Diffusion characteristics of OPC concrete of various grades under accelerated test conditions

Construction and Building Materials 24 (2010) 346–352

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Diffusion characteristics of OPC concrete of various grades under accelerated test conditions J. Prabakar a,*, P. Devadas Manoharan b, A. Chellappan a a b

Structural Engineering Research Centre (SERC), Taramani, Chennai 600113, India Anna University, College of Engineering, Guindy, Chennai 600025, India

a r t i c l e

i n f o

Article history: Received 11 August 2009 Accepted 12 August 2009 Available online 8 October 2009 Keywords: Concrete Chloride profile Diffusion coefficient Voltage

a b s t r a c t Concrete durability mainly depends on the diffusion characteristics of concrete. Chloride ion diffusion is one of the main parameters affecting the durability of Reinforced Concrete Structures. The chloride ion penetration is determined under accelerated diffusion test condition with 12 V (Norwegian method). Depending upon the concrete quality, the diffusion test duration will vary. Generally, high grade concrete will have longer test duration as compared to lower grade concrete. In this paper, OPC concrete of M30, M40, M50, M60 and M75 grades were studied for diffusion properties under different voltages 12, 20, 30, 40, 50 and 60 V. A comparative study was made with standard Rapid Chloride Permeability Test with respective voltages. It has been observed that chloride profile and diffusion coefficient were high in low grade OPC concrete and low for high grade concrete. Minimum test duration was observed at higher voltages. With the increased voltage, the chloride profile and diffusion coefficient were found to be high. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Generally, concrete is a very durable material and the environmental factors such as weathering action, chemical attack, abrasion and other deterioration process may change the properties of concrete with time [3,4]. The premature deterioration of Reinforced Concrete Structures was mainly due to steel reinforcement corrosion. The major aggressive ion causing severe reinforcement corrosion is the chloride ion [10]. The penetrated chloride ion destroys the natural passivity of the surface of reinforcing steel and this leads to the corrosion of steel which causes cracking and spalling of concrete. The resistance of steel corrosion is superior when the thickness of concrete cover is large but too much a cover could result in larger and more cracks allowing direct access of aggressive agents to steel reinforcement [5]. The microstructure of concrete is strongly affected by several factors including chemical composition of cement, water cement ratio, aggregate size and particle distribution. Appropriate mix proportion should be selected for increasing the structure durability [3,7]. The impermeability of concrete can be represented by the rate of flow of chloride ions through unit area of concrete [2]. To determine the chloride penetration within a reasonable time, a test method that accelerates the process was needed [8]. The Rapid Chloride Permeability Test was the rapid qualitative measure of chloride ion penetration of concrete [1,9]. The

* Corresponding author. Tel.: +91 44 24962099; fax: +91 44 22541508. E-mail address: [email protected] (J. Prabakar). 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.08.027

Rapid Chloride Permeability Test gives the total electric charge passed through the concrete specimen [13]. An accelerated steady state diffusion test gives the direct measure of chloride concentration with time [14–17]. It was reported that in accelerated diffusion test, the applied voltage was increased from 6 V to 40 V and thus the test duration was reduced from 25 days to 7 days [10]. In this paper different grades of OPC concretes were studied under six different voltages namely, 12, 20, 30, 40, 50 and 60 V. Based on the test results, the chloride flux, diffusion coefficient, RCPT values and initiation period were determined and compared.

2. Methodology The main objectives of the study are:  Determination of diffusion characteristics of OPC concrete of various grades under accelerated test conditions.  Estimation of test duration for OPC concrete of various grades.  Estimation of initiation period from diffusion coefficient.

2.1. Materials preparation Commercially available 53-grade Portland cement (Specific gravity 3.15), river sand (Fineness modulus 2.79) and crushed granite aggregate (Fineness modulus 6.81) were used for preparing the mixes. Table 1 illustrates the engineering properties of these materials. Sulphonated naphthalene based super plasticizer was used to obtain desired workability and the water used was the ordinary potable ground water.

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J. Prabakar et al. / Construction and Building Materials 24 (2010) 346–352 2.2. Preparation of test specimens The ingredients were weighed according to the mix proportions. Initially aggregate with one third of water were added to the mixer and allowed to mix for 60 s. The fine aggregates (sand) and cement were added to the mixture and were mixed for 60 s and the remaining two third of water mixed with chemical admixture (Super plasticizer) was added to the mixer and mixed for another 90 s. The concrete mix was removed from the mixer and placed in the molds (cube 150  150  150 mm and concrete cylinder of size 100 mm diameter  200 mm height) in three layers placed on the vibrating table and each layer was vibrated for 30 s. The next day, de-molding was done and the specimens were immersed in saturated calcium hydroxide solution (2% by weight of water). After 28 days of curing, each concrete cylinder was removed and sawed to three numbers of 100 mm diameter  50 mm height cylinder neglecting top 15 mm and bottom 35 mm of the cylinder [11].

2.3.3. Rapid Chloride Permeability Test In this test, the concentration of the solution was 0.3 N NaOH and 3% NaCl (ASTM-C-1202). Apart from the solution concentration the test setup and method were same as diffusion test. The test was conducted for 6 h under various voltages temperature and the current reading was monitored during the test. From the current readings, total charge passed was calculated using the Eq. (4) [12]

Q ¼ 900ðI0 þ I360 þ 2ðI30 þ I60 þ    þ I300 þ I330 ÞÞ

ð4Þ

where, Q is the charge passes (Coulomb), I0 the current (ampere) immediately after voltage is adapted and I360 is the current (ampere) at t min after voltage is applied.

2.3. Experimental program The diffusion characteristics of concretes were studied by diffusion test (Norwegian method) and the Rapid Chloride Permeability Test (ASTM-C-1202 method) respectively. Concrete surface water permeability values were obtained using Germanns Water Permeability Test apparatus. 2.3.1. Permeability test The equipment used for this test is Germanns Water Permeability Test apparatus. The chamber is attached to the concrete surface and water is filled into the pressure chamber. A specified water pressure is applied to the surface area (i.e. 3018 mm2). The pressure is kept constant using micro-meter gauge with attached pin (10 mm diameter). The test can be made either on vertical or on horizontal faces. ‘‘g1” is the initial micro-meter reading in mm and ‘‘g2” is the final micro-meter gauge readings in mm after the test is performed in ‘‘t” seconds, ‘‘q” is the flux, ‘‘b” is the percentage of the concrete cement matrix, and p is the pressure selected and ‘‘L” is the pressure applied over 15 mm thickness of the pressure gasket. The surface permeability (Kcp) can be assessed by the following Eqs. (1) and (2)

q ðmm=sÞ ðbðDp=LÞÞ Bðg 1  g 2 Þ q¼ ðmm=sÞ At K cp ¼

ð1Þ ð2Þ 2

where B is the area of micro-meter gauge (78.57 mm ), A is the surface area of pressure applied (3018 mm2), p is the pressure applied (4 kg/cm2) and L is the length of pressure applied (15 mm). 2.3.2. Diffusion test In this study, increasing the applied voltage from 12 V to 20, 30, 40, 50 and 60 V accelerated the diffusion test. The concrete specimen sized 100 mm diameter  50 mm thick was fixed between two diffusion cells (anode: 0.25 N NaOH solution and cathode: 0.25 N NaCl solution) of 250 ml capacity, such that the top surface of the specimen facing the upstream NaCl container as shown in Fig. 1. The cells were connected to DC power supply and a constant voltage was maintained according to the requirements. The quantity of chloride ions that migrate from cathode to anode solution through the specimen was determined by measuring the concentration of the chloride ions in the anode solution periodically. This volumetric analysis for determining chloride concentration was performed with 0.02 N silver nitrate solution and ammonium ion (III) sulphate indicator against 0.02 N ammonium thio cyanate solution. A data logger was used to record current and temperature for downstream NaOH solution. The effective chloride ion diffusion coefficient was determined from the chloride concentration flux using Eq. (3) [6].

Dc ¼ bo

temp.), L is the specimen thickness (5 cm), V is the volume of chloride collecting cell (250 cm3), Z is the ion valance in the testing chloride salt [1], eo is the charge of proton (4.8  1010 e.s.u.), w is the applied electric voltage (V), Co is the initial chloride concentration in chloride source solution, Ao is the cross sectional area of specimen (78.5 cm2), dc/dt is the steady state migration rate of chloride ions (m mol/cm2s).

300KT LV dc ðcm2 =sÞ Zeo DW C o Ao dt

ð3Þ

where bo is the correction factor for ionic interaction (1.46 for 0.25 M NaOH) (1.22– 1.70 for 0.1–0.5 M NaCl), Dc is the diffusion coefficient (cm2/s), K is the Boltzman constant (1.38  1016 evrgs/K/ion), T is the temperature, 300 K (273 + 27 room

3. Results and discussions 3.1. Chloride profile under steady state condition The chloride ion concentration determined using volumetric analysis was plotted against time. Fig. 2 shows a typical chloride profile of M40 grade OPC concrete under 40 V. For a constant voltage, the chloride concentration of the anode sodium hydroxide (NaOH 0.25 N) solution increases gradually with time. After few hours, the chloride concentrations were observed to be increasing linearly from 2.6 m mol/cc to 13.6 m mol/cc. In the steady state condition, it was observed that the top specimen of the cylinder has higher chloride concentration when compared to middle and bottom specimen of the same cylinder. The bottom specimen of the cylinder has lesser chloride concentration due to proper material distribution and impermeability and moreover higher compaction of the bottom specimen leads to dense microstructure with lesser pores than middle or top specimens. The chloride flux (dc/ dt) was calculated only from the steady state chloride profile. The calculated chloride flux was used to determine the steady state diffusion coefficient. 3.2. Effect of voltage on chloride profile The diffusion test was conducted for various voltages from 12 V to 60 V and the influence of voltage on the chloride concentration profile was plotted against the test duration. Typically, the chloride concentration of M40 grade OPC concrete under 12 V, 20 V, 30 V, 40 V, 50 V and 60 V is represented in Fig. 3. Under steady state condition, it has been noticed that the chloride profile was increased from 1.5 m mol/cc to 3 m mol/cc for the applied voltage of 12 V and from 1.5 m mol/cc to 13 m mol/cc for 60 V power supply, which indicates that the steady state chloride profile was achieved within a short test period for higher voltages. The steady state condition was achieved within 20 h for 60 V power supply and 80 h for 12 V power supply for M40 OPC concrete. The test duration depends on the time period for attaining the steady state chloride flux. In M40 OPC concrete, the steady state chloride flux was observed at 6, 16, 22, 36, 54 and 72 h when the specimens were set

Table 1 Physical and engineering properties of raw materials. Sl. No.

Physical properties

Cement

Sand

Aggregate

1 2 3 4 5 6 7

Specific gravity Bulk density (kg/m3) Fineness modulus Standard consistency (%) Initial setting time in minute Final setting time in minute Compression strength of mortar cubes N/mm2

3.15 1440 – 31.0 110 224 53.22

2.68 1700 2.79 – – – –

2.69 1550 6.81 – – – –

Source: CEL, SERC.

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DATA LOGGER (+)

(-)

(+)

(-)

PERSONAL COMPUTER

DIFFUSION CELL SETUP

NaOH

PRINTER

NaCl Fig. 1. Layout diagram of diffusion and RCPT setup.

under 60, 50, 40, 30, 20 and 12 V respectively. Similar trends were obtained for all other grades of concretes. In general, it was also observed that the time taken to achieve the steady state condition for M75 OPC concrete was higher than that of M60 and M40 OPC concrete.

at short duration. In general, it was observed that the test duration was increased when the grade of concrete was enhanced. The test duration was 1.5 times more for M75 of OPC when compared to M40 of OPC. With the increased voltage 12–60 V, the test duration was reduced to 3.5, 2.2 and 2 times for M40, M60 and M75 of OPC concretes.

3.3. Effect of voltage on temperature The temperature variations of the concrete specimens set under various voltages for diffusion test was represented in Fig. 4. For a constant power supply, the solution temperature was increased with the increased test duration. Maximum temperature was observed when the test was performed at 60 V. During the test, the maximum temperature attained was 37 oC for M30 OPC concrete and it was 1.20 times higher than M75 OPC concrete. As reported by the various researchers, the maximum temperature obtained was within the limits, which indicates that the test can be carried out with 60 V. 3.4. Effect of voltage on diffusion test duration

Chloride concentration in m mol / cc

The duration of the test conducted to obtain diffusion coefficient of various grades of concretes under different voltages are represented in Fig. 5. It was observed that the test duration was enhanced when it was tested at 12 V and at 60 V, it was determined

16

3.5. Effect of concrete grades on diffusion coefficient The diffusion coefficient was calculated from the steady state chloride flux for all specimens varying voltages. The calculated chloride diffusion coefficient was plotted against the applied voltages. Fig. 6 represents the diffusion coefficient of OPC concretes of M30, M40, M50, M60 and M75 observed under various voltages. It was seen that the Diffusion coefficient of M30 grade concrete was higher than other concrete grades for all the voltages. Lesser diffusion coefficient was observed when the concrete grade was enhanced. The graph also shows that the Diffusion coefficient of concrete increases for the higher voltages. The Dc of M40 OPC increases from 4.59E13 to 1.72E12 m2/s for 12 V and 60 V respectively. Similarly the Dc of M60 concrete increases from 1.11E13 to 6.459E13 m2/s for 12 V and 60 V respectively. The Dc of M75 concrete was very less when compared to the lower grade concretes and it was lowered 10 times than M40 of OPC.

TOP MID BOT

14 12 10 8 6 4 2 0 0

10

20

30

40

Time in hr Fig. 2. Chloride profile of M40 grade OPC concrete under 40 V.

50

349

Chloride concentration in m mol / cc

J. Prabakar et al. / Construction and Building Materials 24 (2010) 346–352

18

12V

16

20V

14

30V 40V

12

50V

10

60V

8 6 4 2 0

0

20

40

60 Time in hr

80

100

120

Fig. 3. Influence of voltage on chloride profile of M40 grade OPC concrete.

3.6. Concrete surface water permeability

perature variations of concretes during RCPT was measured and found to be within the limits.

The permeability test results of OPC concretes are 1.30E7, 1.21E6, 1.10E6, 9.86E7 and 7.23E7 mm/s for M30, M40, M50, M60 and M75 respectively as shown in Fig. 7 (Source: CEL, SERC). From results, it was observed that the permeability value of low grade concrete was maximum as when compared to high grade of concrete.

3.7. Effect of concrete grades on RCPT The standard Rapid Chloride Permeability Test (RCPT) was conducted for 6 h for various grades of concretes Fig. 8 represents the RCPT results for M30, M40, M50, M60 and M75 grade of OPC concrete with 60 V and lowered voltages. The RCPT value had decreased with higher grade of concrete. The coulomb of M30 OPC concrete was 2164 coulomb and for M75 grade concrete it was reduced to 892. The standard 60 V was reduced to 50, 40, 30, 20 and 12 V. The RCPT value, Q for each grade of concrete was observed to be decreasing with lesser voltage and for M30 grade OPC concrete Q decreases from 2164 to 418 for 60 V and 12 V respectively. Similar trend was observed for all other grade of OPC concretes. Tem-

38

The observed 6 h current flow for the varied voltages was plotted against time for various grades of concretes as shown in Fig. 9. From the study, it has been seen that M30 grade concrete has higher current flow when compared to other higher grade concretes. It was also noticed that when the voltage was reduced the current values also reduced linearly. From the observed current flow the total charge passed Q in coulomb was calculated using ASTM Eq. (4). The calculated Q values were plotted against the applied voltages. 3.9. Comparison between charge passed and diffusion coefficient of OPC concretes The relation between Diffusion coefficient (12 V) and the RCPT values (60 V) for different grades of OPC concrete was plotted in Fig. 10. The diffusion coefficient was observed to increase linearly with the increase in Q values. This plot can be used only for those concretes having RCPT values ranging from 800 to 2200 coulomb.

M30

37 Temperature in oC

3.8. Effect of voltage on current flow of RCPT

M40

36

M50

35

M60 M75

34 33 32 31 30 29 28

0

10

20

30 40 Applied Voltage in V

50

Fig. 4. Effect of voltage on temperature during diffusion test.

60

70

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J. Prabakar et al. / Construction and Building Materials 24 (2010) 346–352

180

M30 M40 M50 M60 M75

160

Time in hr

140 120 100 80 60 40 20 0

0

10

20

30

40

50

60

70

Applied Voltage in V Fig. 5. Effect of voltage on diffusion test duration.

Diffusion coefficient, Dc in m2/sec

3.0E-12

M30 M40 M50 M60 M75

2.5E-12 2.0E-12 1.5E-12 1.0E-12 5.0E-13

0.0E+00 0

10

20

30

40

50

60

Applied Voltage in V

70

Fig. 6. Effect of voltage on diffusion coefficient of OPC concretes.

Surface water Permeability in mm / sec

1.40E-06 1.20E-06 1.00E-06 8.00E-07 6.00E-07 4.00E-07 2.00E-07 0.00E+00 30

40

50

Concrete grade

S1

60

Fig. 7. Concrete water surface permeability.

75

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J. Prabakar et al. / Construction and Building Materials 24 (2010) 346–352

Charge Passed, Q in Coulomb

2500

M30 M40 M50 M60 M75

2000

1500

1000

500

0 0

10

20

30

40

50

60

70

5

6

7

Applied Voltage in V Fig. 8. Effect of voltage and grade on RCPT.

100

M75-OPC-12V M75-OPC-20V M75-OPC-30V M75-OPC-40V M75-OPC-50V M75-OPC-60V

90

Current in mA

80 70 60 50 40 30 20 10 0

0

1

2

3

4

Time in hr Fig. 9. Effect of voltage on current flow of RCPT.

2

Diffusion coefficient (Dc), m /sec

9.00E-13

y = 5E-16x - 4E-13

8.00E-13 7.00E-13 6.00E-13 5.00E-13 4.00E-13 3.00E-13 2.00E-13 1.00E-13 0.00E+00 0

500

1000

1500

2000

Charge passed (Q), coulombs Fig. 10. Comparison between charge passed and diffusion coefficient of OPC concrete.

2500

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3.10. Initiation period based on diffusion coefficient The initiation period (to) was calculated from Diffusion coefficients using Eq. (5). The Dc (mm2/year) and chloride concentration obtained at 24 h under a constant 12 V power supply was used. The value of ‘X’ was determined using the error function chart. The initiation period of M30, M40, M50, M60 and M75 grade OPC concrete was 7, 14, 24, 55 and 122 years respectively, The value of initiation period depends on the Diffusion coefficient of the respective grade and type of concrete and the Dc of M75 OPC grade concrete was very high when compared to M30 OPC concrete. Similar variation was observed in initiation period also.

to ¼

L2 4  Dc  X 2

ð5Þ

4. Conclusion The chloride concentration was high for low grade concrete and low for the high grade concrete. The chloride concentration profile was high when subjected to higher voltage and vice versa. The steady state was attained at earlier test period with higher voltages and the test period was prolonged under lower voltage. Temperature variation during the test period was observed to be within the limits. The diffusion coefficient was high for low grade concrete and low for the high grade concrete. For all grade of concretes, the diffusion coefficient was high when it was subjected to higher voltage, vice versa. The RCPT value, Q was high for low grade concrete and low for the high grade concrete. In RCP test the Q value decreases with the decrease in voltages. The time for initiation, calculated from Dc, indicates that the lower grade concrete has minimum initiation period and the higher grade concrete have higher initiation period. From the above study, it was concluded that by increasing the resistivity of concrete using higher grade concretes, the durability of concrete was enhanced. In general, the diffusion coefficient value of concrete was studied with 12 V for lower resistivity of concrete. Concrete having higher resistivity required longer test duration. Therefore, by increasing the voltage the Dc can be determined within the reasonable time. Acknowledgement The authors are grateful to the Director Structural Engineering Research Centre (SERC), Chennai for his valuable support and per-

mission for publishing this paper. The authors are also acknowledge the help and support rendered by the staff of Construction Engineering Lab (CEL) of SERC.

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