1. Chapter 2
LITERATURE REVIEW
2.1 CONCRETE BOND
Bond stress is the shear stress acting parallel to the bar on the interface
between the bar and the concrete. Bond stress may be considered as the rate of transfer of
force between concrete and steel. In other words, if there is bond stress there is change in
steel stress and vice-versa. Bond is due to combined effect of adhesion, friction and
bearing (for deformed bars).
Concrete, on its own, is strong in compression but weak in tension. As a
matter of fact, the compressive strength of concrete is about ten times greater than its
tensile strength. This negative trait is remedied by placing steel reinforcing bars into the
concrete to form reinforced concrete (RC). This approach allows a material with much
higher tensile strength, such as steel, to take on the tensile load that the concrete cannot
support. In order for this relationship to work, however, the concrete and the reinforcing
steel must have a sufficient bond between them so the tensile load can be transferred
effectively to the steel. There are three different aspects that contribute to bond strength:
chemical adhesion, friction, and mechanical interlock. The chemical adhesion is a bond
between the concrete and the steel, the friction is caused by the bar deformations, or ribs,
slipping along the concrete, and the mechanical interlock is a bearing force caused by the
ribs bearing against the concrete (Swenty, 2003).
In order to insure an adequate bond, ACI 318 (2008) regulates how long a bar
must be imbedded into the concrete based on factors such as concrete type, concrete
strength, bar diameter, and bar type. This regulated factor is called the development
length of the bar, and prevents a bond failure from being the controlling failure mode of a
structure.
Bond failure usually occurs in two different ways. In structures, the most
common is known as a splitting failure. A splitting failure occurs when a small clear
cover or small spacing between reinforcing bars exists. The small amount of concrete
around the bars can crack or split, exposing the reinforcement and ultimately leading to
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2. bond failure. Also contributing to a splitting failure are the mechanical properties of the
surrounding concrete such as concrete tensile strength, bar geometry, and the presence of
transverse reinforcement such as stirrups (ACI Committee 408, 2003). This result tends
to be the more catastrophic of the bond failure modes (Swenty, 2003). Another common
bond failure type is pull-out. This mode occurs when the reinforcing bar slips, and as a
result, the concrete between the bar deformations is crushed, leading to a simple pulling
out of the bar. Usually pull-out controls when there is a larger concrete clear cover and
spacing between the reinforcing bars making splitting less likely. A less common bond
failure is known as a conical failure. This occurs when the concrete cracks propagate
outward from the ribs on a reinforcing bar, and the bar ultimately pulls out along with a
“cone” of concrete upon failure.
Bond slip behavior of reinforcement bars in reinforced concrete members has
a pronounced influence on the design of anchorage of reinforcing bars and their splice
lengths and on the structural ductility. Parameters that affect the concrete-steel bond
properties include concrete density, concrete cover, aggregate type, confinement
conditions (e.g. transverse reinforcement), type, diameter, location and orientation of the
reinforcing bar, and mix additives such as silica fume or fibers (Dancygier, 2010). The
main source of bond of deformed bars is the mechanical interlocking between the
concrete and the lugs of the rebar. Plain bars are more sensitive to the voids beneath
horizontal reinforcement because of the decrease of the contact area between concrete
and steel and hence the adhesion. The bond behavior is significantly affected by the
concrete type in specimens with deformed bars (Soylev, 2011).
The study of Harajli (2004) concentrated on the analytical evaluation of the
average bond strength at failure, or development strength of reinforcing bars embedded in
plain HSC in comparison with NSC. The analysis adopted a numerical solution scheme
of the bond problem and incorporated an experimentally derived local bond stress-slip
response, applicable for both NSC and HSC. The bond strength results predicted by the
analysis were in very good agreement with a collection of experimental data for both
NSC and HSC. The analytical results demonstrated that the average bond stress
distribution along the bar development length at bond failure is generally non-uniform,
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3. and that the degree of non-uniformity in the corresponding bond stress distribution is
more pronounced for HSC as compared to NSC and increases with an increase in the
development length. Increasing the development length leads to a concentration of the
bond force in only a portion of the bar at the loaded end.
2.2 BOND TESTING
Testing for bond strength is carried out in a variety of ways. The most
common and traditional method is the standard pull-out test. One issue with the pull-out
test is that a compressive stress is induced on the bond that normally does not exist in an
actual structure. To remedy this, ACI 408R-03 outlines several other methods such as the
beam anchorage, beam end, and splice tests that place the bond in situations that are more
similar to those present in the field (ACI Committee 408, 2003). Note that the following
ACI bond tests do not have specimen dimensions. This is because ACI does not specify
specific dimensions.
The pull-out test is popular due to its ease of construction and testing. ASTM
C234 was developed to standardize the testing method, but was later disbanded due to the
high level of inconsistency that the test yields. RILEM, however, has provided a set of
recommendations for the test in order to provide some form of uniformity and minimize
some of the inconsistencies. The RILEM test recommends casting a single reinforcing
bar into a concrete cube with only half of the bar inside the specimen actually bonded to
the concrete, as shown in Figure 2.1 (RILEM 7-II-28, 1994). This approach is to prevent
a conical bond failure at the bottom and is achieved using a bond breaker of some type.
The bar is fed through a metal plate and a pulling force is applied to the bar while the
metal plate pushes up on the concrete block until a bond failure occurs. Usually a device
is installed on the unloaded end of the reinforcing bar in order to measure slip. While this
test has been modified by RILEM, it is still not accepted as an accurate way of
determining development lengths for reinforcement (ACI Committee 408, 2003).
Therefore, this test is commonly used as a means of comparison between a control
specimen of known development requirements and an experimental specimen. Data for
this test is often compiled into force vs. slip and stress vs. slip plots.
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4. Figure 2.1: Typical pullout specimen (db=bar diameter)
The IS: 2770 (PART IV) 1967 covers the method for the comparison of bond resistance
of different types of reinforcing bars with concrete by means of a pullout test. It states the
whole test procedure with method to calculate the bond stress.
The moulds for bond test specimens shall be of size suitable for casting
concrete cubes of dimensions specified. According to that, 150mm size of cubes is used
for 16mm diameter bars. Apparatus shall be provided for measuring the movement of the
reinforcing bar with respect to the concrete at both loaded end and 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 0.0025mm and having a range of not less than 2.5mm shall
be used.
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5. Figure 2.2 Pullout test specimen with LVDTs
The cube shall be reinforced with a helix of 6mm diameter plain mild steel
reinforcing bar such that the outer diameter of the helix is equal to the size of cube. To
study the effect on bond stress in the situation of without transverse reinforcement, this
helix reinforcement is not used. Instead of that, various doses of steel fiber are used and
its effects are studied. The slip at the loaded end of the bar shall be calculated as 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.
2.3 EFFECTS OF STEEL FIBERS ON BOND
Fiber reinforced concrete (FRC) may be defined as a composite material made
with Portland cement, aggregate and incorporating discrete discontinuous fibers. Plain,
unreinforced concrete is a brittle material, with a low tensile strength and a low strain
capacity. The role of randomly distributes discontinuous fibers is to bridge across the
cracks that develop provides some post-cracking ductility. If the fibers are sufficiently
strong, sufficiently bonded to material, and permit the FRC to carry significant stresses
over a relatively large strain capacity in the post-cracking stage. The real contribution of
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6. the fibers is to increase the toughness of the concrete, under any type of loading. The
fibers tend to increase the strain at peak load, and provide a great deal of energy
absorption in post-peak portion of the load vs. deflection curve. When the fiber
reinforcement is in the form of short discrete fibers, they act effectively as rigid
inclusions in the concrete matrix. The fiber reinforcement may be used in the form of
three – dimensionally randomly distributed fibers throughout the structural member when
the added advantages of the fiber to shear resistance and crack control can be further
utilized. The fiber concrete may also be used as a tensile skin to cover the steel
reinforcement when a more efficient two – dimensional orientation of the fibers could be
obtained (Nguyen)
The effect of fibers on the variation of bond between steel reinforcement and
concrete with casting position has not been sufficiently studied. Bond strength decreases
as concrete depth beneath horizontal reinforcement increases. This phenomenon is known
as top-bar effect and bleeding is considered to be the most important factor behind this
phenomenon. As the heavier materials settle in fresh concrete, bleed water moves
upwards and it is trapped under large aggregates and horizontal reinforcement. The void
formation due to concrete settlement and water accumulation under the reinforcement
causes reduction in bond strength.
The main source of bond of deformed bars is the mechanical interlocking
between the concrete and the lugs of the rebar. Plain bars are more sensitive to the voids
beneath horizontal reinforcement because of the decrease of the contact area between
concrete and steel and hence the adhesion. The bond behavior is significantly affected by
the concrete type in specimens with deformed bars. The results of compressive strength
and splitting tensile strength tests indicate increase in compressive strength but no
increase in splitting tensile strength by steel fiber addition with respect to the control
specimen. Steel fiber reinforced concrete specimens remained integral after the pullout
failure. The strength term does not change for steel fiber reinforced concrete as the tensile
strength did not change by steel fiber addition. Steel fiber has a confinement effect and
the development length can be reduced by confinement factor. However, as the
contribution of steel fibers to bond strength depends on crack length and width. The
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7. confinement with fiber decreases as the concrete depth increases due to segregation of
steel fibers. Therefore, there is need for an additional top-bar factor to define the decrease
of confinement for top-cast bars. The steel fiber reinforced concrete had higher bond
strengths. However, the decrease in bond strength from bottom cast to top cast bar was
higher, mainly due to segregation of steel fibers. The superiority in the bond strength was
attributed to the improvement in the fracture behavior by the presence of steel fibers
(Soylev 2011).
Concrete is most widely used construction material in the world. However, it
has some deficiencies such as low tensile strength, low post cracking capacity, brittleness
and low ductility, limited fatigue life, not capable of accommodating large deformations,
low impact strength. The weakness can be removed by inclusion of fibers in the mix. The
fibers can be imagined as an aggregate with an extreme deviation in shape from the
rounded smooth aggregate. The fibers interlock and entangle around aggregate particles
and considerably reduce the workability, while the mix becomes more cohesive and less
prone to segregation. Fibers help to improve the compressive strength, flexural strength,
tensile strength, post peak ductility performance, pre-crack tensile strength, fatigue
strength, impact strength and eliminate temperature and shrinkage cracks. Fibers act as
crack arrester restricting the development of cracks and thus transforming an inherently
brittle matrix into a strong composite with superior crack resistance (Shende, 2011).
It is known that the addition of steel fibers leads to a reduction of crack
width of bending elements in reinforced concrete. It is however, not established whether
this effect is only due to prove transfer of tensile force by the fibers across cracks or also
because of an improvement of bond of the embedded deformed bar reinforcement by
fibers. As the cover decreases, the bond strength decreases. The corner position of the bar
leads to a lower bond strength than the edge position. The results also show that the bond
splitting strength depends primarily on the relative cover irrespective of fiber addition
within the investigated range of fiber contents. However the post-peak ductility after
reaching the bond splitting strength is markedly enhanced by fiber addition. The observed
reduction of crack width and deformation of reinforced concrete bending members with
steel fiber addition is caused by the transfer of tensile force across primary cracks by the
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8. fibers, acting as randomly oriented reinforcing bars. The post-peak ductility of specimens
failing by splitting is greatly improved by fiber addition (Rostasy, 1988).
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