W2 - L5 - MRCS - Embedded Metal Corrosion - 5.pdf

Embedded metal corrosion

1. Radhakrishna G. Pillai
Department of Civil Engineering
Indian Institute of Technology Madras, Chennai, India
Repair and Maintenance of Concrete Structures
Embedded Metal Corrosion
Courtesy: Some images are sourced from the internet for demonstration purposes.
Lecture 5
Corrosion in prestressed concrete
NPTEL –MOOC Course on Maintenance and Repair of Concrete Structures

2. Outline of
Module on corrosion of embedded metal
•Significance
•Fundamentals of corrosion
•Carbonation-induced corrosion
•Chloride-induced corrosion
•Different types of steel reinforcement and precautions to be
taken
–Bare steels
–Coated steels
–Non-metallic rebars
•Corrosion in prestressed concrete

3. Illustration of prestressing of a stack of
books…
PCAP

4. •Strands are stressed before the loads are applied
–Pre-tensioning -strands are stressed before the concrete is cast
–Post-tensioning -strands are stressed after the concrete is hardened
Two types of prestressed concrete systems
Pre-tensioned systemPost-tensioned system
PCAP

5. •Strands are embedded in concrete
What is the strand protection system in
pre-tensioned concrete bridge?
Chloride ingress rate through the concrete cover will
govern the corrosion initiation and service life
Concrete
Strands
Cl-
Cl-Cl-Cl-Cl-

6. Pitting corrosion on strands and deformed
bars due to chlorides
•Deformed bar
–Early expansive
stresses, cracking and
manifestation of
corrosion products at
the concrete surface
•7-wire strand
–Initial corrosion
products occupy the
space between the
seven wires
–Delayed expansive
stresses, cracking and
manifestation of
corrosion products at
the concrete surface
Li et al. (2011)

7. Unnoticed localized corrosion can lead to catastrophic
failure mechanisms-Why?
Cross sections through the top points ( ) of each
wire within one pitch
Effective cross
section of one pitch
Clear cover
One pitch = 200 mm
Rengarajuet al. (2018)
No visible
signs of
distress until
6% cross
sectional loss
Stress concentration & early structural
distress
Sigitet al. (2007)

8. What is the strand protection system in
post-tensioned, segmental concrete bridges?
•A segmental concrete bridge consists of
many precast concrete segments
•Concrete segments are constructed in a
precast yard and then transported to the
construction site for installation
•Segments are connected using
high-strength steel tendons
Precast concrete segment
http://www.jacobsschool.ucsd.edu/news/news_releases/release.sfe?id=247
Steel
tendon
Box-girder
segment
Tendon

9. •Strands are embedded in plastic ducts and the interstitial
space is supposed to be filled with grout
•Types of grouted tendons
–Internal tendons
–External tendons
Strand protection systems in
post-tensioned concrete systems?
Tendon
Duct
Grout
StrandsHighly congested
tendon systems

10. Free-flow paths for grouts due to tendon
profiles and possible locations of voids
FHWA, TxDOT2004
A simplified schematic of a simply supported PT bridge span
(Not to scale)

A simplified schematic of a cast-in-place, continuous PT bridge span
(Not to scale)

11. Free-flow paths for grouts in
internal and external tendons
http://www.fhwa.dot.gov/bridge/pt/pt.pdf
Depending on the profile, the strands could be eccentrically
located, creating free and large spaces. Grout flows though
such free-flow paths; but, do not fill in the space between the
strands. Such exposed strands corrode prematurely

12. Grout must be pumped from the lowest points
to get complete filling of the duct
Vent
Dead end anchorage
Duct
Stressing anchorageInlet
Vents
Inlet
Vent Min.
1 mGroutflow
Grout
flow
Groutflow
Groutflow Increasing gradient
•Injection of grout should be from the lowest point of the
tendon to achieve complete filling of the duct
•Inappropriate grouting practices can significantly
reduce the quality of the PT grout and result in void
formation
http://www.fhwa.dot.gov/bridge/pt/pt.pdf

13. Keep anchors “face-down” and appropriate vents to
avoid voids at anchorage and moisture ingress
ûü
FHWA-NHI-13-026
Void formation at the ends àmoisture ingress àeventual corrosion

14. Cementitious cover/cap can attract humidity and
moisture and cause corrosion near the ends
http://www.fhwa.dot.gov/bridge/pt/pt.pdf
PT systems with ends covered with
cementitious materialsû
Heavily corroded strands
near anchorage
•Cementitious caps/coversàmoisture ingress àcorrosion
•Cementitious end caps should be covered with water-proofing material/coating
and then, if possible, a plastic cap and concrete.
•Polymeric coating may not last for the desired life of the structure –requiring
repeated applications
•Metallic caps are the best protective measure

15. Moisture ingress should be avoided by sealing
the anchor/end cap
http://www.fhwa.dot.gov/bridge/pt/pt.pdf
Crack-resistant bituminous
coating is better than
cementitious coatingü
Metallic caps are the best protective measure

16. •Rainwater was
flowing through
these joints in a
metro rail bridge
Use sufficient epoxy such that it is squeezed out and
the joints between the segments are sealed
Inadequate joints àmoisture ingress àeventual corrosion of strands

17. Water can enter the box girders through
openings at the expansion joints
http://www.fhwa.dot.gov/bridge/pt/pt.pdf
•Stagnant water can ingress the tendons through
possible cracks/damage in the ducts àstrand corrosion
•Provide drain pipes for the water to flow out of the boxes
Stagnant water
inside box girder

18. Tendon re-stressing and replacement can
become very challenging later
http://www.fhwa.dot.gov/bridge/pt/pt.pdf
Quality of grouting is important to avoid the expensive and challenging
repair later….

19. •Chances are very less that the
grout will entirely displace the
water
•Strands may get corroded and left
as it is
–There may be significant delays
between the placing of strands in
the duct and the prestressing
Avoid the filling of duct with water before
grouting
Signs of strand corrosion in a
duct that was filled with water
Stop the practice of filling the duct with water
and then expect the grout to push the water
forward, until the “entire” duct is filled with grout

20. Use of poor quality, hardened cement and
excess water
Poor grouting practices àexcessive bleedingàeventual corrosion
Pre-hydrated lump of cement

21. Other issues associated with segmental
concrete bridges
A typical precast yard
Failure of shear key

22. Perception of good performance is because
the most segmental PT bridges are young
•Studies indicate that most segmental bridges are performing well
•However, these conclusions are are heavily based on the condition of
the “young” structures
•Strand corrosion can be a major issue
7%
(31 to 40 years)
24%
(21 to 30 years)
19%
(11 to 20 years)
50%
(0 to 10 years)
Percentage of segmental bridges
within different age groups
Total number of segmental bridges
constructed in the USA
0
100
200
300
400
1970 1980 1990 2000 2010
Data 8
Total number of bridges
Year
25
0
103
165
329
FHWA

23. Premature corrosion has been observed in
many bridges
1950 1960 1970 1980 1990 2000 2010 2020
Bickton Meadows (UK)
Kohlbrand (Germany)
Angel road (UK)
Taf Fawr (UK)
Ynsys-y-Gwas (UK)
Hale Boggs (USA)
Mandovi (India)
Folly new (UK)
Melle (Belgium)
Bob Graham (USA)
Niles channel (USA)
Saint Stefano (Italy)
Lowe speedway (USA)
Mid bay (USA)
Varina-Enon (USA)
Cline avenue (USA)
Ringling causeway (USA)
Time taken for tendon failure
Time line (years)
Name of the PT bridge (country)
(20)
(17)
(17)
(05)
(12)
(33)
(21)
(30)
(15)
(16)
(08)
(13)
(30)
(40)
(XX)
(08)
(02)
(02)
•447 bridges in UK
•Ban during 1992 –1996

24. •Many tendons are not adequately
protected due to inadequate designof
grout materials and/or poor construction
practices
•Such situations have led to tendon failures
in critical bridges on the US highway
network
Issues associated with segmental concrete
bridges
For bridges, not only the
look from far away, but
also the performance
are important
Tendons in PT bridges have failed at 4, 8, 13, and 16 years of service

25. Premature corrosion has been observed
in many bridges
•Varina-Enon, Virginia
•At 17 years
FDOT report, 2003
Anchorage zone in a PT girder

26. Premature corrosion has been observed
in many bridges
•Sunshine Skyway, Florida
•At 8 years
FDOT 2001
Anchorage zone in PT column

27. Damaged ducts, voids in ducts, water in
bridges, and corroded strands
Strands in segmental bridges can be susceptible to
corrosion at relatively young ages
Damaged ducts of tendons
Water near tendons in bridgeCorroded strands in tendons
Voids in tendons
TxDOT2004

28. An example of PT bridge collapse due to
strand corrosion
Collapsed Ynys-y-Gwas Bridge
YouTube
Strand corrosion in the girder led to the collapse of Ynys-y-Gwas
bridge in the United Kingdom in 1985 (after 33 years of service)

29. Direct exposure to moisture/chlorides accelerates
corrosion and have led to failure of tendons
Hansen 2007
Corrosion in piers of Sunshine-skyway bridge in Florida
Pitting corrosion on a prestressing strand

30. Direct exposure to moisture/chlorides accelerates
corrosion and have led to failure of tendons
Hansen 2007
•Tendons with voids have failed at 4, 8, 13, and 16 years of service
•Occurred without warning, raising concerns about the safety
•and time-variant reliability of these bridges
Corrosion in the girder of Varina-Enon bridge in Virginia

31. Why such premature corrosion?
Voids can form inside the ducts and expose the
strands
Pillai, R.G., 2009, “Electrochemical Characterization and Time-Variant Structural Reliability Assessment of Post-Tensioned,
Segmental Concrete Bridges”, PhD thesis, Texas A&M University, Texas.
Anchorage zone in a PT girderAnchorage zone in PT column
End of strand
Semi-circular
grout face
•A metro rail bridge

32. Why such premature corrosion?
Chloride ingress àCorrosion of exposed strands at
the voids
End of strand
Semi-circular
grout face
Chlorides
Corroded strands at the upper portion where
grout not present

33. Corrosion mechanisms in prestressed
systems
TendonStrand
Wire
An autopsied tendon with
segregated grouts,
congested strands, and
corroded strands

34. Are the existing PT grouts good enough?
-Formation of soft grout should be avoided
Inadequate specifications àVoids àPremature corrosion

35. •If the tendon is fully grouted, then one may not see the
semi-circular end face
Visual observations as performance tests
End of strand
Semi-circular
grout face
An end-cap –top portion of
the tendon has been
corroded
An evidence of partially-filled tendon
http://www.fhwa.dot.gov/publications/research/infrastructure/structures/bridge/13028/

36. Tendons are the backbone of
pre-and post-tensioned concrete structures

We must do whatever we can to prevent corrosion
of these expensive and important systems
–an extra care in achieving this is really worth in
the long term.

37. •Prestressed concrete can be more vulnerable to corrosion and lead to
catastrophic failure
•Strand corrosion will take more amount and time to manifest as stains
on concrete surfaces
•Good grouting practices are very important to ensure durability of post-
tensioned structures
Summary

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