Reviews Chlorine Ion Transport Mechanisms in
Concrete and Factors Affecting it
Pooneh Shahmalekpour
1, Mir Abdolhamid Mehrdad
2, Meisam Peikarnegar
31,2. Department of Civil Engineering, University of Guilan, Rasht, Iran
3. Department of Civil Engineering, Ahrar Institute of Technology and Higher Education, Rasht, Iran
ABSTRACT: The goal of current research is considering mechanism of transporting chloride ion in concrete and factors affecting it. In this direction 6 cases of methods of transporting chloride ion has been considered. Also parameters such as conditions of loading, conditions of confrontation, regards of designing and implementation and components and plan of mixing on penetration of concrete generally and penetration of chloride ion specifically are effective.
INTRODUCTION
As we know chloride ion enters concrete from external environment, in case of contact with polluted soil or sea water, and from internal environment (instrument having chloride ion). However regarding high importance of chloride ion as an important factor in reducing reliability of concrete, mechanism of chloride ion into concrete has ben known well. It should be paid attention that penetration of chloride ion into concrete is not affected by a factor but it different factors may interfere in this action simultaneously.
Mechanism of transport of chloride ion into concrete
Mechanism of penetration of chloride ion into concrete can be limited into the fallowing cases:
Ion diffusion: ion diffusion is the movement of matter affected by difference of concentration and or more exactly it can be said that ion distribution is a chemical potential from an environment with high concentration into an environment with low concentration. It should be paid attention that when we talk about chloride transport only free chloride ion can participate in this difference of concentration or chemical potentials.
Capillary suction: capillary suction is movement (transport) of water and ions into porous concrete environment due to surface tension of capillary pores.
Permeation: fluid transport under the act of gradient or change of pressure is called penetration. Migration: migration means move of ions under the effect of gradients of electrical potentials. Absorption: move of ions under the effect of gradients of humidity is called absorption.
Combined transport: in tidal zone and coverage region that is in situations that concrete are not saturated completely and are continuously wet and dried, permeation of chloride ion into concrete is more rapid than pure distribution (T259). In this direction simultaneously salts move with mechanism of distribution for increasing depth of permeation and simultaneous effect of distribution and capillary suction creates combined mechanism of movement of ions into concrete. In concretes exposing to sea environment, permeation of chloride ions can be three time more than usual mechanism (Nordtest Method, 1995). In sea mechanism much amount of sea water having chloride are sauced into it while concrete is being wet and when concrete is exposed to air humidity of its surface is dried and much amount of chloride ion remains on surface of concrete (T259). Rapid dryness of concrete (due to much environmental) causes much oxygen exist in pores of concrete and create much negative pressure so while concrete is being wet, much water is absorbed. Repeat of wet and dryness of concrete increases concentration of salt at internal environment of concrete and may concentration of chloride ion even in pores become more than concentration of chloride ion of sea water. Due to feature of hydrophilic of chloride ions, degree of humidity inside concrete increase by increase of ions and so if there is sufficient humidity chloride ions enter deeper places through distribution. Generally it can be said that except component of concrete that continuously drawn in sea wearthere is combination of all ways of concrete transport in most conditions (T259).
168 chloride can act as a factor for preventing or delaying erosion with reduction of chloride ion in permeated solution (T259).
Effective factor on chloride ion transport
Generally it can be said that reduction of concrete permeation is solution and key problem of reliability and it should be paid attention that effective factors inconcrete permeation is not limited to its mixing plan but in concrete with good mixing plan factors such as incorrect penetration like incomplete mix of concrete (incorrect mixing), lack of sufficient concentration and improper production leads to high penetration. Generally it can be said that main parameters that affect concrete permeation generally and permeation of chloride ion specifically are mentioned in table 1 schematically (T259).
We explain some of the most important factors.
Table1.parameters affecting chloride ion permeation into concrete (Nordtest Method, 1995)
Constituent component and mix design
Regards of designing and implementing Conditions of confrontation
Conditions of loading
W/C
Type of cement Mineral additives Criteria of cement Maximum size of grain Type of aggregation Concrete coverage
density production
surface coverage on concrete constructive joints and cold joints concrete temperature
humidity
Degree of temperature Conditions of confrontation Time of confrontation Surface tension
Features of cracks
Constituent materials and concrete mix design Ration of water to cement
As we know permeation of chloride ion into concrete depends on amount of capillary pores of cement matrix for distinct conditions of production that it has much dependency to water into cement. Generally it can be said that for a distinct hydration degree permeation into concrete has lower ration of water to cement (T260).
Type of cement
As we know cement type 5 due to low degree of C3A in it acts weak regarding reliability of concrete in polluted environments by presence of chloride ion. On the other hand type of consumptive cement affects degree of connective chloride.
Additives (pozzolans)
As we know pozzolans such as silica ash against chloride ion in comparison with usual concrete are less and it can be said that permeation of usual concrete against chloride is 20 times more than concrete having silica ash.
Regards of designing and implementing Production
obviously proper production for achieving resistance, low penetration and concrete reliability is necessary. Since penetration of concrete is generally depending on the volume and size of connected capillary pores of concrete and also tiny crack in border layer of paste and aggregate, concrete should be produced as cement while hydrating change to cement matrix with small capillary pores during the first confrontation (T259).
Surface protection
Nowadays using surface protection has been prevalent for preventing advent of injury due to permeation of corrosive matters or for stopping damage of mechanism that are exposed to corrosive matters. Variety in division of surface protection is a lot. A kind of division is based on interaction between protective matters and concrete that are divided into 4 groups as below (W hithig, 1991).
Penetrants,
169
Conditions of confrontation Humidity
since penetration of chloride ion into concrete is done through penetrative liquid in capillary pores, cracks, in case of saturation of concrete, chloride ions are penetrated into chloride ion easier. Therefore degree of chloride ion and depth of its penetration depend on environmental conditions at concrete surface extensively (T259).
Temperature
Another important parameter in penetration of chloride ion into concrete is temperature; in research it is observed that increase of temperature from 120c to 300 or from 20. To 50 causes increase of distribution of chloride ion to the degree of 50% and 120% (T259). However information about effect of temperature n chloride ion is limited and by more study in warm regions at the south of country we can achieve more data (T259).
Loading conditions
In this section, the effects of stress and cracks on the penetration of chloride ion into the concrete are briefly described.
tension level
The effect of loading on the penetration of chloride ion has been investigated by different people (Andrade, 1993; Ozyildirim, 1994). In these studies, it is found that increasing the stress up to 65-70% for ultimate compressive strength has no significant effect on the penetration of chloride ion. Also, chloride ion penetration are more observed in the areas under the stress, in which there is cracking due to the mechanical and thermal loadings as well as shrinking. It can be related to the damage of aggregate border area and pate due to the loading that results in the increasing of porosity in the border area. This phenomenon is commonly referred to as stress-corrosion effect (T259).
Cracks
Cracks caused by static and dynamic loads as well as sediment even in the concretes with high quality can cause to penetrate chlorine ions into concrete (Andrade, 1993). In any case, regardless of the cracks at early ages, such as cracks due to plastic shrinkage and heat of hydration, in the hardened concretes, cracks and micro-cracks resulting from shrinkage, drying, thermal stress, wetting and drying and the loading and unloading are of the most important factors in chloride ion penetration in tropical regions such as the Persian Gulf (MrGrath and Hoot on, 1996).
The equations governing the transfer of chloride
As we know, chloride penetration into concrete has a complex process that includes diffusion, capillary suction, and translational motion with water. Sometimes external electrical potential is applied in order to chlorine ions movement on the concrete to evaluate diffusion coefficient in an accelerated way. In this section, the diffusion mechanism and the rules governing them were taken into consideration (Stanish et al, 1996).
Figure1. Chlorine ions diffusion in a liquid system (Stani sh et al , 1996 )
Diffusion Function
By taking a fluid system into account such as figure 1, in which chlorine ions diffusion occurs, the chemical potential function of chlorine ions can be defined as follows (Blomberg, 2003):
cl -cl
-cl
-cl -cl -cl -cl
-cl- cl- cl
-cl -cl
-cl -cl -cl -cl -cl
-cl -cl
-cl
-cl -cl -cl -cl
-cl- cl- cl
-cl -cl
-170
c
RTLn
.
0
Under the influence of the chemical potential gradients, ions are moving in the direction of this potential difference, and since chlorine ions cannot exist alone, so it is clear that the presence of cations must also be considered. As a result of the chlorine ions movements, an electric field contour is produced between chlorine ions and surrounding cations. And as a result of chlorine ions movements, chlorine ions are drawn back, therefore, the average rate of the chlorine ions is:
x u x Ln x c Ln BRT x u x B Or x u c Ln Ln x c Ln BRT 1
Where, B is Proportionality Factor, and
u
is ion mobility.Considering DBRT, the tone of ion flow in per unit area (flux) is as follows:
x u c c Ln Ln x c Ln c D c
Jd
. . 1
Or x u c c c x c D
Jd
. 1
In which activation coefficient and the electric field counter are dependent on the concentration of free chloride. As
solving the problem of
c
c
.
andc
u
x
is hard, (Although the solutions proposed by some researchers), thesestatements are generally removed, and Fick's first law is defined as follows:
x
c
D
J
d
Free chloride and bound chloride
This is free chloride that can move from one point to another. And destroys resistant layer on the surface of rebar and corrosion will start. When free chloride ions penetrate into the concrete from surrounding solutions, some of them may become trapped by hydration products that are called bound chloride. Although the mechanisms of bounding chloride are not so clear, we believe that both physical and chemical reactions are involved in the formation of chlorine bond. Wide ranges of hydrated gels provide a suitable space for the formation of physical bound chloride. While Friedel's salt are normally known as a great product of Chemical Bonding.
Bounded chlorides are usually considered as harmless for rebar, and in any way, chloride bounding can delay free chloride movements. Therefore, at the time of studying chloride ion movements, the effect of Bounded chloride should be taken into account (Stanish et al, 1996).
CONCLUSION
171 control concrete, concrete containing micro silica and concrete containing metakaolin, concrete containing rice hull ash and, concrete containing fly ash tolerate more reduction compared to increasing the portion of water to the cement from 0.35 to 0.40. But this process cannot be observed in concretes containing pozzolan compositions. Salt destructive environment reduces compressive strength of all mixing plans compared to ordinary water. Also, the effect of Salt destructive environment under wet and dry cycles in compressive strength reduction is more than immersion salt destructive environment; by increasing the age of concrete, the difference of compressive strength in destructive salt environments and in ordinary water environments will reduce.
REFERENCES
Andrade, C. (1993). Calculation of Chloride Diffusion Coefficients in Concrete From Ionic Migration Meas urements, C ement and C oncrete R es earc h, Vol. 23, N o. 3, pp. 724 -742.
Blomberg, J. M. (2003). Laboratory Testing of Bridge Deck mixes, RDT 03-004, Missouri Department of Transportation Research, Development, and Technology.
M rG r at h , P. , H o ot on , R . D . ( 1 9 9 6) . I nf l u en c e of V ol t ag e on C h l or i d e D if f us i on Coefficients From Chloride Migration Tests , Cement and Concrete Research, Vol.26, No. 8, pp. 1239-1244.
Nordtest Method, (1995). Accelerated Chloride Penetration into H ardened C oncrete, N ordtest, Espoo, Finland, Proj. 1154-94. Ozyildirim, C. (1994). Rapid Chloride Permeability Testing of Silica-fume Concrete, Cement, Concrete and Aggregates, CCAGPD, Vol. 16, No.
1, pp. 53-56.
St an i s h , K. D . , H o ot on , R . D ., T h om as , M . D . A . ( 1 9 9 6) . T es ti n g t h e c h l or i d e p en et r at i on R es is t anc e of C onc r e t e: A lit er at ur e R e vi ew " , FHW A C ontr ac t D T FH 6 1 - 9 6-R - 0 02 2, Department of Civil Engineering, University of Toronto, Canada. T259, (1980). Standard Method of Test for Resistanc e of Concrete to Chloride Ion Penetration, Americ an Association of Stat e
Highway and Transportation Officials, W ashington, D.C ., U.S.A.
T 260, (1994). Standard Meth od f or Sampling and T es ting f or C hloride Ion in C onc ret e and C onc r et e R ea M at eri als , Am er ic an As s oc i ati on of St at e H ig h w ay and T r ans p ort ati o n Officials, Washington, D.C., U.S.A.
