uct condass potentialmapping2013-01-29 [kompatibilitätsmodus] · 2016-04-14 · - normally given...

1

CE

Technical Workshop

Condition Assessment using classical and d N D t ti T ti th dmodern Non-Destructive Testing methods

Potential Mapping

Alexander Taffefrom

in cooperation withChristian Sodeikat

Schießl – Gehlen – Sodeikat (Munich, Germany)

1Cape Town / Johannesburg 14./15.02.2013 - [email protected]

Condition Assessment using NDT-CE – Potential Mapping

CEContentContent

Damage pattern: Chloride induced and carbonation induced corrosion

Electrochemical basics:Galvanic half-cell, Galvanic element, steel in concrete

Application of potential mapping:Preparation, measurement procedure, interpretation of results

Examples with potential mapping



2

CEWhy important? Why a challenge?Why important? Why a challenge?

Huge amount of damages from chloride induced corrosion (de-icing salts or seawater)=> concentrated loss of cross section in rebars

Difficult location of chloride induced corrosion => in many cases no spalling

Potential mapping:Quick and reliable method for the detection of areas of active corrosion – mainly chloride inducedy

Easy to handle but correct interpretation needs experience!



CEChloride induced corrosionChloride induced corrosion

XD

urfa

ce

De-

icin

g Pitting corrosion Rust-colored pattern

Up to total loss of section o sp

allin

g at

the

s

D

Fractured tendon ducts

Ofte

n no



3

CECarbonation induced corrosionCarbonation induced corrosion

XCn

Typical spalling

One-sided slow loss of cross

arbo

natio

n

Changing climate: Dry and wet

loss of cross section over large areas

Ca



CEGalvanic halfGalvanic half--cellcell

Balance at the interface metal/electrolyte- iron-ions at steel surface- electrons in steelElectric voltage

Maintenance of the balance by difference of electric voltage between electrolyte (concrete) and metal (reinforcement)

=> Permanent electrochemical potential

(metal)

halfcell 1

Fe++ Fe++ Fe++ Fe++ Fe++

e- e- e- e- e- e- e- e- e-

e- e- e- e- e- e- e- e- e-

e-

e-

e-

e-

e-

e-

Fe++ Fe++ Fe++ Fe++ Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

potential

=> Measureable with reference electrode as potential difference

From: Schießl, Gehlen, Sodeikat



4

CEGalvanic elementGalvanic element

Galvanic element = two half-cells= electrical connection of two different metals in an electrolyte

electric current

voltage U2voltage U1

U1 < U2

Electric current becomes measurable

Lower precious metal dissolves, higher precious metal keeps stable

met

al 1

met

al 2

Corrosion process of reinforcement in concrete:- passive reinforcement- active areas


halfcell 1 halfcell 2

galvanic element (two halfcells)



CEPreconditions for corrosionPreconditions for corrosion

anodic iron dissolution is possible; e.g. due to chlorides

electric conductivity of reinforcement

electrolytic conductivity of concrete;(water, humidity > 85 … 90 % r.h.)

oxygen in concrete (except for water saturated concrete always available)

potential differences (due to differences in ventilation or local depassivation always present)

--



5

CECorrosion process in concreteCorrosion process in concrete

Diffusion von Sauerstoff durch die Betondeckung

diffusion of oxygen through the concrete cover

Luftair

Porenwasser pore water H O + 1/2 O= Elektrolyt= electrolyte

steel

anodischerT il ß

anodischerTeilprozeßanodischerT il ß

anodic process kathodischer TeilprozeßS ff

kathodischer TeilprozeßS t ff d kti

kathodischer TeilprozeßS ff

cathodic processd ti f

H2O + 1/2 O 2

2(OH) -

2e -

Fe ++

potential -

difference


Teilprozeß

Eisenauflösung

Fe Fe ++

+ 2 e-

Teilprozeß

Eisenauflösung

Fe Fe ++

+ 2 e-

Teilprozeß

Eisenauflösung

Fe Fe ++

+ 2 e-

dissolution of iron

Fe Fe ++

+ 2 e-

Sauerstoffreduktion

2e-

+ H 2O + 1/2 O 2 2(OH)

Sauerstoffreduktion

2e-

+ H 2O + 1/2 O 2 2(OH)-

Sauerstoffreduktion

2e-

+ H 2O + 1/2 O 2 2(OH)

reduction of oxygen

2e-

+ H 2O + 1/2 O 2 2(OH)-



CEDifferent forms of corrosionDifferent forms of corrosion

Micro-corrosion element = no separation of anode and cathode

Macro-corrosion element= local separation of anode and cathodeanode and cathode anode and cathode

steel

H2O + 1/2 O2

2(OH)-2e-

Fe++

potential-

difference

steel

H2O + 1/2 O2

2(OH)-

2e-

Fe++

potential-difference

anode cathode


Carbonation induced Chloride induced



6

CEPotential mapping Potential mapping -- principleprinciple

high ohmic voltmeter

mobile reference electrode

concrete cover

- 500

- 400- 300

-100

0

100

s. A

g/A

gC

l ge

s.

0-100

-100-0

-200--100

Colour coded results

- 600500

cathode cathode

reinforcement

electric connection

0

+100

mV

]

local corrosion- 700 mV

From Guideline B3 DGZfP (2008) in German

-500

-400

-300

-200

Po

ten

tia

l [m

V]

vs

-300--200

-400--300

-500--400

2,75 m

2,75 m

- 400

- 300

- 200

- 100

po

ten

tial

[m



CEPotential mapping Potential mapping –– application Iapplication I



concrete cover

- 500

- 400- 300

- 600500

cathode cathode

reinforcement

electric connection

0

+100

mV

]


Photo: Schießl

bar electrode(spot measurement)

- 400

- 300

- 200

- 100

po

ten

tial

[m

Photo: Taffe

Photo: Schießl Gehlen Sodeikat



7

CEPotential mapping Potential mapping –– application IIapplication II



concrete cover

- 500

- 400- 300

- 600500

cathode cathode

reinforcement

electric connection

0

+100

mV

]


Photo: Proceq

- 400

- 300

- 200

- 100

po

ten

tial

[m

Results: Schießl Gehlen Sodeikat

Wheel electrode(area measurement)



CEInfluences Influences –– concrete coverconcrete cover

concrete cover

- 600- 500

- 400- 300

Concrete cover

damping of detectable potentials

at same corrosion activity:

cathode cathode

reinforcement

0

+100

V]

local corrosion

- 700 mV

at same corrosion activity:

- more noble (positive) values with large concrete cover

- less noble (negative) values with small concrete cover


--- 400

- 300

- 200

- 100

po

ten

tial

[m

V



8

CEInfluences Influences –– electric resistanceelectric resistance

Electric resistance of concrete (resistance of the electrolyte)

repair layer with big electrolytic resistanceSmall electric resistance of the concrete causes

- large corrosion currents

- flat equipotential lines

Measuring of higher potential-gradients by

- very dry concrete for

- the same concrete cover

--

- the same corrosion activity

negative potentials may not reach the surface

risk that corrosion is not detectedFrom: Schießl, Gehlen, Sodeikat



CEInfluences Influences -- summarysummary

Active corrosion ↑

Increasing Chloride content ↑

Increasing concrete cover ↓Increasing concrete cover ↓

Increasing dampness ↑

Wet concrete (without O2-access) ↑ ↑

Coating / layer with increased resistivity ↓ ↓

CEM III vs. CEM I ↑

Higher temperature ↑↑ = shift to more negative potentials --



9

CEPreparation of the measurementPreparation of the measurement

Removal of dirt and non-conductive layers (e.g. mastic asphalt) or coating

Moisten of the surface about 20 min before, l th h l id d i t d ievenly over the whole area, avoid drying-out during

measurement! Measurement on dry surface will shift the potentials 100 to 200 mV in positive direction

Specify measuring grid before measurementTypical grid: 25 cm x 25 cm, with accuracy of 10 cmup to 10 cm grid in areas of suspicionException: 50 cm x 50 cm with smaller grid in areas with high gradients (“Mexican head”)

--



CEProcedure of potential mappingProcedure of potential mapping

Calibration of the reference electrode (using 2nd electrode)

Connection with correct polarity (potential in the negative mV range)

Electric connection to the rebar; verifying of the electrical connection to the reinforcement in the measured area with a resistance test (values < 1 Ω)- normally given within one slab or deck- mostly not given in case of a joint

Collect data by positioning the calibrated reference electrode y p gon the prepared surface in the specified grid using bar electrode (small areas) or wheel electrode (large areas)



10

CEInterpretation of results IInterpretation of results I

Common potential range for the reference electrode Cu/CuSO4

Potential [mV]200 - 200 - 400 - 600 - 800- 0

ref.: Cu/CuSO4passive – without chlorides

normal aerated

pitting corrosion

uniform corrosion

normal range of potentials

=> Absolute potential is a not reliable criterion=> Only reasonable in the case of chloride corrosion

--



CEInterpretation of results IIInterpretation of results II

US Standard ASTM876Linkage of potentials with probability of corrosion

Potential difference to Cu/CuSO4-electrode

Probability of active corrosion atthe moment of measuring

Only valid for US conditions (formally coated bridges) with limited transfer to European

-250 mV < 10 %

-250 mV bis –350 mV No assessement possible

-350 mV > 90%

From: ASTM C876-91 Standard Test Method for Half-Cell Potentials of uncoated Reinforcing Steel in Concrete

conditions or car parks

Limited value for condition assessment --



11

CEBasic principles for interpretationBasic principles for interpretation

Potential mapping is a “snapshot” of current corrosion activity

Boundary conditions (e.g. water saturation) have to be known

More negative potentials indicate greater probability of corrosion activityy

General limiting values for the occurrence of corrosion activity can not be formulated

High potential gradients indicate high probability of corrosion activity

In suspicious areas a smaller grid down to 5 or 10 cm should be applied

In suspicious areas rebars should be excavated representatively In suspicious areas rebars should be excavated representatively together with evaluation of the state of corrosion and loss of section

Additional investigations (e.g. concrete cover, location of delaminations, chloride profiles) are essential for correct interpretation.



CEExample IExample I

Crack in concrete floor

0,00

0,250 50

0,00

0,25

0,50

0,75

1,00

1,25

1,50

1,75

2,00

2,25

2,50m

minimalpotential

E CSE,min = -285 mV

maximalpotentiallanes

Example: lay-by

0,50

0,751,00

1,251,50

1,752,00

2,25

2,502,75

3,003,25

potential

E CSE,max = 100 mV

meanpotential

ECSE,m = -95,8 mV

standarddeviation

s = 75,4 mV

potential[mV]

potential map

3,503,75

4,00

4,254,50

4,755,00

crack

[mV]

-250- 249 to -200- 199 to -150- 149 to -100- 99 to - 50-- 49 to 0

> 0



12

CEExample IIExample II

Helicline of an underground car park 0.021.0

dividing cracks

dividing cracks



CEExample IIIExample III

Low potential due to:

Deck of a highway bridge

exposed reinforcement

rests of coating

„self potential“ coating

cutting area no chloride inducedcorrosion

current corrosion

not chloride induced

current corrosion

exposed reinforcement

concrete cover potential mapping

current corrosion

chloride induced

cutting area



uct condass potentialmapping2013-01-29 [kompatibilitätsmodus] · 2016-04-14 · - normally given...

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