Hydraulicjack

Rigid beam

Applied load

Support reaction

Support reaction

Testzone

Figure 4. Test setup for bond specimens

measured steel stress at splitting failure for B1 (withouttransverse reinforcement at the splice zone) was 855 MPa.Using confining transverse reinforcement for B2 andB3, the measured stresses at failure were 27% and 42%

higher than that measured for B1, respectively. Thisbehavior was typical for the second and third group ofbeams.

2.5. Mode of Failure

Failure of the spliced beams was almost identical. Allbeams without confining transverse reinforcementalong the splice length failed suddenly after theinitiation of the splitting cracks without warning orpropagation of the cracks accompanied by loss of theconcrete cover. Failure of the beams with confinementreinforcement was also due to splitting of the concretecover over the splice length. The splitting failure ofthese beams was more ductile and allowed propagationof the splitting cracks prior to failure as shown inFigure 6.

2.6. Stresses in the Spliced Bars

The stresses in the spliced bars were evaluated based onthe measured strains from the strain gages attached tothe longitudinal bars located immediately outside thesplice zone. The measured strains were used tocalculate the stresses in the spliced bars based on thegiven stress-strain relationships in Eqns 1 and 2.Furthermore, using crack section analysis, the stressesin the MMFX corresponding to the measured ultimateload were determined for each tested beam. Thestresses in the MMFX steel before failure given inTable 2, indicate that adding confining transversereinforcement increases the measured stresses in thelongitudinal bars and thus increasing the ultimate loadcarrying capacity.

2.7. Analysis of Test Results

Test results of the concrete beams without confiningtransverse reinforcement were used to examine therelationships proposed by previous researchers



0

50

100

150

200

250

300

350

0 5 10 15 20 25 30 35 40 45 50 55 60 65

Group 1

Group 2

Group 3

Load

(kN

)

Mid-span deflection (mm)

0

50

100

150

200

250

300

350

0 5 10 15 20 25 30 35 40 45 50 55 60 65

Load

(kN

)


0

50

100

150

200

250

300

350

0 5 10 15 20 25 30 35 40 45 50 55 60 65

Load

(kN

)


B1B2B3

B4B5B6

B7B8B9

Figure 5. Load-deflection behaviour of bond specimens

Splice length

B7

Figure 6. Typical splitting failure for bond specimens

(Orangin et al. 1977; Darwin et al. 1996; Esfahani andRangan 1998; Zuo and Darwin 2000) for conventionalsteel. It should be noted that these empirical modelsdeveloped by various researchers (Orangin et al. 1977;Darwin et al. 1996; Esfahani and Rangan 1998; Zuoand Darwin 2000) were originally developed forconventional steel reinforcement. The dimensions andmaterial characteristics of the beams tested at AinShams University and at N.C. State University (Hosny2007) used in the current study are given in Table 3.For a detailed list of symbols used, refer to the ACI408R-03 (2003). The predicted stresses for differentbeams using various descriptive equations proposed byresearchers for conventional steel are given in Table 4.It should be noted that the current expression proposedby the ACI Committee 408 (2003) provided anacceptable level of accuracy when applied to highstrength steel reinforcement. The analysis showed thatthe proposed model by Esfahani and Rangan (1998)did not account for the confinement effect provided bythe transverse reinforcement and hence yielded thehighest standard deviation for the ratio of the measuredto predicted stress at the onset of splitting failure.Furthermore, the expression proposed by Orangunet al. (1977) and that proposed by Zuo and Darwin(2000), overestimates the stresses in the HSlongitudinal bars.

3. SHEAR BEHAVIOR OF CONCRETEBEAMS WITH TRANSVERSE HS STEELREINFORCEMENT

The second phase of the current study included testingtwelve medium scale concrete beams at the StructuralLaboratory of Ain Shams University. Eight beams werereinforced with HS steel stirrups; three beams had nostirrups and a control beam with conventional steelstirrups. Beams without shear reinforcement were usedto evaluate the concrete contributions at different shear

span-to-depth (a/d) ratios. All beams were T-sectionwith a total height of 500 mm, a flange of 600 mm wideand 80 mm thick. The thickness of the web was 200 mm.The length of all beams is 3.0 m. The span of the beamwas divided into two simply supported spans of 1.35 meach except for B07, B08 and B09, which had a 2.70 mspan as shown in Figure 7. All beams had identicalbottom longitudinal reinforcement consisted of six 25 mm diameter conventional bars. The beams weredesigned to achieve a brittle shear failure prior toreaching the ultimate flexural capacity. Anchorage ofthe flexural reinforcement was provided using U-shapedbars to prevent any possible slippage. Details of thetested specimens as well as the concrete compressivestrength of different specimens are given in Table 5. Tomonitor the behavior of the tested beams under theapplied loading, the instrumentation included LVDTsfor deflections, electrical resistance strain gauges forsteel strains, and displacement gauges (Demic points)for concrete strains on the concrete surface. Demic-points were mounted on the web surface of each beamin three directions (rosette shape) and at differentlocations to evaluate the shear deformations in terms ofthe shear crack width and the concrete strain. Eachrosette consisted of three demic lines of a 200 mmgauge length. One demic line was placed horizontal.The other two demic lines were placed vertically anddiagonally at a 45° angle, respectively as shown inFigure 7.

3.1. Test Setup

All beams were tested under three point bendingconfiguration according to its (a/d) ratio. For beams of(a/d) ratio of 1.00 and 1.50, the span of the beam was1.35 m and the remaining portion of the beam wascantilevered, and therefore, unstressed so that thebeam can be tested again at the far end. The load wasapplied using 1500 kN capacity hydraulic jack. The



Table 2. Measured strains and the corresponding stresses in the spliced bars

Stresses in spliced bars

Measured Strain gages Cracked section

db ls/db Confinement strain readings analysis

Group Beam ID (mm) ratio level (%) (MPa) (MPa)

B1 C0 0.65 855 848Group 1 B2 13 30 C1 1.18 1083 1145

B3 C2 2.61 1211 1193B4 C0 0.42 654 731

Group 2 B5 13 18 C1 0.74 908 758B6 C2 0.85 969 972B7 C0 0.34 628 717

Group 3 B8 19 30 C1 0.50 779 834B9 C2 0.76 931 1000



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of 150 mm from the right end of the beam, while theother was positioned at a specific distance from theleft end according to the span of the beam. The beamswere monotonically loaded up to failure usingdisplacement control. A summary of the test results ispresented in Table 5 including shear cracking load,shear capacity, angle of shear crack and mode offailure. All the tested beams failed in shear beforeflexural capacity is reached. No slip of the flexuralreinforcement was observed during any of the beamtests.

3.2. Deflections

The load-deflection plots for all beams are shown inFigure 8 according to its a/d ratio and concretecompressive strength. For beams B01, B02 and B03with a/d = 1.00, no significant increase in the shearcapacity was observed for beams with shearreinforcement (B02 and B03) compared to the beamwithout shear reinforcement B01. This is attributed tothe small a/d ratio of those beams, which makes thearch action behavior dominant. A significant increasein the shear capacity for beams with HS shearreinforcement (B05 and B06) compared to the beamwithout shear reinforcement (B01) and the controlbeam (B00). The same behavior was also observed forbeams with a/d ratio of 2.25. For beams B10 and B11having a/d = 1.50 but with fc

′ = 45 MPa, the plots arevery close up to failure, where B11 provides highershear capacity than B10. It is clear that for different(a/d) ratios, adding web reinforcement to the concretebeams increased both stiffness and shear capacitysignificantly except for beams with a/d = 1.00 wherethe increase in the shear capacity is not significantbecause the shear resisting mechanism is a pure archaction, which is almost independent on the presence ofshear reinforcement. Also, increasing the shearreinforcement ratio slightly increases the shearcapacity and decreases the deflection at failure withoutsignificant increase in the stiffness. The control beamB00 shows that using HS steel stirrups for beamsimproved the behavior and provided significantincrease in both shear capacity and stiffness thanconventional steel stirrups.

3.3. Crack Pattern

As shown in Figure 9, it was observed that shearcracks initiated with a steep angle at the tension sideof the beam and approached the compression flange ata relatively flat angle. The angle of shear crack wasdetermined as the angle at which the shear crackintersects the mid-height of the web. The angle of the



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shear cracks in the beams without shear reinforcementranged between 39 and 41 degrees. This relatively lowrange for shear crack angles is due to the absence ofweb reinforcement where only longitudinalreinforcement exists. The angle of the shear cracksfor beams with shear reinforcement ranged between43 and 47 degrees which is typical for reinforcedconcrete beams. It was also observed that, byincreasing the shear reinforcement ratio, the crackangle increases. For all beams, shear crackingoccurred before or at the same time as the flexuralcracking except for beams of a/d = 2.25. Beams B01,B02 and B03 with a/d = 1.00, almost have the samecrack pattern at shear cracking load. The crack patternconsisted of one major shear crack from the support tothe load, followed by another shear crack appearednearly parallel to the major one with other minorflexural cracks. These two shear cracks can beconsidered to identify the concrete strut goingdiagonally from the support to the load and the shearresisting mechanism in this case is “Arch Action”.Also, for beam B04, which had no shearreinforcement, crack pattern is similar to beams witha/d = 1.00. For beams B05 and B06, one major shearcrack developed from the support to the load.Increasing the applied shear resulted in developmentof small flexural cracks near the loading area at thebottom of each beam. The shear transfer mechanismfor both beams can be identified as a combinationbetween the arch action and the contribution of shearreinforcement in a beam action. For beam B07, whichhas no shear reinforcement, only one major shearcrack appears. Increasing the applied shear, the crackwidens rapidly causing failure of the beam. For beamsB08 and B09, one major shear crack appeared at themiddle of the tested shear span with some flexuralcracks. Some of the existing flexural cracks changedits direction to become flexural-shear cracks. Thecrack patterns for both beams became more irregularat higher load levels. The shear transfer mechanismfor both beams can be identified as a typical beamaction. The crack patterns of B10 and B11 aresomehow similar to the crack patterns for B02 andB03. The shear transfer mechanism for both B10 andB11 also can be identified as a combination betweenthe arch action and beam action but with moreparticipation of the arch action than that for beamsB05 and B06 having lower concrete strength. Thismay be attributed to the high concrete compressivestrength of these beams, which increased both strengthand stiffness of the diagonal concrete strut in the archaction and lead the shear mechanism to approachtowards the pure arch action.

3.4. Crack Width

Total deformations within the strain rosetteconfiguration were used to determine the width of theshear cracks. For a typical strain rosette at a specificlocation and considering diagonal and vertical

measurements, the summation of shear crack widths canbe determined. Each beam had one or two strain rosettesin the shear span according to its a/d ratio. The width ofthe shear crack was determined based on the observednumber of shear cracks passing within each rosette. The



aP

a

T1

L

S Cantilever end

200

6∅25

6∅12

600

d =

500

80

∅10@spacing (s)

∅8@spacing (s)

LVDT(1) LVDT(2)Span

LVDT(3)

P

200

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 fc′′ Span Spacing vfv capacity cracking crack Ioad Ptest capacity of

Specimen type (MPa) ((mm)) a/d (mm) (MPa) Ver (kN) Vm (kN) (deg) (kN) Vtest (kN) failure

B00 Conventional 1.50 200 1.57 171.00 171.00 43 741.00 370.50 SYB01 — — 0.00 266.00 266.00 41 798.00 532.00 DCB02 High-Strength 1.00 250 2.51 228.00 266.00 46 855.00 570.00 DC

& Corrosive-B03 Resistant 1.35 200 3.14 228.00 304.00 46 912.00 608.00 DCB04 — — 0.00 199.50 199.50 39 627.00 313.50 SC

25B05 High-Strength 1.50 250 2.51 171.00 199.50 45 912.00 456.00 SR

& Corrosive-B06 Resistant 200 3.14 171.00 228.00 46 969.00 484.50 SCB07 — — 0.00 150.00 100.00 39 260.00 162.50 STB08 2.70 2.25 250 2.51 142.50 106.88 44 627.00 391.88 SCB09 High-Strength 200 3.14 142.50 71.25 44 684.00 427.50 SC

& Corrosive-B10 Resistant 250 2.51 199.50 199.50 45 1132.00 566.00 SR

45 1.35 1.50B11 200 3.14 199.50 228.00 47 1236.00 618.00 SR

DC: Shear failure initiated by crushing of diagonal concrete strutSC: Shear compression failureSR: Shear failure initiated by rupture of steel stirrupsST: Shear failure initiated by diagonal tension of concreteSY: Shear failure initiated by yielding of steel stirrups

ρρ

applied shear force versus the width of shear crack forall specimens is shown in Figure 10. It is clear thatincreasing the amount of shear reinforcement, reducesthe shear crack width considerably. Narrower crackswere observed using high strength stirrups compared toconventional stirrups at the same load level. This maybe attributed to the better bond characteristics of thehigh-strength steel than that for the conventional steel.

3.5. Failure Modes

All the tested beams failed in shear. In general, theobserved mode of failure was either shear-tension orshear-compression failure. Control beam (B00) failedin a shear-tension mode due to the yielding of thestirrups that led to the development of a major shearcrack. Beams B01, B02 and B03 having a/d = 1.00,failed in a shear-compression mode due to the crushingof the diagonal concrete strut in the arch actionmechanism. Beams B04, B05 and B06 having a/d = 1.50, failed in a shear-compression mode due tothe crushing of the concrete compressive zone underthe applied load, except beam B05 where the mode of

failure was a shear-tension failure due to the rupture ofthe HS steel stirrups. Beams B07, B08 and B09 havinga/d = 2.25, failed in a shear-compression mode due to the crushing of the concrete compressive zone underthe applied load, except beam B07 that has no shearreinforcement, which failed in direct diagonal tensionof the concrete web. Shear tension failure was alsoobserved for beams B10 and B11 having a/d = 1.50 butwith fc

′ = 45 MPa. The shear-tension failure due to therupture of the HS steel stirrups is attributed to thelimited ductility of the HS steel relative to conventionalsteel. For higher concrete compressive strength, thistype of failure is attributed to the redistribution of aportion of the high shear carried by the arch actionmechanism when the concrete strut softens near failurecausing higher stresses in the stirrups that lead torupture. Great attention should be paid to thedetermination of the minimum amount of shearreinforcement for the HS steel to account for itsductility as well as the effect of high concretecompressive strengths to avoid this brittle type offailure.



700

600

500

400

She

ar (

kN)

300

200

100

00.00 2.00 4.00

@200

@250

No stirrups

6.00 8.00Deflection (mm)

10.00 12.00 14.00 16.00

B01B02B03

700

600

500

400

She

ar (

kN)

300

200

100

00.00 2.00 4.00

@200@250

No stirrupsControl


10.00 12.00 14.00 16.00

B00B04B05B06

700

600

500

400

She

ar (

kN)

300

200

100

00.00 2.00 4.00

@200

@250

No stirrups


10.00 12.00 14.00 16.00

B07B08B09

700

(a) (b)

(c) (d)

600

500

400

She

ar (

kN)

300

200

100

00.00 2.00 4.00

@200

@250


10.00 12.00 14.00 16.00

B10B11

a/d = 1.00fc′ = 25 MPa

a/d = 1.50fc′ = 25 MPa

a/d = 2.25fc′ = 25 MPa

a/d = 1.50fc′ = 45 MPa

Figure 8. Shear deflection relationships for test specimens

3.6. Stirrup Contribution

The stirrup contribution, Vs, can be determined basedon the measured stirrup strain and the mechanical andgeometric properties of the stirrups using the conceptof smeared reinforcement in concrete beams asfollows:

(3)

where Av is the area of one leg of stirrups, bw is the webwidth, s is the spacing between stirrups, εv is themeasured strain in stirrups crossing the shear crack, Es

is the modulus of elasticity of the shear reinforcementand d is the distance from extreme compression fiber tocentroid of longitudinal tension reinforcement. Therelationships between the applied shear and thecomponents of the shear resisting mechanism Vc and Vs

are presented in Figure 11. It can be seen that the

VsA

b sE b dv

wv s w= ε

concrete contribution component, Vc, at any load levelwas almost the same as the shear force at the initiationof the first shear crack, Vcr, except for beams with a/d = 1.00 where the concrete contribution increased byincreasing the applied shear. This is attributed to thepure arch action mechanism for those beams where the diagonal concrete strut is the main component of the mechanism with the longitudinal reinforcementacting as a tie. In addition, the concrete contributioncomponent, Vc, at failure decreased due to softening ofconcrete strut. It can also be seen that the contribution ofshear reinforcement is higher for beams with a/d = 1.50and 2.25 than that for beams of with a/d = 1.00.Therefore, it can be concluded that stirrups in concretebeams contribute to the shear carrying capacity in twodifferent ways:

(1) Creating the stirrup contribution component, Vs;(2) Maintaining the concrete contribution

component, Vc, almost constant up to failure by



B102/12/2006

B402/12/2006

B725/11/2006

B1004/12/2006 B11

04/12/2006 B005/12/2006

B823/11/2006

B926/11/2006

B506/12/2006

B206/12/2006

B303/12/2006

B603/12/2006

Figure 9. Crack patterns at failure

controlling the shear cracks and therebyimproving the shear resisted by aggregateinterlock.

3.7. Effect of Steel Type

The influence of using HS steel stirrups can beevaluated by comparing test results of the control beamB00 to beam B06. The only difference between thesebeams 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 shearcapacity of each beam, B06 had an increase in the steelcontribution by 57% compared to that for B00. The useof HS steel instead of conventional steel for stirrupschanged the mode of failure from a shear-tension failuremode for B00 to a shear-compression failure mode forB06.

3.8. Effect of Shear Reinforcement Ratio

The influence of the shear reinforcement ratio wasexamined by changing the stirrup spacing from 250 to200 mm in addition to one beam without shearreinforcement for each a/d ratio. From Table 5, it is

clear that the shear cracking load wasn’t affected bythe shear reinforcement ratio. However, the shearcapacity increased by increasing the shearreinforcement ratio. After isolating the concretecontribution from the shear capacity, beams withstirrup spacing of 200 mm experienced an increase inthe steel contribution by 10 to 14% compared to otherbeams having stirrup spacing of 250 mm. The width ofshear crack was also reduced by increasing the shearreinforcement ratio. In all beams reinforced by HSsteel stirrups, the concrete contribution was almost thesame as its value at the initiation of the first shearcrack.

3.9. Effect of a/d Ratio

From Table 5, it is clear that increasing a/d ratio, theshear capacity decreases. For the same shearreinforcement ratio, beams with a/d = 1.00, the steelcontribution is insignificant compared to that predictedfor other beams with larger a/d ratios. The crack widthfor beams dominated by the arch action mechanism isrelatively smaller than those for beams dominated bythe beam action mechanism. For all beams, the



700

600

500

400

She

ar (

kN)

300

200

100

00.00 0.50 1.00

@200

@250No stirrups

Crack width (mm)2.502.001.50 3.00

B01B02B03

(a)

a/d = 1.00fc′ = 25 MPa

700

600

500

400

She

ar (

kN)

300

200

100

00.00 0.50 1.00

@200@250

No stirrups

Crack width (mm)2.502.001.50 3.00

B07B08B09

(c)

(b)

(d)

a/d = 2.25fc′ = 25 MPa

700

600

500

400

She

ar (

kN)

300

200

100

00.00 0.50 1.00

@200@250

Control

No stirrups

Crack width (mm)2.502.001.50 3.00

B00B04B05B06

a/d = 1.00fc′ = 25 MPa

700

600

500

400

She

ar (

kN)

300

200

100

00.00 0.50 1.00

@200

@250

Crack width (mm)2.502.001.50 3.00

B10B11

a/d = 1.50fc′ = 45 MPa

Figure 10. Applied shear versus crack width for different specimens

concrete contribution was almost the same as the shearvalue at the initiation of the first shear crack.Nevertheless, for beams with a/d = 1.00, the concretecontribution continued to increase after the initiationof the first shear crack then it begins to decrease nearfailure due to softening of the concrete in the diagonalconcrete strut.

3.10. Effect of Concrete Strength

From Table 5, it can be seen that beams with fc′ = 45 MPahad an increase in its shear capacity ranging from 24 to28% compared to other identical beams withfc′ = 25 MPa. It should be noted that those beams withhigher concrete compressive strength demonstratedhigher stiffness compared to other beams with lowerconcrete strength at the same load level. Increasing theconcrete compressive strength resulted in an increase inthe stiffness of the diagonal concrete strut and enhancedthe contribution of the arch action in the shear resistingmechanism. Furthermore, using higher concretestrength altered the mode of failure to shear-tensionfailure governed by rupture of the stirrups.

3.11. Code Predictions

Predictions based on the current shear design codeprovisions were carried out to evaluate theapplicability of these approaches in the design ofconcrete beams reinforced with HS steel stirrups. Thecodes investigated in the current study included ACI318-08 (2008), AASHTO LRFD (2005), CSA A23.3-04(2004), EC-2 (2003), DIN (2001) and BS 8110 (1997).Most of the mentioned code provisions includeadditional considerations for the relatively deepconcrete members that have shear span-to-depth ratios(a/d) less than 2.00. These additions are taken intoconsideration in predicting the shear capacity of thetested beams with a/d ratio less than 2.00. Predictionsof the shear capacity of the tested beams were carriedwithout using any material safety factor for bothconcrete and steel contributions. The results of codeapplication are summarized in Table 6 with idealizedyield strength of 830 MPa for HS steel. Most of thedesign codes included in the current studyconservatively predict the shear capacity for HS steelstirrups with an associated yield stress of 830 MPa.



700

600

500

400

She

ar c

ompo

nten

t (kN

)

300

200

100

00 100 200 300 400 500 600

@200

@250

Applied shear (kN)

Applied shear

Vs

Vc Vc

Vs

700

B08B09Shear

(a)

a/d = 2.25

700

600

500

400

She

ar c

ompo

nten

t (kN

)

300

200

100

00 100 200 300 400 500 600

@200

@250

Applied shear (kN)

Applied shear

Vc

Vs

700

B08B09Shear

(b)a/d = 1.00

700

600

500

400

She

ar c

ompo

nten

t (kN

)

300

200

100

00 100 200 300 400 500 600

@200@250

Control (B00)

Applied shear (kN)

Applied shear

Vc

Vs

700

B00B05B06Shear

(c)

a/d = 1.50

Figure 11. Applied shear vs. shear components

The analysis shows that the high yield strength cannotbe fully utilized under the umbrella of the currentdesign codes.

4. CONCLUSIONSBased on the findings of the current study, the followingconclusions can be drawn:

(1) Members reinforced with high strength steelwithout confining transverse reinforcementexhibit brittle failure. Using transversereinforcement, in the form of stirrups, increasesthe flexural carrying capacity and ductility offlexural members.

(2) Test results indicate that using a splice lengthof 30db, stresses up to 850 MPa and 630 MPacan be achieved for bar sizes 13 mm. and19 mm without confining transversereinforcement, respectively. Using transversereinforcement at the splice region allowed thehigh strength rebars to develop its yieldstrength.

(3) The current expression by the ACI 408R-03 canbe used to estimate the stresses in thelongitudinal high strength bars using a reductionfactor of 0.92.

(4) The use of HS steel as shear reinforcement forconcrete beams did not affect the shear crackingcapacity (initiation of the first diagonal crack).The angle of shear cracks in concrete beamsreinforced with HS steel stirrups varied between44 to 47 degrees, which are typical values forconcrete beams reinforced with conventionalstirrups.

(5) Direct replacement of the conventional steelstirrups by a similar amount of HS steel stirrups

increases the shear capacity of concrete beams,increases their stiffness and reduces the shearcrack width due to the better bondcharacteristics of the HS steel.

(6) HS steel stirrups allowed the concrete contributionto remain unchanged at its initial value at theinitiation of the first shear crack, up to failure.

(7) Increasing the shear reinforcement ratio of HSsteel stirrups increases the shear capacity ofconcrete beams and enhances their stiffness.

(8) The use of HS steel stirrups as a shearreinforcement for concrete beams becomesmore effective for shear span-to-depth ratiogreater than 1.50.

(9) The use of high concrete compressive strengthfor beams reinforced with HS steel stirrupsincreases the shear capacity of the beams,improves their stiffness, and decreases the shearcrack width. It also changed the mode of failurefor the beams from shear-compression failuremode to shear-tension-failure mode.

(10) Most of the design codes included in the currentstudy conservatively predict the shear capacityfor HS steel stirrups. The analysis shows thatthe high yield strength cannot be fully utilizedunder the umbrella of the current design codes.

ACKNOWLEDGMENTSThe authors would like to thank MMFX TechnologiesCorporation, CA, USA for the donation of the materialsused in this study. The authors are grateful to NSF for itsfinancial support for this research project. Specialthanks are extended to the technicians at the StructuralLaboratory at the Faculty of Engineering at Ain ShamsUniversity.



Table 6. Code predictions

Experimental

shear

capacity Test/ACI Test/AASHTO Test/CSA Test/EC-2 Test/DIN Test/BS-Specimen (kN) (%) (%) (%) (%) (%) 8110(%)

B00 370.50 101.17 110.12 110.12 77.90 65.69 80.43B01 532.00 70.46 82.78 82.78 36.65 30.55 41.54B02 570.00 65.76 87.72 87.72 65.45 46.05 63.16B03 608.00 61.65 83.88 83.88 68.68 50.49 59.21B04 313.50 96.83 90.29 90.29 41.36 34.47 46.88B05 456.00 82.20 87.38 87.38 87.00 77.08 78.95B06 484.50 77.37 84.21 84.21 95.66 86.33 74.30B07 162.50 47.08 71.02 70.79 60.00 50.00 68.00B08 391.88 80.10 99.34 101.59 73.70 93.46 88.77B09 427.50 87.31 109.31 112.19 84.45 102.15 84.21B10 566.00 88.85 124.76 124.76 75.36 65.53 73.02B11 618.00 81.38 116.05 116.05 79.83 70.82 72.82

Average % 78.08 99.08 99.72 78.77 73.99 74.30

REFERENCESACI Committee 408 (2003). Bond and Development of Straight

Reinforcing Bars in Tension (ACI408R-03), 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. 114–122.

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. 95–107.

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. 96–106.

Zuo, J. and Darwin, D. (2000). “Splice strength of conventional and

high relative rib area bars in normal and high-strength concrete”,

ACI Structural Journal, Vol. 97, No. 4, pp. 630–641.

EL-Hacha, R. and Rizkalla, S.H. (2002). Fundamental Material

Properties of MMFX Steel Rebars, Report No. 02–04,

Constructed Facilities Laboratory (NCL), North Carolina State

University (NCSU), USA.

Hassan, T., Seliem, H., Dwairi, H., Rizkalla, S. and Zia, P. (2008).

“Shear behavior of large concrete beams reinforced with high-

strength steel”, ACI Structural Journal, Vol. 105, No. 2,

pp. 173–179.

Hosny, A. (2007). Bond Behavior of High Performance Reinforcing

Bars for Concrete Structures, Master Thesis, North Carolina

State University, Raleigh, USA.

ACI Committee 318 (2008). Building Code Requirements for

Structural Concrete (ACI 318-08) and Commentary (318R-08),

American Concrete Institute, Farmington Hills, MI, USA.

AASHTO LRFD (2005). Bridge Design Specifications and

Commentary, 3rd Edition, American Association of State and

Highway Transportation Officials, Washington, DC, USA.

CSA Committee A23.3 (2004). Design of Concrete Structures,

Canadian Standards Association, Rexdale, Ontario, Canada.

Eurocode 2 (2003). Design of Concrete Structures, Part 1: General

Rules and Rules for Buildings, European Committee for

Standardization, Brussels, Belgium.

DIN 1045-1 (2001). 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 8110 (1997). Structural Use of Concrete, Part 1, British

Standard Institute, London, UK.

NOTATIONa shear spanAb area of bar being developed or spliced Atr area of each stirrup or tie crossing the potential

plane of splitting adjacent to the reinforcementbeing developed, spliced, or anchored.

Av area of one leg of stirrups crossing the shear crackbw width of the web of a beamc cover dimension = cmin + db/2cb bottom concrete cover for reinforcing bar being

developed or splicedcmax maximum (cb, cs)cmed median (cso, cb, csi + db/2) cmin minimum (cso, cb, csi + db/2)cs minimum [cso , csi + 6.35 mm]csi 1/2 of the bar clear spacingcso side concrete cover for reinforcing bard distance from extreme compression fiber to

centroid of longitudinal tension reinforcementdb diameter of barEs modulus of elasticity of steel reinforcementf_c specified compressive strength of concretefsM measured stress in reinforcing barfsP predicted stress in reinforcing barfy yield strength of steel being developed or splicedfyt yield strength of transverse reinforcementld development or splice lengthM constant used in expressions for the bond

strength of bars not confined by transversereinforcement

M cosh (0.0022ld ) (Esfahani and Rangan 1998)n number of bars being developed or splicedN the number of transverse stirrups, or ties, within

the development or splice lengthRr relative rib area of the reinforcements spacing of transverse reinforcementtd 0.028db + 0.28 (Darwin et al.1996)td 0.03db + 0.22 (Zuo and Darwin 2000)tr 9.6Rr + 0.28 (Darwin et al. 1996; Zuo and Darwin

2000)Vc nominal shear strength provided by concreteVcr shear force at the initiation of the first shear crackVs nominal shear strength provided by shear

reinforcementεv measured strain in stirrups crossing the shear crackρv shear reinforcement ratio



bond characteristics and shear behavior of concrete beams

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