International Journal of Engineering Trends and Technology (IJETT) – Volume3 Issue 6 Number2–Dec 2012
EXPERIMENTAL INVESTIGATION ON CORROSION OF COTTED REBARS IN
REINFORCED CONCRETE
K. Sathish kumar, Assistant Professor
Department of civil engineering , Bharath University, Chennai
ABSTRACT
Reinforcement corrosion in concrete is regarded as the predominant factor in the premature degradation of
reinforced concrete structures. The failure does not occur due to structural collapse, but also due to loss of
serviceability characterized by cracking, spalling, debonding and excessive deflection.
The reinforcement corrosion in concrete structures is attributed to combination of factors which includes
concrete composition, service environments and loading conditions. It is evident that substantial understanding of
corrosion in concrete structures is of foremost importance. Therefore this can only be gained through experiment on
concrete structural members under both corrosion process and service loads.
For this thesis work, two types of loads are applied
1. 5% above crack load
2. 5% below crack load
The beams subjected to the above load conditions are allowed for accelerated corrosion process.
Monitoring of corrosion of beam specimens is carried out regularly. There are various types of monitoring of
corrosion; of which half-cell potential is one effective method. It is based on the electrode potential of steel rebar
with reference to a standard electrode undergoing changes depending on a corrosion activity.
The parameters like cover thickness, bar diameter and crack width should be taken in to consideration for
investigating their effect on reinforcement corrosion. The effect of rice husk ash and silica fume on the
reinforcement corrosion is studied.
INTRODUCTION
Corrosion is a physiochemical interaction between a metal and its environment which results in changes in the
properties of the metal and which may often lead to impairment of the function of the metal, the environment or the
technical system of which these form a part.
Corrosion is a natural process and is a result of the inherent tendency of metals to revert to their more stable
compounds, usually oxides. Most metals are found in nature in the form of various chemical compounds called ores.
In the refining process, energy is added to the ore, to produce the metal. It is this same energy that provides the
driving force causing the metal to revert back to the more stable compound. Reinforcement corrosion is the main
cause of damage and early failure of reinforced concrete structures worldwide with subsequent enormous cost for
maintenance, restoration and replacement.
The process of corrosion sets in due to ingress of moisture, oxygen and other deleterious substances into the body
of concrete which is unsound, permeable and absorbent. Cracks due to structural and other causes such as creep,
shrinkage, etc., also allows ingress of moisture and other harmful ingredients and thus accelerate the rate of
corrosion. There are several interactive factors both external and internal, which leads to corrosion of reinforcement
and ultimately failure of structures.
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Corrosion mechanism
Corrosion of steel in concrete is initiated and maintained generally by two mechanisms. Presence of de-passivating
ions in large amounts to destroy passivating films locally. Reduction in alkalinity of concrete is due to the effect of
atmosphere carbon-di-oxide. Once the ability of concrete to maintain steel reinforcement in a passive condition has
been lost, then rusting takes place.
(1) Fe + ½ O2 + H2O

Fe (OH)2
The above equation shows the combined action of moisture and dissolved oxygen ion to produce ferrous
hydroxide. This equation is an overall representation of the corrosion process.
(2) Fe 
Fe2+ + 2e-
(3) ½ O2 + H2O + 2 e-

2 OH-
Fig.Mechanism of Corrosion
The above equation represents the reactions at anode and cathode. The equation (2) represents the oxidation of
ion from uncharged pieces to a positively charged ion together with the liberation of electrons. The equation (3)
describes the reduction of the non metallic solution piece. This is a process in which electrons are consumed as
shown in Fig. Mechanism of Corrosion
In electrochemical terminology, oxidation reactions are termed as anodic processes. They processed at sites on
the metal termed as anodes. Reduction reactions are termed as cathode processes and take place at cathodes. Each
electron, which is released into metal as a result of an anodic reaction, is consumed in cathodic reaction. This
maintains electrical neutrality, a fact which is consistent with the observation that corroding metals do not change
up.
Iron oxidation and dissolution proceed at the anodic side, liberating electrons, which flow through the metal to
be involved in the reduction of oxygen at the cathodic site. Ionic current passing through the electrotype completes
the electrical circuit.
When aqueous corrosion occurs, under the influence of oxygen reduction, the metal spontaneously adopts an
electrode potential known as the corrosion potential. Although electrode potential cannot be measured absolutely, it
is relatively straight forward matter to measure the potential difference between the electrode of interest and a
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standard reference electrode, the potential of which has been arbitrarily fixed. For example, a typical value for the
corrosion potential of the steel in sea water would be in the region of -0.65 volt measured with respect to a
Silver/Silver Chloride/ sea water reference electrode. The same corrosion potential would be read as -0.7 V if a
copper/ copper sulphate reference electrode was used. The corrosion potential of well-passivated steel rebar in
concreter is around +0.01 V vs. CuSO4.
Monitoring methods of corrosion
The monitoring methods of corrosion are also known as insitu tests for concrete. They have been developed
primarily to help access physical condition of concrete qualitatively with reference to corrosion affected, corrosion
prone, and not corrosion prone locations in a member or a structure.
In order to identify the presence of corrosive environment within the concrete, extent and the severity of
corrosion, chemical and electrochemical tests are required. The test methods and general guidelines for
interpretation are described in the following sections.
Chloride content
Chloride content can be determined by collecting the broken concrete samples from the core concrete. Primarily the
chloride content of concrete in the cover portion is of prime importance. The test consists of powdering the sample,
obtaining the water extracts and conducting standard titration experiment for determining the water soluble chloride
content, which is expressed by weight of concrete or by weight of cement if the mix ratio is known. This method
gives the average chloride content in the cover region; whereas the level of chloride near the steel-chloride interface
is of more importance. Further, a chloride profile across the cover thickness will be a more useful measurement and
this can help to make a rough estimation on chloride diffusion rate. One recent development for testing of chloride
content includes the use of chloride ions sensitive to electrode. This is commercially known as “Rapid chloride test
kit” and the test consists of obtaining powdered sample by drilling, collecting the sample from different depths
(every 5mm), mixing the sample (of about 1.56 gm weight) with a special chloride extraction liquid and measuring
the electrical potential of the liquid by chloride sensitive electrode.
With the help of calibration graph relating electrical potential and chloride content, the chloride content of
the sample can be directly determined. As the quantity of sample required is very little, a chloride profile for every 5
mm depth across the cover concrete up to steel-concrete interface can be established. As already mentioned, the
corrosive environment with in concrete gets established once on the pH value is lowered to 12 and less or the
chloride level reaches the threshold value of about 0.4% to 0.6% by weight of cement. The quantitative guidelines
for identification of corrosion prone locations based on pH values and chloride content is given Table
Guidelines for Identification of Corrosion Prone Location Based on Chemical Analysis
Sl.No
1
2
3
4
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Test results
High pH values greater than 12.0 and very low
chloride content
High pH values and high chloride content greater
than threshold values(0.4 to 0.6 % by weight of
cement)
Low pH values and high chloride content (greater
than 0.4 to 0.6 % by weight cement)
Low pH values and high chloride content
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Interpretations
No corrosion
Corrosion prone
Corrosion prone
Corrosion prone
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Half-cell Potential Survey
Corrosion being an electrochemical phenomenon, the electrode potential of steel rebar with reference to a standard
electrode undergoes changes depending on corrosion activity. A symmetric survey on well defined grid points gives
useful information on the presence or probability of corrosion activity. The grid points used for the other
measurements, namely, rebound hammer and UPV can be used for making the data more meaningful.
1.
2.
3.
Copper-Copper Sulphate Electrode(CSE)
Silver-Silver Chloride Electrode(SSE)
Saturated Calomel Electrode(SCE)
The measurement consists of giving an electrical connection to the rebars and observing the potential difference
between the rebar and reference electrode in contact with concrete surface.
Generally the potential values become more negative as the corrosion becomes more and more active. However,
less negative potential values may also indicate the presence of corrosion activity, if the pH values of concrete are
less. The general guidelines for identifying the probability of corrosion based on half-cell potential values as
suggested in ASTM C 876 are given in Table
Corrosion Risk by Half-Cell Potential
corrosion
> 95%
50%
<5%
Potential
More negative than -350 mV
-200 to -350 mV
More negative than -200 mV
DESCRIPTION OF MATERIALS AND MIX USED
Introduction
The materials used such as cement, fine aggregate, coarse aggregate, water, rebar, Rice Husk Ash, Silica fume and
description of the specimen etc, are discussed in detail in this section.
Materials
Cement
Cement used for all the nine beams and companion specimens was Ordinary Portland Cement (Ultratech 53 grade).
The cement was in standard gunny bags, later on placed in airtight container to avoid the lumps.
Aggregate
1. Fine Aggregate:
The fine aggregate used for all the specimens was the river sand which is available in Karur. The sand was
sieved through the I.S. Sieve No. 300.
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2. Coarse Aggregate:
Graded crushed hard blue granite jelly of 20 mm size available in and around Coimbatore was used.
Water
Potable water available in the campus was used for mixing and curing of concrete specimens.
Steel
TMT (Thermo Mechanically Treated) steel bars of size 10 mm, 8 mm have been used as main reinforcement and
hanger bars respectively. 6 mm diameter bars have been used for shear reinforcement of beams.
Rice Husk Ash
Rice husk ash is a by-product from combustion of rice- shells produced during the de-husking operation of paddy in
industrial furnaces and must be ground to fine particle sizes to develop pozzolanic property. Rice husk ash cement is
manufactured and supplied by M/S KC-CONTECH, Chennai, in the name of HYPER2000. Silica (SiO2) content is
85-97% .
Silica fume
Silica fume is the most used mineral admixture in high strength high performance concrete. As defined by ACI
116R, silica fume is of very fine particles of crystalline silica, produced in electrical furnaces, as a by product of the
production of elemental silicon or alloy containing silicon; also known as condensed silica fume or micro silica. It is
mainly amorphous silica with high SiO2 content.
Selection of the mix proportion
The mix adopted for all beams were 1: .82:2.59 by weight of cement: fine aggregate: coarse aggregate and water
cement ratio of 0.38. The actual material required for each specimen was weighed and mechanically mixed and table
vibrated for compaction.
DESCRIPTION OF TEST SPECIMENS
Eighteen numbers of concrete beams of 1000 mm length and 100 mm x 150 mm in section were cast.
Companion specimen
Totally eighteen beams were cast and tested. Along with each beam three cubes of 150 x 150 x 150 mm size, were
cast with the same concrete mix. The companion specimens were tested and cube compression strength was
determined. The companion specimens were compacted and cured similar to the beams. The specimens were tested
according to the IS Code 516- 1964.
Cube crushing test
The cube-crushing test was done in compression testing machine. The load was applied as per IS Code 516-1964.
The rate of loading was about 14 N/mm2 per minute and the ultimate loads were noted. The results are given in
Tables.
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TEST RESULTS
Compressive Strength of Cube Specimens (OPC+10%SF) - 7th day
No. of
Compressive
SL. No
Days
Load (kN)
Strength (N/mm2)
1
7
620
27.55
2
7
650
28.88
3
7
640
28.44
Compressive Strength of Cube Specimens (OPC+10%SF) - 28th Day
No. of
Compressive
SL .No
Days
Load (kN)
Strength (N/mm2)
1
28
1000
44.44
2
28
1020
45.33
3
28
990
44
Compressive Strength of Cube Specimens (OPC + 10% RHA) - 7th Day
No. of
SL. No
Load (kN)
Compressive
Days
Strength (N/mm2)
1
7
640
28.44
2
7
620
26.66
3
7
660
29.33
Compressive Strength of Cube Specimens (OPC + 10% RHA) - 28th Day
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No. of
SL. No
Load (kN)
Compressive
Days
Strength (N/mm2)
1
28
1020
45.33
2
28
960
42.66
3
28
990
44
Compressive Strength of Cube Specimens (OPC) - 7th Day
No. of
SL. No
Load (kN)
Compressive
Days
Strength (N/mm2)
1
7
780
34.66
2
7
760
33.77
3
7
790
35.11
Compressive Strength of Cube Specimens (OPC) -28th day
SL. No
No. of
Load (kN)
Days
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Compressive
Strength (N/mm2)
1
28
1080
48
2
28
1070
47.55
3
28
1100
48.88
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COMPRESSIVE STRENGTH
IN MPA
COMPARISON CHART FOR
OPC,OPC+RH,OPC+SF(28th DAY)
50
OPC
OPC+RH
OPC+SF
45
40
35
1
2
3
48
47.55
48.88
OPC+RH 45.33
42.66
44
44.44
45.33
44
OPC
OPC+SF
NO OF SPECIMENS
METHODOLOGY
Preparation of the beam specimens and testing methodology
Introduction
In this chapter, the preparation of the mould for casting of the beam specimens, vibration methods, demoulding,
curing, acceleration of corrosion etc are explained in detail.
Mould
The beam mould is made of teak wood. The inside dimensions of the mould are 100 mm wide 150 mm deep and
1000 mm long. The mould was cleaned and mould oil is applied to avoid adhesion of concrete for easy removal.
The steel mould is used for all cubes and cylinders. These moulds were cleaned and gaps were filled with plaster
of paris putty and oiled before casting.
Reinforcement details
The details of the reinforcement for all the beams are shown in Fig. Details of reinforcement
Casting
The actual quantity of materials were weighed and kept ready before mixing. The moulds were kept ready on table
vibrator with reinforcement placed in position with cover blocks. The concrete was mixed by concrete mixer and
filled in the mould by three layers and compacted well by table vibrator each time for duration of 10 seconds. After
24 hours the beam and companion specimens were removed from the moulds and placed outside and cured by
putting the specimen in fiber curing tank for 28 days.
Testing methodology
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Mix proportion for M40 grade of concrete
Water
Cement
F.A
C.A
0.38
1
.82
2.59
185.4
487
401
1262
The mix proportion is 0.38: 1: .82: 2.59
Fig Details of Reinforcement
Details of the Test Specimen
For the investigation of chloride induced reinforcement corrosion in concrete structures under service loads, 18
beam specimens were cast using M 40 grade of concrete. All these specimens were kept in salt free water for a
period of 28 days for curing.
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There are many factors, which influence the corrosion of reinforcement. Some of these factors are observable and
some are not. The specimens of the test programme were designed in such a way that the effect of observable factor
is measured. The scope of the present work deals with the finding of the effect of following variables on corrosion of
rebars in concrete.
Variables Considered
1.Ordinary Portland Cement .
Cement
2. Ordinary Portland Cement replaced partially by Rice Husk Ash (RHA) (10% by
weight of cement)
3. Ordinary Portland Cement replaced partially by Silica Fume (SF) (10% by weight
of cement)
Coating
Uncoated and coated
Conditions
Stressed and Unstressed
Loading
5% above crack load and 5% below crack load
Mix
M 40
Size of beam
1000 mm x 100 mm x 150 mm
Beam specifications and loading details
I set (OPC )
- 2 Beams (Unstressed)
- 2 Beams (Stressed) – 5% ACL
- 2 Beams (Stressed) – 5% BCL
II set (OPC + RHA)
- 2 Beams (Unstressed)
- 2 Beams (Stressed) – 5% ACL
- 2 Beams (Stressed) – 5% BCL
III set (OPC + SF)
- 2 Beams (Unstressed)
- 2 Beams (Stressed) – 5% ACL
- 2 Beams (Stressed) - 5% BCL
Beams are cast with fresh concrete (as per design mix – M40) using admixture.
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Use of rice husk ash cement
It has been noted that the Rice Husk ash Cement along with Ordinary Portland Cement (OPC) improves the
impermeability of concrete/ mortar and improve their resistance to corrosion of rebar. The details of Rice Husk Ash
Cement are given in Table
Details of Rice Husk Ash Cement
Sl. No
Application
Dosage
1
Plastering work
5% - 15% by weight of cement
2
Water proof coatings
30% by weight of cement
3
In concrete or mortar when used in severe condition
such as costal structures, structures in chemical
industries, effluent treatment plants, etc.
30% - 60% by weight of
cement.
Specific gravity
:
1.89
Packing
:
Available in 15 kgs
Shelf life
:
8 Months
Use of silica fume
It has been noted that the Silica fume along with Ordinary Portland Cement (OPC) improves the impermeability of
concrete/ mortar and there by improve resistance to corrosion of rebars. The details of Silica fume are given in
Table
Details of Silica Fume
Sl. No
Application
Dosage
1
Plastering work
5% - 10% by weight of cement
2
Water proof coatings
20% by weight of cement
3
In concrete or mortar when used in severe condition
such as costal structures, structures in chemical
industries, effluent treatment plants, etc.
20% - 40% by weight of
cement.
Specific gravity
:
2.2
Packing
:
Available in 25 kgs
Shelf life
:
12 Months
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TEST PROCEDURE
To investigate the initiation of chloride induced reinforcement corrosion of concrete structures in aqueous chloride
solution and the simultaneous service loads, the specimens were clamped back to back for the purpose of simulating
the loaded condition of the beam. The test setup for the beams under stressed and unstressed condition is shown in
Fig below
The loading points are placed at 300 mm centre to centre, by using angle and bolt connection and the beams were
kept in correct position one over the other. Angles of size 75 mm x 75 mm x 8 mm, bolts of diameter 25 mm and
length 450 mm and nuts of diameter 26.5 mm were used. The angles, nuts and bolts were coated with Zinc-Copper
Alloy to avoid the Galvanostatic Corrosion (usually this occurs when two dissimilar materials are present).
Beams under stressed condition
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Beams under unstressed condition
TESTING ARRANGEMENT AND TESTING
Introduction
This chapter deals with the instrumentation and testing of beams.
Preparation of beams for testing
Before testing, all the beams were marked by lines with 50 mm spacings from the edge of two sides and the central
bottom portion was marked to determine the deflection. The bottom surfaces at the supports and at the middle of the
bottom were cleaned and levelled to accommodate the support and deflectometer.
Details of load bed
After preparation, the beams were lifted to the testing bed, which was a steel loading frame of capacity 250 kN. The
support points are provided with a hinged support at one end and roller support at other end by steel rod welded to
base plate.
Load distribution arrangements
The load is applied by means of 250 kN capacity hydraulic jack powered by hand operated hydraulic pump. The 100
kN capacity-proving ring (No 86027) was kept on the beam. The complete test setup is shown in Fig.7.1
Testing of beams
All the instruments used were completely checked before testing and loading. Initial reading of deflectometer and
proving rings were observed correctly.
Load was applied gradually by the hand operated hydraulic jack. The following readings were observed and
recorded at different stages of loading up to the initial crack and ultimate load.
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Test Setup
1. Deflectometer readings
2. Proving ring readings
During testing, formation and growth of cracks were recorded on the beam by drawing line along the crack and
marking the corresponding loads in divisions at each point.While taking readings, extreme care was taken not to
touch any of the testing and measuring equipments.
Load Test on R.C. Beams
After the completion of accelerated corrosion process, the RC beams were subjected to two-point load to initiate
pure bending in middle one third spans. The beam was supported on a hinge at left hand side and roller at right hand
side. To measure the deflection, the dial gauge was fixed at mid span of the beam. Gradual static loads were applied
and corresponding deflections were observed. Load at initiation of cracks, crack propagation and crack width were
noted. The beams were tested up to failure as shown in Fig .6.3.
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Failure Patter of Beam Specimen
DISCUSSION OF TEST RESULTS
Results of load test
After the accelerated corrosion process for a period of 15 days, the beams were subjected to load test upto ultimate
load and corresponding deflections were observed. The beams with combination OPC are taken more load than the
beams with Silica fume and beam with Rice Husk Ash on all conditions. The Load-Deflection curves are shown in
fig.nos 7.1,7.2 and 7.3.
60
Load in KN
50
40
OPC
30
OPC+RH
20
OPC+SF
10
0
0
1
2
3
4
5
6
7
8
9
10
Deflection in m m
Load Vs Deflection Curve (Stressed Condition - 5% Above Crack Load)
70
Load in KN
60
50
OPC
40
OPC+RH
30
OPC+SF
20
10
0
0
1
2
3
4
5
6
7
8
9 10
De fle ction in m m
Load Vs Deflection Curve (Stressed Condition - 5% Below Crack Load)
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60
Load in KN
50
40
OPC
30
OPC+RH
OPC+SF
20
10
0
0
1
2
3
4
5
6
7
8
9
10
Deflection in mm
Load Vs Deflection Curve (UnStressed Condition)
Determination of weight loss of steel
After undergoing the corrosion process, the beams were subjected to load test. Subsequently they were broken and
the reinforcement cage was taken out separately. From the cage the main rods were separated.
Initial preparation consists of removal of M-seal , binding wires etc. Then all the rods were placed in the
chemical solution known as Reebaklens (FOSROC Chemicals India Ltd.) to remove loose rust particles.
The rods were placed in the solution for 5 minutes and were taken out, cleaned and wiped. The weights of the
rods were determined to estimate the weight loss in rebar. The comparisons of weight loss of rebars are shown in
Table
Ultimate Load and Percentage of Weight Loss
Sl. No
Beam Specification
Ultimate
load (kN)
Weight of Rebars
(gms)
Initial
Final
Weight loss
(%)
1
OPC(ACL-UC1)
60.63
2644
2601
.963
2
OPC(ACL-C30)
33.72
2606
2510
2.98
3
OPC(BCL-UC10)
50
2622
2610
.457
4
OPC(BCL-C29)
40.68
2698
2632
8.18
5
OPC+RH(ACL-UC7)
57.46
2653
2340
3.82
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6
OPC+RH(ACL-C20)
46.36
2654
2629
.95
7
OPC+RH(BCL-UC13)
50
2668
2620
.687
8
OPC+RH(BCL-C26)
31.39
2610
2570
7.18
9
OPC+SF(ACL-UC3)
57.95
2677
2475
9.69
10
OPC+SF(ACL-C16)
31.89
2644
2620
.907
11
OPC+SF(BCL-UC15)
55.71
2606
2510
3.68
OPC+SF(BCL-C17)
35.34
2618
2545
2.78
13
OPC(UC-6)
61.70
2648
2632
.685
14
OPC(C28)
43.02
2655
2340
11.86
15
OPC+RH(UC9)
60.63
2654
2629
.946
16
OPC+RH(C25)
47.39
2668
2620
1.832
17
OPC+SF(UC11)
59.56
2610
2570
1.532
18
OPC+SF(C18)
40.78
2677
2475
7.54
12
CONCLUSION
In this experimental study, the beam specimens with different percentages of opc,rise husk ash and silica fume were
subjected to accelerated corrosion process. The efficiency was decided based on the weight loss of rebars.
A comprehensive test programme designed to determine reinforcement corrosion in concrete under simulated
conditions of sea water and simultaneous service loads has been carried out in this work. From the experimental
studies, the following conclusions were drawn.
1.
The rate of deterioration of concrete, under stressed condition was more than that of concrete under
unstressed condition.
2. The load-induced specimens with initial cracks escalate chloride penetration and hence expedite the
corrosion initiation.
3 Stressed condition:
A. 5% above crack load
a.The specimen cast with silica fume shows a weight loss of 90.06% less than that of the specimen with
OPC.
b. The specimen cast with rice husk ash shows a weight loss of 74% less than that
of the specimen with
OPC.
c. The specimen cast with rice husk ash shows a weight loss of 60% less than that of the specimen with silica
fume.
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B. 5% below crack load:
a. The specimen cast with silica fume shows a weight loss of 87% less than that of the specimen with OPC.
b. The specimen cast with rice husk ash shows a weight loss of 33% less than that of the specimen with OPC.
c. The specimen cast with rice husk ash shows a weight loss of 81% less than that of the specimen with silica
fume.
4. UnStressed condition:
a. The specimen cast with silica fume shows a weight loss of 55.28% less than that of the
specimen with OPC.
b. The specimen cast with rice husk ash shows a weight loss of 27.58% less than that of the
specimen with OPC.
c. The specimen cast with rice husk ash shows a weight loss of 38.25% less than that of the specimen
with silica fume.
REFERENCES
1.Divakar, Y., Manjunath, S. and Aswath, M.U. (2012) “Experimental Investigation on Behaviour of Concrete with
the use of silics fume”, International Journal of Advanced Engineering Research and Studies, Vol. 1 Issue 4, pp. 8487.
2.Ilangovan, R., Mahendran, N. and Nagamani, K. (2008) "Strength and durability properties of concrete containing
rice hush ash", ARPN Journal of Engineering and Applied Science, Vol.3(5), pp.20-26.
3.Subramanian, S. (2007) "corrosion - Challenges and solutions", The Indian Concrete Journal, December, pp.39-50.
4.Michael D. Lepech, Victor C. Li, Richard E. Robertson, and Gregory A. Keoleian (2008) “Design of Engineered
Cementitious Composites for Improved corrosions”, ACI Materials Journal, vol. 105, no. 6, pp. 567-575
5.”concrete technology” ,M.S CHETTY Hi tech publications 2007
ISSN: 2231-5381
http://www.ijettjournal.org
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