Experimental evaluation of corrosion rate of rebars in M25 concrete with water proofing admixture.
Corrosion, in general, is the destruction or deterioration of materials because of the reaction of materials with its environment surrounding it . Large numbers of existing concrete structures are being damaged with time due to reinforcement corrosion due to environmental exposure and changes. Reinforcement corrosion is a dangerous activity that takes place in the re-bars of the concrete structures and leads to delamination because of the expansive action of corrosion product .
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 cement paste determines the durability of concrete structure. Concrete is normally reinforced with steel rods. The reinforcing steel rods provide strength and ductility only through the bond strength and anchorage with the concrete [3, 4]. The effectiveness of bond and anchorage is reduced due to the deterioration of concrete or steel or both. The durability of concrete structure depends on the resistance of concrete against physical and chemical attack and its ability to protect the embedded reinforcement against corrosion.
Corrosion control of steel reinforced concrete can be done by various methods like steel surface treatment, use of admixture in concrete, surface coating on concrete and cathodic protection. Among the above said methods, mixing admixture with concrete is very effective and cheaper .
The improvement of structural properties can be achieved either by adding mineral admixtures such as fly ash, blast furnace slag, silica fume, methyl cellulose, carbon fibres or by solid particle dispersions such as latex . Preparing dense and impermeable concrete by reducing water-cement ratio can prevent the reinforcement corrosion [7,14,15,16,17]. Adding mineral admixtures like silica fume and blast furnace slag to concrete improve the compressive strength, capillary efficiency and corrosion current density, but the slump of the concrete is decreased [9,10,13]. Amino alcohol based organic inhibitors reduce the corrosion rate of reinforcing steel in concrete with 2 grms./lit. of NaCl and 4% of inhibitors . Multifunctional Organic Inhibitors consisting Amines and Fatty acids, significantly reduces Chloride ingress which is predominant in corroding the reinforcement regardless of concrete quality. The addition of multifunctional organic inhibitors is very effective in mitigating sulphate attack than silica fume addition in concrete . Now-a-days chemical admixtures like water proofing compounds are added to concrete in the mixing stage itself, so that concrete with less water content can be obtained with increased workability and durability. Though the literatures have reported a numerous studies [23,24] on the strength properties and corrosion resistance behavior of concrete with mineral admixtures like fly ash and blast furnace slag etc., the corrosion resistance of concrete with rebars by adding water proofing admixtures, is yet to be studied.
Therefore it is attempted to investigate the corrosion resistance of reinforced M25 concrete mix with various percentage of water proofing admixture in this wok. Since it is specified that the percentage of admixtures in concrete should not exceed 1.5% by volume  and M25 concrete mix, mild steel rod, CTD rod and TMT rod are most commonly used in concrete structures, in the present investigation, the corrosion rate of mild steel rod, CTD rod and TMT rod when used in M25 concrete and with 0%, 0.5%, 0.75%,1% and 1.25% of water proofing admixture, were measured by electro-chemical tests namely A.C. Impedance (ACI) test, Linear Polarisation Resistance (LPR) test, Open Circuit Potential (OCP) test, and by weight loss test (destructive test).
Materials and Concrete Mix Design
The materials used in this study and their properties are given in Table 1. The M25 concrete mix was designed with the water cement ratio of 0.43 as per ACI 211-1-91 standard. The proportion of ingredients by weight of cement is arrived as 1: 2.31: 2.57. After 28 days of curing, the actual strength of this M25 grade concrete mix was obtained as 34.27 N/[mm.sup.2] which is greater than the required design strength specified by ACI 211-1-91 standard. Cubes of size 150 mm were cast with 0%, 0.5%, 0.75%,1% and 1.25% of water proofing admixture.
Preparatory work for Steel Rods
As per ASTM G1-03 , the rods were pickled in a solution that comprises 500 ml concentrated hydrochloric acid, 500 ml distilled water and 3.5 gms of hexamethylene-tetramine (Hexamine). After pickling, the rods were weighed using an electronic balance of four decimal accuracy.
Three rebars whose corrosion rate is to be studied and three copper wires were taken. One end of the copper wire is tied with the rebar by letting it through a hole drilled at 1 cm from one end of rebar. The copper-mild steel connection was sealed with epoxy material for avoiding galvanic corrosion between mild steel rod and copper wire. All the rebars are suspended vertically in to the concrete mould in such a way that one end of copper wires is extending out of the mould and the rebars to have 25mm cover at the top and bottom. The extended end of copper wire is connected to an impedance meter.
The concrete added with admixture was poured in to the mould, compacted well and kept at room temperature for 24 hrs. Then the cubes were demoulded and cured in a water pond. After 28 days of curing, the cubes were subjected to alternate wetting and drying with 3% (by mass of water) sodium chloride (NaCl) solution so as to get accelerated corrosion. The specimens for weight loss test were cast to have rebars without any copper wires. Since the initiation of corrosion over rebar was slow, the Electrochemical tests (ACI, LPR, OCP tests) and the gravimetric test (weight loss) were started after six months from casting and then the tests were conducted for every three months periodically.
AC Impedance Test
AC Impedance technique is an electrochemical, non-destructive technique to quantify the corrosion of steel re-bars embedded in concrete . Impedance 'Z' is the ratio of A.C. Voltage ([DELTA]E) to A.C. current ([DELTA]I). In this technique an alternating voltage ([DELTA]E) of 20 mV is applied to the rebar and the resultant current ([DELTA]I) and phase angle ([phi]) are measured for various frequencies. The general electrical circuit system followed is shown in the Fig. 1. where, [R.sub.s]--Solution resistance, [R.sub.p]--Polarization resistance, [C.sub.dl]--Double layer capacitance and W--Warberg's Impedance. The response to A.C. input is a complex impedance that has both real (resistive) and imaginary (capacitive or inductive) component Z' and Z" respectively, as shown in Fig. 2.
By studying the variation of the impedance with frequency, an equivalent electrical circuit can be determined, which would give the same response as the corrosion system being studied. The real part in X-axis and the imaginary part in Y-axis in the 'Nyquist plot' as in Fig.3 was obtained, with the diameter equal to [R.sub.p]. The semi circle is an offset from the origin by a value Rs (solution resistance), which is the ohmic resistance of the concrete cover zone between the reference cell and the reinforcing bar. From the Nyquist plot, the impedance can be represented as a vector of length [absolute value of Z].
Nyquist plot is directly recorded for the frequencies from 0.01 Hz to 100 KHz by using the software provided with the electro chemical analyzer when A.C. current is applied to the specimen. The experimental set up is shown in Fig.4. From the plot, Rp, the polarization resistance value which is the difference between the values of ([R.sub.s] + [R.sub.p]) and [R.sub.p] were obtained, then the corrosion rate ([I.sub.corr]) is calculated  using the formula given below.
[I.sub.corr] = B/[R.sub.p] micro amps /[cm.sup.2]
Where, B is a constant from anodic and cathodic Tafel slopes (26 mV for actively corroding steel rod). The corrosion rate in terms of mm / year was obtained by multiplying the [I.sub.corr] value with the factor K = 11.7 (mm/yr)/(mA/[cm.sup.2]).
In this electrochemical 3-electrodes system, the embedded rebar acts as working electrode, stainless steel plate acts as counter electrode and the saturated calomel electrode acts as reference electrode. Concrete surrounding the rebar is the electrolyte and all the elements are kept wet for effective conduction of current between them. The above three electrodes are connected to the electro-chemical analyzer, CHI604C.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
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Linear Polarization Resistance (LPR) test
Linear polarization resistance technique is the rapid, non intrusive technique which plays major role in finding the rate of corrosion of rebars embedded in concrete . In LPR measurements, the rebar is perturbed by a small amount from its equilibrium potential.
The term linear polarization refers to the linear regions of the polarization curve, in which slight changes in applied current in an ionic solution cause corresponding changes in the potential of the rod . The Fig. 5 shows that for a simple corroding system, polarization curve holds good for a few millivolts and obeys the quasi--linear relationship. The slope of this relationship is called 'Polarization Resistance'.
[R.sub.p] = [DELTA]E/[DELTA]I
This slope is related to the instantaneous corrosion rate through the Stern Geary equation .
[I.sub.corr] = [[beta].sub.a][[beta].sub.c]/ (2.3 [R.sub.p] ([[beta].sub.a]--[[beta].sub.c])) = B/[R.sub.p]
Where, [[beta].sub.a] = Anodic Tafel slope, [[beta].sub.c] = Cathodic Tafel Slope, [R.sub.p] = Polarization Resistance,
B = Stern--Geary Constant.
The value B is a constant containing the anodic and cathodic Tafel slopes, i.e., the slopes of the polarization curves. Usually, B is 26 mV for actively corroding steel in concrete as per Stern et al. . Electrochemical analyzer, CHI604C, is the instrument used to measure the [R.sub.p] value, which is working based on '3LP' method. The term '3LP' represents the "Three Electrode Linear Polarization". The three electrodes are used to monitor the corresponding changes in potential of the steel--concrete interface. By knowing the area of steel rebar embedded in concrete, the current applied is converted into corrosion current density.
The experimental set up is the same like the AC Impedance test. The potential is scanned from initial 'E' to final 'E', at a scan rate of 0.1667 mv/s and the current potential curve is obtained. The slope of this curve represents the polarization resistance ([R.sub.p]) value. Fig. 6 shows the typical slope of a current potential curve. The corrosion rate in micro Amps/ cm2 is calculated by [I.sub.corr] = B/[R.sub.p] and the corrosion rate is calculated in mm/yr by multiplying [I.sub.corr] value with a factor 'K' which is equal to 11.7 (mm/yr) / (mA/cm2).
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Open Circuit Potential Technique
Open Circuit Potential (OCP) Technique is the most common electrochemical technique for diagnosing the corrosion risk of reinforced concrete structures. Open Circuit Potential is the potential of an electrode measured with respect to a reference electrode when no current is flowing through it. The tendency of any metal to react with an environment is indicated by the potential difference between them . In reinforced concrete structures, concrete acts as an electrolyte and the reinforcement will develop a potential depending upon the properties of concrete environment. The reference electrode was connected to the common terminal, whereas the working electrode are connected to the +ve terminal of the voltmeter . According to this method, if the potential of steel rebar becomes more negative than -270 mV when saturated calomel electrode is used as reference electrode, the probability of corrosion is 90% . The same experimental setup used for ACI test and LPR test is used for this test also.
Weight Loss Test (Gravimetric Technique)
Weight Loss Test is a destructive technique in which the re-bars are weighed before embedding into the concrete and after the corrosion attacks the rebars. Periodically the concrete cubes are broken open and the rods are taken out for weighing. Pickling in a solution consists of 500 ml hydrochloric acid +500 ml distilled water +3.5 grams of hexamethylene tetramine is done before weighing the rebars. The weight difference is a measure of corrosion rate . The rods are weighed with an electronic balance of 4 decimal accuracy. From the weight loss, the corrosion rate is calculated by the formula given below .
Corrosion rate = KW/ ATD (mm/yr)
Where, K is 8.76 x [10.sup.4] a constant. W is the weight loss in grms, T is the exposure time in hours, A is the surface area in [cm.sup.2] and D is the density of the rod (7.85 gm/[cm.sup.3]).
Results and Discussion
AC Impedance measurement test
Figure 7, 8 and 9 show the rate of corrosion of M.S, TMT and CTD respectively. From Fig. 7, it is understood that for M.S. rods embedded in concrete, the corrosion rate decreases for the first 8 months and then increases with time. The same trend is true for TMT and CTD rebars also as shown in Fig. 8 & 9. This is because of the reason that the passive film formed over the surface of rod is active for 8 months and after that the film started deteriorating paving way to corrosion process. It is also known that corrosion rate of M.S. rod decreases with the increasing percentage of admixtures added. Among the MS rods embedded in concrete with different % of admixtures, for 1.25 % of admixture corrosion rate is lesser after 11th month from curing. This nature is true for the case of other rebars also. In general, the corrosion rate is minimum in the case of TMT rebars. This is due to the reason that the manufacturing process of TMT bars leads to lesser torsional residual stresses in the rebars and then lesser corrosion rate. Whereas as in the case of CTD bars, due to severe plastic deformation during its manufacture, the corrosion rate is found to be higher.
[FIGURE 7 OMITTED]
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[FIGURE 9 OMITTED]
L.P.R measurements test
The corrosion rate of MS, TMT and CTD rebars determined using LPR technique, are shown in Fig. 10, 11 and 12 respectively. From these Figures, it is inferred that the corrosion rate of all rebars increases after 8th month of curing. This nature of trend is similar to the trend shown by ACI test. However, the rebars embedded in concrete mixed with 1% and 1.25% of water proofing admixture showed a decrease in corrosion rate after 11th month, whereas other cases of admixtures show the increasing corrosion rate. According to LPR test, TMT bars are showing better corrosion rate compared to other two rebars and behaviour of MS rebars and CTD rebars is similar.
[FIGURE 10 OMITTED]
[FIGURE 11 OMITTED]
[FIGURE 12 OMITTED]
OCP measurement test
As per ASTM C876,-270mV vs Saturated Calomel Electrode (SCE) has been taken as threshold potential for the active condition of rebar. Fig. 13,14 and 15 show the potential (in volts)--time (in months) behaviour of concrete for different admixture proportions of MS, TMT and CTD rebars respectively. The Table 2 shows the range of OCP values and corresponding probability for corrosion in rebars. From Figures 13-15 and Table 2, it is understood that the probability of corrosion in rebars increases, as the OCP value approaches the threshold value. From the OCP values for different rebars, it is also inferred that the effect of admixture addition is almost same as for all the rebars.
After 14th month from curing, the OCP value for MS rebars in control concrete only reaches the threshold value, therefore the probability of corrosion is about 90%. For the same MS rebars used in concrete added with different % of admixtures, the probability of corrosion is uncertain.
After 14th month from curing, the OCP value for TMT rebars in control concrete and concrete admixed with 0.75% admixture reaches the threshold value, therefore the probability of corrosion is about 90%. For the same TMT rebars used in concrete added with 0.5 %, 1.0% and 1.25% of admixtures, the probability of corrosion is uncertain.
After 14th month from curing, the OCP value for CTD rebars in control concrete only reaches the threshold value, therefore the probability of corrosion is about 90%. For the same CTD rebars used in admixed concrete, the probability of corrosion is uncertain.
[FIGURE 13 OMITTED]
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[FIGURE 15 OMITTED]
Weight Loss measurements
Figures 16,17 an 18 show the corrosion rate measured by weight loss measurement test for MS, TMT and CTD rebars. The effect of admixture on corrosion rate when added in 1.0% and 1.25% are almost same in TMT rebars. The effect of admixture on corrosion rate when added in 1.25% is better in CTD and MS rebars. In TMT rebars in concrete with both1.0% and 1.25% of admixture, the change in corrosion rate is found to be very little. Thereore, the plot is almost a horizontal line. For other cases, the corrosion rate increases with time. In CTD rebars in concrete with 1.25% of admixture, the change in corrosion rate is found to be very little. For other cases, the corrosion rate increases with time. Whereas in MS rebars, for other cases, the corrosion rate increases with time.
[FIGURE 16 OMITTED]
[FIGURE 17 OMITTED]
[FIGURE 18 OMITTED]
Effect of admixtures
From the results of above four tests and discussion, it is understood that the control concrete and concrete with admixture protects the rebars upto 8th month. After 8th month, the condition of rebar changes its state from cathodic to anodic, where the corrosion process starts. This is due to the reason that the passive film started deteriorating after the 8th month. It is also observed that the admixtures reduce the corrosion rate in all rebars. As the percentage of admixture increases the corrosion rate decreases. However, in TMT rebars, the concrete with 1.0% admixture and 1.25% admixture acts almost same in reducing the corrosion rate. Therefore, it is better to use concrete with 1.0% water proofing admixture. Though the increasing percentage of admixtures reduces corrosion rate, the concrete is not stable for the admixtures that exceed 1.25%. Therfore for the MS and CTD rebars, it is better to use the concrete with 1,25% admixture for minimum corrosion rate.
Based on the experimental study, the following conclusions are made:
* Corrosion rate in rebars is reduced due to the addition of water proofing admixture.
* Corrosion rate of TMT bars is always less when compared with MS and CTD bars.
* From the OCP test, it is concluded that even after 14 months, the corrosion rate in TMT rebars is well below compared to other rebars.
* When TMT rebars are used, the use of admixture in 1.0% will be effective in reducing the corrosion rate.
* When MS and CTD rebars are used, we must add 1.25% of water proofing admixture for better corrosion resistance.
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R. Manoharan (a) *, P. Jayabalan (b) and K. Palanisamy (b)
(a) * Department of Civil Engineering, J. J. College of Engg. & Technology, Trichy--620009, E-mail: email@example.com, firstname.lastname@example.org
(b) Department of Civil Engineering, National Institute of Technology, Trichy--620015.
Table 1: Properties of materials used. SI. No. Material Properties Remarks 1. Cement Specific Gravity--3.06 43 grade--O.P.C. 2. Fine Specific Gravity--2.67 Cauvery river sand, aggregate Fineness modulus--2.52 Tamilnadu, India 3. Coarse Specific Gravity--2.78 Thuvakkudi Quarry, aggregate Fineness modulus--7.28 Trichy, Tamilnadu, Bulk density--1523 India kg/m3 4. Water Potable--as per IS456- Cauvery river water, 2000 Tamilnadu, India 5 Water Brown colour liquid Obtained from a proofing Ligo-sulphonated- chemical supplying admixture napthalene polymer company, Bangalore, Conplast India. X421 IC 6. Reinforcement 10mm Dia, 76mm long Obtained from a MS,TMT & CTD ,Mild steel, Thermo steel producing Rods. mechanically treated company, Trichy, Cold twisted deformed Tamilnadu, India. Rebars--Fe415 Table 2: Threshold values of OCP. S.No OCP Values Corrosion (%) (From Milli volt vs SCE) 1 - 270 threshold 2 > - 270 90 3 - 270 to - 125 Uncertain 4 < - 125 nil
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|Author:||Manoharan, R.; Jayabalan, P.; Palanisamy, K.|
|Publication:||International Journal of Applied Engineering Research|
|Date:||Jun 1, 2009|
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