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Accelerated AC impedance testing for prequalification of marine construction materials
DESALINATION ELSEVIER
Desalination 165 (2004) 377-384
www.elsevier.com/locate/desal
Accelerated AC impedance testing for prequalification of marine construction materials A. Husain*, S. A1-Bahar, S. Abdul Salam, O. A1-Shamali Department of Buildings and Energy Technologies, Kuwait Institutefor Scientific Research, PO Box 24885, Safat 13109, Kuwait Tel. +965 483-6100; Fax +965 484-5763; email: [email protected] Received 2 February 2004; accepted 11 February 2004
Abstract
The AC impedance technique was used in this study for the evaluation and prequalification of concrete materials prepared with chemical corrosion inhibitors and pozolanic admixtures of GGBS slag. In the Arabian Gulf region, marine seawater corrosion has caused severe concrete deterioration over the past decades. Major industrial buildings such as power generation plants, desalination plants and off-shore structures, and oil or water piers deteriorate severely in the marine industrial seawater environment due to the deleterious effect of chloride ions and CO2 gas. It has only recently been appreciated in this difficult region that concrete with supplementary cementing materials exhibit a very significant reduction in permeability and corrosion effects. Assessment of the corrosion condition of steel rebar in concrete for newly proposed projects can be carried out with different techniques. Most of the testing methods suggested are based on the ASTM standards G-109, ASTM C876, and ASTM C1202. The duration for ASTM testing requires at least a minimum of 1 to 3 years of exposure in simulated weathering conditions before any reliable conclusion can be drawn. In the present study, accelerated AC impedance measurements were carried out over a wide frequency range on reinforced Lollipop specimens of GGBS slag with different degrees of compaction of the concrete mix. The AC impedance technique allows detection of the breakdown of passivity and performance of the steel reinforcement in concrete within a much shorter time than with other tests. The correlation between the AC impedance technique and traditional ASTM standards indicated concurring results of the benefit of the application of multicomponent corrosion protection systems under the prevailing conditions of the marine environment in the Arabian Gulf region. Keywords:
AC impedance technique; Concrete corrosion inhibitor; Pozolanic admixtures; Desalination plants; Concrete oil and water piers; Concrete rebar corrosion evaluation
*Corresponding author. Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Office National de l'Eau Potable, Marrakech, Morocco, 30 May-2 June, 2004. 0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved doi; 10.1016/j.desal.2004.06.043
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1. Introduction Concrete structures in the marine environment can suffer from cracks and sapling due to expansion of reinforcement as corrosion takes place. In a marine environment, unprotected steel corrodes rapidly in the oxygen-rich splash zone. In the State of Kuwait, surveys of marine concrete structures have shown an alarming degree of corrosion within 10 to 15 years of construction. Large expenditures are allocated every year to maintain and repair corroded concrete structures that have prematurely deteriorated and barely served half of their expected life [1,2]. Twenty years of investigation and research on the performance of reinforced concrete in the Gulf region concluded that corrosion prevention measures must be an essential and integral part of any concrete practices, and certain corrosion protection systems must be included in new concrete structures [3]. The use of corrosion protection systems are rarely used because of their high cost when compared to ordinary Portland cement. Over the last decade, this region has become an open market and research field for the application of a number of corrosion protection systems. Though they have been used for construction in very limited projects in the region, they have not been standardized for the national building code of practice. This is attributed to a lack of familiarity or comprehensive database on the long-term effectiveness of the systems under the environmental and service conditions prevailing in the region. On the other hand, many investigators have reported that blended cement containing grain granulated blast furnace slag (GGBS) is superior to ordinary Portland cement in cases of attack by chloride, due to their capacity to bind chloride to their hydration products, which reduced chloride content in the concrete. An added advantage of GGBS in the Kuwaiti environment is its rate of hydration where the associated heat evolution and temperature gradient is reduced [4,5].
Many routine tests of DC electrochemical techniques were performed on various slabs. Work on linear polarization techniques, corrosion potential and cathodic protection of reinforcing steel has indicated that some information can be obtained [6]. A major obstacle to the use of the DC technique is the relatively high dielectric property of the concrete itself [7]. Another major obstacle to the study of steel reinforcement corrosion in concrete has been the lack of a reliable and satisfactory accelerated corrosion monitoring technique. There is thus an increasing demand for reliable methods of testing the corrosion susceptibility of rebar steel in a particular type of concrete and in a particular environment. However, not many attempts have been made to observe the corrosion behavior and/or screening of the new supplementary cementing materials under the natural curing conditions of the hot Kuwaiti climate using an electrochemical impedance (EIS) technique. EIS has a distinct advantage over DC techniques in that measurement can be obtained for high resistivity environments of the types under consideration. An advantage of the EIS technique is the very small excitation amplitudes, generally in the range of 5 to 10 mV peaks to peak. This minimally disturbs the sample, attached corrosion products, or absorbed species during testing. Consequently, it seems convenient to characterize GGBS performance and develop its systematic electrochemical behavior in this study. The first objective of this study was to measure the corrosion parameters in concrete with different cement content according to ASTM standards in order to draw some preliminary consequences about the effect of concrete composition on its corrosion behavior. The second aim was to examine the viability of the EIS technique to be employed as an accelerated, nondestructive, and repetitive diagnostic tool for the study of corrosion evolution of steel reinforcement bars embedded in supplementary cementing concrete specimens. The overall results were
A. Husain et al. / Desalination 165 (2004) 377-384
correlated with the AC impedance technique in order to draw some preliminary conclusions and criteria of the effect o f these additives on concrete corrosion performance.
379
activities and corrosion rate. The results are reported after 1 year of ASTM specified exposure conditions in sodium chloride solution. 2. I. A S T M G109 time-to-corrosion test
2. Experimental A comparative evaluation was conducted in the laboratory between ordinary Portland cement (PC) concrete and GGBS concrete (50% cement replacement). In addition, the study involved the evaluation of the Kuwait Scientific Center's construction project that uses GGBS material in its concrete structure and is referred to as "siteconcrete mix". The chemical compositions o f the mixed concrete structure are presented in Table t. Concrete specimens were fabricated on site as well as in the laboratory in order to assess the corrosion-resistance performance according to ASTM standards in terms o f time-to-corrosion Table 1 Laboratory and site-prepared concrete mix proportions Parameter
Concrete type PC concrete
Slag concrete
Concrete grade 28-day cube strength, kg/cm z W/C ratio
K450 450 0.36
K450 450 0.36
Cement type Cement, kg/m3
I 480
I 240
GGBS, kg/m3
--
240
Washed sand, kg/m 3 20 mm aggregate (dry), kg/m 3 10 mm aggregate (dry), kg/m3 Free water, l/m3
550 770 380 170
550 770 380 170
Caplast super special, l/m3 Target slump, mm
5.5-8.5 a 95
5.5-8.5 a 95
aA range for variable climatic conditions. W/C = water cement ratio; GGBS = ground granulated blast furnace slag.
A test specimen with reinforced steel rebar (13 mm dia.) o f concrete blocks 300×300× 200 mm in dimensions embedded with two steel bars as the upper mat anode were ponded with 15% NaC1 solution for 28 days. In the same block, four steel bars were embedded as the lower mat cathode in a chloride-free environment. The concrete cover on each side o f the block is 25 mm. A weekly test cycle was conducted on each specimen. Measurements proceeded after 96 h (4 days) o f saltwater ponding, followed by vacuum removal o f saltwater and an immediate freshwater rinse, followed by vacuum removal. The weekly corrosion potential readings were recorded with respect to Cu/Cu SO 4 (CSE) reference electrode with a half-cell potential of -350 mV. The weekly corrosion readings were followed by 72 h (3 days) o f air drying at 20°C +5. These weekly test cycles were repeated 48 times. 2.2. Corrosion rate with lollipop test specimens
This test is suitable for evaluating the effect of the marine environment on corrosion since it simulates the wicking of chlorides by concrete in seawater. The test is used to evaluate corrosion activities that occur in a localized area on the reinforced steel bar embedded in the concrete. Specimens used for this test were 330x200x 76 mm prisms. Each specimen has a single reinforcing steel bar (13 mm dia.) centered and positioned 5 cm from the bottom. Specimens were immersed in a 3% NaCI solution to a depth of one-half the specimen height. Corrosion rate measurements were determined every month using the GECOR6 corrosion rate meter vs. the Cu/CuSO4 reference electrode.
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Table 2 Concrete specimenused for electrochemicalimpedancespectroscopy(EIS) testing from February 1999-February2001 Specimentype
Specimen c o d e
Concretespecifications
Preparation location
Portland cement concrete
PC
Laboratory (GGBS) concrete
SLG
Site (GGBS) concrete
SIT
Inhibited concrete
CNI
Concrete with Portland cement Concrete mix with 50% slag Concrete mix with 50% slag Calcium nitrite inhibit/concrete
Laboratory Laboratory Site Laboratory
2.3. Electrochemical impedance test
Electrochemical impedance spectroscopy (EIS) was used in this study to characterize four different compositions of duplicated cement concrete samples. Table 2 shows the chemical composition of the concrete specimens used for the impedance testing. The test specimens of concrete blocks (300x300x200 rnm) were used. Each specimen has a single reinforcement steel bar (13 mm dia.) embedded in concrete, centered and positioned 5 cm from the bottom. For the sake of having an EIS spectrum characteristic of different admixtures of concrete, calcium nitrite corrosion-inhibiting admixtures (CN) were also studied. This enhanced the stability of the passivating layer on the surface of the reinforcing steel if presented at an optimum concentration. The samples were immersed in a 3% saline solution approximately 15 cm deep. Graphite rods were used as counter electrodes. A saturated Ag/AgCI was used as a reference electrode. Data acquisition was controlled using a Solarton Potentiostat-Galvanostat (Model 1287) interfaced with a Solarton Frequency Response Analyzer (Model 1260) to provide sweep frequency measurements. Both instruments were interfaced with a computer for data logging, storage, and analysis. EIS measurement in the frequency range of 100 kHz to 10 mHz was applied. The system responded to the excitation signal with complex current waveforms, which yields impedance information for each discrete frequency. The resultant data are presented in the
form of complex plots of Nyquist, Bode, and phase angle, which reflect the spectrum of frequency domain. Equivalent circuit and data analysis software known as Solarton Z-plot was used for analysis of the resulting spectrum.
3. Results and discussion
The results of the accelerated time-tocorrosion test are presented in Figs. 1 and 2 in terms of corrosion potential and corrosion current density vs. time. Fig. 1 shows differences in corrosion potential of the steel reinforcement for PC and GGBS slag concrete (SLG). According to the ASTM criterion of corrosion, risk given by the standard test method for half-cell potentials of un-coated reinforcing steel in concrete (ASTM C 876, 1991), the slag concrete performance fell in the area of greater than 90% probability that no corrosion activity was occurring on the steel surface. This effect was indicated by the steel corrosion potential values of less than -200 mV during 48 weeks of exposure. In contrast to the slag concrete, the behavior of corrosion potential values (Eoo,~) of steel bars in the PC concrete changed during the exposure cycles from 90% probability of no steel corrosion activities to uncertain corrosion activities at the 35th week. These findings are consistent with the results of corrosion current density vs. time shown in Fig. 2, which indicates higher values for PC concrete throughout the 48-week cyclic exposure to a 15% NaCI solution.
A. Husain et al. / Desalination 165 (2004) 377-384 -600-
-500- r
~
.as0~
-100.
0 50
lOO
150
2o0
25o
300
350
Time(d)
Fig. 1. Corrosion potential vs. time for steel bars in PC, SLG, and SIT concrete.
l.\
10::t 0.05] 0. 0
50
100
150
200
250
300
350
Tin~ (d)
Fig. 2. Corrosioncurrent density vs. time for steel bars in PC, SLG, and SIT concrete specimens after the lollipop test.
The increase in the corrosion current density (Ioo,) values are attributed to the relative reduction in the electrical resistivity of the PC concrete around the steel reinforcement bar in comparison to the slag concrete (SLG). At the end of the 48th exposure cycle, the electrical resistivity of the PC and slag concrete were 14 and 31 kf2.cm2, respectively. A follow-up on the results of testing carried out on the slag (SIT) concrete corrosion activities prepared at the Scientific Centre's project site were monitored for a period of 1 year. The results
381
of corrosion potential are consistent with the earlier experimental findings. The corrosion potential on steel reinforcement after 1 year of exposure was -500 mV, which indicates that there is >90% probability that reinforcing steel corrosion was occurring in the area at the time of measurement. Unlike the previous results in the comparative study of corrosion test activities for PC, and SLG, the SIT did not demonstrate the benefit of reducing the risk of corrosion. This was believed to be due to the poor compaction practice of the mix concrete materials when specimens were prepared at site. As a result, the concrete's permeability to chloride ion penetration was very high. A similar analogy can be derived from the corrosion rate measurement test (the lollipop test). The results indicate that both concrete falls in the area of 90% probability that no steel corrosion was occurring when test specimens were immersed in a 3% NaC1 solution. As explained earlier and as indicated by the results shown in Fig. 2, the increase in corrosion current density was attributed to the reduction in the electrical resistivity of the concrete. After 1 year exposure of the lollipop specimens, the electrical resistivity of the PC and slag concrete were 72 and 98 kf2.cm2, respectively. These findings are in agreement with the results of the standard methods of testing for electrical indication of concrete's ability to resist chloride ion penetration (ASTM C 1202, 1991). This indicates the effectiveness of slag concrete in reducing chloride ion penetration based on the total charge in coulombs passed through the concrete for 6 h. The charge transfer continued to be reduced as the curing period increased, indicating improving resistance to chloride ion penetration with the concrete's maturation. It gave an average value of 1567 Coulombs for PC and 636 Coulombs for laboratory slag (SLG) after 56 days of curing. In the case of SIT, it gave an average value of 1775 Coulombs, which is in agreement with the above hypothesis of poor permeability resistance.
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The results of the EIS analysis (Figs. 3 and 4) indicated an impedance spectrum in the form of Nyquist plots for the plain concrete (PC), slag concrete (SLG), site concrete (SIT) and an additional sample of calcium nitrite inhibited concrete (CNI). Fig. 3 represents the EIS measurement results for the month of February (1999) while Fig. 4(a) and (b) represent the same test obtained after a duration of 1 and 2 years, respectively, that is February, 2000, and February, 2001. A qualitative analysis of the Nyquist impedance spectra indicates that the corrosion performance of the different concrete mixtures are in agreement with the ASTM test results of corrosion rate and potential values obtained in the previous section. The impedance results show that the laboratory-prepared slag concrete structure exhibited the highest impedance and capacitance values of all the specimens. This is shown clearly in Figs. 3 and 4 before and after I and 2 years of exposure, respectively.
As was expected from the protective nature of the 50% laboratory slag granulated replacement (SLG), the GGBS concrete slowed down the diffusivity of the chloride species to reach the steel substrate. In other words, the presence of a Warburg diffusion response indicated that the
-30O0 SLG --41--sff -o-PC
-'-Q-- SLG --B--SIT --O--FC ---e-- CN
-2O00 f
=a z
f
-1000
I
r
0
,
1000
t
,
2000
3000
Z'
-100000
--o-- S.G .--II--
/
-2000
SIT
1000 ...o.,.SLG •,4=.. SIT -r~O ,..o.....PC •-e- CNt
--o--FC -75000
~-500
-1500
- - D - SLG --B-- SIT --o-PC
.2~o
-50OO0 ~1-1000
M
o
~,o ~
-25000
,
0
I
25000
i
I
J
I
I
I
--e-- CN
//
-500
2~(X]0
~ , O ~ S
i
5 0 0 0 0 75000 100000 12500 Z'
Fig. 3. EIS Nyquists plot for steel bars in GGBS concrete (SLG) and (SIT), Portland cement (PC), and inhibited concrete (CNI) during February 1999.
0
500
1000 Z'
1500
2000
Fig. 4. EIS Nyquists plot for steel bars in GGBS concrete (SLG) and (SIT), Portland cement (PC), and inhibited concrete (CNI). (a) February, 2000; (b) February, 2001.
A. Husain et al. / Desalination 165 (2004) 377-384
slow diffusion processes of the corrosive species through porosity in the material and corrosion product were the rate-controlling factor for the mechanism of concrete corrosion protection for this particular sample [8]. Water-saturated concrete tends to show resistivity on the order of a few ~.cm 2 while oven-dried concrete or adequately prepared compacted GGBS can approach resistivity values typical of an electrical insulator as reflected on the EIS spectrum after 1 year of exposure. The impedance values recorded for SLG are in the range of 60× 103, 497, 407 cm2 and capacitance reaching 1.06 to 0.853 nF/cm2 after 1 and 2 years. In contrast, SIT concrete had a reduction in impedance (Rp) and capacitance (Cdl) values with 406.31,494 fLcm 2 and 0.753 nF/cm 2 and 69.7 mF/cm 2 after 1 and 2 years, respectively. In contrast to the SLG concrete, the CNI and PC samples showed the lowest impedance and capacitive behavior. The Rp values for PC and CN after 2 years (Fig. 4a and b) was 87.94 and 41.97 ~.cm 2 and capacitance 4.97 and 16.09 nF/cm 2. This is due to the high rate of permeability and adsorption of the chloride corrosive species to the steel substrate surface. In the case of the inhibited concrete sample, the attempt of studying such an inhibitor with concrete is to show the validity of EIS to determine that inadequate application of inhibitor concentration and or variation in specimen solution submersion may cause the corrosion process to become even worse than ordinary concrete (PC). The hypothesis behind the unexpected behavior of the inhibited concrete sample largely was related to improper film formation and dependence on the rate of adsorption of the competed chloride/nitrite species to the passive oxide layer on steel surface with subsequent formation of unprotected localized corrosion cells. The effectiveness of the calcium nitrite admixture, therefore, is dependent on an accurate prediction of the chloride loading of the structure over its expected design life and, hence, on the
383
selection of an appropriate dosage of the admixture [9]. If chloride ions arrive as a contaminant to the surface of the steel, the surface tends to become active when the molar ratio of C1- to nitrite and OH- ions in the pore solution reaches a critical value, exceeding 0.9. Microscopically, the concrete at the interface acts as a non-homogeneous electrolyte, which varies greatly with the overall moisture content, governed by the degree of diffusivity of corrosive and inhibited ions such as oxygen, chloride, and nitrite.
4. Conclusions
Slag concrete with 50% cement replacement can provide the requirements for a high level of corrosion protection in coastal areas and marine facilities in Kuwait; however, poorly prepared, uncompacted and inadequately cured slag concrete will be as poor as any ordinary prepared concrete. The corrosion evaluation of reinforced concrete with supplementary cementing materials by means of sophisticated methods of accelerated EIS measurement and analysis is conceptually possible. Evidence of accelerated results for optimum material selection of concrete mix design for the Kuwaiti marine environment was established using the EIS technique. Equivalent circuit analysis of the impedance spectra showed the presence of a constant phase element that was found in some cases to be approximately equal to the Warburg diffusion impedance. In other spectra, it indicated a depressed semi-circle. The laboratory slag concrete (GGBS) exhibited very high impedance and capacitance values that exceeded the performance of other concrete samples, which is in agreement with ASTM C876 and G 109. The degree of protection for concrete with supplementary cementing materials can be arranged under the prevailing
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condition in the following order: from highest to lowest protective properties SLG > SIT > PC > CNI.
Acknowledgments The authors are grateful for the financial support provided by the Kuwait Foundation for the Advancement of Sciences (KFAS) and the Kuwait Institute for Scientific Research (KISR).
References
[1] F. A1-Matrouk, A. AI-Hashem, F. Al-Kharafi and M. E1-Khafif, Proc. 2nd Arabian Corrosion Conference, Kuwait, 1996, pp. 567-579.
[2] S. AI-Bahar and E. Attiobe, Proc. Int. Conf. CONSEC '95, Sapporo, Japan, 1 (1995) 564-573. [3] O.S.B.A1-Amoudi,ACI Mat. J., 92(3) (1995) 236245. [4] D.M. Roy, A. Kumar and J. Rhodes, ACI SP-91, V.M. Malhotra, ed., Vol. 1423, 1986. [5] Z.G. Matta, Concrete Intemat., 14(5) (1992) 47-48. [6] S.E.HussainandRasheeduzzafar, ACIMat. J., 91(3) (1994) 264-272. [7] J.L. Dawson, L.M. Callow, K. Haldky and J.A. Richardson, Proc. NACE Corrosion '78, Houston, 125 (1978) 1-19. [8] C. Andrade, C. Alonso and J.A. Gonzalez, Proc. 3rd Intemat. Symp. Electrochemical Methods in Corrosion Res., Materials Science Forum, B. Elsnor, ed., Zurich, 45 (1988) 330-335. [9] Federal Highway Administration,US Departmentof Transportation,Time to corrosionof reinforcingsteel in concrete containing calcium nitrite, Publication No. FHWA-RD-99-145, 1999, pp. 1-38.
Desalination 165 (2004) 377-384
www.elsevier.com/locate/desal
Accelerated AC impedance testing for prequalification of marine construction materials A. Husain*, S. A1-Bahar, S. Abdul Salam, O. A1-Shamali Department of Buildings and Energy Technologies, Kuwait Institutefor Scientific Research, PO Box 24885, Safat 13109, Kuwait Tel. +965 483-6100; Fax +965 484-5763; email: [email protected] Received 2 February 2004; accepted 11 February 2004
Abstract
The AC impedance technique was used in this study for the evaluation and prequalification of concrete materials prepared with chemical corrosion inhibitors and pozolanic admixtures of GGBS slag. In the Arabian Gulf region, marine seawater corrosion has caused severe concrete deterioration over the past decades. Major industrial buildings such as power generation plants, desalination plants and off-shore structures, and oil or water piers deteriorate severely in the marine industrial seawater environment due to the deleterious effect of chloride ions and CO2 gas. It has only recently been appreciated in this difficult region that concrete with supplementary cementing materials exhibit a very significant reduction in permeability and corrosion effects. Assessment of the corrosion condition of steel rebar in concrete for newly proposed projects can be carried out with different techniques. Most of the testing methods suggested are based on the ASTM standards G-109, ASTM C876, and ASTM C1202. The duration for ASTM testing requires at least a minimum of 1 to 3 years of exposure in simulated weathering conditions before any reliable conclusion can be drawn. In the present study, accelerated AC impedance measurements were carried out over a wide frequency range on reinforced Lollipop specimens of GGBS slag with different degrees of compaction of the concrete mix. The AC impedance technique allows detection of the breakdown of passivity and performance of the steel reinforcement in concrete within a much shorter time than with other tests. The correlation between the AC impedance technique and traditional ASTM standards indicated concurring results of the benefit of the application of multicomponent corrosion protection systems under the prevailing conditions of the marine environment in the Arabian Gulf region. Keywords:
AC impedance technique; Concrete corrosion inhibitor; Pozolanic admixtures; Desalination plants; Concrete oil and water piers; Concrete rebar corrosion evaluation
*Corresponding author. Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Office National de l'Eau Potable, Marrakech, Morocco, 30 May-2 June, 2004. 0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved doi; 10.1016/j.desal.2004.06.043
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A. Husain et al. / Desalination 165 (2004) 377-384
1. Introduction Concrete structures in the marine environment can suffer from cracks and sapling due to expansion of reinforcement as corrosion takes place. In a marine environment, unprotected steel corrodes rapidly in the oxygen-rich splash zone. In the State of Kuwait, surveys of marine concrete structures have shown an alarming degree of corrosion within 10 to 15 years of construction. Large expenditures are allocated every year to maintain and repair corroded concrete structures that have prematurely deteriorated and barely served half of their expected life [1,2]. Twenty years of investigation and research on the performance of reinforced concrete in the Gulf region concluded that corrosion prevention measures must be an essential and integral part of any concrete practices, and certain corrosion protection systems must be included in new concrete structures [3]. The use of corrosion protection systems are rarely used because of their high cost when compared to ordinary Portland cement. Over the last decade, this region has become an open market and research field for the application of a number of corrosion protection systems. Though they have been used for construction in very limited projects in the region, they have not been standardized for the national building code of practice. This is attributed to a lack of familiarity or comprehensive database on the long-term effectiveness of the systems under the environmental and service conditions prevailing in the region. On the other hand, many investigators have reported that blended cement containing grain granulated blast furnace slag (GGBS) is superior to ordinary Portland cement in cases of attack by chloride, due to their capacity to bind chloride to their hydration products, which reduced chloride content in the concrete. An added advantage of GGBS in the Kuwaiti environment is its rate of hydration where the associated heat evolution and temperature gradient is reduced [4,5].
Many routine tests of DC electrochemical techniques were performed on various slabs. Work on linear polarization techniques, corrosion potential and cathodic protection of reinforcing steel has indicated that some information can be obtained [6]. A major obstacle to the use of the DC technique is the relatively high dielectric property of the concrete itself [7]. Another major obstacle to the study of steel reinforcement corrosion in concrete has been the lack of a reliable and satisfactory accelerated corrosion monitoring technique. There is thus an increasing demand for reliable methods of testing the corrosion susceptibility of rebar steel in a particular type of concrete and in a particular environment. However, not many attempts have been made to observe the corrosion behavior and/or screening of the new supplementary cementing materials under the natural curing conditions of the hot Kuwaiti climate using an electrochemical impedance (EIS) technique. EIS has a distinct advantage over DC techniques in that measurement can be obtained for high resistivity environments of the types under consideration. An advantage of the EIS technique is the very small excitation amplitudes, generally in the range of 5 to 10 mV peaks to peak. This minimally disturbs the sample, attached corrosion products, or absorbed species during testing. Consequently, it seems convenient to characterize GGBS performance and develop its systematic electrochemical behavior in this study. The first objective of this study was to measure the corrosion parameters in concrete with different cement content according to ASTM standards in order to draw some preliminary consequences about the effect of concrete composition on its corrosion behavior. The second aim was to examine the viability of the EIS technique to be employed as an accelerated, nondestructive, and repetitive diagnostic tool for the study of corrosion evolution of steel reinforcement bars embedded in supplementary cementing concrete specimens. The overall results were
A. Husain et al. / Desalination 165 (2004) 377-384
correlated with the AC impedance technique in order to draw some preliminary conclusions and criteria of the effect o f these additives on concrete corrosion performance.
379
activities and corrosion rate. The results are reported after 1 year of ASTM specified exposure conditions in sodium chloride solution. 2. I. A S T M G109 time-to-corrosion test
2. Experimental A comparative evaluation was conducted in the laboratory between ordinary Portland cement (PC) concrete and GGBS concrete (50% cement replacement). In addition, the study involved the evaluation of the Kuwait Scientific Center's construction project that uses GGBS material in its concrete structure and is referred to as "siteconcrete mix". The chemical compositions o f the mixed concrete structure are presented in Table t. Concrete specimens were fabricated on site as well as in the laboratory in order to assess the corrosion-resistance performance according to ASTM standards in terms o f time-to-corrosion Table 1 Laboratory and site-prepared concrete mix proportions Parameter
Concrete type PC concrete
Slag concrete
Concrete grade 28-day cube strength, kg/cm z W/C ratio
K450 450 0.36
K450 450 0.36
Cement type Cement, kg/m3
I 480
I 240
GGBS, kg/m3
--
240
Washed sand, kg/m 3 20 mm aggregate (dry), kg/m 3 10 mm aggregate (dry), kg/m3 Free water, l/m3
550 770 380 170
550 770 380 170
Caplast super special, l/m3 Target slump, mm
5.5-8.5 a 95
5.5-8.5 a 95
aA range for variable climatic conditions. W/C = water cement ratio; GGBS = ground granulated blast furnace slag.
A test specimen with reinforced steel rebar (13 mm dia.) o f concrete blocks 300×300× 200 mm in dimensions embedded with two steel bars as the upper mat anode were ponded with 15% NaC1 solution for 28 days. In the same block, four steel bars were embedded as the lower mat cathode in a chloride-free environment. The concrete cover on each side o f the block is 25 mm. A weekly test cycle was conducted on each specimen. Measurements proceeded after 96 h (4 days) o f saltwater ponding, followed by vacuum removal o f saltwater and an immediate freshwater rinse, followed by vacuum removal. The weekly corrosion potential readings were recorded with respect to Cu/Cu SO 4 (CSE) reference electrode with a half-cell potential of -350 mV. The weekly corrosion readings were followed by 72 h (3 days) o f air drying at 20°C +5. These weekly test cycles were repeated 48 times. 2.2. Corrosion rate with lollipop test specimens
This test is suitable for evaluating the effect of the marine environment on corrosion since it simulates the wicking of chlorides by concrete in seawater. The test is used to evaluate corrosion activities that occur in a localized area on the reinforced steel bar embedded in the concrete. Specimens used for this test were 330x200x 76 mm prisms. Each specimen has a single reinforcing steel bar (13 mm dia.) centered and positioned 5 cm from the bottom. Specimens were immersed in a 3% NaCI solution to a depth of one-half the specimen height. Corrosion rate measurements were determined every month using the GECOR6 corrosion rate meter vs. the Cu/CuSO4 reference electrode.
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A. Husain et al. / Desalination 165 (2004) 377-384
Table 2 Concrete specimenused for electrochemicalimpedancespectroscopy(EIS) testing from February 1999-February2001 Specimentype
Specimen c o d e
Concretespecifications
Preparation location
Portland cement concrete
PC
Laboratory (GGBS) concrete
SLG
Site (GGBS) concrete
SIT
Inhibited concrete
CNI
Concrete with Portland cement Concrete mix with 50% slag Concrete mix with 50% slag Calcium nitrite inhibit/concrete
Laboratory Laboratory Site Laboratory
2.3. Electrochemical impedance test
Electrochemical impedance spectroscopy (EIS) was used in this study to characterize four different compositions of duplicated cement concrete samples. Table 2 shows the chemical composition of the concrete specimens used for the impedance testing. The test specimens of concrete blocks (300x300x200 rnm) were used. Each specimen has a single reinforcement steel bar (13 mm dia.) embedded in concrete, centered and positioned 5 cm from the bottom. For the sake of having an EIS spectrum characteristic of different admixtures of concrete, calcium nitrite corrosion-inhibiting admixtures (CN) were also studied. This enhanced the stability of the passivating layer on the surface of the reinforcing steel if presented at an optimum concentration. The samples were immersed in a 3% saline solution approximately 15 cm deep. Graphite rods were used as counter electrodes. A saturated Ag/AgCI was used as a reference electrode. Data acquisition was controlled using a Solarton Potentiostat-Galvanostat (Model 1287) interfaced with a Solarton Frequency Response Analyzer (Model 1260) to provide sweep frequency measurements. Both instruments were interfaced with a computer for data logging, storage, and analysis. EIS measurement in the frequency range of 100 kHz to 10 mHz was applied. The system responded to the excitation signal with complex current waveforms, which yields impedance information for each discrete frequency. The resultant data are presented in the
form of complex plots of Nyquist, Bode, and phase angle, which reflect the spectrum of frequency domain. Equivalent circuit and data analysis software known as Solarton Z-plot was used for analysis of the resulting spectrum.
3. Results and discussion
The results of the accelerated time-tocorrosion test are presented in Figs. 1 and 2 in terms of corrosion potential and corrosion current density vs. time. Fig. 1 shows differences in corrosion potential of the steel reinforcement for PC and GGBS slag concrete (SLG). According to the ASTM criterion of corrosion, risk given by the standard test method for half-cell potentials of un-coated reinforcing steel in concrete (ASTM C 876, 1991), the slag concrete performance fell in the area of greater than 90% probability that no corrosion activity was occurring on the steel surface. This effect was indicated by the steel corrosion potential values of less than -200 mV during 48 weeks of exposure. In contrast to the slag concrete, the behavior of corrosion potential values (Eoo,~) of steel bars in the PC concrete changed during the exposure cycles from 90% probability of no steel corrosion activities to uncertain corrosion activities at the 35th week. These findings are consistent with the results of corrosion current density vs. time shown in Fig. 2, which indicates higher values for PC concrete throughout the 48-week cyclic exposure to a 15% NaCI solution.
A. Husain et al. / Desalination 165 (2004) 377-384 -600-
-500- r
~
.as0~
-100.
0 50
lOO
150
2o0
25o
300
350
Time(d)
Fig. 1. Corrosion potential vs. time for steel bars in PC, SLG, and SIT concrete.
l.\
10::t 0.05] 0. 0
50
100
150
200
250
300
350
Tin~ (d)
Fig. 2. Corrosioncurrent density vs. time for steel bars in PC, SLG, and SIT concrete specimens after the lollipop test.
The increase in the corrosion current density (Ioo,) values are attributed to the relative reduction in the electrical resistivity of the PC concrete around the steel reinforcement bar in comparison to the slag concrete (SLG). At the end of the 48th exposure cycle, the electrical resistivity of the PC and slag concrete were 14 and 31 kf2.cm2, respectively. A follow-up on the results of testing carried out on the slag (SIT) concrete corrosion activities prepared at the Scientific Centre's project site were monitored for a period of 1 year. The results
381
of corrosion potential are consistent with the earlier experimental findings. The corrosion potential on steel reinforcement after 1 year of exposure was -500 mV, which indicates that there is >90% probability that reinforcing steel corrosion was occurring in the area at the time of measurement. Unlike the previous results in the comparative study of corrosion test activities for PC, and SLG, the SIT did not demonstrate the benefit of reducing the risk of corrosion. This was believed to be due to the poor compaction practice of the mix concrete materials when specimens were prepared at site. As a result, the concrete's permeability to chloride ion penetration was very high. A similar analogy can be derived from the corrosion rate measurement test (the lollipop test). The results indicate that both concrete falls in the area of 90% probability that no steel corrosion was occurring when test specimens were immersed in a 3% NaC1 solution. As explained earlier and as indicated by the results shown in Fig. 2, the increase in corrosion current density was attributed to the reduction in the electrical resistivity of the concrete. After 1 year exposure of the lollipop specimens, the electrical resistivity of the PC and slag concrete were 72 and 98 kf2.cm2, respectively. These findings are in agreement with the results of the standard methods of testing for electrical indication of concrete's ability to resist chloride ion penetration (ASTM C 1202, 1991). This indicates the effectiveness of slag concrete in reducing chloride ion penetration based on the total charge in coulombs passed through the concrete for 6 h. The charge transfer continued to be reduced as the curing period increased, indicating improving resistance to chloride ion penetration with the concrete's maturation. It gave an average value of 1567 Coulombs for PC and 636 Coulombs for laboratory slag (SLG) after 56 days of curing. In the case of SIT, it gave an average value of 1775 Coulombs, which is in agreement with the above hypothesis of poor permeability resistance.
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The results of the EIS analysis (Figs. 3 and 4) indicated an impedance spectrum in the form of Nyquist plots for the plain concrete (PC), slag concrete (SLG), site concrete (SIT) and an additional sample of calcium nitrite inhibited concrete (CNI). Fig. 3 represents the EIS measurement results for the month of February (1999) while Fig. 4(a) and (b) represent the same test obtained after a duration of 1 and 2 years, respectively, that is February, 2000, and February, 2001. A qualitative analysis of the Nyquist impedance spectra indicates that the corrosion performance of the different concrete mixtures are in agreement with the ASTM test results of corrosion rate and potential values obtained in the previous section. The impedance results show that the laboratory-prepared slag concrete structure exhibited the highest impedance and capacitance values of all the specimens. This is shown clearly in Figs. 3 and 4 before and after I and 2 years of exposure, respectively.
As was expected from the protective nature of the 50% laboratory slag granulated replacement (SLG), the GGBS concrete slowed down the diffusivity of the chloride species to reach the steel substrate. In other words, the presence of a Warburg diffusion response indicated that the
-30O0 SLG --41--sff -o-PC
-'-Q-- SLG --B--SIT --O--FC ---e-- CN
-2O00 f
=a z
f
-1000
I
r
0
,
1000
t
,
2000
3000
Z'
-100000
--o-- S.G .--II--
/
-2000
SIT
1000 ...o.,.SLG •,4=.. SIT -r~O ,..o.....PC •-e- CNt
--o--FC -75000
~-500
-1500
- - D - SLG --B-- SIT --o-PC
.2~o
-50OO0 ~1-1000
M
o
~,o ~
-25000
,
0
I
25000
i
I
J
I
I
I
--e-- CN
//
-500
2~(X]0
~ , O ~ S
i
5 0 0 0 0 75000 100000 12500 Z'
Fig. 3. EIS Nyquists plot for steel bars in GGBS concrete (SLG) and (SIT), Portland cement (PC), and inhibited concrete (CNI) during February 1999.
0
500
1000 Z'
1500
2000
Fig. 4. EIS Nyquists plot for steel bars in GGBS concrete (SLG) and (SIT), Portland cement (PC), and inhibited concrete (CNI). (a) February, 2000; (b) February, 2001.
A. Husain et al. / Desalination 165 (2004) 377-384
slow diffusion processes of the corrosive species through porosity in the material and corrosion product were the rate-controlling factor for the mechanism of concrete corrosion protection for this particular sample [8]. Water-saturated concrete tends to show resistivity on the order of a few ~.cm 2 while oven-dried concrete or adequately prepared compacted GGBS can approach resistivity values typical of an electrical insulator as reflected on the EIS spectrum after 1 year of exposure. The impedance values recorded for SLG are in the range of 60× 103, 497, 407 cm2 and capacitance reaching 1.06 to 0.853 nF/cm2 after 1 and 2 years. In contrast, SIT concrete had a reduction in impedance (Rp) and capacitance (Cdl) values with 406.31,494 fLcm 2 and 0.753 nF/cm 2 and 69.7 mF/cm 2 after 1 and 2 years, respectively. In contrast to the SLG concrete, the CNI and PC samples showed the lowest impedance and capacitive behavior. The Rp values for PC and CN after 2 years (Fig. 4a and b) was 87.94 and 41.97 ~.cm 2 and capacitance 4.97 and 16.09 nF/cm 2. This is due to the high rate of permeability and adsorption of the chloride corrosive species to the steel substrate surface. In the case of the inhibited concrete sample, the attempt of studying such an inhibitor with concrete is to show the validity of EIS to determine that inadequate application of inhibitor concentration and or variation in specimen solution submersion may cause the corrosion process to become even worse than ordinary concrete (PC). The hypothesis behind the unexpected behavior of the inhibited concrete sample largely was related to improper film formation and dependence on the rate of adsorption of the competed chloride/nitrite species to the passive oxide layer on steel surface with subsequent formation of unprotected localized corrosion cells. The effectiveness of the calcium nitrite admixture, therefore, is dependent on an accurate prediction of the chloride loading of the structure over its expected design life and, hence, on the
383
selection of an appropriate dosage of the admixture [9]. If chloride ions arrive as a contaminant to the surface of the steel, the surface tends to become active when the molar ratio of C1- to nitrite and OH- ions in the pore solution reaches a critical value, exceeding 0.9. Microscopically, the concrete at the interface acts as a non-homogeneous electrolyte, which varies greatly with the overall moisture content, governed by the degree of diffusivity of corrosive and inhibited ions such as oxygen, chloride, and nitrite.
4. Conclusions
Slag concrete with 50% cement replacement can provide the requirements for a high level of corrosion protection in coastal areas and marine facilities in Kuwait; however, poorly prepared, uncompacted and inadequately cured slag concrete will be as poor as any ordinary prepared concrete. The corrosion evaluation of reinforced concrete with supplementary cementing materials by means of sophisticated methods of accelerated EIS measurement and analysis is conceptually possible. Evidence of accelerated results for optimum material selection of concrete mix design for the Kuwaiti marine environment was established using the EIS technique. Equivalent circuit analysis of the impedance spectra showed the presence of a constant phase element that was found in some cases to be approximately equal to the Warburg diffusion impedance. In other spectra, it indicated a depressed semi-circle. The laboratory slag concrete (GGBS) exhibited very high impedance and capacitance values that exceeded the performance of other concrete samples, which is in agreement with ASTM C876 and G 109. The degree of protection for concrete with supplementary cementing materials can be arranged under the prevailing
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condition in the following order: from highest to lowest protective properties SLG > SIT > PC > CNI.
Acknowledgments The authors are grateful for the financial support provided by the Kuwait Foundation for the Advancement of Sciences (KFAS) and the Kuwait Institute for Scientific Research (KISR).
References
[1] F. A1-Matrouk, A. AI-Hashem, F. Al-Kharafi and M. E1-Khafif, Proc. 2nd Arabian Corrosion Conference, Kuwait, 1996, pp. 567-579.
[2] S. AI-Bahar and E. Attiobe, Proc. Int. Conf. CONSEC '95, Sapporo, Japan, 1 (1995) 564-573. [3] O.S.B.A1-Amoudi,ACI Mat. J., 92(3) (1995) 236245. [4] D.M. Roy, A. Kumar and J. Rhodes, ACI SP-91, V.M. Malhotra, ed., Vol. 1423, 1986. [5] Z.G. Matta, Concrete Intemat., 14(5) (1992) 47-48. [6] S.E.HussainandRasheeduzzafar, ACIMat. J., 91(3) (1994) 264-272. [7] J.L. Dawson, L.M. Callow, K. Haldky and J.A. Richardson, Proc. NACE Corrosion '78, Houston, 125 (1978) 1-19. [8] C. Andrade, C. Alonso and J.A. Gonzalez, Proc. 3rd Intemat. Symp. Electrochemical Methods in Corrosion Res., Materials Science Forum, B. Elsnor, ed., Zurich, 45 (1988) 330-335. [9] Federal Highway Administration,US Departmentof Transportation,Time to corrosionof reinforcingsteel in concrete containing calcium nitrite, Publication No. FHWA-RD-99-145, 1999, pp. 1-38.