CIVE1173 Service Life Modelling

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CIVE1173 Service Life Modelling
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2013-06-19 21:20:35
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Service LifeModelling
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Week 7 lecture notes summary. Exam
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  1. Damage Propagation vs Time
  2. SLM Limit States/Evidence
    • Entrance of aggressive agents/Cores or Drilling
    • Initiation of corrosion/LPR or half-cell
    • Rust staining/visual
    • Cracking/visual
    • Spalling/visual
    • Structural failure/closure
  3. SLM Phases
    • Initiation
    • Propagation
  4. Initiation phase
    • Deterministic; measured values, well established models, single output value
    • Probabilistic; distribution curves/mean and standard deviation, probability/reliability as output
  5. SLM - chloride models
    • Data from cores, labs, literature
    • NaCl ponding test, seal all faces except top with epoxy, determine chloride profile after 90 days
  6. Chloride Initiation Phase - Diffusion model


    • Cx,t = chloride depth x, at time t
    • Cs = surface chloride
    • Dapp = chloride diffusion coefficient apparant
    • T = time
    • x = cover depth
    • erf = error function
    • requires concentration of chlorides at bar to initiate corrosion to predict service life.  Chloride % by weight can be expected to decrease exponentially as depth increases
    • Reaches max. value
  7. Chloride diffusion coefficient, Dapp
    • Dtm chloride diffusion coefficient at time tm and n is the material coefficient. e.g. tm at age zero is 1
    • n factor; OPC -0.264, PFA -0.699, GGBS -0.621
  8. Chloride Diffusion Model; surface chloride Cs
    • where Cs < 0.06 (or any other pre-set value)
    • alpha is a constant depending upon exposure conditions
    • C0 is the initial surface concentration
    • t time in years
    • e.g. mean values OP cement 0.36%, Blended cements 0.51%
  9. Chloride Diffusion Model
    Rate of chloride build up at surface in different marine environments
  10. Using Chloride Diffusion Model to determine cover required
  11. Chloride Propagation Phase - Diffusion model
    • Diffusion Model can be used as basis for propagation but considerable uncertainty exists.
    • Uncertainty increases as the propagation continues from cracking to spalling to structural failure.
    • If using then predict corrosion rate first. Actual measurements or:
    • CR = a.ebCxmicron/year
    • CR – corrosion rate, a and b are constants and Cx is the chloride content
  12. Calculating chloride corrosion rate for propagation phase for uncracked concrete
    • CR = 0 (Cx <0.46% by weight of cement)
    • CR = 0.55e1.564Cx (0.46%<Cx <3.0% by weight of cement)
    • CR = 60 (3.0%<Cx by weight of cement)
  13. Chloride corrosion propagation. Crack penetration
    • pcr = (83.4 + 7.4c/db - 22.6fct)/1000
    • c – cover
    • db – bar diameter
    • fct – tensile splitting strength of concrete
  14. Chloride corrosion propagation. Crack propagation
    • wcr = 0.05 + Beta(p(t) - pcr) >0
    • Where
    • Beta – 12.5 top cast, 10 bottom cast bars
    • p(t)– mean corrosion penetration at time t
  15. Chloride corrosion propagation. Residual cross section
    Ares = pi(db – 2p(t))2/4
  16. SLM - carbonation modelling
    • Depth of Carbonation =
    • Q + k1.k2.k3.k4.k5.C.tx
    • Q = Instantaneous Carbonation
    • C = Carbonation Coefficient for Concrete (Assumptions for C are made for differing grades of concrete)
    • t = time
    • k1 = factor - effect of CO2 concentration in the air
    • k2 = factor - effect of Exposure
    • k3 = factor - effect of orientation (N,S,E,W)
    • k4 = factor - effect of Curing
    • k5 = factor - effect of supplementary cementitious materials (e.g. - flyash, BFS)
    • x = factor- effect of asset type and method of construction
  17. SLM - carbonation initiation phase
    • xc(t) = Kt0.5
    • t – time
    • xc(t) – carbonation depth at time t
    • K – constant
    • K can be determined from site data given carbonation depth at time t.
  18. Carbonation initiation impacted by:
    The environment. Time of wetness & temperature
  19. Carbonation initiation, rates for various concrete strengths
  20. SLM Initiation Phase - Probabilistic models
    • Distribution function for each variable
    • Normal, t, chi squared, Wei-Bull, etc
    • Surface chloride, chloride diffusion coefficient, cover depth (age factor, temperature)
    • Monte Carlo simulation (or similar) to build up a distribution pattern for results
    • Analyse probability that result gives desired service life
  21. SLM Initiation Phase - probabilistic models examples
    • RMIT research project
    • Model uses analogy between heat transfer and diffusion
    • Time dependant analysis
    • Variation in models
    • Model replicates at concrete pile with different rates of chloride build up
    • FEA Mesh
    • Comparison at point y (mid-point)
    • Other models. Questionable accuracy:
    • Freeze/Thaw
    • Sulphate attack
    • ASR

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