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1.3 Steel fibers reinforcement contribution on theductility of concrete masonry

The presence of steel fibers in a concrete elementwill produce tensile stresses across a cracked sectionin tensile zone. The distribution of these stressesacross the section depends upon the type of fiber,percentage of fiber and magnitude of strain.

The RILEM Task Group 162 published a recom-

mendation about design method for steel fiberreinforced concrete, which is based on the same fun-damental as the design of normal reinforced concrete(Rilem 2000a). In this reference, it is proposed amodel of real stress block for structural elementswith ordinary reinforcement (Fig. 4), which can beapplied to the masonry wall.

Figure 4. Element with ordinary reinforcement (Rilem 2000a)

The bending moment of masonry wall filled withthe SFRSCC, can be evaluated using the simplifiedmodel of real stress blocks (Fig.5) proposed as de-sign assumptions to the SFRS (Vandewalle, M.

1994).

Figure 5. SFR simplified model of real stress block

In this study the equivalent flexural tensilestrength, which is an important parameter character-izing the post-cracking behaviour of steel fiber rein-forced concrete, is accounted as a contributor of theincreasing strength capacity on the reinforced ma-sonry wall. The equivalent flexural tensile strength

is determined in terms of area under the load-deflection curve obtained by testing a simply sup-ported beam under three-point loading.

Assuming a linear stress distribution on the fail-ure section of the test beam, the equivalent flexural

tensile strength feq,2 and feq,3 can be determined bymeans of the following expressions:

2

2,

2,50,02

3

bh

lDf

f

BZ

eq = (2)

2

3,

3,50,22

3

bh

lDf

f

BZ

eq = (3)

where, DfBZ,2 and DfBZ,3 are the areas under the load-

deflection curve up to a deflection 2 and 3, (Fig.6), l = span of the specimen (mm), b = width of thespecimen (mm) and h = height of the specimen(mm).

After obtain the self-compacting properties forthe steel fibers reinforced concrete infill, the equiva-lent flexural tensile strength is determined to pre-view, by this way, the contribution part of the fiberson flexural behaviour of the masonry wall.

Figure 6. Load-deflection diagram (Amorim 2002)

2 MATERIALS AND METHOD2.1 MaterialsOne reference mixture for the C20/25 concrete classwas developed to obtain the self compacting charac-teristics. First, it was assumed that the fibers wouldlow the workability of the mixture, what impose theadjustments on the mortar ratio of the reference mix-ture due the increasing of the fibers volume.

Cement type II/B-L 32,5 and fly ash were the onlypowder materials used for the mixtures. A superplas-ticizer (Viscocrete 3000) based on a modified car-boxylate was applied to achieve a better slump re-tention ability.

Two different sands and two different coarse ag-gregate were used to compose the aggregates mix-ture. Each type of sand was quantified as 50% of to-tal sand weight. The sands fineness modulus were1,64 and 2,97. The coarse aggregates mixture was

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composed of 70% of the crushed granite 9,5 mmmaximum aggregate size and 30% of 19 mm maxi-mum aggregate size. A cylindrical steel fiber typewith hooked ends and glued in bundles (Dramix RC65/30-BP) was used in the experiments.

2.2 Test methodsAn experimental campaign was carried out to evalu-ate the self-compacting properties of the masonryconcrete infill. To characterize the fresh state of theSFRSCC was applied a combination of qualitativeobservations and quantitative measurements. Theslump flow indicates the free deformability of themixture. The funnel V indicates the stability, segre-gation resistance and restricted deformability of themixture. The L box was used to determine the pass-ing ability of the SFRSCC and the capacity to fill thenarrows holes of the masonry units.

The hardened properties were determined by the

compressive and flexural strengths. The compressivestrength was determined on the 150 mm cubes at 28days. The SFRSCC flexural strength was determinedin according with the RILEM TC 162-TDF recom-mendations (Rilem 2000b), on the beams of a 100 x100 mm cross section and a span length of the 450mm.

3 RESULTS AND DISCUSSION3.1 SFRSCC fresh behaviourIt was observed that the steel fibers modify theslump flow of the reference mixture. The productionof SFRSCC is simplified and possible with an ade-quate mortar ratio, which increase when is addedmore fibers volume in the concrete. Table 1 showsthe mortar ratio values for the slump flow valuespresented at table 2. The necessary mortar ratio is di-rectly proportional to the fiber ratio.

Table2. Test results of the fresh SFRSCCSlump flow* L box* Funnel

V*

Mixtures

D final(mm)

T500(sec)

H2/H1 T40(sec)

T(sec)

A 765 100 0.88 - 273B 735 138 1,00 100 322C 730 185 1,00 200 402D 760 102 0.90 213 336

* see description at part 1.1

The figure 7 shows the mortar ratio need for differ-ent fiber ratio. The funnel V allows measuring thedeformation speed of flowing concrete. The figure 8

shows the effect of fibers volume and mortar ratioon the flow time through the funnel V. The morefibers were added it was also necessary increase themortar ratio in relation of the reference mixture.

The passing ability verified by the L box test wasdone with a bar spacing, simulating the dimensionsof the hollow concrete block with a steel bar put inthe middle of the hollow section.

Figure 7. Influence of fibers on the mortar ratio

Figure 8. Influence of fibers on the flow time at funnel V

The passing behaviour is affected by the segrega-tion resistance of the mixture, mainly by the contentof coarse particles and by the maximum size of ag-gregate (Grnewald & Walraven. 2001). The valuesof the H2/H1 presented at the table 2 indicates thatthe mixtures did not cause blocking, but when morefibers were added, the flow-time T40 increase (Fig.9). This behaviour denotes the effect of the freshconcrete weight increasing.

All mixtures remained self-compacting even withthe addition of the 120 kg of steel fibers per cubicmeter of concrete.

Figure 9. Influence of fibers on the flow time T40 L-box

0,0

0,5

1,0

1,5

2,0

2,5

0 20 40 60 80 100 120

Fiber ratio l/d*Vf (%)

FlowtimeT40(sec)

62

64

66

68

70

72

74

0 20 40 60 80 100 120

Fiber ratio (l/d * Vf ) in %

Mortarrati

o(As%)

2,00

2,50

3,00

3,50

4,00

4,505,00

0 20 40 60 80 100 120

Fiber ratio l/d*Vf (%)

FlowtimeatVfunnel(

sec)

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3.2 SFRSCC hardened behaviourThe compressive strength of 150-mm cubes after 28days was 28, 28, 26 and 27 MPa for mixtures A,B,Cand D, respectively. The compressive strength re-sults denote any influence of the steel fibers volumefor mixtures with the same cement content.The equivalent flexural strengths determined accord-

ing to RILEM TC 162-TDF recommendations(Rilem 2000b) are presented at the table 3.

Table 3. The SFRSCC equivalent flexural tensile strengthsMixture Vf

(%)2,eqf

(N/mm2)

3,eqf

(N/mm2)B 0,5 2,98 2,51C 1,0 6,48 5,84D 1,5 12,12 6,97

Vf = volume of fibers (%)

The equivalent flexural tensile strengths resultswere the average of three beam samples test for eachmixture. The results at table 3 indicate that the in-creasing of equivalents flexural strengths is around100 % per each 0,5% fiber volume augmentation.

Figure 10 shows the typical load-deflectioncurves obtained and confirm the performance of theSFRSCC mixtures.

Figure 10. Load deflection curves of different fibers volume

3.3 Theoretical contribution of the SFRSCC on themasonry walls bending moment.

Using the equivalent flexural tensilestrengths 2,eqf , and applying the design assumptionsto the SFRS (Vandewalle, M. 1994) and based onthe simplified model of real stress blocks (Fig. 4),the theoretical bending moment was determined fortwelve masonry walls.

From these theoretical assumptions it is possibleto expect a fiber volume contribution on the ma-sonry wall bending moments. These contributionswill be verified by the flexural test performed on thereinforced masonry walls.

4 CONCLUSIONThe major findings of the study are:

First, a simple parameter was proposed to designSCC reinforced with steel fibers. It was verified that

it can be a satisfactory control parameter of the self-compactibility. Qualitative observations indicatedthat a homogenous fibers distribution was given inthe mixtures.

Second, it was found that is possible to obtain thedesired self-compacting properties of the masonryreinforced concrete infill till 1,5% fibers volume.However, the increasing of fibers volume needsmore fine materials or mortar augmentation in theconcrete.

Next, the increase of fibers volume is responsibleof important augmentation in the flexural capacity of

the SFRSCC.Finally, it was preview that the contribution car-

ried by the each 0,5% volume fibers addition is verysignificant and can be appreciate as an equivalent re-inforcement bar area.

REFERENCES

Amorim, J.A. 2002. Beto de custo competitivo reforado comfibras de ao para pavimentos industriais. Masters thesis.University of Minho, Portugal

Grnewald, S. & Walraven, J.C. 2001. Parameter-study on theinfluence of steel fibers and coarse aggregate content onthe fresh properties of self-compacting concrete. Cementand Concrete Research 31(2001) 1793-1798.

Nawa, T et al.1998. State-of-the-art report on materials anddesign of self-compacting concrete. Proc. Intern. Workshopon Self-compacting Concrete. August 1998; Kochi Univer-sity of Technology, Japan. pp 160-190.

Oliveira, L. A. P. & Dotreppe, J.C. 1993. Theoretical and ex-perimental research on masonry walls with a new reinforc-ing system. Masonry Intrnational, Vol. 7, N2, pp 51 -54.

Oliveira, L. A. P. 2001. A formulao do beto auto-compactvel reforado com fibras de ao. In Branco F. et

al (eds), Construo 2001: Por uma construo sustentvel;Proc. Nation.Congress, Lisbon, 17-19 December 2001. pp463-470

Rilem TC 162-TDF Committee. 2000a. Test and design meth-ods for steel fibre reinforced concrete. Recommendationsfor design method. Materials and Structures, 33 (3)75-81.

Rilem TC 162-TDF Committee. 2000b. Test and design meth-ods for steel fibre reinforced concrete. Recommendationsfor bending test. Materials and Structures, 33 (1-2) 3-5.

Skarendahl, A & Petersson O (ed) 2000. Self compactingconcrete. State-of-the-Art report of Rilem Technical Com-mittee 174 SCC. Cachan: RILEM Publications S.A.R.L

Vandewalle, M. 1994.Tunnelling the World. Zwevegem: NVBekaert S.A

0

2

4

6

8

10

12

14

0,0 2,0 4,0 6,0

Deflection at mid-span (mm)

Load(kN)

F0,5

F1,0

F1,5

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ACKNOWLEGEMENTSThe authors wish to acknowledge the financial sup-port of the Fundao para a Cincia e Tecnologiathrough the concerned action POCTI 36025/99.