Study on durability of concrete by using accelerated chloride migration test

Transcription

1 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.35 to.65) and different slag contents (ranging from % to 7%) 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 9 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 C122. 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 C122 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

2 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: 1 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.35,.45,.55,.65 respectively, and the number is the percentage of cement replaced by slag. All cylindrical specimens (Φ1 mm 2 mm) were cast and cured in water for 9 days. Two 5 mm thick discs for the ponding test and the RCPT and one 3 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 A A A A A B B B B B C C C C C D D D D D

3 2.2 Ponding Test The ponding test is similar to the test described in ASTM C1543. Each 5 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 9 days. After the ponding, all crystallized salts on the surface were completely removed by wire brushing, and three cylindrical specimens (Φ2 mm 5 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 3μm 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 D is derived from the regression of the relation curves by using the Fick s second law. p 2.3 Accelerated chloride migration test (ACMT) The accelerated chloride migration test is a modified version of the ASTM C122 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 25 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 1 days. From this test method, the chloride ion diffusion coefficient D is calculated by the Nernst-Plank equation. 3. RESULTS AND DISCUSSIONS s 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 4% 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 1] that the mineral admixtures may change the

4 RCPT, Total charge passed (coulomb) 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 Diffusion coefficient ( 1-8 m 2 /hour) passed (coulomb) Ponding, D p ACMT, D s A A A A A B B B B B C C C C C D D D D D %Slag 4%Slag 7%Slag 2%Slag 5%Slag W/B Fig. 1 The relationships between the total charge passed and W/B

5 Diffusion coefficient from ponding ( 1-8 m2 /hour) /hour) Diffusion coefficient from ponding ( 1-8 m2 /hour) /hour) Experiment results Previous study [14] Ref. [15] Ref. [16] Total charge passed (coulomb) Fig. 2 The correlation between the chloride diffusion coefficient from the ponding test and the total charge passed Experiment results Ref. [15] Ref. [16] 2 1 y =.9x R² = Total charge passed (coulomb) Fig. 3 The correlation between the chloride diffusion coefficient from the ponding test and the total charge passed (concrete mixes without mineral admixtures)

6 Diffusion coefficient from ACMT ( 1-8 m 2 /hour) 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 2% 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 steadystate 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 ( D s ) and the non-steady-state diffusion coefficient from the ponding test D ). By linear regression, the empirical relationship between D and D is statistically ( p derived as: s p 8 Y.246 x (1) 2 The R for this regression Equation (1) is.972. It appears that D s correlates linearly with D regardless of concrete with or without containing mineral admixtures. When p 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 %Slag 4%Slag 7%Slag 2%Slag 5%Slag Fig. 4 The relationships between the steady state chloride diffusion coefficient from the ACMT and W/B W/B

7 Diffusion coefficient from Ponding ( 1-8 m 2 /hour) ACMT, Diffusion coefficient ( 1-8 m2 /hour) 6 5 Experimental result Total charge passed (coulomb) Fig. 5 Correlation between the steady state chloride diffusion coefficient from the ACMT and the total charge passed Experimental result Previous study [14] y =.246x R² = Diffusion coefficient from ACMT ( 1-8 m 2 /hour) 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

8 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 M is gratefully appreciated. REFERENCES D. Whiting, Rapid Measurement of the chloride permeability of concrete, Public Roads 45(3) (1981) 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) 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) (2) 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., 3(2) (2) 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) (23) 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) (24) P. McGrath, and R. D. Hooton, Re-evaluation of the AASHTO T259 9-day salt ponding test, Cem. Con. Res., 29 (1999) M. R. Sherman, D. B. McDonald, and D. W. Pfeifer, Durability aspects of precast prestressed concrete Part 2: Chloride permeability study, PCI J., (1996)