perfomance evaluation of rebar in accelerated corrosion
DESCRIPTION
Evaluation of the performance of reinforcement rebar under corrosive environment.TRANSCRIPT
-
www.cafetinnova.org
Indexed in
Scopus Compendex and Geobase Elsevier, Chemical
Abstract Services-USA, Geo-Ref Information Services-USA
ISSN 0974-5904, Volume 05, No. 01
February 2012, P.P. 154-159
#02050121 Copyright 2012 CAFET-INNOVA TECHNICAL SOCIETY. All rights reserved.
Performance Evaluation of Rebar in Accelerated Corrosion by
Gravimetric Loss Method
AKSHATHA SHETTY1, KATTA VENKATARAMANA
1 and INDRANI GOGOI
2
1Department of civil Engineering, NITK, Surathkal.Srinivasnagar-575025, India
2Assam Engineering Institute Chandmari, Guwahati-781003
Email: [email protected], [email protected], [email protected]
Abstract: Corrosion of concrete occurs due to chloride ingress into concrete and is a major cause of steel corrosion.
Rebar corrosion in concrete is one of the major problems in the durability criteria. This paper explains, experimental
investigations carried out on smaller specimens for evaluating the performance of two types of cement with three
types of rebar by gravimetric method. From the results of gravimetric loss obtained by different types of steel with
different types of concrete, it is concluded that blended cement, i.e. PPC performs better compared to OPC against
accelerated rebar corrosion in concrete. Amongst steel types, CTD steel resulted in higher gravimetric loss compared
to MS followed by TMT steel.
Keywords: Gravimetric Loss, Reinforcement Corrosion, Accelerated corrosion.
Introduction:
Reinforcement corrosion is the most important cause of
deterioration of reinforced concrete (RC) structures.
Rebar corrosion in concrete is a major problem faced by
civil engineers. The breakdown of the passive film and
consequently corrosion initiation takes place most
frequently in the presence of chloride ions at the rebar
level (Pradhan and Bhattacharjee, 2009). The rust
produced as a result of corrosion has volume 2 to 6
times than that of steel; it causes volume expansion
developing tensile stresses in concrete (Bhaskar
et.al.,2010). The undesired effect of corrosion include
cracking and spalling of the concrete cover, reduction,
and eventually loss of bond between concrete and
corroding reinforcement, and reduction of cross-
sectional area of reinforcing steel. Consequently the
load carrying capacity of the structure is reduced and
brittle failure of structure may occur without prior
warning. Hence these effects of corrosion need to be
studied for improving the performance of structures.
The magnitude of reinforcement corrosion has a
significant effect on flexural strength, deformational
behavior, ductility, bond strength and mode of failure of
reinforced Concrete structures.
Corrosion of steel embedded in concrete is an
electrochemical process. With the attention of
researchers focusing towards the prediction of the
residual life of RC structures affected by the
reinforcement corrosion, the use of electrochemical
techniques for the determination of relevant parameters
in this regard becomes a major area of durability study.
Therefore nowadays the electrochemical techniques are
widely used for the study of rebar corrosion in
laboratories together with their application to real life
structures (Andrade and Alonso 1996, Pradhan and
Bhattacharjee, 2009).
Previous Experimental Investigations Regarding
Gravimetric Loss:
Pradhan and Bhattachajee (2009):
Researchers carried out their investigation on large
number of specimens for evaluating the performance of
different types of rebar in chloride contaminated
concrete made with different types of cement through
different corrosion rates technique. Steel bars of 12mm
diameters were used. Three types of steel reinforcement
used were cold twisted deformed (CTD) bars, Tempcore
TMT bars and Thermax TMT bars. Types of cement
used were OPC, PPC and PSC.
The slab Specimens of size (300mmx300mmx52mm)
were prepared with a centrally embedded steel
specimen. Water cement ratio of 0.45, 0.5, and 0.55
were used; cement content was kept constant at
210kg/m3. Chloride was admixed in to concrete as NaCl
of analytical reagent grade. Concentrations of sodium
chloride used were 0%, 1.5%, 3% and 4.5% by mass of
cement and the corresponding chloride concentration
were 0%, 0.91%, 1.82% and 2.73% respectively. Linear
polarization Resistance test and AC Impedance
spectroscopy test were performed. It was concluded that
blended cement, i.e. PPC and PSC performed better as
compared to OPC against chloride induced rebar
corrosion in concrete whereas, amongst steel type
-
155 AKSHATHA SHETTY, KATTA VENKATARAMANA and INDRANI GOGOI
International Journal of Earth Sciences and Engineering
ISSN 0974-5904, Vol. 05, No. 01, February 2012, pp. 154-159
Tempcore steel resulted in lower corrosion rate as
compared to Thermax steel followed by CTD steel.
Amleh (2000):
Cylinder specimens of height 305mm, embedment
length of 279.6mm and 20mm diameter rebar was used
for the study. Specimens were prepared for a normal
concrete mix with water cement ratio of 0.32. During
corrosion process specimens were immersed in an
electrolyte solution, which contained 5% NaCl by
weight of water. Impressed current technique method
was adopted. It was observed that the available bond
strength decreases almost linearly with increase in the
mass loss.
Fang et al. (2006):
Totally, 24 specimens were tested, for both confined
and un-confined conditions. Corrosion percent was
varied from 0 to 6%. Deformed steel bars of 20mm
diameter and 420mm in length were used. For those
specimens with stirrups supplying lateral confinement,
round steel of 6mm diameter with a c/c spacing of
40mm was used. Concrete compressive strength of 56.2
MPa was achieved. Direct electric current technique
was impressed on the main steel bars. Specimens were
immersed in 5% NaCl solution. Actual corrosion level
was determined. It is observed that the increase in
corrosion level will cause significant reduction in bond
capacity under cyclic loading.
Apostolopoulos and Michalopous (2006):
In this study, BSt 500s tempcore steel of 12mm diameter
was used. These ribbed rebars were artificially corroded
in a specially designed salt spray corrosion chamber,
according to the ASTM B 117-94 standard, for 10, 20,
and 30, 45, 60 and 90 days. To accelerate the corrosion,
specimens were sprayed with 5% sodium chloride and
95% distilled water solution with pH range of 6.5 to 7.2
and spray chamber temperature was maintained around
35 (1.1 to 1.7) C for different corrosion levels. It was
concluded that a considerable reduction in fatigue limit
took place due to a reduction of the exterior hard layer
of martensite (%) and a drastic drop in energy density
on the corroded specimens.
Almusallam (2001):
In this study reinforced steel bars were embedded in the
concrete specimens prepared with ASTM C 150 Type V
cement. A coarse to fine aggregate ratio of 1.68 and a
water cement ratio of 0.45 were kept invariant in all the
concrete mixtures. Two groups of concrete specimens
were prepared. First groups of specimens were prepared
with 6mm diameter and the other groups were prepared
with 12 mm diameter steel bars. Corrosion of
reinforcing steel was accelerated by impressed current
technique. The specimens were partially immersed in a
5% NaCl solution. After desired level of reinforcement
corrosion was obtained, concrete specimens were split
along the line of steel bars. The gravimetric loss in
weight is determined. The tensile test were conducted
on both clean and corroded reinforcing steel bars so that
the influence of degree of reinforcement corrosion on
the tensile properties of reinforcing steel bars could be
assessed. Results indicated that level of reinforcement
corrosion does not influence the tensile strength of the
steel bars, calculated on the actual area of cross section.
However when nominal diameter is utilized in the
calculation, tensile strength is less than the ASTM A
615 requirement of 600 MPa. The degree of corrosion
obtained was 11 and 24% for 6 and 12 mm diameter
steel bars respectively. Furthermore reinforcing steel
bars with more than 12% corrosion indicates a brittle
failure. Based on review of past literature, following
topics are considered for this study.
1. Determine mass loss rate for different types of
rebar's embedded in concrete.
2. Corrosion Performance Appraisal for concrete with
OPC and PPC.
3. Develop an empirical relation to determine the
applied current required for a specific corrosion
percentage.
Preparation of Test Specimens:
Smaller sizes of (7.09cm7.09cm7.09cm) cube
specimens are used for the present study. M20 grade of
concrete is used. Mix proportion of 1:1.871:3.291 is
used for both OPC. Cement used is conforming to BS-
12-1978 & ASTM C-150 Type 1 and PPC with water
cement ratio of 0.496. The length of 8.5cm rod was
centrally embedded in concrete. Initial weight of the
steel bar is noted for different types of steel rebars such
as Cold Twisted Deformed bars, Thermo Mechanically
Treated (TMT) and Mild Steel (MS) bars; chemical
composition is according to IS1786. Diameter of 16mm
is used for CTD bars, and 20mm used for MS and TMT
rebars. The Slump test is conducted to ascertain the
workability of the mix; slump obtained is between 55 to
60 mm. After 24 hours concrete cubes are demoulded
and the specimens were kept for 28 days curing in
water. For each level of corrosion 3 samples are tested
and average value is noted. The test set up used for the
experiment is shown in Fig.1. After the curing,
specimens are kept for accelerated corrosion (Fig.2) by
direct impressed technique method. Exposed part of the
top rod is covered with M-seal (Fig.3), which prevents
the corrosion at that location. Specimens are partially
immersed in 3.5% concentrated NaCl solution in
corrosion tank for a duration of 5 days, during which
known amount of current is applied (Iapp). The amount
of current applied for TMT and Mild Steel rebars is
0.125A, 0.251A, 0.377A, 0.5A, and for CTD bars
-
156 Performance Evaluation of Rebar in Accelerated Corrosion by Gravimetric Loss Method
International Journal of Earth Sciences and Engineering
ISSN 0974-5904, Vol. 05, No. 01, February 2012, pp. 154-159
0.08A, 0.16A, 0.24A, 0.32A for the different levels of
excepted corrosion rates of 2.5%, 5%, 7.5% and 10%
respectively. After completion of corrosion process, the
specimens are broken (Fig. 4). Rebars are cleaned
properly to remove rust and the final weight is noted.
From results of initial and final weight, gravimetric loss
is evaluated. For the occurrence of corrosion, oxygen
and moisture are the essential factors. In the case of
controlled specimens, the condition immediately after
curing will be considered as the condition of no-
corrosion or 0% corrosion.
Calculation of Degree of Induced Corrosion and
Mass Loss:
The impressed current technique, also called as
galvanostatic method, consists of applying a constant
current from a DC source to the steel embedded in
concrete to induce significant corrosion in a short period
of time. After applying the current for a given duration,
the degree of induced corrosion can be determined
theoretically using Faradays law, or the percentage of
actual amount of steel lost in corrosion can be
calculated with the help of a gravimetric test conducted
on the extracted bars after subjecting them to
accelerated corrosion. The mass of rust produced per
unit surface area of the bar due to applied current over a
given time can be determined theoretically using the
following expression based on the Faradays law
(Ijsseling 1986)
F
TIW app .. (1)
Where Mth=theoretical mass of rust per unit surface area
of the bar (g/cm2); W=equivalent weight of steel which
is taken as the ratio of atomic weight of iron to the
valency of iron (27.925g); Iapp=applied current density
(Amp/cm2); T=duration of induced corrosion (sec); and
F=Faradays constant (96487Amp-sec).
The actual mass loss of rust per unit surface area may be
determined as
( )LD
wwM
fi
ac
=
(2)
Where Mac =actual mass of rust per unit area of the bar
(g/cm2); Wi =initial weight of the bar before corrosion
(g) Wf = weight after corrosion (g) for a given duration
of induced corrosion (T); D = diameter of the rebar
(cm); and L=Length of the rebar sample (cm).
The degree of induced corrosion also expressed in terms
of the percentage weight loss ( is calculated as
(Ahamad, 2009).
(3)
Compressive Strength Test:
The objective of the present study being the
determination of mass loss rate for different types of
rebar's embedded in concrete, the compressive strength
of the companion cubes are given in Table 1.
Gravimetric Test Results:
To achieve different percentage corrosion, the number
of days is kept constant and the current applied, Iapp, is
varied. Iapp is obtained by equating Eqs. 1 and 2.The
specimen identification name with the time in days,
current applied (Iapp), rate of corrosion and average
of Gravimetric loss (%) for both OPC and PPC concrete
are presented in Table 2. Here all the rebar samples are
of size 8.5cm and CTD, MS and TMT rebar of diameter
16, 20, 20mm respectively are embedded in the cubes.
Table 2 represents the Deviation (%) wrt corrosion (%)
required for rebars embedded in OPC and PPC concrete.
Variation of Iapp (A) against Gravimetric Loss (%) for
MS, TMT, CTD bars in OPC and PPC Concrete are
shown in Fig.5. These figures show the effect of
corrosion on gravimetric loss of (MS, TMT and CTD)
rebars.
A linear regression analysis is carried out to give an
expression to predict Iapp for required gravimetric loss. The expressions obtained for OPC and PPC are as
follows:
y=19.349x-0.227, R2=0.996 (MS-OPC) (4)
y=19.349x-0.227, R2=0.996 (TMT-OPC) (5)
y=30.463x-0.108, R2=0.999 (CTD-OPC) (6)
y=17.608x-0.071, R2=0.996 (MS-PPC) (7)
y=17.054x-0.104, R2=0.998 (TMT-PPC) (8)
y=27.625x+0.086, R2=0.996(CTD-PPC) (9)
Where y=gravimetric loss (%); x=applied current.
From Fig 5, it is observed that experimental results vary
linearly. As Iapp varies with the gravimetric loss
MS,TMT, CTD bars in OPC concrete resulted in higher
gravimetric loss as compared to the PPC concrete for
the respective steel bars. This is because in Blended
concrete there will be reduction in permeability due to
pore refinement.
Conclusions:
1. A linear regression analysis is carried out to obtain
expressions to predict Iapp for required gravimetric
loss. These expressions can be used for prediction of
values within the range of experimental data.
2. Gravimetric loss in CTD bar embedded in OPC is
higher than MS and TMT bars. It can be further
concluded that there is higher deviation in
gravimetric loss of TMT bars.
3. It is observed that as the Iapp varies with the
gravimetric loss MS, TMT and CTD bars in OPC
-
157 AKSHATHA SHETTY, KATTA VENKATARAMANA and INDRANI GOGOI
International Journal of Earth Sciences and Engineering
ISSN 0974-5904, Vol. 05, No. 01, February 2012, pp. 154-159
concrete resulted in higher gravimetric loss as
compared to the PPC (blended) concrete.
4. Experimental results vary linearly. The obtained
equations can be used for the prediction of values
within the range of graph values.
5. From Gravimetric loss results obtained, it is
concluded that the blended cement, i.e. PPC is better
as compared to OPC against accelerated rebar
corrosion in concrete whereas, amongst steel type
CTD (16mm diameter) steel more susceptible to
corrosion than MS (20mm diameter) followed by
TMT steel. This may be because TMT bar is more
ductile material than MS and CTD bars.
References:
[1] Ahamad, S.,(2009). Techniques for inducing
accelerated corrosion of steel in concrete. The
Arabian Journal of science and Engineering,(
34),Number 2C.
[2] Ahmad, S., (2003), Reinforcement Corrosion in
Concrete Structures, Its Monitoring and Service
Life PredictionA Review, Cement & Concrete
Composites, 25, 459471.
[3] Almusallam, A.A., (2001). Effect of degree of
corrosion on the properties of reinforcing steel bars.
Journal of Construction Building Materials, 21(15),
361-368.
[4] Amleh, L., (2000), "Bond Deterioration of
concrete, Departmental Report, McGill University,
Montreal, Canada.
[5] Andrade, C. and Alonso, C., (1996). Corrosion
rate monitoring in the laboratory and on site.
Journal of Construction building materials.10, 315-
328.
[6] Apostolopoulos, Ch, Alk and Michalopoulos, D.,
(2006). Effect of corrosion on mass loss, and high
and low cycle fatigue of reinforcing steel. Journal
of materials Engineering and Performance. 15(6),
742-749.
[7] Bhaskar, s., Bharatkumar, B.H., Ravindra, Gettu
and Neelamegam. M. (2010). Effect of corrosion
on the bond behavior of OPC and PPC concrete.
Journal of structural Engineering 37(1), 37-42.
[8] Pradhan, B. and Bhattacharjee, (2009).
Performance evaluation of rebar in chloride
contaminated concrete by corrosion rate. Journal of
Construction and materials, 23, 2346-2356
[9] Fang, C., Lundgren, K., Plos, M., Gylltott, K.,
(2006). Effect of corrosion on bond in reinforced
concrete. Journal of Cement and Concrete
Research. 36, 548-555.
[10] Ijsseling, F.P., (1986). Application of
Electrochemical Methods of corrosion Rate
Determination to System Involving Corrosion
Product Layers. Journal of British corrosion,
21(2), 95-101
Acknowledgements:
The Partial financial support from BRNS is gratefully
acknowledged.
Figure 1: Schematic Representation of the Test Set Up Used For the Experiments
-
158 Performance Evaluation of Rebar in Accelerated Corrosion by Gravimetric Loss Method
International Journal of Earth Sciences and Engineering
ISSN 0974-5904, Vol. 05, No. 01, February 2012, pp. 154-159
Figure 2: Accelerated Corrosion Process
Figure 3: Exposed Surface Covered with M-Seal
Figure 4: Destructive Testing
Table 1: Compressive Strength of OPC and PPC
Concrete
SI
NO
Curing
duration
Compressive strength
(N/mm2)
OPC
1 7 days 18.2
2 28days 32.2
PPC
1. 7 days 15.52
2 28days 27.33
Figure 5: Effect of Applied Current on Gravimetric Loss of MS, TMT and CTD Bars in both OPC and PPC
Concrete
-
159 AKSHATHA SHETTY, KATTA VENKATARAMANA and INDRANI GOGOI
International Journal of Earth Sciences and Engineering
ISSN 0974-5904, Vol. 05, No. 01, February 2012, pp. 154-159
Table 2: Deviation (%) with Respect to Corrosion (%) Required for Rebars Embedded in OPC and PPC
Type of
Steel and
Concrete
Expected
Corrosion Rate
(%) (C)
Gravimetric loss
(%) (G)
Deviation (%)
S=
MS-OPC
0.0 0.00 0.00
2.5 2.21 11.6
5.0 4.49 10.2
7.5 7.02 6.4
10 9.84 1.6
TMT-OPC
0.0 0.00 0.00
2.5 2.1 16
5.0 4.39 12.2
7.5 6.91 7.87
10 9.71 2.90
CTD-OPC
0.0 0.00 0.00
2.5 2.34 6.4
5.0 4.58 8.40
7.5 7.11 5.2
10 9.80 2.00
MS-PPC
0.0 0.00 0.00
2.5 2.00 20.00
5.0 4.23 15.40
7.5 6.92 7.73
10 8.56 14.40
TMT-PPC
0.0 0.00 0.00
2.5 1.98 20.8
5.0 3.92 21.6
7.5 6.57 12.40
10 8.38 16.20
CTD-PPC
0.0 0.00 0.00
2.5 2.3 8
5.0 4.49 10.2
7.5 7.08 5.6
10 8.66 13.4