IRJET- A Review on R.C.C. Structure with Fully, Partially and without Infille...
project
1. PROJECT REPORT
On
Evaluation & Improvement
of Bond Strength in Concrete
By
NAIYER RAFIQUE CE1202
FAHEEM QADIR CE1229
AAQIB AMIN CE1230
MOHAMMAD OWAIS CE1238
MEHDI CE1255
Under the supervision of
ER. RABIA GUL
Assistant Professor
Department of Civil Engineering
(2016)
DEPARTMENT OF CIVIL ENGINEERING
ISLAMIC UNIVERSITY OF SCIENCE AND TECHNOLOGY
2. 2
1-University Avenue, Awantipora, Pulwama, Jammu & Kashmir 192122
ISLAMIC UNIVERSITY OF SCIENCE AND TECHNOLOGY
Awantipora 192122 (INDIA)
CERTIFICATE
We hereby certify that the work which has being carried out in this project entitled
“BOND STRENGTH OF CONCRETE”, in partial fulfilment of the requirement for the
award of degree of “Bachelors of Technology in Civil Engineering” submitted to
Department of Civil Engineering, Islamic University of Science and Technology,
Awantipora is an authentic record of our own work carried out under the supervision of
Er. Rabia Gul.
NAIYER RAFIQUE CE1202
FAHEEM QADIR CE1229
AAQIB AMIN CE1230
MOHAMMAD OWAIS CE1238
MEHDI CE1255
This is to certify that the statements made above by the candidates are correct and true to
the best of my knowledge.
Supervisor
Er. Rabia Gul
Assistant Professor
Department of Civil Engineering
3. 3
DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING
ISLAMIC UNIVERSITY OF SCIENCE AND TECHNOLOGY
Awantipora 192122 (INDIA)
ACKNOWLEDGEMENT
We are thankful to our guide Er. Rabia Gul and all the other faculty members who guided
us at every step of this work. We are also thankful to the our Head of Department
Er. Misbah Gul for her unwavering support. This work could not have been completed
without the important insight into the subject by Er. Muttahir Hussain.
Naiyer Rafique
4. 4
ABSTRACT
In this work we have analyzed the performance of various parameters namely Grade of
Concrete, Presence of kink and diameter of bars affecting the Bond Strength of
Concrete. Our aim is to verify the values provided in the IS 456-2000 in case of bond
strength in concrete. The values are obtained using Pull-out test on a number of samples
of concrete where bonding strength is recorded. The test methodology is discussed
extensively in the project report. The data obtained will be used for plotting a graph for
verifying our result.
5. 5
Table of Contents
1 INTRODUCTION………………………………………………………..9
1.1 BACKGROUND……………………………………………………………….9
1.2 IMPORTANCE AND FEASIBILITY…………………………………………10
1.3 OBJECTIVES…………………………………………………………………………………………………….11
2.LITERATURE REVIEW .......................................................................... 14
2.1 CONCRETE; COMPOSITION AND IMPORTNCE..........................................14
2.2. MECHANISM OF BOND RESISTANCE.........................................................14
2.3 BOND STRESS…………………………………………………………………15
2.4 TYPES OF BOND………………………………………………………………15
2.5 BOND FAILURE AND BOND STRENGTH………………………………….21
2.6 TEST SPECIMEN………………………………………………………………26
2.7 PREPARATION OF TEST SPECIMEN……………………………………….28
2.8 TEST PROCEDURE……………………………………………………………29
2.9 CALCULATION OF BOND STRESS…………………………………………30
2.10 RECORD OF RESULTS………………………………………………………30
3 WORK CARRIED OUT ……………………………....……...……………………….31
3.1 COLLECTION OF RAW MATERIAL………………………………………...31
3.2 TESTING………………………………………………………………………..31
3.3 WORKABILITY TEST…………………………………………………………31
3.4 COMPRESSION TEST…..……………………………………………………. 32
3.5 CONSISTENCY TEST FOR CEMENT………………………………………..32
3.6 WATER ABSORPTION TEST FOR AGGREGATES………………………...34
3.7 PULLOUT TEST………………………………………………………………..34
4. EXPERIMENTAL RESULTS AND COMPARISON …………………...…38
4.1 STANDARD CONSISTY FOR CEMENT…………………………………….38
4.2 INITIAL AND FINAL SETTING TIMES……………………………………..38
6. 6
4.3 WATER ABSORPTION FOR COURSE AGGREGATES……………………38
4.4 EXPERIMENTAL RESULTS FOR WEIGHT OF CONSTITUENT USED IN
EACH RATIO PER BLOCK………………………………………………………39
4.5 EXPERIMENTAL RESULT OF PULLOUT TEST FOR SIMPLY DEFORMED
BAR………………………………………………………………………………..39
4.6 EXPERIMENTAL RESULT OF PULLOUT TEST FOR DEFORMED BARS
WITH KINK……………………………………………………………………….40
4.7 INFERENCES FROM THE PULLOUT TEST CARRIED OUT ON THE
SAMPLE…………………………………………………………………………..42
5. CONCLUSION, SUMMARY AND FUTURE SCOPE ………………..….43
6. REFERENCES…………………………………………………………..44
7. 7
List of Figures
Figure 1. Bond stress in a beam…………………………………………….16
Figure 2. Effect of flexural cracks on flexural bond stress in constant
movement region…………………………………………………………...18
Figure 3. Anchorage bond stress…………………………………………...19
Figure 4. Typical bond splitting crack patterns…………………………….22
Figure 5. Stirrups resisting tensile force due to bond………………………23
Figure 6. Pullout test apparatus…………………………………………….27
Figure 7. sieve shaker………………………………………………………31
Figure 8. Pullout test setup…………………………………………………37
8. 8
List of Tables
Table 1. Consistency results……………………………………………….38
Table 2. Initial and final setting time………………………………………38
Table 3. Water absorption results…………………………………………..38
Table 4. Various grades used for testing with water-cement ratio 0.5……..39
Table 5. Experimental values of bond strength for deformed bars…………39
Table 6. Experimental values of bond strength for deformed bars with
kink ………………………………………………………………40
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1. INTRODUCTION
1.1 Background
Bond in reinforced concrete refers to the adhesion between reinforcing steel and
surrounding concrete. It is this bond which is responsible for the transfer of axial force
from a reinforcing bar to the surrounding concrete, thereby providing strain compatibility
and composite action of concrete and steel. If this bond is inadequate, slipping of the
reinforcing bar will occur, destroying full composite action. Hence, the fundamental
assumption of the theory of flexure, viz. plane sections remain plane even after bending,
becomes valid in reinforced concrete only if the mechanism of bond is fully effective. It
is through the action bond resistance that the axial stress (tensile or compressive) in a
reinforcing bar can undergo variation from point to point along its length. This is required
to accommodate the variation in bending moment along the length of the flexural
member. Had the bond been absent, the stress at all points on a straight bar would be
constant, as in a string or a straight cable.
Bond is necessary not only to ensure adequate level of safety allowing composite action
of steel and concrete, but also to control structural behavior along with sufficient ductility
Bond resistance is achieved by the development of tangential (shear) stress components
along the interface (contact surface) between the reinforcing bar and the surrounding
concrete. The stress so developed at the interface is called bond stress and is expressed in
terms of the tangential force per unit nominal surface area of the reinforcing bar.
The mechanisms that initiate bond failure may be any one or combination of break-up of
adhesion between the bar and the concrete, longitudinal splitting of the concrete around
the bar, crushing of the concrete in front of the bar ribs (in deformed bars) and shearing
of the concrete keyed between the ribs along a cylindrical surface surrounding the ribs (in
deformed bars). The most common type of bond failure mechanism is the pulling loose of
the reinforcement bar, following the longitudinal splitting of the concrete along the bar
embedment. Occasionally, failure occurs with the bar pulling out of the concrete, leaving
a circular hole without causing extensive splitting of the concrete.
The bond strength in case of concrete varies as per the grade of concrete, the
development length, and the diameter of the reinforcement bar. Thus we analyze a wide
variety of the variables which may increase or decrease the value of bond strength
provided in the IS code. These values shall be experimentally obtained as per the
laboratory guidelines provided in IS 2770:1967. Hence in reference to these values
10. 10
obtained we will be able to devise a stress-strain curve of the bond strength of concrete
for deformed steel bars. The results derived from the experiments on the bond strength of
concrete are verified in comparison to the standard values of Bond strength of concrete
provided in the IS code. This approach will not only ensure the applicability of the
methodology used in devising the above mentions results but will also help us in
verification of the values of bond strength of the concrete. This experimental data will be
based on the variation of parameters involved with the increase and decrease of Bond
Strength of concrete.
After all the test result have been performed on the deformed bar and accurate data has
been produced to verify the test methodology and other factors, we will provide the bars
with a kink (small zig-zag bend) in our bars and verify and compare these test results on
the kinked bars. That may result in an increase in the bond strength of the concrete which
is to be found.
1.2 Importance and Feasibility
The cognizance of bond strength of concrete is of utmost importance in design and
construction of small as well as large scale structures. The bond between steel and
concrete is more or less a manifestation of the strain compatibility between the two.
Calculation of bond strength is necessary as the data obtained is quite useful in
assessing the failures that can be developed by slippage, cracking etc.
Feasibility of the experiments that are used to assess bond strength is quite evident as
very simple techniques and common procedures as well as machinery are used in the
basic experimental analysis. Data thus obtained can be easily cross verified with the IS
Code and applied in construction works, be it minor or major procedures.
The credibility of the project can also be validated by the fact that the data collected
can be used whenever we like without any further expenses. Moreover the dilemma of
using various types of bars like deformed and kinked bars as well as varying diameter
of bars can be eliminated by knowing the basic strength values of the bond of any kind
of steel with the given grade of concrete. This reduces the effort of engineers to a great
extent and they can utilize the energy in more important matters of construction. Since
the exact data that is needed to provide reinforcement in already available, overuse of
material to achieve the desired level of safety can be considerably avoided. The
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reduction in labor as well as the optimum use of steel in concrete can help in making
the project much more economic and sustainable.
As the requirements of the project are very nominal, it can be carried out at minimal
costs and schedule so as to accommodate the possibility of the best result by carrying
out multiple tests on similar samples. This increases the possibility of acquiring a very
accurate result accompanied by detailed graphical representation of the same.
The operational feasibility however has to be taken care of by using raw material that
has not remained stagnated or unused for long periods of time. The basic materials
such as sand and cement should be used immediately so as to avoid any variation in
their properties especially when using design mixes. The steel should also be taken
care of and any kind of corrosive action on it should be avoided as it might affect the
bond between steel and concrete.
Pullout test is the most common and important test that is carried out in the calculation
of bond strength of concrete. It requires the casting of multiple number of cubes which
need to be properly soaked in a small water tank so as to avoid damage due to heat of
hydration and other effects. A simple 50 Ton UTM is used in the test and a simple
collar is devised so as to hold the cubes while being tested in the UTM. The whole
procedure is quite simple, practical as well as economical and has long term
advantages when applied in major civil engineering projects.
The fact of the matter is that the design bond strength of concrete is of utmost concern
in the field of civil engineering and needs to be properly investigated so as to make
buildings and various other type of constructions safe and act as a testimony to the
comprehensive approach in the subject of strength of materials.
1.3 Objectives
The design and verification of bonding strength of concrete in the light of the values
provided by the IS 456 2000 underlies a number of objectives. Some of the main
objectives have been discussed below.
1.3.1 Calculation of Bonding Strength in concrete
The bonding strength in case of concrete varies as per the grade of concrete, the
development length, and the diameter of the reinforcement bar. Thus we analyze a
wide variety of the variables which may increase or decrease the value of bonding
12. 12
strength provided in the IS code. These values shall be experimentally obtained as per
the laboratory guidelines provided in IS2770.1.1967. Hence in the reference to these
values obtained we will be able to devise a stress-strain curve of the bonding strength
of concrete for deformed steel bars.
1.3.2 Verification/Comparison of Bond strength values
The results derived from the experiments on the bonding strength of concrete are
verified in comparison to the standard values of Bonding strength of concrete provided
in the IS code. This approach will not only ensure the applicability of the methodology
used in devising the above mentions results but will also help us in verification of the
values of bonding strength of the concrete. This experimental data will be based on the
variation of parameters involved with the increase and decrease of Bonding Strength of
concrete.
1.3.3 Variation of Bonding Strength by differing various parameters as:
1.3.3.1 Grade of Concrete
Grade of the concrete is the most decisive parameter in the bonding strength of the
concrete. This project will require a number of different test specimens based on the
grade of concrete. The results which shall be derived from the experimentation on
these different specimens shall be of M20, M25 and M30. These test specimens will be
prepared as per the Design Mix Ratio of the different concrete grades. From the
previous guide lines it has somewhat been established that higher concrete grades
results in increased bonding strength but the objectives of this project demands a
tabulated proofs in the form of graphs and other data, so we look forward to alter the
grades of concrete to achieve a maximum accuracy of our laboratory generated data.
1.3.3.2 Type of the Bar
Another differentiable parameter that also plays an important role in the bonding
strength of the concrete is the type of the bar. The main type of the reinforcement bars
that shall make different combinations with the provided steel grades are deformed
bars and plain bars. The IS 456 designates a specified bonding strength in case of plain
bars. These days industry demands the production and reliability of deformed bars only
13. 13
and we will direct or tests based on these deformed bars. This will help in
standardization of the experimental test results.
1.3.3.3 Diameter of bar
Bars of various diameter will be used in order to verify its impact on bonding strength
of concrete. Recent studies have shown that bonding strength increases when bars of
smaller diameter are used for same grade of concrete.
The above discussed parameters will help us to generate large amount of variable data
and when this data shall be presented in the form of graphs, the results that are
obtained will be accurate.
After all the test result have been performed on the deformed bar and accurate data has
been produced to verify the test methodology and other factors, we will provide the
bars with a kink (small zig-zag bend) in our bars. After this more tests and analysis we
will be able to verify and compare these test results on the kinked bars. That may result
increase of the bonding strength of the concrete which is to be found.
14. 14
2. LITERATURE REVIEW
2.1 Concrete; composition and importance.
Concrete is a composite material composed of coarse granular material (the aggregate or
filler) embedded in a hard matrix of material (the cement or binder) that fills the space
between the aggregate particles and glues them together. We can also consider concrete
as a composite material that consists essentially of a binding medium within which are
embedded particles or fragments of aggregates. Concrete contains water, aggregate, and
cement. Often, additives and reinforcements are included in the mixture to achieve the
desired physical properties of the finished material. When these ingredients are mixed
together, they form a fluid mass that is easily molded into shape. Over time, the cement
forms a hard matrix which binds the rest of the ingredients together into a durable stone-
like material with many uses.
Concrete is the most widely used construction material in the world. It is used in many
different structures such as dam, pavement, building frame or bridge. Also, it is the most
widely used material in the world, far exceeding other materials. Its worldwide
production exceeds that of steel by a factor of 10 in tonnage and by more than a factor of
30 in volume. The present consumption of concrete is over 10 billion tons a year, that is,
each person on earth consumes more than 1.7 ton of concrete per year. It is more than 10
times of the consumption by weight of steel. Famous concrete structures include the
Hoover Dam, the Panama Canal and the Roman Pantheon. The earliest large-scale users
of concrete technology were the ancient Romans, and concrete was widely used in the
Roman Empire. The Colosseum in Rome was built largely of concrete, and the concrete
dome of the Pantheon is the world's largest unreinforced concrete dome. After the Roman
Empire collapsed, use of concrete became rare until the technology was re-pioneered in
the mid-18th century. Today, concrete is the most widely used man-made material
(measured by tonnage).
2.2 Mechanisms of Bond Resistance
Bond resistance in reinforced concrete is achieved through the following mechanisms:
1. Chemical adhesion: due to a gum-like property in the products of hydration (formed
during the making of concrete).
15. 15
2. Frictional resistance: due to the surface roughness of the reinforcement and the grip
exerted by the concrete shrinkage.
3. Mechanical interlock: due to the surface protrusions or ribs (oriented transversely to
the bar axis) provided in deformed bars.
Evidently, the resistance due to 'mechanical interlock' (which is considerable) is not
available when plain bars are used. For this reason, many foreign codes prohibit the use
of plain bars in reinforced concrete except for lateral spirals, and for stirrups and ties
smaller than 10 mm in diameter. However, there is no such restriction, as yet in the IS
Code.
2.3 BOND STRESS
Bond resistance is achieved by the development or tangential (shear) stress components
along the interface (contact surface) between the reinforcing bar and the surrounding
concrete. The stress so developed at the interface is called bond stress and is expressed in
terms of the tangential force per unit nominal surface area of the reinforcing bar.
2.4 Types of Bond
There are two types of loading situations which induce bond stresses, and accordingly
bond is characterized as:
2.4.1 Flexural bond
Flexural bond is that which arises in flexural members on account of shear or a variation
in bending moment, which in turn causes a variation in axial tension along the length of a
reinforcing bar [Fig. 8.l(d)l. Evidently, flexural bond is critical at points where the shear
(V = dM/dx) is significant.
Variation in tension along the length of a reinforcing bar, owing to varying bending
moment, is made possible through flexural bond. The flexural stresses at two adjacent
sections of a beam, dx apart, subjected to a differential moment dM, is depicted in Fig.
2.1.1(b). With the usual assumptions made in flexural design, the differential tension dT
in the tension steel over the length dx is given by
16. 16
𝑑𝑀 =
𝑑𝑇
𝑧
...............................................1
where z is the lever arm.
(a)
Fig 1. Bond Stress in a beam
This unbalanced bar force is transferred to the surrounding concrete by means of flexural
bond developed along the interface. Assuming the flexural (local) bond stress𝑢𝑓to be
17. 17
uniformly distributed over the interface in the elemental length dx,equilibrium of forces
gives
𝑢𝑓(∑𝑜)𝑑𝑥 = 𝑑𝑇..............................................................(2)
Where ∑𝑜 is the total perimeter of the bars, at the beam section under consideration
From Eq. 8.2, it is evident that the bond stress is directly proportional to thechange in the
bar force. Combining Eq. 8.2 with Eq. 8.1, the following expression forthe local bond
stress 𝑢𝑓is obtained:
u 𝑓 =
𝑽
𝒛(∑𝒐)
.......................................(3)
where 𝑉 = 𝑑𝑀 ⁄ 𝑑𝑥
It follows that flexural bond stress is high at locations of high shear, and that this bond
stress can be effectively reduced by providing an increased number of bars of smaller
diameter bars (to give the same equivalent A,,).
It may be noted that the actual bond stress will be influenced by flexural cracking, local
slip, splitting and other secondary effects - which are not accounted for in Eq. 3. In
particular, flexural cracking has a major influence in governing the magnitude and
distribution of local bond stresses.
2.4.1.1 Effect of Flexural Cracking on Flexural Bond Stress
From Eq.3, it appears that the flexural (local) bond stress 𝑢𝑓hasa variation that is similar
to and governed by the variation of the transverse shear force V. In fact, it would appear
that in regions of constant moment, where shear is zero, there would be no bond stress
developed at all. However, this is not true. The tensile force T in the reinforcement varies
between flexural crack locations, even in regions of constant moment, as indicated in Fig.
2.1.1.1. At the flexural crack location, the tension is carried by the reinforcement alone,
whereas in between the cracks, concrete carries some tension and thereby partially
relieves the tension in the steel bars. As local bond stress is proportional to the rate of
change of bar force local bond stresses do develop in such situations.
18. 18
(a)
Constant moment region between flexural cracks
(b) probable variation of bar tension T
(c) variation of flexural (local) bond stress 𝑢𝑓
Fig 2. Effect of flexural cracks on flexural bond stress in constant moment region
The bond stresses follow a distribution somewhat like that shown in Fig.2.1.1.1(c),with
the direction of the bond stress reversing between the cracks. The net bond force between
the cracks will, of course, be zero in a region constant moment.
19. 19
When the moment varies between the flexural cracks, the bond stress distribution will
differ from that shown in Fig.2.1.1.1(c), such that the net bond force is equal to the
unbalanced tension in the bass between the cracks.
Beam tests show that longitudinal splitting cracks tend to get initiated near the flexural
crack locations where the local peak bond stresses can be high. The use of large diameter
bars particularly renders the beam vulnerable to splitting and/or local slip.
Flexural bond is less critical in a compression bar, compared to a tension bar with an
identical axial force.
2.4.1.2 Anchorage (development) bond
Anchorage bond (or Development bond) is that which arises over the length of anchorage
provided for a bar or near the end (or cut-off point) of a reinforcing bar:
This bond resists the ‘pulling out' of the bar if it is in tension(Fig. 2.1.1(e)) or conversely,
the 'pushing in' of the bar if it is in compression.
(a) cantilever beam
(b) probable variation (c)assumed uniform
of anchorage bond stress𝑢 𝑎average bond stress 𝑢 𝑎𝑣
Fig 3. Anchorage bond stress
This situation is depicted in the cantilever beam of Fig. 2.1.2, where it is seen that the
tensile stress in the bar segment varies from a maximum (fs) at the continuous end Dto
practically zero at the discontinuous end C.
20. 20
The bending moment, and hence the tensile stress are maximum at the section atD.
Evidently, if a stress fsis to be developed in the bar at D, the bar should not be terminated
at D, but has to be extended ('anchored') into the column by a certain length CD. At the
discontinuous end C of the bar, the stress is zero. The difference in force between C and
D is transferred to the surrounding concrete through anchorage bond. The probable
variation of the anchorage bond stress u, is as shown in Fig. 2.1.2(b) - with a maximum
value at D and zero at C.
An expression for an average bond stress uav can be derived by assuming a uniform bond
stress distribution over the length L of the bar of diameter Ø [Fig. 2.1.2(c)], and
considering equilibrium of forces as given below:
(𝜋∅𝐿)𝑢 𝑎𝑣 = (𝜋 ∅2
4⁄ ) 𝑓𝑠 → 𝑢 𝑎𝑣 =
∅𝑓𝑠
4𝐿
...............................................4
This bond stress may be viewed as the average bond stress generated over a length L in
order to develop a maximum tensile (or compressive) stress fs, at a critical section; hence,
this type of bond is referred to as development bond.
2.4.1.3 Development length
The concept underlying 'development length' is that a certain minimum length of the bar
is required on either side of a point of maximum steel stress, to prevent the bar from
pulling out under tension (or pushing in, under compression). However, when the
required bar embedment cannot be conveniently provided due to practical difficulties,
bends, hooks and mechanical anchorages can be used to supplement with an equivalent
embedment length. The term anchorage length is sometimes used in lieu of development
length in situations where the embedment portion of the bar is not subjected to any
flexural bond.
The expression given in the IS 456:2000 (Cl. 26.2.1) for development length, Ld follows
from Eq. 4:
𝐿 𝑑 =
∅𝑓𝑠
4𝜏 𝑏𝑑
.............................................................5
where 𝜏 𝑏𝑑, is the design bond stress, which is the permissible value of the average
anchorage bond stress𝑢 𝑎𝑣. The values specified for 𝜏 𝑏𝑑(CI. 26.2.1.1 of the IS Code
456:2000) are 1.2MPa, 1.4MPa, 1.5MPa, 1.7MPa and 1.9MPa for concrete grades M20,
M25, M30, M35 and M40 and above respectively for plain bars in tension, with an
21. 21
increase of 60 percent for deformed bars in tension, and a further increase of 25 percent
for bars in compression.
2.5 BOND FAILURE AND BOND STRENGTH
2.5.1 Bond Failure Mechanisms
The mechanisms that initiate bond failure may be any one or combination of the
following:
break-up of adhesion between the bar and the concrete;
longitudinal splitting of the concrete around the bar;
crushing of the concrete in front of the bar ribs (in deformed bars); and
shearing of the concrete keyed between the ribs along a cylindrical surface
surrounding the ribs (in deformed bars).
The most common type of bond failure mechanism is the pulling loose of the
reinforcement bar, following the longitudinal splitting of the concrete along the bar
embedment [Fig.3.1.1]. Occasionally, failure occurs with the bar pulling out of the
concrete, leaving a circular hole without causing extensive splitting of the concrete. Such
a failure may occur with plain smooth bars placed with large cover, and with very small
diameter deformed bars (wires) having large concrete cover.
However, with deformed bars and-with the normal cover provided in ordinary beams,
bond failure is usually a result of longitudinal splitting.
In the case of ribbed bars, the bearing pressure between the rib and the concrete is
inclined to the bar axis [Fig. 3.1.1(b)]. This introduces radial forces in the concrete
(wedging action), causing circumferential tensile stresses in the concrete surrounding the
bar (similar to the stresses in a pipe subjected to internal pressure)and tending to split the
concrete along the weakest plane. Splitting occurs along the thinnest surrounding
concrete section, and the direction of the splitting crack (bottom splitting or side splitting)
depends on the relative values of the bottom cover, side cover and bar spacing as shown
in Fig. 3.1.1(b).
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(a) bottom and side splitting cracks
(b) splitting forces with deformed bars
Fig 4. Typical bond splitting crack patterns
Splitting cracks usually appear on the surface as extensions of flexural or diagonal
tension cracks in flexural members, beginning in regions of high local bond stress. With
increased loads, these cracks propagate gradually along the length of embedment
(longitudinal splitting) with local splitting at regions of high local bond stress and
associated redistribution of bond stresses. It is found that in a normal beam, local splitting
can develop over 60 - 75 percent of the bar length without loss of average bond strength
and without adversely affecting the load-carrying capacity of the beam The presence of
stirrups offers resistance to the propagation of continuous longitudinal splitting cracks
[Fig. 3.1.2]. However, in beams without stirrups, the failure due to bond can occur early
23. 23
and suddenly, as the longitudinal split runs through to the end of the bar without the
resistance offered by the stirrups.
Fig 5. Stirrups resisting tensile forces due to bond
2.5.2 Factors affecting Bond Strength
Bond strength is enhanced when the following measures area adopted:
Deformed bars are used instead of plain bars
Smaller bar diameters are used
Higher grade of concrete is used
Increased cover is provided around each bar
Increased length of embedment: bends and/or hooks are provided
Mechanical anchorages are employed
Stirrups with increased area, reduced spacing and/or higher grade of steel are used
Termination of longitudinal reinforcements in tension zones is avoided
Any measure that will increase the confinement of concrete around the bar is
employed.
2.5.3 Recent Studies
Several research studies have been reported on the influence of deformation patterns and
rib geometry on bond (Rehm, 1961; Goto, 1971). For bars with rib face angles, bond
behaviour is influenced by the rib face angle. However, when the rib face angle is less
than 30 degrees, the bond behaviour is different. In bars with small rib spacing and small
rib height the bond strength is reduced. Mathey and Watstein (1961) reported that the
24. 24
bond strength decreases as the embedment length increases, and decreases as the bar
diameter increases. Hansen and Liepins (1962) reported an increase in the bond strength
under dynamic loading over static loading. Also progressive bond failure and large slip
were expected from large repeated loading. Ferguson and Thomson (1962) reported on
development length of rebars and effect of confinement. Bond stress varies as a function
of development length rather than bar diameter. Ultimate bond stress varies as a function
of c f ' , with other factors being constant, since the bond strength is related to concrete
tensile strength. The nature of bond failure and factors influencing splitting, importance
of bar spacing
Lutz and Gergely (1967) studied the action of bond forces and the associated slip and
cracking using rebars with different surface properties. The slip was found to be due
primarily to the relative movement between concrete and steel along the surface of the
ribs and also due to crushing of mortar. Goto (1971) studied the primary and secondary
cracking by injecting ink around the deformed rebars in axially loaded tests. Nilson
(1972) estimated the bond stress from the slope of the steel strain curve. The strain in
concrete and steel was measured internally and the bond slip was calculated from the
displacement functions obtained by numerical integration of strains. Jiang et al. (1984)
developed new test method by cutting the reinforcing bars into two halves and placing in
two opposite sides of the cross section to study the local slip, secondary cracking and
strain distribution in concrete surrounding the interface. A simple one-dimensional
analysis predicts the stresses in steel and concrete, local bond slip, tensile stiffening and
total elongation of the reinforcing bar. Ueda et al. (1988) studied the beambar anchorage
in exterior beam-column joints. A model has been proposed to predict the load-lead end
deformation and anchorage length of rebars extended from beams into exterior columns
and subjected to large inelastic loadings. Effects of anchored bar diameter, confinement
of joint and compressive strength of concrete on the hook behaviour in exterior beam-
column joints have been studied (Soroushian, 1988)). An analytical model has been
developed for predicting the overall pullout behaviour of rebars, which has been
recognized by ACI–318-83 for development of standard hooks in tension. Soroushian and
Choi (1989) reported on local bond strength of deformed bars with different diameters in
confined concrete. The bond strength decreases as the bar diameter increases. Soroushian
et al. (1991) studied the influence of strength of concrete with different confinements.
Confinement influences local bond of deformed bars. The ultimate bond strength
increases as square root of concrete compressive strength. Abrishami and Mitchell (1992)
25. 25
formulated a new testing technique to simulate uniform bond stress distribution along a
rebar to determine bond stressslip response. Malvar (1992) tested specimens with varying
confining pressure using confining rings with rebar ribs normal and inclined to the
surface and obtained consistent bond-slip response over a short embedded length.
Mathematical model for bond-slip behaviour of a reinforcing steel bar embedded in
concrete subjected to cyclic loading was reported by Yankelevsky et al. (1992). Bortolotti
(2003) proposed models to predict the tensile strength of concrete from pullout load. The
confinement improved the bond strength slightly but ductility was improved significantly
(Harajli et al. 2004). Somyaji et al. (1981) and Jiang et al. (1984) conducted several
experimental and theoretical studies on bond in NSC. The secondary cracks as well as the
distribution of strain in concrete in the vicinity of rebar have been studied.
Darwin et al. (1996) reported development length criteria for conventional and high
relative rib area of reinforcement. On the basis of a statistically based expression, the
development length of reinforcement and splice strength in concrete for compressive
strength varying between 17 and 110MPa with and without confinement have been
investigated. The effects of cover, spacing, development/spliced reinforcement were
incorporated in design equation. The effects of concrete compressive strength, splice
length and casting position on the bond strength of rebars have been studied (Azizinamini
et al. 1993; 1999a)). Increasing the development length in HSC in tension does not seem
to increase the bond strength of deformed rebars, when concrete cover is small. Concrete
crushing occurred in front of the ribs in NSC, whereas there was no indication of concrete
crushing in front of the ribs in HSC with the first few ribs being more active. In HSC
with small cover, failure occurred due to splitting of concrete prior to achieving uniform
load distribution Azizinamini et al. (1993). Azizinamini et al. (1999b) in another study
reported that when calculating the development length in HSC for tension splice, a
minimum number of stirrups should be provided over the splice region. Statically based
on the experimental data an expression has been proposed to calculate the extra number
of stirrups required. Eligehausen et al. (1983) reported comprehensive study on the effect
of bar diameter embedded in
NSC. The maximum bond capacity decreases slightly with increasing bar diameter. The
frictional bond resistance was not influenced by the bar diameter, lug spacing or relative
rib area. Larrard et al. (1993) investigated the effect of bar diameter on bond strength.
The bond strength increases with tensile strength of concrete at a higher rate with smaller
bar diameters. A parameter which accounts for the ratios of side cover and bottom faces,
26. 26
and spacing of the spliced bars was introduced. CEB-FIB report (2000) presented a
general description of the local bond law for tensile forces. Six main stages have been
recognized in local bond stress-slip response. Goto (1971) carried out tests to clarify the
propagation of different types of cracks around the tensile reinforcing bars. The internal
cracks develop around the reinforcing bars in concrete cylinders. The inclination of
internal cracks and the direction of compressive forces on the bar ribs vary between 45
and 800
2.6 Test Specimens
2.6.1 Size of the Test Specimen
The test specimens shall consist of concrete cubes of size given below, with a single
reinforcing bar embedded vertically along a central axis in each specimen. The bar shall
project down for a distance of about 10 rom from the bottom face of the cube as cast, and
shall project upward from the top face whatever distance is necessary to provide
sufficient length of bar to extend through the bearing blocks and the support of the testing
machine and to provide an adequate length to be gripped for application of load:
Diameter of the Bars
Mm
Size of the Cube
mm
Upto and including 12 100
Over 12 upto and including 25 150
Over 25 225
27. 27
Fig 6. Pull-Out Test Apparatus
2.6.2 The cube shall be reinforced with a helix of 6 mm diameter plain mild steel
reinforcing bar conforming to Grade I of IS: 432 (Part 1)- 1966· or IS: 226-1962t at 25
mm pitch, such that the outer diameter of the helix is equal to the size of the cube, each
end of the helix being welded to the next turn.
2.6.3 The average compressive strength of three cubes representing the concrete used for
test specimen in 2.1, made and tested in accordance with relevant requirements of IS:
516-1959· shall be 200 to 300 kg/cm2 at the time of making the pull-out tests. If the
range of the compression strength of three cubes tested exceeds 50 kg/cm2, the test series
shall be discarded. All test specimens and the control cubes required to establish the
strength of concrete shall be cured under similar conditions.
28. 28
2.6.4 For the purpose of comparing bond resistance of deformed bars and plain bars, the
concrete used in both tests should be of the same mix, strength, age and curing. The bars
to be tested shall also be of same cross-sectional area and have similar surface conditions.
2.7 Preparation of Test Specimen
2.7.1Bars- Loose scale and rust shall be thoroughly removed from the bars by wire
brushing and bars inspected to ensure that they are free from grease, paint, or other
coatings which would affect their bond. Suitable solutions may also be applied, if
necessary, to clean the grease or oil. The end of the reinforcing bars on which the stem of
the dial gauge is to bear in the test, shall be ground to a reasonably smooth surface
normal to the axes of the bars.
2.7.2 Mixing Concrete - Except in those tests for which the method of mixing concrete
is a controlled variable, the concrete shall be mixed in accordance with the relevant
requirements of the method of making and curing concrete compression test specimens in
the laboratory specified in IS : 516·1959. The consistency of each batch of concrete shall
be measured immediately after mixing. When the air content of the freshly mixed
concrete is also required to be known, the determination shall be made in accordance
with the relevant requirements of IS : 1199-1959.
2.7.3 Moulding and Curing Specimens - Except in those tests for which the method of
placing concrete in moulds is a controlled variable, the specimens shall be moulded and
cured in accordance with the requirements of the method of making and curing concrete
compression test specimen in laboratory (compaction by hand) specified in IS :
516·1959. After the top layer has been rodded, the surface shall be struck off with a
trowel and covered with damp burlap to prevent evaporation.
2.7.4 Number of Specimen - At least three specimens of the deformed bar submitted for
test and three comparative specimens of plain bars of the same effective cross-sectional
area as the deformed bars under test, shall be prepared and tested.
29. 29
2.7.5 Preparing Specimen for Testing - Top surface of the cube, which is the bearing
surface in the pull-out test, shall be capped with a thin layer of neat cement paste at least
24 h prior to testing, or a thin layer of high strength gypsum plaster shall be applied at
least 2 h prior to testing.
2.8 TEST PROCEDURE
The procedure is as follows:
1) The test specimen shall be mounted in a suitable testing machine in such a manner that
the bar is pulled axially from the cube. The end of the bar at which the pull is applied
shall be that which projects from the top face of the cube as cast.
2) In assembling the testing apparatus on the specimen the distance between the face of
the concrete and the point on the loaded end of the reinforcing bar at which the device for
measuring slip is .attached, shall be carefully measured so that the elongation of the bar
over this distance may be calculated and deducted from the measured slip.
3) The load shall be applied to the reinforcing bar at a rate not greater than 2 250 kg/min,
or at no-load speed of the testing machine head of not greater than 1·25 mm/min,
depending on the type of testing machine used and the means provided for ascertaining or
controlling speeds.
4) The movement between the reinforcing bar and the concrete cube, as indicated by the
dial micrometers shall be read at a sufficient number of intervals throughout the test to
provide at least 15 readings by the time a slip of 0·25 mm has occurred at the loaded end
of the bar. The dial micrometers shall be read at the loaded and unloaded ends and
reading recorded to an estimated 0·1 of the least division of the dial.
The loading shall be continued and readings of movements recorded at appropriate
intervals until:
a) the yield point of the reinforcing bars has been reached,
b) the enclosing concrete has failed (the type of failure shall be noted ), or
c) a minimum slippage of 2.5 mm has occurred at the loaded end.
30. 30
5) The maximum load for each type of failure shall be recorded.
6) For the purpose of comparison the bond resistance of deformed bars and plain bars, the
comparison of bond strengths shall be made on the basis of the average bond stresses
calculated from the loads at a measured slip of 0.025 mm at free end. It is recommended
that when comparing plain and deformed bars, the complete load-slip curves of both
should also be plotted. The following details shall be recorded:
a) The load at a slip of 0·025 mm at the free end, and
b) The load at a slip of 0'25 mm at the free ends.
2.9 CALCULATION OF BOND STRESS
1) The slip at the loaded end of the bar shall be calculated as the average of the readings
of the two dial gauges, corrected for the elongation of the reinforcing bar in the distance
between the bearing surface of the concrete block and point on the reinforcing bar at
which the measuring device was attached.
2) For the purpose of this test, the average bond stress shall be the value obtained for each
specimen, by dividing the applied load at the slip specified, by the surface area of the
embedded length of the bar; and then taking the average value for the group of each type
of bar in the test series.
3) For deformed bars, the surface shall be calculated from the nominal size of the
deformed bar as specified in the relevant standard specification.
2.10 RECORD OF RESULTS
The following details shall be recorded:
a) The crushing strength of the concrete cube at an age corresponding to the age of the
specimen at the time of making the pull-out tests,
b) The age of specimen,
c) The load at a slip of 0-025 mm at the free end,
d) The load at a slip of 0-25 nun at the free end,
e) The slips at free and loaded ends at regular intervals of loading, and
f) The maximum load at failure and the type of failure.
31. 31
3. WORK CARRIED OUT
3.1 Collection of raw materials.
The constituent materials used in this investigation were procured from local sources.
Ordinary Portland cement of C53 grade was used. The cement and sand are easily
available in the market. Well graded river sand passing through 2mm and retained on
1.18mm sieve was used as fine aggregate.
Further we used reinforcement bars of three different diameters viz, 10mm, 12mm,
16mm. The bars used were deformed in nature. Several samples of each dia were
used and half of them were provided with a kink. The kink was provided at an angle
of 45 degrees with a pitch of 1.5 inch by using electronic press for 10 and 12mm bars
and gas welding was done on the 16mm bar.
Fig 7. Sieve shaker
3.2 Testing.
The testing program includes a variety of tests that address workability of the fresh
concrete and compressive and flexural strengths of the samples. The compressive
strength was checked using compression testing machine in which the samples were
tested for 7 days and 28 days. The UTM (universal testing machine) was used to
check the flexural strength of samples 28 days. Slump test was performed for each
mix design of concrete to check the workability of concrete on every partial
replacement of coarse and fine aggregates.
3.3 Workability test
Measurement of workability was done by performing the slump cone tests on every
concrete mix. The metallic slump mould has a top dia of 10 cm and bottom dia of 20
32. 32
cm with a height of 30 cm. The thickness of the mould sheet is 1.6 mm. The mould is
filled in four layers with freshly prepared concrete and each layer is tamped 25 times
by using a tamping rod. Difference in the height of mould and height of subsided
concrete after removing the mould gives us the slump value which is a measure of
workability of the concrete.
3.4 Compression tests.
Out of the many tests applied to the concrete, compression test is of the utmost
importance which gives an idea about all the characteristics of the concrete.
Compression tests were performed on cubic samples of size (15×15×15)cm in a
compression testing machine. Samples were tested both for 7 day and 28 day
compressive strengths. Maximum loads at the time of the failure of each sample were
noted and then the compressive strength is determined.
3.5 Consistency test for cement
In the project we weighed approximately 400gm of cement and mixed it with a
weighed quantity of water (w/c ratio = 0.26). The mixing was done for a time
between 3 to 5 minutes. The following procedure was then followed: Fill the Vicat
mould with paste and level it with a trowel. Lower the plunger gently till it touches
33. 33
the cement surface. Release the plunger allowing it to sink into the paste and note that
water content at which the plunger penetrates upto a depth of 33-35 mm.
3.5.1 Initial and final setting times.
Procedure:
a paste of standard consistency.
5
minutes and the gauging shall be completed before any sign of setting occurs.
commencing to fill the mould.
e top of the mould.
Initial Setting Time: Immediately place the test block with the non-porous resting
plate, under the rod bearing the initial setting needle. Lower the needle and quickly
34. 34
release allowing it to penetrate in to the mould. In the beginning the needle will
completely pierce the mould. Repeat this procedure until the needle fails to pierce the
mould for 5 + 0.5mm. Record the period elapsed between the time of adding water to
the cement to the time when needle fails to pierce the mould by 5 + 0.5mm as the
initial setting time. Final Setting Time: Replace the needle of the vicat apparatus by
the needle with an annular ring. Lower the needle and quickly release. Repeat the
process until the annular ring makes an impression on the mould. Record the period
elapsed between the time of adding water to the cement to the time when the annular
ring fails to make the impression on the mould as the final setting time. This
procedure is repeated again by replacing 10 percent and 20 percent of cement by glass
powder and the effects of glass as replacement to cement on initial and final setting
time are noted down.
3.6 Water absorption test for aggregates.
The sample of aggregates was thoroughly washed to remove the finer particles and
dust, weighed and then placed in a basket and were immersed in water for a period of
24 hours. After that the aggregates were removed from the water and allowed to 44
drain for a few minutes and then were gently surface dried with a piece of cloth. The
aggregates were again weighed after surface drying. Formula used in water
absorption: Percentage water absorption = ( A-B/B ) × 100 Where; A is weight of
aggregates before immersion. B is weight of aggregates after surface drying
3.7 Pull-Out Test
3.7.1 Apparatus Required
3.7.1.1 Moulds for Bond Test Specimens
The moulds shall be of size suitable for casting concrete as specified in 5.2.1 and shall
conform to the requirements of compression test specimens specified in IS:516.1959. The
mould shall be watertight. Water tightness may be accomplished by using grooved joints,
or a sealing compound may be applied at the joints after assembly. The moulds shall be
designed to hold the bars rigidly in place and shall allow for easy removal without
disturbance of embedded bars.
35. 35
3.7.1.2 Measuring Apparatus
Apparatus shall be provided for measuring the movement of the reinforcing bar with
respect to the concrete at both the loaded and unloaded (free) ends of the bar. Dial
micrometers shall be used at both locations. At the free end of the bar a dial micrometer
graduated to read in 0·0025 mm and having a range of not less than 2·5 mm shall be
used. At the loaded end, dial micrometers graduated in 0·025 mm will be satisfactory, but
a range of at least 12·5 mm should be provided, and a range of 25 mm is desirable
3.7.1.3 Testing Machine
The testing machine may be of any reliable type, of sufficient capacity for the tests and
capable of applying the load at the rate specified in 4.2. The bearing surface of the
concrete cube shall be supported on a square machined steel plate of size not less than the
size of the test cube and 20 mm thick, with a hole drilled through its centre of sufficient
diameter to accommodate the reinforcing bar. If a cross-bar measuring apparatus similar
to that shown in Fig. 5.2.1 is used, this plate should be supported on a steel block at least
125 mm in diameter and 75 mm thick. This block should have a central hole to
accommodate the reinforcing bar, and in addition, on its top side should have a diametral
slot and central hole of dimensions sufficient to accommodate the cross-bar. This slotted
block shall rest in turn on a spherically seated bearing block at least 125 mm in diameter
and having a central hole at least 40 mm in diameter (see Fig. 5.2.1)
3.7.1.4 Tamping Rod
The tamping rod shall be a round, straight steel rod 15 mm in diameter and approximately
0·6 m in length, having the tamping end rounded to a hemispherical tip, 15 mm in
diameter.
The work carried out in evaluating the bond strength of concrete can be a visually simple
but yet very long and tiresome process. In this process first concrete blocks of dimensions
150 x 150 x 150 mm are prepared by making special steel moulds with a hole drilled in
the bottom so as to accommodate the reinforcement.
36. 36
In the beginning we took random ratios of sand, cement and concrete and prepare simple
concrete blocks. These blocks were then tested on a UCT to determine their compressive
strength for the purpose of tallying the results through the process of interpolation, the
details of which will be explained further. Once the compressive strengths were
determined, reinforcements were added to the concrete blocks with the predefined ratios.
The reinforcements used were of different thickness of 10, 12 and 16 mm and the ratios
used were A(1 : 1.5 : 3), B(1 : 1.3 : 2.6) and C(1 : 1.2 : 2.4). Moreover the types of
reinforcements used were deformed bars and deformed bars with kick at 45 degrees to
understand the pattern of change in the bond strength when the pullout test was
conducted. The samples were prepared for a full period of 28 days so that the concrete
would gain its full strength and the bonding would give the best possible results. Two
samples of each type of reinforced concrete block were made so as to make the results
more effective and accurate.
Once the concrete blocks were ready, they were taken for testing and the pullout test
procedure was carried out in which the blocks were fixed in a UTM to make a suitable
arrangement. The pattern of the test was recorded on the graph with the help of a
computer which was used to monitor the change in bond strength with the applied stress.
After the test procedures were complete, the results of the simply deformed bars were
interpolated so as to compare them to the results on the IS code and the variation in the
results was checked. After this the results were compared with the bond strength in
deformed bars with a kink it was verified whether kinked bars improved the bond
strength of concrete or not.
38. 38
4. EXPERIMENTAL RESULTS AND
COMPARISON
4.1 Standard Consistency for cement
sample Consistency(%) Mean
Standard concrete
0.32
0.280.26
0.31
Table 1. Consistency results
4.2 Initial & Final setting times
sample Initial setting
time (hrs).
Final setting
time (hrs).
Mean initial
setting time
(hrs).
Mean final
setting time
(hrs).
Standard
concrete
0.6 5.63
0.54 5.510.55 5.41
0.49 5.5
Table 2. Initial and final setting time
4.3 Water absorption for course aggregated
Sample Course aggregate (%) Mean absorption value
Course aggregates
0.72
0.680.65
0.68
Table 3. Water absorption results.
39. 39
4.4 Experimental results for weight of constituents used in each ratio
per block.
Cement (kg) Sand (kg) Concrete (kg) Compressive
Strength
(N/𝑚𝑚2
)
A(1 : 1.5 : 3) 1.47 2.2 4.4 21
B(1 : 1.3 : 2.6) 1.65 2.14 4.29 23
C(1 : 1.2 : 2.4) 1 1.2 2.4 24
Table 4. Various grades used for testing with water-cement ratio 0.5
4.5 Experimental results of pullout test for simple deformed bars.
10mm Mean(τ) 12mm Mean(τ) 16mm Mean(τ)
A (ratios)
σ = 10.367 KN,
τ =2.2 N/𝑚𝑚2
2.1 N/mm2
12.157 KN,
2.15 N/mm2
2.03 N/mm2
13.348 KN,
1.85
N/mm2 1.95 N/mm2
9.42 KN,
2.0 N/mm2
10.8 KN,
1.91 N/mm2
15.456 KN,
2.05 N/mm2
B
9.89 KN,
2.1 N/mm2
2.208 N/mm2
11.42 KN,
2.02 N/mm2
2.12 N/mm2
14.92KN,
1.98N/mm2
2.11N/mm2
10.83 KN
2.3 N/mm2
12.55 KN,
2.22 N/mm2
16.89KN,
2.24/mm2
C
10.6KN,
2.25 N/mm2
2.42 N/mm2
12.83 KN,
2.27 N/mm2
2.36 N/mm2
16.66 KN,
2.21 N/mm2
2.28 N/mm2
12.2 KN,
2.58 N/mm2
13.85 KN,
2.45 N/mm2
17.72 KN,
2.35 N/mm2
Table 5. Experimental values of bond strength of deformed bars derived from the formula
τ =
𝜎
𝜋𝑑𝑙
, where ‘l’ is the embedded length of bar and ‘d’ is the dia of the bar
42. 42
4.7 Inferences from the pullout tests carried out on the sample
1. The grade of concrete affects the bond strength. Higher the grade of concrete,
greater is the bond strength.
2. Type of reinforcement is also vital in changing the bond strength. Deformed bars
result in more bond strength than plain bars and deformed bars with kink lead
to higher bond strength than simply deformed bars.
3. The dia of bars does not play a vital role in changing the bond strength of
concrete. The bond strength is not consistently affected by dia at all as per
experimental data.
43. 43
5. CONCLUSION AND FUTURE SCOPE
At the end of this project we conclude that bond strength plays a significant role in
building constructions. A slight discrepancy in the analysis of bond strength can cause
major damage to vital structures. Therefore, there needs to be a proper mechanism to
determine how this concept can be utilized in making buildings safer. This project is a an
attempt in this direction. Various studies need to be done in this regard if we take into
account the improvement our results have shown. The tremendous improvement, simple
changes like ribs and kinks have caused when added to steel bars, is a proof that this
subject has enormous future scope.
We conclude that the increase in the grade of concrete or introduction of a kink in steel
bar causes the bond strength to increase by a large extent. Furthermore, our attempt was
to properly explain the concept in its original form. The improvement in results is an
indication that introducing a kink or improving the grade or changing the diameter is vital
for making the structures stronger.
The future scope can be assessed by the fact that the derived results of this project can be
repeatedly used as a reference by technical professionals anywhere. Moreover, this report
can be helpful in making future project more economical.
44. 44
References
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89(2): 161-168
[2]ACI 318-2005. Building code requirements for structural concrete and commentary,
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[3]Azizinamini, A.Stark, M. Roller, J. J. and Ghosk, S.K. 1993. Bond performance of
reinforcing bars embedded in high strength concrete. ACI StrJl, 90(5): 554–561.
[4]Azizinamini, A. Pavel, R. Hatfield, E. and Ghosh, S.K.1999a. Behavior of spliced
reinforcing bars embedded in highstrength concrete. ACI Str Jl 96(5): 826–835.
[5]Azizinamini, A. Darwin, D. Eligehausen, R. Pavel, R. and Ghosh, S.K. 1999b.
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strength concrete. ACI Str Jl 96(6): 922–926.
[6]Bortolotti. 2003.Strength of concrete subjected to pull out load. ASCE Mat, Jl 15(5):
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[7]Darwin, D. Zuo, J. Tholen, M.L. and Idun, E.K. 1996. Development length criteria for
conventional and high relative rib area reinforcing bars. ACI Str Jl 93(3): 347–359.
[8]De Larrard, F. Schaller, D. and Fuchs, J. 1993. Effect of bar diameter on the bond
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[9]Eligehausen, R. Popov, E.G. Bertero, V.V. 1983. Local bond stress-slip relationships
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