BOND in Concrete

8/9/2019 BOND in Concrete

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BOND

in ConcreteEngr. Ahsan


2/19

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


10/19

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]


12/19

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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BOND in Concrete