riyad aboutaha, phd, f-aci syracuse university0 100 200 300 400 500 600 700 800 900 1000 1100 1200...

Syracuse University

Riyad Aboutaha, PhD, F-ACI

ASCE Expo 2013 Syracuse, NY

November 11, 2013

• 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.


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.


• 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.

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


Type – 2

8 Φ12

Stirrups Φ6/150

fcu =39.4 MPa

fy = 550 MPa


Type – 3

Original Column Information

4Φ8

Stirrups Φ6/150

fcu = 35.8 MPa

fy = 550 MPa


Type – 1

4Φ16

Stirrups Φ6/150

fcu = 35.6 MPa

fy = 550 MPa


Type – 2

8 Φ12

Stirrups Φ6/150

fcu =39.4 MPa

fy = 550 MPa


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)

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%


12 # 9


fcu = 4 ksi

fy = 60 ksi

C/d = 1.0, 1.5, 2.0, and 2.5

As = 2.08%


12 # 11


fcu = 4 ksi

fy = 60 ksi

C/d = 1.0, 1.5, 2.0, and 2.5

As = 3.25%


12 # 11


fcu = 4 ksi

fy = 60 ksi

C/d = 1.0, 1.5, 2.0, and 2.5

As = 3.25%


12 # 14


fcu = 4 ksi

fy = 60 ksi

C/d = 1.0, 1.5, 2.0, and 2.5

As = 4.69%


12 # 14


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.

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

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.


deterioration is at tension side of the column section.

Effects of Amount of Corrosion and Loss of Concrete


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)


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)


deterioration is at left side of the column section.


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%).


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


deterioration is at extreme tension side of the column section

(Amount of Corrosion = 10%).


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%


Clear Cover = 2.26"

Pn (

kip

s)

Mn (kip-ft)


Case – IV : Corrosion at all bars


deterioration is at all sides of the column section (Amount of

Corrosion = 10%).


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%


Clear Cover = 2.26"

Pn (

kip

s)

Mn (kip-ft)


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.



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.



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


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


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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