CHAPTER 4 BEHAVIOUR OF REINFORCED CONCRETE CORRODED …shodhganga.inflibnet.ac.in/bitstream/10603/43138/9/09... · 2018-07-03 · proportions and hand mixing were adopted for making

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CHAPTER 4

BEHAVIOUR OF REINFORCED CONCRETE

CORRODED SQUARE COLUMN SPECIMENS

4.1 GENERAL

In the pervious chapter, the effects of corrosion on circular corroded

and uncorroded columns were discussed in detail. In this chapter the

behaviour of conventional and corroded square columns with Fly Ash as

corrosion inhibitor and behaviour of column under loading conditions of

corrosion have been studied.

4.2 EXPERIMENAL PROGRAMME

The experimental programme was divided into two phases. In the

first phase, investigation was done on the effect of fly ash and bagasee ash

(added separately) as supplementary cementitious materials and the influence

of these products on the strength and resistance to chloride ion penetration of

concrete made with three different cement replacement levels (10%, 20% and

30%). By conducting various tests on the cylindrical and cubical specimens

with various proportions, the better corrosion inhibitor was chosen and its

optimum level obtained. In the second phase, using this corrosion inhibitor

and its optimum level the behaviour of square columns were studied.

115

4.2.1 Fly Ash

Fly Ash (FA) is the residue from combustion of pulverized coal

collected by the mechanical or electrostatic separators from fuel gases of

thermal power plants. Its composition varies with the type of fuel burnt, load

on boiler and type of separator etc. The carbon content in fly ash should be as

low as possible whereas the silica content should be as high as possible.

There are about 80 thermal power plants functioning in India generating

around 100 million tons of fly ash per annum. Storage and disposal of fly ash

is a problem for thermal power stations as it creates severe pollution

problems. In view of these serious considerations, lot of investigations are

being carried out for utilization of fly ash. The fly ash from Mettur power

plant was used in the present investigation.

4.2.2 Bagasse Ash

Bagasse is the residue fiber remaining when sugarcane is pressed to

extract the sugar. In India, there are about 391 sugar units generating 81.36

million tonnes of bagasse. Bagasse is also used as raw material for paper

making due to its fibrous content and about 0.3 tonnes of paper can be made

from one tonne of bagasse. When this waste is burnt under controlled

conditions, it also gives ash having amorphous silica, which has pozzolanic

properties. The bagasse from Shakthi sugars, Appakoodal, Erode was used in

this test. The Bagasse Ash (BA) was obtained by burning the bagasse at

controlled temperature of 800oC for more than one hour. These ashes were

prepared as a fine powder using a pulverizer and kept in polythene bags

before use. The physical and chemical properties of BA and FA are presented

in Tables 4.1 and 4.2.

116

Table 4.1 Properties of the Constituent Materials

Sl.No Parameter OPC used Fly Ash Bagasse

Ash Fine

Aggregate Coarse

Aggregate 1 Normal

Consistency 29 % 30% 30 % - -

2 Fineness by Sieving (%) 45 micron

80 78.60 91 -

-

3 Initial Setting Time (minutes)

38 84 67 - -

4 Final Setting Time(minutes)

300 395 350 - -

5 Specific Gravity

3.15 2.10 1.93 2.55 2.69

6 Bulk Density 1747 1590 7 Fineness

Modulus - - - 2.755 6.86

8 Water Absorption

- - - 1.0% 0.52%

Table 4.2 Chemical Composition of Cement and Ashes

Material SiO2 Al2O3 Fe2O2 CaO MgO SO3

OPC 21.4 5.3 3.2 61.6 0.8 2.2 Fly Ash 59.62 26.43 6.61 1.20 0.76 0.58 Bagasse

Ash 64.50 6.93 5.69 5.2 1.69 -

4.2.3 Mix Proportions and Casting

A mix M30 grade was designed as per IS 10262-1981 and the

same was used to prepare the test samples. The cement replacement level was

117

varied from 10%, 20% and 30% labelled six series as FA1, FA2, FA3 and

BA1,BA2,BA3. All the materials were taken by weight as per the mix

proportions and hand mixing were adopted for making concrete.

4.2.4 Details of Specimen

The experimental program involves casting and testing of 117

concrete specimens with and without FA and BA. The different specimens

considered in this study include 54 numbers of 150 150 150 mm cubes for

7, 28 and 56 days cube compressive strength, 21 numbers of 150 mm

diameter and 300mm height cylinders for 28 days split Tensile strength,

modulus of elasticity value and 21 numbers of (100 100 500 mm size)

prisms for 28 days flexural strength. All the test specimens were cast in steel

moulds on a table vibrator and allowed to set for 24 hours. Then the

specimens were removed from the moulds and kept in the curing pond. The

strength characteristics were examined as per IS 516-1959. Three specimens

were tested for the required age and the average values were taken. Chloride

permeability of the concrete mixes were evaluated as per ASTM C1202

(100 mm dia, 50 mm thick cylindrical sample) at the age of 28 days.

Figure 4.1 shows the one batch of specimens after 28 days curing.

Figure 4.1 Batch of Specimens after 28 Days Curing

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The bond strength of concrete was estimated from pull out test.

The pull out test was carried out on 150 mm concrete cubes with 12 mm

diameter reinforcing bar centrally embedded in the cube with a cover of

25 mm as shown in Figure 4.2. Totally 24 specimens were cast and cured.

12 of these specimens were placed in the tank filled with an electrolytic

solution, which contains 3.5 % Sodium chloride (NaCl) by weight of water,

and was changed on a weekly basis to eliminate any change in the

concentration of NaCl. Three specimens were used as control specimens and

were not placed in the concentrated NaCl solution. The direction of the

current was arranged so that the reinforcing bar served as the anode while the

bare steel bar served as the cathode. A voltage of 5V was applied to accelerate

the corrosion for a period of 30 days. After the accelerated corrosion, the pull

out test was performed on the specimens. Figure 4.3 shows the accelerated

corrosion process of the specimens.

Figure 4.2 Pull Out Test Specimen

119

Figure 4.3 Accelerated Corrosion Process of the Specimens

4.3 RESULTS AND DISCUSSION

4.3.1 Physical and Chemical Properties of Cement and Ashes

Physical properties of cement, Fly Ash and Bagasse Ash are given

in Table 4.1. From the fineness value it confirms that the ashes are very fine

particles. The chemical analysis values are given in Table 4.2 for OPC, FA

and BA showed the silica content of BA is 64.50% and FA is 59.62%

against the cement value of 21.4%. These confirmed that basically FA and

BA are silica rich material which can be used as supplementary cementitious

material.

4.3.2 Workability

The mixed fresh concrete was tested for its workability parameters

and the results are given in Table 4.3. From the observed values it is evident

that the workability performance of blended cement concrete is better than

conventional concrete The spherical shaped particle of fly ash acts as

miniature ball bearings within the concrete mix and improves the

workability.

120

Table 4.3 Workability of the Concrete

Slump Value

CRL FA1 FA2 FA3 BA1 BA2 BA3 2.0 3.2 3.2 3.7 2.0 2.5 2.7

4.3.3 Mechanical Properties

The results of compressive strengths at the age of 28 days are

reported in Table 4.4 and variation with age is shown in Figure 4.4. It has

been observed that the compressive strength decreases with increase in

replacement of cement by fly ash at 7 and 28 days in FA1 and FA3

specimens. The reason for the above observation is that pozzolonic reaction in

fly ash concrete starts only after the release of free lime from hydration of

cement which takes more time. This is quite in agreement with similar

observations reported by earlier researchers on development of compressive

strength of fly ash concrete.

05

101520253035404550

0 7 14 21 28 35 42 49 56Age of Concrete in Days

Com

pres

sive

Str

engt

h (N

/ SQ

.MM

)

CRLFA1FA2FA3

Figure 4.4 Compressive Strength Vs Age of Concrete - FA Series

Specimens

121

The variation of cube compressive strength for the partial

replacement of cement by BA at different curing periods is given in

Figure 4.5. It can be seen that the compressive strength increases with

increase in replacement of cement by BA at 7 and 28 days in BA1 and BA2

specimens. From the foregoing discussion, it can be concluded that the BA

containing amorphous silica increases the compressive strength. Flexural

strength test, split tensile strength and the cylinder compressive strength

were conducted for various mixes on test specimens at the age of 28 days and

the results are given in Table 4.4. The flexural strength, and cylinder

compressive strength decreases with increase in replacement of cement by fly

ash at 28 days, whereas the strength results are increased with increase in

replacement of BA in BA1 and BA2 specimens at 28 days. Figure 4.6 shows

the experimental test set up and Figure 4.7 shows the failure mode of the

specimens. The percentage variation of strength results for different

percentage of replacement of cement by FA and BA were compared with the

conventional specimens and are presented in Table 4.5.

05

101520253035404550

CRL BA1 BA2 BA3

Specimens

Com

pres

sive

Str

engt

h (N

/SQ

.MM

)

7- Days28 Days

Figure 4.5 Compressive Strength Vs Percentage Cement Replacement -

BA Series Specimens

122

Table 4. 4 Strength Results for the Concrete

Parameters CRL FA 1 FA 2 FA 3 BA 1 BA 2 BA 3 Specimens Cured

for 28 days

Cube Compressive Strength (MPa)

36.08 32.15 33.41 30.58 42.14 43.12 34.51

Split Tensile Strength (MPa)

4.8 4.22 4.50 4.11 4.57 5.18 4.657

Flexural Strength (MPa)

5.98 5.28 5.65 5.19 6.37 6.59 5.77

Cylinder Compressive

Strength (MPa) 31.69 28.19 27.30 26.81 36.95 36.52 30.26

Chloride Permeability Coulombs

1646 876 850 830 978 953 940

Table 4.5 Comparison of Test Results

Specimens

Cube Compressive

Strength (Days) Split

Tensile Strength

Flexural Strength

Cylinder Compressive

Strength

Chloride Permeability

Test

7 28 56 28 Days 28 Days 28 Days

CRL - - - - - - - FA 1 -18 -11 -10 -12 -12 -11 -47 FA 2 -15 -7 +10 -6 -6 -14 -48 FA 3 -25 -15 -16 -14 -13 -15 -50 BA 1 +7 +17 - -5 +7 +17 -41 BA 2 +12 +20 - +8 +11 +15 -42 BA 3 -10 -4 - -3 -4 -5 -43

123

Figure 4. 6 Experimental Test Set - up - of Concrete Prisms

Figure 4.7 Failure Mode of Prisms

124

4.3.4 Pull Out Test

The bond between concrete and rebars plays a vital role to give

strength to the concrete structures. Pull out strength was determined by

measuring the maximum force required to pull the rebar from the concrete

after the accelerated corrosion process. The test was carried out in pull out

testing machine. Average of three specimen results are shown in Table 4.6.

UC and C referred as Uncorroded and Corroded specimens. FA1 to FA3

represents the cement replacement by fly ash as 10%, 20% and 30%. The

test results show that the at 20% replacement of cement by fly ash gives less

variation than the conventional and corroded specimens. The higher bond

resistance of fly ash concrete is a result of the lower corrosion rate of the

reinforcing bars.

Table 4.6 Pull Out Test Results

Sl.No Specimens Pullout test

in kN 1 UC 8.44

2 C 7.68

3 FAUC1 7.59

4 FAUC2 7.76

5 FAUC3 7.42

6 FAC1 7.21

7 FA C2 7.29

8 FA C3 7.04

4.3.5 Rapid Chloride Permeability Test

Rapid chloride Permeability test was conducted as per ASTM C

1202 standards. It is given in Figure 4.8. The values of chloride permeability

125

in terms of total charge passed through the specimen for various mixes are

given in Table 4.3. It was observed that addition of mineral admixtures has

excellent resistance to chloride ion penetration and the charge passed in

coulombs was below 1000, which is well below the normal concrete.

Figure 4.8 Chloride Permeability Test Set up

4.3.6 Conclusions

The following conclusions were arrived from the first phase:

i) Fineness of FA and BA indicates that these ashes are smaller

size particles when compared to OPC

ii) Workability performance of blended cement concrete is

better than conventional concrete. This is due to lubricating

effect of ash particles while mixing.

iii) The mechanical properties of FA series specimens decrease

in early days strength, whereas increased percentage was

observed in BA series specimens as compared to controlled

concrete.

iv) The percentage increase in strength results observed in

bagasee ash concrete was higher than fly ash concrete. But

126

the resistance to chloride ion penetration of fly ash concrete

was higher than bagasee ash. Therefore, in this research, fly

ash has been used as corrosion inhibitor. The optimum

percentage of fly ash was evaluated from the strength results

as 20%.

4.4 BEHAVIOUR OF SQUARE COLUMN

4.4 .1 Parameters of Study

From the previous study, the better corrosion inhibitor was chosen

as fly ash with 20% replacement for cement. The other parameters considered

were

i) Cover to Diameter ratio (C/D): Specimens were cast with

two C/D ratios 2 and 2.5 with a cover of 20 and 25mm.

The diameter of the rod was kept constant as 10 mm.

ii) Columns were reinforced with 4 Nos. of 10 mm diameter.

(1.40%Ag and 2.01% Ag )

iii) Columns were subjected to loading condition of corrosion

process described as “L”.

iv) The columns were subjected to one level of corrosion. Thus

“UC” for uncorroded and “C” for corroded specimens.

Totally thirty columns were tested. The size of the column and

reinforcement details are given in Table 4.7 and in Figures 4.9 and 4.10. The

columns S3UC had a middle test region of 750 mm long and two enlarged

capitals at their ends. The capital of height 125 mm was provided at both ends

of the column to distribute the load evenly. Figures 4.11 and 4.12 show the

specimens after 28 days curing and Figures 4.13 and 4.14 represented the

accelerated corrosion process.

127

Table 4.7 Details of Test Specimen - Square Column

Specimen No of Specimen

Size of the Column

Cover in mm

Main Reinforcement

Lateral Ties

S1UC 2 150 150

500 mm Height

25

4 Nos - 10 mm diameter

6 mm dia at 150 mm c/c

S1C 2

S1FAUC 2

S1FAC 2

S2UC 2 125 125

500 mm Height

20 4 Nos - 10 mm diameter


S2C 2

S2LUC 1

S2LC 1

S3UC 2 125 125

1000 mm Height



S3C 2

S3LUC 1

S3LC 1

S 4UC 2 150 150

2000 Height



S4 C 2

S4FAUC 2

S4FAC 2

S5LUC 1 150 mm dia 300 mm

Height 20 4 Nos - 10 mm

diameter 6 mm dia at 73 mm c/c S5LC 1

128

Figure 4.9 Reinforcement Details - S3UC Specimen

Figure 4.10 Reinforcement Details - S4UC Specimen

129

Figure 4.11 Specimens after 28 Days Curing –S3UC

Figure 4.12 Specimens after 28 Days Curing - S2UC

130

Figure 4.13 Specimens Subjected to Accelerated Corrosion Process -

S4C

Figure 4.14 Specimens Subjected to Accelerated Corrosion Process -

S3C

131

4.4.2 Sustained Load and Corrosion Process

A load of 45 kN was applied to the specimens. The loading was

exerted by keeping two 2.5 cm thick steel plates one at the top and the other at

the bottom of the specimens and the whole assembly is tightened with

threaded rod and nuts which was controlled by a torque wrench. Figures 4.15

and 4.16 show the sustained loading of the specimens. The load decreases

as the corrosion proceeds (Creep effects) with increase in time. To maintain

the loads constant throughout the duration of the test, continuous monitoring

and adjustment of nuts were required. The steel plates, nuts and bolts were

plated with Zinc - coating to avoid the galvanostatic corrosion.

Figure 4.15 Sustained Loading of the Specimens - S5LUC

132

Figure 4.16 Sustained Loading of the Specimens - S2LUC and S3LUC

4.4.3 Instrumentation and Testing Procedure

All the specimens were loaded until failure under axial compression

in a testing frame. The specimens were externally confined by 10 mm thick

mild steel collars in the top during testing to prevent premature failure.

Longitudinal strains were manually measured using DEMEC gauge with

150mm length having a least count of 0.002mm.Pellets were glued on the

concrete surface. Pellets were fixed at mid height of the column for strain

measurements in the longitudinal direction. Strains were noted down for

every 100kN increment of load. The Test set up for the columns is shown in

Figures 4.17 to 4.19.

133

Figure 4.19 Test Set - up - of Column - S2C

Figure 4.17 Test Set - up - of

Column S4UC

Figure 4.17 Test Set - up - of

Column S3UC

134

4.5 RESULTS AND DISCUSSION

The corrosion activity of column specimens were evaluated by Non-

Destructive and destructive tests. In the Non-Destructive test the effect of

corrosion on electrochemical study (OCP measurement), Physical study

(Rebound value, UPV value), and Chemical study (Chloride content, pH

value) of all the specimens were studied. In the Destructive test the effect of

corrosion on ultimate load, axial strain, energy absorption capacity and

weight loss of steel of all the specimens were studied.

4.5.1 Estimation of Corrosion Potential by Open Circuit Potential

Measurement

Figure 4.20 shows the corrosion potential of the specimens versus

the time of accelerating steel corrosion. It can be seen that concerning the

bars in the S1C specimens the potential drops to -276 mV after 16 days of

accelerated exposure, whereas those noted in the S2C is 12 days of exposure.

The potential drops to -276 mV in S3C specimens after 15 days and S4C

is 18 days of exposure.

Exposure Time

-450

-400

-350

-300

-250

-200

-150

-100

-50

00 10 20 30

OC

P in

mV

S2CS1CS3CS4C

Figure 4.20 OCP Vs Exposure Time- S Series Specimens

135

From Figures 4.21 and 4.22 It can be observed that the bars in the

S1FC specimens the potential drops to -276 mV after 18 days of accelerated

exposure, whereas those noted in the S4FC is 22 days of exposure. The

potential drops to -276 mV in S2SC and S3SC specimens after 12 days

and S5SC is 8 days of exposure. The average potential values measured at

the end of the accelerated corrosion process in a well defined grid points are

shown in Table 4.8 and Figure 4.23. It can be seen that the column with

20mm cover shows lower potential value than the column with 25 mm cover.

-400

-350

-300

-250

-200

-150

-100

-50

00 5 10 15 20 25 30

ExposureTime

Ocp

in m

V

S1FC

S4FC

S1C

S4C

Figure 4.21 OCP Vs Exposure Time - S1 and S4 Series Specimens

136

-600

-500

-400

-300

-200

-100

00 10 20 30 40

Exposure Time

OC

P in

mV

S2CS3CS2LCS3LCS5LC

Figure 4.22 OCP Vs Exposure Time - S2 and S3 Series Specimens

Figure 4.23 Grid Points for Measurement of OCP Value

137

4.5.2 Rebound Hammer Test

The rebound test was carried out after the accelerated corrosion

process on the points where the OCP measurement was taken. Table 4.8

gives test results. It can be seen that conventional specimens had a rebound

number of 40 which indicates that the cover concrete was good. For corroded

specimens the values were in the range of 26-30. Locations showing very low

rebound number have weak surface concrete and such locations are defined as

corrosion prone locations.

Table 4.8 Test Results of Square Column

Specimens

Open Circuit Potential

Value (mV)

Rebound Hammer

Value

UPV Value (km/s)

Chloride Content % by Weight of

Cement

pH Value

S1UC - 40 4.50 - - S1C -376 28 3.90 0.209 10.6 S1FAUC - 40 4.50 - - S1FAC -366 30 3.95 0.188 10.6 S2UC - 40 4.50 - - S2C -399 28 3.90 0.221 10.3 S2LUC - 40 4.50 - - S2LC -431 26 3.60 0.237 10.2 S3UC - 40 4.50 - - S3C -385 28 3.84 0.213 10.6 S3LUC - 40 4.50 - - S3LC -428 27 3.70 0.214 10.3 S4UC - 40 4.50 - - S4C -369 30 3.95 0.187 10.6 S4FAUC - 40 4.50 - - S4FAC -354 31 4.16 0.175 10.6 S5LUC - 40 4.50 - S5LC -510 26 3.68 0.274 10.1

138

4.5.3 Ultrasonic Pulse Velocity Test

The UPV test was carried out after the accelerated corrosion

process and the values are given in Table 4.8. It can be seen that conventional

specimens have a higher UPV than corroded specimens which indicates that

there is no loss of integrity. Low velocity indicates a really weak surface

concrete. For the corroded specimens the UPV was 3.8 km/s, which may be

due to initiation of delamination of outer portion of concrete (cover concrete).

Figure 4.24 shows the Grid points of the UPV value.

Figure 4.24 Grid Points for Measurement of UPV Value

4.5.4 Chloride Content Test

Chloride content can be determined by collecting broken samples.

The samples were taken on the points based on the OCP value. Table 4.8

gives the chloride content of the cover concrete. It can be seen that the

column with 20mm cover shows higher percentage of chloride content than

the column with 25 mm cover.

139

4.5.5 pH Value Test

pH value is very important as the alkalinity of concrete maintains

the passivity of steel. Based on the Open Circuit Potential (OCP) values,

powder samples were collected from the cover concrete at the selected

locations. The pH value of the filtered solution was then measured using a

calibrated pH meter and presented in the Table 4.8.

4.5.6 Discussions on the Effect of Corrosion on Non-Destructive

Test

The effect of corrosion on Non-Destructive Test (NDT) results of all

the corroded columns were compared with the respective threshold (or

conventional column) value. The percentage decrease in the value are

presented in Table 4.9.

The percentage variation depends on the corrosion rate. The

corrosion of reinforcement is initiated when the chloride content exceeds a

threshold value that depassivates the embedded steel in concrete. Once the

protective oxide layer around the reinforcement bars were broken, the

corrosion propagation depends on moisture and oxygen availability in the

neighborhood of the rebars.

The performance of S1FAC and S4FAC was better than all the

specimens. The results of this specimen show that fly ash addition to concrete

mixes improves the corrosion resistance of concrete. The fly ash cement mix

has a denser microscopic pore structure and relatively impermeable concrete.

Water normally enters concrete by capillary action: the fewer and smaller the

capillary pores are, the less water enters the concrete and the lower the rates

of oxygen diffusion and carbonation. Due to the high alkalinity in concrete

there are few H+ ions to be consumed in the cathodic reaction. Therefore,

140

when oxygen reduction rate is reduced, the corrosion rate is reduced.

Increasing the electrical resistivity of the concrete slows down the ion

transfer and therefore increases the corrosion resistance of rebar in concrete.

The performance of S4FAC was better than S1FC, due to the increased

surface area of steel.

Table 4.9 Comparison of Effect of Corrosion on Concrete - Percentage

Decrease in NDT Results

Specimens Open Circuit

Potential Value

Rebound Hammer

Value

UPV Value

Chloride Content

pH Value

S1C 36 30 13 39 12

S1FAC 33 25 12 25 12 S2C 45 30 13 47 14

S2LC 56 35 20 58 15

S3C 40 30 15 42 12

S3LC 55 33 18 43 14

S4C 34 25 12 25 12

S4FAC 28 23 8 17 12

S5LC 85 35 18 83 16

Test results show that the NDT results of S5LC, S2LC and

S3LCdrops faster than other specimens. This was due to the formation and

propagation of a crack of the specimens. As expected the column with

25 mm cover (S1C and S4C) thickness showed lower corrosion current and

higher resistance than the column with 20 mm cover (S2C, S3C and S5LC).

The comparative NDT results on corroded column are presented in

Table 4.10.

141

Table 4.10 Percentage Decrease in NDT Results - Corroded column

Specimens Open Circuit

Potential Value

Rebound Hammer

Value UPV

Chloride Content

pH Value

S1C - S1FAC 3 7 1 10 - S2C - S2LC 8 7 8 7 1 S3C - S3LC 11 4 4 1 3 S4FAC - S4C 4 3 5 6 -

4.5.7 Distribution of Cracks and Corrosion Activities

After the accelerated corrosion process the surface cracks were

noticed on the surface of the specimens and the crack measurements are

recorded and shown in Table 4.11. The size and extent of cracking has an

important role on the effect of corrosion. The specimens S2LC and S3LC

showed significant longitudinal cracking and transverse cracking because of

combined effect of loading and corrosion process. Figure 4.25 shows the

crack map for the concrete column.

Table 4.11 Details of Crack Measurement - Square Column

Specimen No of Longitudinal

Crack

Average Length of

Longitudinal Crack (mm)

Average width of

Longitudinal Crack (mm)

No of Transverse

Cracks

Average Width of Transverse

Crack (mm)

S1C - - - 3 0.13-0.16 S1FAC - - - 3 0.11-0.13 S2C 2 50 0.12 4 0.15-0.16 S2LC 3 80 0.14 6 0.16-0.18 S3C 1 50 0.14 3 0.14-0.16 S3LC 3 60 0.14 5 0.13-0.17 S4C - - - 3 0.14-0.17 S4FAC - - - 3 0.11-0.14 S5LC 3 80 0.19 13 0.26-0.29

142

Figure 4.25 Crack Map - Square Column

4.5.8 Effect of Corrosion on Axial Behaviour

The effect of corrosion on the axial load carrying capacity, energy

absorption capacity, and ductility of reinforced columns are examined and

discussed in detail in the following section. All the specimens were tested

under monotonic loading. The first set includes S1UC, S1C, S1FAUC,

S1FAC. S1UC and S1FAUC served as control columns. Specimens S1C and

S1FAC were subjected to first level of corrosion. The control specimen failed

due to shear and this failure was indicated by inclined cracks appearing on

the sides of the column. The specimen attained the peak load of 825 kN.

The axial stains were measured at eighty percentage of loading and energy

143

absorption capacity was calculated for the area under the stress-strain curve.

Figures 4.26 to 4.28 compare their ultimate load, axial strain and ductility.

825

747

787

757

700

720

740

760

780

800

820

840

S1UC S1C S1FAUC S1FAC

Specimens

Ulti

mat

e L

oad

in k

N

Figure 4.26 Comparison of Ultimate Load Between Corroded and

Uncorroded Column - S1 Series Specimens

Specimen S1C reached an ultimate load of 747 kN which is 10 %

less than SIUC. This can be ascribed to the deterioration of the bond at the

steel to concrete interface caused by corrosion and reduction in the cross

sectional area of the reinforcing bars. This is supported by the observation of

a strain in about 1000 µ€ (micro strain) which is 24 % less than the virgin

specimens. The energy absorption capacity is reduced by 15%.

144

1310

1000

1150 1130

0

200

400

600

800

1000

1200

1400


Specimens

Axi

al S

trai

n (M

icro

Str

ain)

Figure 4.27 Comparison of Axial Strain Between Corroded and

Uncorroded Column -S1 Series Specimens

Specimen SIFAUC exhibited an ultimate strength of 787 kN

which is 5% less than the estimated virgin strength of 825 kN. The reason

for the above observation is that the delay in the pozzalonic reaction of fly

ash. Less percentage variation was observed in axial strain and energy

absorption capacity when compared to conventional specimens.

The behaviour of S1FAC was better than S1C. The ultimate load in

this specimen was 757 kN which is 4% less than S1FAUC and 1 % higher

than S1C. The percentage decrease in axial strain was 2% less than that of

S1FAUC and 11% higher than that of S1C.

Numerous authors have reported that fly ash exhibited better

resistance to corrosion damage than normal concrete. The reduced level of

corrosion in fly ash concrete is attributed to the higher resistivity of the

concrete mainly due to the denser microstructure of concrete.

145

0.03

0.0255

0.0267

0.0259

0.023

0.024

0.025

0.026

0.027

0.028

0.029

0.03

0.031


Specimens

Ener

gy A

bsor

ptio

n ca

paci

ty

Figure 4.28 Comparison of Energy Absorption Capacity Between

Corroded and Uncorroded Column -S1 Series Specimens

The second set specimens includes S2UC, S2C, S2LUC and S2LC.

S2UC attained the maximum load of 734 kN followed in sequence by

S2LUC , S2C and S2LC. Specimen S2C reached an ultimate strength of

642 kN, which is 1.14 times less than the conventional column. The axial

strain decreases by a factor of 1.33 and energy absorption decreases by 1.36

times than the uncorroded column. Figures 4.29 to 4.31 compare their

ultimate load, axial strain and ductility.

The ultimate load observed in the specimen S2LUC was 653 kN.

which is 1.12 times less than the conventional specimen. The axial stain and

energy absorption are observed as 1.2 times less than S2UC.

146

733.5

642.12 653

481

0

100

200

300

400

500

600

700

800

S2UC S2C S2LUC S2LC

Specimens

Ulti

mat

e L

oad

in k

N



Specimen S2LC attained an ultimate load of 481kN which is 25%

less than that of S2C and 27% less than S2LUC. The axial strain in S2LC

decreases by 39% when compared to S2LUC and 32% when compared to

S2C. At the same time 42% and 35% variations were observed in the

energy absorption capacity. Sustained load expedited the initiation of

reinforcement corrosion in the specimens by cracks in the concrete. These

cracks might connect flow paths facilitating water and ion penetration to the

surface of reinforcing steel and further accelerate the corrosion of

reinforcement. Once the corrosion progresses, it leads to further surface

cracking, delamination and debonding, resulting in reduction of the ultimate

capacity and axial rigidity.

The third set of specimens include S3UC, S3C, S3LUC and S3LC.

Specimen S3C sustains an ultimate strength of 623 kN, which is 11% less

than the estimated virgin strength of 696 kN. The axial strain and energy

147

absorption capacity decreases by 24% than the conventional specimen.

Figures 4.32 to 4.34 compare their ultimate load, axial strain and ductility.

1260

9471050

645

0

200

400

600

800

1000

1200

1400

S2UC S2C S2LUC S2LC

Specimens

Axi

al S

trai

n (M

icro

Str

ain)


Uncorroded Column -S2 Series Specimens

0.0287

0.02110.0237

0.0138

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

S2UC S2C S2LUC S2LC

Specimens

Ene

rgy

Abs

orpt

ion

Cap

acity


Corroded and Uncorroded Column - S2 Series Specimens

148

695.88623.28 626

488

0

100

200

300

400

500

600

700

800

S3UC S3C S3LUC S3LC

Specimens

Ulti

mat

e L

oad

in k

N



950

720

820

595

0

100

200

300

400

500

600

700

800

900

1000

S3UC S3C S3LUC S3LC

Specimens

Axi

al S

trai

n (M

icro

Str

ain)



149

The ultimate load taken by the specimen S3LUC was 626 kN,

which is 10% less than the conventional specimen. The axial stain and energy

absorption are observed as 14 % and 18% less than S3UC.

Specimen S3LC attained an ultimate load of 488 kN which is 22%

less than that of S3C and S3LUC. Notable percentage variation was

observed in axial strain. The value of 27% was noted when compared to

S3LUC and 17% when compared to S2C. At the same time 20 % and

14 % variations were observed in the energy absorption capacity.

0.0193

0.01480.0158

0.0127

0

0.005

0.01

0.015

0.02

0.025

S3UC S3C S3LUC S3LC

Specimens

Ene

rgy

Abs

orpt

ion

Cap

acity



Specimen S4C reached an ultimate load of 616 kN which is 8 %

less than the S4UC. The observation of strain is about 745 µ€ (micro

strain) which is 20 % less than the virgin specimens. The energy absorption

capacity is reduced to 9% when compared to S4UC. The control column

failed due to buckling and this failure was indicated by a lump of concrete

spalling from the sides of the column at approximately mid height at the

150

ultimate load level. The specimen attained the peak load of 672 kN. The

comparisons are presented in Figures 4.35 to 4.37.

672

616

634

622

580

590

600

610

620

630

640

650

660

670

680


Specimens

Ulti

mat

e L

oad

in k

N



Specimen S4FAUC exhibited an ultimate strength of 634 kN

which is 6% less than the estimated virgin strength of 672 kN. The

percentage decrease in axial train and energy absorption were noted as 11%

and 5% compared to S4FAUC. The behaviour of S4FAC was better than

S4C. The ultimate load in this specimen was 622 kN which is 2% less than

S4FAUC and 1 % higher than S4C. The percentage decrease in axial strain

was 2% less than that of S4FAUC and 9% higher than that of S4C.

Likewise the percentage decrease in energy absorption capacity was 1% less

than that of S4FAUC and 3% higher than that of S4C.

151

927

745824 810

0

100

200

300

400

500

600

700

800

900

1000


Specimens

Axi

al S

trai

n (M

icro

Str

ain)



0.0145

0.0132

0.01380.0136

0.0125

0.013

0.0135

0.014

0.0145

0.015


Specimens

Ene

rgy

Abs

orpt

ion

Cap

acity


Corroded and Uncorroded Column -S4 Series Specimens

Specimen S5LC reached an ultimate load of 429 kN which is 36 %

less than the S45LUC. The observation of strain is about 245 µ€ (micro

strain) which is 70 % less than the virgin specimens. The energy absorption

152

capacity is reduced to 83% when compared to S5LUC. The comparisons

are shown in Figures 4.38 to 4.40 and in Table 4.12.

675

429

0

100

200

300

400

500

600

700

800

S5LUC S5LC

Specimens

Ulti

mat

e L

oad

in k

N



815

245

0

100

200

300

400

500

600

700

800

900

S5LUC S5LC

Specimens

Axi

al S

trai

n (M

icro

Str

ain)



153

0.02

0.0035

0

0.005

0.01

0.015

0.02

0.025

S5LUC S5LC

Specimens

Ene

rgy

Abs

orpt

ion

Cap

acity



Table 4.12 Comparison of Effect of Corrosion on Concrete- Percentage Decrease in Destructive Test Results

Specimens UL AS EA S1UC-S1FAUC 5 12 11 S1UC-S1C 10 24 15 S1FAUC - S1FAC 4 2 3 S1FAC-S1C 1 13 2 S2UC-S2LUC 11 19 17 S2UC-S2C 13 25 27 S2LUC - S2LC 27 39 42 S2LC-S2C 25 32 35 S3UC-S3LUC 10 14 18 S3UC-S3C 10 24 23 S3LUC - S3LC 22 27 20 S3LC-S3C 22 17 14 S4UC-S4FAUC 6 11 5 S4UC-S4C 8 20 9 S4FAUC - S4FAC 2 2 1 S4FAC-S4C 1 9 3 S5LUC- S5LC 36 70 83

154

4.5.9 Stress-Strain Response

Typical Stress- Strain curves for uncorroded and corroded

specimens are shown in Figures 4.41 to 4.45. Each curve is an average of the

stress-strain curves for the three identical specimens with the same level of

corrosion. The axial rigidity of the column decreases due to corrosion. The

above behaviour can be related to the development of cracks which may

affect the failure mode and ultimate strength of the specimen through the

significant loss of bond between concrete and reinforcement.

0

5

10

15

20

25

30

0 0.0005 0.001 0.0015

Axial Strain

Axi

al S

tres

s MPa S1UC

S1CS1FAUCS1FAC

Figure 4.41 Axial Stress Vs Axial Strain for S1 Series Specimens

155

0

5

10

15

20

25

30

35

0 0.0005 0.001 0.0015

Axial Strain

Axi

al S

tres

s MPa S2UC

S2CS2LUCS2LC


0

5

10

15

20

25

30

35

0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012

Axial Strain

Axi

al S

tres

s M

Pa S3LUCS3UCS3LCS3C


156

0

5

10

15

20

25

0 0.0005 0.001

Axial Strain

Axi

al S

tres

s MPa

S4UCS4CS4FAUCS4FAC


0

5

10

15

20

25

30

35

0 0.0005 0.001 0.0015

Axial Strain

Axi

al S

tres

s MPa

S5LUCS5LC


157

4.5.10 Failure Mode of Square Column

In corroded column, the failure started in the lower corroded portion

and extended over the entire specimen and disintegration of the concrete

cover and core especially in the lower half of the columns. The spalling of

concrete cover was caused due to corrosion cracks and it occurs all over the

length. This premature mode of failure was accompanied by simultaneous

buckling of longitudinal reinforcement. In the case of convention column,

the fine irregular cracks were noticed in the surface of the columns. These

cracks get widened with increase in load level and the concrete was crushed

as the longitudinal reinforcement had yielded. Figures 4.46 to 4.51 show the

failure mode.

Figure 4.46 Failure Mode of

S2C Specimen

Figure 4.47 Failure Mode of S1UC

Specimen

158


S3C Specimen


S2UC Specimen


S4C Specimen


S3UC Specimen

159

4.5.11 Measurement of Weight Loss

The loss of weight of the rebars were calculated by the gravimetric

method. Figure 4.52 shows the percentage weight loss versus level of

corrosion for various C/D ratios. Figure 4.53 shows the corroded rebars.

0

1

2

3

4

5

6

7

8

9

10

S1C S1FAC S4C S4FAC S2C S2LC S3C S3LC S5LC

Specimens

Perc

enta

ge W

eigh

t Los

s

Figure 4.52 Comparison of Weight Loss in S Series Specimens

Figure 4.53 Corroded Rebars

160

4.6 CONCLUSIONS

The following conclusions are arrived at based on this study:

The rebound Value, UPV value and pH of concrete pore

solution decreases in the corroded specimens.

The chloride content expressed as percentage by weight of

cement increases with decrease in the concrete cover.

The corrosion potential decreases with decrease in the

concrete cover.

The load carrying capacity, ductility and energy absorption

capacity of the conventional column increases with the

increase in the cross section of the column (S1UC and

S2UC).

The load carrying capacity, ductility and energy absorption

capacity of the conventional short (S1UC) column increases

with respect to long column (S4UC) of same cross section.

The percentage decrease in load carrying capacity, ductility

and energy absorption capacity of the corroded column

decreases with the increase in the cover concrete (S1C and

S2C).

The percentage decrease in load carrying capacity, ductility

and energy absorption capacity of the corroded short column

increases when compared to the percentage decrease in long

column of the same cover (S1C and S4C).

The percentage loss of weight in steel increases with

decrease in cover and the percentage decrease in load

161

carrying capacity of the corroded column increases with

increase in percentage weight loss of steel.

The percentage loss of weight in steel and load carrying

capacity of the corroded column increases with decrease in

surface areas of steel embedded.

Significant increase in strength and ductility was observed in

fly ash concrete corroded column when compared to that of

conventional concrete corroded column.

The sustained load and accelerated corrosion procedure

results in a loss of ultimate load carrying capacity of about

27 %.

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CHAPTER 4 BEHAVIOUR OF REINFORCED CONCRETE CORRODED …shodhganga.inflibnet.ac.in/bitstream/10603/43138/9/09... · 2018-07-03 · proportions and hand mixing were adopted for making