ACI MATERIALS JOURNAL TECHNICAL PAPER

Shear Behavior of Macro-Synthetic Fiber-ReinforcedConcrete Beams without Stirrupsby Salah Altoubat, Ardavan Yazdanbakhsh, and Klaus-Alexander Rieder

Twenty-seven large-scale beams were instrumented and testedunder monotonic centerpoint loading to determine the effect of anewly developed high-modulus macrosynthetic fiber on the shearstrength and failure behavior of longitudinally reinforced concrete(RC) beams without stirrups. Slender and short beams withrespective shear span-depth ratios (aid) of 3.5 and 2.3 were tested.The length of the beams varied between 1.9 and 3.2 m (75 and126 in.), and the macrosynthetic fibers were added at volumefractions of 0.50, 0.75, and 1.0%. Deflection of the beam, strain inthe concrete and in the flexural reinforcing bars, and the crackingpattern were monitored during the test at different stages of themonotonic loading until failure. The results showed that theaddition of macrosynthetic fibers significantly improved the shearstrength and ductility of the RC beams and modified the crackingand failure behavior. The ultimate shear strength of slenderand short beams was increased up to 30% compared to thecontrol RC beams.

INTRODUCTIONThe addition of discrete fibers to concrete mainly

improves its post-cracking behavior (residual strength,toughness, and ductility). The improved material propertiesof fiber-reinforced concrete (FRC) tend to improve the flexuraland shear behavior of FRC structures. As such, FRCbecomes an attractive material for several applicationsincluding beams, slabs-on-ground, pavement, and elevatedmetal composite slabs. It has been recognized that fiberreinforcement is an effective way to enhance the fracturetoughness of concrete in all modes of failure. Research overthe past three decades has clearly established the potentialuse of fiber reinforcement for enhancing the shear capacityof reinforced concrete (RC) beams.l-s The effect of fiberreinforcement on shear strength of concrete is attributed totwo main factors: 1) a direct factor imposed by the post-cracking strength at the inclined shear crack (in a similar wayto stirrups); and 2) an indirect factor that increased thecontribution of concrete to shear strength by improvingaggregate interlock and dowel action of flexural reinforcement.The indirect factor is attributed to improved control ofcracking (width and distribution of cracks) and the ability todistribute stresses. Consequently, the shear strength of theconcrete, ultimate shear capacity, and ductility of FRCbeams are improved. Several research studies that involvedsmall- and lar~e- scale testing ofFRC beams have confirmedthis theory. 6-1

Many research studies carried out over the past threedecades have focused on the shear behavior of FRC. Most ofthe published work, however, has focused exclusively onsteel fiber-reinforced concrete (SFRC). The results showedthat steel fibers can be used to boost the shear capacity ofconcrete and to improve the shear crack distribution; therefore,

they are capable of replacing some of the vertical stirrups inRC structural members. This helps to reduce the problemsassociated with congestion of shear reinforcement such asinterference with concrete compaction that results inhoneycombing and poor quality of concrete, particularly atcritical sections such as beam-column junctions. A largedatabase of test results for shear strength of steel fiber-reinforcedconcrete (SFRC) beams obtained by mani investigators wasrecently compiled by Parra-Montesi nos. 1 Based on this, thelist of beams exempt from the minimum shear reinforcementrequirement in Section 11.4.6.1 of ACI 318-0812 nowincludes beams that are constructed with SFRC. This addition toACI 318 represents the first permitted structural use ofSFRC in the ACI Building Code.

Unlike SFRC, there are only few studies reporting resultson shear with synthetic fibers.21

,13-IS The limited research onshear behavior with synthetic fibers is attributed perhaps tothe small increase in toughness and associated structuralperformance of concrete when low-modulus synthetic fibersare added to concrete. Synthetic fibers, typically made ofpolypropylene, have primarily been used in concrete materials tocontrol shrinkage cracking and, to a limited extent, toimprove toughness and impact resistance. In recent years,however, increasing efforts have been devoted toward thedevelopment of new generation of macrosynthetic fibers thatimpart significant toughness and ductility to concrete comparableto commonly used steel fibers. Accordingly, the application ofmacrosynthetic fibers in the concrete industry has extendedbeyond shrinkage and thermal cracking control to structuralapplications. Large-scale testing of slabs-on-ground hasdemonstrated that the tested macrosynthetic fiber cansignificantly increase the flexural and ultimate load-carrying capaci~ of concrete slabs-on-ground relative to plainconcrete slabs.16, 7 It was also observed that macrosynthetic andsteel FRC slabs behaved similarly at different stages of cracking.

The increasing use of macro synthetic fibers for structuralapplications necessitates the understanding and the evaluation ofthe effect of fiber addition on the shear behavior of concretebeams. This study is part of a comprehensive experimentalprogram that focused on the shear behavior of beamsreinforced with a new type of macro synthetic fiber. 18,19 Thisfiber has a higher modulus of elasticity compared to regularpolypropylene fibers and an optimized geometry to enhancethe bond between the fiber and the concrete matrix, whichleads to an increase in the toughness properties of concrete.The current study contributes a new set of large-scale RC

ACi Materials Journal, V. 106, No.4, July-August 2009.MS No. M-2008-337 received October 12,2008, and reviewed under Institute publication

poliCIes. Copynght © 2009, Amencan Concrete Institute. All rights reserved, including themak.in~ of copIes unless penmsslOn IS obtained from the copyright proprietors. PertinentdiSCUSSIonIncludIng authors' closure, if any, will be published in the May-June 2010 ACiMaterials Journal if the discussion is received by February 1,2010.

ACI member Salah Altoubat is an Assistant Professor of civil engineering at theUniversity of Sharjal~ United Arab Emirates. He received his PhD in civil engineeringfrom the University of Illinois at Urbana-Champaign, Urbana, IL, in 2000. Hereceived the AC/ Wason Medal for Most Meritorious Paper in 2003. He is a memberof AC/ Commillees 440, Fiber Reinforced Polymer Reinforcement, and 544, FiberReinforced Concrete. His research interests include structural applications offiber-reinforced concrete, durability aspects of cancrete, creep, shrinkage, andcracking of concrete.

ACimember Ardavan Yazdanbakhsh is a PhD Student of civil engineering at TexasA&M University, College Station, TX. He received his MSc in civil engineering fromthe University of Sharjah United Arab Emirates, in 2008. His research illterests include theshear capacity of concrete elemellts, stmctural behavior of fiber-reinforced concrete, and thedesign and developmelll of srress·relaxillg cementitious composites.

AC/ member Klaus·Alexander Rieder is a Senior Principal Scientist at GraceConstruction Products. He received his SeD in physics from the Technical Universityof Vienna, Vienna, Austria, in /995. He is a member of AC/ Commiuees 209, Creepand Shrinkage in Concrete; 215, Fatigue of Concrete; and 544, Fiber ReinforcedConcrete; and Joint AC/·ASCE Commillee 446, Fracture Mechanics. Hisresearch interests include all durability- and cracking-related aspects of concreteand the development of high-performance fibers for concrete applications.

beam tests of plain and macrosynthetic FRC to the existingliterature. Results obtained from testing 27 large-scale beamsunder shear loading are presented. The beams were reinforcedwith longitudinal flexural reinforcement and were designed tofail in shear under monotonic centerpoint loading in a simplysupported configuration. Macrosynthetic fibers were added tothe concrete at three different volume fractions, namely, 0.50,0.75, and 1.0%. Control beams without fiber reinforcement wereused as reference. The other study parameters were cross-sectional dimension (230 x 390 mm and 280 x 460 mm

oI

~y ·ex_6/~2

n.~

Fig. I-Schematic outline of location and orientation of (a)embedded strain gauges; (b) external strain gauges; and (c)details of beam cross section. (Note: 1 mm = 0.0394 in.)

Table 1-Details of large-scale beams testedin study

Test No. of h, d, b, Length, Span,series beams mm mm mm m m aId p,% V/,%

Ll-O.O 3 460 400 280 3.2 2.82 3.5 2.15 0.00

L1-0.50 2 460 400 280 3.2 2.82 3.5 2. IS 0.50

L1-0.75 2 460 400 280 3.2 2.82 3.5 2. IS 0.75

L2·0.0 2 390 330 230 2.7 2.31 3.5 3.18 0.00

L2-0.50 2 390 330 230 2.7 2.31 3.5 3.18 0.50

L2-0.75 2 390 330 230 2.7 2.31 3.5 3.18 0.75

L2-1.0 2 390 330 230 2.7 2.31 3.5 3.18 1.00

Shl·O.O 2 460 400 280 2.2 1.82 2.3 2.15 0.00

Shl·0.50 2 460 400 280 2.2 1.82 2.3 2.15 0.50

Shl·0.75 2 460 400 280 2.2 1.82 2.3 2.15 0.75

Sh2-0.0 2 390 330 230 1.9 1.5 2.3 3.18 0.00

Sh2-0.50 2 390 330 230 1.9 1.5 2.3 3.18 0.50

Sh2·0.75 2 390 330 230 1.9 1.5 2.3 3.18 0.75

[9.1 x 15.3 in. and II x 18.1 in.]), flexural reinforcement ratio(2.15 and 3.18%) and (aid) (2.3 and 3.5). The beams wereinstrumented with embedded and external strain gauges toobtain the concrete strain at particular points within the beamand the flexural steel strain at midspan. Load-deflectionmeasurement, failure and cracking behavior, load-strainmeasurements, and the shear capacity of the beams arepresented and discussed in this paper.

RESEARCH SIGNIFICANCEThe objective of this research was to investigate the effect

of a macrosynthetic fiber on the shear behavior, strength, andductility of RC beams without stirrups. A large-scale testingprogram was conducted on simply supported concrete beamsreinforced with fibers. The study contributes a set of 27 large-scale beam tests to the existing literature on shear behaviorof FRC. The observations and results presented in this paperestablish the effectiveness of the investigated macrosyntheticfiber in increasing the shear strength and in preventing abrittle failure of the concrete beam. The test results willparticularly be valuable to help the ongoing effort of ACI 318 toinvestigate the viability of fibers as an alternative to theminimum shear reinforcement. The results can also help tovalidate and/or calibrate existing theoretical models topredict shear capacity of synthetic FRC beams.

EXPERIMENTAL PROGRAMLarge beams testing program

Twenty-seven large-scale concrete beams were designed,instrumented, and tested in displacement control mode undera monotonic three-point loading system in a simplysupported configuration. The 2005 edition of the ACI 318design code was adopted for the design of the beams and todetermine the amount of flexural reinforcement such thatshear failure would occur. The main experimental parameters inthe test series were aid, flexural reinforcement ratio p,synthetic fiber content VJ' and the size of the beam. The aidused in the study were 3.5 and 2.3 to reflect the behavior ofslender and short beams, respectively. The 27 beams werecast in two phases, of which 13 beams were cast in Phase Iand 14 beams in Phase n. The cross section of the beams (b x h),as shown in Fig. l(c), was 280 x 460 mm (II x 18.1 in.) inPhase I and 230 x 390 mm (9.1 x 15.3 in.) in Phase II. Allbeams were reinforced with three longitudinal reinforcing barswith a diameter of 32 mm (1.26 in.), which corresponds to asteel ratio of 0.0215 and 0.0318 in Phase I and n, respectively.The main flexural bars were extended straight at least 100 mm(4 in.) beyond the end-support points to provide anchoragefor the reinforcing bars, and an addition L-shaped (diameterof 12 mm [0.5 in.]) bar with a length of 300 mm (12 in.) eachway were attached to the main reinforcement at the end ofthe beam to enhance the bar anchorage. The average cylindercompressive strength of the concrete used in this study wasapproximately 41 MPa (6000 psi). Macrosynthetic fibers wereadded at volume fractions of 0.50, 0.75, and 1.0%. Table 1presents details of the beams. The beams were labeled toindicate the type of beam (short or slender), the phase inwhich they were cast (lor 2), and the fiber content. Theletter "L" denotes long or slender (aid = 3.5), whereas"Sh" means short (aid = 2.3). For example, the beam labeledLl-0.50 stands for a slender beam cast in Phase 1 (I) with afiber volume fraction of 0.50%.

For each phase, the large beams and the companion labspecimens, including the 100 x 200 mm (4 x 8 in.) cylinders

for compressive strength measurement and the 150 x 150x 550 mm (6 x 6 x 22 in.) beams for flexural toughnessmeasurement were all cast in 1 day in a modem precastfactory. Three sets of beams were cast in each phase. Eachset consisted of a pair of identical slender beams and a pairof identical short beams. The beams of the first set werenonfibrous (control), whereas the second and third sets werereinforced with macrosynthetic fibers at volume fractions of0.50% and 0.75%, respectively. Also, two identical slenderbeams with a volume fraction of 1.0% were cast and testedin the second phase. Duplicate beams were tested to enhancethe reliability of the results. The identical beam (duplicate)of each pair are labeled "a" and "b."

Material propertiesThe main components of the polymeric fiber used in the

study are polypropylene and polyethylene. This synthetic"macro" fiber's mechanical properties and geometry aresignificantly different from existing synthetic "micro"fibers, which are used to control plastic shrinkage cracking.The fiber's nominal length is 40 mm (1.57 in.) and has anaspect ratio of 90 and a specific gravity of approximately0.92. The fiber has a rectangular cross section with anaverage width of 1.4 mm (0.055 in.) and average thicknessof 0.105 mm (0.004 in.). The average tensile strength of thefiber is 620 MPa (90 ksi) with a modulus of elasticity of9500 MPa (1380 ksi). The fibers were added at volume fractionsof 0.50%, 0.75%, and 1.0%, which correspond to 4.6, 6.9,and 9.2 kg/m3 (7.75,11.6, and 15.51b/yd\ respectively.

The mixture proportions and properties of the concreteused for casting the beams are presented in Table 2. The finalwater-cement ratio (w/c) was approximately 0.47. Thecoarse-to-fine aggregate ratio was targeted to be close to50:50 to maintain workability and to have sufficient pastevolume for coating the fibers. The workability of the PRCconcrete mixtures was maintained by adjusting the dosage ofhigh-range water-reducing admixture to offset the possiblereduction in the slump, particularly for the mixtures withhigh fiber content. The coarse aggregate used in this concretemixture was a Gabro gravel with a maximum aggregate sizeof 20 mm (0.79 in.) and a specific gravity of 3.1. The fineaggregate constituents were natural washed sand with aspecific gravity of 2.6 and dune sand with specific gravityof 2.63. The 28-day compressive strength values measuredaccording to ASTM C39-05 and flexural strength valuesmeasured according to ASTM C78-08 are summarized inTable 2 (results based on the average of six standard testspecimens). The flexural and compressive strength resultsdid not vary significantly between the mixtures, whichshowed that the precast plant was able to produce concretewith consistent quality. The average values of the equivalentflexural strength,fe 3, measured according to JSCE-SF420 at3 mm (0.12 in.) ct{~flection were also reported (based ontesting six standard beam specimens), which can be obtainedfrom bending of beams under third-point loading with asupport span of 450 mm (18 in.). The equivalent flexuralstrength,fe 3, is calculated by inserting the average load intothe formuhl for the modulus of rupture. The average load isequal to the area under the load-versus-deflection curve (alsocalled toughness) measured up to a beam deflection of3 mm(0.12 in.) divided by 3 mm (0.12 in.). The Ie,3value is directlyproportional to the toughness value Tl~8 defined in ASTMC1609-0721 flexural beam test using a 150 x 150 x 550 mm(6 x 6 x 22 in.) beam.

Details of instrumentation and testingThe large-scale RC beams were instrumented with

embedded and external strain gauges to monitor the strain inthe concrete and in the reinforcing steel bars at differentstages of loading. Figures lea) and (b) show the schematiclocations of the embedded and external strain gauges.Embedded concrete strain gauges with a gauge length of100 mm (4 in.) were installed in the shear span of all beamsand aligned at 45 degrees perpendicular to the potentialdirection of a diagonal shear crack. Some of the beams werealso instrumented with embedded concrete strain gaugesplaced parallel to the reinforcing steel bars at midspan of thebeam. In addition, embedded strain gauges with a gaugelength of 10 mm (0.39 in.) were attached to the longitudinalreinforcing bars at the section of maximum bending momentto monitor the state of stress in the reinforcing bars. Externalstrain gauges with a gauge length of 20 mm (0.79 in.) werealso attached to the surface of the beam to measure thecompressive concrete strain at midspan and at differentpoints within the shear span. The deflection of the beams atmidspan was measured with two linear voltage displacementtransformers (LVDTs) with a range of ±25 mm (±1 in.).Results from the strain at different locations and deflectionmeasurements were used to explain the cracking pattern andoverall structural response of the beams.

Testing and measurementsTesting of beams in each phase began 28 days after casting

and was completed over a period of 3 days. The test setupconsisted of a simply supported loading configuration withroller supports to prevent restraint to axial elongation. Thebeams were loaded at midspan (centerpoint) and tested indisplacement control mode using a hydraulic actuator with acapacity of 500 kN (112 kips) (Fig. 2). The loading rate was0.6 mm/min. (0.024 in.lmin.) during the first testing phaseand was reduced to 0.48 mm/min. (0.019 in.lmin.) for the secondtesting phase to allow for better observation of crack development.

The parameters measured and recorded during themonotonic testing were beam deflections, strain in theconcrete at different locations, strain in the reinforcing flexural

Table 2-Mixture proportions and averageproperties of concrete

V1= v1= v1= V1=Materials 0.0% 0.50% 0.75% 1.0%

Coarse aggregate, kg/m3 943 943 943 943

Fine aggregate, kglm3 942 942 942 942

Cement, kglm3 380 380 380 380

Water, kg/m3 195 195 195 195

High-range water-reducing admixture,2.15 4.55 6.70 7.45

kg/m3

wlc 0.474 0.474 0.474 0.474

Slump,mm 100 100 90 80

Cylinder compressive strength, MPa 40.9 41.9 41.9 35.6"

Compressive - standard deviation, MPa 1.4 2.9 3.8 1.9

Flexural strength, MPa 5.63 5.64 5.68 4.75

Flexural standard deviation, MPa 0.39 0.46 0.47 0.18

Equivalent flexural strength (fe.3), MPa 0.18 1.53 2.38 2.63

le.3 - standard deviation, MPa 0.08 0.26 0.45 0.15

'This mixture exhibited signs of segregation due to high contents of fiber and high-range water-reducing admixture.Note: I kg/m3 = 1.686 Ib/yd3; 25.4 mm = 1 in.; I MPa = 145 psi.

steel bars, and the applied load. With the aid of visualobservation and the load deflection and strain measurementdata, the load levels for the beams corresponding to formation ofdiagonal shear cracking as well as the ultimate shear capacitywere determined. The formation of the first diagonal shear crackwas associated with a sudden reduction in load-carryingcapacity of the RC beam. The maximum load carried by theRC beam before failure was used to calculate the ultimateshear capacity of the beam.

EXPERIMENTAL RESULTS AND DISCUSSIONLoad-deflection results

The load versus midspan deflection curves of the slenderbeams tested in the two phases, and the short beams tested inPhase II are presented in Fig. 3(a) to (c). The load-deflectioncurves for the individual beams (not average response of thetwo identical beams) are shown in Fig. 3. The load-versus-deflection response of all beams was similar up to the load atwhich the first diagonal shear crack was formed in thecontrol beams. As mentioned previously, the first diagonalcrack was associated with a sudden load reduction in theload-deflection curve. For control beams, the load did notincrease beyond the first diagonal cracking load, and thus itmarked their ultimate capacity. The addition of macrosyntheticfibers increased the first diagonal crack load relative to thecontrol beams, as can be seen from the results shown in Fig. 3.After the formation of the diagonal shear crack, different typesof global behavior were distinguished. The first type ofbehavior was related to the control concrete beams thatexhibited sudden shear failure as indicated by the abruptdrop in the load-deflection curve followed by a completeloss of the load-carrying capacity. The second type ofbehavior was related to the fiber concrete beams, which didnot fail when the first diagonal shear crack was formed andthe post-cracking behavior was dependent on the fiberdosage. The beams with 0.50% of macrosynthetic fibersexhibited little load reduction after the first diagonalcracking and then continued to resist similar or higher loadsuntil another shear crack formed at a much higher deflectionbefore failure. The beams with a higher dosage of fibers (0.75 and1.0%) exhibited no reduction in load after the first diagonal crackand continued to resist higher load, and more shear cracks weredeveloped before failure. The plateau in the load-deflection curvein Fig. 3(b) for the beams with 0.75 and 1.0% of the fibers reflect

the multiple shear cracks developed in the beams beforefailure and showed the improvement in ductility.

Table 3 presents a summary of the loads corresponding tothe formation of first diagonal shear crack and ultimate loadcapacity of the tested beams. This table also includes thepercent increase of the load-carrying capacity of the SNFRCrelative to the corresponding control RC beams. The resultsshow that the addition of macro synthetic fibers to theconcrete beams significantly increased the first diagonalcracking load and the ultimate load.

Table 4 presents a summary of the average deflectionresisted by the RC beams at ultimate load and the percentageincrease in the deflection for the SNFRC beams relative tothe control beam. The macrosynthetic fiber addition of0.50% and 0.75% to the concrete increased the slenderbeams resistance to deflection at ultimate load by 63% and

o500

Deflection (in.)0.2 0.3

~ 300

".•.3 200

...oOl60 Q.

;;:."40 ~

_ .. Control

-Vf"0.50%

-Vf"0.75%

o400

Deflection (in.)0.2 0.3

Z:.~ 200o...J

60 •..oOlQ.

40 ~~

468Deflection (mm)

o400

Deflection (in.)

0.2 0.3

z'":;; 200.•o...J

60 •..oOlQ.

40 ~~

4 6 8 10Deflection (mm)

Fig. 3-Load-versus-deflection curves of' (a) slender beamsin Phase I; (b) slender beams in Phase ll; and (c) shortbeams in Phase ll.

93%, respectively. The increase in deflection resistance dueto macrosynthetic fibers is more pronounced in short beams-93% and 138% for 0.5% and 0.75%, respectively. The deflectionresults indicated the significant improvement in the ductility ofthe beams due to the addition of macrosynthetic fibers.

The shape of the load-deflection curves of the FRC beamsrelative to the control beams (Fig. 3) demonstrates theimproved global structural response that discrete macrosyntheticfibers add to RC beams. The benefit can be immediately seen inthe first diagonal cracking load, the ultimate load-carryingcapacity, and the ductility of the beams before failure.Furthermore, the toughness, which is proportional to the areaunder the load-deflection curve, is another indicator of thebenefit of macrosynthetic fibers.

Load-strain responsesThe measured strains in this study were used primarily to

judge the type of failure of the RC beams (that is, shearfailure versus flexural failure) to explain the cracking patternand to approximate the load at which shear cracking started.These measurements were also used to further explain thedifference in behavior between the control and FRC beams.

Figure 4 shows the tensile strain in the steel and thecompressive strain in the concrete at the section of maximum

Table 3-Loads corresponding to first diagonalcrack and ultimate capacity of controland SNFRC beams

First diagonal Ultimate load, Increase inBeam crack load, kN kN ultimate load, %

Ll-O.O-a 347 347 -

Ll-O.O-b 339 339 -

Ll-O.O-c 341 341 -

Ll-0.50-a 360 * -

Ll-0.50-b 372 386 13

Ll-0.75-a 393 429 25

Ll-0.75-b 423 426 24

Shl-0.0-a 405 405 -

Shl-O.O-b 358 376 -Shl-0.50-a 398 439 12

Shl-0.50-b t >500 >28

Shl-0.75-a t >500 >28

Shl-0.75-b t >500 >28

L2-0.0-a 230 230 -

L2-0.0-b 236 236 -

L2-0.50-a 263 263 13

L2-0.50-b 261 267 15

L2-0.75-at 227 245 5

L2-0.75-b 266 279 20

L2-1.0-a§ 259 265 14

L2-1.0-b 260 303 30

Sh2-0.0-a 270 270 -Sh2-0.0-b 270 270 -

Sh2-0.50-a 289 335 24

Sh2-0.50-b 281 300 11

Sh2-0.75-a 305 337 25

Sh2-0.75-b 310 341 26

*This test was stopped after fIrst crack load.tActuator capacity was exceeded before fIrst diagonal cracking.*Outlier based on strain data.§Beam was severely segregated.Note: I kN = 224.809 Ibf.

bending moment (gauge em,l and gauge ex,I). The RCbeams with a higher fiber dosage rate (Vf= 0.75% and 1.0%)are selected for discussion as they experienced greater deflectionsand, thus, greater strain in concrete and reinforcing bars. Theincrease in the measured longitudinal strain of the steel withincreasing fiber volume fraction is attributed to the ability offibers to better (more fibers per unit volume) arrest crackingand distribute stresses and, thus, attract greater loads.Consequently, the strain in the steel of the SNFRC beamsat ultimate load was increased with increasing fiber volumefraction. The maximum measured strain in the reinforcingsteel bars at ultimate load was approximately 1500 microstrain(less than the yield strain of the steel), which indicates thatflexural yielding did not occur prior to the shear failure. Themaximum concrete strain in the extreme compression fiberat the section of the maximum bending moment (gauge ex, 1)was not more than 1500 microstrain, which is less than thecompressive failure strain of the concrete. Therefore, thestrain in the concrete and the reinforcing steel bars at ultimateload indicate that the mode of failure of the RC beam wasshear as it was designed.

The tensile strain in the concrete as measured by the diagonalembedded strain gauge em,2 in slender beams of Phase I arepresented in Fig. 5. The test results show that gauge em,2responded similarly in all beams up to a load level ofapproximately 170 kN (38.2 kips), which is much lowerthan the load at which the first visible diagonal shear crackoccurred (340 kN [76.4 kips] for control beams and 360 to

Table 4-Average values of beam deflectionsbefore ultimate load

Control Vf= 0.50% VrO.75% Vf= 1.0%

Deflec- Deflec- Deflec- Deflec-tion, tion, Increase, tion, Increase, tion, Increase,

Beams mm mm % mm % mm %

Slenderbeams, 5.3 7.4 40 9.6 81 - -Phase I

Slenderbeams, 3.7 6.0 63 7.1 93 7.3 100Phase II

Shortbeams, 2.6 4.0 54 - - - -Phase I

Shortbeams, 1.7 3.5 103 4.1 138 - -Phase II

Z 300~"co.3 200

,...oIII60 Q.-;:-6'<II

40 -

-Vf=0.75%j 20-Vf=1.0%

o500 1000 1500 2000

o·2000 ·1500 ·1000 ·500 0

Fig. 4-Load versus strain in reinforcing bars (gauge em,l)and concrete top fiber (gauge ex,]) at midspan.

400 kN [80.9 to 89.9 kips] for fiber RC beams). One possibleinterpretation for this observation is that the flexural crackstarted to incline at this load level, which defmed the initiationof flexural shear crack. The flexural shear crack initiation isdifferent from the flexural shear crack formation, whichfully developed at a much higher load level. Control RCbeams and SNFRC beams exhibited the same response up tothe point of flexural shear crack initiation, suggesting that

500Slender beams - Phase I gauge em,2

100

400

80

Z 300,...0:.

60III.., c-

eo '"0 -ij'...J200 ~40

100 -Control20

-Vf=0.50%

-Yf=0.75%0 0

0 100 200 300 400Microstrain

Fig. 5-Tensile strain responses of the gauge em,2 inslender beams of Phase I.

Z 150:...,eo.3 100

,...o

30 ~

-Control

-Vf=0.50%

-Yf=0.75%o

300

Fig. 6-Load points at which diagonal crack reachedmidheight in slender beams in Phase II.

macrosynthetic fibers do not alter the load at which the shearcrack initiates. The short beams of Phase I and all the beamstested in Phase II also behaved in a similar way. This observationis consistent with conventional wisdom that fibers do notincrease the tensile strength of concrete. The point of shearcrack initiation cannot be clearly seen from the load-deflectioncurves. This point, however, as indicated by the strain data,represents a turning point in the beam's response after whicha clear behavioral difference is exhibited by the SNFRCbeams relative to the control beams.

The strain measured by the gauge, however, is stronglyinfluenced by the location of the diagonal crack relative tothe location of the strain gauge. Two strain behaviors werecaptured: one when the diagonal crack went through thegauge (or passed in very close proxirllity), and the otherwhen the crack path was not close to the location of the straingauge. In the first case, the rate of increase in strain changeddramatically and the gauge recorded increasing strain at aconstant rate after the crack had passed the strain gauge position,whereas in the second case, the strain started to decrease.The rate of the strain response to increased loading and thedegree of unloading is influenced by the fibers and is used tohighlight the difference between the control and SNFRCbeams. The general trend in the first case is that the strain inthe control beam increased at a rapid rate after the initiation ofthe diagonal crack, whereas the strain in the FRC beamsincreased at a lower rate, indicating a kind of strain-hardeningresponse (refer to Fig. 5). This is attributed to the fibers' abilityto distribute stresses and to slow down the crack propagationprocess. A similar behavior can also be seen when the crackpath was not close to the location of the strain gauge by lookingat the level of strain reduction (gauge unloaded). For thecontrol beams, the strain gauge unloaded rapidly and ceased tomeasure any increase in strain, whereas for the SNFRC beams,the gauge unloaded at a much lower rate and this phenomenonoccurred at relatively higher loads and deflections.

Another important feature that can be obtained from thestrain measurement is the load at which the diagonal crackreached the rlliddle of the beam (location of gauges em,2 andem,3). According to some researchers,22 the load at whichthe diagonal crack reached the rlliddle of the beam defines itsshear strength. As mentioned previously, for the case whenthe diagonal crack passed through the gauge, the rate ofincrease in strain changed as the crack approached the gauge

Fig. 7-Cracking sequence and patterns of (a) slender control beam (L2-0.0-a); (b) SNFRCslender beam with Vf = 0.50% (L2-0.50-a); (c) SNFRC slender beam with Vf = 0.75%(L2-0.75-b); (d) control short beam (Sh2-0.0-a); (e) short beam with Vf = 0.50% (Sh2-0.50-b); and (f) short beam with V f = 0.75% (Sh2-0.75-a).

and the strain increased at a constant rate afterward. Figure 6presents strain data for the control RC and the SNFRC beamswhere the diagonal crack propagated through the straingauge. The point at which the rate of strain reduced but thestrain continued to increase at a constant rate can be definedas the point (circled in the graph) the diagonal crack reachedthe middle of the beam. The curves in Fig. 6 clearly showthat the SNFRC beams generally reached this level at ahigher load compared to the control RC beams.

The data obtained from the strain gauges demonstrate thatmacro synthetic fibers increase the load at which the shearcrack forms and the fibers also increase the strain capacity.They are also consistent with the findings obtained from theload-versus-deflection response and further explained theglobal response of the RC beams.

Cracking and failureCracking pattern and sequence were carefully monitored

and mapped during testing. Figure 7 presents pictures ofcracking pattern and sequence for slender and short beams.During the test of slender beams, it was observed thatflexural cracks started at the midspan and spread out to theshear span, where the flexural cracks (with increasing load)began to incline as flexural shear cracks (diagonal crack).The number of flexural cracks, the inclination of thediagonal crack that lead to failure, and the degree of splittingalong the longitudinal reinforcement characterize thecracking pattern of the tested RC beams. The control RCbeams failed with a single and steep diagonal shear crack,which was associated with obvious splitting along thelongitudinal reinforcement as can be seen in Fig. 7(a).Moreover, the control beams had fewer flexural cracksbefore the formation of the first diagonal crack, whichmarked the failure of the beams. Unlike the control RCbeams, the SNFRC beams developed multiple flexural anddiagonal shear cracks before failure occurred, as can be seenin Fig. 7(b) and (c). Furthermore, the formation of the rustdiagonal crack did not mark the failure of the SNFRC beams.They continued to resist higher loads and developed morediagonal cracking before failure, which occurred in a gradualand stable manner. The creation of multiple flexural andshear cracks in the SNFRC beams contributed to the increaseof the ductility as reflected in the load-deflection curves,particularly for the RC beams reinforced with 0.75% offibers. The synthetic fibers significantly reduced splittingalong the longitudinal reinforcement relative to the controlbeams as shown in Fig. 7. The beams with 0.75% of fibersdid not exhibit obvious splitting along the reinforcing bars.This is attributed to the fact that fibers improved the confinementstress around the reinforcing bars and thus reduced splitting.

As can be seen in Fig. 7(b) and (c), the primary diagonalcrack that leads to failure of the SNFRC beams was flatterrelative to that of the control beam (Fig. 7(a» and extendedfarther toward the support. Fenwick and Paulay23 postulatedthat appreciable arch action develops when the diagonalcrack extends to the support, thereby separating the tensionand compression zones of the shear span and allowing therelatively large translational displacement associated witharch action to occur. The cracking patterns of the SNFRCbeams show that macro synthetic fibers improve the archaction in slender RC beams and thus lead to an increase ofthe shear strength.

Figure 7(d) and (e) show the cracking pattern andsequence for short beams. The short control RC beams

Table 5-Average values of increase in first shearcracking load and ultimate load of beams due toaddition of macrosynthetic fibers

Increase in first Increase inVj,% aid crack load, % maximum load, %

0.50 3.5 10 14

0.75 3.5 18 231.00 3.5 12 30

0.50 2.3 7 >20

0.75 2.3 14 >28

0.40

•• 0.35no::E~. 0.30~~.r.

0.25g,c~U; 0.20...••Q).r.00 0.15

0.10

I,

•: LoINer Lim~

I " rSFRCAC1~1a-4a;-R 1.4.6.1(1)

- - -- - -- --- - -- - .-ACI~1.con -.ild=i"sr

DX1gn Shee!I~~~S nglh Lim~

4.8

4.44.0 00:r

CIl3.6 ~

III

3.2..•(;::J

2.8 19.:r

2.4 ""--0'"

2.0 ""--"C

1.6 !!!.

1.20.00 0.25 0.50 0.75 1.00 1.25

Fiber volume fraction (V,), %

Fig. 8-Values of shear strength normalized by JfZ forbeams tested in this study.

shown in Fig. 7(d) developed web-shear cracks that led to asudden and brittle shear failure. Unlike the control RCbeams, the SNFRC beams developed flexural shear cracksbefore failure, as can be seen in Fig. 7(e) and (f). The addition ofmacro synthetic fibers changed the mode of failure of shortbeams from web-shear cracking to flexural shear cracking.This can be attributed to the fact that macro synthetic fibersincreased the shear strength of the beam to a level that wassufficient to mobilize flexural cracking prior to shear failure.

Shear strength analysisAs mentioned previously, macrosynthetic FRC increased

the first diagonal cracking strength and the ultimate shearstrength relative to the control RC beams, as summarized inTable 5. The increase in shear strength above the referencebeams ranged between 14 and 30% for slender SNFRCbeams, depending on the dosage of fibers and by more than28% for short beams. This significant improvement in theultimate shear strength is consistent with published resultsobtained by previous investigatorsl4,15 who reported testresults on limited number of RC beams using another type ofsynthetic macrofibers.

To account for the real material properties and sectiondimensions, the average shear strength of the RC beamswas calculated from the ultimate loads using actual dimensionsof the beams. The average shear strength then was normalized tothe square root of the compressive strength. It is well establishedthat the shear strength of concrete beams is directly proportional tothe square root of concrete compressive strength. Thenormalized results are shown in Fig. 8 for all beams tested inthis program. The graph also shows the ACI 318-08 limit of theplain concrete shear strength (0.17 JJ7[2JJ7]) and thelower limit (0.3JJ7[3.5JJ:]) that was established for

SFRC to be considered as an alternative to the ACI 318-08minimum requirement for shear reinforcement (refer toSection R11A.6.1(t) of ACI 318-08). The results obtainedfrom this study show that macrosynthetic fibers at volumefraction greater than 0.75% can meet the shear strengthlower limit established by ACI 318-08 for SFRC. The beamductility, however, must be investigated further to explore thepotential of macrosynthetic fibers as an alternative for theminimum shear reinforcement, which will be a topic foranother paper.

SUMMARY AND CONCLUSIONSStructural testing of 27 large-scale beams under center-

point loading were conducted to determine if newlydeveloped macrosynthetic fibers could enhance the shearstrength and failure behavior of longitudinally reinforcedconcrete beams without stirrups. Slender and short beamswith aid of 3.5 and 2.3, respectively, were tested. The beamswere instrumented with embedded and external straingauges to monitor the strain in the concrete and in thereinforcing steel bars. The length of the beams was variedbetween 1.9 and 3.2 m (75 and 126 in.), and the macrosyntheticfibers were added at volume fractions of 0.50, 0.75, and1.0%. Global deflection of the RC beams, the strain in theconcrete and in the flexural reinforcing bars at particularlocations within the beam, and the cracking pattern weremonitored during the shear tests at different stages ofloading. The experimental results showed that the additionof macrosynthetic fibers significantly improved the shearstrength and ductility of the RC beams and modified thecracking and failure behavior of the RC beams. Specificfindings and conclusions are summarized as follows:

The addition of macrosynthetic fibers to concreteimproved the first diagonal shear cracking strength andthe ultimate shear strength of the reinforced concretebeams. Synthetic fibers at 0.50, 0.75, and 1.0% volumefraction increased the ultimate shear strength of slenderbeams by 14%,23%, and 30%, respectively, comparedto the control beams. Similarly, macrosynthetic fibersat 0.50 and 0.75% increased the ultimate shear strengthof short beams by at least 20% and 28%, respectively;The addition of macrosynthetic fibers to RC beamschanged the cracking pattern and mode of failure. Thecontrol RC beams failed with a single and steep shearcrack that led to a mode of failure that was brittle. TheSNFRC beams exhibited multiple diagonal cracksbefore failure occurred. Furthermore, the failurediagonal crack extended further toward the support ofthe SNFRC beams, suggesting that macro syntheticfibers improved the arch action in slender beams andthus increased the shear strength. The addition ofmacrosynthetic fibers to short RC beams changed themode of failure from web-shear cracking (observed incontrol RC beams) to flexural shear cracking;The load-versus-deflection curves showed that theSNFRC beams are remarkably more ductile comparedto the control RC beams. The control beams failedabruptly upon the formation of the first diagonal crackwhereas the SNFRC beams continued to resist higherload and deflection after the formation of the firstdiagonal crack. The addition of macrosynthetic fibersat 0.50, 0.75, and 1.0% by volume increased the deflectionat maximum load for slender beams by 63, 93, and100% compared to the control beams. Similarly, macrosyn-

thetic fibers at 0.50 and 0.75% increased the deflectionat maximum load for short beams by 103 and 138%relative to the control RC beams. This increase in thedeflection capacity of the SNFRC beams demonstrates thesignificant improvement in ductility that macrosyntheticfibers could impart to the concrete beams; and

• The load-versus-strain curves showed that macrosyntheticfibers effectively distributed stresses in the concrete beam(by transferring stresses across diagonal cracks) andimproved its strain capacity. The data further showedthat macrosynthetic fibers did not change the load atwhich the diagonal crack initiated, but they did slowdown the propagation and widening of the diagonalcrack and thus increased the load at which the majordiagonal crack fully developed. The strain data alsoverified that the formation of the first diagonal crackdid not mark the failure of the SNFRC beams and thebeam continued to distribute stresses and, thus, increasedthe strain capacity of the beam and its shear strength.

ACKNOWLEDGMENTSSupport for this project was provided in part by W.R. Grace, Cambridge,

MA, USA; Juma AJmajid Company, Dubai, United Arab Emirates CUAE);and the Hazard and Risk Management Research Group, University ofSharjah, Sharjah, UAE. The authors would like to acknowledge supportprovided for this project.

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