Prediction of a Chloride Ion Penetration Profile for a Concrete


photo here

Prediction is based on a one-dimensional finite difference solution of Fick's second law of diffusion, with a variable external chloride concentration and a two-layer representation of the concrete. Surface treatment option has been added in 2006 and mill and fill option added in 2007 in collaboration with Prof. Guthrie of Brigham Young University. Ability to input a measured chloride profile added in 2017.


Please supply the following parameters (defaults provided)


Environmental Parameters

Specify external chloride concentration and temperature as a function of month of the year:

  Month     Ext. chloride      Temperature (oC)
         conc. (moles/liter)
January                
February               
March                  
April                  
May                    
June                   
July                   
August                 
September              
October                
November               
December               
Beginning month of exposure is:

Total duration of exposure days

Unexposed boundary condition is


Structural Design Parameters

Member thickness m

Depth of Reinforcement mm


Concrete Mixture Parameters

w/c ratio

Degree of hydration

Volume fraction of aggregate %

Air content %

Initial chloride concentration of concrete g chloride/g cement Guidance


Diffusion Coefficients (D) of Original Concrete

Click here to view database of concrete diffusivities from literature.

Note that all diffusion coefficients are apparent diffusivity values, as we are modelling transport in the pore space of the concrete based on Fick's 2nd law.

Time dependent diffusion coefficient for bulk concrete takes the form of D=Dinf+Di*t-m

Be sure that you use values of m in the range (0,1).

Reference: Mangat and Molloy, Materials and Structures, Vol. 27, 338-346, 1994. Note- Mangat and Molloy found m values ranging from 0.44 to 0.86 and also that approximately:

m=2.5(w/c)-0.6

To have a constant D value with time, simply set Dinf to this desired value and Di to zero

Dinf m*m/s at 25 C

Di m*m/s

m

Curing time before exposure of concrete to chlorides days (Recommended > 0 )

The surface layer of the concrete may have a different (lower or higher) D value than the bulk concrete due to carbonation or poor curing practices. Input the surface layer D value relative to the bulk D value and the thickness of this layer. To bypass this feature, set the skin layer thickness to 0.0 or use D(surface concrete)/ D(bulk concrete)=1.0.

Ratio D (surface concrete)/D (bulk concrete)

Thickness of surface layer mm

Activation Energy for diffusion kJ/mole


Chloride Binding Parameters for Original Concrete Guidance

Based on a Langmuir isotherm of the form:
  C(bound)=(alpha*C(free))/(1.+beta*C(free))

where C(bound) is in (mole Cl-)/kg cement and
C(free) is in (mole Cl-)/L

Alpha

Beta

Rate constant for binding s-1

Chloride Reaction Parameters Guidance

Assuming the formation of Freidel's salt from all of the 
C3A and C4AF initially available in the cement powder.
C3A content of cement % on a mass basis

C4AF content of cement % on a mass basis

Rate constant for aluminate reactions with chloride s-1

New Feature (March 2006)

Allows application of a surface treatment at a specific time, beyond which further transfer of chlorides into/out of the top surface is prohibited.

Set this time to a time greater than the total exposure time to turn off this feature

Time at which surface treatment is applied days

Mill and Fill Concrete Options (February 2007)

Time at which milling and filling is performed days

Depth of milling m

Thickness of new (filling) concrete m

w/c ratio of new concrete

Degree of hydration of new concrete

Volume fraction of aggregate for new concrete %

Air content of new concrete %

Initial chloride concentration of new concrete g chloride/g cement Guidance

Diffusion Coefficients (D) of New Concrete

Time dependent diffusion coefficient for bulk concrete takes the form of D=Dinf+Di*t-m

Be sure that you use values of m in the range (0,1).

To have a constant D value with time, simply set Dinf to this desired value and Di to zero

Dinf m*m/s at 25 C

Di m*m/s

m

Curing time before exposure of new concrete to external chlorides days (Recommended > 0)

The surface layer of the concrete may have a different (lower or higher) D value than the bulk concrete due to carbonation or poor curing practices. Input the surface layer D value relative to the bulk D value and the thickness of this layer. To bypass this feature, set the skin layer thickness to 0.0 or use D(surface concrete)/ D(bulk concrete)=1.0.

Ratio D (surface concrete)/D (bulk concrete)

Thickness of surface layer mm


Chloride Binding Parameters for Overlay Concrete Guidance

Based on a Langmuir isotherm of the form:
  C(bound)=(alpha*C(free))/(1.+beta*C(free))

where C(bound) is in (mole Cl-)/kg cement and
C(free) is in (mole Cl-)/L

Alpha

Beta

Rate constant for binding s-1

Chloride Reaction Parameters Guidance

Assuming the formation of Freidel's salt from all of the 
C3A and C4AF initially available in the cement powder.
C3A content of cement % on a mass basis

C4AF content of cement % on a mass basis

Rate constant for aluminate reactions with chloride s-1


Measured Chloride Profile Option (January 2017)

The user can input a measured Cl- profile for the original 
concrete before any sealing or milling and filling operations were performed. 
Cl- profile will be initialized to these values and time-based 
simulation will start from there.

Time at which Cl- profile was measured days

                Midpoint         Measured Cl- (free)
                Depth (m)       (kg Cl-/m3 concrete)
Top layer                
2nd layer                
3rd layer                
4th layer                
5th layer                
6th layer                
7th layer                
8th layer                

-Linear interpolation applied between measured points.

-Bound chloride set based on input isotherm parameters for original concrete.

Assumed reaction percentage to form Freidel's salt (0 to 100 %) %

The results of submitting this form will be sent to you by e-mail,
since the program will take several (1-3) minutes to execute.  Please 
provide your e-mail address below (optional).

E-mail address



For more information, contact:

Mr. Dale Bentz

dale.bentz@nist.gov


Bibliograpy:

Bentz, D.P., Clifton, J.R., and Snyder, K.A., Predicting Service Life of Chloride-Exposed Steel-Reinforced Concrete, Concrete International, 18(12), 42-47, 1996.

Birdsall, A.W., Guthie, W.S., and Bentz, D.P., Effects of Initial Surface Treatment Timing on Chloride Concentrations in Concrete Bridge Decks, Transportation Research Record, Journal of the Transportation Research Board, No. 2028, Design of Structures 2007, 103-110, 2007.

Guthrie, W.S., Nolan, C.D., and Bentz, D.P., Effect of Initial Timing of Scarification and Overlay Treatment on Chloride Concentrations in Concrete Bridge Decks, Transportation Research Record, Journal of the Transportation Research Board, No. 2220, Maintenance and Preservation of Structures and Equipment, 66-74, 2011.