16 comparison of aashto t-277 (electrical) and aashto t...

16 COMPARISON OF AASHTO T-277 (ELECTRICAL) AND AASHTO T-259 (90D PONDING) RESULTS by C. ANDRADE* and D. WHITING** * Instituto de Ciencias de la Construction "Eduardo Torroja", Madrid, Spain ** Construction Technology Laboratories, Skokie, Illinois, USA

SUMMARY

Several concrete types based in OPC or containing polymers have been submitted to AASHTO methods T-277 (Rapid Chloride Permeability Test) and T-259 (90 days ponding). Chloride profiles recorded after completion of both test types are compared and analyzed by means of the application of Nerst-Plank or Nernst-Einstein equations. It is shown that there is a relationship for conventional concretes, between coefficients calculated fiom chloride concentration gradients produced, either by long-term ponding or by accelerated ingress of chloride into the specimens. However, the Diffusion Coefficient calculated fiom any of the migration equations yields lower values than those obtained from longer-term ponding, although relative ranking of concrete types is the same. These differences may be the consequence of the short testing time of the AASHTO T-277 procedure as well as the relatively high voltage used. These circumstances induce a surface chloride concentration lower than in the case of natural difision (90 days ponding). Large differences are computed when the concrete is too impermeable as is the case of polymer concretes.

INTRODUCTION

With the aim of comparing concretes of various compositions with respect to their relative resistance against chloride penetration, in the late 1970's a technique for determining the relative permeability of concretes to chloride ions was developed under contract to the Federal Highway Administration (FHWA). The method was later standardized by the American Association of State Highway and Transportation Officials (AASHTO) as AASHTO T 277 [l] and in 1993 a revised version was standardized by ASTM as C 1202 [2]. Both methods rely on measurement of the amount of electrical charge passing through a saturated 100 mm diameter slice of concrete 50 mm thick during a 6 hour test period under an applied potential of 60 Vdc. The initial development report [3] noted that the test was primarily qualitative in nature and should be used only to rank concretes in terms of their relative ability to permit ingress of chloride ions and not to predict the quantitative amount of chloride which could pass into the concrete. However, the test has been used, especially in the USA, for specification purposes, using the data presented in the original report to construct limits of acceptance based on amount of charge passed (in coulombs) over the six hour test period. Variants of the test developed by Zhang, et. al [4] and Detwiler, et.al [S] have also been proposed. Because of this,considerable criticism of the

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1st RILEM workshop on Chloride Penetration into Concrete15-18 october 1995, St Rémy lès Chevreuse, France

(c) 1995 RILEM, Cachan, France

test has recently occurred in both the U.S. [6] and Europe [7].

Recent work [8] has been carried out with the aim of clarifjring the hndamental meaning of this test and to contribute to the understanding of the processes occurring when an electrical field is applied to the concrete, such as occurs during cathodic protection, chloride removal or realkalization. In the cited study Nernst-Plank and Nernst-Einstein equations were used to calculate difhsion coefficients from a migration test (AASHTO T 277) either in steady-state [8] or non-steady-state conditions [9]. Therefore, it appears feasible to calculate chloride diffsuion coefficients (D values) from chloride profiles or from electrical resistivity of the concrete.

The objective of the study described in this paper was to determine the relationships between diffision coefficients calculated fiom electrical parameters of the AASHTO test and calculated fiom the profiles of chloride actually entering the concrete under short-term (i.e. using 60 Vdc potential) and long-term (90 day ponding) exposures. As the authors had access to the original data used in the AASHTO T 277 test development, it was felt that this would be especially pertinent to current discussions relating to significance and use of the AASJATO procedure as well as to suggest modifications that may improve its suitability as a predictive indicator of chloride penetration into concrete.

EXPERIMENTAL

Materials

Eight types of concrete were selected for this comparative study (see Table 1). Three were conventional concretes (CON6, CON4, CON32) with water-to-cement (wlc) ratios ranging fiom 0.6 to 0.32. The other five concrete types were modified by addition of different types of polymers and special treatments.

All concretes, with the exception of the single polymer concrete (PC) mixture, were produced with ASTM C 150 Type I portland cement. Coarse aggregate was a dense, crushed limestone. Fine aggregate was a natural siliceous sand. Polymer modifier used in the latex-modified concrete (LMC) was a 48% aqueous emulsion of styrene-butadiene latex. Beads of a 75:25 blend of parailin and montan wax were used in the internally sealed concrete (ISC).

An air-entraining agent based on neutralized Vinsol resin was used in all concretes except the LMC and ISC mixes. A lignin-based water reducer was used in the low wlc ratio concrete. The PC mix was produced with a polyester monomer binder and minus 2 mm silica sand in proportion of 20:80 by mass. A methyl ethyl ketone (MEK) accelerator was used at 2 percent by weight of resin binder. The polymer impregnated concrete (PIC) was produced using a methyl methacrylate (MMA)/trimethyl propylyl tri methacrylate (TMPTMA) monomer mix in proportion of 95:5 by mass. The monomer system was catalyzed with 1% benzoyl peroxide initiator.

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Mix proportions for all concretes produced for this study are given in Table 1.

From each concrete mix a series of slabs 600 X 300 X 150 mm in dimension were cast. Two of these were plain concrete slabs, one of which was used for 90 day ponding tests and the other for extraction of cores for AASHTO T 277 testing.

Test procedures

- Aashto T 277

Test slices 50 mm thick were removed fiom the top of each 100 mm diameter core taken from the concrete slabs. The slices were allowed to air dry for 1 hour, then prepared for test in accordance with the standard AASHTO method [l]. Just prior to test, the electrical resistance of the specimen was measured using a 100 Hz AC soil resistance meter applied across the screen electrodes within the test cell (Figures 1 and 2). Upon conclusion of test the resistance of each specimen was again measured using the AC meter m. Each test specimen was then sealed in a thick polyethylene bag and transferred to a freezer maintained at - 18" C in order to prevent further migration of chloride ion within the specimens until slices could be obtained for analysis of chloride gradients.

Each 50 mm X 100 mm test specimen was rapidly transferred to a water-cooled diamond saw and a nominal 6 mm thick slice obtained from the top surface. Slices were removed until each specimen had been sectioned into 5 slices. Slices were fractured in a jaw crusher, then ground to pass a 150 pm sieve. The resulting powder samples were analyzed for total chloride ion in accordance with AASHTO T 260.

- Aashto T 259 (90-day ponding)

Slabs companion to those from which the 100 mm cores were obtained were ponded with a 3 percent solution of sodium chloride for a period of 90 days. At the end of this period powder samples were obtained by drilling at the following increments: 0-10 mm, 10-16 mm, 16-22 mm, 22-29 mm, and 29-41 mm. Powder samples were analyzed for total chloride ion.

Calculation of diffusion coefficients

Three theoretical procedures have been used to calculate diision coefficients: 1) the fitting of equation [4] into the chloride profiles obtained either in the AASHTO T 277 (60V) test or fiom the 90 d ponding test; 2) by introducing resistivity values; or 3) fiom charge passed (coulombs) introduced into the Nernst-Einstein equation.

- From Chloride Profiles

The fitting of chloride profiles into equation [4] was camed out by regression analysis. The

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computer program allows the simultaneous calculation of surface concentration C, and D values. These D values are the "apparent" diffision coefficients, D,, as they represent effects both of diffision and of interaction of chloride ions with the cement matrix [l01

- From Nerst-Einstein Equation (AC Resistivity)

As shown by Andrade [S], by using this equation only effective diffusion coefficients, Deff, can be calculated, as the equation holds only for ions d i s i i g under steady-state conditions with no reaction [10]. The equation used is:

The coulombs passed during the 6 h of the AASHTO test may be converted into a "mean" electrical current value by dividing the coulombs by the time (21,600 seconds). The 60 V applied are then divided by this mean calculated current to obtain a mean resistance. Finally, resistivity is obtained using the cell geometry parameters given in (2).

K, being the AC electrical resistance measured before (R,,,,d the 60 V AASHTO test.

The resistivity value is then introduced in [l] in order to calculate D,, Therefore, the following D,, are otained:

Coulombs (A.sg) P @.cm) D,, (cm2/sec)

In addition to these three theoretical parameters, an empirical approach suggested by Berke [l l] has also been used. Berke's equations are:

From coulombs D= 0.0 103 X 1 o - ~ From resistivity D= 54.6~1 0-8 p-'.''

RESULTS

Results of AASHTO T 277 testing are shown in Table 2. In addition to the amount of charge passed through the slices over the 6-hour test period, the electrical resistance of each specimen before and after test was also recorded.

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Chloride contents of the drill samples taken fiom the 90-day ponding slabs after completion of the test are shown in Table 3. Significant amounts of chloride have diffused into the specimens over the 90-day period of test. In a number of the test concretes, chloride ions were detected at depths more than half-way through the test slice. For all concretes large amounts of chloride were detected within the first 3 to 5 mm of the surface, indicating that even highly impermeable concretes are not able to keep chloride from penetrating into the near-surface skin under the conditions of the ponding test.

Chloride contents of slices taken fiom the AASHTO T 277 test specimens are shown in Table 4. No data were obtained from the PC specimen. For comparative purposes, the chloride profiles resulting from 90 day pondiig and the 60 V test are shown in Figure 3. For two of the sets in Table 4 (CON6 and ISC-A), chloride content at the second sampling increment is greater than from the first slice. This may be attributed to depletion of the chloride concentration in the test cell reservoir as the test proceeded, such that the amount of chloride entering the cell was reduced over the period of test.

In general, it can be seen fi-om Figure 3 that for ordinary concretes the profiles reached are of similar shapes for both tests. This is especially true when the sampling interval closest to the surface is disregarded. For polymeric concretes (LMC and ISC-A), agreement between chloride profiles for the two tests is reasonably good. However, for the PIC mix there is a significant difference between the two profiles, that arising from the 60 V test being anomalously "flat". For PC no profiles were obtained from the 60 V test specimens, so no comparisons can be made.

Values of C, calculated from chloride profiles in both tests are compared in Figure 4, which shows that the C, achieved in the 60 V test are much smaller for ordinary, latex-modified, and internally sealed concretes. That is, in spite of the fact that the profiles seem similar, the more rapid 60 V test apparently does not allow sufficient time for a similar degree of chloride interaction with the concrete.

Calculated Diffusion Coefficients

The data developed in the original FHWA test program [l21 presented in Tables 3 and 4 were used to calculate diffusion constants in accordance with the expressions previously developed in Section 4 of this paper. Results are presented in Table 5 and Figure 5 . As no chloride concentration profiles were obtained for the PC mix specimen subsequent to the AASHTO T 277 test, no apparent diffusion constants were calculated in this case.

It is evident from table and figure 5 that D,, is always greater than D, ,,, indicating that the 60 V test does not absolutely reproduce the 90-day ponding test, as was already seen in Figure 4. However, in terms of relative ranking the comparison is more optimistic. Accordingly, rankings from 90 day D values compare favorably to those fi-om the rest of the derived diffusion coefficients shown in Table 5, except for the case of those obtained fiom 60 V profiles. For this last method of calculation, some anomalies are produced due the flatness of the ISC and PIC chloride profiles. For these two concretes, the D,, values

139



calculated from the 60 V profiles are too high.

DISCUSSION

Comparison Between Different Methods of Calculation of D Values

Several methods of calculation of the D values have been presented in Table 5. In order to compare them, the differences between what have been termed D,, and D,, should be clarified.

An apparent (D,) value has been attributed [10][8] to the value obtained in a non-steady state experiment such as 90-day ponding, where chloride binding is occurring siiultaneouslly with diision. On the other hand, an effective (D,,) value is obtained when the experiment is performed under steady-state condition in concretes already saturated with chlorides, therefore no binding is developed during diffision. Usually D,,, is greater than D,,. From the results shown in Table 5, only D values calculated from the Einstein equation (resistivity values) yield D,, In the other methods D, is calculated.

For any given concrete, the values obtained using Berke's equations (from coulombs, Q, or resistivity, &d do not agree. This discrepancy may be attributed to a possible error in the curve fitting used to derive the initial equations. While the values obtained appear reasonable, the fact that the equations are empirical, make them less attractive than the use of profile fitting or the Einstein equation.

Comparison Between 90-Day Ponding and 60 V Tests

Quite good similitude of chloride profiles obtained from 90 d and 60 V tests was found. However, the fact that (C, D,) ,, are greater than (C,, D,) ,. indicates that that the 60 V test is either too short, or the voltage too high, to fblly reproduce the 90 day test. In a previous paper [9] it was found that a voltage drop of 12 V and a testing time of one week, were able to reproduce profiles of ponding periods of one year.

In addition to the short duration of the 60 V test, two other reasons may help explain the differences between the profile shapes of the 90 d and 60 V tests:

1) If the testing time is too short, the binding or reaction of chlorides with the cement matrix may be less than in longer-term tests.

2) The D value may be an inverse function of the chloride concentration, D= f (l/C,).

Therefore, if one wishes to obtain similar absolute D values from 90 day and electrical migration tests, the test duration and voltage have to be optimized in order to minimize the development of other mechanisms such as absence of binding, or enhancement of the dependence of D values on chloride concentration.

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This optimization is especially critical in concretes made with polymers, as it is seen in Table 5 that they may exhibit anomalous behaviour, which indicates different mechanisms of chloride penetration in these concrete. Such differences may be due to "skin" effects or non-constant D values.

CONCLUSIONS

The conclusions that can be drawn from present study include the following:

1. Migration concepts explain and just@ the application of a voltage drop across the concrete in order to accelerate the passage of chloride ions.

2. Chloride Diffusion coefficients calculated by means of curve fitting of Migration Coefficient @mig) into chloride profiles, have agreed quite well with those obtained from Nernst-Einstein equation applied to the coulomb's or resistivity's results.

3. The Difision Coefficients calculated from the AASHTO T-259 (ponding) test are higher to those obtained from the AASHTO T-277, which has been attributed to the higher surface concentrations (CS) achieved in the natural ponding test. However, the concrete ranking is similar in both test types.

4. AASHTO T-277 should be modified in order to better model and predict real conditions: Either migration concepts are applied in order to calculate Diffusion Coefficients or simple concrete resistivity values are measured and introduced into Nernst-Einstein equation.

REFERENCES

1. AASHTO T 277-93, 'Standard Method of Test for Electrical Indication of Concrete's Ability to Resist Chloride', American Association of State Highway and Transportation Officials (AASHTO), Standard Specifications for Transportation Materials and Methods of Sampling and Testing, Sixteenth Edition, 1993, Part 11- TEsts, Washigston, DC, pp. 876-88 1.

2. ASTM C 1202-94, 'Standard Test Method for Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration', American Society for Testing and Materials, 1994 Book of ASTM Standards, Volume 04.02-Concrete and Aggregates, Philadelphia, PA, 1994, pp. 620-625.

3. Whiting, D. 'Rapid Determination of the Chloride Permeability of Concrete', FHWAIRD-8 111 19, Federal Highway Administration, Washington, DC, 198 1, 174 pp.

4. Zhang, M.H. and Gjorv, O.E., 'Permeability of High-Strenght Lightweight Concrete', ACI J 88 (1991) pp. 463-469.

5. Detwiler, R.J., Kjellsen, K.O., and Gjorv, O.E., 'Resistance to Chloride Intrusion of Concrete Cured at Different Temperatures', ACI J. 88 (1991), pp. 19-24.

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6. Pfeifer, D.W., McDonald, D.B., and Krauss, P.D., 'The Rapid Chloride Permeability Test and Its Correlation to the 90-day Ponding Test', PC1 J, no 1 (1994) pp. 38-47.

7. Arup, H., Sorensen, B., Frederiksen, J., and Thaulow, N., 'The Rapid Chloride Permeation Test- An Assessment', CORROSIONl93, paper no 334, March 7-12, 1993, New Orleans, Louisiana.

8. C. Andrade, 'Calculation of chloride diffision coefficients in concrete from ionic migration measurements' Cement and Concrete Research, 23, 724-742 (1993).

9. C. Andrade, M.A Sanjuh, A. Recuero and 0. Rio 'Calculation of chloride diffusivity in concrete fiom migration experiments in non-steady-state conditions' Cement and Concrete Research, 24, 12 14-1 228 (1 994).

10. A. Atkinson, A.K. Nickerson The diffision of ions through water - saturated cmeent' Journal of Materials Science 19.3068-3078 (1984).

11 N.S. Berke, M.C. Hicks 'Estimating the life cycle of reinforced Concrete decks and marine piles using laboratory diffusion and corrosion data' Corrosion forms and control of infiaestructure, ASTM STP 1137, V. Chacker Ed. ASTM, Philadelphia, 207-23 1 (1992).

12. Whiting, D. 'In-Situ Measurement of the Permeability of Concrete to Chloride Ions', In-situ Nondestructive Testing of Concrete, ACI SP-82, American Concrete Institute, Detroit, Michigan, 1984, pp. 501-524.

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Table 1 Mix proportions and characteristics of fresh concretes used in the study

Table 2 Results of AASHTO T 277 testing on slices obtained from cores

Mixture

CON6

CON4

CON32

LMC

ISC

PC

PIC

Type

w/c=O.60

w/c=O.40

wIc4.32

latex modified

internally sealed

polym-

po lym~ impregnated

ISC

PC

PIC

Cement (kg m-')

390

390

490

390

390

resin - 20 % wlw

390

sealed-heated

internally sealed- unheated

polymer

polymer impregnated

Water @g m-)

234

156

160

94

216

-

195

43 10

0

3 5

Additive (S) @g m*') .

- - WRA-1.1

SBR latex - 58

wax beads - 71

catalyst - 0.4 % wlw

PMMA - 5% w/w

455

> l @

56 000

320

5 X 10'

23 000

Slump (mm)

198

50

0

182

86

-

8 1

I

Air Content ("/.l

m

8.0

7.9

6.7

7.7

3.7

--

5.2

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Table 3 Chloride contents of slices obtained fiom 90-day ponding specimens

144



Table 4 Chloride contents of slices obtained from AASHTO T277 specimens

145



Table 5 Values of resistivity, p,surface concentration C, and diffusivity, D, for the different test types.

I

CON6

CON4

CON32

LMC

ISC

ISC-A

PC

PIC

EINSTEIN EQUATION Dca X 10" cm2/s

FROM Cl- PROFILES DM ~ l 0 ' ~ c r n ~ l s

BERKE EQUATIONS D x l~ -~crn~ / s

R~~~~ KQ.CM

4.16 7.01

10.81 2.70

15.14 1.92

24.83 1.17

12.89 2.26

5.12 5.70

5x 10' 0.00074

368.46 0.079

.PP

FROM Q

12

4.9

3.89

2.26

4.33

11.63

0.204

60 V

0.258 9

0.30 4.32

0.26 1 0.95

0.233 0.3

0.068 122.5

0.245 13.1

-

0.008 523

Ponding FROM

RINT

5.74

3.31

2.75

1.26

2.64

7.34

0.057

Coulombs AASHTO

C D"

CS D

CS D

CS D

CS D

CS D

CS D

CS D

Q D

Q D

Q D

Q D

Q D

Q D

Q D

Q D

P(KQ.CM)

0.552 24.56

0.75 7.90

0.47 4.20

0.59 3.44

0.082 6.26

0.51 18.34

0.079 10.74

0.22 6.46

4570 6.53

1540 2.20

1170 0.67

613 0.87

1330 1.37

4310 4.40

35 0.036

P D

P D

P D

P D

P D

P D

P D

P D

9.3 3.13

16.02 1.82

19.224 1.51

41.65 0.7

20.02 1,45

7.29 4.00

> 106 <0.0001

897.1 0.032

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Figure 1. Cell used in AASHTO test. Electrical resistance measurements.

Two units req8d.

Glue join screen unit to cell

t I I I

Mesh soldered between shims

Figure of the cell. 147



%CL- total

0.6 -1 o 90 days

m GO Volts

CON 32

CON 4

LMC -

Figure 3. profiles from 90 d and 60 V tests. 148



line of / equality / Y /

/ C 0 ~ 3 2 m / *m =*

/ LMC -

Figure 4. Values of CS from 60 V versus CS 90 d calculated from chloride profiles.

I S C P I C

0 6 O V d c 0 Einstein in

Einstein fin 0 Berke Q X Coulombs

2 4 6 8 10 12 14 16 18 2 0 22 24 26 PONDING Dap X 10'@cm2/s

Figure 5. calculated from different sources. 149



16 comparison of aashto t-277 (electrical) and aashto t...

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