Corrosion Failure of Post -Tensioned Tendons in Presence ...· INTRODUCTION: Post-Tensioned Strand

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  • Corrosion Failure of Post-Tensioned Tendons in Presence of Deficient Grout

    Kingsley Lau, Florida International UniversityIvan Lasa, FDOT; S. Permeh and K.K. Krishna Vigneshwaran, FIU

    Tom Maze Transportation Seminar, Iowa State University, February 26, 2016

  • INTRODUCTION: Post-Tensioned Strand Corrosion

    Several documented cases of corrosion-related failures of post-tensioned (PT) strand in FL.

    Corrosion identified in early 2000s 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 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

  • Segregated Chalky Grout

    Corrosion Product

    A

    B

    C

    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.

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    26 27 Bottom

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    32 Top Soft

    33 Top

    34 Top

    34 Top

    Black

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    47 Top

    47 Bottom

    48 Top

    48 Bottom

    Moi

    stur

    e 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

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    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

    Moi

    stur

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    nten

    t /%

    Tendon Section

    Corrosion Failure/ Wet Plastic Grout

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    12 Bot 27 Bot 33 Top 35 33A 33B Soft

    W. Anch. Soft

    Soft Soft Soft Soft Soft Soft

    405-6 404-9 401-9U 401-9D 402-9 403-6 403-9 CT

    Pore

    Wta

    er p

    H

    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

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    Tota

    l Chl

    orid

    e /m

    g Clg

    -1gr

    out

    Moisture Content /%

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    Free

    Chl

    orid

    e Co

    ncen

    trat

    ion

    (ppm

    )

    Segregated Grout Properties: Chloride Content

    Low chloride content in grout. Below conventional chloride corrosion threshold concentrations.

    Apparent 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 (12).

    Assuming 67% cement content in grout, upper range 0.3mg/g would correspond to 0.05% cement which is

  • Segregated Grout Properties: Grout Content

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    Cum

    ulat

    ive

    Frac

    tion

    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 4 44 1 46 4 1 6 4 4

    1 1 14

    3 4 4 42

    6 42 2

    3 2 4 4 6 1

    A3

    A 4

    Grout

    Crystalline Material

    Pore Outline

    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 vicinity of segregated grout as well.

    TopBottom

    + Gypsum Ettringite None

    Possible indicator of enhanced sulfate presence segregated grout free water.

    10 60

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    12 Bot

    27 Bot

    33 Top

    35 33A 33B W. Anch

    405-6 404-9 401-9 402-9 403-6 403-9 CT

    Conc

    entr

    atio

    n (p

    pm)

    Tendon Section

    Ca 2+Cl-

    SO4 2-

    Segregated Grout Properties: Pore Water Content

    Sulfate concentrations as high as 9700 ppm measured in pore water.

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    12 Bot

    27 Bot

    33 Top

    35 33A 33B W. Anch

    405-6 404-9 401-9 402-9 403-6 403-9 CT

    Conc

    entr

    atio

    n (p

    pm)

    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

    0.47 mgCl/gdrypH 9.87800 ppm SO42-

    0.25 mgCl/gdrypH 11.97100 ppm SO42-

    0.14 mgCl/gdrypH 12.755% moisture4300 ppm SO42-

    0.18 mgCl/gdrypH 12.744% moisture1900 ppm SO42-

    0.09 mgCl/gdrypH 12.714% moisture310 ppm SO42-

    Corrosion

  • -

    -

    -

    -

    Corrosion

    I

    II

    III

    IV

    V

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    12

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    0.01

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    1

    4021

    0L I

    4021

    0L II

    4021

    0L II

    I

    4021

    0L IV

    4021

    0L V

    pH

    Tota

    l Chl

    orid

    e Co

    nten

    t / m

    g Cl-

    mg d

    ry-1

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    NEW

    Moi

    stur

    e Co

    nten

    t %

    Conc

    entr

    atio

    n (p

    pm)

    Ca 2+K +Na +Cl -NO3 2-SO4 2-Moisture Content

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    ulat

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    tion

    Moisture Content, %

    Hard Grout

    Deficient Grout

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    ulat

    ive

    Frac

    tion

    Chloride Content, lb/yd3

    Hard Grout

    Deficient Grout

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    Cum

    ulat

    ive

    Frac

    tion

    Sulfate Content, ppm

    Hard Grout

    Deficient Grout

    At least five other bridges contained grout with some form of deficiency. Extent of grout deficiencies vary greatly by bridge. Grout leachate remains alkaline. High chlorides found in some bridges (but thought to be unrelated to segregation). Deficient grout shows higher moisture and free sulfate concentrations.

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    ulat

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    pH

    Hard Grout

    Deficient Grout

    Grout Condition of Recently Inspected Florida PT Bridges

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    Sulfa

    te C

    onte

    nt, p

    pm

    Moisture Content, %

    Hard Grout

    Deficient Grout0.01

    0.1

    1

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    0 10 20 30 40 50

    Chlo

    ride

    Cont

    ent,

    lb/y

    d3

    Moisture Content, %

    Hard Grout

    Deficient Grout

    High chlorides found in some bridges (but thought to be unrelated to segregation).

    Deficient grout shows higher moisture and free sulfate concentrations.

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    -350 -300 -250 -200 -150 -100 -50 0

    Cum

    ulat

    ive

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    tion

    E /mVCSE

    12 Inside

    20 I

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