LNECdurati.lnec.pt/pdf/ICDS12_Presentations/B1_5_JAppleton.pdf · fib bulletin 34 (2006) RC3 2,0 -...

DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012

Durability assessment of concrete structures exposed to marine environment

– case study –

António Costaa, Júlio Appletonb and Manuel Coitoc

a ICIST, A2P; b ICIST, A2P; c EDP


Industrial concrete infrastructures

ICDS12 International Conference DURABLE STRUCTURES: from construction to rehabilitation LNEC • Lisbon • Portugal • 31 May - 1 June 2012

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Durability assessment:

– Visual inspection to classify the exposure conditions of the structures, identify deterioration and execution defects;

– Laboratory tests on concrete samples extracted from the structures to characterize concrete quality, chloride ingress and concrete deterioration;

– In situ test to characterize concrete cover and carbonation depth;

– Probabilistic analysis to characterize the durability.


Offshore Structures

Environmental exposure classes

XS1

XS3

XS2


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Onshore Structures


XS1XS3


XS3

XS1

Onshore Structures



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Visual Inspection

No significant damage was observed except irregular cracking on the top of the caissons


Core drilling

Laboratory tests:

Petrographic analysis

Potential AAR

Potential DEF

Carbonation resistance

Chloride diffusion


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Laboratory tests

Petrographic analysis

– Potential reactive aggregates were identified

– No signs of gel resulting from the alkali aggregate reaction were visible

Irregular cracks in greywacke particlesGreywacke contain chert and quartzite


Potential alkali aggregate reaction - AAR

Laboratory tests


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Potential internal sulphate reaction – delayed ettringite formation - DEF

samples B

samples A

Laboratory tests


Chloride profiles

Laboratory tests


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In situ tests

Concrete cover measurements


In situ tests

Concrete cover measurements

Structure 1 – zone 1

Structure 1 – zone 2

Structure 1

Concrete cover (mm)

Rel

ativ

e fr

eque

ncy

(%)


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StructureConcrete cover (mean values)

[mm]

Standard deviation

[mm]

Concrete cover (5% fractile)

[mm]Exposure Class

1 49.09 4.55 41.6 XS1

2 51.07 4.22 44.1 XS1

3 57.36 6.12 47.3 XS3; XS1

4 58.65 8.29 45.0 XS3; XS1

5 60.88 3.72 54.8 XS3; XS1

6 53.45 5.76 44.0 XS3

7 62.41 6.13 52.3 XS3

8 54.23 7.53 41.8 XS3

9 50.70 7.89 37.7 XS3; XS1

In situ testsConcrete cover measurements

Normative requirements XS1: Cmin = 35 mm Cnom = 45 mm

XS3: Cmin = 45 mm Cnom = 55 mm


In situ tests

Carbonation depth

dc 0 (in general)

dc,max 3 mm


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Durability assessment

S(t) – action

R(t) - resistance

pf = p{[R(t) - S(t) < 0]}T < ptarget

ptarget – target failure probability

tL – service life

tg – target service life

pf = p(tL< tg) < ptarget

2 methodologies:

Service period design

Lifetime design

Probabilistic approach

Time

R

S

pf = 0,5

f (tL)

R;

S

R(t)

S(t)

pf

pftg

tL,m

TL


Durability assessmentProbabilistic analysis

– Define the deterioration mechanism and the limit state –

reinforcement depassivation by chlorides

For exposure classes XS1 and XS3: tp 0 tL = ti

Lev

el

of

det

erio

rati

on

Time of exposureinitiation propagation

ti tp

1

3

4

2

1 despassivation2 cracking3 spalling4 collapse

Limit states

tL = ti + tp


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– Define a model to describe the chloride ingress into concrete –

model defined in LNEC E465

C (x,t): chloride concentration at the depth x and at the time t ;

C0 : initial concentration of chlorides in the concrete mix;

CS: chloride surface concentration;

Dapp(t): apparent coefficient of chloride diffusion of concrete at time t;

t0: reference time;

t: time of exposure

n: ageing exponent taking the time dependent behaviour of Dapp



– Define the analysis methodology and the limit state function –

service period design or lifetime design

- Service period design

limit state function:

R (t): concrete cover (m);

S (t) = XCR : penetration depth of the critical chloride content (m)

g(z) = R(t) – S(t)

pf = p (S (t) > R(t)) = p(g(z) < 0) = (-)

(-): standard normal distribution : reliability index


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– Characterize statistically the parameters of the environmental action S(t)

and the material resistance R(t)

R ; Dapp ; CS ; CR ; n

– Perform the statistical analysis - numerical simulation technique –

Monte Carlo method

pf = Nf / N

Nf is the number of failures: n (g(z) < 0)

N is the number of trials



VariableDistribution

typeMean value

Standard deviation

R (mm) Normal 53.45 5.76

Dapp (10-12 m2/s) Normal 1.25 0.28

Cs (wt.- % /c) Normal 2.57 0.82

CR (wt.- % /c) Normal 0.4 0.048

C0 (wt.- % /c) Deterministic 0.05 -

n Normal 0.65 0.05

Example for one of the offshore structuresExposure class: XS3Concrete mix: Water/binder = 0,40Binder: CEM IV/A (fly ash)


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reliability index

probability of failure

Probabilistic analysis Reliability

classes

LNEC E465

(2005)

EN 1990

(EC 0)

fib bulletin 34 (2006)

RC3 2,0 - 1,3

RC2 1,5 1,5 1,3

RC1 1,2 - 1,3

Minimum values of the reliability index (β) for ELS – reference period of 50 years

t (anos) P(S>R)(%) β

5 0.02 3.52

10 0.26 2.79

15 1.00 2.32

20 2.41 1.97

25 4.53 1.69

30 7.32 1.45

35 10.67 1.24

40 14.45 1.05

45 18.56 0.89

50 22.87 0.74

60 31.72 0.47

70 40.39 0.24

80 48.51 0.037

90 55.88 ‐0.148

100 62.43 ‐0.316



0

5

10

15

20

25

30

35

40

45

50

55

60

0 10 20 30 40 50 60 70 80 90 100

Xcr

[m

m]

Time [years]

concrete cover - mean value

XCR (t) – mean penetration depth of the critical chloride content (mm)

Deterministic analysis

Service life- mean value


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