bond in concrete
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8/9/2019 BOND in Concrete
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BOND
in ConcreteEngr. Ahsan
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Presentation outline• Bond• Mechanism of Bond transfer
• Development length
•
Tests on Bond• Influencing parameters
• Bond in different types of concretes
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Bond
• Concrete being weak in tension requires services of
reinforcement to overcome that shortcoming.
• In order to pass on tensile forces successfully to reinforcement
however it is essential that the good B!D between the two
material is present in the system.
• Many researchers have studied the interface and also have
made efforts to improve B!D behavior of "einforced
concrete members by playing with various parameters like
cover# relative rib area etc.
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FBD
$% & '
(T) * uavg .+.d b.l , T- & '
(s . f s) , s. f s- & uavg .+.d b.l
/f s- ( f s)0 s & uavg .+.d b.l
1f s . +23. d b- & uavg .+.d b.l
uavg .+.d b.l & 1f s . +23. d b-
uavg .l & 1f s . d b 23
%or l & d4
3u2d b& df s2d4
5here u is the true bond stress acting in the length d4.
Bond stress at any point is proportional to the slope of the steel stress diagram at that same point.
Source: [1]
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Average bond stresses in beam in terms of Shear force
T & M26d
1T & uavg .+.d b. 14
1T & 1M26d
1M214 & uavg .+.d b . 6d
uavg & 72 /+.d b06d
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True Bond stresses in Beam
Source: [1]
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Mechanism of Bond Transfer
• 8mooth bar
• dhesion• %riction
− 9uickly lost due to :oisson;s effect
Source: [1]
• Deformed bar
• dhesion
• %riction
• Mechanical interlock /Bearing on
deformations of bar0
•
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• The splitting cracks follow the
reinforcing bar along the bottom or side
surfaces of the beam
•
nce these cracks develop# the bondtransfer drops rapidly.
• 8plitting load is function of
• Tensile strength of concrete
• verage bond stress
•
Min. dist. from bar to concretesurface or to ne4t bar.
• :ullout failure can occur if cover and
bar spacing are large.
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Development length• Because the actual bond varies along the length of the bar anchored in =one of tension#
the CI code uses concept of development length
• The Development length # ld# is the shortest length of bar in which the bar stress can
increase from =ero to the yield strength# f y.
• Distance lesser than >ld? will result in pull out failures.
Taking and
5e have
•
fs=0
fs= fy
ld
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Design e4pressions
CI @)A
The design provision for development and splice length of straight
reinforcement in tension are based on the e4pressions developed by
rangun# irsa and Breen in year )E(
Source: [2]
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Source: [3]
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Comparison of design expressions
Source: [3]
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Tests• 7arious test configurations are in practice.
:ullout specimens / 8TM C-@30
5idely used
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Bond behavior in different type of concretes
• L8Co Bond strength increases with increasing concrete
compressive strength.Balaguru# CI(3'A()-N
o vraham et al in -')'# performed pullout tests of
deformed bars in !8C and L8C specimens. They
observed the Ligher bond values for L8C specimens.
o They also established that this increase is not necessarily
accompanied by the similar increase in concrete strength.
• %iber reinforced concreteo %iber reinforcement# especially steel fibers# tend to act as
transverse reinforcement providing increased bond
strength to reinforcing steel.
o Lara6ili et al /-''-0 tested @- small scale beam specimens
to evaluate the local bond stress(slip response. Le
observed that adding fibers in ) and -O by volume
fraction increased the splitting bond strength# on average#
by -H and @@O# respectively# and resulted in a significant
improvement in the ductility of bond failure.
Source: Harajili et al -2002
Source: Ezeldin & Balaguru -1989
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• Fightweight aggregate concrete
o Fightweight aggregate concrete develops lower
bond strength than normalweight concrete with the
same compressive strength.CI(3'A()-N
o CI @)A incorporates factor P&'.E in design
e4pression to reflect the lower tensile strength of
the lightweight concrete.
• 8elf Compacting Concrete.
o Increased cohesiveness of 8CC leads to reduction
in bleeding# segregation and reduce the structuraldefects resulting from porosity under embedded
reinforcement.
o Desnerck et al /-')'0 tested total of @H specimens.
were cast using @ different concrete typesQ one
conventional vibrated concrete /C7C0 and two
powder(type 8CCRs. The bond stressSslip
behaviour of reinforcing bars with diameters
ranging from )- to 3' mm were recorded. %rom
these tests# it can be concluded that the bond
strength of 8CC is even higher than it is for C7C.
Source: Desnerck et al -2010
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References• )N 5ight# . G.# and Macregor# . .# Reinforced Concrete Mechanics & Design# :earson :rentice Lall# !ew ersey#
-''# )'@)pp.
• -N CI Committee @)A# >Building Code "equirements for 8tructural Concrete /CI @)A('A0 and Commentary#?
merican Concrete Institute# %armington Lills# MI# -''A# 3@ pp.
• @N CI Committee 3'A# -''@# > Bond and Development of 8traight "einforcing Bars in Tension /CI 3'A"('@
/"eapproved -')-00#? merican Concrete Institute %armington Lills# Mich.# E@ pp.
• .
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Danke
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About me
• 2002 B.E Civil
• 2003-2006 Field Engineer
• 2006-2008 MS Structural Engineering
•
2008-2010 Structural Engineer• Fr! 2010 Assistant "r#essr
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