Effect of Concrete Components on Bond Strength of Rebar and Concrete at Concrete of Admixtures 3

7/27/2019 Effect of Concrete Components on Bond Strength of Rebar and Concrete at Concrete of Admixtures 3

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Effect of Concrete Components on Bond Strength of Rebar with Concrete at Concrete of Admixtures

Kemal T. YCEL1, Cengiz ZEL211 Suleyman Demirel University, Faculty of Engineering and Architectural, Civil Engineering Department

2 Suleyman Demirel University, Faculty of Technical Education, Construction Education Department

Abstract

The paper reports the results of an experimental study on investigation of the effects of admixtures on the bondcharacteristics between concrete and reinforcement. This aim, device is developed by our carry out pull out test,flexural tests and flexural-pull out test on concrete specimens of mineral (silica fume, C and F type fly ash),

polypropylene fiber and plasticizer admixtures (based on polynaphthalene and polycarboxylate) includingnatural or crushed aggregates at two cement dosage, 350 kg and 400. While the concretes was used rebar ofdiameter of 18 mm ribbed, the concretes of fiber admixtures were used three rebar, diameter of 18 mm ribbed, of14 mm ribbed and of 14 mm smooth.

The results showed that concretes including admixtures are raising bond strengths. While type of aggregate andsurface geometry with diameter of rebar is most importance affect on properties of adherence, type of aggregateis only importance affect on properties flexural-adherence. Although the relationships are existing among thetests, interactions arent acceptable.

Keywords: Adherence, rebar, bond strength and admixtures

1. INTRODUCTION

In recent days, the classical concrete production is replaced with admixture added concrete production as calledalso technological concrete production. The admixture added concrete are produced as fiber, mineral andchemical admixtures added concrete. Polymer and steel fibers are added into the concrete mixes with aim of forincrease to mechanical strength and especially, gain to ductility to high strength concrete which have high brittle.

The property of adherence, which is the bond characteristics between concrete and reinforcement, is fairly animportant in parts of construction exposed to loads of both static and dynamic. Transmittances of stress between

rebar with concrete surround of rebar take places relative motion or resist of slipping intersection concrete withsurface of embedded rebar. The resist to slipping of rebar is called as adherence or adherence stress (Trk, 2002).

Stress of adherence is slip stress as parallel directional to rebar of axis. Adherence stress occur on the rebar is becontrolled with chemical adhesion, friction resistance and mechanical bond. Behavioral changes to importantdegree to according to type of rebar, after chemical adhesion.Result of shear affect has happened intersection rebar and concrete at the smooth rebar, come into being togetherelusion and collapse, risking in point of construction. If ribbed bar is used, increasing load will causedmechanical bond as radial and longitudinal loads. If rebar is arrived to maximum shear as depend on the regionalmicro crushing of concrete coating of pores in front of ribbed, flexural cracking will occurred. Namely, coatingconcrete cover of rebar is come off from concrete at ribbed rebar. Thus, the adherence problem of ribbed rebarturn into pull out-shear problem in the intersection of concrete to concrete (Akman, 1992).

One of the commonly used methods of characterizing the bonds in concrete systems is the direct pull-out test,where a rod which fully penetrates a block or cylinder of concrete is pulled out by direct tension while appliedload as well as displacement at the loaded and/or free end of the rod is recorded. In addition to this, manymethods are developed or offered by many researchers. (Bakis vd., 1998; Zhang vd., 2001; Lau vd., 2001;Lorenzis vd., 2002; ACI 408R-03 ).

2. MATERIAL AND METHOD

2.1. Material

In this study, crushed and natural river stone was used as coarse aggregates, maximum size of 20 mm, specificgravities were 2,76 kg/dm3 and 2,70 kg/dm3, respectively. Natural river sand were used as fine aggregates, aspecific gravity of 2,56 kg/dm3.

1 [email protected]


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Two different series concretes were produced in this investigation. In the first series concrete (R series); 45%crushed stone and 55% sand and in the second series concrete (A series); 10% crushed stone, 35% natural stoneand 55% sand were used.

In experiments, CEM I PC 42.5 R type cement (TS EN 196-1 and TS EN 197-1), properties of cement is shownin Table 1, was used as cement dosage 350 and 400 kg/m3 for each series concrete.

Two different chemical admixtures (polycarboxylate H and polynaphthalene N) were used as plasticizer foreach cement dosage of each series concrete. Chemical admixtures were used as ratio of binder and same slump(202 cm) in mixture. The polynaphthalene (N) is used as superplasticizers (ASTM C 494 Type G) with densityat 20C; 1.21 kg/l, solid mass fraction of 41.5%. The polycarboxylate (H) is used as hyperplasticizers (TS EN9342) with density at 20C; 1.10 kg/l, solid mass fraction of 22.0%.

Fly ash (C and F type) and silica fume (S) (ASTM C 618) were used in mixes as cementitious and fine materials.Mineral admixtures were added to mixtures. Properties of mineral admixtures are shown in Table 1.

Table 1. Properties of mineral admixtures and cementSpecificgravity

(g/cm3

)

FinenessBlaine

(cm2/gr)Passing

90 (%)

Passing

200 (%)Cement 3.12 0.1 2.5 3110F typefly ash

2.00 2.8 15.7 2580

C typefly ash

2.13 2.9 12.3 5230

Silicafume

2.10 16.4 39.2 --

The composition of mixtures for each plasticizer type (N or H) of each cement dosage (350 and 400) of eachseries concrete (R and A series) can be summarized as follows: Water/binder (W/B) ratio was used as constant 0.38 all of the mixtures Control mixtures (O) were produced without the inclusion of mineral admixture,

Dosage of mineral admixture: silica fume (S) 10% of cement, C type fly ash (C) 20% of cement, F type flyash (F) 20% of cement, both silica fume 10% of cement and C type fly ash 10% of cement (SC), both silicafume 10% of cement and F type fly ash 10% of cement (SF) as adding cement by mass.

Dosage of plasticizer admixtures: ratios were used in mixtures showed as solid ratio of chemical admixturesin Fig. 1.

In addition polypropylene fiber admixture (P), specific gravity was 0.91 g/cm 3, length and diameter of fiberswere 12 mm and 18 micron, respectively, was used, 600 gr/m 3 in concrete. In concrete series of fiber admixtureswas produced as 18 which is ribbed rebar and 14 which are ribbed and smooth rebar as known round steelrebar.

0.00

0.20

0.40

0.60

0.80

1.00

1.20

O S F C SF SC P O S F C SF SC P

ement osage ement osage

atooastczermxture

Polynaphthalene (N) at R Series

Polynaphthalene (N) at A Series

Polycarboxylate (H) at R Series

Polycarboxylate (H) at A Series

Figure 1. Ratio of plasticizer admixtures in mixture

Three type of rebar, ribbed rebar the diameter of 18 mm (18N) and 14 mm (14N) with smooth rebar the

diameter of 14 mm (14D), were used at the experimental works. Properties of rebars are shown in Table 2below.


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Table 2. Properties of rebars was used in experimentDiameterof rebar(mm)

Yieldstrength

(kgf)

Tensilestrength

(kgf)

Ratio ofelongation

at break (%)

ElasticityModule

(kgf/mm2)18N 12347 15231 21.7 2041614N 7936 9618 21.4 1901314D 4976 7422 31.1 21917

2.2. Method

The apparatus used to measure the properties of adherence (bond strength), flexural and flexural-adherence(same time both adherence and flexural) is shown in Figure 2. It is consist of three main section and it wasdeveloped by our. First section was used to measure of adherence, which is called as pull out test. Anothersection was used to measure of flexural properties and last section was used to measure of flexural-adherence

properties.

Concrete specimens as a prism of dimension at 15x15x60 cm was prepared for all adherence tests, flexural testsand flexural-adherence tests. Rebar for adherence and flexural-adherence tests was covered with fresh concreteafter it was put into in centre of mould. Before testing, specimens were cured in standard curing conditions at202 C for period of 28 days.

The LVDT was used to displacement or extension at the all adherence and flexural tests. Data obtained fromLVDT was transferred to data logger via indicator and was transferred to computer from data logger.

At adherence test was made use of hydraulic cylinder attached to hydraulic pump. Its loading capacity is 30.000kgf and controllable from computer or as digital.

Figure 2. Device of experiment

According to TS 500 is determined that length of put in to place in concrete of rebar. It is shown as Equation 1below.

20f

f12.0L

ctd

yd

b

= (1)

Lb= length of rebar, as adherence strength to resistible without yield, embedded in concrete (mm)fyd= yield strength of rebar (MPa)fctd= tensile strength of concrete (MPa)

= diameter of rebar (mm)In addition to this, length of rebar as embedded in concrete must be 20 times bigger than or equal diameter ofrebar. Hence, it was to determination as Lb is 360 mm.

1

3

2

LVDT

LVDT

8 MPaHydrologicpump

LVDT

30.000 kgfhydrologic pump

Digital or computer control


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According to data was obtained from pull out test, adherence strengths are to determination of Equation 2 below.

L

P

b

=

(2) = adherence strengths

P = load obtained from pull-out testLb= length of rebar embedded in concrete= diameter of rebar (mm)

In addition to adherence strength, extension was measured as slip off of rebar from concrete by LinearVariable Displacement Transducer (LVDT).

In this study, it was to investigation also properties of flexural and flexural-adherence together with properties ofadherence obtained from pull out. During the flexural and flexural-adherence tests, displacement was measuredfrom centre of specimen in flexural test and other edge without load in flexural-adherence test by LVDT.

Specimens was pre tensioned from rebar with load of 8 MPa (80 bar) before flexural-adherence tests, andflexural strength was to determination as cantilever load.

Flexural strength is calculated as Equation 3 below.

( )( )221

ed.d2

l.P3= (3)

e= flexural strengthP = loadl = distance of between two abutmentd1 and d2 = respectively width and height of specimen

3. RESULTS of EXPERIMENT

Specimens after testing are shown in Figure 3. Breaking is parallel to used load at the flexural and adherence

test. Although cracks are parallel to rebar and used load, specimens are breaking split into two from height andconcretes of cover on rebar isnt break off. Slipping of rebar is occurred, because of the fact that concrete layersat between ribs of rebar is crushing.

Breaking is angular containing both partly flexural breaking and partly adherence breaking, at the flexural-adherence test. This breaking is similar to sprain breaking or shearing. If specimens are exposed to both flexuraland adherence load, its behavior is like this. Actually, component of construction is exposed to these loads.

a) Flexural b) Adherence

c) Flexural-AdherenceFigure 3. Specimens after testing

Cracks


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3.1. Results of Flexural Tests

It is showed that result flexural strength and deformation of flexural test in Figure 4. According to result of alltests, including crushed and natural aggregate, although highest flexural strength is obtained from RSCN40 (7.83MPa) included crushed aggregate, silica fume, C type of fly ash and polynaphthalene admixture at 400 cementdosage, highest deformation is obtained from APN40 (11.8 mm) included natural aggregate, silica fume, fiber

and polynaphthalene admixture at 400 cement dosage. Lowest flexural strength and deformation are obtainedfrom RH40 (3.1 MPa and 1.8 mm) included crushed aggregate polycarboxylate admixture at 400 cement dosage.

02468101214

0123456789

10

O S F C SFSC P O S F C SFSC P O S F C SFSC P O S F C SFSC P

350 Dosage 400 Dosa ge 350 Dosa ge 400 Dosage

Polynaphthalene Admixtures Polycarboxylate Admixtures

FlexruralDeformation(mm)

FlexruralStrenght(MPa)

Strenght at A Series Strenght at R Series

Deformation at A Series Deformation at R Series

Figure 4. Flexural strength and deformation at flexural test

According to flexural tests, flexural strength of concretes of natural aggregates is generally lower than concretesof crushed aggregates, except without mineral admixture and including silica fume. Addition of mineral or fiber

admixtures and increasing of cement dosage are enhancing flexural strength. Although silica fume is enhancingflexural strength at lowest ration, ration is to a higher level in concretes used together silica fume with fly ash.Because the same slump concretes are produced, affect of plasticizer admixtures not to observation at theflexural strength. Flexural deformation on concrete including mineral or fiber, unlike flexural strength, atconcretes of natural aggregates is generally bigger than concretes of crushed aggregates.

Relationship between flexural strength with flexural deformation is shown in Figure 5. Correlation coefficient ofR series concretes is higher than A series concretes. At concretes of high strength, deformation ratio is lowerthan the others. It can be result from the pore in concrete.

Figure 5. Deformation-strenght relationships at the flexural test

3.2. Result of Pull Out Tests (Adherence Tests)

They are showed that result of pull out tests in Figure 6 for including natural aggregate and crushed aggregate.The highest adherence strength is obtained from APH35(15297 kgf) included natural aggregate, fiber andpolycarboxylate admixture at 350 cement dosage. The highest extension is obtained from ACH40 (103.2 mm)


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included natural aggregate, C type of fly ash and polycarboxylate admixture at 400 cement dosage. The lowestadherence strength and extension are obtained from APN35 (3310 kgf and 33.5 mm) included natural aggregate,fiber and polynaphthalene admixture at 350 cement dosage.

At concrete series of crushed aggregate, highest adherence strength is obtained from RSFH35 (17102 kgf)included silica fume, F type of fly ash and polycarboxylate admixture at 350 cement dosage. The highest

extension is obtained from RSCH35 (143.2 mm) included silica fume, C type of fly ash and polycarboxylateadmixture at 350 cement dosage. The lowest adherence strength and extension are obtained from RPN35 (4833kgf and 9.5 mm) included fiber and polynaphthalene admixture at 350 cement dosage.

020406080100120140160

02000400060008000

1000012000140001600018000

OS F CSF SC

P18NP14NP14D

OS F CSF SC

P18NP14N

O CSF SC

P14NP14D

CSF

P18N350 Dosage 400 Dosage 350 Dosage 400 Dosage


Extension(mm)

AdherenceStrenght(kgf)

Adherence Strenght at A Series Adherence Strenght at R Series

Extension bar at A Series Extension at R Series

Figure 6. Result of pull out test

According to pull out tests, like to flexural tests, adherence strength of concretes of crushed aggregates is higherthan concretes of natural aggregates. Addition of mineral or fiber admixtures and increasing of cement dosage

are enhancing adherence strength. However, ratio of enhancing at concretes of 350 cement dosages bigger thanat concretes of 400 cement dosages. Diameter of rebar or ribbed more affect than type or amount of mineraladmixtures. Extension of concretes of crushed aggregates is generally higher than concretes of naturalaggregates.

Figure 7. Extension-strenght relationships at the adherence test

Relationship between adherence strength with extension is shown in Figure 7. As similar to flexural test,correlation coefficient of R series concretes is higher than A series concretes. Although extension is decreasingwhile strength is increasing at R series concretes, this state is to contrary at the A series concretes.

3.3. Result of Flexural-Adherence Tests

They are showed that result of flexural-adherence tests in Figure 8 for both including natural aggregate andcrushed aggregate.


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At concrete series of natural aggregate, highest flexural-adherence strength and deformation are obtained fromAPH40 (5270 kgf, 80 mm) included fiber and polycarboxylate admixture at 400 cement dosage. Although lowestflexural-adherence strength is obtained from APN35 (3403 kgf) included fiber and polynaphthalene admixture at350 cement dosage, lowest deformation is obtained from ASFN40 (8.6 mm) included silica fume, F type of flyash and polynaphthalene admixture at 400 cement dosage.

At concrete series of crushed aggregate, highest flexural-adherence strength is obtained from RPH40 (6520 kgf)included fiber and polycarboxylate admixture at 400 cement dosage. The highest deformation is obtained fromRSCN40 (79.7 mm) included silica fume, C type of fly ash and polynaphthalene admixture at 400 cementdosage. Although lowest flexural-adherence strength is obtained from RPN35 (3566 kgf) included fiber and

polynaphthalene admixture at 350 cement dosage, lowest deformation is obtained from RPH40 (41.5 mm)included fiber and polycarboxylate admixture at 400 cement dosage.

0

153045607590

01000200030004000500060007000

OS F CSF SCP18NP14NP14D




350 Dosage 400 Dosage 350 Dosage 400 Dosage


Deformation(mm)

Flexrural

-AdherenceStrenght(kgf)

Strenght at A Series Strenght at R Series

Deformation at A Series Deformation at R Series

Figure 8. Result of flexural-adherence testsAccording to flexural-adherence tests, flexural-adherence strength of concretes of natural aggregates is generallylover than concretes of crushed aggregates. Addition of mineral or fiber admixtures and increasing of cementdosage are enhancing flexural-adherence strength. Because of approximate flexural-adherence strength, affect ofadmixtures type isnt clear observation. At the same time, flexural-adherence strength of rebar of diameter 14mm is highest than result of other test, as particularly ribbed. Namely, flexural-adherence properties of ribbedrebar of diameter 14 mm is similar to ribbed rebar of diameter 18 mm or including mineral admixtures.However, although flexural-adherence deformation of concretes of natural aggregates is generally lover thanconcretes of crushed aggregates, it is changeable according to type of concrete.

Figure 9. Deformation-strenght relationships at the flexural-adherence test

Relationship between flexural-adherence strength with deformation is shown in Figure 9. Although strength isincreasing while deformation is increasing, correlation coefficient isnt important.


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3.4. Comparasion of Tests

Relationships among the tests are shown in Figure 10. Adherence strength is interaction with both flexural andflexural-adherence test. However, in this study, because concretes of many different components were prepared,acceptable correlation coefficient wasnt obtained.

a) Relation of Adherence and Flexural Strenght

a) Relation of Flexural and Flexural-Adherence Strenght

c) Relation of Adherence and Flexural-Adherence StrenghtFigure 10. Comparasion of Tests

Table 3. Correlation coefficient among the strengths

Flexural AdherenceFlexural-

Adherence

FlexuralR = 0.012

(R = 0.141)**R = 0.451

(R = 0.189)**

AdherenceR = 0.339

(R = 0.001)*R = 0.011

(R = 0.920)**Flexural-Adherence

R = 0.192(R = 0.014)*

R = 0.149(R = 0.773)*

* These coefficients are among the concretes of A series including rebar at diameter of 18 mm** These coefficients are among the concretes of R series including rebar at diameter of 18 mmCoefficients in parenthesis is among the concretes of rebar at diameter of 14 mm

Concretes includingrebar of 14 mmdiameter

Concretesincluding rebar of14 mm diameter

Concretesincluding rebarof 18 mmdiameter

Concretesincluding rebar of18 mm diameter


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4. CONCLUSIONS

According to flexural, adherence and flexural-adherence test, most importantly factor is type of aggregate.Strengths of concretes including crushed aggregates are higher than concretes including natural aggregates.However, deformation or extension is concretes including natural aggregates higher than concretes including

crushed aggregates.

Addition of mineral admixtures is increasing the flexural strength, adherence strength and flexural-adherenceflexural. According to reference concretes, although strength increasing at lowest ratio is concretes of silicafume, concretes including both silica fume and fly ash are higher than from it.

Addition of polypropylene fiber is obtained near to strength of concretes including mineral admixtures.

Although increasing of cement dosage is raised properties strength, deformation and extension properties isgenerally decreased.

Because the same slump concretes are produced, affect of plasticizer admixtures not to observation at thesestrengths.

Surface geometry, whether ribbed or not, diameter, and yield stress of rebar and are most important a factorinfluence to adherence. Rising diameter of rebar is increase to adherence strength. On the other hand, particularlyat flexural-adherence strengths, surface geometry is effective rather than diameter of rebar. Extension atadherence test is to observation changeable according to component of concretes and properties of rebar.Deformation of flexural-adherence is similar to one another.

Breakings are parallel to used load at the flexural and adherence test, and are to direction component-force ofused load at the flexural-adherence test. Correlation coefficient of R series concretes is higher than A seriesconcretes at between strength with deformation or extension.

According to relationships among the tests, adherence strength is interaction with both flexural and flexural-adherence test but correlation coefficient in these relation isnt acceptable.

Acknowledgment

This study was made possible by the financial support from TBTAK (The Scientific and TechnologicalResearch Council of Turkey) as Project no. 104I040 and 106M155.

References

Trk K., 2002. According to Concrete Features, The Investigation of bond Strength of Reinforcement inReinforced Concrete Elements Subject to Combined Bending. PhD Thesis, 111 p., Elaz, TURKEY.

Akman M.S., 1992. Concrete Technology on Sea Construction, Istanbul Technical University, 74-105 p..stanbul, TURKEY.

Bakis vd., 1998, Bakis, C. E., Uppuluri, V. S., Nanni, A., Boothby, T. E., 1998. Analysis of BondingMechanisms of Smooth and Lugged FRP Rods Embedded in Concrete, Composites Science andTechnology, 58, 1307-1319

Bakis, C. E., Uppuluri, V. S., Nanni, A., Boothby, T. E., 1998. Analysis of Bonding Mechanisms of Smoothand Lugged FRP Rods Embedded in Concrete, Composites Science and Technology, 58, 1307-1319

Lau, K. T., Dutta, P. K., Zhou, L. M., Hui, D., 2001. Mechanics of Bonds in an FRP Bonded Concrete Beam,Composites Part B: Engineering, 32, 491-502.

ACI 408R-03, 2003. Bond and Development of Straight Reinforcing Bars in Tension, American ConcreteInstitute, 49 pp. USA.

TS EN 196-1 (equivalence EN 196-1), 2002. Methods of testing cement-Part 1: Determination of strength,Turkish Standards Institution, 24 p., Ankara, TURKEY.


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TS EN 197-1 (equivalence EN 197-1), 2002. Cement- Part 1: Compositions and conformity criteria for commoncements, Turkish Standards Institution, 25 p., Ankara, TURKEY.

ASTM C 494a, 1999. Standard Specification for Chemical Admixtures for Concrete. Annual Book of ASTMStandards, , p. 9, USA.

TS EN 934-2 (equivalence EN 934-2), 2002. Admixtures for concrete, mortar and grout - Part 2: Concreteadmixtures; Definitions, requirements, conformity, marking and labeling, Turkish StandardsInstitution, 18 p., Ankara, TURKEY.

ASTM C 618, 2000. Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use inConcrete, Annual Book of ASTM Standards, p. 4, USA.

TS 500, 2000. Requirements for design and construction of reinforced concrete structures. Turkish StandardsInstitution, 79 p., Ankara, TURKEY.

Effect of Concrete Components on Bond Strength of Rebar and Concrete at Concrete of Admixtures 3

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