The corrosion investigation of rebar embedded in the fibers reinforced concrete Saeid Kakooei a,b , Hazizan Md Akil c,⇑ , Abolghasem Dolati b,d , Jalal Rouhi e,1 a Department of Mechanical Engineering, Universiti Teknologi PETRONAS, Perak, Malaysia b Kish University, Kish Island, Iran c School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Malaysia d Department of Materials Engineering, Sharif University of Technology, Tehran, Iran e Nano-Optoelectronic Research (NOR) Lab, School of Physics, Universiti Sains Malaysia, Malaysia highlights " The polypropylene fibers had caused delay in starting the degradation process. " Reinforcement potential increased as the amount of fibers increased from 0 to 2 kg m 3 . " The corrosion rate in coral concrete is more than twice that in siliceous concrete. " We concluded coral aggregates are not suitable for using in concrete structures. " We used NDT methods (electrical resistivity, permeability) for concrete examine. article info Article history: Received 19 December 2011 Received in revised form 8 March 2012 Accepted 25 April 2012 Keywords: Corrosion Permeability Fibers reinforced concrete Polypropylene abstract One effective method for preventing corrosion of steel reinforcement and improving the mechanical properties of concrete is changing the physical nature of concrete by adding different materials. In this study, we have used polypropylene fibers as an additional material. We have compared the corrosion rate of rebar using different volume ratios and sizes of polypropylene fibers. Reinforcement potential increased as the amount of fibers increased from 0 to 2 kg m 3 . The polypropylene fibers delay the initial corrosion process by preventing cracking, thereby decreasing permeability of the concrete. In addition, the corrosion rate of concrete samples made with Kish Island coral aggregate was compared to samples made with a siliceous aggregate. The corrosion rate in this concrete is more than twice that in siliceous concrete. We concluded that coral aggregate is improper for making concrete and using in concrete struc- tures in the onshore atmosphere. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction It is generally understood that the initially protective environ- ment that concrete provides for steel is due to the alkaline nature of concrete. This is usually reflected in passive or nobler corrosion potentials (or E corr values) for steel in concrete. When aggressive ions such as chlorides penetrate into concrete, the environment can become favorable for steel corrosion [1,2]. When aggressive ions reach the critical amounts, the potentials become more active, and thus a shift in E corr values is often associated with the intrusion of aggressive ions. This will cause the corrosion rate to increase. When rebar embedded in concrete corrodes, the corrosion prod- ucts slowly gather around the surface of the bar, taking more space and applying pressure to the encasing concrete, leading to cracks. The crack widths propagate in proportion to the degree of loss of rebar mass [3]. There are observable cases where concrete will be exposed in marine environments or bridge decks subjected to deicing salts that are replete with chloride ions [4–7]. Diffusion of chloride into concrete that is exposed to the marine environ- ment will decrease over time due to the hydration of the concrete, making the concrete pore structure more dense, or removing chlo- ride ions by binding [8]. Other factors can also have affected this process, such as the amount of cover, the method and length of consolidation and curing, and the water/cement ratio (w/c), but it seems that permeability plays the main role. We can potentially decrease this permeability by the addition of polymer fibers and the encasement of concrete in a polymer resin or polymer compos- ite [5]. Additionally, fiber reinforced concrete has a longer service life in comparison with other types of concrete because of its resis- tance to corrosion and chemicals [9]. Payrow et al. mentioned that the appearance of post-peak residual strength of concrete mem- bers in bending was the best benefit of the using fibers in concrete as reinforcement [10]. 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.04.051 ⇑ Corresponding author. Tel.: +60 45996161; fax: +60 45941011. E-mail address: [email protected](H.M. Akil). 1 Present address. Construction and Building Materials 35 (2012) 564–570 Contents lists available at SciVerse ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Transcript
Construction and Building Materials 35 (2012) 564–570
Contents lists available at SciVerse ScienceDirect
a Department of Mechanical Engineering, Universiti Teknologi PETRONAS, Perak, Malaysiab Kish University, Kish Island, Iranc School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Malaysiad Department of Materials Engineering, Sharif University of Technology, Tehran, Irane Nano-Optoelectronic Research (NOR) Lab, School of Physics, Universiti Sains Malaysia, Malaysia
h i g h l i g h t s
" The polypropylene fibers had caused delay in starting the degradation process." Reinforcement potential increased as the amount of fibers increased from 0 to 2 kg m�3." The corrosion rate in coral concrete is more than twice that in siliceous concrete." We concluded coral aggregates are not suitable for using in concrete structures." We used NDT methods (electrical resistivity, permeability) for concrete examine.
a r t i c l e i n f o
Article history:Received 19 December 2011Received in revised form 8 March 2012Accepted 25 April 2012
One effective method for preventing corrosion of steel reinforcement and improving the mechanicalproperties of concrete is changing the physical nature of concrete by adding different materials. In thisstudy, we have used polypropylene fibers as an additional material. We have compared the corrosion rateof rebar using different volume ratios and sizes of polypropylene fibers. Reinforcement potentialincreased as the amount of fibers increased from 0 to 2 kg m�3. The polypropylene fibers delay the initialcorrosion process by preventing cracking, thereby decreasing permeability of the concrete. In addition,the corrosion rate of concrete samples made with Kish Island coral aggregate was compared to samplesmade with a siliceous aggregate. The corrosion rate in this concrete is more than twice that in siliceousconcrete. We concluded that coral aggregate is improper for making concrete and using in concrete struc-tures in the onshore atmosphere.
� 2012 Elsevier Ltd. All rights reserved.
1. Introduction rebar mass [3]. There are observable cases where concrete will
It is generally understood that the initially protective environ-ment that concrete provides for steel is due to the alkaline natureof concrete. This is usually reflected in passive or nobler corrosionpotentials (or Ecorr values) for steel in concrete. When aggressiveions such as chlorides penetrate into concrete, the environmentcan become favorable for steel corrosion [1,2]. When aggressiveions reach the critical amounts, the potentials become more active,and thus a shift in Ecorr values is often associated with the intrusionof aggressive ions. This will cause the corrosion rate to increase.When rebar embedded in concrete corrodes, the corrosion prod-ucts slowly gather around the surface of the bar, taking more spaceand applying pressure to the encasing concrete, leading to cracks.The crack widths propagate in proportion to the degree of loss of
ll rights reserved.
60 45941011.
be exposed in marine environments or bridge decks subjected todeicing salts that are replete with chloride ions [4–7]. Diffusionof chloride into concrete that is exposed to the marine environ-ment will decrease over time due to the hydration of the concrete,making the concrete pore structure more dense, or removing chlo-ride ions by binding [8]. Other factors can also have affected thisprocess, such as the amount of cover, the method and length ofconsolidation and curing, and the water/cement ratio (w/c), butit seems that permeability plays the main role. We can potentiallydecrease this permeability by the addition of polymer fibers andthe encasement of concrete in a polymer resin or polymer compos-ite [5]. Additionally, fiber reinforced concrete has a longer servicelife in comparison with other types of concrete because of its resis-tance to corrosion and chemicals [9]. Payrow et al. mentioned thatthe appearance of post-peak residual strength of concrete mem-bers in bending was the best benefit of the using fibers in concreteas reinforcement [10].
S. Kakooei et al. / Construction and Building Materials 35 (2012) 564–570 565
Moreover, electrochemical techniques such as polarizationcurves and potentiometer data applied to steel-reinforced concretecan be useful in assessing the effect of the intrusion of aggressiveions on the corrosion rate of the steel inside [11,12]. The controlof the crack width for improving concrete durability can be a keyparameter for designing reinforced concrete structures [13–15].In particular, unexpected cracking can be seen as a consequenceof steel corrosion, leading to a prematurely reduced service lifefor a reinforced concrete structure [16,17]. Also the use of polypro-pylene can reduce shrinkage cracking in mortar mixture or concrete[13,14]. Pelisser et al. investigated the use of different fibers inconcrete, and they found that crack opening can be significantlyreduced [14]. Ismail et al. studied the relation between cracks
and chloride diffusion. Their result suggested that if crack openingswere greater than the threshold value, chloride diffusion along thecrack path depends on mortar age [18]. In this research paper, theuse of polypropylene and its effect on corrosion rate of rebar in con-crete was investigated. Following this, the study of corrosion ratesby electrochemical methods, and measuring the permeability ofthe concrete cover of the reinforcement and concrete electricalresistivity were investigated.
Fig. 1. Dimensions of the specimens.
Table 2Physical characteristics of polypropylene fibers.
An ordinary Portland cement, equivalent to ASTM type II, wasused to prepare the mortar specimens. Siliceous sand rangingbetween 0 and 6 mm diameter and tap water were employed atlaboratory temperature (20–25 �C). Cylindrical reinforced concretespecimens 10 cm in diameter and 20 cm in height were used. Thecomposition of the concrete is given in Table 1. As shown in Fig. 1,an air–tape–concrete–steel interface and electroplater’s tape wereused for minimizing crevice corrosion. The main physical charac-teristics of the polypropylene fibers are listed in Table 2. In this re-search, concrete samples were prepared with volumetric ratios offibers: 0, 0.5, 1, 1.5 and 2 kg m�3 as mentioned in our previouswork [19]. Usable aggregate must be in the special range for suit-able mixing design and least permeability. Aggregate size was se-lected similar to ASTM C33 [20]. The ratio of water to cementwas 0.48. All experiments were run in a simulation of marine envi-ronmental conditions. For accelerating the corrosion process, con-crete samples were put in a container of Persian Gulf seawater.Chemical analysis demonstrates that the amount of Cl� ion’s in thiswater is about 250–270 g/l. Concrete samples mix with coralaggregate were named as CSP0. Where C: coral aggregate, S: seawater environment, and P: volumetric ratio of polypropylene fibers(this is 0 in this mix). Concrete samples mix with the siliceousaggregate were named as MSP0–2L19. Where M: siliceous aggregate,S: sea water environment, and P: volumetric ratio of polypropylenefibers (0, 0.5, 1, 1.5 and 2 kg m�3).
2.2. Experimental test
A triple-electrode system was used for electrochemical mea-surements. The electrode consisted of a reinforced concrete speci-men as the working electrode. A saturated calomel electrode (SCE)
Table 1The raw materials used in the presented mixture design.
Water 190 kg m�3
Cement 400 kg m�3
Aggregate 1760 kg m�3
wc 0.48
and a platinum electrode were used as reference and counter,respectively. An EG&G Model ZAHNER IM6 potentiostat, was usedfor electrochemical measurements. In addition, a computer pro-gram was used with a scan rate of 1 mV/s in order to analyze theresistance to polarization. The variations of corrosion potential(Ecorr) with time were recorded with respect to a saturated calomelreference electrode at room temperature. In the beginning, thesemeasurements were taken every day, until balanced conditionswere established. The measurements were carried out on the 5th,30th, 60th, and 100th day of the exposure period. The variationsof open circuit potentials (Ecorr) with time were recorded withrespect to a Cu/CuSO4 reference electrode at room temperature.A RESI electrical resistivity meter (made in Switzerland) was usedfor electrical resistivity measurement. Furthermore, permeabilitymeasurements were carried out with a TORENT permeability set.
3. Results and discussion
3.1. Polarization curves
3.1.1. The effect of volumetric ratio of polypropylene fibers oncorrosion behavior of concrete samples
The effect of volumetric ratio of polypropylene fibers on corro-sion behavior of concrete samples with different volumetric ratiosof polypropylene fibers that were exposed to sea water for 5, 50,100, and 150 days are shown in Figs. 2–5. Initially, the concrete sam-ple with 0.5 kg m�3 of fibers had the least potential in comparison to
Fig. 2. Polarization curves of concrete samples with different volumetric amount of polypropylene fibers after 5 days exposure in sea water. Numbers (1–5) indicatevolumetric amount of polypropylene fiber as follows: 1 = 0.5 kg m�3, 2 = 1 kg m�3, 3 = 1.5 kg m�3, 4 = 2 kg m�3, and 5 = 0 kg m�3 polypropylene fibers.
Fig. 3. Polarization curves of concrete samples with different volumetric amounts of polypropylene fibers after 50 days exposure in sea water. Numbers (1–4) indicatevolumetric amount of polypropylene fiber as follows: 1 = 0.5 kg m�3, 2 = 1 kg m�3, 3 = 1.5 kg m�3, and 4 = 2 kg m�3 polypropylene fibers.
Fig. 4. Polarization curves of concrete samples with different volumetric amounts of polypropylene fibers after 100 days exposure in sea water. Numbers (1–4) indicatevolumetric amount of polypropylene fiber as follows: 1 = 0.5 kg m�3, 2 = 1 kg m�3, 3 = 1.5 kg m�3, and 4 = 2 kg m�3 polypropylene fibers.
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the other concrete samples. This sample was very prone to corro-sion. The concrete sample with 1 kg m�3 of fiber also had a lowertendency toward corrosion and was in a passive state. As the curvesshow increasing polypropylene fiber ratio caused a noticeable
increase in corrosion potential. Therefore, the curves shift to valuesthat are more positive. Also, current density of corrosion shifts to theleft side. It shows that there was lower current flowing on the rein-forcement surface.
Fig. 5. Polarization curves of concrete samples with different volumetric amount of polypropylene fibers after 150 days exposure in sea water. Numbers (1–5) indicatevolumetric amount of polypropylene fiber as follows: 1 = 0.5 kg m�3, 2 = 1 kg m�3, 3 = 1.5 kg m�3, 4 = 2 kg m�3, and 5 = 0 kg m�3 polypropylene fibers.
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Increasing the number of fibers has a direct relation to perme-ability. It causes concrete samples to become denser with lowerpermeability. It means that a progressively smaller number of ionsenter the concrete samples. So corrosion potential will be morepositive and current density of corrosion will be lower [20,21]. Cor-rosion of rebar in the concrete sample with 2 kg m�3 fiber contentwas severely increased after 150 days exposure in sea water. Thecorrosion potential was decreased to the same level as in the con-crete sample without fibers. Fig. 5 shows the concrete sample with1.5 kg m�3 fibers content after 150 days exposure in seawater. Itwas more passive and had a more suitable degree of corrosionresistance compared to the other concrete samples. This is becausethe amount of fibers has a direct influence on the efficiency of fiberreinforcement and the degree of rebar corrosion in concrete [17].
3.1.2. The effect of fiber size on corrosion behavior of concrete samplesConcrete samples with similar volumetric ratios of polypropyl-
ene fiber (2 kg m�3) and different fiber sizes (6 mm,12 mm,19 mm)were exposed to sea water for 5, 50, 100 and 150 days. Polarizationcurves of these samples were compared in Figs. 6–9. As polariza-tion curves in Fig. 7 show, the concrete samples with fiber size of6 mm showed more active corrosion after 50-days compared tothe other samples. The Concrete sample with fibers sizes of12 mm had more positive potential as compared to two other sam-ples. After 100 and 150-days exposure in sea water, the concretesample having 12 mm fibers showed more positive potential andwas the most passive in relation to corrosion.
Fig. 6. Polarization curves of concrete samples with different sizes of polypropylenepolypropylene fiber as follows: 1 = 6 mm, 2 = 12 mm, and 3 = 19 mm.
3.2. Open circuit potential changes of concrete samples versus time
One of the well known methods for measuring rebar corrosionin concrete is the half cell potential technique (Ecorr). In this meth-od, the difference in potential between rebar embedded into con-crete and a reference electrode (copper/copper sulphate electrode(CSE)) is measured in accordance with ASTM C-876 [22,23].Fig. 10 shows the result of the measurement of the potential for4 months. With increasing volumetric ratio of fiber, the potentialof samples shifted to more positive Ecorr values. This is noticeablein concrete sample with 1.5 kg m�3 fibers content.
3.3. Corrosion current density (icorr) versus time
Corrosion current density of concrete samples was studied for3 months (Fig. 11). The concrete sample with coral aggregate wasshown to have more current density corrosion than the other sam-ples. The concrete sample without fibers had variable corrosiondensity. The current density decreased over 50 days and reacheda steady amount. In the case of concrete samples with polypropyl-ene fibers, the MSP1L19 sample was shown to have a lower corrosioncurrent density in comparison with other samples. But, because theeffect of potential is more significant than current density (cfSection 3.1.1), the concrete sample with 1.5 kg m�3 of fibers wasshown to have a better result. In this case, reading only the corro-sion currents can lead us to incorrect understandings if the above
fibers after 5 days exposure in sea water. Numbers (1–3) indicate the size of
Fig. 7. Polarization curves of concrete samples with different size of polypropylene fibers after 50 days exposure in sea water. Numbers (1–3) indicate the size ofpolypropylene fiber as follows: 1 = 6 mm, 2 = 12 mm, and 3 = 19 mm.
Fig. 8. Polarization curves of concrete samples with different size of polypropylene fibers after 100 days exposure in sea water. Numbers (1–3) indicate the size ofpolypropylene fiber as follows: 1 = 6 mm, 2 = 12 mm, and 3 = 19 mm.
Fig. 9. Polarization curves of concrete samples with different size of polypropylene fibers after 150 days exposure in sea water. Numbers (1–3) indicate the size ofpolypropylene fiber as follows: 1 = 6 mm, 2 = 12 mm, and 3 = 19 mm.
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factors are not taken together with other parameters related to thecorrosion kinetics.
3.4. Corrosion rate of concrete samples
Fig. 12 shows a comparison of corrosion rate of concrete sam-ples with different volumetric amount of polypropylene fibers. Asshown, the concrete sample with 2 kg m�3 of fiber has the lowest
corrosion rate. And after 40-days of exposure in sea water, concreteproduced with fibers was shown to have a lower corrosion ratecompared to the samples without fibers. It should be mentionedthat polypropylene fibers delay the initial corrosion process as wellas decreasing permeability, and decreasing volumetric expansionand contraction. In addition, it shows the ability of fibers to arrestcrack formation and to control crack propagation [11,17,22,24–25].Also, the concrete sample with coral aggregate showed a higher
Fig. 10. Open circuit potential changes of concrete samples (potentiometry);Numbers (1–5) indicate volumetric amount of polypropylene fiber as follows:1 = sample with coral aggregate without polypropylene fiber, 2 = 1.5 kg m�3,3 = 0.5 kg m�3, 4 = 1 kg m�3, and 5 = 0 kg m�3 polypropylene fibers.
Fig. 11. Corrosion current density (icorr) versus time (days); Numbers (1–5) indicatevolumetric amount of polypropylene fiber as follows: 1 = sample with coralaggregate without polypropylene fiber, 2 = 0 kg m�3, 3 = 0.5 kg m�3, 4 = 1 kg m�3,and 5 = 1.5 kg m�3 polypropylene fibers.
Fig. 12. Corrosion rate of concrete samples with different volumetric amounts ofpolypropylene fibers; Numbers (1–5) indicate volumetric amount of polypropylenefiber as follows: 1 = sample with coral aggregate without polypropylene fiber,2 = 0 kg m�3, 3 = 0.5 kg m�3, 4 = 1 kg m�3, 5 = 1.5 kg m�3, and 5 = 2 kg m�3 polypro-pylene fibers.
Fig. 13. Corrosion current changes of concrete samples with different fiber sizes;Numbers (1–3) indicate the size of polypropylene fiber as follows: 1 = 6 mm,2 = 12 mm, and 3 = 19 mm.
Table 3Electrical resistivity of concrete samples with different volumetric ratio of polypro-pylene fiber.
MSP0.5L19 MSP1L19 MSP1.5L19 MSP0 CSP0
p (KX cm) 13–14 14–19 11–18 14–15 11–14
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corrosion rate compared to the concrete sample with siliceousaggregate, which was related to the presence of chloride com-pounds in coral aggregate [26,27].
3.5. Corrosion current changes in concrete samples with different fibersizes over time
Fig. 13 shows corrosion current changes with time for concretesamples with 6 mm, 12 mm and 19 mm fiber sizes. According toFig. 13, sample MSP2L12, was shown to have corrosion current low-er than the two other samples.
3.6. Electrical resistivity of concrete samples with different volumetricratio of polypropylene fiber
Measuring of electrical resistance is clearly described in a scien-tific paper by Ferreira and Jalali [28]. Electrical resistance of con-crete samples was shown in Table 3. Electrical resistance isdependent on the continuous hydration of cement and it varies incorrespondence to the changes in concrete [28]. Electrical resis-tance of produced concrete samples consisting of 1 kg m�3 and1.5 kg m�3 fibers had maximum values compared to other samples.This is because of the decrease in corrosion induced by the electricalcurrent.
The concrete sample with coral aggregate was shown to havethe lowest electrical resistance. The decrease in electrical resis-tance is related to presence of chlorides components in coralaggregate.
3.7. Oxygen permeability of concrete samples with different volumetricratio of polypropylene fibers
The importance of measuring the concrete moisture is to deter-mine the gas permeability. Therefore, it should be conducted insuch a way as to neutralise the moisture effects. In this regard theelectrical resistivity q should be determined, which can be mea-sured by the four electrode method. For this purpose, the circuitwas between two external electrodes and the potential reductionwas measured between the two internal ones. A repeatable labora-torial method was properly obtained by comparing the results ofKT, q and Ko (oxygen permeability factor) which can be appliedfor columns, the samples made up of different concrete mixturesof different amounts of moistures. Formulas (1) and (2) were usedfor dry concrete and moisture concrete, respectively.
Table 4Permeability factor of different concrete samples in 3 months exposure in sea water.
Time (day) Concrete samplesMSP0.5L19 MSP1L19 MSP1.5L19 MSP2L19 MSP0 CSP0
where Ko(T) is the oxygen permeability calculated for dry concrete(E�16 m2), Ko(F) is oxygen permeability calculated for moistureconcrete (E�16 m2), KT is gas permeability calculated by Torrent per-meability test (E�16m2), and q is the electrical resistivity by 4-pinWenner method (KX cm).
The explanation for formula (2) is that a concrete cover beinglow in quality has high gas penetration (KT) and therefore thelow electrical resistivity q in (KT0.4/q0.7) of formula (2) is affectedby the quality. If the concrete cover is moisturized, the KT and qare lower and therefore (KT0.4/q0.7) is less affected. Results of thesecomparisons are illustrated in Table 4. [17,21,29–30]. According tothis result, it can be said that permeability is the main factor that isresponsible for diffusion of chloride ion as an aggressive element inconcrete.
4. Conclusions
The following conclusions can be drawn from this research:
i. Polarization curves became more positive with increasingvolumetric ratio of polypropylene fibers.
ii. Samples with fiber volumetric ratios of 1.5 kg m�3 indicatedbetter corrosion resistance compared to the other samples.
iii. In this research, using coral aggregate for producing concretesamples showed that this concrete composition was not apractical composition. Corrosion rate in this concrete wasat least twice that was shown in siliceous concrete.
iv. The results show that 6 mm length fibers were not the suit-able size to be used in concrete. The result of using fiberswith length of 12 and 19 mm was approximately the same,with the optimum size being 12 mm.
v. Apart from increasing corrosion resistance, the presence ofpolypropylene fibers decreased the permeability, volumetricexpansion and contraction of concrete, which in turn hadreduced the chance of concrete cracking.
Acknowledgments
The authors would like to thank Mr. Ray Lingel for the Englishediting of this manuscript. Facilities and funding for this studywere provided by Kish University, Iran. Also authors would like
to thank Universiti Sains Malaysia for supporting the researchwork (8640013).
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