concrete shear research

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DECLARATION OF THESIS / UNDERGRADUATE PROJECT PAPER AND COPYRIGHT

ANG TING GUAN Authors full name :

Date of birth : 10 / 02 / 1982

Title : THE INFLUENCE OF THE ANCHORAGE OF INDEPENDENT BENT-UP BAR ON ITS SHEAR CAPACITY

2007/2008 Academic Session:

I declare that this thesis is classified as :

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820210-01-6267

8 MAY 2008

P.M. DR. RAMLI ABDULLAH

I hereby declare that I have read this thesis and in my opinion to this thesis is sufficient

in terms of scope and quality for the award of the Master of Civil Engineering.

Signature : ...

Name of Supervisor : PM. DR. RAMLI ABDULLAH

Date : 9 MAY 2008

THE INFLUENCE OF THE ANCHORAGE OF

INDEPENDENT BENT-UP BAR ON ITS

SHEAR CAPACITY

ANG TING GUAN

A project report submitted in partial fulfillment of the

requirement for the award of degree of

Master of Engineering (Civil - Structure)

Faculty of Civil Engineering

Universiti Teknologi Malaysia

APRIL, 2008

ii

I declare that this thesis entitled The Influence of the Anchorage of Independent Bent-

up Bar on its Shear Capacity is the result of my own research except as cited in the

references. The thesis has not been accepted for any degree and is not concurrently

submitted in candidature of any other degree.

Signature : _________________

Name : ANG TING GUAN

Date : 8 MAY 2008

iii

Dedicated to

To my beloved parents

Thanks for your support

iv

ACKNOWLEDGEMENT

The successfully completion of this project is the result of many people have

given me a helping hand. I have learnt a many things other than from text or note

from my engineering course. The experience that I gain through this project will

become the valuable treasure in my life.

First of all, I wish to express my grateful thanks to my project supervisor,

P.M Dr. Ramli Abdullah for his invaluable guidance, suggestions and helpful advices

for various types of analysis and comparison processes to be carried out as well as

contents improvements. Thanks for his contribution in the progress of doing this

project. In addition, deep appreciation to his for spending time in our frequent

discussions and questioning and answering which led to the accomplishment of this

project.

I also wish to acknowledge those who lend me a hand when I encounter some

problem in my project. For example the lecturers that have taught me the knowledge

about reinforced concrete and other related. Without their help, I may not finish the

project on time. In addition, I also wish to express my great appreciation to my

friend, who gives the information and also guidance to accomplish this project.

Finally, I wish to convey my heartful thanks to my parents and wife who have

given me constant support and encouragement throughout the dissertation. For the

last but not the least, I want to give my gratitude for those people that help me in this

project but not stated above, their contributions will not be forgotten.

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ABSTRACT

The use of independent bent-up bars as parts of shear reinforcement has been

shown to be effective. Laboratory tests revealed that beams provided with a

particular amount of such reinforcement in conjunction with nominal links achieved

higher shear resistance than beams with the normally adopted vertical design links.

In the conventional bent-up bars system, it was required that the length of the

horizontal portion of the bars after the bend be at least the anchorage length of the

bar. In many cases, this requirement has limited the provision of closely spaced

multiple system of bent-up bars. This project presents the results of experimental

investigation on five rectangular beams in which the effect of using short anchorage

of the independent bent-up bars on the capacity of the beam in carrying shear was

studied. The influence of various amounts of bent-up bars was also investigated. All

the beams were provided with identical longitudinal reinforcement but with different

sets of shear reinforcement. In one beam, the shear reinforcement was in the form of

closely spaced vertical links, while the other three beams had nominal links

combined with independent bent-up bars of various amount and anchorage lengths.

Test results indicate that the anchorage length of the bent-up bars is insignificant to

the capability of the bars in carrying shear. It also suggests that the provision of the

large amount of the bent-up bars does not produce the corresponding advantage to

the beams shear capacity. It may therefore be concluded that independent bent-up

bars be used effectively and economically in reinforced concrete beam design to

resist shear.

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ABSTRAK

Kegunaan bar condong bebas sebagai tetulang ricih adalah berkesan. Ujikaji

makmal mendapati rasuk yang dibekalkan dengan sesetengah bilangan tetulang

bersama dengan nonimal perangkai untuk mencapai rintangan ricih yang lebih tinggi

daripada perangkai pugak yang biasa digunakan. Dalam sistem tradisional bar

condong, kepanjangan bahagian bar melintang selepas dibengkokkan mesti

sekurang- kurangnya kepanjangan tambatan bar. Dalam banyak situasi, kelayakan ini

telah mengehadkan bekalan gandaan sistem yang berjarak rapat bagi bar condong.

Kajian ini memaparkan keputusan dari ujikaji makmal yang telah dijalankan ke atas

lima rasuk bersegi empat dimana kesannya menggunakan tambatan yang pendek

dalam bar condong terhadap keupayaan menanggung ricih telah dikaji. Pengaruh dari

pelbagai bilangan bar yang dibengkok juga pun diujikaji. Semua rasuk dibekalkan

dengan tetulang bergarisan bujur yang sama tetapi set ricih tetulang yang berlainan.

Dalam satu rasuk, ricih tetulang adalah dalam bentuk perangkai pugak yang berjarak

dekat, manakala tiga rasuk yang berlainan mempunyai nominal perangkai yang

bergabungan dengan bar condong bebas dengan pelbagai bilangan dan panjang

tambatan. Keputusan ujikaji menunjukkan kepanjangan tambatan dari bar condong

adalah tidak berkesan untuk menanggung ricih. Ujikaji ini juga mencadangkan

dengan membekalkan banyak bar condong bebas tidak akan menghasilkan kebaikan

yang sama kepada keupayaan ricih rasuk. Oleh yang demikian, bar condong bebas

adalah bekesan dan ekomoni untuk digunakan dalam mencorakkan rasuk konkrit

bertetulang untuk menahan ricih.

vi

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENT vii

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF SYMBOLS xiv

LIST OF APPENDICES xvi

1 INTRODUCTION

1.1 General 1

1.2 Objectives of Studies 3

1.3 Scope of Works 4

2 LITERATURE REVIEW

2.1 Shear Stress Variation in Reinforced Rectangular Beams 5

2.2 Shear Failure in Beams without Shear Reinforcement 7

2.3 Types of Shear Failure 10

2.3.1 Case 1: (av/d > 6) 10

2.3.2 Case 2: (6> av/d > 2) 11

2.3.3 Case 3: (av/d > 2) 12

vii

2.3.4 Case 4: (av/d = 0) 12

2.4 Shear Reinforcement 13

2.4.1 Vertical Links 14

2.4.2 Bent-up Bars 17

2.5 BS8110 Requirement for Designing the

Shear Reinforcement 20

2.6 Summary 21

3 EXPERIMENTAL INVESTIGATION

3.1 Introduction 23

3.2 Design of Experiment 24

3.3 Details of Test Specimens 25

3.2.1 Beam B1 25

3.2.2 Beam B2 26

3.2.3 Beam B3 27

3.2.4 Beam B4 28

3.2.5 Beam B5 29

3.4 Materials of Reinforced Concrete Beam 30

3.4.1 Steel Reinforcement 31

3.4.2 Concrete 32

3.5 Preparation of Test Specimens 33

3.5.1 Formwork 33

3.5.2 Reinforcement 34

3.4.3 Concreting and Curing 36

3.6 Compression Tests: Cube test 38

3.7 Test Procedure 40

3.8 Instrumentation 42

4 TEST RESULTS

4.1 Introduction 44

4.2 Beam B1 45

4.2.1 Specimen Behavior during Test 45

4.2.2 Test Results 46

4.3 Beam B2 47

viii



4.4 Beam B3 50



4.5 Beam B4 52



4.6 Beam B5 55



4.7 Summary of Specimen Behavior and Test Results

for All Specimens 58

5 ANALYSIS AND DISCUSSION 63

6 CONCLUSION AND RECOMMENDATIONS

6.2 Conclusion 67

6.3 Recommendation 68

REFERENCES 70

APPENDIX A 71

APPENDIX B 78

APPENDIX C 79

APPENDIX D 89

x

LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Value of ultimate shear stress vc (N/mm2)for a concrete

strength of fcu = 25 N/mm2 (BS8110 : 1997) 17

3.1 Details of specimens 25

3.2 Proportion of concrete mix design 33

4.1 Concrete compression strength for all beams 45

4.7 The Results of the all specimens 60

5.2 The comparison between testing result

and calculation value 65

xi

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Principal stresses in a beam 6

2.2 Shear stress variation in reinforced rectangular beams. 6

2.3 Failure due to av/d ratio 8

2.4 Case 1: (av/d > 6) 10

2.5 Case 2: (6> av/d > 2) 11

2.6 Case 3: (av/d > 2) 12

2.7 Case 4: (av/d = 0) 12

2.8 Shear reinforcement 13

2.9 Types of shear reinforcement 14

2.10 Vertical links 14

2.11 Vertical links and the analogous truss 15

2.12 Bent-up Bars 18

3.1 Dimension of Beam Specimen 25

3.2 Beam B1 26

3.3 Beam B2 27

3.4 Beam B3 28

3.5 Beam B4 29

3.6 Beam B5 30

xii

3.7 Several size of steel reinforcement 31

3.8 Independent bent-up bar with different amountof anchorage length. 32

3.9 Ply woods formwork 34

3.10 Bar bender machine Takeda TBN-32 35

3.11 Steel cutter machine branded Kingsland 35

3.12 Reinforcement cast into formworks 36

3.13 Mechanical drum mixer 37

3.14 Steel cube moulds 37

3.15 Casting completed 37

3.16 Specimens coated with white pain layer 38

3.17 Compressive strength test machine 39

3.18 Flow chart of research methodology 41

3.19 Testing machine and set up 42

3.20 Testing instruments 43

4.1 Appearance and cracking configuration of specimen beam B1 46

4.2 Graph load versus deflection for specimen beam B1 47







4.9 Appearance and cracking configuration of

xiii

specimen beam B5 56


4.11 Appearance and cracking configuration of all specimens 59

4.12 Graph compressive strength of concrete for all specimens beams 60

4.13 Graph load versus deflection for all specimen beams 61

4.14 Graph maximum ultimate load for all specimen beams 61

4.15 Graph maximum deflection for all specimen beams 62

xiv

LIST OF SYMBOLS

A - Area of cross-section

As - Area of tension reinforcement

Asb - Area of steel in bent-up bars

As,prov - Area of tension reinforcement provided

As, req - Area of tension reinforcement required

Asv - Total cross-sectional area of links at the neutral axis

av - Shear span

b - Width of a section

bv - Breadth of member for shear resistance

c - Cover to reinforcement

d - Effective depth of the tension reinforcement

fcu - Characteristic concrete cube strength at 28 days

fs - Service stress in reinforcement

ftt - Design tensile stress in concrete at transfer

fy - Characteristic strength of reinforcement

fyb - Characteristic strength of inclined bars

fyv - Characteristic strength of link reinforcement

L - Effective span of a beam

Mmax - Maximum bending moment

sb - Spacing of bent-up bars

sv - Spacing of links

V - Shear force at ultimate design load

Vb - Design ultimate shear resistance of bent-up bars

Vc - Design ultimate shear resistance of a concrete section

v - Shear stress

xv

vb - Design shear stress resistance of bent-up bars

vc - Design ultimate shear resistance of a singly reinforcement concrete

beam

- Angle between a bent-up bar and axis of a beam

- Bond coefficient

- Bar diameter

- Angle

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Analysis of singly reinforcement rectangular section 71-77

B Concrete mix design Grade 30 78

C Detail for Testing Result 79-88

D Calculation of Shear Stress Analysis 89-94

1

CHAPTER 1

INTRODUCTION

1.1 General

Reinforcement concrete beam is concrete in which reinforcement bars have

been incorporated to strengthen a material. The shear reinforcement must be

provided when the value of actual shear stress exceeds the permissible shear stress of

the concrete used. Shear reinforcement is used to prevent failure in shear; this

increases the ductility of the beam and considerably reduces the chances of a sudden

failure. Furthermore, the anchorage is the embedment of a bar in concrete so that it

can carry load through bond between the steel and concrete.

Generally, an inclined bar is one that is employed to resist tension in the

bottom of the beam near mid- span and is then bent up at 45o into the top of the

member, where it may provide resistance over the support. In such a case, the force

in the horizontal parts of the bar must balance the horizontal components of the force

in the inclined part and the complementary compressive resistance.

Any form of effectively anchored reinforcement that intersects these diagonal

cracks will be able to resist the stress to a certain extent3. In practice, shear

2

reinforcement is provided whether in the form of vertical links, inclined links or

combination system of links and bent-up bars. Bent- up bars is normally used to

carry heavy shear forces.

Vertical links are simple in fabricating and installing, therefore it is most

common used as shear reinforcement in building construction. Links are arranged

closely or sometimes double or more shear links are used to resist high shear stress.

Congestion near the support of reinforced concrete beam due to the presence of the

closely spaced links can increase the cost and time required in fixing the

reinforcement.

The use of bent- up bars along with vertical links had been practical before,

where all the tensile reinforcement is not required to resist bending moment; some of

the bar was bent- up in the region of high shear to form the inclined legs of shear

reinforcement. However, inclined bars are seldom used in present- day practice. This

is because the cost of bending is not insignificant. They are also difficult to

manipulate and fix compare with straight bars, especially in congested situations. In

beams with small number of bars provided, the bent- up system is not suitable due to

insufficient amount of reinforcement would be left to continue to the support as

required by the code of practice.

However, where large concentrated loads must be supported, their use should

be considered in order to avoid the congestion that can arise when the multiple- link

systems, that would otherwise be necessary, are employed. Inclined bars used as

shearing reinforcement must be checked for anchorage and bearing.

In this project, total of five reinforced concrete beams which were contained

different types of shear reinforcement and anchorage length were designed and used

in the laboratory testing. All the beams were designed to fail in shear. Thus the

tension reinforcement and concrete properties recommended need to be considered

3

and calculated to give a sufficient of bending moment resistance. Furthermore, same

grade of concrete and the size are applied to the entire beam. All the beams will be

tested and the result will be compared.

1.2 Objective of the Studies

The main objectives of this study are:

a) To study the effectiveness of independent inclined bars as shear reinforcement in

rectangular beams.

b) To investigate the anchorage length of the independent bent up bar on its

capacity in carrying shear.

c) To study the influence of the amount of the independent bent up bar on its

capacity in resisting shear.

4

1.3 Scope of Works

This study was based on the experimental investigation carried out within the

scope as below:

(a) Five reinforced concrete beams which are contained different types of shear

reinforcement and anchorage length were designed and used in the laboratory

testing.

(b) Grade C30 concrete are applied the entire beam and the size of the beam were

200mm width x 250mm height x 2300mm length

(c) Each of the beams was provided with 3T16 as tension reinforcement and 2T10 as

top of reinforcement and the variable of vertical links and inclined bars as shear

reinforcement system.

(d) All specimens were tested to failure with two point loads near the support.

.

(e) The inclination of the independent bent-up bar was 45 degree from the

longitudinal axis and provided within 500mm from support beam.

5

CHAPTER 2

LITERATURE REVIEW

2.1 Shear Stress Variation in Reinforced Rectangular Beams

Shear is a technical term used when the two sections on either side of a plane

in a member have a tendency to slide along that plane. The tendency is caused by the

shear forces that act in the opposite directions. Figure 2.1 are represents the

distribution of principal stresses across the span of a homogeneous concrete beam.

The form of an arch is taken on the principal compressive stress while the tensile

stresses have a curve of a suspended chain. The shear is low and the bending

stresses are dominant the direction of the stresses tends to be parallel to the beam

axis at the mid-span. Near the supports, the shearing forces are greater and the

principal stresses are inclined, so that the tensile stresses are liable to cause a

diagonal cracking. Therefore, Shear reinforcement must be provided when the

diagonal tension exceeds the limit tensile strength of the concrete.

The variation of bending moment along the span causes the shear. The

relationships between shear V and bending moment M is shown below. Once the

shear V is known, the shear stress v can be obtained by dividing the shear force with

the area of cross section on which the shear is acting:

6

V = dM/dx Equation 2.1

Figure 2.1: Principal stresses in a beam

As shown in Figure 2.2, shear stress variation in reinforced rectangular

beams is in the forms of parabolic and uniform. The section above neutral axis is in

compression, while below neutral axis is in tension. As T + T is greater than T, the

lower portion has a tendency to slide towards right, while the upper portion has a

tendency to slide towards left due to C + T greater than C.

Figure 2.2: Shear stress variation in reinforced rectangular beams.

Normally, calculation is used to determine dimension of the cross- section of

a reinforced concrete beam and area of longitudinal steel, which consider as

resistance to bending moment at the ultimate limit state. If load in a plain concrete

beam is increased without reinforcement, tension crack will form at the largest

7

tension stresses location and will cause immediate fails to the beam. However, the

situation is different when tension reinforcement is provided; much higher loads can

be carried. This is due to the flexural tension strength will resist by the steel. Even

though tension cracks will still form in the concrete, it does not cause fail to the

beam.

Shear stresses increase proportionally to the loads. In the result, the diagonal

tension stresses of significant intensity are created in the regions which close to the

support, also known as regions of high shear force. Diagonal cracks formed is due to

the longitudinal tension reinforcement does not reinforce the tensional weak concrete

against the diagonal tension stresses that caused by shear or combined effect of shear

and flexure.

2.2 Shear Failure in Beams Without Shear Reinforcement

Shear reinforcement is important in preventing failure in shear. If the

reinforcement is not provided, the behavior of beams must be fully understood. In

beam without shear reinforcement, inclined cracking formed is depending on the

conditions of load and support. Once the load is increased, the inclined cracks may

develop and extend.

Diagonal tension stresses reach the tensile strength of the concrete is the

cause of diagonal cracks to develop. From Figure 2.3, the shear mechanism is

complex and depends in the shear span ratio ad.

8

Figure 2.3: Failure due to av/d ratio

There are three actions form the mechanism of resisting shear in the beam,

which are:,

a) Shear stresses in the compression zone with a parabolic distribution.

b) Aggregate interlock along the cracks

c) Dowel action in the bars where the concrete between the cracks transmits shear

forces to the bars.

According to BS 8110: Part I (7), behavior of reinforced concrete beams is

much influenced by the shear stresses. It is also depending on configuration, support

conditions and load distribution. At a location of large shear force V and small

bending moment M, there will be little flexural cracking.

v = V / bvd Equation 2.2

Where, V = the ultimate shear force

bv = the beam width (bv = b for rectangular beam)

d = the effective depth

9

This value is then use as guild to determine whether the section is of the

correct size and whether shear reinforcement is required. The design concrete shear

stress vc depend on the percentage of the steel in the member, the depth and the

concrete grade. An increase in the amount of tension steel as well as increase in the

dowel action component increases the aggregate interlock value by resisting the

width of the shear crack.

The design concrete stress is given by the following formulae:

Equation 2.3

Where 100As (bvd) should not be greater than 3.0 and 400/d should not be

less than unity. The code notes (clause 3.4.5.4 in BS 8110) that for tension steel to be

counted in calculating As it must continue for a distance at least equal to d beyond

the section being considered. This formula gives values of vc concrete grade 25. For

higher grades of concrete values should be multiplied by (fcu/25)1/3. The value of fcu

should not be greater than 40 and value of m is 1.25.

Shear failure in beam sections without shear reinforcement normally occurs

at about 30o to the horizontal. The shear capacity is increased when the angle is

steeper due to the load or the section is close to the support. The increase is due to

the concrete in diagonal compression resist shear. For a maximum shear stress

requirement, the nominal shear stress v = V/ bvd must exceed 0.8 fcu1/2 or 5 N/mm2

even if the beam is reinforced to resist shear. This upper limit prevents failure of the

concrete in diagonal compression.

11430.79 100 /( ) (400 / ) /c s v mv A b d d

10

2.3 Types of Shear Failure

The vertical cracks are produced by the bending moment and these are linked

to diagonal cracks produced by the shear forces. Rectangular beam fails eventually

as the shear forces V is increased and the failure mode is strongly dependent on the

shear-span/depth ratio av/d. There are five types of shear failure, which four are

related to shear span ratio av/d. When ratio increases, the shear resistance will be

decreases.

2.3.1 Case 1: (av/d > 6)

The bending moment is significantly large compare to the shear force and

mode of failure is similar to pure bending. The initial vertical bending cracks

become inclined due to the action of the shear stresses and failure occurs in the

compression zone as shown in Figure 2.4. The stresses in the tensile steel are

approximately at yield and only minimum reinforcement is required in design

situations.

Figure 2.4: Case 1: (av/d > 6)

11

2.3.2 Case 2: (6> av/d > 2)

The initial bending cracks become inclined early in the loading sequence and

at collapse horizontal cracks from running along the line of the tensile reinforcement,

which shown in Figure 2.5. The horizontal cracks reduce the shear resistance of the

section by destroying the dowel force and reducing the bond stresses between the

steel and the concrete. The steel in the tension zone does not reach yield.

Figure 2.5: Case 2: (6> av/d > 2)

2.3.3 Case 3: (av/d > 2)

The influence of shear tends to be critical and the diagonal crack form

independently. The beam continues to carry additional shear force and failure takes

place by crushing of concrete in compression near the loads as the cracks penetrates

into this zone. Therefore, in this case bending cracks do not develop but shear cracks

(at approximately 45o) suddenly appear and often run through to the compression

zone and produce collapse as shown in Figure 2.6. The steel in the tension zone does

not reach yield.

12

Figure 2.6: Case 3: (av/d > 2)

2.3.4 Case 4: (av/d = 0)

Punching shear failure occurs when the plane of failure is forced to run

parallel to the shear forces as shown in Figure 2.7. This can occur when the opposing

shear forces are close together or, if shear links have been added, when the failure

plane forms which does not intercept the shear links. When this type of failure

occurs, the shear resistance of the section is at a maximum. The additional of shear

reinforcement in the form of vertical links does not increase the shear resistance

above the punching shear value and if a section is unable to resist this force, it must

be increased in size.

Figure 2.7: Case 4: (av/d = 0)

13

2.4 Shear Reinforcement

The addition of shear reinforcement increases the shear resistance for cases of

failure I, II and III. Numerous diagonal cracks develop as shown in Figure 2.8 and at

failure the shear reinforcement and the longitudinal steel yield, provided that the

steel is anchored and the member is not over- reinforced.

Figure 2.8: Shear reinforcement

There are three types of shear reinforcement, which are vertical links,

inclined links and bent-up bars as shown in Figure 2.9. Commonly at the

construction practice, the vertical links are used as shear reinforcement because of

their simplicity in fabricating and installing. By the way, the inclined links are used

at an angle 40 to 60 degrees. Bent-up bars are normally used to carry heavy shear

forces. The inclination of the bent-up bars should be at 30 to 60 degree but usually

the angles are 45 degree in order to ensure the reinforcement intercepts the crack

effectively. Furthermore, bent-up bars are most effective in deep beams with low

span/ depth ratios where the straight inclined portions of the bars extend over a

considerable length of beam (Standard Reinforced Concrete Details, 1973).

14

Figure 2.9: Types of shear reinforcement

2.4.1 Vertical Links

When ultimate limit state at the average shear stress v exceeds the design

shear stress vc + 0.4 N/mm2 shear reinforcement is added. Vertical links is the most

common type of shear reinforcement which strengthens the web of a beam and

prevents dowel failure. Vertical links in a beam intercept at approximately 45o in the

concrete is shown in the Figure 2.10.

Figure 2.10: Vertical links

15

In order to derive simplified equations the action of a reinforced concrete

beam in shear is represented by an analogous truss in which the longitudinal

reinforcement forms the bottom chord, the links are the vertical members and the

concrete acts as the diagonal and top chord compression members as indicated in

Figure 2.11.

Figure 2.11: Vertical links and the analogous truss

In the analogous truss, let

Asv be the cross- sectional area of the two legs of the vertical links

fyv be the characteristic strength of the links reinforcement

V be the shear force due to the ultimate loads

Using the method of section it can be seen at the section XX in the figure that

at the ultimate limit state the force in the vertical links member must equal the shear

force V, that is

0.95 fyv Asb = V Equation 2.4

0.95 fyv Asb = vbd Equation 2.5

16

where v = V/ bd is the average shear stress on the section,

when the links spacing is less than the effective depth, a series of superimposed

equivalent trusses may be considered, so that the force to be resisted by the link is

reduced proportionally. Thus if sv = the links spacing, the equation above will

becomes

0.95 fyv Asb = vbd (sv/d)

Hence, Equation 2.6

the equation will be rewritten since the concrete is also capable of resisting a limited

amount of shear as

Equation 2.7

where the vc is the ultimate shear stress that can be resisted by the concrete. Value of

vc are in table 2.1. As a simplified approach for the beam carrying mainly uniformly

distributed loads, the critical section for design may be taken at a distance d from the

face of the support using the value if vc. The average shear stress should never

exceed the lesser of 0.8fcu or 5 N/mm2.

yv

cvvs f

vvbsA

95.0/

17

Table 2.1: Value of ultimate shear stress vc (N/mm2) for a concrete strength of

fcu = 25 N/mm2 (BS8110 : 1997)

2.4.2 Bent-up Bars

Bent- up bars is normally used to carry heavy shear forces. The resistance to

shear of a beam reinforced with bent- up bars is based on the assumption that the

inclined bars form the tension members and the concrete the compression members

of a lattice girder system. The local bearing stress inside the bend in the bar may be

the limiting condition.

Inclined bars such as bent- up bar is another type of shear reinforcement.

Some of the bars may be bent up to form shear reinforcement while all the tensile

reinforcement is not required to resist the bending moment at the ultimate limit state.

In this case, bent- up bars must be adequately anchored at the top of the beam to

18

prevent bond failure. The bent- up bars can located at a distance from support from

L/5 to L/7 where L is the effective span of beam.

The bent- up bars resist diagonal tension and act as support to hanger bars

which are provided to give sufficient support to vertical stirrup. Bars may be bent up

near the supports to resist the shearing forces which shown in Figure 2.12. The bent

up bars and the shear resistance of the bars is determined by taking a section XX

through the girder.

Figure 2.12: Bent-up Bars

From the geometry of part (a) of the figure, the spacing of the bent up bars is

sb = (d-d)[cot + cot] Equation 2.8

and at the section XX the shear resistance of the single bar is

V = 0.95 fyv Asb sin Equation 2.9

19

Where Asb is the cross-sectional area of the bent-up bar.

For a multiple system of bent-up bars, as in part (b) of the figure, the shear resistance

is increased proportionately to the spacing sb. Hence

Vb = 0.95 fyv Asb sin [cot + cot] (d-d)/ Sb Equation 2.10

The code requires that the spacing sb has a maximum value of 1.5d and the angles

and should be greater than or equal to 45. With = = 45 and sb = (d d), so

the equation above becomes

V = 1.34 fyv Asb Equation 2.11

And this arrangement is commonly referred to as a double system.

2.9 BS8110 Requirement for Designing the Shear Reinforcement

Calculate the design shear stress v at any cross section from

v = V / bvd

The shear stress must never exceed the lesser of 0.8fcu or 5 N/mm2.

v > or 5 N/mm2 fcu8.0

20

If v < 0.5vc, then:

a) No shear reinforcement is required for members of minor structural

importance, such as lintels.

b) For all other structure members, provide minimum links which are

defined as shear links that will provide a shear resistance of 0.4 N/ mm2.

If 0.5vc < v < (vc + 0.4)

If v is between 0.5vc and (vc + 0.4), so the minimum links are needed to

provide for whole length of beam. So the area of shear reinforcement to be

provided is

(vc + 0.4) < v < 0.8fcu or 5 N/mm2

If v is between in the range of (vc + 0.4) and 0.8fcu or 5 N/mm2, provide

links or links combined with bent-up bars. Not more than 50% of the shear

resistance provided by the steel may be in the form of bent-up bars. The links

are provided as follow:

For the use of bent-up bars:

The shear resistance Vb of the system:

Vb = Asb(0.95fyv)(cos + sincot) d-d/sb

0.4/

0.95v

s vyv

bA s

f

yv

cvvs f

vvbsA

95.0/

21

Where Asb is the cross-sectional area of the bent-up bars, fyv the characteristic

strength, and are the angles, d the effective depth, d the concrete cover to the

centers of the top reinforcement and sb is the spacing of the bent-up bars.

2.6 Summary

From the literature review carried out in this chapter, the following summaries are

raised:

a) Near the supports, the shearing forces are greater and the principal stresses

are inclined, so that the tensile stresses are liable to cause a diagonal cracking.

Therefore, Shear reinforcement must be provided when the diagonal tension

exceeds the limit tensile strength of the concrete.

b) If load in a plain concrete beam is increased without reinforcement, tension

crack will form at the largest tension stresses location and will cause

immediate fails to the beam. However, the situation is different when tension

reinforcement is provided; much higher loads can be carried. This is due to

the flexural tension strength will resist by the steel.

c) Shear failure in beam sections without shear reinforcement normally occurs

at about 30o to the horizontal. The shear capacity is increased when the angle

is steeper due to the load or the section is close to the support.

22

d) Diagonal cracks formed is due to the longitudinal tension reinforcement does

not reinforce the tensional weak concrete against the diagonal tension

stresses that caused by shear or combined effect of shear and flexure.

e) Rectangular beam fails eventually as the shear forces V is increased and the

failure mode is strongly dependent on the shear-span/depth ratio av/d.

f) The bent- up bars resist diagonal tension and act as support to hanger bars

which are provided to give sufficient support to vertical links. In BS8110, at

least 50% of the shear resistance must provided by the vertical links.

23

CHAPTER 3

EXPERIMENTAL INVESTIGATION

3.1 Introduction

The purpose of this project is to investigate the use of the inclined bar as

shear reinforcement with different types of bar arrangement and anchorage length.

Therefore, due to the problems of conventional shear reinforcement, the uses of

independent inclined bars provided in the high shear region are recommended in this

project. In order to experimental the investigation of testing, the system was carried

out at Structure Laboratory of the Faculty of Civil Engineering.

First of all, total five reinforced concrete beams which were contained


in the laboratory testing. All the beams were designed to fail in shear. Thus the

tension reinforcement and concrete properties recommended need to be considered

and calculated to give a sufficient of bending moment resistance. Furthermore, same

grade of concrete and the same size cross section are applied to the beam. All the

beam were tested and compared.

24

3.2 Design of Experiment

In this investigation, five reinforced concrete beams which are contained


in the laboratory testing. Concrete grade C30 are applied to all the beam and the size

of the beam were 200mm width x 250mm height x 2300mm length shown in Figure

3.1 below. Each of the beams was provided with identical amount of main

reinforcement and the variable of vertical links and inclined bars as shear

reinforcement system, all details of specimens are shown in Table 3.1.

To study the behavior of the shear failure in reinforced concrete beam, Beam

B1 was designed with sufficient amount of nominal links and shear link to resist the

shear failure and served as a control specimen. The beam was provided with 3T16 as

tension reinforcement and 2T10 as hanger bars. Beam B2, B3 and B4 were designed

with the same nominal links and different independent inclined bars and anchorage

length as shear reinforcement. This 3 beam are designed to compare with the control

specimen for the behavior and carrying shear. Beam B5 was cast only with

independent inclined bars as shear reinforcement and to investigated the result for

the ultimate load its can carry without combination of any vertical shear links. The

inclination was 45 degree from the longitudinal axis and provided within 500mm

from support. The spacing of the inclined bar for Beam B2, B3 and B4 were 200mm

while the Beam B5 was 100mm.

25

Table 3.1: Details of Specimens

Vertical links Inclined Bars

Beam Cross-section

(mm)

Main

Reinforcement

Shear

Region

Nominal Amount Anchorage

length

B1 3T16 R6-50 R6 -150 - -

B2 3T16 R6-150 R6 -150 4T16 75

B3 3T16 R6-150 R6 -150 6T16 75

B4 3T16 R6-150 R6 -150 6T16 150

B5

200 x 250

3T16 - - 8T16 150

Figure 3.1: Dimension of Beam Specimen

3.3 Details of Test Specimens

3.3.1 Beam B1

In practical, vertical links are most commonly used in construction site as

shear reinforcement compared to the inclined links and bent-up bars. For the beam

B1 (as shown in Figure 3.2) were designed and prepared according to BS 8110 with

sufficient amount of nominal links and shear links to resist the shear failure. This

beam also served as control specimen.

26

As conventional shear reinforcement, this beam is using closely vertical links

at shear region. That is mean; links 6mm with spacing 150mm (R6-150) was used as

nominal links at middle and link 6mm with spacing 50mm (R6-50) as shear links at

the support. Shear links are arranged closely at shear region with 500mm length near

the support. By the way, at the middle of the beam span, nominal links are arranged

within 1000mm. Beam B1 was provided with 3T16 as tension reinforcement and

2T10 as top reinforcement.

Figure 3.2: Beam B1

3.3.2 Beam B2

From Figure 3.3, Beam B3 was designed with nominal links and independent

inclined bars as shear reinforcement. Top and bottom reinforcement were provided

similar as beam B1, which is 2T10 and 3T16. This beam is designed as bent-up bars

system in the form of independent bars. Compared to other specimen, independent

inclined bars are most efficient because these shear reinforcement were placed in the

27

direction of the principle tensile stresses. The inclination of bent-up bars is 45 from

the longitudinal axis and provided 4 bars with 16mm size (4T16) of reinforcement

within 500mm length near the support. Furthermore, spacing between these inclined

bars was 200mm and the anchorage length for inclined bar is 75mm. This beam also

provided with nominal links with R6-150 along the beam span.

Figure 3.3: Beam B2

3.3.3 Beam B3

Beam B3 (as shown in Figure 3.4 below) was designed in order to investigate

the performance of an increased amount of additional bars in resisting shear forces.

At beam B3, the amount of inclined bars was increased from 4T16 to 6T16 at shear

region. The spacing between these inclined bars was 200mm and the anchorage

length for inclined bar is 75mm. This beam also provided with nominal links with

R6-150 along the beam span. The top and bottom reinforcement were provided

similar as other specimens.

28

Figure 3.4: Beam B3

3.3.4 Beam B4

Beam B4 as shown in Figure 3.5 below, was designed in order to investigate

the anchorage length of the independent bent up bar on its capacity in carry shear. At

beam B4, the amount of anchorage length was increased from 75mm to 150mm at

shear region. The arrangement and positions of the independent bent-up bars are

same as Beam B3. The spacing between these inclined bars was 200mm and also

provided with nominal links with R6-150 along the beam span. The top and bottom

reinforcement were provided similar as other specimens.

29

Figure 3.5: Beam B4

3.3.5 Beam B5

To study the influence of the amount of the independent bent up bar on its

capacity in resisting shear. Beam B5 was designed without any vertical links but only

provided with the independent inclined bars as shear reinforcement as shown in

Figure 3.6 below. Only 3 nominal links were provided at the middle and each edge of

the beam length, in order to ease the installation of main reinforcement. Top and

bottom reinforcement were provided similar as other beams, which is 2T10 and

3T16. The inclination of bent-up bars is 45 from the longitudinal axis and provided

8 bars with 16mm size (4T16) of reinforcement within 500mm length near the

support. Furthermore, spacing between these inclined bars was 100mm and the

anchorage length for inclined bar is 150mm.

30

Figure 3.6: Beam B5

3.4 Materials of Reinforced Concrete Beam

Reinforced concrete is a composite material of steel bars embedded in a

hardened concrete. Normally, plain concrete whilst strong in compression is

relatively weak in tension and is therefore generally unsuitable for structure use. For

this reason, most concrete structures are of reinforced concrete in which the steel

bars are positioned in the concrete to resist the tensile forces, with the concrete

resisting the compressive forces. Reinforced concrete beam is designed as specified

materials, which are required to provide adequate safety during their services.

31

3.4.1 Steel Reinforcement

Commonly, concrete strong in compressive but weak in tension and steel are

good in compressive and tension. Therefore, steel is important reinforcing material

for concrete. Normally, type of reinforcing steel is form of round bars. The strength

of high steel is 460 N/mm2 and 250 N/mm2 for mild steel. In this project, high steel

reinforcement was used as tension steel, compressive steel and for additional bar in

resisting shear forces. While mild steel were used as shear and nominal links.

Furthermore, 3 bars with size 16mm (3T16) are chosen at tension

reinforcement and 2 bars with size 10mm (2T10) are chosen at compression

reinforcement as shown in Figure 3.7. R6 were provided for each beams for shear

and nominal links. All reinforcement bars used were in clean condition with only

minor rust.

For the inclined bar as shown in Figure 3.8, T16 was used to resist the shear

forces. In design consideration, a reinforce bars must be adequate anchored;

otherwise it will withdraw from the concrete before it has reached its full tensile

strength. So 75mm and 150mm of anchorage length are be using in this investigation.

Figure 3.7: Several sizes of steel reinforcement

32

Figure 3.8: Independent bent-up bar with different amount of anchorage length.

3.4.2 Concrete

Concrete is composed mainly of three materials, namely cement, water and

aggregate and an additional material, known as an admixture is sometimes added to

modify certain of its properties. Plain concrete is made by mixing cement, fine

aggregate, coarse aggregate and water. In this project, the target mean strength to be

obtained at 28days is 30 N/mm2 with a medium workability.

Concrete mix design was carried out for this investigation to ensure each of

specimens will satisfy the compressive strength specification. The purpose of mixing

is to produce an intimate mixture content of cement, water, fine and coarse

aggregate. Coarse aggregate with the size of 10 mm, fine aggregate and Ordinary

Portland Cement (OPC) are used as the major constituent of the concrete mix. The

ratio for free-water/cement was 0.54. Furthermore, water is needed for the chemical

process or hydration for making structural concrete, which the concrete will hardens

to reach sufficient strength. Therefore, several trial mixes need to be carried out to

determine the correct proportion of material. After prepared all the material, mixing

will be carry on by using a rotated mechanical mixer as shown in figure 3.12. The

mixing shall be continued until there is a uniform distribution of the materials.

33

In order to get good concrete, curing is needed because it let the process of

keeping the concrete moist to gain full strength. All the specimens will be covered

with plastic to sure that the water wont evaporate. In order can precisely achieving

the required strength for the grade of concrete at later stage, several trial mixes had

been conducted. 10 steel mold which with each dimension of 100mm x 100mm x

100mm had been used repeatedly to conduct the trial mix. Compaction test then was

carried up onto those trial cubes to measure the compressive strength of the concrete.

The amount of the material for concrete grade C30 for typical one meter cube is

shown in Table 3.2, it needed 410kg of cement, 220kg of water, 805kg of sand and

945kg of coarse aggregate and the calculations of concrete grade are shown in

Appendix B.

Table 3.2: Proportion of concrete mix design

Material

Grade fcu

(N/mm2)

Quantities

(m3)

Water

(kg)

Cement

(kg)

Fine

Aggregate

(kg)

Coarse

Aggregate 10mm

(kg)

C30 30 1 220 410 805 945

3.5 Preparation of Test Specimens

3.5.1 Formwork

In this investigation, 5 sets of formworks had been prepared. The material

selected were the 18mm thick plywood which were well-cut into the desired

dimensions. The ply woods were then held together firmly by using nail. The

finished internal dimension of the formworks was 200mm width x 250mm height x

2300mm length. Figure 3.9 shows the specimen preparation for the specimens.

34

Therefore, the inner part of formwork was being saturated and covered by oil before

casting to ease the stripping of formwork after the concrete has hardened.

Figure 3.9: Ply woods formwork

3.5.2 Reinforcement

During the preparation of material and formwork, the steel reinforcement

were cut and bent into required length and shape and they were later combined to

form reinforcement cage. For the bar cutting work, electric machine branded

Kingsland was used as shown in Figure 3.10. The bars with diameter larger than

10mm were cut by using this machine. For mild yield bar R6 cutting, only manual

cutter was used. For bar bending purpose, bar bender machine Takeda TBN-32 with

inner bend diameter of 150mm had used as shown in figure 3.11. This machine was

useful to across with bar bending job concern with high yield strength bar and also

for the independent bent-up bars. On the order hand, a man made mold had been

produced by using wood and nail to ease the vertical link construction work.

Furthermore, the mechanical bar bender was needed to bend the mild yield strength

bar bending task.

35

After the vertical links, independent bent-up bars and main bars have

prepared, the reinforcement caging work began. The manual rebar bender had been

used for all bar tighten task to bond the vertical links and independent bent-up bars to

the main bars upon completion. A day before concreting, the reinforcement cage was

inserted into the formwork (Figure 3.12). The reinforcement cage was hold firmly in

place inside the form with wood spacer at each side and hanging at top by steel wire.

All the spacers were then removed during the compaction work.

Figure 3.10: Bar bender machine Takeda TBN-32

Figure 3.11: Steel Cutter Machine branded Kingsland

36

Figure 3.12: Reinforcement cast into formworks

3.5.3 Concreting and Curing

Concrete drum mixer machine with capacity of 0.3 meter cube (Figure 3.13)

was used in concrete mixing task. Before the day of concrete mix, the materials were

prepared, weighed and stored in laboratory. Drum cleaned and saturated with clean

water before the mixing process. This was to ensure that no water which need for

hydration process later absorbed by drum or contaminants.

Before mixing, materials including cement, sand and coarse aggregate was

firstly being poured into the drum. It then followed by one third of required water, as

to ensure that no cement can adhere to the side of drum. Upon completion, mixer

started for three to five minutes and all the remaining water was slowly poured into

the mixer. At certain period, mixer was stopped and its condition was checked to

ensure its moisture content and mixing condition by visual inspection based on the

experience at trial mix. Upon finish mixing, concrete moved to concreting area along

with drum with aid of forklift.

Slump test then conducted before concreting to determine its workability as

in required condition. At the same time, ten concrete cubes with each 100mm x

37

100mm x 100mm were produced as shown in Figure 3.14. For compacting purpose,

vibrator poker with size of 25mm was used. After 2 days, formwork and mold was

being stripped and removed and specimens were cured by covering a layer of plastic.

In the Figure 3.16 below shown that, specimen was coated with a layer of white paint

before testing for ease of crack observation later.

Figure 3.13: Mechanical drum mixer Figure 3.14: Steel cube moulds

Figure 3.15: Casting Completed

38

Figure 3.16: Specimens coated with white pain layer

3.6 Compression Tests: Cube Test

Compression tests are important in this project and will be carried out for

each concrete mix proportion to determine the compressive strength of the concrete.

Coarse aggregate with the size of 10 mm, fine aggregate and Ordinary Portland

Cement (OPC) are used as the major constituent of the concrete mix to test in the

cube strength of mix design. The fresh concrete was cast in steel or cast-iron moulds.

Test cubes with the dimension of 100 mm x 100 mm x 100mm will be prepared for

each batch of concrete mix and they will be tested on their compressive strength

using compressive strength test machine on day 7, 14, 21 and 28 shown in Figure

3.17 below.

First of all, the mould surface of the mould is ensure that is clean and damp

but free from superfluous moisture before commencing the test. The mould is placed

on a smooth, horizontal rigid and non-absorbent surface free from vibration and

shock. Before filling the mould, a thin layer of oil must be applied to the inside

surface on the mould, in order to prevent the development bond between the mould

39

and the concrete. By reduces leakage of mortar, the mould and its base should be

clamped together during casting.

Figure 3.17: Compressive strength test machine

In the position at the top whilst it is filled in two layers, each approximately

half of the height of the mould when compacted. Each layer of fresh concrete was

compacted 35 times by using the compacting bar. After the top layer has been

compacted, its level with the top of cube is smoothed. All the cube concretes were

curing by covering with plastic and the curing process for cube concrete only takes

one day.

40

3.7 Test Procedure

A research flow chart of methodology has been planned as shown in Figure

3.18. Firstly, information was collected and studies the behavior on shear to design

several of beams to testing in laboratory. Next, to identify the concrete mix

proportion, trial mixes are designed. After that, the material and formwork are

prepared for concrete mix in the cubes with the dimension of 100 mm x 100 mm x

100mm.

After the concrete is harden, all the cube concretes sample were curing by

covering with plastic. After curing process, formworks were removing. When the

compressive strength of the concrete C30 N/mm2, reinforced concrete beam were

prepared for testing.

Prior to testing, the surface of the specimens was painted with while emulsion

so that the detection of the cracks during the test was easier and their making became

clearer. To ease the installation of beam in testing frame, all the lines position of

point load, support and middle of beam were marked. The equipment used for the

testing is the hydraulic jack. The test was carried out with the specimen placed

horizontally in a simple loading arrangement. .

On the testing, the hydraulic jack testing machine with maximum 1000kN

capacity was used as shown in Figure 3.19. The beam was supported by solid round

steel as simply supported member and from support to support the beam effective

length of each beam was 2000mm. The hydraulic jack was placed at two point load

with distance 500mm from point load to support were applied symmetrically to the

beam with av less than 2.5d position above the specimen to provide axial load (N).

While the load cell will be located below the jack and connected to the data logger to

record the total load subjected to the specimen. On the other hand, the Variable

Differential Transducer (LVDT) was set at the point load and mid span of the

41

specimen to measure the deflection. Result on the ultimate load is obtained when the

specimen fail. Result on the ultimate load is obtained when the specimens fail and

the cracking on the specimen was marked by using marker pen. All the data were

collected after the testing and analyzed. Lastly discussion and conclusion were

making in this test procedures.

Figure 3.18: Flow chart of research methodology

42

Figure 3.19: Testing Machine and Set up

3.8 Instrumentation

In brief, all the instruments involved in the testing were as listed as below:

a) Testing Frame - Placement of specimen

b) Hydraulic Jack - Induce the load on specimen.

43

c) Load cell - Read and calibrate the loading from hydraulic jack as

shown in Figure 3.20.

d) Transducer - Measuring the deflection of beam

e) Data logger - Record all the reading of deflection, rotation and loading

act on specimen as shown in Figure 3.20.

f) Marker pen - Mark the cracking on specimen.

Figure 3.20: Testing Instruments

44

CHAPTER 4

TEST RESULTS

4.1 Introduction

The design and the testing method for the specimens were explained in

previous chapter. This chapter presents the results obtained from the testing carried

out on the five specimens. From the experiments, illustrated in terms of the

specimen behavior, crack deformation, deflection and ultimate strength were

recorded and observed during the testing.

To obtain the actual concrete strength for each part of specimen, cube

compression test was carried out on control cubes for all specimens on the testing

day. The results for cube testing are as shown in Table 4.1.

45

Table 4.1: Concrete compression strength for all beams.

Days Sample Concrete Strength, fcu

(N/mm2)

Average, fcu

(N/mm2)

1 19.003

2 19.54 19.27

3 27.937

4 27.94 27.94

5 28.54

6 30.069

7 29.23

29.28

8 30.6011

9 31.66 31.13

10 31.9414

11 32.83 32.38

4.2 Beam B1

The specimen was provided with the main reinforcement and with R6-50

vertical links as shear reinforcement as shown in Table 3.1. The Test procedure

adopted was described in section 3.5.

4.2.1 Specimen Behavior during Test

Beam B1 as served as control specimen was cast with nominal links in the

moment region and vertical links in the shear region with the spacing of 50mm.

46

Upon loading, the first crack was occurred at a load of around 40kN. The crack was

spotted in the constant moment region, originated from the tension face of the beam.

At the same load, some crack also appears in the shear region. More vertical cracks

formed along the length in the constant moment region and extended upwards.

However the propagation of these cracks within the moment region was slow

but steadily with continued loading. Shear cracks will be appeared and take place on

the shear region with respected approximately 45 to the point load with 75kN. As

the load increased the cracks extended towards the support point at the lower end and

towards the applied load at the other end. The development of crack pattern can be

seen in Figure 4.1 below. As expected with the specimen and test method employed

in this testing, the beam was failed due to shear failure when more loads are

increasing.

Figure 4.1: Appearance and Cracking Configuration of Specimen Beam B1

4.2.2 Test Results

The results in terms of ultimate load and deflection obtained from specimens

tested in this investigation were shown in Table 4.2. Totally 3 of the variable

differential transducer are provided to measuring the deflection of the beam. Two are

47

located at the point load and one is located at middle span. Failure was caused by the

shear failure at the load of 205kN and with the maximum deflection at 16.73mm.

Figure 4.2 shows the load-deflection relationship for the specimen beam B1.

As can been see in the figure, deflection occurred as soon as the load was applied but

the main bar not yet yield when the maximum ultimate load reached. The complete

result for the testing in shown in Appendix C.

Graph Load (kN) vs Deflection (mm)

0

40

80

120

160

200

240

0 2 4 6 8 10 12 14 16 18

Deflection (mm)

Loa

d k

N)

Beam B1

Figure 4.2: Graph load versus Deflection for Specimen Beam B1

4.3 Beam B2

The specimen was provided with the identical amount of main reinforcement

as provided in Beam B1, but with vertical links of R6-150 and 4T16 as independent

bent-up bars in the shear region as shown in Table 3.1. Anchorage length of the bent-

48

up bar are 75mm. Furthermore, test procedure adopted for this testing are described

in section 3.5.


Beam B2 was cast with nominal links in the moment region and combination

of vertical links and independent bent-up bar in the shear region. The independent

bent-up bars were approximately 45 with the respect to the longitudinal axis of the

beam. Testing procedure was similar with the previous specimens.

Upon loading, the first flexural crack was occurred at the middle in the

constant moment region at a load of around 40kN. With continue increasing of

loading, the width of crack became lengthened and widened. At the same time more

vertical cracks formed along the length of the constant moment region and extended

upwards. These developments of crack pattern can be seen in Figure 4.3 below.

However the propagation of these cracks within the moment region was slow.

At the load of 60kN, the first cracks appeared within the shear region. The shear

crack respected approximately 45 to the positions of the point load. As the load

increased the cracks extended towards the support point at the lower end and towards

the applied load at the other end.

When the ultimate load reach until 236.2kN, the beam failed in the moment

region on the bar buckle on the top reinforcement due to the very large compression

force. This was because the independent bent up bar was not reach its limit, so the

beam failed in bending not in shear span.

49


4.3.2 Test Results

The ultimate load and deflection for beam B2 are presented in Table 4.1.

Beside that, Figure 4.4 shown that the results in term of graph load versus deflection

relationship. Beam B2 was failed at a load of 236.2kN and with the maximum

deflection 18.97mm. This should that the use of combination system with vertical

links and independent bent-up bar as shear reinforcement were provided stronger

capacity than the conventional shear reinforcement system. Furthermore, the beam

provided high stiffness and yield in main bar. The complete result for the testing in

shown in Appendix C.


0

40

80

120

160

200

240

280

0 2 4 6 8 10 12 14 16 18 20

Deflection (mm)

Loa

d kN

)

Beam B2


50

4.4 Beam B3





in section 3.5.


Beam B3 was cast with nominal links in the moment region and combination

of vertical links and increase amount of independent bent-up bar in the shear region.

The independent bent-up bars were approximately 45 with the respect to the

longitudinal axis of the beam and provided with 75mm of anchorage length. Testing

procedure was similar with the previous specimens. These developments of crack

pattern can be seen in Figure 4.5 below. Form the figure, the first crack was occurred

at the middle in the constant moment region at a load of around 40kN. With continue

increasing of loading, more vertical cracks formed along the length of the constant

moment region and extended upwards.

At the load of 60kN around, the first diagonal cracks developed in the shear

region. As the load increased the cracks extended towards the support point at the

lower end and towards the applied load at the other end. When the load reach to

225.6kN, the beam failed in the moment region on the bar buckle on the top

reinforcement due to the very large compression force. The failure is similar with

beam B2 and at the same location, because the independent bent up bar was not

reach its limit, so the beam failed in bending not in shear span.

51


4.4.2 Test Results

The ultimate load and maximum deflection for Beam B3 was at 225.6kN and

16.17mm respectively are presented in Table 4.2. From the result, it shown that the

independent bent-up bar can carry shear for the experimental testing because the

ultimate load are high than the control specimen.

Figure 4.6 shows the load-deflection relationship for the specimen beam B3.

From the graph, the beam provided high stiffness and yield in main bar. Furthermore,

Beam B3 was failed between the two point loads. The complete result for the testing

in shown in Appendix C.

52


0

40

80

120

160

200

240

0 2 4 6 8 10 12 14 16 18

Deflection (mm)

Loa

d kN

)

Beam B3


4.5 Beam B4





in section 3.5.


Beam B4 was also cast with nominal links in the moment region and

combination of vertical links and independent bent-up bar in the shear region. The

53

amounts of independent bent-up bars are same as beam B3 but difference in

anchorage length. Thats 150mm for each of the independent bent-up bars. on the

other hand, the independent bent-up bars were approximately 45 with the respect to

the longitudinal axis of the beam. Testing procedure was similar with the previous

specimens.

As would be expected with the specimens in this testing, the first flexural

crack was occurred at the middle in the constant moment region at a load of around

40kN. With continue increasing of loading, the width of crack became lengthened

and widened. At the same time more vertical cracks formed along the length of the

constant moment region and extended upwards. These developments of crack pattern

can be seen in Figure 4.7 below.

However the propagation of these cracks within the moment region was slow.

At the load of 60kN, shear cracks appeared within the shear region. As the load


the applied load at the other end. The cracks were around 45 with respect to the

longitudinal axis of the beam increased slowly in width and length as the load

increase.

When the load reach 229.4kN, the beam failed in the moment region on the

bar buckle due to the very large compression force. This was because the

independent bent up bar was not reach its limit, so the beam failed in bending not in

shear span.

54


4.5.2 Test Results

The test results in term of ultimate load and deflection are presented in Table

4.2. The graph load versus deflection relationship is presented in Figure 4.8 and

shows the deflection proportional to the load. From the graph, the beam provided

high stiffness and yield in main bar.

From the Table, the failure was cause by flexure failure at a load of 229.4kN.

Therefore, with the increase amount of the independent bent-up bar and anchorage

length within the shear region was effective a very little increasing of the capacity

and increase the ductility of the beam when compared with beam B3. The maximum

deflection for Beam B4 was 19.68mm. Furthermore, Beam B4 was failed between

the two point loads. The complete result for the testing in shown in Appendix C.

55


0

40

80

120

160

200

240

280

0 2 4 6 8 10 12 14 16 18 20 22

Deflection (mm)

Loa

d kN

)

Beam B4


4.6 Beam B5


as provided in Beam B1, but cast without any vertical links. In this Beam, only 8T16

independent bent-up bars were provided in the shear region as shown in Table 3.1.

Anchorage length of the bent-up bar are 150mm. Furthermore, test procedure

adopted for this testing are described in section 3.5.


Beam B5 was cast without any nominal links and vertical links and only

provided with the independent bent-up bar in the shear region. Totally 8 number of

56

independent bent-up bars and same anchorage length with beam B4 is provided. The

independent bent-up bars were also approximately 45 with the respect to the

longitudinal axis of the beam as designed for previous beams. Testing procedure was

similar with the previous specimens.

During the testing, the first flexural crack formed randomly in the constant

moment region at a load of around 40kN. With continue increasing of loading, the

width of crack became lengthened and widened. At the same time more vertical

cracks formed along the length of the constant moment region and extended

upwards. The propagation of these cracks within the moment region was slow. At the

load of 50kN, the first shear cracks appeared within the shear region. As the load


the applied load at the other end. The cracks were around 45 with respect to the


increase.

When the load reach 239.2kN, the beam failed in the mid span of the moment

region on the bar buckle due to the very large compression force. This independent

bent up bar was provided in the beam also not yet reach its limit, therefore the beam

failed in bending not in shear span. These developments of crack pattern can be seen

in Figure 4.9 below.


57

4.6.2 Test Results

The failure for this beam was cause by flexure failure at a load of 239.2kN

and the maximum deflection for Beam B4 was 24.82mm. Therefore, with only

provide independent bent-up bar as shear reinforcement without any vertical links

with the spacing of 100mm within the shear region was effective stronger capacity

than the conventional shear reinforcement system as control beam. The test results in

term of ultimate load and deflection are presented in Table 4.2. The graph load

versus deflection relationship is presented in Figure 4.10. From the graph, the beam

provided high stiffness and yield in main bar. The complete result for the testing in

shown in Appendix C.


0

40

80

120

160

200

240

280

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

Deflection (mm)

Lo

ad

kN

)

Beam B5


58

4.7 Summary Specimen Behavior and Test Results for All Specimens.

All the beams had the same first crack load around 16.5%-19.5% of the

ultimate load. The crack was spotted in the constant moment region, originated from

the tension face of the beam. With continue increasing of loading, the width of crack

became lengthened and widened. At the same time more vertical cracks formed

along the length of the constant moment region and extended upwards.

However the propagation of these cracks within the moment region was slow

but steadily with continued loading. Shear cracks will be appeared and take place on

the shear region. As the load increased the cracks extended towards the support point

at the lower end and towards the applied load at the other end. All the five specimens

experienced due to the small shear span effective depth ratio (a/d = 2.4), the diagonal

shear crack was observed in all of the beams at the load level from 40 to 75kN.

Specimens Beam B1 showed its shear cracking at 19.5% of ultimate load, Beam B2

at 24% of ultimate load, Beam B3 at 26% of ultimate load, Beam B4 at 26% of

ultimate load while Beam B5 demonstrated the shear cracking at 21% of the ultimate

load in the shear region. The cracks that were around 30 to 45 with respect to the


increases.

Figure 4.11 shows the appearance and cracking configuration of specimens

after the experiment. Shear failure showed by concrete crushing at the loading point

(shear compression), and flexural failure showed by concrete crushing at the

maximum compression zone under the elastic state of tension bar. Only Beam B1

failed in the shear span from the point load to the support due to shear failure.

However, Beam B2, B3, B4 and B5 were failed between the two point loads. This is

due to bending failure by yielding of main reinforcement and bar buckle due to the

very large compression force under the elastic state of tension bar.

59

The five specimens indicated difference in the ultimate load and deflection.

Beam B1 experienced the maximum deflection at 16.73 mm and failed at 205kN.

Beam B2 failed at 236.2kN and deflection at 18.97mm. The ultimate load and

maximum deflection for Beam B3 was at 225.6kN and 16.17mm respectively.

Besides that, Beam B4 broke down at 229.4kN with deflection 19.68mm. Lastly,

Beam B5 showed the maximum deflection at 24.82mm and approached the ultimate

load of 239.2kN. Table 4.2 shows the results of the experiment while Figure 4.12 and

Figure 4.13 illustrated the compressive strength of concrete for all specimen beams

and the load-deflection graph for all specimens respectively. Furthermore, Graph

maximum ultimate load and deflection for all specimen beams as shown in Figure

4.14 and 4.15.

Figure 4.11: Appearance and Cracking Configuration of All Specimens

60

Table 4.2: The Results of the All Specimens

Vertical links Inclined BarsBeam Main

Reinforce

ment

Shear

Region

Nominal Amount Anchorage

length

Ultimate

Load

(kN)

Maximum

Deflection

(mm)

Failure

Mode

B1 3T16 R6-50 R6 -150 - - 205.0 16.73 Shear

B2 3T16 R6-150 R6 -150 4T16 75 236.2 18.97 Bending

B3 3T16 R6-150 R6 -150 6T16 75 225.6 16.17 Bending

B4 3T16 R6-150 R6 -150 6T16 150 229.4 19.68 Bending

B5 3T16 - - 8T16 150 239.2 24.82 Bending

Graph Compressive Strength Of Concrete

0

5

10

15

20

25

30

35

0 2 4 6 8 10 12 14 16

Days

Str

en

gth

, F

cu

(N

/mm

2)

Figure 4.12: Graph Compressive Strength of Concrete for All Specimens Beams

61


0

40

80

120

160

200

240

280

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

Deflection (mm)

Lo

ad

(k

N)

Beam B1 Beam B2 Beam B3 Beam B4 Beam B5

Figure 4.13: Graph load versus Deflection for All Specimen Beams

205

236.2

225.6229.4

239.2

180

190

200

210

220

230

240

B1 B2 B3 B4 B5

Load (kN)

Ultimate load (kN) vs Beam

Figure 4.14: Graph Maximum Ultimate Load for All Specimen Beams

62

16.7318.97

16.17

19.68

24.82

0

5

10

15

20

25

B1 B2 B3 B4 B5

Deflection (mm)

Deflection (mm) vs Beam

Figure 4.15: Graph Maximum Deflection for All Specimen Beams.

63

CHAPTER 5

ANALYSIS AND DISCUSSION

(a) Beam B5 showed the highest value of ultimate strength with 239.2kN,

followed by specimen B2, B4, B3 and lastly B1 which reached the lowest

ultimate strength of 205kN. These happened because the used of the

independent inclined bar as shear reinforcement was much stronger than the

conventional shear reinforcement system.

(b) Beam B1 was designed and prepared with the vertical links as the control

specimen. Theoretically, Beam B1 can carry the actual load of design and it

shows that the use of vertical links that intercepts the crack can reduces the

likelihood of a sudden failure due to shear failure. Beam B2 to Beam B5 was

not fail in shear but fail in moment region based on the bar buckle due to the

very large compression force. This was because the independent bent up bar

was not reach its limit, so the beam failed in bending not in shear span.

(c) The maximum deflection was found at Beam B5 about 24.82 mm with the

ultimate load of 239.2kN. From the theory analysis, when the deflection and

load increased, it means that the allowable deflection is lower than the

permission deflection.

64

(d) It is observed that the specimens actually had achieved and even go more than

the required strength of concrete for 8.27% before the experiment was being

carried out. Although variation among different casts in the same experiment

was unavoidable, they were relatively small. However, these errors only

induced a tiny effect on the result; therefore the influence of the concrete

strength in this analysis may be neglected.

(e) From the analysis, the experimental ultimate load for Beam B2, B3, B4 and B5

failed lower than the calculation value as shown in table 5.1. The amount of the

independent inclined bars increases the shear stresses thus yield reinforcement

crushing on the bending region before the inclined bar could reach it maximum

strain value. All of the four beam specimens were suggested as the most

effective shear reinforcement system. Besides that, Beam B5 has the highest

value of ultimate load at 239.2kN and it does also can carry the shear without

provided at least 50% of shear links as states on the BS8110 code.

(f) Beam B2, B3, B4 and B5 have achieved the ultimate load with the control

specimen as shown in Table 4.1. The percentages of the ultimate load

compared with the control specimen for Beam B2 to Beam B5 was at the range

of 10% to 17% as shown in table 4.2. From the result, four specimens (B2, B3,

B4, and B5) were good at the ultimate load where the amount is larger than the

control specimen.

(g) From the experimental beam B2 to B5, the first crack occurred at the middle of

the beam is in the form of flexural cracks. With increasing loads, diagonal

crack developed in the shear region at approximately 45 with respect to the

longitudinal axis of the beam. The length and width of the shear cracks

increased gradually until failure take place by crushing of the concrete in

compression zone.

65

(h) The anchorage length that provide for Beam B3 and Beam B4 with difference

length of 75mm and 150mm was not significant influence on improving the

capacity of the beams and not effective to carry shear because both beams were

failed almost at the similar ultimate load. Maybe the anchorage length might

not sufficient length to carry load.

(i) From the experimental testing, anchorage length for the independent bent-up

bar can provide shorten length as shear reinforcement. Based on the BS8110

table 3.27, the anchorage bond length for concrete cube strength C30 is 58

number multiples of bar size. So it is not a suitable to provided in the beam

because the inclined bar are never located in the shear region to carry shear.

Furthermore, with provided short anchorage length for the independent bent-up

bar can set more number of bent-up bar and easy to arrangement as a double

system in the shear region to carry shear.

(g) Graph load versus deflection relationship for all the specimen beam shown that

the deflection proportional to the load. All the combination beam for vertical

links and bent-up bar provided less deflection and can carry more high load

when compare with the control beam. So the beam provided with independent

bent-up bar can be used. Furthermore, beam B2 to B5 provided high stiffness

and yield in main bar so that the sudden failure wont be occur.

(h) The use of bent up bar are very popular in Malaysia engineer at 1968. Code

CP114 was the designed handout. Almost all the engineer training will

provided bent-up bar and vertical links to resist shear. When at 1972 in UK, a

new code CP110 was using to be a design reference book. Inside the reference,

the book never prefer for the bent-up bar. All the engineers are providing only

with links to carry shear. Maybe difficult to casting the bent-up bar and not

suitable provided in the shorten beam. Bent up bar only provided in the beam at

least 4 or 5 main bars. After that, the design code was change to BS8110. In

66

this design book, shear reinforcement must be provided at least 50% of vertical

links and bent-up bar to carry shear.

Table 5.1: The Comparison between Testing Result and Calculation Value

Beam Concrete

Strength,

fcu

(N/mm2)

Calculation

Value

(kN)

Test

Result

(kN)

Percentages of Ultimate

Load Compared with

Control Specimen Beam B1

(%)

B1 93.68 102.5 -

B2 158.98 118.1 15.21

B3 210.22 112.8 10.05

B4 210.22 114.7 11.90

B5

32.48

242.86 119.6 16.68

67

CHAPTER 6

CONCLUSION AND RECOMMENDATIONS

6.1 Conclusion

The conclusions that can be drawn from this study are as follows:

(a) The anchorage length of the independent bent-up bars has very little effect on

the capacity of the system in carrying shear in rectangular reinforced concrete

beams.

(b) Shorten length of anchorage length for bent-up bar can be used in the

multiple system and can provide more number in the shear region to increase

the shear capacity.

(b) The shear capacity of the rectangular reinforced concrete increases with the

increase in the amount of the independent bent-up bars up to a certain level,

beyond which the advantage is insignificant.

68

(c) Ultimate load for beam B2 to B5 can achieve high ultimate load than the

contro

concrete shear research

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