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P R O T E C T I O N O F STEEL REBARS IN L I G H T W E I G H T
C O N C R E T E W I T H THE USE O F CORROSION INHIBITORS
G Batis
G Grigoriadis
National Technical University of Athens
C A Meletiou
TITAN Cement Co S.A.
Greece
A B S T R A C T . The use of corrosion inhibitors (CI) is a well-established method for protecting
steel rebars against corrosion at almost all types of constructions, made of reinforced
concrete. The scope of this study was to investigate the effect of CI on the durability of
lightweight concrete made with pumice. Two types of inhibitors were used, based on calcium
nitrite and aminoalcohols, respectively. Specimens made with the use of Cement Kiln Dust
(CKD) as an additive, were also tested and their performance was compared to the respective
one obtained with the use of inhibitors. It has been shown in previous investigations that,
conventional concrete made with cement containing 6% CKD exhibited a reduced corrosion
of steel rebars. All specimens were exposed to a severe, corrosive environment containing
chlorides. The results indicated that, corrosion and carbonation rates were significantly
reduced in all specimens treated with the two inhibitors. They also indicated that CKD
performed remarkably well under highly corrosive conditions and, in some cases, provided
better protection of rebars than the CI's. This last indication provides new potential for further
utilization of CKD in cement and concrete, which may lead to considerable economic and
environmental benefits.
Keywords: Concrete, Durability, Corrosion, Carbonation, Inhibitors, Protection, Calcium
nitrite, Aminoalcohols, Cement kiln Dust (CKD), Steel rebars
D r George Batis is Associate Professor of National Technical University of Athens,
Department of Chemical Engineering, Section of Materials Science and Engineering, Greece.
Grigorios Grigoriadis is a PhD Student at Section of Materials Science and Engineering,
Department of Chemical Engineering, NTUA, Greece.
D r C A Meletiou is a Research Fellow in Concrete Technology Laboratory, Division of
Research & Technology, Department of R&D, TITAN Cement Co., S.A., Greece.
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494 Batis, Grigoriadis, Meletiou
INTRODUCTION
Lightweight concrete (LC) is a widely known type of concrete with many applications in
modern construction technology, such as insulating materials, prefabricated structural
elements, wall panels, grouts, etc. The use of this type of concrete is often quite suitable,
because reduces significantly the overall construction cost while, at the same time, provides
satisfactory mechanical properties. The main disadvantage of LC is that performs poorly
against corrosion. The use of porous aggregates (e.g. volcanic pumice) for the production of
LC, results in a dramatic increase of the final porosity. Thus, the diffusion of both
atmospheric 0 and C 0 through the open pores is increased, along with the absorption of
other corrosive factors, mainly chlorides, due to capillary action. Consequently, reinforced
LC exhibits higher corrosion and carbonation rates than normal concrete [1].
2
2
The most widely used method to protect steel rebars against corrosion, is the application of
corrosion inhibitors (CI). Since their first use in 1978, their utilization in the construction of
reinforced concrete is being continuously increasing. Nowadays, their use is considered to be
one of the most preferable methods of corrosion protection, because it is easy to apply, it
costs less than all the other methods with no maintenance required and increases significantly
the service life of a typical construction - in some cases even up to 100%. In addition,
treatment with CI, normally, does not affect the mechanical properties of the concrete, while
loss of bonding between rebars and the concrete is avoided - a problem which frequently
occurs when rebars are treated with epoxy coatings, Zn or PVC [2].
In this investigation, the effectiveness of using CI in reinforced LC made with volcanic
pumice is evaluated. Two types of inhibitors were used, an aqueous solution of calcium
nitrite and an organic, nitrogenous solution based on aminoalcohols. Specimens were
prepared by using type II cement and volcanic pumice at a high W/C ratio, in order to
increase the already high porosity of the concrete and, thus, to accelerate the corrosion of the
rebars. The experimental tests were carried out under a severe, corrosive environment
containing chlorides at, approximately, sea-water concentration. For comparative reasons, the
performance of Cement Kiln Dust (CKD) as a protective additive, was also investigated. This
is a by-product of low-alkali cement manufacturing process, mainly containing calcium
carbonate. Previous investigations [3, 4] have shown that CKD, can improve the durability of
concrete by stabilizing pH at higher values (>11.9), due to its relatively high alkalinity, thus
preventing the dissolution of the protective passive film surrounding the steel.
Table 1 Chemical composition of cement, CKD and pumice, %
COMPONENT
Si0
A1 0
Fe 0
CaO
MgO
K 0
Na 0
S0
Others
2
2
2
2
2
3
3
3
CEMENT 1135
CKD
PUMICE
27.38
9.10
5.65
45.39
2.73
0.94
0.56
2.71
7.71
13.68
4.36
2.30
42.59
1.23
0.79
0.28
0.10
34.67
70.55
12.24
0.89
2.36
0.10
4.21
3.49
0.03
6.13
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Use of C o r r o s i o n I n h i b i t o r s
495
MATERIALS AND E X P E R I M E N T A L P R O C E D U R E
The materials used in this investigation were: cement (type 1135), volcanic pumice from Yali
island in SE Greece, as an aggregate and CKD. The chemical composition of these materials
is shown in Table 1.
Two types of corrosion inhibitors were used, one based on calcium nitrite (CN) and the other
based on aminoalcohols (AM). Inhibitor AM was applied, both as an admixture (AMI) and
as an impregnation (AM3) by surface spraying. The mix proportions for the preparation of all
series of test specimens, are shown in Table 2.
Each series was consisted of three prismatic specimens (80 mm 80 mm 100 mm) with
four cylindrical steel bars (100 mm 12 mm) embedded in each one, as shown in Fig. 1. On
each steel bar was properly attached a copper wire. The top surface of all specimens and the
part of steel bars which protrudes over the concrete were coated with an epoxy glue to protect
them from atmospheric corrosion.
x
x
x
The specimens were partially immersed in a 3% wt NaCl solution, up to a height of 25mm.
After immersion, the following measurements were carried out at specified time intervals:
Corrrosion half-cell potential, versus Saturated Calomel Electrode (SCE).
Gravimetric mass loss of the rebars, according to ISO/DIS 8407.3 method.
• Mean depth of carbonation, usingg phenolpthalein indicator (RILEM CPC-18 method).
• Porosity after 9 months of exposure, using Hg-intrusion porosimetry method (MIP).
#
#
Table 2 Mix proportions of all series of test specimens
SERIES CODES
MATERIALS
H35 (g)
Pumice (kg)
Water (ml)
CKD (g)
Inhibitor CN (ml)
Inhibitor AM (ml)
Inhibitor AM (ml)*
RF
BD
CN3
CN6
AMI
AM3
AM3/1*
500
1.5
500
500
1.5
500
30
500
1.5
400
500
1.5
300
500
1.5
485
500
1.5
500
500
1.5
500
100
—
—
—
—
200
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
15
15
15
(*) Used externally, as impregnation by surface spraying, before immersing in NaCl
solution. (**) The specimens were treated with inhibitor AM, after 1 month of exposure.
MEASUREMENTS AND RESULTS
C o r r o s i o n Potential M e a s u r e m e n t s
Figures 2-5 present the measurements of corrosion potential versus time of exposure, for all
series of specimens.
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496 Batis, Grigoriadis, Meletiou
mm
100 mm
25 mm
20 mm
"80mm"
Figure 1 Specimens' shape and dimensions
E X P O S U R E TIME, days
Figure 2 Corrosion potential of CN specimens
w
u
(/)
>
>
•RF
•AMI
a
100
200
300
400
500
E X P O S U R E TIME, days
Figure 3 Corrosion potential of A M I specimens
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Use of Corrosion Inhibitors 497
' 0
100
200
300
400
500
E X P O S U R E T I M E , days
Figure 4 Corrosion potential of AM3 specimens
E X P O S U R E T I M E , days
Figure 5 Corrosion potential of CKD specimens
From the potential measurements shown in Fig. 2, it is clear that the reference exhibits a rapid
decrease of potential to a level of -650 mV, which takes place within about 30 days of
exposure. With the addition of the CN inhibitor at 3% wt of cement in the mixture, the time
required to obtain this level of potential is extended to about 100 days, thus indicating the
protecting effect which is provided. In the case where this inhibitor is added at a 6% wt of
cement in the mixture, the reinforcement of steel remains in the passive state (> -350 mV) for
up to 400 days as the results show in Fig. 2. It is important to note that, this extraordinary
protection against corrosion obtained by the use of CN inhibitor at that % level, took place in
spite of the fact that a number of cracks were present in the test specimens. These cracks may
have been caused by the accelerating effect produced by the excessive level of NO2" in
concrete (the recommended maximum addition level of CN inhibitor is about 3.5% wt of
cement).
The application of inhibitor AM, either as an admixture, or as surface impregnation, drives
the corrosion potential to more electropositive values, as shown in Fig. 3, 4. This effect is
more clearly shown in the case where AM is applied as impregnation by surface spraying 30
days after initial exposure (specimens AM3/1). The initial corrosion potential before the
spraying is -650 mV, while under application the potential goes to -350 mV within the next
30 days, indicating the inhibiting effect.
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498 B a t i s , G r i g o r i a d i s , Meletiou
The addition of 6% CKD in the mixture, presented similar performance with the reference, as
far as corrosion potential is concerned. This performance can be seen in Figure 5.
M a s s Loss A n d C a r b o n a t i o n Results
From the test measurements obtained after the initial 9 months of exposure, it can be seen
that, relative to the reference, either the CI or CKD resulted in lower % mass loss of steel and
depth of carbonation, as shown in Figures 6-7. It is noted that, significantly lower % mass
loss and depth of carbonation were obtained with the addition of CN inhibitor at 6% wt. of
cement in the mixture. However, the lowest depth of carbonation was obtained in the
specimens made with the addition of 6% CKD, probably due to the relatively higher
alkalinity, as compared to the other additions.
The above performance was also observed throughout the total test duration of 15 months. It
is important to note that, the specimens made with the addition of CKD exhibited the best
overall performance throughout the total test duration.
2,50
tn
2,00
s
eg
• 9 months
1,50
•
12 months
•
15 months
0,00
RF
BD
CN3
CN6
AMI
AM3
AM3/1
S E R I E S OF S P E C I M E N S
Figure 6 Mass loss of the rebars, %
•9
months
m 12 m o n t h s
d 15 months
CN3
CN6
AMI
AM3
AM3/1
S E R I E S OF S P E C I M E N S
Figure 7 Carbonation depth of all specimens
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Use of Corrosion Inhibitors 499
Porosity Measurements
Porosity measurements show that all series of test specimens have high porosity values,
between 34.0 to 38.0.
DISCUSSION
The experiments indicated that all the additions used in this investigation provided protection
against corrosion in all specimens made of lightweight concrete with pumice. The use of
anodic inhibitors (such as calcium nitrite) and anodic-cathodic ones (such as aminoalcohols)
at a rate of 3% appears to provide about the same degree of protection. At this rate of
addition, the CI exhibited a level of corrosion, similar to that normally obtained in
conventional reinforced concrete made without the use of any inhibitors for corrosion
protection. The use of CN inhibitor at the increased rate of 6% is prohibited, due to the
observed crack formation.
Comments On CKD Anti-corrosion Action
On the other hand, the addition of 6% CKD, with the given composition, surprisingly
exhibited the best protection against corrosion, although this material is not classified as an
inhibitor. The protective action against corrosion of the CKD addition, is mainly attributed to
its fineness and relatively higher alkalinity, which cause a reduction of total porosity and
carbonation, respectively, as shown in previous investigations [3,4].
The effect of nitrite ions
The inhibiting effect of nitrite ions on steel corrosion induced by chlorides has been reported
by other investigations. Several hypotheses have been formulated, concerning steel
passivation by nitrite ions. One among them, suggests that nitrite ions are easily oxidized in
alkaline environments, consuming oxygen according to the following total reaction [5]:
2N0 +0
2
>2N0
2
3
Another hypothesis [6, 7] suggests that nitrite ions are sacrificially reduced to nitrogen with a
simultaneous oxidation of ferrous to ferric ion, according to the following total reaction:
6Fe(OH)
2
+ 2N0
2
+ 3H 0
2
> 6Fe(OH)
3
+ N + OFT
2
The formation of iron oxyhydroxide or other similar products, would decrease the formation
of ferrous or ferric chlorocomplexes and, consequently, would supress iron anodic
dissolution.
According to the most universally accepted hypothesis [8, 12], a stable passive layer could be
formed on the reinforcement, avoiding all the metastable intermediate forms, according to the
following reaction:
2+
2Fe
+ 20H
+2N0 ~
2
->2NOt+2y-FeOOH
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500
Batis, G r i g o r i a d i s , Meletiou
Thus, the nitrite ions compete with chlorides for the ferrous ions at the anode. Nitrite ions
must be present in sufficient quantities and come in immediate contact, by diffusion, with the
structure in order to provide protection.
T h e Use O f Amino-alcohols As C o r r o s i o n I n h i b i t o r s
Amines and alkanolamines and their salts with organic and inorganic acids, have been
described and patented for various applications, such as for the protection of steel in
cementitious matrices [5, 9]. The effectiveness of this class of compounds, has been shown
by means of various experiments [6, 7, 10]. The new technology of spraying CI can be used
in lightweight concrete. Earlier experiments [8, 11] have shown that aminoalcohols
chemisorbs from aqueous solutions onto oxidized steel surfaces. The thickness and the
composition of the absorbate phase, depend upon the composition and the concentration of
the aminoalcohols solutions. The absorbate phase is formed even if chlorides are present. The
corrosion inhibition effect is explained by the fact that aminoalcohols displaces, due to its
strong bonding, ionic species from the oxidized steel surface, in particular chlorides which
cause corrosion and forms a durable passivating film [11].
CONCLUSIONS
Based on all the measurements and results obtained in this investigation, the following main
conclusions can be drawn:
•
•
•
•
Corrosion inhibitors based on CN and AM can be effectively used for corrosion
protection of lightweight concrete made with pumice
Inhibitors applied as admixture exhibited better corrosion protection than as an
impregnation by surface spraying on lightweight concrete
The use of 6% CKD as an addition in lightweight concrete, provides a significant level of
protection against chloride-induced corrosion
The possibility of using CKD for protecting against corrosion constructions made of
lightweight concrete, might have significant economic and environmental consequences.
REFERENCES
1.
BATIS, G., AIDINI, A., LOUVARIS, G., NICOLAIDES, A., «Reinforcement Corrosion
in Pumice Lightweight Concrete)), Concrete 2000, Dundee U.K., 1993, ppl53-162.
2.
NUERBERGER, U., «Korrosionsschutz im Massivebau)), Expert Verlag, Boebligen,
1992, pp 93-102.
3.
BATIS, G., KATSIAMBOULAS, A., MELETIOU, C.A., CHANIOTAKIS, E.,
«Durability of Reinforced Concrete Made with Composite Cement Containing Kiln
Dust)), Concrete in the Service of the Mankind, Vol. 1, Edited R.K. Dhir and T.D. Dyer,
1996, pp 67-72.
4.
TASIOS, TH., ALIGIZAKI, K., «Durability of Reinforced Concrete)), Athens, 1993.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
Use of C o r r o s i o n I n h i b i t o r s
501
5.
JUSTNES, H., NYGAARD, E.C., «The Influence of Technical Calcium Nitrate
Additions on the Chloride Binding Capacity of Cement and the Rate of Chloride Induced
Corrosion of Steel Embedded in Mortars», Corrosion and Corrosion Protection of Steel
in Concrete, Edited R.N. Swamy, Vol II, 1994, pp 491-502.
6.
SAGOE-GRENTSIL, K.K., YILMAZ, V.T., GLASSER, F.P., «Properties of Inorganic
Corrosion Inhibition Admixtures in Steel-Containing OPC Mortars. Part 1: Chemical
Properties)), Advances in Cement Research, Vol. 4, No 15, 1991/1992, pp 91-96
7.
YILMAZ, V.T., SAGOE-CRENTSIL, K.K., GLASSER, F.P., «Properties of Inorganic
Corrosion Inhibition Admixtures in Steel Containing OPC Mortars. Part 2:
Electrochemical Properties)), ibit, pp 97-102.
8.
PROWELL, B.D., WEYERS, R.E., AL-QUADI, I.L., «Evaluation of Corrosion
Inhibitors for the Rehabilitation of RC Structures)), Concrete 2000, Edited R.K. Dhir,
M.R. Jones, Dundee, UK, 1993, ppl223-1238.
9.
MAEDER U., «A New Class of Corrosion Inhibitors)), Corrosion and Corrosio
Protection of Steel in Concrete, Vol II, Edited R.N. Swamy, 1994, pp851-864.
10. MAEDER, U., «A New Class of Corrosion Inhibitors for Reinforced Concrete)), Third
CAMNET/ACI Int. Conf, Edited Malhotra, St. Andrews, Canada, 1996, pp 215-232.
11. WELLE, A., LIAO, J.D., KAIZER, K., GRUNZE, M., MAEDER, U., BLANK, N.,
«Interactions of N,N'-dimethylaminoethanol with Steel Surfaces in Alkaline and
Chlorine Containing Solutions)), Applied Surface Science, 119, 1997, pp 185-198.
12. BERKE, N.S., HICKS, M.C., "Protection Mechanism of Calcium Nitrite", Corrosion
Conference on Understanding Corrosion Mechanism in Concrete: A Key to Improving
Infrastructure Durability - MIT-Cambridge University, Massachusetts, USA, 27-31 July
1997.
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