ACI Structural Journal/March-April 2008 173

ACI Structural Journal, V. 105, No. 2, March-April 2008.MS No. S-2006-398.R1 received October 11, 2006, and reviewed under Institute

publication policies. Copyright © 2008, American Concrete Institute. All rights reserved,including the making of copies unless permission is obtained from the copyright proprietors.Pertinent discussion including author’s closure, if any, will be published in the January-February 2009 ACI Structural Journal if the discussion is received by September 1, 2008.

ACI STRUCTURAL JOURNAL TECHNICAL PAPER

This paper presents test results of six large-size concrete beamsreinforced with either conventional- or high-strength steel andtested up to failure. The beams were constructed without webreinforcement to evaluate the nominal shear strength provided bythe concrete. The shear behavior, ultimate load-carrying capacity,and mode of failure are presented. The applicability of the currentACI design code to large-size concrete beams constructed withoutweb reinforcement is discussed. The influence of the shear span-depth ratio, concrete compressive strength, as well as the type andthe amount of longitudinal steel reinforcement is investigated.The study shows that using high-strength steel alters the mode offailure from diagonal tension to shear compression failure andresults in higher shear strength compared with using conventionalsteel. It was also found that the current ACI shear designprovisions are unconservative for large-size concrete beamswithout web reinforcement.

Keywords: beams; high-performance steel; high-strength steel; shear.

INTRODUCTIONThe demand for high-performance reinforcing materials

has been increasing over the past few years to combatunnecessary repair costs, which are estimated to exceedbillions of dollars a year in the U.S.1,2 Extensive researchhas been conducted in the past few years to evaluate thematerials characteristics of different types of high-performancesteel. Nevertheless, the resulting impact on existing buildingcodes is sparse in relation to the effort put into research. Thelack of information regarding the behavior of concretemembers reinforced with this type of material preventsdesign engineers from using the full strength of the material.Different types of high-performance reinforcing bars arecurrently being examined by different research institutionsand universities worldwide for various structural engineeringapplications. By metallurgically modifying the microstructureof the steel, the bars are less susceptible to corrosioncompared with conventional steel and have a yield strengththat is almost twice that of conventional steel. Severaldemonstration projects including bridge decks, airportcontrol towers, bridge piers, and high-rise condominiumshave been constructed successfully using high-strengthsteel.2 Nevertheless, in most of these applications, the high-strength bars have been used by direct substitution of theamount required for conventional steel and, thus, neglectingthe benefits of the higher yield strength of the material. It hasbeen recently reported that the higher yield strength of thematerial could strongly influence the shear behavior, ultimateload-carrying capacity, and mode of failure of concrete beamsreinforced with this type of material.3 The use of high-performance steel reinforcement in concrete footings andmat foundations has an emergent potential to increaselongevity and, therefore, lead to substantial savings in the

life-cycle cost of concrete structures. The high yield strengthof the material combined with its enhanced corrosion resistancemakes it ideal for substructure applications. In most cases,the depth of footings and mat foundations is controlled byeither one- or two-way shear. Therefore, understanding theshear behavior of concrete members reinforced with high-performance steel is essential for a safe and economic designof foundations.

Despite numerous comprehensive studies over the last 50 years,understanding of the shear behavior of conventionally-reinforcedconcrete beams remains unclear. Several international codes,4-6

including the current ACI Building Code (ACI 318-05),4 arebased on semi-empirical considerations. The calculatedshear strength could vary significantly among different codeapproaches. Discrepancies up to 250% in the allowable shearstress according to different codes of practice have beenreported.7 There is also substantial evidence that the shearstress at failure decreases as the depth of the member increasesand as the aggregate size decreases.8-10 Such a phenomenonraises doubt about the use of current shear design provisionsfor beams without web reinforcement. It should be highlightedthat 86% of all available test data compiled by the Subcom-mittee F of Joint ACI-ASCE Committee 445 pertain to beamdepths less than 500 mm (20 in.).9

One of the main factors affecting the nominal concreteshear strength is the ability of concrete to transfer shearacross cracks in the web of the beams. Softening of theconcrete due to the biaxial state of tension-compression in theweb of beams loaded in shear has been investigated by manyresearchers, and different formulations have been proposed inthe past 25 years. It has been observed that these models varywidely even for concrete beams reinforced with conventionalsteel. Some theories of biaxial softening of concrete do not evenpredict concrete crushing at very high deformations.11

This paper presents test results of six large-size concretebeams reinforced with either high-strength or conventionalGrade 420 MPa (60 ksi) steel and tested up to failure. All thebeams were constructed without web reinforcement to evaluatethe concrete shear strength. The influence of the shear span-depth ratio (a/d) as well as the type and the amount oflongitudinal steel reinforcement is investigated.

RESEARCH SIGNIFICANCEEfficient use of the high tensile strength characteristics of

the high-performance steel is expected to provide durable

Title no. 105-S18

Shear Behavior of Large Concrete Beams Reinforcedwith High-Strength Steelby Tarek K. Hassan, Hatem M. Seliem, Hazim Dwairi, Sami H. Rizkalla, and Paul Zia

ACI Structural Journal/March-April 2008174

concrete structures with smaller reinforcement ratios andsignificantly reduced maintenance requirements comparedwith conventional steel. The present experimental studyallows quantifying the benefits of using high-strength steeland provides experimental evidence of its high strengthcapabilities. The impact of the high tensile strength of thematerial on the shear behavior of concrete beams isdemonstrated. The study also evaluates the limitations of thecurrent shear design provisions in the ACI Code4 to developdesign guidelines that recognize its contribution to the shearstrength of concrete beams.

EXPERIMENTAL INVESTIGATIONTest specimens

Six large-size concrete beams were constructed andloaded to failure under concentrated load acting at midspan.The main variables included in the study are the a/d, concretecompressive strength, and the type and amount of the longitu-dinal steel reinforcement. All the beams had identical nominalcross-sectional dimensions of 460 x 915 mm (18 x 36 in.) with

a total length of 4900 mm (16 ft). The dimensions of thebeams were selected to be much larger than those used indeveloping the ACI shear design provisions established in1962.12 The beams were cast in three batches of differentconcrete strengths, producing three groups (A, B, and C) ofidentical dimensions. One beam of each group was rein-forced with conventional Grade 420 MPa (60 ksi) steelwhile the other beam was reinforced with high-strengthsteel bars. No transverse reinforcement was provided inany of the specimens to evaluate the nominal concrete shearstrength. The first group of beams (Group A) was tested at ana/d of 1.9. The influence of the concrete compressivestrength on the shear behavior was investigated by testingthe second group of beams (Group B) using the same a/d of1.9, but with a lower concrete compressive strength. Thethird group of beams (Group C) was tested using an a/d of2.7 to examine the flexural shear behavior of the beams. Thelongitudinal reinforcement ratio used in the concrete beamsreinforced with high-strength steel was 40% less than thatused in concrete beams reinforced with Grade 420 MPa (60 ksi)steel. This reduction in the reinforcement ratio is based onusing a yield strength of 690 MPa (100 ksi) for the high-strength steel. The beams are identified by a numberingcode. The first letter denotes the type of the longitudinal steel:G for Grade 420 MPa (60 ksi) steel and M for high-strengthsteel. The second term represents the a/d, while the thirdnumber denotes the concrete compressive strength in MPa.Details of the test specimens are given in Table 1. Thelongitudinal steel was evenly distributed along the width ofthe specimens leaving 40 mm (1.5 in.) concrete cover oneach side. Bottom cover was chosen according to ACI 318-054

for beams subjected to interior exposure. The bottomlongitudinal steel bars were hooked upward beyond thesupports to preclude the possibility of anchorage failure.

MaterialsHigh-strength steel—A commercially available high-

performance steel known as microcomposite multistructuralformable (MMFX) steel, which conforms to ASTMA1035,13 was selected for this study. Tension coupons weretested according to ASTM A37014 to determine the materialcharacteristics of the bars. Typical stress-strain behavior ofthe MMFX bars compared with Grade 420 MPa (60 ksi)steel bars is shown in Fig. 1. The high-strength steel barsexhibited a linear stress-strain relationship up to a stresslevel of 690 MPa (100 ksi), followed by a nonlinear behaviorup to failure without a well-defined yield point. According to

Tarek K. Hassan is an Associate Professor in the Department of Structural Engineering,Ain Shams University, Cairo, Egypt. He received his MSc and PhD from the University ofManitoba, Winnipeg, MB, Canada, in 1999 and 2002, respectively. He is currently apart-time Senior Structural Engineer at Dar Al Handasah Consultants, Cairo, Egypt.His research interests include nonlinear analysis and design of concrete structures,and repair and strengthening of concrete structures using advanced composite materials.

ACI member Hatem M. Seliem is a Postdoctoral Research Associate in the Departmentof Civil, Construction, and Environmental Engineering at North Carolina StateUniversity (NCSU), Raleigh, NC. He received his PhD from NCSU in 2007 and hisBSc and MSc from Cairo University, Cairo, Egypt, in 2000 and 2002, respectively. Hisresearch interests include design of concrete structures using innovative materials andretrofitting of reinforced concrete structures using advanced composite materials.

Hazim Dwairi is an Assistant Professor in the Department of Civil Engineering,Hashemite University, Jordan. He received his PhD from North Carolina StateUniversity in 2004. His research interests include design and analysis of concretestructures and behavior of structures under lateral loads.

Sami H. Rizkalla, FACI, is a Distinguished Professor of Civil and ConstructionEngineering in the Department of Civil, Construction and Environmental Engineering;Director of the Constructed Facilities Laboratory (CFL); and Director of the NSFIndustry/University Cooperative Research Center at North Carolina State University.He is also the immediate Past President and the founder of the Network of Centers ofExcellence on Intelligent Sensing of Innovative Structures (ISIS Canada). He is PastChair and a current member of ACI Committees 118, Use of Computers; 440, FiberReinforced Polymer Reinforcement; E803, Faculty Network Coordinating Committee;and a member of Joint ACI-ASCE Committees 423, Prestressed Concrete, and 550,Precast Concrete Structures.

ACI Honorary Member Paul Zia is a Distinguished University Professor Emeritus atNorth Carolina State University. He is a member of ACI Committees 363, High-Strength Concrete, and 440, Fiber Reinforced Polymer Reinforcement; Joint ACI-ASCE Committees 423, Prestressed Concrete, and 445, Shear and Torsion; ACI TACTechnology Transfer Committee; and the Concrete Research Council.

Table 1—Details of test specimens

Specimen

Group A Group B Group C

G-1.9-51 M-1.9-51 G-1.9-38 M-1.9-38 G-2.7-32 G-2.7-32

Shear span-depth ratio (a/d) 1.9 1.9 1.9 1.9 2.7 2.7

Concrete compressive strength, MPa (psi) 51 (7400) 51 (7400) 38 (5500) 38 (5500) 32 (4650) 32 (4650)

Type of longitudinal reinforcement G* M† G* M† G* M†

Bottom reinforcement ratio, % 0.72 0.44 0.72 0.44 0.72 0.44

Top reinforcement ratio, % 0.36 0.22 0.36 0.22 0.36 0.22

Diagonal cracking load, kN (kips) 670 (150) 670 (150) 670 (150) 670 (150) 445 (100) 445 (100)

Failure load, kN (kips) 871 (195) 1560 (350) 753 (170) 1364 (306) 552 (124) 638 (143)

Predicted failure load using ACI 318-05, kN (kips) 1103 (248) 1917 (431) 1103 (248) 1418 (319) 690 (155) 690 (155)

PTest /PACI 318-05 0.8 0.81 0.68 0.96 0.80 0.92*G refers to Grade 60 steel.†M refers to high-strength steel.

ACI Structural Journal/March-April 2008 175

ASTM A37014 offset method (0.2%), the yield strength ofthe bars was determined to be 827 MPa (120 ksi). Themeasured initial modulus of elasticity of the bars was 200 GPa(29,000 ksi) up to a stress level of 690 MPa (100 ksi), beyondwhich a considerable reduction in the modulus of elasticitywas observed. Based on test results, the average ultimatetensile strength was 1120 MPa (162 ksi).

Grade 420 MPa (60 ksi) steel—The Grade 420 MPa (60 ksi)reinforcing bars used in the current study met the requirementsof ASTM A615.15 Based on tension coupon tests, the barshad an average modulus of elasticity and yield strength of200 GPa (29,000 ksi) and 469 MPa (68 ksi), respectively.

Concrete—All beams were cast with normal-strengthconcrete with a maximum aggregate size of 19 mm (0.75 in.)using three different batches of concrete. The compressivestrength of the concrete was determined based on the average ofat least three 100 x 200 mm (4 x 8 in.) cylinders cast from thesame batch of concrete and cured under the same conditions asthe beams. The measured concrete compressive strength fordifferent specimens is given in Table 1.

Test setupThe beams were tested under a concentrated load acting at

midspan, as shown in Fig. 2. The load was applied using a2000 kN (450 kips) hydraulic actuator. The beams weresupported on either steel I-beams or concrete blocks securedto the strong floor. Neoprene pads were placed between theconcrete beam and the supports to allow for rotation at bothends of the beam. A 25 mm (1 in.) thick steel plate wasattached to the actuator to distribute the load to the beam.Neoprene pads were placed between the loading plate andthe concrete surface to prevent local crushing of theconcrete. Each beam was instrumented to measure thevertical deflections at midspan and at the supports usingstring potentiometers. Twelve horizontal and vertical linearpotentiometers were used to measure diagonal crack widths.Two PI gauges were attached to the top and bottom of thebeam at midspan to measure the concrete deformation. All thedata were continuously recorded up to failure. Instrumentationlayout is shown in Fig. 3 and 4 for beams with a/d of 1.9 and2.7, respectively.

EXPERIMENTAL RESULTS AND DISCUSSIONDeflections

The load-deflection behavior of the test specimens ofGroup A having an a/d of 1.9 and a concrete compressive

Fig. 1—Typical stress-strain behavior for conventional andhigh-strength steel.

Fig. 2—Details of test specimens.

176 ACI Structural Journal/March-April 2008

strength of 51 MPa (7400 psi) is shown in Fig. 5. For bothspecimens, linear behavior was observed up to the initiationof the first flexural crack at a load level of 370 kN (83 kips)followed by nonlinear behavior up to failure. Following theformation of the flexural cracks, a major diagonal shearcrack developed and became visible in both specimens at aload level of 670 kN (150 kips). After cracking, themeasured deflections of the concrete beam reinforced withhigh-strength steel (Specimen M-1.9-51) were significantlyhigher than those measured for the concrete beam reinforcedwith Grade 420 MPa (60 ksi) steel (Specimen G-1.9-51).This behavior is attributed to the reduced reinforcement ratio

used in Specimen M-1.9-51, resulting in higher steel strainand probable higher localized bond slip.

The behavior of concrete beams having the same a/d of 1.9but constructed with a lower concrete compressive strengthof 38 MPa (5500 psi) was investigated using Group B testspecimens. To simulate overloading conditions, bothSpecimens G-1.9-38 and M-1.9-38 of Group B were initiallyloaded to a load level of 450 kN (100 kips) and unloaded.The specimens were reloaded up to failure to evaluate thepost-cracking stiffness for both specimens. The load-deflectionbehavior of the second load cycle is shown in Fig. 6. Themeasured post-cracking stiffness of Specimen G-1.9-38,reinforced with Grade 420 MPa (60 ksi) steel, was 50%higher than that of Specimen M-1.9-38, which is reinforcedwith high-strength steel but with a reduced reinforcement ratio.

The load-deflection behavior of Group C beams with an a/dof 2.7 and concrete strength of 32 MPa (4650 ksi) is shownin Fig. 7. Similar to Group B specimens, the two specimens,G-2.7-32 and M-2.7-32, were loaded to 225 kN (50 kips) andunloaded before loading to failure to simulate overloadingconditions. The beam reinforced with high-strength steel(M-2.7-32) exhibited higher deflections than the beamreinforced with Grade 420 MPa (60 ksi) steel (G-2.7-32)at the same load level. This behavior is attributed to thesmaller area of reinforcing steel used and higher inducedsteel strain and probable more localized bond slips.

In general, concrete beams reinforced with Grade 420 MPa(60 ksi) steel reinforcement exhibited less post-crackingdeflection at the same load level compared with beamsreinforced with high-strength steel because the latter had areduced reinforcement ratio. For beams reinforced withGrade 420 MPa (60 ksi) steel, failure was sudden and waspreceded by relatively little cracking. The measured ultimatemidspan deflection for the three beams reinforced withGrade 420 MPa (60 ksi) steel was less than the span/470. Aless brittle behavior was observed for the beams reinforcedwith high-strength steel. Considerable deflections and muchwider cracks were observed for these beams prior to failure.At failure, the deflections for the beams reinforced with thehigh-strength steel and having an a/d of 1.9 were equivalentto the span/230, twice the deflections of the beams reinforcedwith Grade 420 MPa (60 ksi) steel.

Fig. 3—Instrumentation layout of beams with a/d of 1.9.

Fig. 4—Instrumentation layout of beams with a/d of 2.7.

Fig. 5—Load-deflection behavior of Group A specimens. Fig. 6—Load-deflection behavior of Group B specimens.

ACI Structural Journal/March-April 2008 177

Crack patternFor all specimens, flexural cracks were first initiated under

the applied load. With further increase of load, new flexuralcracks formed in the shear spans and curved toward theloading area. The first diagonal shear crack was observed atthe same load level for identical specimens regardless of thetype of reinforcement. For specimens reinforced withGrade 420 MPa (60 ksi) steel with an a/d of 1.9, the firstdiagonal shear crack was observed at a load level equivalent to80% of the ultimate load. A similar crack was observed at aload level of approximately 45% of the ultimate load for thebeams reinforced with high-strength steel. Such a phenomenondemonstrates the reserved capacity for these beams andprovides adequate warning prior to shear failure. Figure 8(a)depicts a typical crack pattern at the initiation of diagonalshear cracks for Specimens M-1.9-51 and M-2.7-32, reinforcedwith high-strength steel and having a/d of 1.9 and 2.7, respec-tively. Diagonal cracking loads for all the specimens are givenin Table 1. The width of the major diagonal shear crack fordifferent specimens is shown in Fig. 8(b). The figure clearlyshows that the major diagonal shear crack in the beamsreinforced with high-strength steel was almost three timeswider than that observed in identical beams reinforced withGrade 420 MPa (60 ksi) steel before failure. This is attributedto the reduced reinforcement ratio used for high-strength steel.

Failure modeBeams reinforced with Grade 420 MPa (60 ksi) steel—

Diagonal tension failure was observed for the three concretebeams reinforced with Grade 420 MPa (60 ksi) steel. Failureoccurred due to extension of the diagonal shear crack rapidlytoward the load point shortly after its initiation. Strainmeasurements at the bottom of the concrete beams atmidspan implied that yielding of the conventional steelreinforcement governed the failure mode. A separatenonlinear finite element study16 confirmed the observedbehavior and indicated yielding of the longitudinal conventionalreinforcement at the location of shear cracks, as shown inFig. 9 for Specimen G-1.9-51. Upon yielding of the longitudinalsteel reinforcement at the location of a shear crack, thesection was no longer capable of resisting any additionalincrease in load and the concrete beams failed abruptly inshear. Detailed information about the finite element modelingis reported elsewhere.16

Beams reinforced with high-strength steel—Failure wascontrolled primarily by the compressive strength of the diagonalstrut (compression shear failure). The high yield strength ofthe materials precluded diagonal tension failure and allowedthe failure to take place in the concrete strut at much higher

Fig. 8—(a) Crack pattern at initiation of diagonal shearcrack; and (b) diagonal crack width of different specimens.

Fig. 7—Load-deflection behavior of Group C specimens.Fig. 9—Typical tensile strain distribution at ultimate of concretebeams reinforced with conventional Grade 60 steel.16

178 ACI Structural Journal/March-April 2008

loads. Typical tensile strains at the midspan section, forGroups B and C, are shown in Fig. 10 and 11, respectively.The figures show that at ultimate, the measured tensile strainin the high-strength steel bars exceeded its yield straindefined by the 0.2% offset method. Typical shear failure ofconcrete beams reinforced with Grade 420 MPa (60 ksi) andhigh-strength steel is shown in Fig. 12.

Concrete shear strengthDespite the reduction in the reinforcement ratio, the shear

strength of concrete beams reinforced with high-strengthsteel was significantly higher than that of the beams reinforcedwith Grade 420 MPa (60 ksi) steel. The high yield strengthof the material maintained the capacity of the tension tie andallowed the beams to resist more load until crushing of thediagonal strut occurred. This behavior was highlypronounced for beams with a small a/d (a/d = 1.9). The ultimateload-carrying capacity of Specimen G-1.9-51, reinforcedwith Grade 420 MPa (60 ksi) steel and having an a/d of 1.9,was 871 kN (195 kips). Test results showed that the concreteshear strength could be increased by 80% but using 40% lessarea of high-strength steel (Specimen M-1.9-51). Similarbehavior was observed for Specimens G-1.9-38 and M-1.9-38.As the tensile stress in the high-strength steel barsapproached its yield strength, wide cracks were observed

and the beneficial effect of using high-strength steel startedto attenuate. Such a phenomenon was clearly observed forbeams with an a/d of 2.7. Using high-strength steel as mainlongitudinal reinforcement in these beams increased theshear strength by only 16% while using 40% less steel.Failure loads for all specimens are summarized in Table 1.

COMPARISON WITH ACI 318-05Beams with a/d of 1.9—A simple strut-and-tie model was

developed as required by the ACI 318-054 for beams with aclear span less than four times the depth of the beam. Thestrut-and-tie model consisted of a direct strut extending fromthe loading plate to the reaction bearing plate, as shown inFig. 13. Failure mechanisms governing the strength weretypically crushing of the compressive strut or yielding of thetie reinforcement. To account for the nonlinear stress-strainbehavior of high-strength steel, an exponential stress-strainrelationship was assumed as reported by the Concrete Innova-tion Appraisal Services.17 The bearing dimensions weresufficiently large to avoid crushing of the concrete at thenodes in the strut-and-tie model. The predicted ultimatecapacities for different specimens are given in Table 1. In

Fig. 10—Load-tensile strain behavior of Group B specimens.

Fig. 11—Load-tensile strain behavior of Group C specimens.

Fig. 12—Typical shear failure for beams reinforced withconventional and high-strength steel.

Fig. 13—Strut-and-tie model for beams with a/d of 1.9.

ACI Structural Journal/March-April 2008 179

general, the analysis indicates that ACI 318-054 overestimatedthe capacity of concrete beams reinforced with eitherconventional- or high-strength steel. For Specimens G-1.9-51and G-1.9-38 reinforced with Grade 420 MPa (60 ksi) steel,failure was controlled by yielding of the tie. The predictedcapacity was independent of the concrete compressive strengthand, therefore, identical capacities were predicted for bothspecimens. Conversely, crushing of the compressive strutwas the governing mode of failure for Specimens M-1.9-51and M-1.9-38, reinforced with high-strength steel. Increasingthe concrete compressive strength by 34% increased thepredicted capacity by the same magnitude. The analysisdemonstrated the influence of the concrete compressivestrength on the shear strength of the concrete beams reinforcedwith high-strength steel. The capacity of the compressive strutwas taken as 0.51fc′, where fc′ is the specified compressivestrength of concrete. It should be highlighted that, accordingto ACI 318-05, the compressive capacity of the strut isindependent of the tensile strain in the reinforcement.

Beams with a/d of 2.7—The nominal concrete shear strengthvc was predicted according to ACI 318-05 using Eq. (1)

vc = 0.167 (MPa) (1 MPa = 145 psi) (1)

The ACI 318-054 design method considerably overestimatedthe shear strength of large-size concrete beams constructedwithout web reinforcement. The diagonal shear crack wasinitiated in the beams with an a/d of 2.7 and became visible at aload level of 445 kN (100 kips), which corresponds to0.11 MPa. The predicted capacity for Specimen G-2.7-32,reinforced with conventional steel, was 25% higher than themeasured value (shear strength corresponds to 0.134 MPa).It should be noted that the current ACI shear provisions werebased on testing shallow beams,13 which did not account for thesize effect of large-size beams.

CONCLUSIONSBased on the findings of this investigation, the following

conclusions can be drawn:1. High-strength steel strongly influenced the shear

behavior of the concrete beams tested in the current studyand constructed without web reinforcement. Ignoring thehigh strength characteristics of the material could provideunreliable predictions of the ultimate load-carrying capacityand mode of failure;

2. The diagonal cracking strength is a measure of theconcrete contribution at ultimate for members reinforced withconventional steel and constructed without web reinforcement.Such a relationship is inappropriate for high-strength steel asthe behavior is strongly influenced by the a/d and the stresslevel in the bars. Test results showed that initiation of diagonalshear cracks is independent of the type or the amount of thelongitudinal reinforcement;

3. Concrete beams reinforced with Grade 420 MPa (60 ksi)steel and constructed without web reinforcement exhibited avery brittle failure due to yielding of the longitudinal steelreinforcement. Shear failure occurred shortly after initiationof the shear crack;

4. Despite the reduction in the reinforcement ratio by 40%,the shear strength of concrete beams reinforced with high-strength steel was significantly higher than that of the beamsreinforced with Grade 420 MPa (60 ksi) steel. The high yieldstrength of the material maintained the capacity of the

tension tie, and thus enabled the beams to resist more loaduntil crushing of the diagonal strut occurred;

5. A significant reserve in strength was observed for beamsreinforced with high-strength steel after diagonal cracking.Failure was due to crushing of the diagonal concrete strut atmuch higher loads compared with beams reinforced withconventional steel; and

6. The ACI 318-05 simplified expression for the shearcontribution of concrete is unconservative for large-size concretebeams without web reinforcement. The expression needs toaccount for the size effect and the reinforcement characteristics.

ACKNOWLEDGMENTSThe authors gratefully acknowledge the donation of the materials provided by

MMFX Technologies Corp., CA. Special thanks are extended to A. Hosny,E. Thorup, and J. Atkinson at the Constructed Facilities Laboratory for theirvaluable help during the experimental program.

REFERENCES1. Reis, H.; Ervin, B. L.; Kuchma, D. A.; and Bernhard, J. T., “Estimation of

Corrosion Damage in Steel Reinforced Mortar Using Guided Waves,”Journal of Pressure Vessel Technology, V. 127, No. 3, 2005, pp. 255-261.

2. Darwin, D.; Browning, J.; Nguyen, T. V.; and Locke, C., “Mechanicaland Corrosion Properties of a High-Strength, High Chromium ReinforcingSteel for Concrete,” SM Report No. 66, University of Kansas, Lawrence,KS, 2002, 106 pp.

3. Seliem, H.; Hosny, A.; Dwairi, H.; and Rizkalla, S., “Shear Behavior ofConcrete Beams Reinforced with MMFX Steel Without Web Reinforcement,”Report No. IS-06-08, Constructed Facilities Laboratory, North CarolinaState University, Raleigh, NC, 2006, 21 pp.

4. ACI Committee 318, “Building Code Requirements for StructuralConcrete (ACI 318-05) and Commentary (318R-05),” American ConcreteInstitute, Farmington Hills, MI, 2005, 430 pp.

5. Commission of the European Communities, “Design of ConcreteStructures. Part 1: General Rules and Rules for Buildings,” Eurocode 2, 2003,330 pp.

6. DIN 1045-1, Deutsche Norm, “Concrete, Reinforced and PrestressedConcrete Structures—Part 1: Design,” Normenausschuss Bauwesen(NABau) im DIN Deutsches Institut fur Normung e. V. Beuth Verlag,Berlin, Germany, 2001, pp. 1-48.

7. Hawkins, N. M.; Kuchma, D. A.; Mast, R. F.; and Reineck, K.,“Simplified Shear Design of Structural Concrete Members,” NCHRPReport 549, Washington, DC, 2005, 54 pp.

8. Kani, G. N. J., “How Safe are Our Large Reinforced ConcreteBeams?” ACI Structural Journal, V. 64, No. 4, Apr. 1967, pp. 128-141.

9. Bažant, Z. P., and Yu. Q., “Designing Against Size Effect on ShearStrength of Reinforced Concrete Beams without Stirrups: Formulation,”Journal of Structural Engineering, ASCE, V. 131, No. 12, 2005, pp. 1877-1885.

10. Sherwood, E. G.; Bentz, E. C.; and Collins, M. P., “Evaluation ofShear Design Methods for Large, Lightly-Reinforced Concrete Beams,”Proceedings of the Advances in Engineering Structures, Mechanics andConstruction, ON, Canada, 2006, pp. 153-164.

11. Duthinh, D., “Senstivity of Shear Strength of Reinforced Concreteand Prestressed Concrete Beams to Shear Friction and Concrete SofteningAccording to Modified Compression Field Theory,” ACI StructuralJournal, V. 96, No. 4, July-Aug. 1999, pp. 495-508.

12. Joint ACI-ASCE Committee 326, “Shear and Diagonal Tension,”ACI JOURNAL, Proceedings V. 59, No. 1-3, Jan.-Mar., 1962, pp. 1-30, 277-344,and 352-396.

13. ASTM A1035-07, “Standard Specification for Deformed and Plain,Low Carbon, Chromium, Steel Bars for Concrete Reinforcement,” ASTMInternational, West Conshohocken, PA, 2007, 5 pp.

14. ASTM A370-07, “Standard Test Methods and Definitions forMechanical Testing of Steel Products,” ASTM International, WestConshohocken, PA, 2007, 47 pp.

15. ASTM A615-06, “Standard Specification for Deformed and PlainCarbon-Steel Bars for Concrete Reinforcement,” ASTM International,West Conshohocken, PA, 2006, 6 pp.

16. Hassan, T., “Behavior of Concrete Deep Beams Reinforced withHigh Strength Steel,” Ain Shams Scientific Bulletin, V. 41, No. 3, 2007,pp. 109-127.

17. Concrete Innovation Appraisal Services, (CIAS), “Structural DesignCriteria for High-Strength MMFX Microcomposite Reinforcing Bars,”CIAS Report: 04-1, 2004, 34 pp.

fc′

fc′

fc′