I would like to ask about the surface chloride concentration(Cs) in concrete exposing to chloride environment . The model of estimate of Cs in structures addressing to sea or marine condition . Can we used these models in computing the Cs for concrete structures that exposes to deicing condition only (structures in center of cities)?
Yes you can, you however have to consider that chloride transport models for marine structures are defined based on exposure to chlorides from seawater in four exposure conditions:
- Permanently submerged zone
- Tidal zone
- Splash and spray, and
- Coastal zone (up to 1 km from coastline).
Diffusion model:
Diffusion is a transfer of mass of free molecules or ions in the pore solution resulting in a net flow from regions of higher concentration to regions of lower concentration of the diffusing substance. This mode of transport operates in fully saturated media such as fully submerged concrete structures. For porous material like concrete, the diffusion coefficient, D, is the material characteristic property describing the transfer of a given substance driven by concentration gradient. In steady-state chloride diffusion, the effective driving force is the gradient of the free chloride ions in the pore solution; the diffusion coefficient is referred as Dfree or Df. Df can be determined from the difference in concentration of chloride ions in the two cells separated by the concrete. This type of test is used only in research as it is extremely time-consuming and thin sections of mortar or concrete are tested. In the transient or non-steady-state diffusion process, the mass balance equation describes the change of concentration in a unit volume with time. This can be accounted for by Fick’s second law of diffusion
Cx = Cs [1 – erf (x/2(Dt)^0.5)]
where
Cx is the chloride concentration at distance x,
x is the distance from the exposed surface,
Cs is the surface chloride concentration,
D is the chloride diffusion coefficient,
t is the exposure time, and
erf is the error function
ASTM C 1556 is a standard method for determining the apparent chloride diffusion coefficient of cementitious mixtures by bulk diffusion. Precision estimates are given for both apparent diffusion coefficient and surface chloride concentration. It refers to a precision data source from an interlaboratory study of NORDTEST NT Build 443.
In a non-steady-state diffusion process, the gradient of the free chloride ions in the pore solution is the effective driving force; the diffusion coefficient derived from measured free chloride concentration profiles is the effective diffusion coefficient, Deffective or De. The effective diffusion coefficients can also be derived from measured concentration profiles in existing structures and serve as a basis for estimates on the future progress of the chloride penetration.
Capillary suction and absorption model
Capillary absorption is the transport of liquids due to surface tension acting in capillaries. It is influenced by the viscosity, density and surface tension of the liquid and the pore structure (radius, tortuosity and continuity of capillaries) and surface energy of the concrete. Chloride can therefore be transported in the liquid solution.
When concrete in not in permanent contact with a liquid such as in the tidal zone, a non-steady state transport of the liquid prevails. In this case, the amount of liquid absorbed at the surface of the concrete as well as the amount of liquid transported at any distance from the surface is a function of time.
For short-term contact of the concrete with a liquid, the velocity of the take-up is referred to as initial absorption rate, a, given by the mass of the liquid absorbed per unit area and a function of the contact time, t:
a = dm/ A x f (t^n)
where
a = absorption rate (g/m2 sn)
dm = take-up of liquid (g)
A = area in contact with water (m2)
f (t^n) = time function
A time function = t^0.5 is usually valid but functions other than t^0.5 may also be valid
In practice, non-steady state capillary absorption is the mode of transport measured as sorptivity or initial surface absorption test (ISAT).
Different standard tests are available for measuring the transport of chloride ions. Where appropriate, a number of parameters and corresponding test methods can be examined related to various chloride transport mechanisms, as follows:
Diffusion tests
Chloride diffusion coefficient, De.28 (10–12 m2/s) Test method: ASTM C 1566
Rapid Chloride Permeability Test, RCPT (Coulomb) Test method: ASTM C1202
Capillary suction and absorption tests
Absorption (%)
Sorptivity (mm/min^0.5), Test method: ASTM C1585
Initial Surface Absorption Test, ISAT (10–2 ml/m2/s) Test method: Dhir R K, Jones M R, Byars E A and Shaaban I G ‘Predicting concrete durability from its absorption’, Third International Conference Durability of Concrete, SP-145, 1994, pp. 1177–1194”
Volume of Permeable Voids, VPV (%) Test method: ASTM C642
Water Permeability Coefficients, K (m/s) Test method: DIN 1048
I think you can establish your model for de-icing chloride ion ingress/intake based on the test results from a selected test method relating to capillary suction and absorption category.
As far as I know, you can fit the Fick's 2nd law of diffusion to your chloride ion profile to calculate the diffusion parameters, including the surface chloride content (Cs) and chloride diffusion coefficient (Dc). Actually there are several simplifying assumptions in this method. The same equation is typically used for analyzing data from natural exposures like marine environments or accelerated conditions like the concrete immersed in salt water, etc. Case of de-icing condition is also an accelerated environment with high chloride ion concentration at surface of specimen.
I thank you about your comments , I think the paper (Hoffman PC, Weyers RE. Predicting critical chloride levels in concrete bridge decks. In: Schueller GI, Shinozuka
M, Yao JTP, Balkema AA, editors. Structural safety and reliability: Proceedings of ICOSSAR'93. Rotterdam, 1994. p. 957-959.) is focuing on Cs concentration of structures exposing to accelerated conditions or de-icing condition .
Please, could I get this paper by library of your university.
with regard
Abbas
Yes you can, you however have to consider that chloride transport models for marine structures are defined based on exposure to chlorides from seawater in four exposure conditions:
- Permanently submerged zone
- Tidal zone
- Splash and spray, and
- Coastal zone (up to 1 km from coastline).
Diffusion model:
Diffusion is a transfer of mass of free molecules or ions in the pore solution resulting in a net flow from regions of higher concentration to regions of lower concentration of the diffusing substance. This mode of transport operates in fully saturated media such as fully submerged concrete structures. For porous material like concrete, the diffusion coefficient, D, is the material characteristic property describing the transfer of a given substance driven by concentration gradient. In steady-state chloride diffusion, the effective driving force is the gradient of the free chloride ions in the pore solution; the diffusion coefficient is referred as Dfree or Df. Df can be determined from the difference in concentration of chloride ions in the two cells separated by the concrete. This type of test is used only in research as it is extremely time-consuming and thin sections of mortar or concrete are tested. In the transient or non-steady-state diffusion process, the mass balance equation describes the change of concentration in a unit volume with time. This can be accounted for by Fick’s second law of diffusion
Cx = Cs [1 – erf (x/2(Dt)^0.5)]
where
Cx is the chloride concentration at distance x,
x is the distance from the exposed surface,
Cs is the surface chloride concentration,
D is the chloride diffusion coefficient,
t is the exposure time, and
erf is the error function
ASTM C 1556 is a standard method for determining the apparent chloride diffusion coefficient of cementitious mixtures by bulk diffusion. Precision estimates are given for both apparent diffusion coefficient and surface chloride concentration. It refers to a precision data source from an interlaboratory study of NORDTEST NT Build 443.
In a non-steady-state diffusion process, the gradient of the free chloride ions in the pore solution is the effective driving force; the diffusion coefficient derived from measured free chloride concentration profiles is the effective diffusion coefficient, Deffective or De. The effective diffusion coefficients can also be derived from measured concentration profiles in existing structures and serve as a basis for estimates on the future progress of the chloride penetration.
Capillary suction and absorption model
Capillary absorption is the transport of liquids due to surface tension acting in capillaries. It is influenced by the viscosity, density and surface tension of the liquid and the pore structure (radius, tortuosity and continuity of capillaries) and surface energy of the concrete. Chloride can therefore be transported in the liquid solution.
When concrete in not in permanent contact with a liquid such as in the tidal zone, a non-steady state transport of the liquid prevails. In this case, the amount of liquid absorbed at the surface of the concrete as well as the amount of liquid transported at any distance from the surface is a function of time.
For short-term contact of the concrete with a liquid, the velocity of the take-up is referred to as initial absorption rate, a, given by the mass of the liquid absorbed per unit area and a function of the contact time, t:
a = dm/ A x f (t^n)
where
a = absorption rate (g/m2 sn)
dm = take-up of liquid (g)
A = area in contact with water (m2)
f (t^n) = time function
A time function = t^0.5 is usually valid but functions other than t^0.5 may also be valid
In practice, non-steady state capillary absorption is the mode of transport measured as sorptivity or initial surface absorption test (ISAT).
Different standard tests are available for measuring the transport of chloride ions. Where appropriate, a number of parameters and corresponding test methods can be examined related to various chloride transport mechanisms, as follows:
Diffusion tests
Chloride diffusion coefficient, De.28 (10–12 m2/s) Test method: ASTM C 1566
Rapid Chloride Permeability Test, RCPT (Coulomb) Test method: ASTM C1202
Capillary suction and absorption tests
Absorption (%)
Sorptivity (mm/min^0.5), Test method: ASTM C1585
Initial Surface Absorption Test, ISAT (10–2 ml/m2/s) Test method: Dhir R K, Jones M R, Byars E A and Shaaban I G ‘Predicting concrete durability from its absorption’, Third International Conference Durability of Concrete, SP-145, 1994, pp. 1177–1194”
Volume of Permeable Voids, VPV (%) Test method: ASTM C642
Water Permeability Coefficients, K (m/s) Test method: DIN 1048
I think you can establish your model for de-icing chloride ion ingress/intake based on the test results from a selected test method relating to capillary suction and absorption category.
I am not quite sure if you got any useful answer to your original question. My answer would be YES!
Using the erfc-solution to Fick's second law is just a curve-fitting procedure and you are of course free to do that also for chloride profiles from de-icing salt environments. The fit will not be very good in the surface near region; you will most probably get a peak at some depth. If you have a bad concrete quality the chloride profile may also be "disturbed" by carbonation releasing most of the bound chloride so you will get one profile of free chlorides in the carbonated zone and then a (higher) profile of total chloride in the uncarbonated zone.
We have determined a lot of profiles from road environments, made the curve-fitting and published that in papers and reports, even a PhD-thesis. You can see how we dealt with the various shapes of profiles in the enclosed reports.
Hi Nilsson
I thank your attention about my question , also I repeat my thank about your comments and the reports which have been sent .
I would like to continue in contact with you in future to help me by your advises about the durability of concrete if you do not have any mind in this .Also, you are famous person and very interest in topics.
If you accept on contact with me, I will need you email .
with regard
Abbas
Which are the factors that affect the surface chloride concentration (Cs) ? Is it possible specimens exposed to the same conditions in laboratory have different Cs?
Dear Ms. Dalla,
Sorry I missed your question. Didn't you get any answer in five months?
My answer would be YES! The Cs-value in a certain exposure depends on the type and amount of binder and the water-binder ratio. We found a simple relationship with w/b: Cs=k*w/b!
For estimating Cs you would need the chloride binding isotherm, but you must also consider leaching, depending on how frequent you replace the exposure solution.
I enclose a report where you can find expressions for Cs in various environments. The ones for marine environments include leaching.
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