Tailor Made Concrete Structures – Walraven & Stoelhorst (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8

Replacement of shear reinforcement by steel fibres in pretensionedconcrete beams

P. De Pauw & L. TaerweMagnel Laboratory for Concrete Research, Department of Structural Engineering, Faculty of Engineering, GhentUniversity, Ghent, Belgium

N. Van den Buverie & W. MoermanWilly Naessens Industriebouw nv., Wortegem-Petegem, Belgium

ABSTRACT: By means of loading tests up to failure, the shear behaviour of precast pretensioned concretebeams made with steel fibre concrete and without conventional shear reinforcement is compared with the shearbehaviour of a standard beam made with concrete without fibres but with stirrups as shear reinforcement. Abeam made of plain concrete without shear reinforcement is used to investigate the effect of both types of shearreinforcement. The beams are designed according to Eurocode 2 and, especially for the steel fibre concretebeams, according to the guidelines of the “σ − ε − design method” recommended by the RILEM TC 162-TDFtechnical committee and according to information found in literature. From the test results it can be concludedthat, for the beams considered, ordinary shear reinforcement can be eliminated by using steel fibre reinforcedconcrete. However, due attention should be paid to the mixing and casting procedures of the steel fibre concrete.

1 INTRODUCTION

Structural concrete elements like beams always con-tain shear reinforcement if they are designed accordingto Eurocode 2. If no shear reinforcement is requiredaccording to the equilibrium equations, minimumshear reinforcement has to be provided. The produc-tion and placing of conventional shear reinforcementlike stirrups is quite labour intensive and represents anon negligible part in the production time and produc-tion cost of concrete elements. Therefore, the precastconcrete industry is looking for methods to avoid theapplication of traditional shear reinforcement. Dif-fused steel fibre reinforcement might be a suitableanswer. Not only could steel fibre concrete lead toa reduction or replacement of the conventional shearreinforcement like stirrups, it would also make theconcrete less brittle, especially in the precast concreteindustry where high strength concrete is often used.The addition of steel fibres increases the toughnessof the concrete and reduces the width and spacing ofpossible cracks, Taerwe (1992).

A possible reduction in the workability of the con-crete, especially for high fibre contents, and possiblevariations in the material characteristics due to a nonhomogeneous distribution of the fibres, might be thetwo most important obstacles for the breakthrough ofsteel fibre concrete in the precast concrete industry.

In literature, a few studies can be found whichseem to confirm the efficiency of steel fibres toincrease the shear strength of concrete beams oreven to replace the conventional stirrups: Adebar,Mindess, St-Pierre & Olund (1997), Batson, Jenkins &Spatney (1972), Belletti, Bernardi & Meda (2004),Campione & Mindess (2003), Casanova, Rossi &Schaller (1997), Di Prisco, Iorio & Plizzari (2003),Kwak, Eberhard, Kim & Kim (2002) Meda, Minelli,Plizzari & Riva (2002), Narayanan & Darwish (1987),Parra-Montesinos (2006) and Rosenbusch & Teutsch(2002).

Several parameters affect the shear strength of con-ventional or steel fibre reinforced concrete beams.Amongst these parameters are: the beam depth d, theshear span to depth ratio a/d, the flexural reinforce-ment ratio ρ, the concrete compressive strength fc, thegeometry of the beam, the volume fraction of steelfibres Vf , the aspect ratio of the fibres λf , the shapeof the fibres and the tensile strength of the fibres.Most of the available formulas and design methodsfor shear strength calculations take into account theseparameters.

In order to investigate the feasibility of steel fibreconcrete as shear resisting element in precast pre-tensioned beams, an experimental test-program wascarried out in cooperation between the Magnel Labora-tory for Concrete Research of Ghent University and the

391

Megaton company which is a Belgian company activein the precast concrete sector and which is a member ofthe Willy Naessens Group. The Bekaert company pro-vided the steel fibres and assisted in the mix-design ofthe steel fibre concrete. The test-program is discussedin the following paragraphs.

2 TEST PROGRAM

To judge steel fibre concrete as a possible replacementfor conventional stirrups in precast pretensioned con-crete beams, four I-shaped beams with a total lengthof 10.9 m were manufactured and subjected to a load-ing test up to failure. The beams are geometricallyidentical. They have a constant depth of 900 mm, aflange width of 300 mm and a web width of 80 mm.This beam type (I 90/30) is one of the types whichare available in the standard production range of theMegaton company. Figure 1 shows the elevation andtwo cross sections of the beams. The major part of thebeam shows an I-shaped cross-section, while at theend-blocks with a length of 1050 mm the cross sectionis rectangular. The transition from the I-shaped sectionto the rectangular section is realised over a distance of100 mm.

The beams are simply supported and have a spanlength of 10.3 m. The dead weight of the beams equalsabout 3.6 kN/m and they are designed to carry auniform service load of about 31 kN/m. A concreteof strength class C 50/60, passive reinforcement ofstrength class BE 500 S and seven-wire prestressingstrands with a nominal diameter of 12.5 mm, giv-ing a nominal surface area of 93 mm2 per strand, ofstrength class 1860 N/mm2 are used. In the referencebeam without steel fibres, vertical two-legged stir-rups, made out of reinforcement bars with diameter8 mm, are provided as shear reinforcement.Taking intoaccount seven prestressing strands, stressed at 135 kNeach, a stirrup spacing of 300 mm is required in theI-shaped part of the beam according to Eurocode 2.Theend-blocks contain an adequate passive reinforcementto withstand the anchorage forces of the prestressingstrands and the shear forces at the beam ends.

To test the shear behaviour of a beam, it is loadedwith a point load F located at a distance of 2.75 m fromone of the beam supports. Just applying the point loadat the given position instead of a uniform load over theentire beam length however does not guaranty a shearfailure of the beam. On the contrary, the beam will stillfail in bending. To avoid bending failure of a beam,its flexural capacity is increased by applying a higherprestressing force. Hereto, the number of prestressingstrands in the bottom flange of each beam is increasedfrom seven to eleven. As a result of this increasedprestressing force, the allowable tensile stress at thetop fibre of the beams is exceeded at the moment of

Figure 1. Elevation and cross-sections of the beams.

Figure 2. Passive reinforcement in the reference beam withstirrups and in the beams without stirrups.

prestressing.To resist the tensile force in the top flangeafter prestressing, four longitudinal reinforcement barswith a diameter of 16 mm are provided in the top flangeof each beam.The adapted beams are more likely to failin shear during the loading test but their top flange willshow bending cracks due to the prestressing action.The longitudinal reinforcement bars and the bendingcracks in the top flange of the beams will have an influ-ence on the shear behaviour of the beams as comparedto traditional prestressed beams where the longitudinalbars and the cracks are not present. However, a rela-tive comparison between beams with different shearresisting elements is still possible.

A lateral view of the reference beam with traditionalstirrups every 300 mm in the I-shaped section of thebeam is given in figure 2. The other three beams areidentical as far as the prestressed and passive reinforce-ment are concerned, except for the vertical two-leggedstirrups in the I-shaped section of the beam which aremissing, as shown in figure 2.

A first beam without stirrups is made with plainconcrete without steel fibres to be able to investigatethe effect of the two types of shear reinforcement onthe shear strength afterwards. The two other beamswithout stirrups are made with steel fibre concretecontaining the same fibre type but in different vol-ume fractions. The fibre type used is the DRAMIXRC 80/60 BP from the Bekaert company. It concerns ahigh strength steel fibre with hooked ends, a length of60 mm and a length to diameter ratio of 85. In the firstbeam with steel fibre concrete, the fibres are added inan amount of 40 kg/m3. In the second beam with steelfibre concrete this amount is 60 kg/m3. The amount of

392

Figure 3. Strand layout in the bottom flange of the I-shapedcross-section.

fibres is chosen based on design models and based oninformation found in literature.

High strength steel fibres made from a steel wirewith a minimum tensile strength of 2300 N/mm2 werechosen to obtain a ductile failure of the steel fibre con-crete.The high tensile strength of the fibres guaranteesthat the fibres are pulled out of the concrete and thatthey do not break due to tension failure when they areused in concrete of higher strength classes. A disad-vantage of the high strength RC 80/60 BP fibre is that,for the moment, it is only available in non-galvanisedform.

The four beams were cast and pretensioned at theMegaton company. Beside the four beams for the testprogram, some other beams for commercial purposeswere present on the prestressing bench. Because theheaviest commercial beam needed fifteen prestress-ing strands, these strands had also to run through thetest beams. Two of those strands were added left andright of the three strands already present in the secondstrand layer of the test beams, while the two remainingstrands were added as extra strand layers with only onestrand above the three strand layers already present.The extra strands were provided with a special plasticcover so that they would not adhere to the concrete ofthe test beams and would not influence the requiredprestressing force for the test beams. Figure 3 showsthe layout of the strands in the bottom flange of theI-shaped cross-section.

3 CONCRETE MIX-DESIGN

For both the plain concrete and the fibre concrete oneconcrete mix design is chosen. The only differencebetween the plain and the fibre concrete is the pres-ence of the steel fibres in the latter concrete and apossible increase in the amount of superplasticizer tomaintain a sufficient workability. As constituents thefollowing materials are used: Portland cement CEMI 52.5 R, mixed sand 0/4, broken limestone 2/6.3,

Figure 4. Test set-up for checking the filling capacity of thefibre concrete.

broken limestone 6.3/14, water and superplasticizerGlenium 51.

The aim is to obtain a concrete in the strength classC 50/60. For the steel fibre concrete some specialrequirements concerning the mix-design have to befulfilled.

For both the plain and the steel fibre concrete, thestandard mix design from the Megaton company couldbe used. The only things that still had to be checkedwere the amount of superplasticizer needed to obtaina workable steel fibre concrete and whether the steelfibre concrete would pass through the web area ofthe beam and through the openings of about 30 mmbetween the prestressing strands to completely fill thebottom flange of the formwork. A preliminary test wascarried out to check these requirements.

The bottom flange of a test beam was simulated bymeans of a small formwork consisting out of woodenside panels and Plexiglas end-panels. In the Plexiglasend-panels eighteen openings were created to allow for16 mm longitudinal reinforcement bars to be placed inthe bottom flange of the test-element. The reinforce-ment bars simulate the strands in the real beams andthe Plexiglas allows checking the filling of the form-work. The formwork of the test element is placed on avibrating table. Figure 4 shows the test set-up.

In the daily production of traditional precast pre-tensioned beams with plain concrete, both formworkvibrators and poke vibrators are used to compact theconcrete in the formwork. For casting steel fibre con-crete however, the use of the poke vibrators shouldbe avoided because they might influence the orienta-tion of the fibres. Therefore the formwork vibratorsshould be able to realise the compaction of the steelfibre concrete by themselves.

As a consequence, in the preliminary test, the tablevibrator on which the test set-up was placed should beable to compact the steel fibre concrete without extrahelp of poke vibrators.The test was carried out with thefibre concrete containing 60 kg/m3 of fibres. By meansof slump measurements it was found that he amount ofsuperplasticizer for this concrete had to lie in the rangeof 2.2 l/m3 to 2.4 l/m3 to obtain a workable concrete.In a first attempt it was tried to fill the formwork and

393

compact the concrete by activating the vibrating tableonly a few times for a few seconds during casting.Very soon, bridge-forming of fibres above the rein-forcement bars occurred and the bottom flange of thetest element was not completely filled. This could beseen through the Plexiglas and was confirmed by theextra compaction that could be realised by using a pokevibrator afterwards. In a second attempt, the vibratingtable was activated at the start of the casting opera-tion and it was left on during the entire casting period.The concrete was also poured slower into the form-work. Now the formwork was completely filled andthe concrete compaction was good. The applicationof poke vibration afterwards did not show any furthercompaction of the concrete.

It was concluded that the fibre concrete could beused to realise the beams if due attention is paid to thecasting and compacting procedures. Only formworkvibrators in a sufficient number could be used and theyshould be active during the entire casting operation.Furthermore the pouring of the fibre concrete shouldbe executed with an adapted speed.

4 PRODUCTION OF THE BEAMS

The four test beams were produced by the Megatoncompany on four different days over a period of abouta week.Table 1 gives an overview of the beam designa-tion, the beam characteristics and the most importantdates related to the production and testing of thebeams.

The casting of the beams posed no real problems.The fibres of the concrete with a dosage of 60 kg/m3

showed bridge forming on the reinforcement bars inthe top flange of the beams. By the action of the form-work vibrators and the adapted pouring speed however,the concrete with the fibres passed the reinforcementbars in the top flange and the strands in the bottomflange and the formwork was filled correctly.

Beside the four beams, also a number of smallscale test specimens were cast. It concerns cubeswith a rib length of 150 mm for compression tests

Table 1. Overview of the test beams and production relateddates.

Beam Production Testnumber Characteristics date date

Beam 1 No stirrups 21/03/2007 23/04/2007no fibres

Beam 2 Stirrups 22/03/2007 25/04/2007no fibres

Beam 3 No stirrups 27/03/2007 27/04/2007fibres 40 kg/m3

Beam 4 No stirrups 28/03/2007 03/05/2007fibres 60 kg/m3

at 1, 3, 7, 14 and 28 days of concrete age. Cylin-drical test specimens with a diameter of 150 mmand a height of 300 mm to determine the secantmodulus of elasticity at the moment of prestress-ing and at the moment of the loading test on thebeam. Prismatic test specimens with dimensions150 mm × 150 mm × 600 mm for shrinkage tests andwith dimensions 150 mm × 150 mm × 500 mm forcreep tests and prismatic test specimens with dimen-sions 600 mm × 150 mm × 150 mm for two types ofdisplacement controlled bending tests. All small scalespecimens were compacted on a vibrating table.

5 TEST RESULTS

5.1 Small scale tests

All small scale test specimens were stored for 24 hoursat the Megaton company. The specimens were kept intheir moulds and stored in conditions similar to theconditions in which the beams were kept. After 24hours the test specimens were demoulded just like thebeams. While the beams were kept at the Megatoncompany until a few days after prestressing, the smallscale test specimens were transported to the MagnelLaboratory for Concrete Research where they werestored in the main test hall or in a climate chamber.

Table 2 gives an overview of some test resultsobtained during the compression tests and the tests todetermine the secant modulus of elasticity. The lowercompressive strength and lower secant modulus ofelasticity for the steel fibre concrete with 60 kg/m3

fibres might be due to a lower degree of compactionof the concrete. The concrete with the highest amountof fibres showed the lowest density, while the concretewith 40 kg/m3 fibres and the plain concrete of beam 2showed the highest densities.

Shrinkage and creep tests were performed in a cli-mate chamber at 20◦C and with a relative humidity of60%. The creep tests were started at the day that thebeams were prestressed. The load level for the creeptests was chosen equal to the compressive stress at thecentre of gravity of the beams.The shrinkage deforma-tions of the different concrete mixes were more or lessthe same. The creep deformations were more or less

Table 2. Average concrete compressive strength and secantmodulus of elasticity.

Secant modulusfcm,cube, 7days fcm,cube, 28days of elasticity(N/mm2) (N/mm2) (N/mm2)

Beam 1 60.9 63.0 31500 at 27 daysBeam 2 55.6 65.4 31700 at 26 daysBeam 3 59.3 62.4 34000 at 38 daysBeam 4 50.9 55.7 26900 at 37 days

394

similar for the concrete of beams 1 and 2 but higherfor beam 3 and the highest for beam 4. This is due tothe difference in concrete age at the moment the creeptests were started.

Two types of deformation controlled bending testswere performed to determine the post crackingbehaviour of the steel fibre concrete. Four point bend-ing tests according to the Belgian standard NBNB15-238 were performed on prismatic test specimenswith a span length of 450 mm and a cross section of150 mm × 150 mm. Three point bending tests accord-ing to the RILEM method were performed on notchedprismatic test specimens with a span length of 500 mmand an area of 150 mm × 125 mm in the notched crosssection.

Out of the four point bending tests, the flexural ten-sile strength at first cracking fct,fl and the conventionalflexural tensile stresses ff150 and ff300 were determined.Table 3 gives the test results.

Out of the three point bending tests, the flexuraltensile strength at first cracking fct,fl, the equivalentflexural tensile stresses feq2 and feq3 and the residualflexural tensile stresses fR1 and fR4 were determined.Table 4 gives the test results. For the concrete withoutfibres, the moment of first cracking also represents themoment of failure of the prismatic specimen. For plain

Table 3. Results from the four point bending tests on smallscale test specimens.

fct,fl ff150 ff300 Number of fibres(MPa) (MPa) (MPa) in failure section

Beam 1A 5.32 – – –Beam 1B 5.62 – – –Beam 2A 5.90 – – –Beam 2B 5.00 – – –Beam 3A 5.48 5.97 5.62 70Beam 3B 6.10 4.55 4.37 68Beam 4A 5.55 4.74 4.69 94Beam 4B 6.66 6.31 6.34 114

Table 4. Results from the three point bending tests on smallscale test specimens.

Fibres infct,fl feq2 feq3 fR1 fR4 failure(MPa) (MPa) (MPa) (MPa) (MPa) section

Beam 1A 4.47 – – – – –Beam 1B 4.37 – – – – –Beam 2A 4.09 – – – – –Beam 2B 4.61 – – – – –Beam 3A 6.45 6.31 7.36 6.56 7.87 84Beam 3B 4.72 3.75 4.42 3.88 4.83 65Beam 4A 4.79 3.95 4.42 4.03 4.41 91Beam 4B 4.61 5.07 5.58 5.06 5.23 84

concrete only the flexural strength at first cracking isdetermined.

As can be seen from tables 3 and 4, the addition ofsteel fibres to the concrete has almost no effect on theflexural tensile strength at first cracking. Furthermoreit can be seen that the three point bending test giveslower values for the flexural tensile strength at crack-ing then the four point bending test. The conventional,equivalent and residual flexural tensile stresses showimportant variations for similar concrete specimens. Itcan also be seen that the addition of 60 kg of fibres doesnot lead to better results than the addition of 40 kg offibres to one cubic meter of concrete. Also the numberof fibres present in the failure section shows importantvariations. Part of the variations might be explained bythe difficult situation in which the concrete had to besampled from the pouring container in the productionplant.

5.2 Tests on the full scale beams

During the loading test to failure, each beam wasloaded stepwise by means of a hydraulic jack witha capacity of 1000 kN at 2.75 m from one of the sup-ports. Both electronic and manual measurements werecarried out. Vertical displacement measurements werecarried out at the supports, at the section of the pointload, at mid-span and at the section 7.725 m away fromthe support closest to the point load. Deformation mea-surements were carried out over the depth of the beamin the area under the point load. The cracks present inthe top flange and the counter flare of each beam wereregistered before starting the load test. The formationof bending cracks in the bottom flange of the beam andof shear cracks in the web region between the pointload and the closest support were observed and regis-tered during the load test. Once the load approachedthe estimated failure load, the manual measurementswere stopped and the load was gradually increaseduntil failure.

Table 5 gives an overview of the most importantload values for each beam. It contains the load corre-sponding with the first bending crack in the bottomflange, the load corresponding with the first shear

Table 5. Overview of the most important load values.

Beam 1 Beam 2 Beam 3 Beam 4

Load at first 400 kN 400 kN 480 kN 400 kNbending crackLoad at first 405 kN 400 kN 490 kN 420 kNshear crackLoad at final 460 kN 640 kN 680 kN 620 kNmanualmeasurementFailure load 597 kN 721 kN 740 kN 695 kN

395

Figure 5. View of the beams 1 to 4 after failure.

crack in the web, the load at which the last manualmeasurements were carried out and the failure load.

All beams showed shear failure in the area betweenthe point load and the closest support. The lowest fail-ure load was obtained for the beam without shearreinforcement. The three beams with shear resistingreinforcement showed failure loads that lie in the sameorder of magnitude. The beam with a fibre dosage of40 kg/m3 showed the highest failure load, while thebeam with a fibre dosage of 60 kg/m3 showed thelowest failure load of the beams with shear resistingreinforcement. The latter beam however also showedthe lowest compressive strength.

Figure 5 shows the beams after failure. In the beamsmade with concrete without steel fibres, the first shearcrack appeared in the top left corner of the web. Con-secutive cracks at higher loads appeared more andmore downward in the web. For beam 2 with stirrups,at a load of 550 kN a number of parallel shear cracksappeared simultaneously.

The critical crack for beams 1 and 2 had an incli-nation of about 17◦. In the beams made with steelfibre concrete, the shear crack formation was com-pletely different. The first shear crack appeared muchlower in the web than for the beams made with plainconcrete. Consecutive cracks at higher loads appearedmore upward in the web. The critical crack in beam 3had an inclination of about 36◦ in the web and about18◦ in the bottom flange. The critical crack in beam 4had an inclination of about 42◦ in the upper part of theweb and about 28◦ in the lower part of the web.

The bending cracks of beam 2 ran much higherinto the web than the bending cracks of the steel fibreconcrete beams 3 and 4.

Figure 6 shows the vertical displacements of thedifferent beams in the section under the concentratedload.

Up to a load of 400 kN all beams showed more orless similar vertical displacements. For higher loadvalues, the vertical displacements of beams 1 and 2were higher than the vertical displacements of beams3 and 4.

Figure 6. Vertical displacements of the beams in the sectionunder the concentrated load.

6 CONCLUSIONS

Loading tests have shown that it is possible to replacetraditional stirrups by steel fibre concrete to resistshear forces in precast pretensioned beams. Further-more it was shown that higher volume fractions offibres do not necessarily lead to a higher shear resis-tance. The latter phenomenon is probably due to thelower workability and lower compactability of steelfibre concrete with high fibre volumes. When usingsteel fibres in structural concrete elements, specialattention should be given to the concrete mix design,the mixing operation and the pouring and compactingprocedures.

REFERENCES

Adebar P., Mindess S., St-Pierre D., Olund B., 1997, Sheartests of fibre concrete beams without stirrups, ACI Struc-tural Journal, 94(1), 68–76

Batson G., Jenkins E., Spatney R., 1972, Steel fibres asshear reinforcement in beams, ACI Journal Proceedings,69(10), 640–644

Belletti B., Bernardi P., Meda A., 2004, Shear behaviour ofprestressed beams reinforced with steel fibres, 6th RILEMsymposium on fibre-reinforced concrete (FRC) – BEFIB2004, Italy, 925–934

Campione G., Mindess S., 2003, Fibres as shear rein-forcement for high strength reinforced concrete beamscontaining stirrups, High performance fibre reinforcedcement composites (HPFRCC 3): proceedings of the thirdinternational RILEM workshop, 519–528

Casanova P., Rossi P., Schaller I., 1997, Can steel fibresreplace transverse reinforcements in reinforced concretebeams? ACI Materials Journal, 94(5), 341–353

Di Prisco M., Iorio F., Plizzari G., 2003, HPSFRC Prestressedroof elements, RILEM TC 162 – TDF Workshop: Testand design methods for steel fibre reinforced concrete,Germany, 161–188

Eurocode 2: ENV 1992 – 1-1: 1991 and EN 1992 – 1-1: 2004:Design of concrete structures – Part 1-1: General rules andrules for buildings

Kwak Y-K., Eberhard M.O., Kim W-S., Kim J., 2002, Shearstrength of steel fibre-reinforced concrete beams withoutstirrups, ACI Structural Journal, 99(4), 530–538

396

Meda A., Minelli F., Plizzari G.A., Riva P., 2005, Shearbehaviour of steel fibre reinforced concrete beams, Mate-rials and Structures, 38, 343–351

Narayanan R., Darwish I.Y.S, 1987, Use of steel fibres asshear reinforcement, ACI Structural Journal, 84, 216–227

Parra-Montesinos G.J., 2006, Shear strength of beams withdeformed steel fibres: evaluating an alternative to min-imum transverse reinforcement, Concrete International,November 2006, 57–66

Rilem TC 162 TDF, 2000, Test and design methods forsteel fibre reinforced concrete: σ− ε – design method:Recommendation, Materials and Structures, 33, 75–81

Rilem TC 162 TDF, 2003, Test and design methods forsteel fibre reinforced concrete: σ− ε – design method:Final recommendation, Materials and Structures, 36,560–567

Rosenbusch J., Teutsch M., 2002, Brite EuRam ProjectBRPR – CT 98 – 0813: Sub task 4.2: Trial beams in shear,Technical University Braunschweig, Germany

Taerwe L., 1992, Influence of steel fibres on strain-softening of high strength concrete, ACI Materials Jour-nal, January–February 1992, (Title no. 89-M7), Detroit,54–60

397