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Service Life Prediction of Reinforced Concrete exposed to Chloride

Environment based on Corrosion Initiation

Ankur Bansal 09010408

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Points to be Discussed• Cause of Reinforcement Corrosion• Phases involved in corrosion of reinforcement.• Factors affecting the diffusion of chloride into concrete.• Models to determine the time for corrosion initiation• Experimental Work done to determine the critical

chloride content.• Conclusion and Future Work

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Cause of Reinforcement Corrosion• The high alkalinity of the concrete pore

solution provides a protection of steel bars in concrete against corrosion

• Corrosive ions and molecules such as chloride ions and carbon dioxide reach the surface steel and break the passivation film and the corrosion starts.

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• Corrosion Initiation• Corrosion Propagation

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Parameters involved in chloride diffusion

Diffusion Equation:

• D = Diffusion coefficient.• = Surface Chloride content.

• Ccr = Critical Chloride content.

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Factors affecting Chloride Transport

• Concrete Mix Type of cement used. Water binder ratio. Aggregate Size.

• Curing Conditions • Exposure Conditions

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Models to determine corrosion initiation

period

Deterministic• An Exact Time period is determined

Probabilistic• A range of Values is determined with a specific

probability distribution.

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Deterministic Approach

Time taken for chloride content at near reinforcement to reach the critical chloride content value:

Ccr

D is given as

D for the above equation=

And m is the age factor, which is given by

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Probabilistic Approach• Experiments are performed

many times and a distribution of the values are found.

• Random numbers are generated according the distribution achieved(2 lakhs in our case).

• These values are put into the diffusion equation.

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Probabilistic Approach• Then at a given time period, no. of

times the chloride content at reinforcement surface exceeds the critical chloride content is given by n and total number of experiments are N.

• Thus, Probability of failure is given by Pf = n/N

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Simulation Results

Variables Units Mean Standard

Deviation

Distribution

Function

D 3.77 0.91 Normal

Cs% of binder 2.83 0.72 GEV (General

Extreme

Value)

Ccrit% of binder 0.69 0.30 Gumbel

Distributions of different parameters for a specific project were found by (Ha-Won, et al., 2009)

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Generated Random Numbers

1 2 3 4 5 6 7 8 9 100

0.5

1

1.5

2

2.5x 10

4

Surface Chloride Content

No.

of

sam

ple

s

General Extreme value distribution for values of Surface Chloride Content.

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Generated Random Numbers

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 40

0.5

1

1.5

2

2.5x 10

4

Critical Chloride Content

Num

ber

of

Sam

ple

s

Gumbel Distribution for values of critical chloride content.

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Generated Random Numbers

-1 0 1 2 3 4 5 6 7 8

x 10-12

0

2000

4000

6000

8000

10000

12000

14000

Diffusion Coefficient

Num

ber

of

Sam

ple

s

Generated values of Diffusion Coefficient with normal distribution.

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Probability of corrosion initiation v/s

time

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20 40 60 80 100 120 140 1600

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

X: 130Y: 0.05114

Time Period

Pro

babili

ty o

f fa

ilure

After 130 years, 5 % of the samples will fail.

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Experimental Work• Concrete mixes of different w/c ratio and different

cement type are planned to be prepared.• 3 Cubes and 3 beams of different cover depths

i.e. 25mm, 35mm and 44mm were casted.• 28 days Compressive Strength of the cubes was

determined• Beams were Kept in a 9 % NaCl Solution with a

potential difference of 12.0 Volts between reinforcement bar and surrounding solution.

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Test Results• Compressive Strength For concrete Mix with w/c 0.5 using OPC

Cement, compressive strength was found to be 26.07 MPa.

For concrete Mix with w/c 0.55 using OPC Cement, compressive strength was found to be 23.6 MPa.

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Test Results• Accelerated Corrosion Test(for OPC and w/c 0.5 )

0 10 20 30 40 50 60 70 800

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

25mm35mm44mm

Exposure Time (hours)

Curr

ent(

A)

Plot of Current v/s Exposure Time

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Test Results• Accelerated Corrosion Test (w/c = 0.55, OPC)

0.00136566666666667 0.001475 0.0013875 0.001318666666666670

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

35 mm25 mm

Time (hours)

Curr

ent(

A)

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Conclusion and

Future Work