PROBABILISTIC MODELLING OF CONCRETE STRUCTURES DEGRADATION

B. Teplý, P. Rovnaníková, P. Rovnaník, D. Vořechovská

Brno University of Technology, Czech Republic

REC 2012, Brno

Outline•Degradation of RC structures

…Time aspect…Codes and regulations…Durability limit states

•Software tool•Evaluation of some degradation

models …Carbonation m. vs. in situ measurement on cooling tower

…Chloride ingress m. vs. in situ measurement on bridge in Italy…Corrosion m. vs. numerical m.

•Conclusions

Degradation mechanisms vs. time

Degradation mechanisms vs. time

carbonation and/or chloride ingress

Degradation mechanisms vs. time

carbonation and/or chloride ingress

steel depassivation

Degradation mechanisms vs. time

carbonation and/or chloride ingress steel corrosion

steel depassivation

Codes and regulations• ISO 13832 (2008)General principles on Design of Structures for Durability

• fib Model Code (2010) Bulletins 55, 56 (First complete draft)Bulletin 34 Model Code for Service Life Design (2006)

• EN 206-1 (2000)Concrete – Part 1: Specification, performance,

production and Conformity

Durability limit states

1. Service life format

1a. Simplified/initiation LS

2. Limit state format

PS Dt t

D DR t S t

lim DS S t

ULS

SLS

ini Dt t

,where PS ini propt t t

Durability limit states

( ) 0

( ) 0

f D c D d

f D cr a D d

P t P a x t P

P t P C C t P

( ) 0f D a D dP t P r r t P

Steel depasivation (tini):

Steel corrosion (tprop):

( ) 0f D cr a D dP t P w w t P

Software toolFReET-D probabilistic modeling of degradation processes in time www.freet.cz(Teplý, Vořechovská, Chromá, Rovnaník – Brno University of Technology)

Utilizes probabilistic platform of FReET (Vořechovský, Rusina, Novák – Brno University of Technology)

Features: random input variables, MC, LHS, FORM, statistical parameters of result, sensitivity analysis, estimation of probability Pf and reliability index , automatic parametric studies, Bayess updating, number of degradation models

FReET-D: implemented models

 No. of models

Outputs

Carbonation 13carbonation depth, time to depassivation (concretes with Portland and blended cements)

Chloride ingress1 depth of chlorination, time to depassivation2 concentration of chlorides

Frost attack 3actual degree of saturation, relative dynamic modulus of elasticity, loosening of concrete at surface

Sulphate attack 1 rate of concrete corrosionAcid attack 1 depth of concrete corrosion

Reinforcement corrosion

1 net rebar diameter (uniform cor.)1 pit depth (pitting cor.)1 net cross sectional area (pitting cor.)2 time to cracking (uniform cor.)1 crack width (uniform cor.)

1stress intensity factor (prestressed reinforcement, pitting corrosion, fracture mechanics approach)

1 strength and ductility of corroded steel

Evaluation of carbonation models

Cooling tower (Keršner et al. 1996)•height of 206 m •in-site investigation at the age of 19.1 years•the depth of carbonation measured at 75 locations (on both the internal and external surfaces)

Analytical modelsa)fib Model Code (Bulletin No. 34)b)Papadakis et al. (1992), based on mass conservationc)Papadakis et al. (1992), R.H. function updatedd)Morinaga (1992), R.H. function updated

Evaluation of carbonation models

(Teplý et al. 2010)

Evaluation of carbonation models

Carbonation depth

(19.1 years)

External surface (RH = 70%)

Internal surface (RH = 93%)

Mean [mm]

COV [%] Mean [mm] COV [%]

fib Model Code 10.8 48 4.4 60

Papadakis et al. (1992)

8.2 24 1.9 43

Papadakis et al. (1992)

R.H. function updated12.7 18 8.3 51

Morinaga (1992), R.H. function updated

11.9 21 7.7 53

In situ measurements (Keršner et al. 1996)

14.9 56 8.0 29

(Vořechovská et al. 2010)

Evaluation of carbonation models

Carbonation depth

(19.1 years)

External surface (RH = 70%)

Internal surface (RH = 93%)

Mean [mm]

COV [%] Mean [mm] COV [%]

fib Model Code 10.8 48 4.4 60

Papadakis et al. (1992)

8.2 24 1.9 43

Papadakis et al. (1992)

R.H. function updated12.7 18 8.3 51

Morinaga (1992), R.H. function updated

11.9 21 7.7 53

In situ measurements (Keršner et al. 1996)

14.9 56 8.0 29

(Vořechovská et al. 2010)

Evaluation of chloride ingress models

Bridge in ItalyWendner, R., Strauss, A., Guggenberger, T., Bergmeister, K.

and Teplý, B., 'Ansatz zur Beurteilung von chloridebelasteten

Stahlbetonbauwerken mit Bewertung der Restlebensdauer',

Beton- und Stahlbetonbau 12/2010

Chloride concentration measured in the different distances from the nearest surface (from 10 to 50 mm)

Evaluation of chloride ingress models

a) fib Model Code (Bulletin No. 34)

b) fib Model Code; substituted surface concentration

c) Crank´s solution of Fick´s 2nd law

d) Thomas and Baumfort (1999)

e) Nilsson and Carcasses (2004) and Tang and Gulikers (2007)

Evaluation of corrosion model

Analytical model (Li et al. 2006)

vs.

Numerical computation based on nonlinear FE(Vořechovská and Vořechovský 2010)

Evaluation of corrosion modelAnalytical model (Li et al. 2006)

no corrosion filling of

porous zone

crack initiation

crack on concrete surface, spalling

concrete

rust

crack

Constant geometry: thick-wall cylinder (Bažant 1979, Pantazopoulou and Papoulia 2001, Tepfers 1979)

steel

porous zone

Evaluation of corrosion modelNumerical model

►ATENA program (Červenka Consulting)

► constitutive model based on smeared crack approach

► used fracture plastic model NLCEM

► expansion of corrosion products: (negative) shrinkage of reinforcement

Evaluation of corrosion modelComparison of models

Identical parameters: geometry, E, ft

Extras for numerical model: GF, fc,…

Conclusions• Durability design of RC structures is now actual

in engineering practice

• Durability/performance based approach to the specification of concrete durability may be based on predictive (proven!) models utilization

• FReET-D is a software tool for fully probabilistic performance based design; the other proves of the degradation models are needed

• FReET-D can be effectively utilized in combination with FReET and ATENA software