Study on durability of concrete by using accelerated chloride migration test

* Shih-Wei Cho 1) and Chung-Chia Yang 2)

1) Department of Architecture, China University of Science and Technology, Taipei,

Taiwan, R.O.C. 1)

swcho@cc.cust.edu.tw 2)

Institute of Materials Engineering, National Taiwan Ocean University, Taiwan, R.O.C. 2)

ccyang@mail.ntou.edu.tw

ABSTRACT

An experimental investigation is conducted to study the relationship between the chloride diffusion coefficient and the charge passed. In this study, the concrete specimens made with different w/c (ranging from 0.35 to 0.65) and different slag contents (ranging from 0% to 70%) were used in the RCPT, ACMT, and ponding tests. From the test results, the charge passed shows no direct correlation with the diffusion coefficient obtained from the ponding test on the concrete containing slag, but it indicates fair correlation on the concrete without mineral admixtures. However, no matter slag is added, it shows a linear correlation between the steady-state diffusion coefficients from ACMT and the non-steady-state diffusion coefficient from the ponding test. It appears that the ACMT may be a more useful method than RCPT, when the long-term chloride diffusion test is concerned. 1. INTRODUCTION M Both the RCPT and ponding test are widely used to predict the chloride penetration resistance of concrete. The ponding test is standardized by AASHTO and ASTM as AASHTO T259 and ASTM C1543. This test must take 90 days and the procedures are very complicated. In 1981, Whiting developed the RCPT method to specify the chloride permeability of concrete, which later is standardized by AASHTO and ASTM as AASHTO T277 and ASTM C1202. This test used electrical conductive to classify the chloride permeability without analysis and the test takes only 6 hours. From early researches [1], they found the total charge passed of the RCPT is close correlated with the integral chloride content of the ponding test. However, if mineral admixtures such as fly ash and slag are added into concrete, some researchers [2, 3] pointed out that both test results were poorly correlated and the RCPT might produce misleading results. Therefore ASTM C1202 recommends which test method to be applied for different types of concrete. The correlation between the RCPT and the ponding test is also established in this standard. Moreover, because the ponding test is

1)

Assistant Professor 2)

Professor

not a simple and short term test, several modified methods have been created to predict chloride penetration [4, 5]. Apply an external electrical field to accelerate chloride migration is the most used in the modified methods. The objective of this study is to determine the chloride permeability of the different concrete mixes with various test methods. The modified method of the accelerated chloride migration test (ACMT) is used to determinate the steady-state diffusion coefficient, the total charge passed is obtained from the RCPT, and the non-steady state diffusion coefficient is calculated from the ponding test. These test results are compared and correlated. 2. EXPERIMENTAL PROGRAM

2.1 Mixture Design In this study, concrete is made with Type 1 cement, water, slag, river sand, and crushed stone (maximum size: 10 mm). The mixtures are listed in Table 1. The specimen indexes are defined as follow: the first letter, A, B, C, and D, is the water/binder ratio (W/B) of 0.35, 0.45, 0.55, 0.65 respectively, and the number is the percentage of cement replaced by slag. All cylindrical specimens (Φ100 mm × 200 mm) were cast and cured in water for 90 days. Two 50 mm thick discs for the ponding test and the RCPT and one 30 mm thick disc for the ACMT were obtained by cutting from the central portion of the cylinder.

Table 1. Mixture proportions

Mix No. W/B Unit content: kg/m

3

Water Cement Slag Fine Aggregate Coarse Aggregate

A0 0.35 243 695 0 593 733

A20 0.35 241 550 137 593 733

A40 0.35 238 408 272 593 733

A50 0.35 237 338 338 593 733

A70 0.35 234 201 468 593 733

B0 0.45 243 540 0 719 743

B20 0.45 241 428 107 719 743

B40 0.45 239 318 212 719 743

B50 0.45 237 264 264 719 743

B70 0.45 235 157 366 719 743

C0 0.55 243 442 0 801 743

C20 0.55 241 351 88 801 743

C40 0.55 239 261 174 801 743

C50 0.55 238 216 216 801 743

C70 0.55 236 129 300 801 743

D0 0.65 243 374 0 858 743

D20 0.65 241 297 74 858 743

D40 0.65 239 221 147 858 743

D50 0.65 239 184 184 858 743

D70 0.65 237 109 255 858 743

2.2 Ponding Test The ponding test is similar to the test described in ASTM C1543. Each 50 mm thick disc was air dried and the lateral surface of discs were coated with epoxy. After epoxy was hardened, a container for the 1.5 cm deep 3% sodium chloride solution was created by sealing one edge of the specimens with an acrylic ring. The duration of the ponding test is 90 days. After the ponding, all crystallized salts on the surface were completely removed by wire brushing, and three cylindrical specimens (Φ20 mm × 50 mm) were cored out from the ponding disc. Each cylindrical specimen was then cut into ten 5mm thick slices. Thereafter, the slices were grinded to powders which passed a 300μm sieve for the chloride analysis. The results of chloride analysis are plotted in the relation curves of the chloride contents and the depths. The non-steady-state chloride diffusion coefficient pD is derived from the regression of the relation curves by using the

Fick’s second law.

2.3 Accelerated chloride migration test (ACMT) The accelerated chloride migration test is a modified version of the ASTM C1202

method. The major difference between the RCPT and the ACMT is the testing chamber. The chamber for the ACMT is 4.5 liter, but is about 250 ml for the RCPT. The increased volume is to avoid the Joule effect during the test, and make the periodically and continuously measurement of the accumulative chloride ion concentration by the potentiometric titration method practical. The duration of the test is depended on the quality of the concrete, which usually takes 7 to 10 days. From this test method, the chloride ion diffusion coefficient sD is calculated by the Nernst-Plank equation.

3. RESULTS AND DISCUSSIONS

3.1 Total charge passed The total charge passed is obtained from the RCPT are listed in Table 2. The

relations of the total charge passed and the W/B ratio with different slag contents are shown in Fig. 1. For a given W/B ratio, the total charge passed is decreased with increasing the slag content. This means that the charge passed decreases with concrete containing mineral admixtures. For the concrete containing more than 40% slag, the total charge passed is significantly decreased.

3.2 Non-steady-state diffusion coefficients from the ponding test From the ponding test results, the non-steady-state diffusion coefficient is decreased

with the slag added into the concrete. A comparison between the results of the ponding

test and the RCPT is shown in Fig. 2. The test data from the researches of McGrath

and Hooton [7], and Sherman et al. [8] are also plotted. From Fig. 2, it shows no

obvious correlation between the non-steady-state diffusion coefficient and the total charge passed if the mineral mixtures are added. However, if only the concretes

without mineral admixtures are considered, Fig. 3 shows a linear relationship between

the non-steady-state diffusion coefficient and the total charge passed. Several researches pointed out [7, 9 and 10] that the mineral admixtures may change the

chemical composition of the pore solution and the microstructure of the concrete and hence affect the measuring current of the RCPT.

Table 2. Test results from the RCPT, ACMT, and the ponding test

Mix No. RCPT, Total charge

passed (coulomb)

Diffusion coefficient (× 10-8

m2/hour)

Ponding, Dp ACMT, Ds

A0 5311 2.44 17.51

A20 4685 -- 13.92

A40 2208 -- 7.06

A50 1543 1.36 6.19

A70 1034 -- 5.2

B0 7568 5.51 29.14

B20 5630 -- 20.29

B40 2387 -- 12.43

B50 1825 1.47 7.29

B70 1122 -- 6.98

C0 9639 8.37 39.48

C20 6355 -- 20.47

C40 2709 -- 13.37

C50 2148 1.69 10.66

C70 1350 -- 7.73

D0 10442 12.74 56.02

D20 7114 -- 31.41

D40 2992 -- 15.83

D50 2565 2.34 15.61

D70 1686 -- 13.87

Fig. 1 The relationships between the total charge passed and W/B

0

2000

4000

6000

8000

10000

12000

14000

0.35 0.4 0.45 0.5 0.55 0.6 0.65

RC

PT,

To

tal c

har

ge p

asse

d (

cou

lom

b)

W/B

0%Slag 20%Slag

40%Slag 50%Slag

70%Slag

Fig. 2 The correlation between the chloride diffusion coefficient from the ponding test

and the total charge passed

Fig. 3 The correlation between the chloride diffusion coefficient from the ponding test

and the total charge passed (concrete mixes without mineral admixtures)

0

2

4

6

8

10

12

14

0 2000 4000 6000 8000 10000 12000

Dif

fusi

on

co

effi

cien

t fr

om

po

nd

ing

(×1

0-8

m2

/h

ou

r) /

ho

ur)

Total charge passed (coulomb)

Experiment results

Previous study [14]

Ref. [15]

Ref. [16]

y = 0.0009x - 1.1514

R² = 0.9472

0

1

2

3

4

5

6

7

8

9

0 2000 4000 6000 8000 10000 12000

Dif

fusi

on

coef

fici

ent

from

pon

din

g

(×10

-8 m

2 /

hou

r) /

hou

r)

Total charge passed (coulomb)

Experiment results

Ref. [15]

Ref. [16]

3.3 Steady-state diffusion coefficients from the ACMT

The diffusion coefficients obtained from the ACMT are listed in Table 2. Fig. 4 is the

relationship between the steady-state diffusion coefficients and W/CM with different slag contents. The previous research results [14] are also plotted in this figure. It can be seen that the steady-state diffusion coefficient is decreased with the increasing of the slag content. For the concrete containing 25% fly ash, the steady-state diffusion coefficient is lower than that of the concrete containing 20% slag.

In Fig. 5, the steady-state diffusion coefficients from the ACMT are compared with

the total charge passed from the RCPT. The results of the concrete adding fly ash [14] are also shown in this figure. It shows that no obvious correlation between the steady-

state diffusion coefficient and the total charge passed, such as Fig. 3.

Fig. 6 shows the relationship between the steady-state diffusion coefficients from

the ACMT ( sD ) and the non-steady-state diffusion coefficient from the ponding test

( pD ). By linear regression, the empirical relationship between sD and pD is statistically

derived as:

810235.12406.0 xY (1)

The 2R for this regression Equation (1) is 0.972. It appears that sD correlates linearly

with pD regardless of concrete with or without containing mineral admixtures. When

comparing Fig. 6 and Fig. 3, it shows that the ACMT may be a more useful method than

the RCPT, when the long-term chloride diffusion test is concerned.

Fig. 4 The relationships between the steady state chloride diffusion coefficient from the

ACMT and W/B

0

10

20

30

40

50

60

0.35 0.4 0.45 0.5 0.55 0.6 0.65

Dif

fusi

on

co

eff

icie

nt

fro

m A

CM

T

(×1

0-8

m2

/ho

ur)

W/B

0%Slag 20%Slag

40%Slag 50%Slag

70%Slag

Fig. 5 Correlation between the steady state chloride diffusion coefficient from the ACMT

and the total charge passed

Fig. 6 Correlation between the steady state chloride diffusion coefficient from the ACMT

and the non-steady state chloride diffusion coefficient from the ponding test 4. CONCLUSIONS

The conclusions of the experimental investigation are summarized as below: 1. The charge passed obtained from the RCPT and the steady-state diffusion

coefficients obtained from the ACMT are decreased with the increasing of the slag

0

10

20

30

40

50

60

0 2000 4000 6000 8000 10000 12000

AC

MT

, D

iffu

sion

coeff

icie

nt

( 10-8

m2 /

hou

r)

Total charge passed (coulomb)

Experimental result

y = 0.2406x - 1.235

R² = 0.972

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60

Dif

fusi

on

co

effi

cien

t fr

om

Po

nd

ing

( 1

0-8

m2

/ho

ur)

Diffusion coefficient from ACMT ( 10-8 m2 /hour)

Experimental result

Previous study [14]

content. 2. For the concrete containing mineral admixtures, there is no obvious correlation

between the non-steady-state diffusion coefficient obtained from the ponding test and the total charge passed obtained from the RCPT. But a good correlation was observed in the concrete without mineral admixtures.

3. There is no obvious correlation between the steady-state diffusion coefficient obtained from the ACMT and the total charge passed obtained from the RCPT.

4. A good correlation is observed between the steady-state diffusion coefficients from the ACMT and the non-steady-state diffusion coefficient from the ponding test. It appears that the ACMT may be a more useful method than RCPT, when concerning the long-term chloride diffusion test.

5. ACKNOWLEDGEMENTS

The financial support of National Science Council, ROC, under the grants NSC 100-2625-M-157-001 is gratefully appreciated. REFERENCES D. Whiting, “Rapid Measurement of the chloride permeability of concrete”, Public Roads

45(3) (1981) 101-112. R. Feldman, L. R. Prudencio, and G. Chan, “Rapid chloride permeability test on blend

cement and other concretes: correlations between charge, initial current and conductivity”, Con. Bulid. Mater. 13 (1999) 149-154.

T. H. Wee, A. K. Suryavanshi, and S. S. Tin, “Evaluation of rapid chloride permeability test (RCPT) results for concrete containing mineral admixture”, ACI Mater. J. 97(2) (2000) 221-232.

O. Truc, J. P. Oilvier, and M. Carcasses, “A New Way for Determining the Chloride Diffusion Coefficient in Concrete from Steady State Migration Test”, Cem. Con. Res., 30(2) (2000) 217-226.

C. C. Yang, and S. W. Cho, “An electrochemical method for accelerated chloride migration test of diffusion coefficient in cement-based materials”, Mater. Chem. Phy. 81(2) (2003) 116-125.

C. C. Yang, and L. C. Wang, “The diffusion characteristic of concrete with mineral admixture between salt ponding test and accelerated chloride migration test”, Mater. Chem. Phy., 85(2) (2004) 266-272.

P. McGrath, and R. D. Hooton, “Re-evaluation of the AASHTO T259 90-day salt ponding test”, Cem. Con. Res., 29 (1999) 1239-1248.

M. R. Sherman, D. B. McDonald, and D. W. Pfeifer, “Durability aspects of precast prestressed concrete Part 2: Chloride permeability study”, PCI J., (1996) 76-95.