Experimental Study on Corrosion Resistance of TMT Bar ...
ICCBT2008
Experimental Study on Corrosion Resistance of TMT Bar in Concrete R. Manoharan*, National Institute of Technology, Trichy, INDIA. P. Jayabalan, National Institute of Technology, Trichy, INDIA. K. Palanisamy, National Institute of Technology, Trichy, INDIA.
ABSTRACT Emphasis was made to study the durability of reinforced concrete by adding commercially available chemical admixtures like Water Reducer and Water Proofer which are normally used with the purpose of improving the workability property of concrete thus producing dense and impermeable concrete. By using Electrochemical Technique, gravimetric technique and chloride diffusion method, corrosion resistance of TMT (Thermo Mechanically Treated) bar embedded in M25 concrete under chloride corrosive environment with various percentages of admixtures and durations were studied. The present investigation reveals that the addition of commercially available chemical admixtures does not add to corrosion and as the contrary the rate of corrosion of TMT bars reduces. Keywords: Corrosion, admixture, TMT bar, water proofing, water reducing A.C Impedance, Gravimetric and Linear Polarisation Resistance.
*Correspondence Author: R. Manoharan, Research Scholar, Department of Civil Engineering, National Institute of Technology, Trichy, TamilNadu, India. E-mail: [email protected]
ICCBT 2008 - A - (22) - pp239-250
Experimental Study on Corrosion Resistance of TMT Bar in Concrete
1.
INTRODUCTION
Concrete is a composite material made of aggregates and porous cement paste, which is the reaction product of mixing water and cement. The structure and composition of the cement paste determines the durability and the long-term performance of concrete. Concrete is normally reinforced with steel bars. The ability to withstand various types of degradation of concrete and protection to rebars embedded in it depends on the structure of the concrete. The processes of deterioration of concrete and corrosion of reinforcement are closely connected to each other. The former provoke destruction of concrete cover or cause micro cracking that compromises its protective characteristics. On the other hand, corrosion attack, because of the expansive action of corrosion products, produces cracking or delamination of the concrete and reduces its adhesion to reinforcement. Therefore, the durability of concrete structures depends on the resistance of the concrete against chemical and physical factors and its ability to protect the embedded reinforcement against corrosion. In view of the fact that a large number of existing structures are being deteriorated with time by reinforcement corrosion due to environmental exposure, corrosion is one of the main causes for the limited service life of reinforced concrete.The corrosion in reinforcements can be prevented by providing dense and impermeable concrete, which could be achieved by reducing the water-cement ratio. Chemical admixtures (water reducers and water proofers) in concrete are generally used with the purpose of increasing the workability by increasing the effectiveness of mixing water. They can also be used with the purpose of increasing the durability of concrete by decreasing the w/c ratio. For instance, they can guarantee the same workability with a remarkable reduction of water. Hou and Chung [3], reported the effect of admixtures in concrete on the corrosion resistance of steel reinforced Concrete was assessed by measuring the corrosion potential and corrosion current density during immersion in Ca(OH)2 and NaCl solutions. They proved that the admixtures improve the corrosion resistance, due to decrease of water absorptivity, and not so much due to increase in electrical resistivity of concrete. [4] examined the breaking down of passive film on embedded steel and concluded the level of chloride content in concrete also influences the electrical resistivity of the concrete. Further, [5] investigated a model for electrical impedance of steel-concrete interface. This model was validated through the experimental results conducted on reinforced concrete specimens immersed in solutions of chloride and sulphate. [6] presented the results of an experimental investigation on the use of acoustic emission during the carrion of steel rebars embedded in mortar and immersed in sodium chloride solution. The process of corrosion is accelerated by various imposed potentials and is followed by acoustic emission coupled to electrochemical techniques. The experimental results show that electrochemical techniques can evaluate the corrosive character of the medium used. [7] studied the actual and predicted weight loss data for a number of mild steel bars contained in OPC concrete, subjected to three different environmental regimes and monitored using potentiostaticaly controlled linear polarisation resistance measurements. The three sets of reinforced concrete specimens were subjected to chloride induced corrosion. Each set of specimens were exposed to a daily regime of wetting and drying in a controlled environment for duration of 1700 days. Two sets of testing were conducted at around 1200 days and 1700 240
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days. Prior to casting the mild steel bars were individually cleaned and weighted. The weight loss for each bar due to corrosion was recorded at the end of the period of exposure. Instantaneous linear polarisation resistance corrosion measurements were taken on each bar at regular intervals for the duration of the exposure time. These resistance measurements were then integrated to evaluate a predicted total weight loss. The results show that the weight loss evaluated from experimental linear polarisation resistance measurements give a significant over-estimate of the actual weight losses recorded. So far the investigations are done by using mineral admixtures only. The limitations of using mineral admixture are 1. There will not be effective mixing between admixture and concrete. 2. The passive film formed due to addition of mineral admixture over the reinforcements against corrosion will be less than that of chemical admixture. So the corrosion rate will be less in using chemical admixture.
2.
EXPERIMENTAL INVESTIGATION
Concrete cubes of size 150mm were used for the present investigation. M25 grade control concretes with two different admixtures of four different percentages (0.5%, 0.75%, 1.00% and 1.25%) were cast. TMT bars of 5cm length were embedded inside the concrete at a cover of 25mm. After casting, the cubes were cured for 28 days. For performing electrochemical tests, the rebars connected with wires were embedded inside, so that one end of wire was connected to rebar and another end was kept free out of the cube so as to connect to the electrochemical analyzer. After 28 days of usual curing, the cubes were ponded with 3% sodium chloride (NaCl) solution on one face of the cube as shown in Figure 1, in order to maintain unidirectional chloride penetration. To accelerate the corrosion, alternate wetting and drying was done periodically. The studies conducted on the specimens are Electrochemical Technique (AC Impedance), Gravimetric Technique and Chloride Diffusion test. After 2 months of alternate wetting and drying, tests were conducted every month to know the ability of concrete to resist the chloride ion penetration, which is the root cause for reinforcement corrosion. The investigations were done over a period of six months.
Figure 1. Test Specimens subjected to unidirectional ponding
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Experimental Study on Corrosion Resistance of TMT Bar in Concrete
3.
TEST DESCRIPTION
3.1 Electrochemical Technique (AC Impedance) A.C Impedance technique, a useful non-destructing technique for quantifying the corrosion of steel rebars embedded in concrete was carried out. In this technique the base potential is kept constant at initial E. Frequency is scanned from high frequency to low frequency, The Embedded rebar acts as working electrode; Stainless steel plate acts as counter electrode and the Saturated Calomel Electrode (SCE) acts as Reference electrode. These three electrodes are then connected to the electrochemical analyzer. The A.C. current is applied through the specimen. The Nyquist plot is directly recorded for the frequencies from 0.01 Hz to 100 KHz by using interface software called CHI604C. From the plot the polarization resistance values are obtained and then the corrosion rate is calculated using the formula: B (microamps/cm2) I corr = Rp Where, Rp = Polarisation Resistance in Ohms and B = 26 mV and 52 mV for active and passive conditions respectively. Corrosion rate in terms of mm/yr was obtained by multiplying the Icorr with the factor K=11.7 (mm/yr)/(mA/cm2). The Nyquist plot obtained for TMT bar is shown in Figure 2.
Figure 2. Nyquist plot for TMT bar
3.2 Gravimetric Technique (Weight Loss Determination) This technique is a destructive method, which consists in weighing the rebar specimens before and after being introduced in the concrete to be tested. Procedures for preparing, cleaning, and evaluating corrosion rate are followed as per ASTM G1-03 [2]. The difference in weight (gravimetric loss) is a quantitative average of the attack of corrosion. For different 242
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percentages of Water reducer and waterproofer admixtures, the corrosion rate in terms of weight loss, were measured every month. Then the corrosion rate is calculated using the formula, (K * W ) (mm/yr) CorrosionRate = (A *T * D) 4 where K is a constant, K =8.76*10 in case of expressing Corrosion rate in mm/yr T is the exposure time expressed in hours, A is the surface area in cm2, W is the mass loss in gram, and D is the density of the corroding metal (7.85g/cm3) 3.3 Chloride Diffusion Test Measuring the chloride content of the concrete at different depths enable to estimate the potential of chloride induced corrosion damage. There are several analytical values considered to designate the chloride content such as acid-soluble content and water-soluble content etc.. Acid soluble is the most widely used measurement and indicates the proportion of chloride, which is soluble in nitric acid. The water-soluble chloride is that extractable in water in defined conditions. The standard test method for determination of chloride is as per Volhard’s method of determination of chlorides by volumetric analysis and followed in this investigation. IS14959 (Part 2): 2001 [1] give a standard test procedure for determination of chloride by either acid soluble or water-soluble chlorides in hardened concrete. The chloride content can be expressed in terms of percent chloride by the mass of cement (% in weight of cement).
4.
TEST RESULTS AND DISCUSSIONS
Conplast SP430, the chemical , water reducing admixture and conplast X421 IC, the chemical, water proofing admixture of FOSROC Company, Bangalore, India, a brown colored sulphonated naphthalene polymers are used in this investigation. 4.1. Effect of Water Reducing Admixture 4.1.1. AC Impedance Technique Figure 3 shows that the corrosion rate decreases with duration for all the percentages of admixture. The corrosion rate of TMT bar is lower for concrete with all percentage of admixtures of at fifth month of testing compared to concrete without admixture. So the trend shows the addition of chemical admixtures restrict the corrosion rate when comparing to normal concrete.
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Corrosion Rate (mm/y)
VARIATION OF CORROSION RATE W ITH DURATION W R-TM T ROD 0.2 0.15 0.1 0.05 0 3 months
4 months
5 months
Duration (months) CC
WR1
WR2
WR3
WR4
Figure 3. Variation of corrosion rate of TMT bar for various percentages of admixtures at different durations (WR-AC Impedance Technique)
4.1.2. Gravimetric Technique Figure 4 shows that when comparing to normal concrete, Up to fourth month the corrosion rate decreases with duration for concrete with admixtures of 0.5%, 0.75% and 1.00%. But after fourth month the corrosion rate increases with duration for concrete with all admixtures. Higher the percentages of admixture (except 1.25 %) and duration, corrosion rate was lower. Among the percentage of admixtures, the higher resistance to corrosion is offered by admixture of 0.5%.
Corrosion Rate (mm/y)
VARIATION OF CORROSION RATE W ITH DURATION W R-TM T ROD 0.005 0.004 0.003 0.002 0.001 0 3 months
4 months
5 months
Duration (months) CC
WR1
WR2
WR3
WR4
Figure 4. Variation of corrosion rate of TMT bar for various percentages of admixtures at different durations (WR-Gravimetric Technique)
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4.1.3. Diffusion of Chloride The diffusion of chloride in concrete decreases as depth increases from the surface of ponding. In case of 0-5cm Depth (Figure 5), it is observed that the chloride diffusion decreases with duration in concrete with admixtures of 0.5%, 0.75% and 1.00%.The diffusion is lower in concrete with admixtures of all percentages compared to concrete without admixtures when the life of concrete increases. In case of 5-10cm Depth (Figure 6), it is observed that the diffusion of chloride in concrete with admixtures of 0.5% and 1.00% the diffusion decreases with duration. The diffusion of chloride is lower in the concrete with admixtures of 0.5%, 0.75% and 1.00% at all the durations of testing compared to concrete without admixtures. In case of 10-15cm Depth (Figure 7), it is observed that the diffusion of chloride in concrete with admixtures of all percentages, chloride diffusion decreases with duration. The chloride diffusion is lower in concrete with admixtures compared to concrete without admixtures when duration goes up.
Streng th o f chlo ride (% )
The diffusion of chloride is much resisted in the concrete with admixtures of 0.5%, 0.75% and 1.00% compared to concrete without admixtures. In the third month penetration of chlorine is more in the 0-5cm level but it is less in 5-10cm level and lesser in the 10-15cm level. In the fourth month, the concentration of chlorine is more at the 5-10 cm level which is higher than 0-5 and 10-15cm levels. This condition is maintained for fifth month also.
0.25 0.2 0.15 0.1 0.05 0 3 months
4 months
5 months
Duration Control Concrete
0.5%WR
0.75%WR
1.00%WR
1.25%WR
Figure 5. Variation of Strength of chloride with the percentage of admixtures for various durations (WR – M25 – 0-5 cm Depth)
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Str e ng th o f c hlo r ide (% )
0.12 0.1 0.08 0.06 0.04 0.02 0 3 months
4 months
5 months
Duration Control Concrete
0.5%WR
0.75%WR
1.00%WR
1.25%WR
Streng th o f chlo ride (% )
Figure 6. Variation of Strength of chloride with the percentage of admixtures for various durations (WR – M25 – 5-10 cm Depth)
0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 3 months
4 months
5 months
Duration Control Concrete
0.5%WR
0.75%WR
1.00%WR
1.25%WR
Figure 7. Variation of Strength of chloride with the percentage of admixtures for various durations (WR – M25 – 10-15 cm Depth)
4.2. Effect of Water Proofing Admixture 4.2.1. AC Impedance Technique Conplast X 421 IC, the chemical, water proofing admixture of FOSROC Company,Bangalore, India, a brown colored ligosulphonated naphthalene polymer is used in this investigation Figure 8 shows that the corrosion rate decreases with increase of duration and percentage of admixture. Thus the higher percentages of admixed concrete up to 1% shows more corrosion 246
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resistive than the lower percentages. But 1.25 % admixture has lower corrosion resistive than 0.75% admixture.
Corrosion Rate (mm/y)
VARIATION OF CORROSION RATE W ITH DURATION W P-TM T ROD 0.02 0.015 0.01 0.005 0 3 months
4 months
5 months
Duration (months) CC
WP1
WP2
WP3
WP4
Figure 8. Variation of corrosion rate of TMT bar for various percentages of admixtures at different durations (WP-AC Impedance Technique)
4.2.2. Gravimetric Technique Figure 9 shows that the corrosion rate decreases with duration in concrete with and without admixtures. Concrete with higher percentages of admixture are more corrosive resistance compared to lower percentages. After fifth month all the admixed concrete gives less corrosion rate than the normal concrete. So when comparing to water reducing admixture, water proofing admixture is more corrosion resistive.
Corrosion Rate (mm/y)
VARIATION OF CORROSION RATE W ITH DURATION W P-TM T ROD 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0 3 months
4 months
5 months
Duration (months) CC
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WP1
WP2
WP3
WP4
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Experimental Study on Corrosion Resistance of TMT Bar in Concrete
Figure 9. Variation of corrosion rate of TMT bar for various percentages of admixtures at different durations (WP-Gravimetric Technique) 4.2.3. Diffusion of Chloride
Strength of chloride (%)
The diffusion of chloride in concrete is decreasing as depth increases from the surface of ponding. In case of 0-5cm Depth (Figure 10), admixed concrete gives less diffusion than the control concrete. In case of 5-10cm Depth (Figure 11), Chloride diffusion is found to decreases with duration in concrete with admixtures of 0.5%, 1.00%and 1.25%. The diffusion of chloride is lower in concrete with admixtures of all percentages at all the durations compared to concrete without admixtures. Concrete with 1.25% of admixture is better in not allowing the chloride into it. In case of 10-15cm Depth (Figure 12), it is observed that the chloride diffusion decreases with duration in concrete with admixtures of 0.5%, 0.75% and 1.00%. The diffusion of chloride is lower in concrete with admixtures of 0.5%, 1.00% and1.25% compared to concrete without admixtures with increase in life of concrete. Higher percentages of admixture are more resistant to diffusion of chloride than the lower percentages.
0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 3 months
4 months
5 months
Duration Control Concrete
0.5%WP
0.75%WP
1.00%WP
1.25%WP
Figure 10. Variation of Strength of chloride with the percentage of admixtures for various durations (WP – M25 – 0-5 cm Depth)
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S t r e n g t h o f c h lo r id e ( % )
0.12 0.1 0.08 0.06 0.04 0.02 0 3 months
4 months
5 months
Duration Control Concrete
0.5%WP
0.75%WP
1.00%WP
1.25%WP
Figure 11. Variation of Strength of chloride with the percentage of admixtures for various durations
S tr e n g th o f c h lo r id e (% )
(WP – M25 – 5-10 cm Depth)
0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 3 months
4 months
5 months
Duration Control Concrete
0.5%WP
0.75%WP
1.00%WP
1.25%WP
Figure 12. Variation of Strength of chloride with the percentage of admixtures for various durations (WP – M25 – 10-15 cm Depth)
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Experimental Study on Corrosion Resistance of TMT Bar in Concrete
5.
CONCLUSIONS
1. Compared to concrete without admixture, the corrosion rate decreases as the duration increases for the admixed concrete. 2. In the electrochemical techniques, the behavior of Water reducing admixture is more effective in resisting the corrosion especially at lower percentage i.e., at 0.5% of admixture for M25 grade of concrete. 3. The performance of Water Proofing admixture is efficient at higher percentages of admixture compared to lower percentages of admixture. 4. TMT rods are found to perform well in concrete with admixtures compared to concrete without admixtures. 5. In general when compared to Water proofing admixture, the resistance to corrosion offered by Water reducing admixture is better. 6. The chemical admixtures are efficient in resisting the corrosion for TMT bars, which are easily prone to corrosion compared to other bars available in market. Admixtures not only give workability but also durability to concrete structures. REFERENCES [1]. Determination of water soluble and acid soluble chlorides in mortar and concrete- Method of test for hardened mortar and concrete, IS 14959 (Part 2): 2001, Bureau of Indian Standards, New Delhi [2]. Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens, ASTM G1 – 03 [3]. Hou, J, & Chung, D.D.L. Effect of Admixtures in Concrete on the Corrosion Resistance of Steel [4]. Reinforced concrete. Corrosion Sci. 2000. 42(9): 1489 – 1507. [5]. Muralidharan, S. Studies on Corrosion Performance of Steel embedded in silica Fume blended concrete. PG Project report, NIT, Trichy, India. 2003. [6]. Hachani, L.D., & Triki, E. Comparing the Steel-Concrete interface state and its Electrochemical Impedance” Journal of Cement and Concrete Research. 1996 26(2): 253-266. [7]. Idrissi, H, & Liman, A. Study and characterization by acoustic emission and electrochemical measurements of concrete deterioration caused by reinforcement steel corrosion. NDT&E International. 2003 36: 563-569. [8]. Law, D.W, Cairns, J.J., Millard, S. G., & Bungey, J.H. Evaluation of corrosion loss of steel reinforcing bars in concrete using LPR measurements. International Symposium (NDT-CE 2003) Nondestructive Testing in Civil Engineering. 2003.
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Experimental Study on Corrosion Resistance of TMT Bar in Concrete R. Manoharan*, National Institute of Technology, Trichy, INDIA. P. Jayabalan, National Institute of Technology, Trichy, INDIA. K. Palanisamy, National Institute of Technology, Trichy, INDIA.
ABSTRACT Emphasis was made to study the durability of reinforced concrete by adding commercially available chemical admixtures like Water Reducer and Water Proofer which are normally used with the purpose of improving the workability property of concrete thus producing dense and impermeable concrete. By using Electrochemical Technique, gravimetric technique and chloride diffusion method, corrosion resistance of TMT (Thermo Mechanically Treated) bar embedded in M25 concrete under chloride corrosive environment with various percentages of admixtures and durations were studied. The present investigation reveals that the addition of commercially available chemical admixtures does not add to corrosion and as the contrary the rate of corrosion of TMT bars reduces. Keywords: Corrosion, admixture, TMT bar, water proofing, water reducing A.C Impedance, Gravimetric and Linear Polarisation Resistance.
*Correspondence Author: R. Manoharan, Research Scholar, Department of Civil Engineering, National Institute of Technology, Trichy, TamilNadu, India. E-mail: [email protected]
ICCBT 2008 - A - (22) - pp239-250
Experimental Study on Corrosion Resistance of TMT Bar in Concrete
1.
INTRODUCTION
Concrete is a composite material made of aggregates and porous cement paste, which is the reaction product of mixing water and cement. The structure and composition of the cement paste determines the durability and the long-term performance of concrete. Concrete is normally reinforced with steel bars. The ability to withstand various types of degradation of concrete and protection to rebars embedded in it depends on the structure of the concrete. The processes of deterioration of concrete and corrosion of reinforcement are closely connected to each other. The former provoke destruction of concrete cover or cause micro cracking that compromises its protective characteristics. On the other hand, corrosion attack, because of the expansive action of corrosion products, produces cracking or delamination of the concrete and reduces its adhesion to reinforcement. Therefore, the durability of concrete structures depends on the resistance of the concrete against chemical and physical factors and its ability to protect the embedded reinforcement against corrosion. In view of the fact that a large number of existing structures are being deteriorated with time by reinforcement corrosion due to environmental exposure, corrosion is one of the main causes for the limited service life of reinforced concrete.The corrosion in reinforcements can be prevented by providing dense and impermeable concrete, which could be achieved by reducing the water-cement ratio. Chemical admixtures (water reducers and water proofers) in concrete are generally used with the purpose of increasing the workability by increasing the effectiveness of mixing water. They can also be used with the purpose of increasing the durability of concrete by decreasing the w/c ratio. For instance, they can guarantee the same workability with a remarkable reduction of water. Hou and Chung [3], reported the effect of admixtures in concrete on the corrosion resistance of steel reinforced Concrete was assessed by measuring the corrosion potential and corrosion current density during immersion in Ca(OH)2 and NaCl solutions. They proved that the admixtures improve the corrosion resistance, due to decrease of water absorptivity, and not so much due to increase in electrical resistivity of concrete. [4] examined the breaking down of passive film on embedded steel and concluded the level of chloride content in concrete also influences the electrical resistivity of the concrete. Further, [5] investigated a model for electrical impedance of steel-concrete interface. This model was validated through the experimental results conducted on reinforced concrete specimens immersed in solutions of chloride and sulphate. [6] presented the results of an experimental investigation on the use of acoustic emission during the carrion of steel rebars embedded in mortar and immersed in sodium chloride solution. The process of corrosion is accelerated by various imposed potentials and is followed by acoustic emission coupled to electrochemical techniques. The experimental results show that electrochemical techniques can evaluate the corrosive character of the medium used. [7] studied the actual and predicted weight loss data for a number of mild steel bars contained in OPC concrete, subjected to three different environmental regimes and monitored using potentiostaticaly controlled linear polarisation resistance measurements. The three sets of reinforced concrete specimens were subjected to chloride induced corrosion. Each set of specimens were exposed to a daily regime of wetting and drying in a controlled environment for duration of 1700 days. Two sets of testing were conducted at around 1200 days and 1700 240
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days. Prior to casting the mild steel bars were individually cleaned and weighted. The weight loss for each bar due to corrosion was recorded at the end of the period of exposure. Instantaneous linear polarisation resistance corrosion measurements were taken on each bar at regular intervals for the duration of the exposure time. These resistance measurements were then integrated to evaluate a predicted total weight loss. The results show that the weight loss evaluated from experimental linear polarisation resistance measurements give a significant over-estimate of the actual weight losses recorded. So far the investigations are done by using mineral admixtures only. The limitations of using mineral admixture are 1. There will not be effective mixing between admixture and concrete. 2. The passive film formed due to addition of mineral admixture over the reinforcements against corrosion will be less than that of chemical admixture. So the corrosion rate will be less in using chemical admixture.
2.
EXPERIMENTAL INVESTIGATION
Concrete cubes of size 150mm were used for the present investigation. M25 grade control concretes with two different admixtures of four different percentages (0.5%, 0.75%, 1.00% and 1.25%) were cast. TMT bars of 5cm length were embedded inside the concrete at a cover of 25mm. After casting, the cubes were cured for 28 days. For performing electrochemical tests, the rebars connected with wires were embedded inside, so that one end of wire was connected to rebar and another end was kept free out of the cube so as to connect to the electrochemical analyzer. After 28 days of usual curing, the cubes were ponded with 3% sodium chloride (NaCl) solution on one face of the cube as shown in Figure 1, in order to maintain unidirectional chloride penetration. To accelerate the corrosion, alternate wetting and drying was done periodically. The studies conducted on the specimens are Electrochemical Technique (AC Impedance), Gravimetric Technique and Chloride Diffusion test. After 2 months of alternate wetting and drying, tests were conducted every month to know the ability of concrete to resist the chloride ion penetration, which is the root cause for reinforcement corrosion. The investigations were done over a period of six months.
Figure 1. Test Specimens subjected to unidirectional ponding
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Experimental Study on Corrosion Resistance of TMT Bar in Concrete
3.
TEST DESCRIPTION
3.1 Electrochemical Technique (AC Impedance) A.C Impedance technique, a useful non-destructing technique for quantifying the corrosion of steel rebars embedded in concrete was carried out. In this technique the base potential is kept constant at initial E. Frequency is scanned from high frequency to low frequency, The Embedded rebar acts as working electrode; Stainless steel plate acts as counter electrode and the Saturated Calomel Electrode (SCE) acts as Reference electrode. These three electrodes are then connected to the electrochemical analyzer. The A.C. current is applied through the specimen. The Nyquist plot is directly recorded for the frequencies from 0.01 Hz to 100 KHz by using interface software called CHI604C. From the plot the polarization resistance values are obtained and then the corrosion rate is calculated using the formula: B (microamps/cm2) I corr = Rp Where, Rp = Polarisation Resistance in Ohms and B = 26 mV and 52 mV for active and passive conditions respectively. Corrosion rate in terms of mm/yr was obtained by multiplying the Icorr with the factor K=11.7 (mm/yr)/(mA/cm2). The Nyquist plot obtained for TMT bar is shown in Figure 2.
Figure 2. Nyquist plot for TMT bar
3.2 Gravimetric Technique (Weight Loss Determination) This technique is a destructive method, which consists in weighing the rebar specimens before and after being introduced in the concrete to be tested. Procedures for preparing, cleaning, and evaluating corrosion rate are followed as per ASTM G1-03 [2]. The difference in weight (gravimetric loss) is a quantitative average of the attack of corrosion. For different 242
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percentages of Water reducer and waterproofer admixtures, the corrosion rate in terms of weight loss, were measured every month. Then the corrosion rate is calculated using the formula, (K * W ) (mm/yr) CorrosionRate = (A *T * D) 4 where K is a constant, K =8.76*10 in case of expressing Corrosion rate in mm/yr T is the exposure time expressed in hours, A is the surface area in cm2, W is the mass loss in gram, and D is the density of the corroding metal (7.85g/cm3) 3.3 Chloride Diffusion Test Measuring the chloride content of the concrete at different depths enable to estimate the potential of chloride induced corrosion damage. There are several analytical values considered to designate the chloride content such as acid-soluble content and water-soluble content etc.. Acid soluble is the most widely used measurement and indicates the proportion of chloride, which is soluble in nitric acid. The water-soluble chloride is that extractable in water in defined conditions. The standard test method for determination of chloride is as per Volhard’s method of determination of chlorides by volumetric analysis and followed in this investigation. IS14959 (Part 2): 2001 [1] give a standard test procedure for determination of chloride by either acid soluble or water-soluble chlorides in hardened concrete. The chloride content can be expressed in terms of percent chloride by the mass of cement (% in weight of cement).
4.
TEST RESULTS AND DISCUSSIONS
Conplast SP430, the chemical , water reducing admixture and conplast X421 IC, the chemical, water proofing admixture of FOSROC Company, Bangalore, India, a brown colored sulphonated naphthalene polymers are used in this investigation. 4.1. Effect of Water Reducing Admixture 4.1.1. AC Impedance Technique Figure 3 shows that the corrosion rate decreases with duration for all the percentages of admixture. The corrosion rate of TMT bar is lower for concrete with all percentage of admixtures of at fifth month of testing compared to concrete without admixture. So the trend shows the addition of chemical admixtures restrict the corrosion rate when comparing to normal concrete.
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Corrosion Rate (mm/y)
VARIATION OF CORROSION RATE W ITH DURATION W R-TM T ROD 0.2 0.15 0.1 0.05 0 3 months
4 months
5 months
Duration (months) CC
WR1
WR2
WR3
WR4
Figure 3. Variation of corrosion rate of TMT bar for various percentages of admixtures at different durations (WR-AC Impedance Technique)
4.1.2. Gravimetric Technique Figure 4 shows that when comparing to normal concrete, Up to fourth month the corrosion rate decreases with duration for concrete with admixtures of 0.5%, 0.75% and 1.00%. But after fourth month the corrosion rate increases with duration for concrete with all admixtures. Higher the percentages of admixture (except 1.25 %) and duration, corrosion rate was lower. Among the percentage of admixtures, the higher resistance to corrosion is offered by admixture of 0.5%.
Corrosion Rate (mm/y)
VARIATION OF CORROSION RATE W ITH DURATION W R-TM T ROD 0.005 0.004 0.003 0.002 0.001 0 3 months
4 months
5 months
Duration (months) CC
WR1
WR2
WR3
WR4
Figure 4. Variation of corrosion rate of TMT bar for various percentages of admixtures at different durations (WR-Gravimetric Technique)
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4.1.3. Diffusion of Chloride The diffusion of chloride in concrete decreases as depth increases from the surface of ponding. In case of 0-5cm Depth (Figure 5), it is observed that the chloride diffusion decreases with duration in concrete with admixtures of 0.5%, 0.75% and 1.00%.The diffusion is lower in concrete with admixtures of all percentages compared to concrete without admixtures when the life of concrete increases. In case of 5-10cm Depth (Figure 6), it is observed that the diffusion of chloride in concrete with admixtures of 0.5% and 1.00% the diffusion decreases with duration. The diffusion of chloride is lower in the concrete with admixtures of 0.5%, 0.75% and 1.00% at all the durations of testing compared to concrete without admixtures. In case of 10-15cm Depth (Figure 7), it is observed that the diffusion of chloride in concrete with admixtures of all percentages, chloride diffusion decreases with duration. The chloride diffusion is lower in concrete with admixtures compared to concrete without admixtures when duration goes up.
Streng th o f chlo ride (% )
The diffusion of chloride is much resisted in the concrete with admixtures of 0.5%, 0.75% and 1.00% compared to concrete without admixtures. In the third month penetration of chlorine is more in the 0-5cm level but it is less in 5-10cm level and lesser in the 10-15cm level. In the fourth month, the concentration of chlorine is more at the 5-10 cm level which is higher than 0-5 and 10-15cm levels. This condition is maintained for fifth month also.
0.25 0.2 0.15 0.1 0.05 0 3 months
4 months
5 months
Duration Control Concrete
0.5%WR
0.75%WR
1.00%WR
1.25%WR
Figure 5. Variation of Strength of chloride with the percentage of admixtures for various durations (WR – M25 – 0-5 cm Depth)
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Str e ng th o f c hlo r ide (% )
0.12 0.1 0.08 0.06 0.04 0.02 0 3 months
4 months
5 months
Duration Control Concrete
0.5%WR
0.75%WR
1.00%WR
1.25%WR
Streng th o f chlo ride (% )
Figure 6. Variation of Strength of chloride with the percentage of admixtures for various durations (WR – M25 – 5-10 cm Depth)
0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 3 months
4 months
5 months
Duration Control Concrete
0.5%WR
0.75%WR
1.00%WR
1.25%WR
Figure 7. Variation of Strength of chloride with the percentage of admixtures for various durations (WR – M25 – 10-15 cm Depth)
4.2. Effect of Water Proofing Admixture 4.2.1. AC Impedance Technique Conplast X 421 IC, the chemical, water proofing admixture of FOSROC Company,Bangalore, India, a brown colored ligosulphonated naphthalene polymer is used in this investigation Figure 8 shows that the corrosion rate decreases with increase of duration and percentage of admixture. Thus the higher percentages of admixed concrete up to 1% shows more corrosion 246
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resistive than the lower percentages. But 1.25 % admixture has lower corrosion resistive than 0.75% admixture.
Corrosion Rate (mm/y)
VARIATION OF CORROSION RATE W ITH DURATION W P-TM T ROD 0.02 0.015 0.01 0.005 0 3 months
4 months
5 months
Duration (months) CC
WP1
WP2
WP3
WP4
Figure 8. Variation of corrosion rate of TMT bar for various percentages of admixtures at different durations (WP-AC Impedance Technique)
4.2.2. Gravimetric Technique Figure 9 shows that the corrosion rate decreases with duration in concrete with and without admixtures. Concrete with higher percentages of admixture are more corrosive resistance compared to lower percentages. After fifth month all the admixed concrete gives less corrosion rate than the normal concrete. So when comparing to water reducing admixture, water proofing admixture is more corrosion resistive.
Corrosion Rate (mm/y)
VARIATION OF CORROSION RATE W ITH DURATION W P-TM T ROD 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0 3 months
4 months
5 months
Duration (months) CC
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WP1
WP2
WP3
WP4
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Figure 9. Variation of corrosion rate of TMT bar for various percentages of admixtures at different durations (WP-Gravimetric Technique) 4.2.3. Diffusion of Chloride
Strength of chloride (%)
The diffusion of chloride in concrete is decreasing as depth increases from the surface of ponding. In case of 0-5cm Depth (Figure 10), admixed concrete gives less diffusion than the control concrete. In case of 5-10cm Depth (Figure 11), Chloride diffusion is found to decreases with duration in concrete with admixtures of 0.5%, 1.00%and 1.25%. The diffusion of chloride is lower in concrete with admixtures of all percentages at all the durations compared to concrete without admixtures. Concrete with 1.25% of admixture is better in not allowing the chloride into it. In case of 10-15cm Depth (Figure 12), it is observed that the chloride diffusion decreases with duration in concrete with admixtures of 0.5%, 0.75% and 1.00%. The diffusion of chloride is lower in concrete with admixtures of 0.5%, 1.00% and1.25% compared to concrete without admixtures with increase in life of concrete. Higher percentages of admixture are more resistant to diffusion of chloride than the lower percentages.
0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 3 months
4 months
5 months
Duration Control Concrete
0.5%WP
0.75%WP
1.00%WP
1.25%WP
Figure 10. Variation of Strength of chloride with the percentage of admixtures for various durations (WP – M25 – 0-5 cm Depth)
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S t r e n g t h o f c h lo r id e ( % )
0.12 0.1 0.08 0.06 0.04 0.02 0 3 months
4 months
5 months
Duration Control Concrete
0.5%WP
0.75%WP
1.00%WP
1.25%WP
Figure 11. Variation of Strength of chloride with the percentage of admixtures for various durations
S tr e n g th o f c h lo r id e (% )
(WP – M25 – 5-10 cm Depth)
0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 3 months
4 months
5 months
Duration Control Concrete
0.5%WP
0.75%WP
1.00%WP
1.25%WP
Figure 12. Variation of Strength of chloride with the percentage of admixtures for various durations (WP – M25 – 10-15 cm Depth)
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5.
CONCLUSIONS
1. Compared to concrete without admixture, the corrosion rate decreases as the duration increases for the admixed concrete. 2. In the electrochemical techniques, the behavior of Water reducing admixture is more effective in resisting the corrosion especially at lower percentage i.e., at 0.5% of admixture for M25 grade of concrete. 3. The performance of Water Proofing admixture is efficient at higher percentages of admixture compared to lower percentages of admixture. 4. TMT rods are found to perform well in concrete with admixtures compared to concrete without admixtures. 5. In general when compared to Water proofing admixture, the resistance to corrosion offered by Water reducing admixture is better. 6. The chemical admixtures are efficient in resisting the corrosion for TMT bars, which are easily prone to corrosion compared to other bars available in market. Admixtures not only give workability but also durability to concrete structures. REFERENCES [1]. Determination of water soluble and acid soluble chlorides in mortar and concrete- Method of test for hardened mortar and concrete, IS 14959 (Part 2): 2001, Bureau of Indian Standards, New Delhi [2]. Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens, ASTM G1 – 03 [3]. Hou, J, & Chung, D.D.L. Effect of Admixtures in Concrete on the Corrosion Resistance of Steel [4]. Reinforced concrete. Corrosion Sci. 2000. 42(9): 1489 – 1507. [5]. Muralidharan, S. Studies on Corrosion Performance of Steel embedded in silica Fume blended concrete. PG Project report, NIT, Trichy, India. 2003. [6]. Hachani, L.D., & Triki, E. Comparing the Steel-Concrete interface state and its Electrochemical Impedance” Journal of Cement and Concrete Research. 1996 26(2): 253-266. [7]. Idrissi, H, & Liman, A. Study and characterization by acoustic emission and electrochemical measurements of concrete deterioration caused by reinforcement steel corrosion. NDT&E International. 2003 36: 563-569. [8]. Law, D.W, Cairns, J.J., Millard, S. G., & Bungey, J.H. Evaluation of corrosion loss of steel reinforcing bars in concrete using LPR measurements. International Symposium (NDT-CE 2003) Nondestructive Testing in Civil Engineering. 2003.
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