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CORROSION DEVELOPMENT OF POST‐TENSIONED TENDONS WITH
DEFICIENT GROUT
Kingsley Lau and Mario Paredes
Florida Department of Transportation, State Materials Office5007 NE 39th Ave. Gainesville, FL 32609
2012 PTI Convention Nashville, Tennessee. May 7, 2012Durability of Post‐Tensioned Infrastructure
INTRODUCTION: Post‐Tensioned Strand Corrosion
• Several Documented Cases of Corrosion‐Related Failures of Post‐Tensioned (PT) Strand in FL.
• Corrosion identified in early 2000’s attributed to grout void formation due to bleed water formation and chloride presence.
• Subsequent specifications in FL include low bleed grout requirements.
• However, corrosion problems persist.
Investigation of recent PT corrosion and repair issues are on‐going. This presentation overviews current findings from field and laboratory explorations of deficient grout in tendons.
Photo courtesy of Parsons Brinckerhoff
BACKGROUND: PT Bridge in Florida built in 2002
• PT segmental bridge with int. and ext. tendons.• Among first FL bridges to use low bleed grouts.• Ext. Tendons placed to reduce tensile stresses in web. Anchor caps at low elevation.
• Severe corrosion in multiple ext. tendons. Failure occurred after ~8 years service.
• Severe corrosion accompanied by wet plastic grout.
Pier
Anchor Cap
PierDeviators
• Grout segregation characterized as:• A. Wet plastic • B. Sedimented Silica• C. White chalky A
A
C
View of tendon top
SEGREGATED GROUT
Corrosion attributed to wet plastic grout but not necessarily to void presence.Grout segregation created environment with dissimilar pore water chemistry and physical properties.
0
10
20
30
40
50
60
70
80
12 Top
12 Bottom
20 Top
20 Bottom
26 27 Bottom
29 Bottom
30 Bottom
31 Bottom
32 Top
32 Top Soft
33 Top
34 Top
34 Top Black
35 Bottom
47 Top
47 Bottom
48 Top
48 Bottom
Moisture Co
nten
t /%
Tendon Section
Corrosion Failure/ Chalky Grout
Segregated Grout Properties: Moisture Content
High moisture content associated with segregated grout.• ~50% moisture in white chalky grout; ~70% moisture in wet plastic grout
0
10
20
30
40
50
60
70
80
East Anchor
E Low Eeviator
36B Top 34B Top/Soft
34B Bot 33B Top/Soft
33B Top 33B Bot 33A Bot 32A Bot 33A Bot/Dev
32A Bot 32B Bot 31B Bot 30A Top 28B Top
Moisture Co
nten
t /%
Tendon Section
Corrosion Failure/ Wet Plastic Grout
10
11
12
13
14
12 Bot 27 Bot 33 Top 35 33A 33B So †
W. Anch. So †
So † So † So † So † So † So †
405‐6 404‐9 401‐9U 401‐9D 402‐9 403‐6 403‐9 CT
Pore W
taer pH
Tendon Section
Segregated Grout Properties: Pore Water pH
Pore water of segregated grout typically retains high pH.• No indication of processes to decrease pH such as carbonation.• Regions with accumulation of corrosion products may contain localized low pH environments.
Regions with severe corrosion
† Prepared by Ex‐situ Leaching Method
0.01
0.1
1
1 10 100
Total Chloride /m
g Clg
‐1grou
t
Moisture Content /%
10
100
1000
Free
Chloride Co
ncen
tration (ppm
)
Segregated Grout Properties: Chloride Content
Low chloride content associated with segregated grout. Below conventional chloride corrosion threshold concentrations.
• Accumulated chloride content in moist grout may be due to ionic transport.• However, total chloride test preparation methods may over‐sample size of segregated grout thus higher reported chloride concentrations.
• Assuming threshold ratio [Cl‐]/[OH‐] = 0.3,and an upper measured free Cl concentration ~50 to 100 ppm, threshold pore water pH (<11.5‐12) < observed pH values (typ. >12).
• Assuming 67% cement content in grout, upper range 0.3mg/g Cl would correspond to 0.05% cement which is <<CT.
Segregated Grout Properties: Grout Content
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1 10 100 1000
Cumulative Fractio
n
XRF Counts Sulfur /103 s‐1
Top
Mix
Bottom
New
Others
10 20 30 40 50 60
Dev/Light
Dev/Dark
32A Bot
32B Top/Soft
32B Bot
31B Top/Dark
31B Top/Dark
31B Top
31B Bot/Dark
31B Bot/Rust
3A 1
1 6
B1A 1 6 1 1 1 A 1
3 A A
2 3
4 4
A
44 4 4 4 444 1 46 4 1 6 4 4
1 1 143 4 4 4 2
6 42 2
3 2 4 4 6 1
A3
A 4
Gypsum Ettringite
• Apparent enhanced presence of sulfurous compounds in segregated grout.
• Gypsum and ettringite identified as sulfur bearing crystalline compounds.
• Ettringite‐filled voids visually predominant in segregated grout; however, identified in grout in vicinity of segregated grout as well.
TopBottom
+ GypsumΔ Ettringite○ None
Possible indicator of enhanced sulfate presence segregated grout free water.
10 60
1
10
100
1000
10000
12 Bot
27 Bot
33 Top
35 33A 33B† W. Anch†
405‐6 404‐9 401‐9† 402‐9† 403‐6† 403‐9† CT†
Concen
tration (ppm
)
Tendon Section
Ca 2+Cl‐
SO4 2‐
Segregated Grout Properties: Pore Water Content
Sulfate concentrations as high as 9700 ppm measured in pore water.• Apparent lower concentrations of Na+ , K+ , and OH‐ and the higher Ca2+concentration in the segregated grout than elsewhere.
100
1000
10000
100000
12 Bot
27 Bot
33 Top
35 33A 33B† W. Anch†
405‐6 404‐9 401‐9† 402‐9† 403‐6† 403‐9† CT†
Concen
tration (ppm
)
Tendon Section
Na+K+
Pier
Anchor Cap
PierDeviators
Corrosion and similar deficient grout characteristics observed at low elevations, too.
Corrosion at Low Elevation Anchor Caps
I
II
III
IV
V
0
10
20
30
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60
70
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90
100
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
40210L I
40210L II
40210L III
40210L IV
40210L V
NEW
Moisture Co
ntent %
Concen
tration (ppm
)
SO42‐
NO32‐Cl‐
Na+
K+
Ca2+
7
8
9
10
11
12
13
0.01
0.1
1
40210L I
40210L II
40210L III
40210L IV
40210L V
pH
Total Chloride Co
nten
t / mg C
l‐mg d
ry‐1
Estimated
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
‐350 ‐300 ‐250 ‐200 ‐150 ‐100 ‐50 0
Cumulative Fractio
n
E /mVCSE
12 Inside
20 Inside
27 Inside
32 Inside
33 Inside
34 Inside
35 Inside
47 Inside
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
‐400 ‐300 ‐200 ‐100 0 100
Cumulative Fractio
n
E /mVCSE
28
29
30
31
34
35
36
45
51
CORROSION CONDITIONS IN SEGREGATED GROUT
Variation in open‐circuit potential characterizing local anodes in wet plastic grout and passive steel elsewhere.
• Differences in OCP develop macrocell corrosion.
Ecorr, active
Icorr,activeIcorr, active- macrocell
ηa
E
I
Total macrocell behavior
‐500‐450‐400‐350‐300‐250‐200‐150‐100‐500
1 10 100
E /m
V SCE
Time /dy
10
100
1000
0 5 10 15 20 25 30
I corr/uA
Time /dy
Macrocell CorrosionHardened Grout
Wet Plastic Interspersed with Hardened Grout
Titanium Ref and Aux Electrodes
Cement Paste
Epoxy
l
Hard
Segregated
Coupled
CoupledSegregated
Hard
• Corrosion activity in wet plastic grout.Anodic polarization of active steel due to coupling with passive steel enhances corrosion rates.
‐50
0
50
100
150
0.01 0.1 1 10 100
I macro/uA
Time /dy
A
B
C
Large anodic macrocell currents developed in lab tests of field samples.
‐0.8
‐0.7
‐0.6
‐0.5
‐0.4
‐0.3
‐0.2
‐0.1
0
0.1
1.00E‐09 1.00E‐08 1.00E‐07 1.00E‐06 1.00E‐05 1.00E‐04 1.00E‐0
E /m
V SCE
I /A
‐0.8
‐0.7
‐0.6
‐0.5
‐0.4
‐0.3
‐0.2
‐0.1
0
0.1
1.E‐09 1.E‐08 1.E‐07 1.E‐06 1.E‐05 1.E‐04 1.E‐03
E /m
V SCE
I /A
E /V
SCE
E /V
SCE
0.0E+00
5.0E‐06
1.0E‐05
1.5E‐05
2.0E‐05
2.5E‐05
3.0E‐05
0 200 400 600 800 1000 1200 1400 1600
I /A
Time /s
0 ppm Na2SO4
20,000 ppm Na2SO4cathodic
anodic
0.1
1
10
100
1000
SPS A SPS B SPS A SPS B SPS A SPS B
0 ppm sodium sulfate
2000 ppm sodium sulfate
20000 ppm sodium sulfate
I at ‐10
0mV S
CE/uA
Clean
Pre‐rusted
Role of Sulfate Ions
High sulfate concentrations appear to have negative impact on steel passivation.
pH 12.6
pH 13.3pH 13.3
Corrosion activity measured in lab samples with high concentration of sulfates.
pH 13.3 pH 13.3
20,000 ppm Na2SO4
0.001
0.01
0.1
1
10
100
0 50 100 150 200 250 300
Macrocell Cu
rren
t / uA
Time / days
C‐G1‐V3‐1 (High RH/12.6 pH + SO4)
C‐G1‐V3‐2 (High RH/12.6 pH + SO4)
High RH pH: 12.6 2,000 ppm Na2SO4
‐0.700
‐0.600
‐0.500
‐0.400
‐0.300
‐0.200
‐0.100
0.000
0 50 100 150 200 250 300
E / V
SCE
Time / days
C‐G1‐V3‐1 (High RH/12.6 pH + SO4)
C‐G1‐V3‐2 (High RH/12.6 pH + SO4)
High RH pH: 12.6 2,000 ppm Na2SO4
Corrosion activity measured in lab samples with high concentration of sulfates.• Research is in progress to resolve the role of sulfates.
TENTATIVE PROPOSED CORROSION MECHANISM
WORK IN PROGRESS
High water content was present in the grout.
High water content carried higher concentrations of ionic species including sulfates and chloride ions. High concentrations of sulfurous compounds (sulfates in solution) were present in the grout.
Segregated grout material provided poor corrosion protection for embedded strand that did not attain uniform stable passivity.
Differential aeration condition present due to easy access to oxygen, vastly different moisture contents in localized regions, and strand interstitial spaces creating crevices.
Accelerated corrosion was caused by macrocell coupling of local anodes in the strand embedded in segregated grout and extended cathode throughout the tendon.
• Identifying physical parameters of grout segregation mechanism.
• Resolving role of chloride, sulfates, and pH on steel depassivation in deficient grout.
• Identifying possible corrosion after repairs of deficient tendons.
CONCLUSIONS Segregated grout material was observed in localized portions of the tendons, characterized
as: wet plastic, white chalky, and sedimented grout.
High water content (up to 70 wt%) was measured in grout from localized portions of the tendons where corrosion occurred.
Chloride accumulation was apparent in grout with high water content but concentrations were lower than conventional critical chloride threshold concentrations.
No significant drop in pore water pH was observed in limited amount of sampling. However, slight depression of pore water (pH>11) from the wet plastic grout was apparent.
Sulfate concentrations in the grout pore water were as high as 9700 ppm.
Corrosion in segregated grout (signified by high moisture content, high porosity, and high sulfate concentrations) was likely accelerated due to macrocell coupling with extended cathode throughout the tendon.
ACKNOWLEDGMENTS
The work and assistance by Dennis Baldi, Will Blanchard, Matt Brosman, Jason Burchfield, Pat Carlton, Charles Ishee, Marc Knapp, Awilda Merced, Richard Nalli, Juan Rafols, Nikita Reed, and Yongyang Tang is acknowledged here.
Cooperative work with Michael Ahern, Andrew Gillis, Richard Lewis, Randy Poston, and Phillip Sharff and assistance by Marcus Lee and John Newton is also acknowledged.
QUESTIONS???