Rashid Reinforced Fibrous Concrete Elements Subjected to uniaxial tensile loading..........................pdf

KSCE Journal of Civil Engineering (2010) 14(4):547-556DOI 10.1007/s12205-010-0547-0

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www.springer.com/12205

Structural Engineering

A Study on the Reinforced Fibrous Concrete Elements Subjected toUniaxial Tensile Loading

Rashid Hameed*, Anaclet Turatsinze**, Frédéric Duprat***, and Alain Sellier****

Received May 28, 2009/Revised September 11, 2009/Accepted November 4, 2009

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Abstract

The structural response of prisms of cross section 100×100 mm and length of 500 mm constructed with reinforced fibrous concreteand subjected to pure tensile loading has been presented in this contribution. The main focus was to study the effects of addingdifferent metallic fibers in mono and hybrid form in the conventionally reinforced concrete on the tension stiffening and straindevelopment. Two metallic fibers with different geometrical, mechanical and physical properties were investigated: amorphousmetallic straight fibers and carbon steel hooked-end fibers. A total of four concrete mixtures: control, single fiber and hybrid fiberreinforced concretes were prepared. The fibers were investigated at content of 20 kg/m3 for single fiber reinforced concretes, and forhybrid fiber reinforced concrete, at content of 40 kg/m3. Through studying load-deformation response of composites and straindevelopment in steel bar and concrete, it has been found that the metallic fibers improve tension stiffening effect and influencesignificantly the strain development. The effect of two metallic fibers on tension stiffening was seen to be different at differentloading stages. On the other hand, when fibers were used in hybrid form, the behaviour of the composite was improved at all loadingstages in terms of tension stiffening and resistance to cracking. Keywords: reinforced fibrous concrete, metallic fibers, cracking, tension stiffening, strain

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1. Introduction

Since concrete is relatively weak and brittle in tension,cracking is expected when tensile stress is induced in a member.Cracking, in almost any case, is an inevitable characteristic ofreinforced concrete and their effects are multiple. Cracks canaffect the durability of structures by facilitating the corrosionprocess in steel reinforcement. They can cause depreciation inthe appearance of concrete structures. They may also affect theserviceability of water retaining structures by leakage of liquid(ACI 224.2R-92; Christiansen and Nielsen, 2001).

When plain concrete is subjected to tension, the initiation andgrowth of inherent micro-cracks reduce the load carrying areaand increases the stress concentration at critical crack tips,causing the cracks to propagate further (Swaddiwudhipong andSeow, 2006). Before cracking, the behaviour of concretecomponent in tension is assumed to be linear up to its tensilestrength in the calculations of reinforced concrete behaviour andafter cracking it is assumed that it drops to zero indicating failure

of member in tension. But in actual practice, even after cracking,the concrete blocks between two adjacent cracks is still capableof resisting tensile forces induced because of bond with thereinforcement. This contribution of concrete resistance betweencracks is known as tension stiffening effect and helps to controlthe member stiffness, deformation and crack widths related tosatisfying serviceability requirements (Prakhya and Morely,1990; Fields and Bischoff, 2004).

Bond between steel bar and concrete is a key aspect of tensionstiffening since it controls the ability of the reinforcement totransfer tensile stress to the concrete. Mindess (1995) found thatfibers enhance the bond between the matrix and reinforcing steelby inhibiting the crack growth emanating from the bar defor-mations. Different types of fibers have been used in the cementmatrix to improve the cracking and deformation characteristics,toughness and tensile strength of the composite. Bischoff (2003)found that the addition of steel fibers to the concrete improvesthe tension stiffening in reinforced concrete because the fiberconcrete is able to carry tensile stress through the crack opening.

*Ph.D. Scholar, Université de Toulouse, UPS-INSA; LMDC (Laboratoire Matériaux et Durabilité des Constructions), F-31077 Toulouse Cedex 04,France (Corresponding Author, E-mail: [email protected])

**Associate Professor, Université de Toulouse, UPS –INSA, LMDC (Laboratoire Matériaux et Durabilité des Constructions), F-31077 Toulouse Cedex04, France (E-mail: [email protected])

***Associate Professor, Université de Toulouse, UPS-INSA, LMDC (Laboratoire Matériaux et Durabilité des Constructions), F-31077 Toulouse Cedex 04,France (E-mail: [email protected])

****Professor, Université de Toulouse, UPS-INSA, LMDC (Laboratoire Matériaux et Durabilité des Constructions), F-31077 Toulouse Cedex 04, France(E-mail: [email protected])

Rashid Hameed, Anaclet Turatsinze, Frédéric Duprat, and Alain Sellier

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Abrishami and Mitchell (1997) found that after yielding of steelbar it is possible to get an increased value of tensile load by theaddition of steel fibers. Studies have also been carried out toinvestigate the crack resisting ability of the matrix by the use offibers in hybrid form (Banthia and Nandakumar, 2003).

Due to increasing structural applications placing greaterdemand on material performance, the need for more fundamentalinformation on the behaviour of concrete and fibre reinforcedconcrete under different types of loads is of paramount im-portance (Mohammadi et al., 2008). In some cases, a reinforcedconcrete member may undergo purely tensile forces. Forexample, a vertical section of a circular tank wall must carry thetensile ring force produced as a result of the internal liquidpressure. Direct tensile forces may also develop in sections dueto restrained volumetric deformations (Kianoush et al., 2008).Although reinforced concrete members subjected to pure tensiondo not often occur in practice, there is always a need to study indetail about such loadings in order to develop the understandingof the basic principles governing the behaviour of some material,especially for some innovative materials like Fiber ReinforcedConcrete (FRC).

It is evident that if the cracks are not properly arrested, they cancause even complete failure of the structures, and in case ofwater retaining structures, even micro cracks can cause seriousdamage. To study the effect of adding metallic fibers on resist-ance to cracking and tension stiffening effect, behaviour ofreinforced concrete elements incorporating metallic fibers sub-jected to pure tension has been experimentally investigated andresults are reported in this paper. Two metallic fibers differing intheir mechanical, geometrical and physical properties were used.The fibers have been studied in both mono and hybrid forms.

2. Experimental Program

2.1 ConcreteFour different concrete mixes, one control and three mixes

containing metallic fibers were studied. The results of com-pressive strength tests performed on the four mixes showed thatthe average compressive strength of plain and metallic fiberreinforced concrete varied between 40 MPa and 45 MPa. For allthe concrete mixes, CEM I 52.5 R type cement has been used.Local natural round sand with maximum particle size of 4 mmwas used. Round gravels with size range of 4-10 mm were usedas coarse aggregate. A Super-plasticizer has been used as anadmixture to improve the workability of the mix in the presenceof metallic fibers. Table 1 show the mix proportion of controlconcrete.

2.2 Type of Fibers UsedTwo types of macro-metallic fibers, 30 mm in length were

used (Fig. 1).Type I fibers are amorphous metallic fibers (named in this

study as MF1). They are composed of (Fe, Cr) 80% and (P, C,Si) 20% in mass. No corrosion is observed in these fiberswhen immersed in HCl (0.1 N) and in FeCl3 (0.4N) for 24hour (Redon and Chermant, 1999). Due to their rough surfaceand large specific surface area, these fibers are characterisedby a high bond with concrete matrix. They are straight andflexible.

Type II metallic fibers (named in this study as MF2) are madeusing carbon steel wires, and are characterised by a weak bondwith the matrix compared to type I fibers due to their smoothsurface and less specific surface area. They are circular and havehooked-ends. They are usually adhered together in clips ofcertain number of wires. When these clips enter in the mix, theadhesive is dissolved and individual fibers are distributed evenlythroughout the mix.

The characteristics of these two types of metallic fibers (i.e.,MF1 & MF2) are given in Table 2, where L, W, T, D and E arelength, width, thickness, diameter and modulus of elasticityrespectively.

Fiber type and dosage for all the concrete mixtures are given inTable 3. Each mixture is given a name according to type andquantity of fibers. For example, RC-20MF1 where RC standsfor reinforced concrete, 20 is quantity of fibers in kg/m3, MF1 istype of fiber. Similarly, RC-40HyF where 40 is quantity offibers and HyF stands for hybrid fiber.

Table 2. Fibers Investigated in this Study

Fiber Fiber TypeDimension (mm)

Geometry E, GPa Tensile strength (MPa) Density Cross Section

L W T D

MF1 amorphous metal 30 1.6 0.03 - Straight 140 2000 7.2 Rectangular

MF2 carbon steel 30 - - 0.5 Hooked-end 210 1200 7.8 Circular

Table 1. Mixture Proportion of the Control Concrete

Cement(Kg/m3)

Sand(Kg/m3)

Gravel(Kg/m3)

Water(Kg/m3)

Super-Plasticizer(Kg/m3)

322 872 967 193 1.61

Fig. 1. Amorphous Metallic Fibers (MF1) and Carbon Steel Fibers(MF2)

A Study on the Reinforced Fibrous Concrete Elements Subjected to Uniaxial Tensile Loading

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2.3. Specimen DetailsSpecimen used to perform uniaxial tensile test had a cross

sectional dimension of 100×100 mm and length of 500 mm asshown in the Fig. 2. A conventional reinforcing steel bar of 12mm diameter with characteristic yield strength of 580 MPa wasused to ensure the cracking of matrix before the yielding of steelbar. In order to study the load deformation response of the steelbar, a control strain gage was pasted on bare portion (out ofconcrete) at a distance of 50 mm from the concrete end face (Fig.3). Similarly five strain gages were also pasted on the steel barportion embedded in the concrete matrix (Fig. 3) to study thelocal strain development at the interface of steel and concrete. Tostudy the strain development on the concrete surface, five straingages were pasted on the concrete surfaces at the same positionscorresponding to the positions of strain gages on steel bar asshown in Fig. 4.

To prevent pulling out of concrete from the ends during puretensile loading, a bond free zone (no contact) between concreteand steel bar was created up to a length of 50 mm from each endof the specimen. This was made possible by passing the steel barthrough the PVC pipe of length 50 mm and fixing the pipe at the

end of the mold as shown in Fig. 5. In each specimen, the overalldeformation was measured using LVDT as shown in Fig. 6. Thedistance between the reference points for the axial deformationmeasurements was equal to the contact length between concreteand steel bar which was 400 mm.

2.4 Testing Procedure and MeasurementAfter 28 days of casting, displacement controlled uniaxial

tensile tests were performed using universal testing machine. Foreach composition, three samples were tested and the resultspresented here are the average of all three samples. The loadingrate of the displacement imposed by the machine was 0.03 mm/

Table 3. Fiber Contents in Different Concrete Mixtures

MixtureName

MixtureType

Content of fibers, kg/m3Total quantity offibers, (kg/m3)MF1 MF2

RC-CONT Control − − −

RC-20MF1Mono fiber

20 − 20

RC-20MF2 − 20 20

RC-40HyF Hybrid fiber 20 20 40

Fig. 2. Test Specimen

Fig. 3. Location of Strain Gages on Steel Bar

Fig. 4. Location of Strain Gages on Concrete



second. All the specimens were gripped in the machine jaws ateach ends of the steel reinforcement (Fig. 1) so that the load istransferred from the reinforcement to the concrete matrix.

Following measurements were made during each test and datawas recorded at a rate of 10 readings/sec using data acquisitionsystem.• Strain in the steel bar at different locations (Fig. 3)• Surface strain in concrete at different locations (Fig. 4)• Axial Load• Axial deformation (global) of the specimen using LVDT (Fig.

6)

3. Experimental Observations

3.1 Load – Deformation BehaviourIn order to understand the load-deformation behaviour of control

and reinforced fibrous concretes, the axial load is plotted versusthe specimen elongation normalized by the initial effectivelength of the specimen (400 mm) and also strain in bare steel bar(control gage reading). Representative curves of load-defor-mation behaviour for all four compositions are shown in Fig. 7.From these curves, following observations have been made:• Behaviour before first crack • First cracking load• Tension stiffening effect before yielding of steel bar• Behaviour after yielding of steel bar

Fig. 5. PVS Pipe used to Prevent Contact between Concrete andSteel

Fig. 6. Measurement of Axial Deformation using LVDT

Fig. 7. Load-Deformation Response of All Composites (Axial strain is in µm/m.)


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Before formation of first transverse crack, control and fibrousconcrete behaved similarly in term of load versus global axialdeformation response. At this stage, the difference between acomposite member and bare reinforcement response show thetension stiffening effect of the un-cracked section.

Tensile load capacity at different stages has been presented inFig. 8. Regarding the first crack load, RC-40HyF containingfibers in hybrid form exhibited maximum value of first crackingload of 42 kN which was 36% more than the composite withoutfibers (RC-CONT). In general, it was observed that in the pre-sence of MF1 fibers, the first crack load is increased. On theother hand, no effect was noticed on the first crack load whenMF2 fibers were used. The minimum load carrying capacity wasobserved with control composition as expected. After the firstcracking, strain increased suddenly along with a drop of axialload for control and all fibrous composites. Load drop is causeddue to release of elastic stored energy at transverse cracking ofconcrete matrix (Ficher and Li, 2002). Load drop after the firstcrack in RC-CONT, RC-20MF1, RC-20MF2 and RC-40HyFwas 6, 3, 4 and 4 kN respectively (Fig. 8).

Tension stiffening has been evaluated in this study based onload sharing between steel bar and concrete matrix and tensionstiffening bond factor, β is calculated by dividing Pc,m with Pcr,where Pc,m is average load carried by cracked concrete and isobtained by subtracting the load carried by steel bar from thecomposite load and Pcr is load carried by concrete at firstcracking (Bischoff, 2003) as shown in Fig. 9.

Tension stiffening bond factor, β was calculated for all com-positions and presented in Fig. 10. Higher value of β shows thatthe member is more stiffened. It can be observed in Fig. 10 thatthe fibers affect the tension carried by the composite since thefibers contribute in carrying tension along with the steel bar atthe crack location. Among different type of fibers used in thisstudy, it was observed that before yielding of steel bar whencrack width is small, effect of MF1 fibers on tension stiffening ismore significant compared to MF2 fibers (Fig. 10). Although upto yielding of steel bar, the effect of MF2 fibers when presentalone is less significant compared to MF1 fibers, but RC-40HyFcontaining fibers in hybrid form exhibited maximum value of βcompared to other composites.

Further increase of applied displacement resulted in yielding ofsteel bar and crack opening was increased dramatically. The loadat yielding stage for all four composites is shown in Fig. 8, whereit is observed that RC-40HyF exhibited maximum yielding load;this can be attributed to the increased tension stiffening in thepresence of metallic fibers in hybrid form. No effect on com-posite axial tensile load carrying capacity was observed in thepresence of MF2 fibers alone. After the yielding of steel bar, itwas noticed that the axial load capacity of RC-20MF1 startdecreasing and approaches to a value similar to RC-CONT. Onthe contrary, axial tensile load capacity of composite containingonly MF2 fibers start increasing and becomes higher than thevalues of both RC-20MF1 and RC-CONT; this showed theeffectiveness of MF2 fibers in restraining the macro cracks fromfurther opening and improving the response in term of ductilityand tensile strength after yielding of conventional reinforcement.

3.2 Local Strain Distribution in SteelA number of observations regarding the strain distribution in

embedded steel bar prior and after the crack formation can bemade from the local strain recorded using strain gages pasted atdifferent section of steel bar as shown in Fig. 3.

Strain distribution at different section of embedded steel barversus axial load for the RC-CONT is shown in Fig. 11, where itcan be observed that stiffness increases with increase of distancefrom the ends of specimen, this shows a gradual transfer of loadfrom the reinforcement to the concrete matrix and this is

Fig. 8. Loading History at Different Stages

Fig. 9. Tensile Response of Axial Tension Member (Bischoff, 2003)

Fig. 10. Effect of Metallic Fibers on Stiffness



generally true before cracking. After cracking, a severe decreasein the stiffness of specimen at 308 mm was observed; thisapproached towards the stiffness value of end sections. Firsttransverse crack was appeared between sections at 192 and 205mm (more close to 250 mm) as shown in Fig. 12. After theformation of crack, strain gages in the vicinity of cracking planewere found to be detached from the steel bar. Due to detachmentof gages at section 250 mm and 192 mm (near the crack),unfortunately, it was not possible to record strain after thecracking of specimen. Strain distribution was affected due tochange in interfacial bond between concrete matrix and steelreinforcement after cracking, this resulted in drastic increase ofstrain reading from 520 µε to 1100 µε at 308 mm section. Atemporary contraction of strain at the control gage section due toelastic unloading was also observed after cracking of specimen.

In reinforced fibrous concrete containing only MF1 fibers(RC-20MF1), first transverse crack was appeared between straingages at 250 and 308 mm as shown in Fig. 12 at an axial load of35.7 kN. Observations similar to RC-CONT have been made.

The plot between local strains and axial load is shown in Fig. 13.After cracking a severe increase in the strain value at sections inthe vicinity of the cracking plane was also observed. Themaximum increase in the strain value from 430 µε to 1220 µεwas recorded at 308 mm. A temporary contraction in the barereinforcement due elastic unloading was also observed aftercracking. Fig. 14 shows strain values at mid section for all thecomposites at different load levels, where it was observedgenerally that for a given load level, resistance to deformation isincreased in the presence of MF1 fibers in comparison ofcomposite without fibers and composite containing only MF2fibers.

In reinforced fibrous concrete containing only MF2 (RC-20MF2), the first transverse cracking was occurred betweenstrain gages at 192 and 250 mm (Fig. 12). Drop in tensile loadand contraction in the bare reinforcement was also observedsimilar to other composites. The plot between local strains inembedded steel at different section and axial load is shown inFig. 15. A dramatic increase at section near the crack plane was

Fig. 11. Local Strain at Different Section of Steel Bar (RC-CONT)

Fig. 12. Location of Transverse Crack

Fig. 13. Local Strain at Different Section of Steel Bar (RC-20MF1)


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observed after the cracking due to sudden transfer of load to steelbar alone. Maximum increase was observed with the strain gagereading at 308 mm which was from 420 µε to 1780 µε.Moreover, the local strain at 308 and 392 mm was observed to bemore than that of bare reinforcement, which is unusual whenconsidering axial load equilibrium (Kianoush et al., 2008). Whena comparison was made with RC-CONT in term of resistance tocracking at mid section, it was observed in Fig. 14 that beforetransverse cracking of matrix, resistance to cracking was im-proved by very small percentage in the presence of MF2 fibersand this small improvement occurred close to first cracking load.

The first cracking in the specimen constructed with hybridfiber reinforced concrete containing MF1 & MF2 fibers (RC-40HyF) occurred between strain gages at 250 and 308 mm (Fig.12). The plot between local strains at different section and axialload is shown in Fig. 16. Strain value after cracking at 192, 250and 308 mm sections was 1340, 1615 and 1610 µε respectively.Maximum increase in the strain value from 586 to 1615 µε wasobserved at 250 mm after the crack formation. In the presence offibers in hybrid form, the resistance to deformation was maxi-mum of all the composites at a given load level (Fig. 14).

3.3 Surface Strain in Concrete MatrixSurface strain for each composition was recorded using strain

gages pasted at the same section as that for steel bar (Fig. 2). The

surface strain versus axial load response of the RC-CONT ispresented in Fig. 17. Based on the assumption that, before microcracking, both matrix and steel bar undergo same deformation,the theoretical value of strain has been calculated using Eq. (1),where F is total axial tension carried by composite i.e., Fs (loadcarried by steel)+Fc (load carried by concrete).

(1)

The difference in strain values for all the composites at thesame section in steel bar and at concrete surface near the com-posite cracking load must not be misleading since it apparentlyviolates the basic assumption for the calculation of theoreticalstrain in concrete. Infect, micro-cracks appear inside the matrixat load lesser than the load at which first macro transverse crackappears in the composite. Due to these micro-cracks, the defor-mation in steel and concrete does not remain same. This resultsin significant difference of strain distribution in concrete andsteel before the occurrence of major transverse crack in com-posite.

The theoretical values of strain have been plotted along withthe experimental values for the purpose of comparison. In case ofRC-CONT, strain at the mid section was almost same as theore-tical value. Before cracking, the load is gradually transferredfrom the steel bar to the concrete and the stress concentration

εthF

EcAc EsAs+( )-------------------------------=

Fig. 14. Comparison of Steel Strain before Cracking at Mid Section

Fig. 15. Local Strain at Different Section of Steel Bar (RC-20MF2)

Fig. 16. Local Strain at Different Section of Steel Bar (RC-40HyF)

Fig. 17. Surface Strain versus Axial Load (RC-CONT)



occurs at the mid section resulting in crack development at ornear the mid section.

Surface strain versus axial load in all fibrous composites: RC-20MF1, RC-20MF2 and RC-40HyF are shown in Figs. 18, 19and 20 respectively. Comparing to RC-CONT, It was observedin Figs. 18 and 20 that at given load level, the MF1 fiberssignificantly affect the surface strain. On the other hand, MF2fibers present alone cause a minor increase in the resistance tosurface strain.

When a comparison is made between recorded strains at themid section and theoretical strains, it can be observed with RC-20MF1 (Fig. 18) and RC-40HyF (Fig. 20) that the recorded

value significantly deviates from the theoretical value in thepresence of MF1 fibers exhibiting an increased resistance tosurface deformation.

In order to illustrate the effects of type of metallic fibers, sur-face strain at mid section by all composites were plotted versusdifferent loading levels and presented in Fig. 21. Maximum re-sistance to surface strain at all loading levels was offered by RC-40HyF containing fibers in hybrid form; this further strengthenedthe idea of hybridization of two metallic fibers used in this study.

4. Discussion

Uniaxial tensile test were performed on reinforced fibrousconcrete composites and results have been analysed in term oftension stiffening effect and strain development in steel bar andconcrete matrix in the presence of two metallic fibers in bothmono and hybrid forms. From the results obtained, the action oftwo metallic fibers, whose geometrical and mechanical proper-ties are different, can be explained as follows:

From SEM coupled with replica technique, Turatsinze andBascoul, (1996) found that the micro-cracking starts inside thematrix before the occurrence peak load. At the time of initiationof micro-cracks, MF1 fibers which are characterised as non-slipping fibers due to rough their surface and larger specificsurface area resulting in high bond strength with matrix, arestretched and arrest the micro-cracking mechanism throughtransferring the stress across the crack edges. By doing this,micro-cracks formation is delayed resulting in high load carryingcapacity at first macro-crack. From the results obtained, it wasobserved that the composites containing MF1 fibers exhibitedhigh values of first crack load compared to composite withoutfibers (RC-CONT) and also offered more resistance to straindevelopment. Moreover, the propagation of micro-crack towardsthe surface of concrete is also obstructed which significantlyinfluences the surface stiffness; this is evident from the resultsobtained about the surface strain at different section in case ofcomposites containing MF1 fibers.

After the first crack, these fibers being fully stressed shared thetension with the steel bar and the composite exhibits high tensileload bearing capacity of cracked matrix, hence increasing the

Fig. 18. Surface Strain versus Axial Load (RC-20MF1)

Fig. 19. Surface Strain versus Axial Load (RC-20MF2)

Fig. 20. Surface Strain versus Axial Load (RC-40HyF)

Fig. 21. Comparison of Concrete Surface Strain before Cracking


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tension stiffening effect which is evident from the results abouttension stiffening factor, β obtained with the composites con-taining MF1 fibers.

With the increase of applied displacement, crack is furtheropened and localised stress in the MF1 fibers keep on increasingwith increasing crack opening and a stage comes when thelocalised stress in fiber exceeds its tensile capacity and the fiberbreaks instead of pulling out from the matrix due to high bondstrength with the matrix.

On the other hand, action of MF2 fibers characterised asslipping fibers mainly depends upon the anchorage in the matrixdue to hooked-ends and this anchorage is further improved withthe increase of crack opening. These fibers do not have anysignificant effect on the first cracking load (almost similar tocontrol composite) since they do not offer notable resistance tomicro-cracking mechanism which is evident from the strainsdevelopment at the concrete-steel interface. After yielding ofsteel bar, when the crack is further opened with the increase ofapplied displacement, these fibers are stressed and arrest themacro cracks resulting in high tensile strength and more ductilityof the composite even after the yielding of steel bar. With furtherincrease of crack opening, a stage comes when these fibers arepulled out from the matrix instead of breaking and their hookedends become straight.

In uniaxial tensile test on reinforced concrete, the load istransferred from steel bar to concrete; as a result, the crack isinitiated at the interface of steel and the concrete due to elonga-tion in steel bar and propagates towards the concrete surface.Since the crack width is very small near the concrete outersurface and relatively large near the steel concrete interface, inthe presence of metallic fibers in hybrid form, when the micro

cracks are fully developed and MF1 fibers break, MF2 fibersstop the further opening of micro crack near the concrete-steelinterface (Fig. 22). On the other hand, near the surface ofconcrete, MF1 fibers restrain the micro crack formation. In thisway, both types of fibers affect the development and propagationof crack through out the cross section of concrete matrix which isnot the case with composites containing single fiber; this resultsin high tension carrying capacity at first crack which is evidentfrom the value of first crack load obtained with RC-40HyFwhich was maximum of all the composites. Moreover, due to thecombined effect of two types of metallic fibers, RC-40HyFoffered high resistance to strain development both at the concrete-steel interface and at concrete surface.

5. Conclusions

Resistance to cracking of reinforced concrete and reinforcedfibrous concrete containing two metallic fibers differing in theirmechanical, physical and geometrical properties have been in-vestigated by performing test on prismatic specimens subjectedto pure tensile loading. The results of this study confirm thefindings of previous research work about the role of fiberproperties to control the cracking mechanism.

Among the type of metallic fibers used in this study; the use ofamorphous metallic fibers (designated as MF1 fibers) appears tobe highly effective in controlling micro-cracks. Given that thesefibers do not face the problem of corrosion when subjected toaggressive environment and are very effective in controllingmicro-cracking, the reinforced concrete containing these fibersmay prove significantly useful as a material for water retainingstructures and sewerage pipes. Moreover, by the use of thesefibers, durability of the reinforced concrete structure may alsoimprove by arresting the micro cracks through which the waterpenetrates and causes corrosion of the main reinforcement.

Carbon steel hooked-ended fibers (MF2) appears to be moreeffective in arresting the macro-cracking which results in im-proved tensile strength and toughness of the composite afteryielding of steel bar.

The results obtained for the hybrid composite showed thatwhen these fibers are used in hybrid form, the composite exhibitsresistance to both micro and macro cracking mechanisms whichimproves the global response in term of resistance to cracking,tensile strength and toughness. Based on this observation, it canbe concluded that it is a good way to use these fibers in hybridform to improve behaviour of composite at all cracking stages(micro cracking and macro cracking).

Future Research

In ongoing research program, different hybrid compositions ofthese fibers are being tested in order to investigate the positivesynergetic effect between them and also to optimize the dosageof fibers in hybrid form.

Fig. 22. Crack Development in Axial Tensioned Member



Acknowledgements

The authors would like to thank Higher Education Commis-sion of Pakistan for the financial support for this experimentalstudy.

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Rashid Reinforced Fibrous Concrete Elements Subjected to uniaxial tensile loading..........................pdf