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THE INVESTIGATION OF EFFECT OF STEEL FIBER ON THESHEAR BEHAVIOR OF SELF COMPACTING CONCRETE

BEAMS WITH NORMAL AND HIGH STRENGTH

M H A Beygi*, Babol University of Technology, IranJ Vaseghi Amiri, Babol University of Technology, Iran

A Rezai Moazen, Allameh Mohades Noori Higher Education Institute, IranN Ranjbar Malidareh, Babol University of Technology, Iran

M Harajpoor, Mazandaran, University of Science and Technology, Iran

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33rdConference on OUR WORLD IN CONCRETE & STRUCTURES: 25 27 August 2008, Singapore

THE INVESTIGATION OF EFFECT OF STEEL FIBER ON THESHEAR BEHAVIOR OF SELF COMPACTING CONCRETE

BEAMS WITH NORMAL AND HIGH STRENGTH

M H A Beygi*, Babol University of Technology, IranJ Vaseghi Amiri, Babol University of Technology, Iran

A Rezai Moazen, Allameh Mohades Noori Higher Education Institute, IranN Ranjbar Malidareh, Babol University of Technology, Iran

M Harajpoor, Mazandaran University of Science and Technology, Iran

Abstract

In order to investigate the effect of steel fiber (with low strength) on the shear behavior of self

compacting concrete beams, 12 specimens had been designed and tested, which 6 of them had the

compressive strength of 30 MPa and 6 of them had the compressive strength of 60 MPa. In each

group 3 beams with distances of 80mm and 3 beams with distances of 100mm used stirrups and also

in each group the volume of fiber to the concrete with percentages of zero, 0.5 and 1 percent had been

changed. All the beams with simple support were loaded by two concentrated load to the stage of

failure. The amount of applied load, the deflection of mid span and all strains in different positions in

different stages of loading have been measured and recorded. In this research corrugated fiber with

low strength (220 MPa) had been used.

The results of investigation shows that not only we can replace fibers with parts of stirrups by saving

their strength but also we can reach to more shear ductility. Also in order to increase cracking shear

strength, the prevention of sudden cracking and increase of absorption of energy, it is more effective

to increase steel fiber rather than to increase stirrups.

Keywords:Steel Fiber with Low Strength, Shear Behavior, Self Compacting Concrete Beams with Normal andHigh Strength

1- Introduction

For the first time in fifteen years ago, self compacting concrete had been developed in Japan in order to obtainstructural durable concrete. Since then many researches were carried out to reach to appropriate mixture, design

and transferring it into standard concrete.Self Compacting Concrete has been defined as a concrete which has no need for any internal vibration orexternal vibration for form work and due to its weight it would be compacted. The usage of this concrete is in

the structural element which too much reinforcement has been used [1].Few skills for workers are needed to obtain appropriate compacting in structure with normal concrete. The

reduction of this number will lead to reduction of compacting and quality of structure. But self CompactingConcrete needs less skillful workers and thats why in compare with normal concrete it is considerably moreeconomical in performance.Although this concrete has a better compaction property to the normal concrete, therefore we will predict that itsproperties as a hardened concrete (such as compressive strength, permeability, absorption) to the normalconcrete have improved considerably.For the other advantages of Self Compacting Concrete we can imply to reduced period of construction ofconcrete structure, assuring about the compaction of structure especially in areas which the usage of vibrator are

difficult and reduction of noise problem due to vibration especially in the factories which produce concretesegments.

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So far many researches have been carried out for this type of concrete which were mostly about the technology

of concrete, and little investigation has been done for the structural behavior.In Iran also some reports are introduced related to this type of concrete and type of mixture design and theinvestigation of structural behavior [2, 3 & 4].

The application of non-reinforced concrete due to its brittle property except for the weight concrete has no otherusage.

In practice this significant draw back of concrete, by using reinforcement will come to an end, but as far asreinforcement will build a small part of section, the usage of concrete section being an isotropic andhomogenous section is not correct.In order to maintain isotropic condition and to reduce brittleness of concrete as much as possible, in recentdecades very relatively long and thin fibers have been used throughout the whole volume of concrete ashomogenize or spread uniformly.All type of fibers (fiber glass, poly etalon, steel, asbestos and plastic) is effective to improve the concreteproperties and in different direction they will create some joints which will prevent the creation of cracks.

Therefore fibers with an active role will restraint the width of cracks and contribute with inducement of manysmall cracks and hence the serviceability of concrete will increase [5].Self compacting concrete has the same mix design of normal concrete including: Cement, aggregate, water and

additive. Although large amount of super plasticizer should be taken in to account to reduce liquid limit ofconcrete and its better application and large amount of fiber as a factor of lubrication of coarse aggregate and

usage of special admixture for increasing viscosity of concrete should be considered. The property of freshconcrete and hardening of self compacting concrete highly depends on its mix design. Figure (1) showsprinciples of producing self compacting concrete [6].

Figure 1- Principles of mix design self compacting concrete

2- The Methods of Testing the Self Compacting Concrete

Since the performance of self compacting concrete is different to the normal concrete, therefore it is necessaryto maintain the concrete is self compacting or not. To measure the self compacting properties there are several

tests which are as follow:

2-1- Self Compacting

In order to maintain whether the concrete is self compacting we will use U tube test, which has been suggestedby Taisei Group (Hyakawa 1993).

The difference between the levels of concrete in the U tube depends on the fluidity of concrete as suggested byTaisei group if the difference mentioned above becomes more than 300mm, the concrete can be considered asself compacting concrete which Figure (2) shows this device.

Limiting the amount of coarse

aggregate

Usage of Super Plasticizer

Reduction of ratio of water

to the fine aggregate w/p

High ductility of mortar

and concrete

Compact ability

High strength of

mortar and concrete

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Figure 2- Testing System U-Flow

2-2- Deformability

Slump test on its way is not suitable for self compacting concrete. Therefore engineering institute of Japan willdefine the slump test as a slump flow equal to the final diameter of fresh concrete after lifting up the slumpcone. This test will determine the ability of concrete to deform under its own weight and cope with internalfriction.Although this test can not evaluate whether the concrete will pass through the existing space between the barswhich shows mortar tend to separate from aggregate.T50= Required time for concrete to reach 50cm diameter.

Dfinal= The final diameter of concrete which can be measured by two orthogonal diameter and it should be morethan 60 cm. Figure (3) shows this device.

Figure 3- Slump flow test

2-3- Filling Ability

In self compacting concrete the presence of high fluidity is not adequate, in fact when concrete flow near a barermight the coarse aggregate be stopped by barrier and short to prevent the flow of mortar of concrete. Hence if

the mix concrete was not properly designed, the coarse aggregate tend to make an arch and flow of concretewould be prevented. In order to measure the passing and filling ability of fresh concrete L-Box test has beenused.T20= The required time for the concrete to reach the horizontal part with the distance of 20cm.T40= The required time for the concrete to reach the horizontal part with the distance of 40cm.h2 = The height of concrete in horizontal part of L-Box.H1 = The height of concrete in the vertical part of L-Box. Figure (4) shows the device.

Figure 4- L-Box

3- Mix Design ApproachThe compaction ability is highly depended on the material signification and mix proportion. A mix design

method for self compacting concrete is required. Okamura and Ogawa suggest a simple mix design systemassuming concrete produce as ready concrete.In this method the amount of coarse and fine aggregates are constant and self compaction would be induced by

the correcting the amount of filler and super plasticizer in the concrete mixture [7].

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4- Production of Steel Fiber

The fibers used in this research are type of steel with low strength (Yield Stress 220MPa). The reason for usingthis type of steel is that its accessible with low cost. These fibers have been used with d = diameter 0.6mm and

length L=42mm (The ration of length to the diameter =d

L70). As long as the influence of this type of

corrugated steel fiber is more than simple fiber [8]. Thats why all the fibers are shaped with machine insinusoidal wave with wave length of 4mm shown in Figure (5).

Figure 5- Geometric characteristics of fibers

5- Description of Test Specimen

In order to carry out this test 12 specimen were made for this purpose in the Structural Laboratory of BabolUniversity of Technology both for self compacting normal and high strength concrete. 6 beams were consideredfor each of these groups. Stirrups spacing in 3 beams were with distance of 80mm and in other 3 beams were

with distance of 100mm. The volume of fiber to the concrete was zero and in the shear span 0.5 and %1 werechanged.The beams have rectangular sections with 18 20 cm dimension and the length of free span was considered

125cm. The ratio of length of shear span to the effective depth was selected =d

a2.5.

Mechanical properties of steel reinforcement are mentioned in table (1) and the type of beams categories and

their abbreviation for normal and high strength self compacting concrete are shown in table (2 & 3).

Table 1- Properties of Steel Reinforcement

Modulus of

Elasticity)Mpa(510

UltimateStrength(MPa)

YieldStrength(MPa)

Diameter(mm)

SteelMaterials

260045014-16Tensile Bars24352608Compressive

Bars

24352606Stirrup23702200.6Steel Fiber

Table 2- The type of Self Compacting Concrete beam with Normal Strength

GroupAbbreviation

BeamAbbreviation

TensileBars

CompressiveBars

Stirrup Ratio of Fiber toConcrete

N

NS80

NS80V0 143 82 mm80@6 0%NS80V0.5

143

82

mm80@6 0.5%

NS80V1 143 82 mm80@6 1%

NS100 NS100V0 143 82 mm100@6 0%NS100V0.5 143 82 mm100@6 0.5%NS100V1 143 82 mm100@6 1%

Table 3- The type of Self Compacting Concrete beam with High Strength

GroupAbbreviation

BeamAbbreviation

TensileBars

CompressiveBars

Stirrup Ratio of Fiber toConcrete

H

HS80HS80V0 163 82 mm80@6 0%

HS80V0.5 163 82 mm80@6 0.5%HS80V1 163 82 mm80@6 1%

HS100HS100V0 163 82 mm100@6 0%

HS100V0.5 163 82 mm100@6 0.5%HS100V1 163 82 mm100@6 1%

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Geometric properties of beams and types of reinforcement pattern are shown in Figure (6). In table (4 & 5)summery of mix design for concrete used in the test are shown.

Figure 6- Details of beams under the applied Load

Table 4- Mix design of SCC with normal strength (kg)

SandGravelStone

PowderWater

Super

Plasticizer

Micro

SilicaCement

779779245140735315

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Table 5- Mix design of SCC with high strength (kg)

SandGravelStone

PowderWater

SuperPlasticizer

MicroSilica

Cement

5177233502001050450

6- Casting and Test of Beams

All the beams will come out of form works after 24 hours and would be covered with humid jute canvas andkept in humidity conditions for 28 days and then they would be tested.For test of compressive strength cubic specimen 100 100 100mm and for the test of indirect tension

(Brazilian Method) cylindrical specimen with 100mm diameter and 200mm height were used.For each beam slump test, L-Box, compressive strength and indirect tension were carried out and thesespecimens were kept in equal conditions with the casting beams. The values of results of these tests are shownin table (6 & 7).

Table 6- The value of slump flow, L-Box, resistance of compressive specimen and indirect tension for selfcompacting normal strength concrete

Amount of fiber toconcrete % 12 h

h

T20

-T40(s)

Dfinal(cm)

T50(s)

Compressive Strength

(kg/cm2)

Indirect TensionStrength (kg/cm2)

0 0.86 0.5 1.1 75 2.1 330 34

0.5 0.84 0.7 1.5 70 2.7 365 42.2

1 0.79 0.8 1.7 68 2.8 391 49

Table 7- The value of the results of slump flow, L-Box, resistance of compressive specimen and indirect tensionfor self compacting high strength concrete

Amount of fiber toconcrete % 1

2h

h T20-T40(s)

Dfinal(cm)

T50(s)

CompressiveStrength

(kg/cm2

)

Indirect TensionStrength (kg/cm2)

0 0.83 0.6 1.5 71 2.6 632 49.8

0.5 0.8 0.9 1.8 67 2.9 674 58

1 0.77 1 - 2 64 3.1 695 63

In order to carry out the test of beams two concentrated load (on the top) were used by a jack with 100 toncapacity. To maintain the beams camber we use a deflect meter which would be connected to the computer.Figure (7) shows the overall design of loading device and the type of loading and the creation of supports on thebeams.

Figure 7- The overall design of loading device

Figure (8) shows the modeling of beams with the situation which load applied and the position of deflect meterand the button testing the strain.

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Figure 8- The type of applying force on the beams and situation of deflect meter and strain gage

7- Measured Parameters

7-1- The Investigation of Behavior of Compressive Strength

In figure (9) the process of relative increscent compressive strength of normal and high strength self compacting

concrete are shown by adding 0.5 and 1% of fibers. As you can see adding the fibers in both type of concretewill lead to increscent of compressive strength, also it is obvious the role of fibers in increasing the relativestrength of normal self compacting concrete are larger. By adding 0.5 and 1% fiber to normal self compactingconcrete it shows that 10.5 and 18.5% and for high strength self compacting concrete 6.5 and 10% increase ofstrength.

Figure 9- The process of relative increscent of compressive strength of concrete by adding fibers

7-2- The Investigation of Indirect Tensile Strength Behavior (Splitting)

In figure (10) the effect of adding fibers on the indirect relative tensile strength of cylindrical specimen has beenshown. For both type of concrete there is increment of strength, but the effect of fiber on the increment ofrelative strength of self compacting concrete is more. By adding 0.5 and 1% fiber for self compacting normal

strength concrete almost 24 and 44% and for concrete with high strength about 16.5 and 26.5% increment havebeen observed.In the developing processes of tensile strength like compressive strength also regarding the slope of curves in

two area zero to 0.5 and o.5 to 1% have been observed, which in initial volumes the increasing process of tensilestrength are more than larger volume.

Figure 10- The increasing process of indirect tensile strength of concrete by adding fibers

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7-3- The Comparison between Compressive Strength and Indirect Tensile Behavior

The comparison for increment of relative strength of compressive specimen and indirect tensile for selfcompacting normal strength concrete is shown in figure (11) and for self compacting high strength concrete isshown in figure (12).

The relative increment of strength due to adding the fibers in indirect tensile both for self compacting concretewith normal and high strength is considerably higher than compressive strength.

This will indicate that the role of steel fibers is more effective in the increment of strength of tensile region ofconcrete structural members than in the compressive region. We should note that in both test compressivestrength and indirect tensile in both self compacting concrete with normal and high strength the process ofstrength growth by adding fibers in the initial volumes are higher.

Figure 11- The comparison of increment of relative strength of compressive and indirect tensile strength of selfcompacting normal strength concrete specimen

Figure 12- The comparison of increment of relative strength of compressive and indirect tensile strength of selfcompacting high strength concrete specimen

7-4- The investigation of Load Behavior Beams Camber

In order to compare all the beams of Group N with each other the load-camber curves of all beams are shown infigure (13). As we can see near the beam linear state that is not much difference in the load-camber curves due

to adding the fibers. Near ultimate state by adding fibers, in addition to increasing ultimate strength, ultimatestrength would be increase considerably, which will indicate the effective role of fiber in the increasing ductilityand energy absorption. Near the load-camber curves of these beams NS100V0.5, NS80V0, NS100V1,NS80V0.5 are not able.Figure (14) also shows load-camber curves for all the beams in group H. By adding the fibers a similar behaviorwith group N would be observed, regarding this difference that the effect of fibers in the increment of relativestrength and ductility is less. This will show that in beams under the test with geometric and mechanicalsignification which was mentioned above by adding 0.5% the volume of fibers and reducing 25% of stirrups

volume almost load-camber curves are still the same (but with more ductility).In both groups N and H the comparison of beams with stirrup spacing of 100mm and 80mm shows adding thefibers to S100 beams which have less stirrups to the S80 beams would lead to sudden rise in the load-camber

curve.

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Figure 13- Load-camber curves of self compacting normal strength concrete beams

Figure 14- Load-camber curves of self compacting high strength concrete beams

7-5- The Investigation of Behavior of Load Flexural Strain at Position A

Load Flexural strain at position A for all self compacting normal strength concrete beams (group N) is shown

in figure (15) and for all self compacting high strength concrete beams (group H) is shown in figure (16).As we can see all the curves shows similar behavior with increasing load (and it does not show too muchdifference for adding fibers in the shear spans beams to its ultimate state). Similar behavior of load-flexuralstrain of beams with stirrup spacing of 100mm and fiber volume of 75% and 1% and for beams with stirrupspacing of 80mm and fiber volume zero and 5% near the ultimate state is considerable.

Figure 15- Load-strain curve for position A for all self compacting normal strength concrete beams

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Figure 16- Load-strain curve for Position A for all self compacting high strength concrete beams

7-6- The Investigation of the Effect of Load Strain for Position B and C

As you can see in figure (8) strain B and C are considered for the investigation of the effect of adding fiber onthe widening of diagonal cracks with 45angle to the horizon.In figures (17 & 18) load strain curve for position B and C are investigated for all self compacting normal

strength concrete beams. Ultimate strength and curves of two beams NS80V0 and NS100V0.5 are similar to twobeams NS80V0.5 and NS100V1. By adding 0.5% to fiber volume and reducing 25% from the stirrup volume inthe shear spans the shear stiffness of the beams would increase a little bit and on the other hand due to beneficialusage of stirrups and fibers without too much change in the ultimate strength of beams, its ductility would

increase.

Figure 17- The comparison of load strain curve in position B for all self compacting normal strength concretebeams

Figure 18- The comparison of load strain curve in position C for all self compacting normal strength concretebeams

In figures (19 & 20) load strain curve for position B and C of all self compacting high strength concrete beamshave been compared. The same behaviors with self compacting normal strength concrete beams which have

same steel shear signification have been observed. With another term there is proportional similarity in thebehavior of beams HS80V0 and HS100V0.5 with beams HS80V0.5 and HS100V1. In general comparing self

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compacting normal strength concrete beams and self compacting high strength concrete beams we would

observe due to higher stiffness of most of the high strength concrete beams, there is a higher slope in the load strain curve of these beams.

Figure 19- The comparison of load strain curve for position B for all self compacting high strength concretebeams

Figure 20- The comparison of load strain curve for position C for all self compacting high strength concretebeams

Table 8 shows shear cracking of beams. As you can see shear cracking would increase by adding fibers. From

the recorded data we can understand if we want to increase shear cracking strength, adding steel fibers are moreeffective than to add stirrups.

Table 8- Shear cracking

Shear Cracking (Ton)Beam2NS100V0

2.25NS100V0.52.6NS100V12.15NS80V02.35NS80V0.52.70NS80V12.5HS100V03HS100V0.5

3.7HS100V12.8HS80V03.2HS80V0.53.9HS80V1

Figures (21 & 22) shows the effect of adding fiber on the relative increment of ultimate strain B and C for selfcompacting normal strength concrete and in figures (23 & 24) this effect have been shown for self compacting

high strength concrete. As you can see by adding 1% fiber, the increment of ultimate strain in beams with stirrupspacing 80mm for strain at position B for self compacting high strength concrete beam would reach to 63.5%

and for self compacting normal strength concrete would reach to 80.5%. This shows that due to widening ofcracks, the transformation of stress in cracks locations takes place through fibers. As though concrete beams a

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larger strain to the past, with another term by using fibers with low strength would increase toughness and

energy absorption ability of concrete (especially for self compacting normal strength concrete).

Figure 21-Relative increment of ultimate strain in position B due to additional fibers in self compacting normalstrength concrete

Figure 22- Relative increment of ultimate strain in position C by adding fibers in self compacting normal

strength concrete

Figure 23- Relative increment of ultimate strain in position B by adding fibers to self compacting high strength

concrete

Figure 24- Relative increment of ultimate strain in position C by adding fibers in self compacting high strengthconcrete

7-7- The Investigation of Ultimate Shear Strength Behavior

Ultimate shear of all beams are indicated in table (9). By observing this table we could understand that the

combination of stirrups with 100mm spacing and 0.5% fiber volume almost shows same ultimate shear strength

with 80mm stirrup spacing without fibers, and also beam shear strength with 100mm stirrup spacing and 1%fiber volume is almost equal to ultimate shear strength with 80mm stirrup spacing and 5% fiber volume.

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Table 9- Ultimate Shear

Figures (25 & 26) shows relative ultimate shear strength by adding fibers for self compacting normal and highstrength concrete respectively in both figures. The increment of shear strength by adding steel fibers in beams

with low stirrup volume (stirrup with larger spacing) is considerable. By other means in beams with weakerstirrups steel fibers would play a significant role in locking the cracks.

Figure 25- The increment of relative ultimate shear strength by adding fibers to self compacting normal strengthconcrete

Figure 26- The increment of relative ultimate shear strength by adding fibers to self compacting high strength

concrete

8- Conclusion1- By keeping the strength and stirrup spacing constant and adding steel fibers in the shear spans, crackingstrength and ultimate shear strength would increase. This means that the steel fiber will play a significant rolebefore and after cracking.2- By adding steel fiber to shear spans, strains and beam cambers (under uniform loading) would reduce. In thearea of load service this effect is more influent anal and beneficial, because in addition to reducing the cracks,the deformation would reduce and shear stiffness would increase. Reinforced beams with stirrups and steel

fibers (in shear spans), in compare with conventional reinforced beams have a stiffer behavior.3- The effect of steel fiber to increase the tensile strength is more than compressive strength.

Ultimate Shear(Ton)

Beam

8.75NS100V010.25NS100V0.5

11.45NS100V110NS80V0

11.25NS80V0.512.25NS80V1

12HS100V013HS100V0.5

13.75HS100V112.75HS80V013.6HS80V0.514.25HS80V1

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4- In addition to maintain ultimate shear strength of beam we could replace the part of stirrups with fibers. We

could obtain more ductility and strength in first crack. In order to increase shear cracking strength, the capacityof absorbing energy, ductility and preventing sudden cracks, adding fibers are more effective than addingstirrups. This phenomenon is an important factor for structure strengthening against the earth quake

(economically).5- The effect of steel fibers on the increment of strength of self compacting normal strength concrete ductility is

more than self compacting high strength concrete, but with regard to the brittle behavior of high strengthconcrete using fiber for this type of concrete is necessary.

9- References

[1] Ouchi, M., Hibino, M., and Okamura, H., Effect of super plasticizer on self-compact ability of freshconcrete, TRR 1574, PP. 37-40 (1996).[2] Beygi, M.H.A., Vaseghi, J and Shafigh, P., Molavi, R., Investigation of Flexural Behavior of ReinforcedBeams Made of Self Compacting Concrete, Second National Congress of Civil Engineering, Iran University of

Science and Technology, PP. 104 (April 2005).[3] Maghsoodi, A. A., Hoornahad, H., Self Compacting Concrete with the Use of Materials from KermanProvince, 2ndInternational Conference on Concrete & Development, Building and Housing Research Center,

PP. 103, (April 2005).[4] Sadrmomtazi, A., Hatami, F., Characteristics of Self Compacting Concrete and its Utilizations, 2

nd

International Conference on Concrete & Development, Building and Housing Research Center, PP. 105, (April2005)[5] HAREX-Stahlfasertechnik information GmbH & Co.KG (1986).[6] Dehn, F., Holschemacher, K., and Weisse, D., Self-compacting concrete-time development of the materialproperties and the bond behavior, Lacer No. 5, PP.115-123 (2000).[7] Berenjian, J., Beygi, M.H.A., Effect of Steel Fiber on the Mechanical Properties of High StrengthConcrete, Proceeding of Articles of Engineering Faculty, Mazandaran University, Vol. 1, First Edition, PP.

50, (2000).[8] Okamura, H. and Ouchi, M., Journal of Advanced Concrete Technology" vol.1, No.1, PP. 5-15 (2003).