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 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 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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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

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

e Co

nten

t /%

Tendon Section

Corrosion Failure/ Wet Plastic Grout

10

11

12

13

14

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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100

1000

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 (<11.5-12) < observed pH values (typ. >12).

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

Segregated Grout Properties: Grout Content

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ulat

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XRF Counts Sulfur /103 s-1

Top

Mix

Bottom

New

Others

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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 4 26 4

2 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

+ GypsumΔ Ettringite○ None

Possible indicator of enhanced sulfate presence segregated grout free water.

10 60

1

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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†

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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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†

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 SO4

2-

0.25 mgCl/gdrypH 11.97100 ppm SO4

2-

0.14 mgCl/gdrypH 12.755% moisture4300 ppm SO4

2-

0.18 mgCl/gdrypH 12.744% moisture1900 ppm SO4

2-

0.09 mgCl/gdrypH 12.714% moisture310 ppm SO4

2-

Corrosion

-

-

-

-

Corrosion

I

II

III

IV

V

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0.1

1

4021

0L I

4021

0L II

4021

0L II

I

4021

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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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Moisture Content, %

Hard Grout

Deficient Grout

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Chloride Content, lb/yd3

Hard Grout

Deficient Grout

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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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Cum

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ive

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tion

pH

Hard Grout

Deficient Grout

Grout Condition of Recently Inspected Florida PT Bridges

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10000

0 20 40 60 80 100

Sulfa

te C

onte

nt, p

pm

Moisture Content, %

Hard Grout

Deficient Grout0.01

0.1

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100

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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E /mVCSE

12 Inside

20 Inside

27 Inside

32 Inside

33 Inside

34 Inside

35 Inside

47 Inside

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E /mVCSE

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51

PRELIMINARY CORROSION TESTING

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

Polarization Resistance

• Surface rust was apparent on samples with deficient grout.

• Rewetting of interstitial spaces may worsen condition; Possible concern for regrouting repair procedures.Differentiation from passive corrosion

conditions was apparent for samples with wet/plastic grout, generally indicating corrosivity of that grout condition.

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Appa

rent

i cor

r/µ

Acm

-2

Time /dys

32

47

21

1A1B1C 2AT

2AB2BT2BB2CT2CB

icorr

A

cm2 = BRP

32

21

471A 1B

1C

Macrocell Corrosion Hardened Grout

Wet Plastic Interspersed with Hardened Grout

Titanium Ref and Aux Electrodes

Cement Paste

Epoxy

l

Anodic polarization of active steel due to coupling with passive steel enhances corrosion rates.

-50

0

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100

150

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I mac

ro/u

A

Time /dy

A

B

C

-500-450-400-350-300-250-200-150-100

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1 10 100

E /m

V SCE

Time /dy

Hard

Segregated

Coupled

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1000

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I corr

/uA

Time /dy

CoupledSegregated

Hard

Evaluation of Corrosion Behavior in Enhanced Sulfates Conditions

Tendon Mockup

• 15 ft tendon mockup cast with varying sulfates and chlorides addition to the mix water.

• 0 %, ~0.09%,~0.9%,~5.5% sulfates by cement weight.• 0.08% , 0.2% chlorides by cement weight.• (Grout was past expiration and mix had excess water).• Crevice and Non crevice steel probes were embedded in

the grout.• The tendons were inclined at 30°.

Studied parameters: corrosion potential (OCP), polarization resistance and anodic characterization

Sulfate Content

• The test results indicate the top part of the tendons tend to have higher levels of sulfate concentrations.

• In comparison to the resolved sulfate content measured in grout from Bridge I and II, it is seen that separation of grout material can allow for accumulation of sulfate ions without external sulfate sources.

Corrosion

Current Density Corrosion Potential

• Corrosion potentials and current density indicative of active corrosion in top portion of tendon (enhanced sulfates).

• Similar corrosion activity with intentional crevice condition.

Corrosion Behavior In Mock-up Tendons

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-400

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-100

0 50 100 150 200 250 300 350 400 450

Corr

osio

n Po

tent

ial /

mV cs

e

Days

As Rcvd Top As Rcvd BottomMed Sulf Top Med Sulf BottomHigh Sulf Top High Sulf BottomLow Sulf Top Low Sulf BottomLow Chlorides Top Low Chlorides BottomHigh Chlorides Top High Chlorides Bottom

0.001

0.01

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10

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Cu

rre

nt

De

nsi

ty I/

µA

cm

-2

Days

Low Sulf Top Low Sulf Bottom

Low Chlorides Top Low Chlorides Bottom

High Chlorides Top High Chlorides Bottom

As Rcvd Top As Rcvd Bottom

Med Sulf Top Med Sulf Bottom

High Sulf Top High Sulf Bottom

Solution Resistance of Groutin Mock-up Tendons

Grout at high elevations have lower solution resistance. Grout deficiency appears more severe at high elevation.

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1000

10000

Top Bottom Top Bottom Top Bottom Top Bottom Top Bottom Top Bottom

As Received Low Sulfates Med Sulfates High Sulfates Low chlorides High Chlorides

Tendon Mockup Steel Probe Position and Admixed Sulfate Levels

Solu

tion

Resis

tane

/ oh

m

Deficient Grout

Deficient Grout

Corroded steel probefrom mockup (Top)

• Indication of passive film development in all conditions. • Larger passive currents in deficient grout in top portion than bottom portion of

tendons.• Abrupt increase in anodic current at lower potentials indicative of breakdown of

passive-like conditions in high admixed sulfate conditions. Indication of possible role of sulfates in steel corrosion initiation.

Sulfates were though to be related to localized corrosion process by reaction i.e. Fe2+ + 2H2O + SO4

2- —-> Fe(OH)2 + H2SO4 (Jones.1992)

Anodic Characterization of Steelin Mock-Up Tendons

Top Bottom

Small cells filled with alkaline solution 1 and 2 (pH ~12.5 and ~13.3)• Steel test samples: 1.5 inch long, 1/4 inch diameter steel wire • Ref. and Ctr. Electrodes: Activated titanium rods• Testing conducted in pre-mixed alkaline solutions with containing

0 ppm, 2,000 ppm or 20,000 ppm sodium sulfate. • Some samples subjected to incremental additions of sodium sulfate

(5,000 ppm weekly for a month (up to 65,000 ppm)) • Other conditions: crevice and elevated temperature

Electrochemical Tests of Steel in Sulfate Solutions

First Set of Testing• Maintained at steady-state open

circuit potential condition.• Corrosion potentials with a

saturated calomel reference electrode (SCE).

• Linear polarization resistance measurements.

• Pre-mixed or incremental admixed sulfate solutions.

Second Set of Testing• Anodic Polarization Measurements starts

from -1VSCE to +500mVSCE with scan rate 0.05mV/s.

• Pre-conditioned at open-circuit potential or at -1VSCE for 1 day were made

• Conditioned at -1VSCE for at least 30 minutes prior to testing

• Sulfates in pH 13.3 showed no effect on initiating or enhancing corrosion in any of the test conditions at OCP.

• The corrosion potential for the elevated temperature condition was more negative

• Stable passive film can develop for steel immersed in pH 13.3 solution with little adverse effect from sulfate ion presence. Sulfate ions cannot destabilize already developed passive film.

RESULTS : LPR AND OCP (pH 13.3)

• Anodic polarization curves were not well differentiated by sulfate ion presence .

• Indication of passive-like behavior for all test scenarios

• Sulfate ions have shown minor indication to increase the anodic current exchange density. Crevice and enhanced temperature did not seem to aggravate corrosion conditions.

RESULTS : ANODIC POLARIZATION (pH 13.3)

1-dy OCP Pre-Conditioning 1-dy Cathodic Pre-Conditioning

No corrosion was observed for the sulfate-free and 2,000ppm sodium sulfate solution and also when incremental sulfates (up to 65,000 ppm sodium sulfate) were added.

In pH 12.5 solutions, sulfates may not be able to depassivate steel, but the early presence of sulfates may destabilize passive film growth.

RESULT : LPR AND OCP (pH ~12.5)

1-dy OCP Pre-Conditioning• Differentiation in anodic behavior was observed with the presence of sulfate ions.• With the admixed presence of 2,000 ppm sodium sulfate, the passive current density

showed an approximate threefold increase.• The polarization graphs show transition from passive to active corrosion behavior of steel

in solution with the admixed presence of 20,000 ppm sodium sulfate.

1-dy Cathodic Pre-Conditioning• Pitting corrosion occurred for samples exposed with 20,000 ppm sodium sulfate in

samples subjected to extended cathodic pre-conditioning.

RESULTS : ANODIC POLARIZATION (pH ~12.5)

1-dy OCP Pre-Conditioning 1-dy Cathodic Pre-Conditioning

Corrosion

Pitting Corrosion

• Enhanced corrosion activity with cathodic pre-conditioning (early passive film instab).

• Generally larger corrosion currents in sulfate solutions with localized corrosion.

• Lower corrosion current density calculated for sample with pitting corrosion due to small pit areas.

Corrosion Behavior In Sulfate Solution30 mins Conditioning 1 - day Conditioning

Sulfate Levels for Corrosion Development

• Important role of:• Solution pH,• Early presence of sulfate ions, • Level of sulfate presence,• and Cement content.

Acknowledgments

Conclusions• Segregated deficient grout typically contains high moisture content that

maintains high pH and low chloride levels.• Enhanced sulfate concentrations in segregated deficient grout coincident

with severe corrosion development.• High sulfates levels can occur even without external sulfate source.• Segregated grout material provided poor corrosion protection for

embedded strand that did not attain uniform stable passivity. • Stable passive film apparent for steel immersed in pH 13.3 solution with

little adverse effect from sulfate ion presence. Sulfate ions not readily able to destabilize already developed passive film.

• In pH 12.5 solutions, sulfates may not be able to depassivate steel, but the early presence of sulfates may destabilize passive film growth.

• Pitting corrosion conditions apparent in initial stage of degradation due to destabilization of passive film. Active corrosion afterwards.

• Accelerated corrosion likely enhanced by macrocell coupling of local anodes in the strand embedded in segregated grout and extended cathode throughout the tendon.

Acknowledgments

Acknowledgment

• This work was supported by the Florida Department of Transportation (FDOT). The opinions, findings, and conclusions expressed in this publication are those of the author(s) and not necessarily those of the Florida Department of Transportation or the U.S Department of Transportation .

• Support from the FDOT State Materials Office and assistance by Dr. H.R. Hamilton is acknowledged here.

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Water T S

Water T M/W

ANY HINTS FROM GROUTING RECORDS?

Records from one construction project.Initial material deficiencies identified and greater awareness made early .

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Pumping Pressure /lb in-2

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Specific Gravity

• Grout Pump Time: Time Pumping Completed-Time Pumping Started.• Apparent Batch Time: Cumulative Time of Mixing Grout Batches/Total # of Bags.• Elapsed Pumping Time: Cumulative Pumping Time of a Series of Tendons.

• 132 tendons reviewed.• 42% used Grout A, 58% used other approved grouts.• Soft grout observed on 14% of 132 tendons (84% of Grout A).

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PROPOSED CORROSION MECHANISM• High water content was present in the grout.

• High water content carried higher concentrations of ionic species including sulfates 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.

0

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-Z" /

ohm

Z' /ohm

day 0

day 1

day 2

day 5

day 9

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-Z" /

ohm

Z' /ohm

day 0

day 1

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day 5

day 9

1 mHz

Lau, Paredes, and Rafols. Corrosion/2012 No. 1738

ZT [ ]Ω = RS + 1

1RC

+ YoC (jω)nC +

11RP

+ YoM (jω)nM

• Two distinct impedance loops was observed for all test samples• High frequency limit impedance corresponds to solution resistance, Rs.• Intermediate frequency limit impedance , Rc, related to moisture content and

permeability of existing grout (including interstitial spaces)• CPE at intermediate frequencies representative of dielectric properties of grout.• Low frequency limit impedance, Rp, corresponds to polarization resistance• CPE behavior at low frequencies representative of steel interfacial capacitance. Impedance equivalent circuit analog considered as a first approach to interpret

impedance behavior.

CORROSION CONDITIONS IN SEGREGATED GROUT: EIS

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Rc /

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Time /dy

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1A1B1C2AT2AB2BT2BB2CT2CB

Grey/Hardened

Segregated

1

10

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Rs /

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Time /dy

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1A1B1C2AT2AB2BT2BB2CT2CB

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3pt R

esist

ance

/oh

m

Rs /ohm

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1A1B1C

Solution Resistance

• High frequency limit impedance corresponded to solution resistance, Rs.

• Rs trend with time representative of cement hydration of the introduced paste.

• Intermediate frequency limit impedance , Rc, showed distinction between hardened grout with low moisture content and segregated grout with high moisture content.

Method 1 Crushed to pass a No. 100 sieve

Dry the powder samples at 55oC

for 24 hours

1:10 leaching volume

Place on Hotplate with 66 C for 15-18

hours

Filter, and Dilute leachate into

100mL solution

Method 2 Crushed to pass a No. 100 sieve

1:10 leaching volume

Stay at room temperature for

1 week

Filter, and Dilute leachate into

100mL solution

Method 3 crushed to pass a No. 100 sieve

1:1 leaching volume

Stay at room temperature for

1 week

Collecting the leachate solution

Sulfate ion Analysis• Enhanced sulfate(SO42-) content was found in deficient grout.• Rapid change of moisture content due to environmental exposure and time of

exposure.

Test methodology for uniform testing of discrepant grout sample conditioning.

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

Top 1 Top 2 Middle Bottom1

Bottom2

Top 1 Top 2 Middle Bottom1

Top Middle Bottom

Zero Sulfate Intermed Sulfate High sulfate SoftGrout

HardGrout

SoftGrout

HardGrout

Expired Grout A Bridge I Bridge II

Sulfa

te C

once

ntra

tion/

gSu

lfate

.gpo

wde

r-1

Method 1

Method 2

Method 3

Max

Min