MODELING OF THE SURFACE CHLORIDE REDUCTION
DUE TO EXCESS ABSORBABILITY
Supakit SWATEKITITHAM, Hajime OKAMURA, Seigo NASU
Kochi University of Technology
ABSTRACT: The chloride attack on concrete structures in the atmospheric marine environment depends onthe mechanism of aerosol chloride particles which are transported to the structural members. Manyliteratures expressed the level of severity on chloride attack based on the available aerosol chloride content ata certain location. The amount of aerosol chloride particles decreases with distance from seashore under theexponential equation. The proposals of severity were also referred to this empirical trend. However, thismodel is proposed a mechanism to against former conclusion by clarifying the statement of ‘Not overall
aerosol chloride can be penetrated into concrete’. This means that the total amount of absorbed aerosol
chloride ions is not equaled to the total chloride ions in concrete. This model is explained about the limitationof aerosol chloride ions penetration due to the saturated degree of moisture which is able to transport inconcrete in a period of time. During a big typhoon, a lot of aerosol chlorides particles are transported to theconcrete surface, however this phenomenon can also result the surface soaking from too much aerosol
chloride ions on the surface. Whenever, concrete can not absorb total water on the surface, the surface issaturated and droplets will be formed. A location where is located very near to seashore, might have a surface
soaking many times in a year. A structure near seashore might be very severe for chloride attack, but theseverity is also limited by the environmental itself. This phenomenon can be thought as similar as the effectof rain-induced surface chloride dissolution.
KEYWORDS: Aerosol chloride, Surface chloride, Excess absorbability, Surface dissolution
1. INTRODUCTION

pressure, and rain. The calculation for this kind ofstructure must be assumed that transported aerosol

The model of chloride penetration is referred to thend

Fick’s 2

law of diffusion, and advection of freechloride ions as theoretically expressed in Eq.1. The

chloride ions are primarily installed within this layer.
The concrete surface at 2mm depth is defined as aboundary layer as shown in Figure 1.

chloride penetration in the atmospheric structure due∂C free ( x ,t )to aerosol chloride ions is different from the∂tmechanism of them in the submerged condition. In
submerged condition, the exposed salt concentration

where,=∂F free ( x ,t )∂x

Cfree(x,t) is free chloride at a depth and time

is constantly averaged 3% of NaCl. The structure in(mol/l)land will be subjected with aerosol chloride particles

Ffree(x,t) is flux of free chloride at a depth

which depend on the environmental loads such asand time (mol/dm2/hr)

wind, sea wave, distance, temperature, atmospheric(1)

Figure 1 Definition of 2mm surface layer for storing all aerosol chloride particles
The total surface chloride content is calculated by

Ctotal ( 0,t ) =where,following expression as follows [Swatekititham
2006],W( h , x )(2)105 ⋅ M cl ⋅ φ ⋅ S ( 0,t ) ⋅ Δx

Dt = Dcl ⋅ φ ⋅ S ( x ,t ) ⋅ e[ 2285 (11−)]
293 273+Tk(4)

Ctotal(0,t) is total chloride concentration on
surface layer at a certain time (mol/l)

where,W(h,x) is the transported aerosol chloride at a

Dcl is diffusion coefficient of chloride ionsin pore water [Ishida 1999]

2Tk is concrete temperature in Kelvin.

certain height and depth (mg/dm )Mcl is molecular weight of chloride ion

φ is concrete pore ratio

Maruya et al, 1998 proposed the relationship

S(0,t) is surface saturation

between total chloride and fixed chloride and theΔx is surface layer of 2mm which has

proposal is used for determining the free chloride

consistent chloride concentration

concentration at any depths in concrete for this[Swatekititham 2004].

study.

Flux of free chloride content is calculated by ionsdiffusion and the advection due to moisture gradient.

2. MECHANISM OF EXCESS CHLORIDEIONS ABSORBABILITY

The flux due to diffusion is calculated by themultiplication of diffusion coefficient and free

Referred to Figure 2, the aerosol chloride particles

chloride gradient. Another flux is chloride movement

which transport to concrete surface will be

together with the moisture flux. The combination ofaccumulated on 2mm surface layer. Considering on

free chloride flux is expressed as follows;

this surface layer, the limitation of aerosol chloride∂C free ( x ,t )penetration is due to the rate of moisture flux

F free ( x ,t ) = Dt .[where,∂x] + Dw

∂S ( x ,t )∂x.C free ( x ,t ) (3)transport. Thus, the surface soaking criteria can beexplained by Eq.5.

Dt is chloride ions diffusion coefficient

Fin-water > Fout-waterin concrete (dm2/day)
S(x,t) is degree of saturation in concrete
Dw is moisture diffusion coefficientin2

concrete (dm /day) [Saeki T. et al, 1990]

where,(5)

Fin-water is flux in the surface layer due toaerosol chloride (water particles
containing 3% salt)

Fout-water is flux out the surface layer due
The diffusion coefficient, Dt is calculated by theto water penetration in concreteFigure 2 Modeling of salt water drop due to excess moisture absorbability

Figure 3 Calculation of maximum water ingression (cm3) and conversion to the limitation of aerosolchloride (mg/dm2) with time-dependent (hours)
For a structure far from seashore, this criterion isexamples were done in 2 cases; case1: w/c = 0.50

happened only in a huge typhoon. For a structureand saturation 0.70, case 2: w/c = 0.50 and saturationnear seashore, it can be happened more frequent

0.90 for whole depth as illustrated in Figure 3. The

either small-size typhoon or during windy period.

maximum water ingression is converted into theWhen concrete surface is soaked, the salt water layer

limited aerosol chloride under the assumption of 3%

is formed and the droplets of salt water are occurred.

salt concentration consistently. The limited aerosol
chloride in low saturated level in concrete is higher

2.1 Modeling on moisture transport in concrete

than that in high saturated level due to the residual

The moisture flux out of surface layer is necessary to

free-pore space for water storage. For example,

calculate as the rate of absorbability. Referred tomaximum water ingression, in the case of w/c = 0.50

Saeki et al’s model, 1990 of moisture transport inand S = 0.70, is approximately 0.52 cm3. This

concrete, the amount of moisture transport with timeamount of water is come from the aerosol chloride

is calculated under the condition that surface layer isparticles which have 3% salt concentration. Thesaturated. The amount of moisture which canlimited aerosol chloride contents were calculated astransport in concrete depends on the pore volume

equaled to 10 (mg/dm2) for the first hour.

and existed degree of saturation. The calculation

lim[C total ( 0,t ) ] = 3% NaCl ( sea )t → ti

2.2 The surface chloride dissolution model
In general, chloride concentration in the 2mm(6)surface layer is increased due to the amount oftransported aerosol chloride ions. Without thewhere,

ti is time to equilibrium state depending on

consideration of excess absorbability, the surface

the surface depth (for 2mm surface depth, ti

chloride is gradually increased through the structural

is within 1-2 hours)

life. For long-term exposure, the surface chloride

3% NaCl(sea) is sea salt concentration which

concentration is increased much higher than thethe average is 3% for overall Japanaerosol salt concentration of 0.51mol/l. If the surface

seashore.

is soaked during a typhoon, the equilibrium of freechloride concentration in surface layer and outside

2.3 Simulation of chloride penetration for Ananai

salt concentration is taken. Afterward, the surface

Bridge (Kochi 150m from seashore)soaking effect is able to limit the surface chloride

According to the calculation in above model using

content to a certain level. The parameter affecting

the environmental condition in Kochi prefecture in

the times of surface soaking per year is the distance

2003, aerosol chloride particles can cause the surface

from seashore and location. This phenomenon is alsosoaking only once a year. The surface chloride

able to explain why the chloride concentration at theconcentration is fluctuated due to the aerosol

surface layer of structures in tidal and splash zones

chloride content attached occasionally and theare not dramatically different from it in thedegree of surface saturation is so scattered. However,atmospheric zone near seashore. Structures exposed

the tendency of the surface chloride content might bedirectly to seawater are also subjected to theeither the inversion of exponential function or thedesalination out of surface layer.

logarithm of time as expressed in Figure 4.

Figure 4 The surface chloride simulation for Ananai Bridge (150m from seashore)

1

Old part: 52 years old

3

New part: 37 years old

Figure 5 The investigation on surface chloride content of Ananai Bridge2

No.2-3: (37 years)

No.1: (52 years)

Figure 6 Comparison between data and computational results of members No.1-3

Figure 7 Comparison between PWRI data and computational results
Next, the further comparison of chloride profile inand chloride content in 3 types of concrete

concrete is essentially proved the accuracy ofspecimens was done all over Japan. The results ofchloride penetration model including this effect. The

aerosol salt measured by plate and concrete areinvestigation of surface chloride content of thiscompared. The aerosol salt collected by plate nearbridge has been done as shown in Figure 5.

seashore is much larger than that in concrete
specimens as illustrated in Figure7 (left). By

According to the data of chloride distribution inapplying this phenomenon of excess absorbability

concrete, there are some concrete specimens werefor 7 data in 0-20m from seashore, the limitation of

cored from this bridge. The variety of surface

maximum aerosol chloride which is able to absorb inchloride distribution for each member is not wellconcrete is calculated and the comparison is shown

understood yet, thus only 3 data of similar surface

in Figure7 (right).

chloride content can be compared. The compressive
strength of 3 selected members is 42, 40.8 and 39.3
2(N/mm ), respectively. The comparison between data

Referred to JSCE design specification in 1999 andmodifieddiffusion

coefficient

in

2002,

theand calculation is expressed in Figure 6, respectively. calculation of corrosion initiation time under theThe calculation for member No.1 is largely

chloride threshold value at 1.2 kg/m3 at 5cm

overestimated at 52 years, but it is slightly deviatedcovering depth is expressed as 6.8 years for thisfor members’ no. 2 and 3 at 37 years

bridge which has w/c = 0.55 and constant surface
chloride content of 3.8 kg/m3 for distance 150m from

2.4 Verification by PWRI investigated data (1985)

seashore. Comparing with the irregular surface

The investigation of aerosol salt by 10x10cm plate

chloride increment including excess of absorbability

Figure 8 The increment of chloride content at w/c = 0.55 and covering depth 5cm in 150m from seashore inKochi Prefecture, Japanand the result of chloride distribution at the same

ACKNOWLEDGEMENT

covering depth in Figure 4 & 8, the corrosion isinitiated after 14.8 years of exposure. The surface
3The research report in this paper was supported bychloride concentration to reach 3.8 kg/m takes 30

the Grant-in-Aid from the 21st century COE Program,years of exposure. This mechanism can help

No.36401 K-4, ‘Social Management Systems’. I

calculatewould extent my appreciation to Mr. Ishikawa Yasuji,

timeofcorrosion

initiation

more

appropriate than the current specification.

Head of management in second division, Tosa
National Highway, Shikoku Bureau who sincerelyprovided the experimental data of Ananai Bridge for

3. SUMMARY

this study.
The current state of simulation model is able tocalculate the chloride profile in concrete due to theREFERENCES

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