perfomance evaluation of rebar in accelerated corrosion

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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



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




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



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




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

perfomance evaluation of rebar in accelerated corrosion

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