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Shear resistant mechanisms of high

strength fibre reinforced concrete beams

G. Campione, L. La Mendola & G. ZingoneDipartimento di Ingegneria Strutturale e Geotecnica, Universita,Viale delle Scienze, 1-90128, Palermo, ItalyEMail: campione@stru. diseg. unipa. it

Abstract

This paper presents results of an experimental investigation carried out on highstrength fibre reinforced concrete beams in the presence of traditional steelreinforcement, subjected to monotonic and cyclic loads. The aim of theinvestigation was to point out the effectiveness of the fibres as shearreinforcement of the beams, and to show the possibility of changing the shearmode of failure, characterised by high brittleness and poor dissipative capacities,into a flexural mode of failure, which is certainly more suitable in structuresdesigned in seismic areas.

1 Introduction

In reinforced concrete structures in seismic zones, dissipative mechanisms withhysteresis stable cycles are suitable in order to balance the input energy due tostrong earthquakes, greatly limiting the consequent damage. When the shearforce in the beams of framed structures is considerable and the shearreinforcement is inadequate, the failure condition can occur without the fullflexural capacity being attained; the consequence of the brittle failure consists ina poor dissipative mechanism.Several experimental and theoretical investigations addressed to analysing theflexural-shear interaction are present in the literature. ̂ The presence of stirrups,interacting with the shear resisting mechanism, improve the failure condition,offering a contribution to the ultimate strength. ̂Recent studies have focused attention on the use of short fibres randomlydistributed in the concrete as shear reinforcement. Many authors have treated theproblem from an experimental point of view, proposing some empiricalexpressions to evaluate the contribution due to the fibres to ultimate shearstrengthen in beams with and without stirrups. The above mentioned

Transactions on the Built Environment vol 38 © 1999 WIT Press, www.witpress.com, ISSN 1743-3509

24 Earthquake Resistant Engineering Structures

investigations about the use of fibre reinforced concrete refer to monotonicloading.In this paper monotonic and cyclic behaviour is analysed considering differenttypes of fibres in high strength concrete. The experimental investigation showed:- an increase in the ultimate and residual shear strengths for monotonic loading; -more sudden loss of bearing capacity with a significant reduction of stiffness incyclic loading compared to the monotonic case; - possibility of changing thebrittle shear mechanism into a more dissipative flexural mechanism by addingfibres.

2 Experimental program

For the development of the experimental investigation reinforced concretebeams, using high strength concrete with and without fibres, were cast. Thedimensions, boundary conditions and loading arrangements are shown in Fig. 1.For all the beams (20 specimens) the same longitudinal reinforcement wasutilised and for each type of material (plain concrete and fibre reinforcedconcrete) two specimens with different percentages of transversal steelreinforcement were arranged.

225 , 400 225 i_

L=1000 mm

P/2 P/2-N.1

125

N.1 - 2 d) 6.35 mm (950)

N.2-2(j)16mm (1100) N.3 - <|> 6.35 mms=98 and 180 mm

Figure 1: Details and loading configuration of test beams.

Types, geometrical and mechanical characteristics of the fibres adopted arereported in Table 1. The volume percentage adopted for all the types of fibresutilised is 2 %. Cylindrical specimens (100x200 mm) were also prepared andtested in both compression and indirect tension to characterise the mechanicalbehaviour of concrete without fibres (plain concrete) and with fibres (fibrereinforced concrete). For the concrete the following components and relativepercentages were utilised: - 400 Kg/m̂ of cement Portland; - 1050 Kg/mr> ofcoarse aggregate with maximum size 10 mm\ - 720 Kg/nr> of sand; - 55 Kg/m* ofsilica fume; - 150 / of water; - 8 Kg/m̂ of superplastlcizer. The latter, in theamount of 2 % by weight of cement, was necessary to ensure sufficientworkability of the mixes, since it is reduced by the presence of high percentages


Earthquake Resistant Engineering Structures 25

of fibres and silica fume added to the composites to ensure better bond betweenfibres and matrices.

Table 1. Types and properties of the fibres.

Type of fibre

PolyolefmCarbon

Crimped steelHooked steel

Shape

—/\/\/\/\"A /—

Equivalentdiameterdf [mm]

0.800.781.000.50

LengthLf

[mm]25203035

Tensilestrengthft [MPa]

37580011151115

Modulus ofelasticity[MPa]

12000100000207000207000

The deformed steel bars, with a diameter of 16 mm, utilised for longitudinalreinforcement of the beams, is type f̂ >275 MPa, with the followingcharacteristics: - Young's modulus = 206000 MPa\ - yielding stress = 300 MPa; -initial strain hardening = 0.0176; - maximum stress = 400 MPa\ - straincorresponding to maximum stress = 0.14. The transversal steel reinforcement isconstituted by stirrups as shown in Fig.l, having a smooth profile with a diameterof 6.35 mm, yielding stress of 550 MPa, and pitches of 98 or 180 mm. The coverof the steel bars was 10 mm thick, having the same dimension as the maximumsize in coarse aggregate. All the controlled displacement tests were carried outusing a universal testing machine. The four point bending tests on reinforcedbeams were carried out using a rigid frame connected to the testing machine bymeans of a spherical joint. Loads were applied to the beams through the rigidframe at concentrated points, in accordance with the static scheme shown inFig.l. A load cell connected to a data acquisition system made it possible torecord the actual load. The deflections of the beams, purged of the settlementsand the crushing effect of the supports, were recorded through LVDT's,appropriately placed on the beams.

3 Mechanical characterisation of fibre reinforced concrete

3.1 Compressive tests

In Fig. 2 results of monotonic and cyclic tests carried on plain concrete and fibrereinforced concrete cylinder specimens and already discussed in detail in aprevious paper^ are shown.The dimensions of the specimens are diameter 100 mm and length 200 mm, withall four types of fibres. For plain concrete, stable cyclic tests in the softeningbranch of the stress-strain curve were not possible, due to the brittleness of thematerial.The strain was evaluated by using two LVDT's on a gauge half the length of thespecimen placed in the middle part of the specimens, to ensure the recording ofdeformations which are not affected by boundary condition effects; for thesoftening branch three additional LVDT's were placed on a gauge the length ofthe specimen.



90

4 / \1 /U** \J fh \ \1&^\

.i,i,-,,in— mi Ofi-itn .A/M-I c^ r-*»f A 'Poiyolefin \

Crimped steel i\ — — Hooked steel |

PoiyoleftnCarbonCrimped steelHooked ' '

>̂ ""̂ ^/S>-

b)

0.00 0.01 0.02 0.030.00 0.01 002 003

Figure 2 : Results of compression tests: a) monotonic loading;b) cyclic loading.

The experimental results show that fibre reinforced concrete has compressivebehaviour characterised by higher ductility than plain concrete. This observation,which is also valid for normal strength concrete, is even truer in the case of highstrength concrete having high brittleness.In all cases examined the adding of fibres involves an increase in the straincorresponding to the peak stress; moreover, high values of ultimate strain andresidual strength were observed. The peak stress, which generally increases, evenif slightly, in the case of polyolefin fibres, exhibits a reduction compared to plainconcrete; this is perhaps connected to the high air amount in the composites.Fig.3 shows the mode of failure of fibre reinforced concrete in compression usingdifferent types of fibres: the adding of fibres determines higher dissipativecapacities with respect to plain concrete, due to the pull-out resistance of singlefibres randomly distributed in the matrices.

Polyolefin Carbon Hooked steel

Figure 3: Failure mode in compression.

3.2 Indirect tensile tests

Fig. 4 shows experimental results of monotonic indirect tensile tests carried outon cylinders having the same dimensions and characteristics as those utilised forcompressive tests.The results obtained show that the use of fibres significantly increases the peak



and residual strength compared to plain concrete; above all, hooked and crimpedsteel fibres ensure better behaviour than polyolefin and carbon fibres. It isimportant to observe that the tensile behaviour of fibre reinforced concrete issubstantially related to the pull-out resistance of single fibres: if the pull-outphenomenon occurs gradually (fibres with good shape, adequate length, etc.) thebehaviour of the composite will be ductile in tension as shown in Figs. 4 and 5.Table 2 gives the meaningful values of the experimental investigation mentionedbefore.

J20

240-

1 GO-

SO-

^ ̂ Hooked steel/ " ->. ̂

/_ __/ ~~ ̂ - __ __ Crimped steel

// J\

~ * CarbonPlain cone re it e "••--r"-~-~-~~-~-~-r.r~ Polyolefin

Jl LVDT" ;u / - r ^ ui—f \ I ) 4—-

10x10x200 mm

1 2 3W (mm)

Figure 4: Results of split tests and testing arrangement.

Plain concrete Polyolefin Carbon Hooked steel

Figure 5 : Failure mode in split tension.

Table 2. Characteristic values of fibre reinforced concrete.Type

Plain concretePolyolefinCarbon


Compressivestrengthf c [MPa]70.2353.0267.3178.4871.14

Strain at peakstressGo

0.00290.00540.00380.00740.0048

Peak tensilestrengthft [MPa]3.203.424.485.757.66

Residual tensilestrengthfr IMP a]

11.301.734.685.70



4 Behaviour of tested beams

Experimental results, extensively discussed in a previous work**, obtained bybending tests on beams, the details of which are shown in Fig.l with s=98 mm,have shown that the ultimate condition is reached for flexural failure: inparticular, the failure of the beams is due to crushing of concrete, since there isonly slight yielding of the steel in tension because of the high percentages oflongitudinal steel reinforcement. For the beams with s=180 mm, the results forwhich are presented in this paper, prevalently shear failure occurs, as shown in

Figure 6: Shear failure of a beam without fibres, with stirrupswiths=180/%/%.

Results of monotonic and cyclic tests are shown in Fig.7, in which P is the loadacting through the rigid frame on the beams, and S is the net deflection.

ISO

CL 120-

80-

40-

a)' Plain concrete

PoiyolefinCarbonCrimped steel

— — Hooked steel

10

b)Plain concretePolyoieflnCarbonCrimped steel

— — Hooked steel

20 30 0 10 20 306 (mm) 6 (mm)

Figure 7: Load-deflection results for beams with s=180 mm: a) monotonicloading; b) cyclic loading.

The curves in Fig, 7a) show that the adding of fibres in all cases produces animprovement in performance compared to the case of plain concrete: thesignificant loss of strength in the softening branch, which shows brittle shear



failure, can be attenuated or wholly eliminate by changing the type of failure,thus allowing the full flexural capacity to be attained.

120

a) b)

10 200 10 20 0d (mm) 6 (mm)

Figure 8: Monotonic and cyclic results for a) beams in plain concrete;b) beams in hooked steel fibre reinforced concrete.

In particular, it can be observed that even if the shear failure happens again, highvalues of residual strength are obtained due to the presence of the fibres. In thecase of steel fibres, especially in the case of hooked steel fibres, characterised byhigh tensile strength and better bond condition, the failure mechanism becomes aflexural mechanism, being more ductile in this way.Under the action of cyclic loads the bearing capacity of the beams, characterisedby shear failure is exhausted more quickly than in the monotonic case, andtherefore the envelope curve is not close to the monotonic curve in the softeningbranch. This circumstance, that can be pointed out by the comparison betweenthe analogous curves in Fig. 7a) and 7b), is due to the significant and progressivereduction in stiffness as number of cycles increases. The difference betweenmonotonic and cyclic behaviour can be attenuated until it fully disappears byadding fibres as shear reinforcement. It can be observed that even after repeatedcycles of unloading and reloading, fibres can transfer forces across the cracks.For clarity's sake in Fig. 8 the above comparison is shown for tested beams inplain concrete and in hooked steel fibre reinforced concrete.

5 Ultimate shear strength

Several analytical and empirical models concerning the shear problem in normalstrength concrete with and without fibres have been reported in literature. In theabsence of fibres the ultimate shear stress v^ is calculated by considering thesimultaneous occurrence of both beam action and arch action mechanisms; theprediction of Bazant & Sun* for normal strength concrete is able to reproduce theexperimental results by means of the following expression:

= A (mMPa) (1)

with A=0.54, B=0.5, C=249.



In eqn (1) vj/ = [l + ̂5.08/d* 1/̂ 1 + d/(25 dj is the size effect factor; d* is the

maximum aggregate size in mm; d is the effective depth of the beam in mm; a isthe shear span; £' is the compressive strength in MPa and finally p=AJ(bd) isthe reinforcement steel ratio (&= area of tensile reinforcement and 6= beamdepth). The success of Bazant's equation for predicting the shear stress Vu in thefield of normal strength concrete encouraged Imam et al.™ to propose anextension to high strength concrete and a modification in order to take intoaccount the effect of the fibre contribution. The expression obtained refers tohooked steel fibres with different geometric characteristics than those used inthis investigation. In any case this expression is not general; for this reason it isnot able to predict the ultimate shear stress in the presence of different types offibres.For the investigation presented in this paper the expression of v% proposed byImam et at.™ can be used only in the case of plain concrete: for high strengthconcrete the values for A, B, C are 0.60, 0.44 and 275 respectively. Severalstudies have been carried out in the literature in order to take into account theeffect of stirrups, if present, on the failure mode. For the case examined here,due to the great clear span of vertical stirrup legs (s=180 mm=1.79 d), thestirrup effect can be considered negligible.In order to obtain an expression for v% able to reproduce the experimental resultsof this investigation in the presence of different types of fibres, a more generalmodel, proposed by Al-Ta'an & Al-Feef, is utilised in order to calculate theshare of ultimate shear stress, due to the presence of the fibres. The latter isexpressed as follows:

•\r — /ft ̂ if \7 i i<\ Wo o\\uf —(o.3 Jv Vf jLf/Qf l/y \Z.)

where L& df are the length and the diameter respectively of the fibres (see Table1), Vf is the volume percentage and K is a factor depending on the fibre shape;K is assumed equal to 1, 1.2 or 1.3 for straight, hooked or crimped fibresrespectively, as in Al-Ta'an & Al-Feel. The model on which the fibrecontribution to shear strength is evaluated is based on Fig. 9 in which the fibrescrossing the cracking surface participate in the shear strength due to the pull-out force.

P/2 i

0.85 fc

Figure 9: Forces within fibre reinforced concrete beamwith diagonal tension crack.

Eqn (2) is based on a regression analysis that refers to 89 beam tests. Takinginto account the considerations made above, for the experimental investigation



presented here the ultimate strength stress is evaluated by:

v,, = v +vuf (3)

in which v% is expressed by eqn (1) with A, B, C relative to high strengthconcrete and Vuf by eqn (2).In Table 3, the calculated and measured shear strength stresses are contained.

Table 3. Calculated and measured shear strength stresses.Type

Plain concretePolyolefinCarbon


Vu [MPa]eqn(l)4.113.894.074.214.12

v̂ [MPa]eqn (2)

/0.590.480.741.58

v'u [MPa] eqn(3)4.114.484.554.955.70

ym_Pu/2[MPa]• bd

4.184.774.725.085.21

The values of v«™ are deduced from the P% values, which are the maximumvalues in the curves of Fig. 7a). The comparison between analytical andexperimental values shows the good approximation level. A more appropriateevaluation can be based on a model: - taking into account the interaction offibres on the beam and arch mechanisms; - considering also a correlationbetween the tensile strength of the fibrous concrete and the characteristic fibreparameter VfLf/df. This correlation must be particularised for any fibre type andcan be obtained on the basis of adequate tests.

6 Conclusions

The experimental investigation presented in this paper, addressed to evaluatingthe effectiveness of fibres as shear reinforcement, made it possible to arrive atthe following conclusions:• the adding of fibres to high strength concrete improves the compressive

behaviour, particularly for the softening phase, and increases the tensilestrength for both the peak value and the residual value (due to fibre pull-outstrength from the concrete);

• the increase in tensile strength involves an improvement in the behaviour ofthe beam; in particular, the adding of fibres can change shear failure intoflexural failure;

• analytical expressions present in the literature made it possible to estimatethe ultimate shear stress with a good level of approximation. A betterevaluation could be obtained on the basis of further experimentalinvestigations carried out with variation in the most significant parameters: -volume of fibre; - percentage of transverse steel reinforcement; - tensilereinforcement ratio; - span-to-depth ratio. In this way the characteristicvalues changing shear failure into flexural failure can be identified and thecombined beneficial effect of stirrups and fibres on the ultimate strength canbe investigated;



• in the presence of several cyclic loads a more dissipative mechanism can beobtained with the adding of fibres, due to the pull-out strength.

References

1. Bazant, Z. & Sun, H., Size effect in diagonal shear failure: influence ofaggregate size and stirrups, ACI Material Journal, 84 (4), pp.259-272, 1987.

2. Amad, S.H., Khaloo, A.R. & Poveda A., Shear capacity of reinforced high-strength concrete beams, ACI Journal, 83 (2), pp.297-305.

3. Russo, G., Zingone, G. & Puleri, G, Flexure-shear interaction model forlongitudinally reinforced beams, ACI Structural Journal, 88 (1), pp. 60-68,1991.

4. ACI Committee 318, Building Code Requirements for Reinforced Concreteand Commentary, ACI 318M-89/ACI 318RM-89, American ConcreteInstitute, Detroit, 1989.

5. Russo, G., & Puleri, G, Stirrup effectiveness in reinforced concrete beamsunder flexure and shear, ACI Structural Journal, 94 (3), pp. 227-238, 1997.

6. Sharma, A.K., Shear strength of steel fiber reinforced concrete beams, ACIJournal, 83 (4), pp. 624-628, 1986.

7. Al-Ta'an, S.A. & Al-Feel, J.R., Evaluation of shear strength of fibre-reinforced concrete beams, Cement & Concrete Composites, 12 (2), pp. 87-94, 1990.

8. Li, V.C., Ward, R. & Hamza, A.M., Steel and synthetic fibers as shearreinforcement, ACI Materials Journal, 89 (5), pp. 499-508, 1992.

9. Ashour, S.A., Hasanain, G.S. & Wafa, F.F., Shear behavior of high-strengthfiber reinforced concrete beams, ACI Structural Journal, 89 (2), pp. 176-184,1992.

10. Imam, M., Vandewalle, L. & Mortelmans, F., Shear-moment analysis ofreinforced high strength concrete beams containing steel fibres, Can. J. Civ.Eng., 22 (3), pp. 462-470, 1995.

ll.Furlan Jr, S. & Bento de Hanai, J., Shear behaviour of fiber reinforcedconcrete beams, Cement & Concrete Composites, 19 (4), pp. 359-366, 1997.

12. Zingone, G., Campione, G. & La Mendola, L., Comportamento ciclico ditravi inflesse in cemento armato fibrorinforzato ad alta resistenza, Atti del 8°Convegno Nazionale L'Ingegneria Sismica in Italia, 1, pp. 167-174, 1997.


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