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

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

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

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.

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

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

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)

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

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

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]

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

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

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

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

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:

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

List of parameters for SLD – Limit State 1, Depassivation Unit µ s Distribution type Xc -concrete cover [mm] 50 5 Beta* Do -Cl-migration coefficient [ 10-12 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

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

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

Example: Olympic Tower

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)

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

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]

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

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

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

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

Example: Parking Deck Allianz-Arena – Uncoated Area

Example: Parking Deck Allianz-Arena - Column

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

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

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

Interrelation between DCl- and Electrolytic Resistance 100,0 10,0 1,0 0,1 100 1000 10000 100000 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

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

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

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)

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