CEMENT and CONCRETE RESEARCH. Vol. 9, pp. 229-238, 1979. Printed in the U.S.A. 0008-8846/79/020229-10502.00/0 Copyright (c) 1979 Pergamon Press.

DIFFUSION OF CHLORIDE IONS FROM SEAWATER INTO CONCRETE

O.E. Gj#rv and @. Vennesland Cement and Concrete Research Institute

University of Trondheim, NTH 7034 NTH Trondheim, Norway

(Communicated by Z.P. Bazant) (Received Nov. 27, 1978)

ABSTRACT

The rates of chloride diffusion from seawater into concrete made with different w/c ratios and different types of cements were studied. The effect of cathodic protection on the chloride pene- tration was also investigated. The effect of w/c ratio on the chloride content was limited mainly to a surface layer of the concrete and to short durations of exposure. For longer durations of exposure and for greater depths of penetration, cement type had a greater influence on the depth of chloride penetration than the w/c ratio. Portland cements gave as much as two to five times higher chloride penetration than blended cements. For the Port- land cements as much as 8.6% C~A was not adequate for reducing the chloride penetration compared Qith zero-content of C3A. It appears that the diffusion of chloride ions into concrete is deDendent not only on the permeability and the capacity of chloride binding but also on the ion exchange capacity of the system.

Les taux de diffusion de chloride d'eau de mer dans le b~ton, faites avec diff~rentes facteur eau-ciment et diff~rents types de ciments, ont ~t~ ~tudi~es. L'effet de protection cathodique sur la p~n~tration de chloride a ~t~ ~galement examine. L'effet du facteur eau-ciment sur le contenu de chloride s'est limit~ principalement ~ une couche superficielle de b~ton et ~ de courtes dur~es d'exposition. Pour des duties d'exposition plus longues et pour des profondeurs de p~n~tration plus longues, le type ciment avait une influence plus grande sur la profondeur de p~n~tration de chloride que le facteur eau-ciment. Les ciments Portland don- naient une p~n~tration de chloride de deux ~ cinq fois sup~rieure aux ciments m~lang~s. Pour les ciments Portland, m~me avec 8,6% C3A on ne parvenait pas ~ reduire la p~n~tration de chloride, en comparaison avec un niveau z~ro de C~A. Iien ressort aue la diffusion des ions de chloride dans Ie b~ton d~pend non seulement de la perm~abilit~ et de la capacit~ d'attache du chloride, mais aussi de la capacit~ d'~change d'ions du syst~me.

229

230 Vol. 9, No. 2 O.E. Gj~rv, 0. Vennesland

Introduction

Reinforced and prestressed concrete are finding increasing application for many important structures in the oceans. For prestressed concrete structures the allowable chloride content in the fresh concrete is strongly limited due to notential corrosion hazard. However, for concrete sea structures diffusion of chlo- rides from seawater could still cause problems. It is imoortant, therefore, to study various aspects of chloride penetration into concrete exposed to seawater.

Most of the published data on chloride penetration is based on investigations which involved exposure of concrete to solutions of individual chloride salts such as calcium chloride or sodium chloride (1-8). However, it is known now that the rate of chlo- ride diffusion into concrete may be considerably affected by the type of cation associated with the chloride. For instance, the chloride penetration is substantially higher from a solution of calcium chloride than from a solution of sodium chloride of the same chloride concentration (9). Experiments also indicate that resistance to chloride penetration is affected by the type of cement ~ 2, 4, 8) . It appears, therefore, that the diffusion of chlorides into concrete is the result of a more complex mechanism where ion exchange between the permeating solution and the Dote solution present in concrete plays an important role. As a con- sequence, it is unappropriate to apply penetration data obtained from solutions of one salt to concrete exposed to another chlo- ride salt or a more complex solution such as seawater.

In order to provide data on the rate of chloride diffusion into concrete from seawater, a number of experiments were carried out, some results of which are presented in the present paper. Since under certain circumstances concrete sea structures are also subjected to cathodic protection (i0), a few results showing the effect of impressed current on the rate of chloride penetration are also included. The present experiments are part of a more comprehensive research program on evaluation and control of steel corrosion in offshore concrete platforms.

Experimental

Materials

For the test program shown in Table i a number of mortar specimens were produced, the mix design of which is shown in Table 2. The maximum aggregate size was 3.0 mm and the aggregate was clean and inert, consisting mainly of siliceous compounds. As a basis for the investigation an ordinary portland cement was used, but in test series 2 four special types of cement including one sulfate resisting portland cement, two slag cements and one trass cement were also investigated. For all cements the chemical analysis is shown in Table 3. For the two portland cements the computed C3A contents were 8.6% and 0%, respectively.

Test specimen 9

Different types of test specimens were used. For test series 1 and 2 cylindrical mortar specimens of ~ 2.5 x i0 cm were produced. After demoulding and curing the specimens were slightly surface dried and embedded in a 5 mm thick layer of epoxy. Then, one end of each specimen was cut leaving a bulk surface free for exnosure.

Vol. 9, No. 2 231 CHLORIDE, DIFFUSION, SEA WATER, CONCRETE

Test series

TABLE 1 Test Program

Variable

Water-cement ratio

Type of cement

Polarization potential

Material

Mortar A,B,C

Mortar D

Mortar B

TABLE 2 Data on Mix Design

Material

Mortar A

Mortar B

Mortar C

Mortar D

Mix Proportions by Weight

Cement (kg/m 3)

1 (650)

1 (490)

1 (480)

1 (630)

Water

0.4

0.5

0.6

0.5

Aggregate

TABLE 3 Different Types of Cement Investigated

SiO 2

Fe203

A!203 CaO

MgO

SO 3

Insoluble residue

Ignition loss

No. 1

Ordinary Portland Cement

(8.6% C3A)

20.60

2.68

4.97

65.12

No. 2 Sulfate

Resisting Portland Cement (0% C3A)

20.50

5.83

3.53

65.31

No. 3 No. 4

Portland Portland Slag Slag

Cement Cement (30% Slag] (80% Slag)

23.50 28.88

1.94 0.61

7.63 12.00

58.23 47.68

1.50

2.62

0.20

1.30

0.80

2.91

0.10

1.34

2.74

2.68

0.66

2.21

4.24

2.61

0.75

1.54

No. 5

Portland Trass Cement

(26% Trass)

21.89

3.22

6.39

51.46

1.42

3.04

8.40

4.03

For test series 3 cylindrical specimens of ~ i0 x 28 cm were produced with a ~ i0 mm steel rod centered. For these specimens a small part of protruding steel (i0 mm) was also subjected to expo- sure.

Before any exposure all test specimens were given appropriate maturity by at least two to three months of water curing.

232 Vo l . 9, No. 2 O.E. G j~ rv , 0. Vennesland

Exposure

All specimens were exposed to fresh circulating seawater. For test series 1 and 2 the specimens were exposed at the Biolo- gical Test Station in Trondheim harbor where the water temperature was 7 ~ 2°C. For these test series all specimens were continuously submerged.

For test series 3 the specimens were exposed at the Seawater Laboratory of the Ship Research Institute of Norway in Sandefjord. In this laboratory the fresh circulating seawater was kept at a temperature of 20 ~ l°C. These specimens were also continuously submerged, and the embedded steel was subjected to three different polarization levels of -780, -1050 and -1350 mV vs SCE, respec- tively. Unprotected specimens were also included.

Chemical and Physical Analysis

For all test series specimens were removed at regular inter- vals for control of chloride penetration. For each specimen the chloride penetration was determined on 3 - 5 mm thick slices suc- cessively cut off from the specimens. In order to avoid any washing out of chlorides only dry cutting was applied. Total chloride contents were determined by the method of Berman (ii) or Volhard (12). If not otherwise stated the chloride contents are expressed as total amounts by weight of dry material.

The pore-size distribution of the specimens was determined by mercury porosimetry.

0,8

u

"~ 0,6

U

On4 - -

0,2 --

a 6 months

12 months

0 24 months

10 20 30 40

Penetration depth (mm)

FIG. 1 Rate of chloride penetration into mortar of water-cement ratio 0.4.

Vol. 9, No. 2 233 CHLORIDE, DIFFUSION, SEA WATER, CONCRETE

Results and Discussion

Effect of Exposure Time

Even the 0.4 w/c mortars showed progressively increasing depths of chloride penetration with time. For instance, the data in Fig. 1 shows that at a depth of 20 mm below the surface there was hardly any chlorides at six months of exposure, while after 12 months and 24 months the chloride content was about 0.13% and 0.3% respectively. These observations support previous experien- ces (13) indicating that it appears to be just a question of time before detrimental amounts of chloride reach embedded steel even through a high quality concrete.

Effect of Water-Cement Ratio

The data in Fig. 2 for various w/c mortars shows that after two years of exposure the influence of w/c on the chloride content is limited mainly to the portion of mortar which is near the sur- face (viz. < i0 mm). At greater depths such as 20 mm and more the mortars of different w/c ratios (from 0.4 to 0.6) did not show significant differences in chloride content. Similar observations were also made on chloride penetration into specimens with diffe- rent maximum aggregate sizes (from 3.0 to 16 mm).

From Fig. 3 it can be seen that increasing w/c ratios from 0.4 to 0.6 increased the porosity of the material from 9.7% to 12.5%, respectively. It appears, therefore, that porosity or permeability mainly affects the chloride penetration near the

§ T. 0

U

1,0

0,8

0 , 6

0,4

0,2

c: w/c = 0 , 4

,,,/c : 0 , 5

o ,,,/c ,, 0 , 6

0 10 2o 3o 4o

P~etratlon depth (ram) FIG. 2

Effect of water-cement ratio on chloride penetration after two years of exposure.

234 Vol. 9, No. 2 O.E. Gj6rv, ~. Vennesland

0 , 0 6

E u v

>.

O_

0,04

0,02

f

/2

i/!

50 1 O0 200 300 400 1000

w/c- ro6o

0,4

0,5

0,6

I 100oo

......----- ..._..... f j - - " - "

I i

Total poros i ty ( % )

9 , 7

11 ,5

12,5

100000

Pore size (4)

FIG. 3 Effect of water-cement ratio on pore-size distribution.

surface, whereas the diffusion into the interior is more affected by other controlling factors such as chloride binding and ion exchange.

Effect of Cement Composition

The data in Fig. 4 shows that at depths greater than 20 mm the cement composition has more influence on the chloride content than the w/c ratio. Already at i0 mm depth the 80% slag cement shows very low chloride content compared with all the other cements used in this study. At depths of 15 mm and more the effect of cement type is more pronounced, the portland cements showing as much as two to five times higher chloride contents than the blen- ded cements.

The data on pore-size distribution in Fig. 5 demonstrates that increasing content of pozzolanic material decreases the pore sizes. For the 80% slag cement as much as 79% of the total porosity was made up by pores being smaller than 200 A, while for the portland cements the corresponding number was only 29%. This again proves the importance of porosity or permeability. On the other hand it can also be seen from Fig. 5 that the microporosity of the 26% trass cement was close to that of the 80% slag cement, whereas the chlo- ride penetration was much higher and close to the more porous 30% slag cement. Fig. 5 also shows that it is not possible to explain the big difference in chloride penetration between portland cements and blended cements only on the basis of different pore-size dis- tributions.

Previous experiments have shown that great amounts of chlo- ride ions may penetrate into concrete in fresh circulating seawater without the penetration of equivalent amounts of sodium ions (14). The importance of the type of cations in the salt solution was pointed out already in the introduction to the present paper. It appears, therefore, that exchange of anions between the permeating salt solution and the pore solution of the concrete is an important

Vol . 9, No. 2 235 CHLORIDE, DIFFUSION, SEA WATER, CONCRETE

§ o_

_c U

0,8

~ Cement I - OPC (8 ,6 % C3A)

• Cement 2 - SRPC (0% C3A)

\ ~, Cement 3 - 30 % Slag

0,6 . ~ ~ " " Cement4- 80%Slag

~ " ~ o Cement5- 26%Trass

O'' I 0,2

I I0 20 3o 40

Penetration depth (ram) FIG. 4

Effect of cement type on chloride pene- tration after six months of exposure.

controlling factor for the chloride penetration. For blended ce- ments the pore solution has a lower concentration of hydroxyl ions Hence, the capacity for exchanging anions with the permeating solu. tion is also lower.

From Fig. 3 it can be seen that there was little difference in chloride penetration between the mortars made with zero-C3A portland cement and 8.6% C3A portland cement. According to Mehta

0,08

"g /

- / f ~

0,04 ~7 i / / / Cemen, Io,al porosity (%) / /I/ - - - - - , opt c8,6 ~ c~ ~ ,4.~ .' / i / 2. SRPC (0 % C]A)_ 14,2

0,02 _ - - ~ . / ~"y .... 3. 30 % Slog 10,9

.,/// / / . . . . . . . 4. 80% Slag '6,4

p~ / / o ~ ; " I J ' J i

50 100 200 300400 1000 10000 I00000 Pore size (A)

FIG. 5 Effect of cement type on pore-size distribution.

236 Vol. 9, No. 2 O.E. Gj~rv, ~. Vennesland

0,3 • Unprotected

o ~" 0 - 7 8 0 m V vs 5CE

"~ [] 1050 mV vs 5CE

0 , 2 ~ - 1350 rnV vs 5C£

U

0,1

0 0 I0 20 30 40

FIG. 6 Penet ra t ion depth (ram)

Effect of cathodic protection on chloride pene- tration after twelve months of exposure.

(15) a chemical binding of the penetrating chlorides cannot be expected unless the C3A content is higher than about 8%. Extensive experiments carried out by Sluijter (16) also indicate that the binding of chlorides in hydrated cement paste is primarily a mat- ter of physical adsorption to the gel surface. This may explain why C3A had such a negligible effect on the chloride penetration.

Effect of cathodic protection

The data in Fig. 6 shows that a cathodic protection potential of -780 mY vs SCE has little influence on the chloride penetration, but for decreasing potentials the effect is more pronounced. At a potential of -1350 mV about 50% lower chloride content was observed near the mortar surface, while at a depth of 15 mm the reduction was about 75% compared with unprotected specimens.

As the penetrating chloride ions are negatively charged, they tend to move in the electrical field directed towards the external positive electrode. Hence, the chloride diffusion into the specimens is slowed down. This effect has also been employed to remove chlorides from salt contaminated bridge decks (17). For this purpose, application of relatively high current densities were necessary. In the present experiments, however, the current densi- ties were too small to overcome the driving force of chloride dif- fusion inwards; they could only reduce it.

Conclusions

Experimental investigations indicate that for ordinary port- land cements it appears to be just a auestion of time before detrimental amounts of chloride reach embedded steel even through a high quality of concrete. The investigations also indicate that the effect of w/c ratio on the chloride content is limited mainly to a surface layer of the concrete and to short durations of chlo- ride exposure. For longer durations of exposure and for greater depths of penetration (viz. 20 mm and more), the cement type has a greater influence on the depth of chloride penetration than the w/c ratio.

Vol. 9, No. 2 237 CHLORIDE, DIFFUSION, SEA WATER, CONCRETE

Portland cements may give as much as two to five times higher chloride penetration than blended cements. For portland cements as much as 8.6% C3A is not adequate for reducing the chloride penetra- tion compared with zero-content of C3A.

It appears that the diffusion of chloride ions into concrete is dependent not only on the permeability and the capacity of chloride binding but also on the ion exchange capacity of the system.

Acknowledgement

The authors wish to acknowledge the financial support which was provided by Bernt Fossum's Foundation, NTNF, A/S Skarpenord, Elf-Aquitaine Norge A/S, Norsk Hydro A/S, Norwegian Contractors, The Norwegian Assoc. of Reinforcing Steel Producers, The Shin Research Institute of Norway, and Statoil.

.

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