1. Potential and Limits of Probabilistic Service Life Design
Rackwitz – Symposium Nov. 2006
Potential and Limits of Probabilistic Service Life Design
Peter Schießl

4. Bild 57:
Rißstellen mit Querschnittsminderungen (infolge Korrosionsabtragungen) ΔF > 0,5 %. Mittlere Querschnittsminderungen ΔF > 0,5 % und Anteil dieser Rißstellen bezogen auf alle Rißstellen. Rippentorstahl Ø 8 mm

6. The History:
1978 CEB –Task group Durability
1982 CEB Bulletin 148 – St-A-R-Durability
1989 CEB Bulletin 182 – Design Guide
1997 CEB Bulletin 238 – New Approach to Durability Design
1999 Brite EURAM Research Project “DURACRETE”
2002 fib Task Group 5.6
2006 fib Bulletin 34 – MCSLD

7. Probabilistic Service Life design
Involves statistics and reliability.
It was a main objective to establish a methodology as close as possible to structural design
fib TG 5.6 has therefore chosen the package of standards associated with Eurocode-0 / Eurocode-2 as the main references
As the new fib Model Code 2006 shapes, the elements will be implemented into this framework.

8. Model Code for Service Life Design (MC SLD)
Gives “Principles” that have to be obeyed
Gives “Application rules” as informative examples on sufficiently verified methods
Chapter

9. Model Code on Service Life Design
assumptions 1
terms of definitions
other administrative provisions
principles of service life 2
design criteria
full probabilistic design
partial factor design
deemed to satisfy design
avoidance of deterioration
probabilistic models -resistances -loads/exposure -geometry
design values -characteristic values -partial safety factors -combination factors
exposure classes
exposure classes 3
limit states
design equations
design provisions
design provisions
in case of non-conformity with the performance criteria, the structure becomes either obsolete
or subject to a redesign
design / verifications
project specification for material selection and execution maintenance plan inspection / monitoring plan 4
quality plan for execution (optional)
inspection of execution 5
maintenance
condition control during service life

10. Safety concept, structural design
P {failure} = P {R - S < 0} < P target
RC/gR - Sc . gs > 0
RC: load bearing capacity based on characteristics values
gR: partial safety factor (resistance)
SC: characteristic value of the influence of loading
gS: partial safety factor (stress/load)

11. Safety concept, durability design
P {failure} = P {R - S < 0} < P target T T
RC/gR - Sc . gs > 0

12. Safety concept, durability design
P {failure} = P {R - S < 0} < P target T T
RC/gR - Sc . gs > 0
Example RC: concrete cover c
SC: ingress of chloride front

13. Design criterion: cover > depassivation depth
RC , SC
Elaboration of Service Life Design BRITE-EURAM project „DURACRETE“
RC,T
RC(t)
SC,T
SC(t)
SC(t) T
exposure period in [a]

14. New design concept - Basis and terms of the safety concept -

15. Basic deterioration model (reinforcement corrosion)
Degree of deterioation 4
Ultimate Limit State (ULS) 3 2 1
Time t [a]
Reliability index 
~ 1.5
~ 4.2 1
Depassivation of reinforcement 2
Formation of cracks
Limit States
Reinforcement corrosion 3
Spalling
(Deterioration period) 4
Failure

16. Example: Westerschelde - Tunnel - NL
Longitudinal Section
Cross Section
180°
Ring: 1 2 3 4 5
Ring 2
"Tübbing"
0,45 m
90°
270°
joints
0° = 360°
11,00 m

17. Geometry of Prefabricated Tunnel-Ring Element
Chlorid attack (Cl-)
Xc (joint) xc xc
Carbonation (CO2)
eventually
chloride attack (Cl-)

18. Example: Westerschelde - Tunnel - NL
Requirement of the owner:
Tender document:
Service Life 100a
Design for depassivation as a serviceability limit state
  1,8
Agreement between
owner and contractor:

19. Material Model (Chloride Diffusion)
2 Do • M • • t
to n t  xc
Ccrit = CSN 1- erf
ccrit : critical corrosion initiating chloride content
cSN : chloride concentration at the surface
xc : concrete cover
Do : chloride migration coefficient
M : model factor
to : reference age
t : service life C
crit C t o t

20. List of parameters for SLD – Limit State 1, Depassivation
Unit s
Distribution
type
Xc -concrete cover
[mm] 50 5
Beta*
Do -Cl-migration coefficient
[ m2/s]
4,75
0,71 ND
Ccrit -critical Chloride content
[M.-%/Binder]
0,70
0,10
n-aging factor
[-]
0,60
0,07
Kt -factor -test conditions
1,00 - D
Ke -factor-environment
Kc -factor-execution
0,20
CSN c(Cl-) - concrete surface
4,00
0,050
to -time of testing
[a]
0,0767

21. Result of Serve Life Design (Limit State 1 - Depassivation)b
in [-]
1,8
3,6
3,4
3,2
3,0
2,8
2,6
2,4
2,2
2,0 20
100
t in [a] 90 80 70 60 50 40 30
= 3,488
pf = 0,0002
= 2,561
pf = 0,0053
= 2,255
pf = 0,0126
= 1,951
pf = 0,0255

22. Update of SLD during Service Life
Degree of deterioation
Time t [a]
Ultimate Limit State (ULS) 1 2 3 4
After construction
- Knowledge of quality achieved
- Measurement of model parameters
- reduction model uncertainty
Before depassivation
- determination of interaction between action and resistance
→ Measurement of the depassivation front
After depassivation
- determination of
corrosion rate
Design
Assumptions rel. to
- actions
- resistance
Improvement of service life prediction

23. Example: Olympic Tower

24. Resistance variable: concrete cover
dc = dc,measured + e
dc: geometrical variable (concrete cover)
dc,measured: measured concrete cover
e: error term (uncertainty of the non destructive measuring device)

25. Stress variable: carbonation depth
xc(t): Carbonation depth at time t
ke: f(RH) – RH function
kc: f(t curing) – Curing function
RACC,0-1: Inverse of the carbonation resistance
kt, t: Test method translation factors
CS: CO2-concentration of the ambient air
t: Time in service
W: f(t, pSR, ToW) – Wetting function
pSR: Probability of surfaces subjected to driving rain in dependency to their orientation
ToW: Time of wetness

26. Update: comparison a-priori/a-posterior
Orientation East, level 8 m above ground
7.0
0.07
probability of occurance
a-posterior
6.0
reliability index,
0.06
a-priori
[%]
5.0
reliability index,
0.05 f
[-]
a-posterior b
4.0
0.04
failure probablility p
reliability index
3.0
0.03
2.0
0.02
1.0
0.01
0.0
0.00 20 40 60 80
100 33
inspection
time of exposure [a]

27. Example: Parking Deck Allianz-Arena
Highway A 9
Allianz-Arena
Bus parking
Highway A 995

28. Example: Parking Deck Allianz-Arena
pedestrian area
Level 3
Level 2
Level 1
Level 0

29. Example: Parking Deck Allianz-Arena
Coating zones
coated surface (cracked zone, bending zone)
uncoated surface

30. Example: Parking Deck Allianz-Arena
Design principle:
S-1: Cl- on uncoated concrete
S-1: Cl- on coated cracks
R-2: Concrete cover plus monitoring
R-1: Coating plus regulary inspection/maintenance

31. Example: Parking Deck Allianz-Arena – Uncoated Area

32. Example: Parking Deck Allianz-Arena - Column

33. Example: Parking Deck Allianz-Arena
Updating procedure (monitoring)

34. Example: Parking Deck Allianz-Arena
Updating procedure
A-Posterior
A-Priori
Expected information from the sensors (slope assumed)

35. Test Methods for the Material Resistance
Basic requirement
for SLD
Simple and reproducible
measurement of the decisive
material resistance parameters
in the lab and at the site
Chloride diffusion resistance
Lab-testing:
At the site: Electrolytic resistance
Rapid migration test
Wenner-Probe

36. Interrelation between DCl- and Electrolytic Resistance
100,0
10,0
1,0
0,1
Migrationcoefficient DCI,M [10-12 m² / s]
Elektrolytic resistance [Ohm]
w/(z + 0,5 f) = 0,50
ohne und mit SFA
Lagerung:
Beton: 20 °C/80 % r. F.
Mörtel: Wasser 20 °C
28 d
91 d
365 d

37. Testing Concept of Concrete Quality
Potential diffusion
resistance of cementes
Deff, cement
Interrelation between
cement and conrete property
Deff,concr. = Deff, cem. × f(concr.)
Potential concrete quality
Deff, concrete
Concrete quality in the structure
Deff, structure
Requirement for
execution
Measured concrete properties

38. Testing Concept of Concrete Quality
Potential diffusion
resistance of cementes
Deff, cement
Interrelation between
cement and conrete property
Deff,concr. = Deff, cem. × f(concr.)
Deff
Deff
Deff
Potential concrete quality
Deff, concrete
Deff
Concrete quality in the structure
Deff, structure
Requirement for
execution
Deff
Measured concrete properties
Deff → fc !
Deff

39. Conclusion 1
SLD on a probabilistic basis applicable for depassivation in uncracked concrete
Design procedure with partial safety factors exists → DURACRETE, fib Tg 5.6
Only a design concept on a probabilistic basic is able to consider material variations, variation of actions and model uncertainties
SLD ready for standardisation in the next generation of Standards (EN, ISO)
→ fib Tg 5.6 has prepared a ModelCode for Service Life Design (fib - Bulletin, Jan 06)

40. Conclusion 2 – other Deterioration Mechanisms
Reinforcement corrosion in the region of cracks
Dissolving chemical attack
Sulphate attack
ASR
Frost - Frost deicing salt
Further developement of deterioration models, quantification by research
DFG Forschergruppe
Design examples exist
For the time being: Design strategy A