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

Evaluation of the bond strength behavior between steel bars and HighStrength Fiber Reinforced Self-Compacting Concrete at early ages

F.M. Almeida Filho, M.K. El Debs & A.L.H.C. El DebsDepartment of Engineering Structures, University of São Paulo, São Carlos, Brazil

ABSTRACT: Self-Compacting Concrete (SCC) appeared to avoid the difficult and onerous work of concretevibration. It can be defined as a material that is capable to flow inside of a formwork, passing by the reinforcementand filling out the formwork, without the use of any vibration equipments. The main objective of this researchwas developing a high strength SCC with steel fibers for use in precast connections and also to evaluate thebehavior of its bond strength with the steel bars of different diameters. The study considered as fundamentalparameters, the steel fibers and the steel bars diameter. According to the results, the incorporation of steel fibersincreased the bond strength of SCC giving it a ductile behavior according to the fiber content and fiber length.The development of a HSFRSCC reveal itself ideal for use in precast connections as a filling material, producinga more monolithic behavior for the precast structures.

1 INTRODUCTION

With the advances obtained in the construction mate-rials and techniques in the last decades, the reinforcedconcrete loses some space in the civil construction.This occurred due to several reasons like construc-tion time and technical staff costs. However, reinforcedconcrete systems are the only system which coulddevelop the best structural performance, due to themonolithic connections of its structural elements. So,the beginning of self-compacting concrete can be asso-ciated to a new situation, with reduced cast time andtechnical staff cost, bringing a new level of competitioncompared to other structural systems.

The self- compacting concrete is an advanced con-struction material that can be defined as a mixture thatcan be cast in any place of the formwork, just throughthe accommodation of its own weight (Okamura 1997,Gomes 2002). Nowadays, the use of self-consolidatingconcrete in structures is target of several researches,since the absence of vibration, could provoke a weak-ness in the bond between the steel bar and the adjacentconcrete.

According to recent results, for low compressivestrength, compared to OC (ordinary concrete), SCCpresents similar bond strength, with some peculiari-ties (Almeida Filho et al 2005, Almeida Filho 2006).Besides, in places with high reinforcement rate, thefresh properties of SCC stood out over OC (Chanet al. 2003). For high strength concrete, however, thereis an absence of information to support the sameconclusion.

The studies of the bond strength including self-compacting concrete with steel bars are made in twoways: pull-out tests at varying heights in mock-upstructural elements and pull-out of single bars placedin small prismatic specimens using the Rilem proce-dure (Rilem-Ceb-Fib 1973). Based on its simplicity,the pull-out models were adopted in this study.Accord-ing to the literature, SCC appears to improve thebond strength, due to its filling ability when involvingthe reinforcement. This improvement was not sig-nificant, but the experimental data proved the useof self-consolidating concrete presents, at least, thesame behavior of similar models of ordinary concrete(Almeida Filho 2006, Domone 2007).

The application of fibers in SCC mixes improves ithardened properties, as for OC. However, SCC mixesmust consider the absence of vibration, which meansthere is a maximum amount of fiber content that couldbe used due the blocking made by the reinforcement.

According to literature, several researchers studythe effects of the application of fiber on the matrix,because the degree of workability is directly influ-enced by the type and content of fibers used and thematrix (SCC). Thus, as the fiber content increases, thedistribution become more difficult to be uniform; butfor an adequate distribution (uniformly) is required toachieve the benefits of its use (Grünewald & Walraven2001).

So, the developing a SCC reinforced with fiberscan be a good response for the improvement ofmoment resisting precast connections due the betterfilling ability, finishing quality and durability. The

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Precast beam

PrecastColumn

HSFRSCC

Figure 1. Idealized application of HSFRSCC for precastconnections.

combination of steel reinforcement to resist externallyapplied forces (as wind or seismic loads) with the useof a filling material that can absorb the cracking orig-inated by those forces, becomes a good solution fordesign and construction. Figure 1 shows the prob-able applications for high strength fiber reinforcedself-compacting concrete (HSFRSCC) for precastconnections.

So, this paper presents an experimental study ofthe behavior of high strength fiber reinforced SCC inpull-out specimens using different steel bar diameters.The main objective was evaluating the bond stress,regarding the influence of the following parameters:fiber content, steel fiber length (13 and 25 mm) andbar diameter (16 and 20 mm).

2 EXPERIMENTAL PROCEDURE

2.1 Materials

The used cement was CP-V (initial high strengthcement). The used quartz sand had specific weightof 2.63 kg/dm3 and the crushed stone (maximumaggregate diameter of 9.5 mm) had specific weight of2.83 kg/dm3. The used superplasticizer was based onpolicarboxylate, which density was 1.08 kg/dm3 with19% of solid content. Two different ribbed steel bardiameters (10 mm and 16 mm) were used, both withyield strength of 500 MPa.

Figure 2 shows the aggregate grading for SCC mix.

2.2 SCC design

The design used for the SCC were based on the UPCdesign, obtaining the SCC mixes through successiveoptimizations for filler and superplasticizer content,aggregate ratio (void content) and paste content inorder to achieve the SCC required fresh properties(Gomes 2002).

0

20

40

60

80

100

0,1 1 10 100Sieve size(mm)

Pas

sing

(%

)

Quartz sand

Crushed stone

Figure 2. Aggregate grading for SCC mix.

Table 1. Mix design and fiber content per cubic meter.

Material SCC1 SCC2 SCC3 SCC4

Cement (kg) 426.8 426.8 426.8 426.8Fine agg. (kg) 789.8 789.8 789.8 789.8Coarse ag. (kg) 864.5 864.5 864.5 864.5Water (kg) 149.4 149.4 149.4 149.4Superplast. (%) 2.0 2.0 2.0 2.0Filler (kg) 85.4 85.4 85.4 85.4Silica fume (kg) 85.4 85.4 85.4 85.4Fiber 1 (%) 0.0 1.0 1.0 0.0Fiber 2 (% ) 0.0 0.0 1.0 1.0

The mix design was obtained by Eq. 1.

Where, wc is the cement content, w/c is thewater/cement ratio, sf/c is the silica fume/cement ratio,f/c is the filler/cement ratio, sp/c is the superplasti-cizer/cement ratio and ρp is the paste specific weight.All parameters, in equation 1, need to be determinatedbased on the needed property (fresh or hardened). Thepaste specific weight is obtained by tests using Marshfunnel and mini-slump flow.

Table 1 shows the mix design (including each steelfiber content, where Fiber 1 refer to 13 mm fiber lengthand Fiber 2 refer to 25 mm fiber length, both withhooked ends) and Table 2 shows the fresh propertiesof each SCC series. Table 3 shows the hardened prop-erties of SCC mixes. For each SCC mix, 9 concretecylinders (100 mm × 200 mm) were made in orderto obtain the compressive strength, splitting tensilestrength and modulus of elasticity.

The tests were divided according to the presence offibers, its amount and the steel bar diameter. Table 4summarizes the experimental program. The pull-outtests were design to be tested at 3 days.

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Table 2. Fresh properties of SCC mixes.

SCC1 SCC2 SCC3 SCC4

Temp. (◦C) 31 31 22 23RH (%) 26 26 65 50Slump test

t50 (s) 3.0 3.0 1.7 1.5D (cm) 71.0 60.0 63.0 69.5V-Funnel test

tv(s) 11.0 10.9 5.0 5.10L-Box test

t60(s) 6.4 10.7 >12.0 >12.0Blocking ratio 0.8 0.6 – –

Table 3. Hardened properties of SCC mixes.

SCC1 SCC2 SCC3 SCC4

fc,3 (MPa) 78.83 74.70 57.72 54.98fct,3 (MPa) 5.41 5.29 5.84 4.21Ec,3 (GPa) 37.00 35.83 36.12 36.37

Table 4. Nomenclature for pull-out specimens.

φ(mm) Fiber 1 Fiber 2

P16-SCC1 16P20-SCC1 20P16-SCC2 16 1% –P20-SCC2 20 1% –P16-SCC3 16 1% 1%P20-SCC3 20 1% 1%P16-SCC4 16 – 1%P20-SCC4 20 – 1%

2.3 Pull-out specimens

The pull-out specimen geometry (Figure 3) was basedin the model established by Rilem procedure. Theinstrumentation was the same of Rilem recommen-dation, where one LVDT was placed at the top of thesteel bar to measure the slip between the steel bar andthe concrete cylinder.

2.4 Pull-out tests

Figure 4 shows the pull-out test set-up at the testmachine, Instron.

The position and the inclination of the bars dur-ing the cast have significant influence in the bondresistance with the specimens cast in the verticaldirection presenting larger bond resistance than theones with horizontal cast. So, for this research, thepull-out specimens were cast in the vertical direction.

Bonded zone

10

5φ

φ

5φ

φ

Unbonded zone10φ

Figure 3. Pull-out specimen geometry and specimen at test.

Machine grips

Load cell

LVDT

Instron's piston

Piston support

Reaction slab

Test support

Pull-outspecimen

Loaddirection

Instron'ssupport

Figure 4. Pull-out test set-up.

They were submitted to monotonic displacement witha rate of 0.016 mm/s for 16 mm steel bar, and a rateof 0.020 mm/s for the 20 mm steel bar, until failure(Almeida Filho 2006).

3 ANALYSIS OF RESULTS

3.1 Fresh and hardened properties

According to the Table 3, the fiber content and itslength had major influence on the fresh properties ofthe SCC mix. However, the air humidity (is some cases,was below 30%), associated with the high environ-mental temperature (in some cases over 30◦C) alsocontributed to the loss of the SCC flowability.

The performed tests for evaluating the passing abil-ity (L-box), had a gap between the reinforcementdefined in function of the coarse aggregate maxi-mum diameter, resulting of 32 mm (Gomes 2002).According to the results, the passing ability was notsignificantly changed when Fiber 1was used; however,

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Fiber 2 presented high blocking ratio and the SCCreached values over 12 seconds for t60 (L-box).

These results showed the high influence of the tem-perature and the air humidity.Also, the presence of sunrays directly incident at the fresh test location resultedin less flowability for the SCC.

Figure 5 shows the fresh tests (L-box and slumpflow) performed for SCC series, showing the blockingratio presented when Fiber 2 (L-box) was used and thespread of the slump test when using Fiber 1.

The hardened properties of each SCC mix showedthe influence of the temperature and the air humid-ity. Those factors affect directly the SCC compressivestrength and the SCC tensile strength (Table 3), butthey do not seem to influence the SCC modulus ofelasticity, which remains around 36 GPa.

Figure 6 shows the variation of SCC hardenedproperties.

According to Figure 6, the SCC1 mix presented bet-ter mechanical behavior when compared to other SCCmixes, but is worth to comment and remember theinfluence of the environmental, that conducted to thesedifferences.

3.2 Pull-out tests

The pull-out tests were performed 3 days after cast toevaluate at an early age its use for precast connections.So, the pull-out specimens remained at the humiditychamber until test and were monotonically tested withdisplacement control. The bond stress evaluation wasmade by using Eq. 2.

Where, F is the pull-out load, φ is the steel bardiameter and ld is the embedment length.

Each SCC mix was evaluated using, at least,2 pull-out specimens in order to obtain a representative

Fiber 1Fiber 2

Figure 5. Fresh tests performed for SCC mixes.

value and its counter-proof. The coefficient of vari-ation is below 10% for pull-out tests, which allowsthis assumption and use a minor number of specimens(Almeida Filho 2006). Also, for achieving better com-parison, the bond stress of test specimens were dividedby the concrete compressive strength (τ/fc).

4,0 4,5 5,0 5,5 6,0

50

55

60

65

70

75

80

85

90

fc

Ec

fct (MPa)f c

(M

Pa)

35,0

35,5

36,0

36,5

37,0

37,5

38,0

SCC4

SCC3

SCC2

SCC1

Ec

(MP

a)

Figure 6. Hardened properties of SCC mixes.

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,00 0,05 0,10 0,15 0,20 0,25

0,0

0,1

0,2

0,3

0,4

0,5

/fc

Slip (mm)

P20-SCC1-1

P20-SCC1-2

P20-SCC1-3

Average

0 1 2 3 4 5 6 7 8 9 10

0 1 2 3 4 5 6 7 8 9 10

0,0

0,1

0,2

0,3

0,4

0,5

0,6

/fc

0,0 0,1 0,2 0,3 0,4 0,5 0,6

0,0

0,1

0,2

0,3

0,4

Slip (mm)

P16-SCC1-1

P16-SCC1-2

P16-SCC1-3

Average

Figure 7. τ/fc vs. slip for SCC1 mix.

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The first tested mix, SCC1, did not present steelfibers to serve as a reference sample. The resultsshowed a brittle rupture (as expected due the highconcrete compressive strength) with low slip values,meaning a poor capacity to redistribute the externallyapplied loads when the design service loads were sur-passed, which may result in extensive cracking for thestructure, compromising its durability. Also, the SCC1mix presented similar behavior for both bar diameters,showing similar values for slippage and bond stress.

Figure 7 shows the τ/fc vs. slip results for SCC1 mixfor each bar diameter.

Figure 8 shows the τ/fc vs. slip results for SCC2 mixfor each bar diameter.

The SCC2 mix presented similar behavior of SCC1mix; however, the use of 1% of steel fiber in the con-crete showed a better behavior, because the fiber seemsto make a mesh reducing the cracking formation, withsimilar results.

The SCC3 mix presented a better behavior due thepresence of larger fibers combined with smaller fiber.According to the test results, the bond stress vs. slip

0 2 3 4 5 6 7 81 9 10

0 2 3 4 5 6 7 81 9 10

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,0 0,5 1,0 1,5

0,0

0,1

0,2

0,3

0,4

0,5

/fc

Slip (mm)

P16-SCC2-1P16-SCC2-2Average

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,0 0,2 0,4 0,6 0,8 1,0

0,0

0,1

0,2

0,3

0,4

0,5

/fc

Slip (mm)

P20-SCC2-1P20-SCC2-2P20-SCC2-3Average

Figure 8. τ/fc vs. slip for SCC2 mix.

behavior had a better post-peak branch which repre-sents a good response for reducing cracking due thebetter absorption of external forces when the designservice loads were surpassed. Figure 9 shows the τ/fcvs. slip results for SCC3 mix for each bar diameter.

The SCC4 mix used 1% of 25 mm length steel fiberto evaluate its application. According to the results, itsuse compromises the bond stress but granted a goodductile behavior. Also, the specimens presented highvariation which may be explained by the rupture pro-cess. The absence of small fiber that act against themicro-cracking provoked a larger variation. The useof small fiber length, as shown at Figures 8 and 9,resulted in lower variation for bond stress at failure.Figure 10 shows the τ/fc vs. slip curve for SCC4 mixfor each bar diameter.

The τ/fc ratio proved to be a good parameter tocompare the test results. According to the results, theτ/fc ratio furnish maximum bond stress near 0.5 forall mixes, which means the bond stress of all speci-mens were equivalent, although the different concretecompressive strength.

Figure 9. τ/fc vs. slip for SCC3 mix.

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Figure 10. τ/fc vs. slip for SCC4 mix.

Tables 5 and 6 shows the test results for the speci-mens with 16 mm and 20 mm steel bar, respectively.

Where Fu is the failure load, su is the slippage atfailure, τ0.01 is the bond stress for slippage of 0.01 mm,τ0.1 is the bond stress for slippage of 0.1 mm, τ1.0 is thebond stress for slippage of 1.0 mm, τm is the averagebond stress for τ0.01, τ0.1 and τ1.0 and τu is the ultimate,or maximum, bond stress.

According to Tables 5 and 6, the reduction of thebond stress was noticed, mainly for SCC3 and SCC4mixes. This might occurred due the lower concretecompressive strength, but it needs more studies toachieve a better conclusion. However, when compar-ing the parameter τ/fc, all specimens presented similarbehavior, showing a good response from specimenswith fibers. The SCC4 mix presented better behaviorfor 20 mm steel bars, but the worst behavior for 16 mmsteel bar. Also, the specimens with 20 mm presentedhigher bond strength when compared to the others with16 mm; this can be explained by the higher stiffnesspresented by the 20 mm steel bar, which induced a

Table 5. Test results for 16 mm steel bar.

SCC1 SCC2 SCC3 SCC4

Fu (kN) 124.8 124.7 100.4 89.58su (mm) 0.30 0.73 0.75 0.82τ0.01 (MPa) 10.53 11.89 9.88 5.68τ0.1 (MPa) 27.74 25.26 16.49 13.78τ1.0 (MPa) 31.04 30.96 24.81 22.17τu (MPa) 31.04 31.01 24.96 22.28τm (MPa) 23.10 22.70 17.06 13.88τm/fc ratio 0.29 0.30 0.30 0.25

Table 6. Test results for 20 mm steel bar.

SCC1 SCC2 SCC3 SCC4

Fu (kN) 214.6 208.0 184.8 159.2su (mm) 0.17 0.56 0.92 0.222τ0.01 (MPa) 10.63 8.95 7.23 14.82τ0.1 (MPa) 32.20 28.28 17.93 25.15τ1.0 (MPa) 34.16 33.11 29.33 25.34τu (MPa) 34.16 33.11 29.42 25.34τm (MPa) 25.66 23.45 18.16 21.77τm/fc ratio 0.33 0.31 0.31 0.39

Figure 11. Failure mode for pull-out specimens (SCC2,SCC3 and SCC4, respectively).

more fragile rupture of the specimen, and, by that,higher load failure.

Figure 11 shows the failure mode presented by pull-out specimens of the developed mixes.

According to Figure 11, the failure mode was almostthe same for all series, i.e., a brittle rupture, exceptionof SCC3 and SCC4 mix, which had a more ductilefailure due the use of a 25 mm fiber length. So, theapplication of fibers with 25 mm provoked a betterpost-peak behavior although a lower bond strength, butthis lower value seems to be originated by the lowerpresented concrete compressive strength.

4 CONCLUSIONS

According to the test results, the following conclusioncan be drawn:

1. According to the test results, the application of steelfibers improves the slip in the test, which conduct toa more ductile behavior.Also, the length of the steel

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fiber had influence in the fresh properties, due theblocking ratio. So, the SCC fresh properties testsneed to be revised in order to obtain better analysesof the results when using fibers;

2. The environmental had major influence on theSCC fresh properties, as shown in the experimentalprogram. The presence of sun rays directly inci-dent over the mixer, high temperatures and low airhumidity contributed to the loss of flowability ofSCC mixes;

3. The different fiber length was used to understandhow the bond stress will behave under monoton-ically tests. According to the tests, the presenceof fiber length of 13 mm contributed to a betterbehavior, similar to those without fiber (SCC1 mix)with the crack control, i.e., no brittle failure. Thus,the post-peak presented by pull-out tests demon-strate the benefits of using steel fibers, as shownby SCC2, SCC3 and SCC4 mixes;

4. The hardened properties of SCC mixes, like themodulus of elasticity which were not significantlychanged with the content or length of the fibers.

5. The τ/fc ratio proved to be a good parameter to com-pare the test results, which means the bond stress ofall specimens were equivalent, although the differ-ent concrete compressive strength. Thus, accordingto this parameter, the specimens with fibers pre-sented similar or even higher bond strength to thosemade without fibers, which means the applicationof fibers, besides do not reduce this parameter,increases the ductile behavior, which reduces thecracking formation and grant a better distributionof external efforts.

Finally, this research is the beginning of a majorstudy where the main objective is the developing andapplication of self-compacting concretes as a filling

material to improve the stiffness of moment resistingprecast connections.

ACKNOWLEDGES

The research group would like to thanks to FAPESPfor the financial support and the technical staff at theLaboratory of Structures of the Engineering StructuresDepartment of São Carlos Engineering School.

REFERENCES

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Domone, P.L. (2007) A review of the hardened mechanicalproperties of self-compacting concrete. Cement &Concrete Composites 29:1–12.

Gomes, P.C.C. 2002. Optimization and characterization ofhigh-strength self-compacting concrete. Doctoral thesis,Universitat Politècnica de Catalunya.

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Rilem-Fip-Ceb 1973. Bond test for reinforcing steel: 1-Beamtest (7-II-28 D). 2-Pullout test (7-II-128):Tentative recom-mendations. Materials and Structures 6(32):96–105.

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