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Syracuse University
Riyad Aboutaha, PhD, F-ACI
ASCE Expo 2013 Syracuse, NY
November 11, 2013
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• Introduction
• Deterioration mechanisms of steel reinforced concrete members (SRCM)
• Effects of corrosion on bond strength of SRCM
• Effects of corrosion of serviceability of SRCM
• M-P Interaction diagrams for deteriorated steel reinforced concrete columns
• Ultimate Strength of Deteriorated Reinforced Concrete Beams • Summary and Conclusion
-
• Corrosion of steel reinforcing bars in concrete
• Freezing and thawing
• Carbonation of concrete
• Alkali-silica reaction
-
Time
Deterioration
Carbonation
Frost attack
Corrosion
ASR
Simplified degradation mechanism in corrosive environment.
Critical
-
Time
Deterioration Carbonation + Frost attack + Corrosion
Simplified degradation mechanism in corrosive environment, showing the
combined deterioration effects of corrosion, frost attack, and carbonation
Critical
Corrosion
-
Primarily due to introduction of deicing salts
-
Concrete Concrete
Exterior Surface Exterior Surface
Original Corroded
Effects of corrosion on serviceability of SRCM
-
Concrete
Exterior Surface
Concrete
Exterior Surface
Corrosion of Closely Spaced Bars (Splitting Crack)
Corroded Stirrup
X < 2C
C
-
Corrosion Cracks along Column’s main bars
-
Major loss of steel section
Major loss of bond between steel rebars
and concrete
Effects of Corrosion on Bond Strength of SRCM
-
Exterior Surface Corroded Stirrup
C
Concrete
X < 2C
-
Visible concrete distress marked on an elevation of a concrete bridge pier
Delamination (hollow sound)
Exposed rebars Cracks parallel to corroded rebars
Flexural-Shear/ Shear cracks
-
Corrosion Leading to Flexural Structural Deficiency
-
Flexural Cracking (Structural)
1
1
Corrosion problem leading to a flexural deficiency
-
Flexural Shear Cracking (Structural)
Corrosion problem leading to a shear deficiency
1
1
-
(a) Affected regions before spalling
of the concrete cover.
(b) Affected regions after spalling
of the concrete cover.
Reinforced Concrete Columns
-
Column Interaction Curve
Tension Side Cover Loss - (As = 2.08% , C/D = 2.0)
0
500
1000
1500
2000
2500
3000
3500
4000
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400
Mu = Kip ft
Pu
= K
ips
CR=0% CR=4.25% CR=10% CR=50% CR=75%
Clear Cover = 2.26"
Pn (
kip
s)
Mn (kip-ft)
-
The Model involves:
Assessment of material properties of deteriorated columns
Buckling length of exposed corroded steel bars
Location of the deteriorated column face
Loss of concrete
Amount of corrosion
-
• Effect of Reinforcement Corrosion on Bond Strength 0-4% the ultimate bond strength increases.
4 to 6%, the bond failure occurs suddenly.
Load carrying capacity decreased with the decrease in the available bond strength.
• Buckling of Reinforcing Bars
Experimental Studies
-
• The residual capacity of corroded reinforcing bars were investigated by Du et al. (2005).
• A simple equation (by Due et al.) was proposed to predict the residual capacity of corroded reinforcing bars.
ycorr fQf 005.01
corrsos QAA 01.01
Material Properties - Steel Rebars
uocorru Q )05.00.1(
yocorry Q )05.00.1(
-
Stress-strain curve of corroded reinforcement
[Du et al., 2005].
Idealized stress-strain curve for corroded
reinforcement.
Material Properties - Steel Rebars
-
Lexp
A2,Es
Deteriorated
Region
A1,Es
P
M
Critical Buckling Stress vs Lexp
(fyoriginal = 60 ksi)
0
10
20
30
40
50
60
70
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Lexp = inches
fycr
(ksi)
CR=0% CR=5% CR=10% CR=50% CR=75%
Effect of amount of corrosion and exposed bar
length on critical buckling stress of reinforcement.
Material Properties - Steel Rebars
-
• Interaction diagrams computed by assuming a series of strain distributions
• Conventional strain compatibility does not apply as-is in computing stresses in corroded reinforcement
• Strain in unbonded reinforcement is calculated using;
,0
Lc
c ave sL
dxL L
-
The interaction diagram calculation process is illustrated below for
one particular deterioration case and strain distribution.
dxLL
L L
cave,cs
0
1
As’
As(cor)
b
d = h c
εcu = 0.003
εsc fsc
a=β1c
0.85 fc’
fs(cor)
(a) Deteriorated Section (b) Strains (c) Stresses
Calculation of stress and strains for a given section and strain distribution.
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Stirrups are used to provide shear resistance, additional bond strength and to confine the concrete and longitudinal reinforcement.
Stirrups are more vulnerable to corrosion than longitudinal bars due to both a lesser cover and a greater surface/cross-sectional area ratio.
Corrosion of stirrups has significant effect on axial, bending, and shear capacities of deteriorated RC columns. Effect of corrosion on confinement effect of stirrups before and
after deterioration.
Effects of Corrosion of Stirrups
-
Rodriguez et al. conducted an experimental study using 24 columns.
Analytical interaction diagrams for three types of tested columns were developed.
Only one set of data is available. 4Φ8
Stirrups Φ6/150
fcu = 35.8 MPa
fy = 550 MPa
Concrete Cover = 20 mm
Type – 1
4Φ16
Stirrups Φ6/150
fcu = 35.6 MPa
fy = 550 MPa
Concrete Cover = 20 mm
Type – 2
8 Φ12
Stirrups Φ6/150
fcu =39.4 MPa
fy = 550 MPa
Concrete Cover = 20 mm
Type – 3
Original Column Information
4Φ8
Stirrups Φ6/150
fcu = 35.8 MPa
fy = 550 MPa
Concrete Cover = 20 mm
Type – 1
4Φ16
Stirrups Φ6/150
fcu = 35.6 MPa
fy = 550 MPa
Concrete Cover = 20 mm
Type – 2
8 Φ12
Stirrups Φ6/150
fcu =39.4 MPa
fy = 550 MPa
Concrete Cover = 20 mm
Type – 3
Original Column Information
-
The model reasonably agrees with the ones developed by Rodriguez et al. (1997).
In compression field the developed model shows lower strength.
Column Interaction Curve - Type 2 - Case 2
(Cover Loss = 34mm)
0
200
400
600
800
1000
1200
1400
1600
1800
0 10 20 30 40 50 60
Mu =kN-m
Pu
= k
N
Case-2 (CL=34mm) Rodriguez's - Type 2 - Case 2
Exp-23
Comparison of the column interaction diagrams for Type – 2, Case – 2 column.
Pn (
kN)
Mn (kN-m)
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Residual structural capacity of RC columns was investigated for several deterioration cases for each column type.
Six different corrosion deterioration cases with different corrosion amounts, cover to depth (C/D) ratios, and exposed bar lengths were investigated.
Column Size = 24in x 24in
12 # 9
Links #4 @ 10 inches
fcu = 4 ksi
fy = 60 ksi
C/d = 1.0, 1.5, 2.0, and 2.5
As = 2.08%
Column Size = 24in x 24in
12 # 9
Links #4 @ 10 inches
fcu = 4 ksi
fy = 60 ksi
C/d = 1.0, 1.5, 2.0, and 2.5
As = 2.08%
Column Size = 24in x 24in
12 # 11
Links #4 @ 10 inches
fcu = 4 ksi
fy = 60 ksi
C/d = 1.0, 1.5, 2.0, and 2.5
As = 3.25%
Column Size = 24in x 24in
12 # 11
Links #4 @ 10 inches
fcu = 4 ksi
fy = 60 ksi
C/d = 1.0, 1.5, 2.0, and 2.5
As = 3.25%
Column Size = 24in x 24in
12 # 14
Links #4 @ 10 inches
fcu = 4 ksi
fy = 60 ksi
C/d = 1.0, 1.5, 2.0, and 2.5
As = 4.69%
Column Size = 24in x 24in
12 # 14
Links #4 @ 10 inches
fcu = 4 ksi
fy = 60 ksi
C/d = 1.0, 1.5, 2.0, and 2.5
As = 4.69%
Type – I column original cross-section investigated for different corrosion cases.
Type – II column original cross-section investigated for different corrosion cases.
Type – III column original cross-section investigated for different corrosion cases.
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The deterioration cases can be summarized as follows:
Case – I Corrosion at the extreme compression layer of bars
Case – II Corrosion at the
extreme tension layer of bars
Case – III Corrosion at the
extreme left or right side bars
Case – IV Corrosion at all bars
Case – V Corrosion at the extreme
compression layer of bars and at the left or right side bars
Case – VI Corrosion at the extreme
tension layer of bars and at the left or right side bars
M M M
M M M
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Type – I column with C/D ratio equals to 2
Interaction diagram for pre-defined deterioration stages when
deterioration is at compression side of the column section.
Interaction diagram for pre-defined deterioration stages when
deterioration is at tension side of the column section.
Effects of Amount of Corrosion and Loss of Concrete
Column Interaction Curve
Compression Side Cover Loss - (As = 2.08% , C/D = 2.0)
0
500
1000
1500
2000
2500
3000
3500
4000
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400
Mu = Kip ft
Pu
= K
ips
CR=0% CR=4.25% CR=10% CR=50% CR=75%
Clear Cover = 2.26"
Pn (
kip
s)
Mn (kip-ft)
Column Interaction Curve
Tension Side Cover Loss - (As = 2.08% , C/D = 2.0)
0
500
1000
1500
2000
2500
3000
3500
4000
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400
Mu = Kip ft
Pu
= K
ips
CR=0% CR=4.25% CR=10% CR=50% CR=75%
Clear Cover = 2.26"
Pn (
kip
s)
Mn (kip-ft)
-
Interaction diagram for pre-defined deterioration stages when
deterioration is at left side of the column section.
Column Interaction Curve
Left Side Cover Loss - (As = 2.08% , C/D = 2.0)
0
500
1000
1500
2000
2500
3000
3500
4000
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400
Mu = Kip ft
Pu
= K
ips
CR=0% CR=4.25% CR=10% CR=50% CR=75%
Clear Cover = 2.26"
Pn (
kip
s)
Mn (kip-ft)
Effects of Amount of Corrosion and Loss of Concrete
Type – I column with C/D ratio equals to 2
-
Case – I : Corrosion at the extreme compression layer of bars
Effect of exposed bar length on load carrying capacity when
deterioration is at extreme compression side of the column section
(Amount of Corrosion = 10%).
Column Interaction Curve
Compression Side - (As = 2.08%, C/D = 2.0, CR = 10%)
0
500
1000
1500
2000
2500
3000
3500
4000
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400
Mu = Kip ft
Pu
= K
ips
CR=0% CR=4.25% CR=10%
CR=10%, Lexp=20in CR=10%, Lexp=40in CR=10%, Lexp=60in
Clear Cover = 2.26"
Pn (
kip
s)
Mn (kip-ft)
Effects of Exposed Corroded Bar Length
-
Case – II : Corrosion at the extreme tension layer of bars
Effect of exposed bar length on load carrying capacity when
deterioration is at extreme tension side of the column section
(Amount of Corrosion = 10%).
Column Interaction Curve
Tension Side - (As = 2.08%, C/D = 2.0, CR = 10%)
0
500
1000
1500
2000
2500
3000
3500
4000
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400
Mu = Kip ft
Pu
= K
ips
CR=0% CR=4.25% CR=10%
CR=10%, Lexp=20in CR=10%, Lexp=40in CR=10%, Lexp=60in
Clear Cover = 2.26"
Pn (
kip
s)
Mn (kip-ft)
Effects of Exposed Corroded Bar Length
-
Case – IV : Corrosion at all bars
Effect of exposed bar length on load carrying capacity when
deterioration is at all sides of the column section (Amount of
Corrosion = 10%).
Column Interaction Curve
All Sides Cover Loss - (As = 2.08%, C/D = 2.0, CR = 10%)
0
500
1000
1500
2000
2500
3000
3500
4000
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400
Mu = Kip ft
Pu
= K
ips
CR=0% CR=4.25% CR=10%
CR=10%, Lexp=20in CR=10%, Lexp=40in CR=10%, Lexp=60in
Clear Cover = 2.26"
Pn (
kip
s)
Mn (kip-ft)
Effects of Exposed Corroded Bar Length
-
The load carrying capacity experiences significant reduction between second and fourth deterioration stages (i.e. between cover cracking and spalling).
Effect of C/D ratio on load carrying
capacity of Type – I columns (Amount
of Corrosion = 0%)
Effect of C/D ratio on load carrying
capacity of Type – I columns (Fourth
deterioration stage (CR = 10%) when
deterioration is at compression side of the
column section).
Effect of C/D ratio on load carrying capacity
of Type – I columns (Second deterioration
stage when deterioration is at compression
side of the column section).
Deterioration Stage (I)
(Corrosion 0%) Deterioration Stage (II)
(Corrosion 2.25%) Deterioration Stage (IV)
(Corrosion 10%)
Effects of “Cover” to “longitudinal Bar Diameter” C/D Ratio
-
Load carrying capacity of deteriorated columns is sensitive to concrete cover spalling.
As the concrete cover to column gross area ratio increases, the load carrying capacity of a column might significantly be affected
Effects of “Cover” to “Column Depth” Ratio
-
Estimation of Load Carrying Capacity Reduction Using Interaction
Diagrams Developed for Deteriorated RC Columns
The level of load carrying capacity reduction for any deterioration stage can be determined using several different approaches.
Two of those methods are; By pre-defined eccentricity lines By pre-defined axial loads
-
First Approach – Defining the eccentricity line
Strength Reduction for Eccentricity = eb,original
(Compression Side Deterioration, As = 2.08%, C/D = 2.0)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0% 10% 20% 30% 40% 50% 60% 70% 80%
Amount of Corrosion (% Diameter Loss)
Red
ucti
on
in
C
ap
acit
y
(% o
f O
rig
inal C
ap
acit
y)
Strength reduction at different deterioration stages (reduction is calculated using pre-defined eccentricity approach).
Estimation of load carrying capacity reduction for defined eccentricity.
Estimation of Load Carrying Capacity Reduction Using Interaction
Diagrams Developed for Deteriorated RC Columns
-
Strength Reduction for Axial Load = 0.4Pu
(Compression Side Deterioration, As = 2.08%, C/D = 2.0)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0% 10% 20% 30% 40% 50% 60% 70% 80%
Amount of Corrosion (% Diameter Loss)
Red
ucti
on
in
C
ap
acit
y
(% o
f O
rig
inal C
ap
acit
y)
Second Approach – Defining the axial loads
Strength reduction at different deterioration stages (reduction is calculated using pre-defined axial load approach).
Estimation of load carrying capacity reduction for defined axial load.
Estimation of Load Carrying Capacity Reduction Using Interaction
Diagrams Developed for Deteriorated RC Columns
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The amount of strength loss depends on the location of the deterioration.
In the compression-controlled region, the compression side
deterioration causes more capacity reduction than left side or tension
side deterioration.
In general, corrosion in tension reinforcement causes more strength
reduction than compression or left/right side deterioration.
Corrosion on all four sides of the column causes significant strength
reduction.
The steel reinforcements reach their usefulness and effectiveness in transferring compression forces when the Lexp/Dcor ratio equals to 30 (i.e. the capacity reduced to 75% of the original value).
M-P Interaction Diagrams for Deteriorated RC Columns
-
L
Lub
-
Effects of Unbonded Length of Corroded Beams
-
FEA Model – Beam Geometry
-
Load-deflection curve of El Maaddawy et al. control beam
(experimental versus FEA)
Verification of FEA Model
-
Moment-deflection curves of
Sharaf & Soudki C1, D1, D3
Model has been verified against 17 different beams
Two fully bonded
15 with different unbond lengths
Cairns and Zhao load-deflection curves are unavailable
Moment-deflection curves of
Cairns and Zhao S2, S4B, S9, S11 (FEA)
Verification of FEA Model
-
Effect of Unbonded Length
Effect of partial unbond on ultimate moment capacity
Beams with relatively short unbonded length (up to 65% of the span length),
the ultimate moment capacity is not affected (Tension steel yields).
When the unbonded length is over the full length of the beam, the ultimate
capacity decreases by 35% of the original capacity
Effects of Various Parameters
-
Effect of Reinforcement Ratio
The increase of reinforcement ratio is associated with a decrease in Mub/Mb for the same unbonded length
For beams with reinforcement ratio of 1.35% (approximately 54% of the maximum reinforcement ratio
suggested by the ACI), and with unbonded reinforcement over the full span of the beam, the loss of
ultimate capacity is about 32%.
When the unbonded length is less than 60% of the beam span, there is no loss of ultimate capacity
reported regardless of the reinforcement ratio.
Effect of reinforcement ratio on ultimate moment capacity
Effects of Various Parameters
-
Effect of corrosion on ultimate capacity
When Lub = 2200 mm or less, which is 73% from the original length (3000), steel yields, and the
reduction of ultimate capacity is due to the reduction of the cross-section of steel bars only. In other
words, if the unbonded length is less than 73% of the span length, the reduction of ultimate capacity
is due to the reduction in cross-section area (because of corrosion). Note that the relationship is
linear
Effect of Corrosion of Reinforcement
Effects of Various Parameters
-
Ultimate Strength of Deteriorated RC Beams
Steel reinforcement ratio has major effect on the ultimate flexural strength of
RC beams with unbonded steel bars
Beams with relatively low reinforcement ratio (about min ACI 318-11) are
able to maintain their original capacity even when the unbonded length is
90% of the span length
Regardless of reinforcement ratio, span to depth ratio, and loading type, all
beams were able to maintain their original capacity when the unbonded
length was less than 50% of the span length.
L
Lub
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