Experimental evaluation of corrosion rate of rebars in M25 concrete with water proofing admixture.
Manoharan, R. ; Jayabalan, P. ; Palanisamy, K. 等
Introduction
Corrosion, in general, is the destruction or deterioration of
materials because of the reaction of materials with its environment
surrounding it [1]. 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 [2].
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 [5].
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 [6]. 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 [11]. 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 [12]. 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 [21] 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).
Experimental Programme
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 [18], 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.
Methodology
AC Impedance Test
AC Impedance technique is an electrochemical, non-destructive
technique to quantify the corrosion of steel re-bars embedded in
concrete [8]. 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 [8] 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]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
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 [25]. 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
[1]. 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 [8].
[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. [1].
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
[20]. 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 [24]. 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% [22]. 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 [19]. 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 [8].
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]
[FIGURE 8 OMITTED]
[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]
[FIGURE 14 OMITTED]
[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.
Conclusions
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.
References
[1] Fontanna, Mars G., 2005, Corrosion Engineering, Tata Mc
Graw-Hill Edition, Third Edition
[2] Luer Bertolini et. al, 2004, Corrosion of steel in concrete
(prevention, Diagnosis, Repair), WILEY-VCH verlag Gmb HRCo. KGaA-2004.
[3] Baskar, S., Prabakar, J., Srinivasan, P., and Chellappan, A.,
2006, Effect of rebar corrosion on the behaviour of bond in reinforced
concrete", The Indian concrete journal, pp. 19-23
[4] Fu, X., and chung, D.D.L., 1997, "Effect of corrosion on
the bond between concrete and steel rebar", Cement and Concrete
Research, Vol. 27, pp.1811-1815.
[5] Jiangynan Hon, and Chung, D.D.L., 2000, "Effect of
admixtures in concrete on the corrosion resistance of steel reinforced
concrete", Corrosion Science, Vol.42, pp.1489-1507.
[6] Chung, D., 2000, "Corrosion control of steel-reinforced
concrete, Journal of Materials Engineering and performance", 9 (5),
pp. 585-588.
[7] Gambhir, ML., 1995, Concrete Technology, Tata Mc Graw-Hill
publishing company Ltd., New Delhi, 2nd edition.
[8] Ha-won Song, and Velu Saraswathy, "Corrosion Monitoring of
Reinforced Concrete Structures--A Review", International Journal of
Electrochemical Science, 2(1), pp. 1-28.
[9] Ibrahim Turkmen, and Mehmet Gavgah, 2003, "Influence of
mineral admixtures on the some properties and corrosion of steel
embedded in sodium sulfate solution of concrete", Material Letters,
Vol. 57, pp.3222-3233.
[10] Benjamin, S.E., Khalid, F.F., and Rizwan A.Khan, 2000,
"Performance of steel in ordinary Portland, fly ash and slag cement
mortars during the hydration period', Journal of Materials
Processing Technology, 103(3), pp. 383-388.
[11] Wombacher, F., marder, U., and Marazzani, B., 2004,
"Aminoalcohol based mixed corrosion inhibitors', Cement and
Concrete Composites, 26(3), pp.209-216.
[12] Nmai, Charles K., 2004, "Multi-functional organic
corrosion inhibitor", Cement and Concrete Composites, 26(3),
pp.199-207.
[13] Dotto, JMR., de Abreu, A.G., Dal Molin, D.C.C., and Muller,
I.L., 2004, "Influence of silica fume addition on concretes
physical properties and on corrosion behaviour of reinforcement
bars", Cement and Concrete Composites, 26(1), pp.31-39.
[14] Scott A civjan, La Fave, Jarmes F., Joanna Trybulski, Daniel
Lovett, Jose Lima, and Donald W.P feifer, 2005, "Effectiveness of
corrosion inhibiting admixture combinations in structural
concrete", Cement and Concrete Composites, 27(6), pp. 688-703.
[15] Ha-won song, and Velu saraswathy, 2006, "Studies on the
corrosion resistance of reinforced steel in concrete with ground
granulated blast-furnace slag--An overview", Journal of Hazardous
Materials, 138(2), pp.226-233.
[16] Saraswathy, V., and Ha-won Song, 2007, "Improving the
durability of concrete by using inhibitors", Building and
Environment, 42(1), pp. 464-472.
[17] Tae-Hyun Ha, Srinivasan Mualidharan, Jeong-Hyo Bae, yoon-cheol
Ha, Hyun-Goo Lee, kyung-wha park, and Dae-kylong kin, 2007,
"Accelerated short-term techniques to evaluate the corrosion
performance of steel in fly ash blended concrete", Building and
Environment, 42(1), pp. 78-85.
[18] ASTM G1-03, A1 Chemical cleaning procedures, 5
[19] ASTM G109 - 07 Standard Test Method for Determining Effects of
Chemical Admixtures on Corrosion of Embedded Steel Reinforcement in
Concrete Exposed to Chloride Environments.
[20] ASTM, C 876 Standard Test Method for Half cell potentials of
Uneoated Reniforceing steel in concrete, 1-6
[21] ASTM C 494 / C 494 m -05, Standard specification for chemical
Admixtures for concrete, 1-10
[22] Vedalakshmi, R., Rajagopal, K., and Panlaniswamy, N., 2008,
"Longterm corrosion performance of rebar embedded in blended cement
concrete under macro cell corrosion condition", Construction and
Building Materials, 22(3), pp. 186-199.
[23] De.Shutter, G., and Lue, L., 2004, "Effect of corrosion
inhibiting admixtures on concrete properties", Construction and
Building Materials, 18(7), pp. 483489.
[24] Linhua jiang, and Zhenquins Liu,Yiqun Ye, 2004,
"Durability of concrete incorporating large volumes of low-quality
fly ash", Cement and Concrete Research, 34, pp. 1467-1469.
[25] J.P.Broomfield, ASTM STP 1276 (1996) 91-106
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: cadsrb@gmail.com,
ranee_mano@yahoo.co.in
(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
