chapter 4 experimental programme for...

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57 CHAPTER 4 EXPERIMENTAL PROGRAMME FOR THE DETERMINATION OF THE OPTIMAL PERCENTAGE OF FLY ASH 4.1 INTRODUCTION For the experimentations, concrete of grade M 20 was adopted. For the above specified grade of concrete, mix proportion was arrived by Indian Standard method of mix design and it is as follows: 1 : 1.464 : 3.210 and water-cement ratio 0.5 Concrete specimens cast adopting the above mix proportion using Ordinary Portland Cement (OPC) for the study of various mechanical, micro structural and durability properties were considered as control specimens. In this proportion, the content of cement was partly replaced by fly ash with increments of 5% by weight of cement upto 30% and similar specimens as that of the control specimens were cast for each percentage increment of fly ash and tested to determine the properties of fresh and hardened fly ash blended cement concrete including its micro structural and durability properties. 4.2 TESTS ON FRESH CONCRETE 4.2.1 Slump Test To measure the consistency of concrete and certain factors contributing to workability, this common test was performed for all the

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57

CHAPTER 4

EXPERIMENTAL PROGRAMME FOR THE

DETERMINATION OF THE OPTIMAL PERCENTAGE OF

FLY ASH

4.1 INTRODUCTION

For the experimentations, concrete of grade M20 was adopted. For

the above specified grade of concrete, mix proportion was arrived by Indian

Standard method of mix design and it is as follows:

1 : 1.464 : 3.210 and water-cement ratio 0.5

Concrete specimens cast adopting the above mix proportion using

Ordinary Portland Cement (OPC) for the study of various mechanical, micro

structural and durability properties were considered as control specimens. In

this proportion, the content of cement was partly replaced by fly ash with

increments of 5% by weight of cement upto 30% and similar specimens as

that of the control specimens were cast for each percentage increment of fly

ash and tested to determine the properties of fresh and hardened fly ash

blended cement concrete including its micro structural and durability

properties.

4.2 TESTS ON FRESH CONCRETE

4.2.1 Slump Test

To measure the consistency of concrete and certain factors

contributing to workability, this common test was performed for all the

58

batches of concrete with various percentages of fly ash and by observing the

manner in which the concrete slumped, it was compared with that of the

control concrete without fly ash.

4.2.2 Compacting Factor Test

Compacting factor test was performed to measure the inherent

characteristics of the concrete which relates very close to the workability

requirements of concrete particularly for concrete mixes of very low

workability and mixes that are relatively dry and insensitive to slump test.

4.2.3 Vee Bee Consistometer Test

This test was ideally suited for very dry concrete mixes whose

slump values were unable to be determined by slump test and it was

performed in the laboratory to measure indirectly the workability of the

various concrete mixes. The time taken in seconds for the conical concrete

specimen to assume full cylindrical shape as observed through a transparent

glass disc by vibrating the concrete was noted as Vee Bee Degree.

4.2.4 Setting Time of Concrete by Penetration Resistance

Setting times of concrete mixes without and with various

percentages of fly ash were determined as per IS: 8142 -1976. Spring

reaction-type penetration resistance apparatus graduated from 50 N to 600 N

and needle of 16mm2 bearing area were used. The penetration resistance was

calculated from the force required to cause 25mm depth of penetration of the

needle divided by the bearing face area of the needle. The results of each trial

were plotted with penetration resistance in N/mm2 as the ordinate and elapsed

time in minutes as the abscissa. Times of initial and final setting were

determined from the curves plotted for each concrete mix at which penetration

resistances were 3.43 N/mm2 (35 kgf/cm2) and 26.97 N/mm2 (275 kgf/cm2)

respectively.

59

4.3 RESULTS OF THE FRESH CONCRETE TESTS

The values obtained from the various tests performed on the fresh

concrete mixes are tabulated below

Table 4.1 Test values of fresh concrete mixes

S.

No.Identification

Slump

‘mm’

Compacting

factor

Vee

Bee

Time

‘secs’

Final

setting

time

‘minutes’

1CM - Control mix

(without fly ash)62 0.88 6 260

2 FA 5 (fly ash 5%) 66 0.89 5 290

3 FA 10 (fly ash 10%) 73 0.91 5 310

4 FA 15 (fly ash 15%) 78 0.92 4 325

5 FA 20 (fly ash 20%) 82 0.94 3 340

6 FA 25 (fly ash 25%) 86 0.95 3 360

7 FA 30 (fly ash 30%) 89 0.95 2 380

Figure 4.1 Variation in slump values

60

Figure 4.2 Variation in compacting factor values

Figure 4.3 Variation in Vee Bee times

61

Figure 4.4 Variation in final setting times

4.3.1 Discussion on Test Results

In fresh state, fly ash plays an important role in the fluidity of

concrete which is commonly expressed in such phenomenological

measurements as workability, compactability, setting and finishability.

Addition of fly ash has significant influence on the rate of hydration reactions

as well as on the effectiveness of chemical admixtures.

Fly ash concrete mixes were found to be more cohesive than plain

concrete mixes. During slump test, the fly ash concrete mixes subsided more

slowly and gradually than the control mix which was without fly ash and it

exhibited abrupt fall or subsidence.

Based on the test results it was observed that the slump, compacting

factor and final setting time of concrete increased whereas on the other hand

the Vee Bee time decreased with increase in percentage of fly ash added.

62

All the above findings indicate that with increase in percentage of

fly ash as partial replacement of cement in concrete increase the workability

of concrete. Though the increase in workability was initially high with

incremental increase of fly ash percentages, it was observed that the rate of

increase in workability gradually decreased later on even though the

incremental increase in fly ash percentage was constant.

Electron microscope photographs have shown that the particles in

fly ash occur as solid spheres of silica glass and are so fine and their sizes

range from < 1 µm to 100 µm. Majority of the particles are of 20 µm only.

Since the surface areas of the particles are also high they contribute to the

increase in workability with increase in their percentages. Hence due to the

increase in fluidity and mobility of the fly ash blended fresh concrete, there

was increase in final setting time of the concrete also.

4.4 MECHANICAL STRENGTH TESTS

4.4.1 Compressive Strength

Since most of the desirable characteristic properties of concrete are

qualitatively related to its compressive strength and moreover the largest

nominal size of aggregate was limited to 20mm, this test was performed on

cubical specimens of 150mm size. Cubes were cast using steel moulds,

demoulded after 24 hours and cured by completely immersing in water. 12

numbers of cubes were cast for each concrete mix with partial replacement of

cement by fly ash in various proportions. Totally 84 cubes were cast for all

the proportions and for the control. 3 cubes from each mix proportion of fly

ash were tested sequentially at a time on the 7th, 28th, 56th and 90th day of

casting and the average values obtained were compared with that of the

control specimen.

63

Figure 4.5 Compression test on concrete cubes

4.4.2 Split Tensile Strength Test

As there are no standardized methods to measure the tensile

strength of concrete directly, an indirect method called cylinder splitting

tension test was performed on cylindrical concrete specimens placed

horizontally between the loading surfaces of a compression testing machine

and the load was applied until failure of the cylinder along the vertical

diameter.

Though the loading condition produces a high compressive stress

immediately below the two generators to which the load was applied, a larger

portion corresponding to depth will be subjected to a uniform tensile stress

acting horizontally. When the load was applied along the generatrix, the

horizontal stress in an element on the vertical diameter of the cylinder is

2P/ LD

where, P is the compressive load on the cylinder

L is the length of cylinder and

D is the diameter of the cylinder.

64

Cylinders of 150mm diameter and 300mm length were cast, cured

and 3 numbers were tested sequentially at a time on the 7th, 28th, 56th and 90th

day of casting and the average values obtained were compared with that of the

control specimen.

Figure 4.6 Split tensile test on concrete cylinders

4.4.3 Flexural Strength Test

Beam tests were performed to measure the flexural strength

property of concrete as per I.S. 516-1959. The value of the modulus of rupture

(extreme fibre stress in bending) depends on the dimension of the beam and

manner of loading. Since two point loading yield a lower value of the

modulus of rupture than the centre point loading, the code specifies two point

loading. Since the largest nominal size of the aggregate was only 20mm,

beam specimens of size 100 x 100 x 500mm were cast, cured and 3 numbers

of beams were tested sequentially at a time on the 7th

, 28th, 56

th and 90

th day

of casting and the average values obtained were compared with that of the

control specimen.

65

The expression adopted for calculating the modulus of rupture:

fb = (P x L)/(b x d2)

P = maximum load in ‘N’ applied to the specimen

L = length in ‘mm’ of the span on which the specimen was

supported

b = measured width of the specimen in ‘mm’

d = measured depth of the specimen in ‘mm’

IS 456 – 2000 specifies following relationship between the

compressive strength and flexural strength. As per the code, flexural strength

= 0.7ckf , where fck is the characteristic compressive strength of concrete in

N/mm2.

4.4.4 Bond Strength Test

Bond strength refers to the adhesive force between steel and

concrete. The roughness of the steel surface and the compressive strength of

concrete are the primary factors that influence bond strength. Also bond

strength is a function of specific surface of gel. The cement used in this

experimentation consisted of higher percentage of C2S thereby giving higher

specific surface of gel and higher bond strength. Moreover with inclusion of

fly ash replacing cement partially, the percentage of C2S increased leading to

still higher specific surface of gel and bond strength.

4.5 RESULTS OF THE MECHANICAL STRENGTH TESTS

The values obtained from the various mechanical tests performed

on the hardened concrete specimens are tabulated below

66

Table 4.2 Compressive strength of concrete cubes

S.

No.Identification

7 Days

N/mm2

28 Days

N/mm2

56 Days

N/mm2

90 Days

N/mm2

1CM - Control mix

(without fly ash)16.31 26.53 28.00 28.89

2 FA 5 (fly ash 5%) 15.11 26.22 27.38 28.58

3 FA 10 (fly ash 10%) 14.35 25.51 27.82 29.02

4 FA 15 (fly ash 15%) 14.18 25.06 28.53 30.18

5 FA 20 (fly ash 20%) 14.22 25.55 28.88 31.64

6 FA 25 (fly ash 25%) 14.60 25.91 29.29 32.35

7 FA 30 (fly ash 30%) 13.87 24.84 27.91 28.84

Table 4.3 Split tensile strength of concrete cylinders

S.

No.Identification

7 Days

N/mm2

28 Days

N/mm2

56 Days

N/mm2

90 Days

N/mm2

1CM - Control mix

(without fly ash)1.68 3.03 4.31 4.62

2 FA 5 (fly ash 5%) 1.50 2.94 4.20 4.55

3 FA 10 (fly ash 10%) 1.38 2.83 4.10 4.49

4 FA 15 (fly ash 15%) 1.27 2.79 4.17 4.58

5 FA 20 (fly ash 20%) 1.22 2.76 4.23 4.61

6 FA 25 (fly ash 25%) 1.14 2.74 4.29 4.65

7 FA 30 (fly ash 30%) 1.11 2.67 4.24 4.61

67

Table 4.4 Flexural strength of concrete beams

S. No. Identification7 Days

N/mm2

28 Days

N/mm2

56 Days

N/mm2

90 Days

N/mm2

1CM - Control mix

(without fly ash)3.20 5.60 7.20 8.00

2 FA 5 (fly ash 5%) 3.20 5.40 7.00 7.80

3 FA 10 (fly ash 10%) 3.00 5.60 6.80 8.00

4 FA 15 (fly ash 15%) 2.80 5.80 7.00 8.40

5 FA 20 (fly ash 20%) 2.40 4.60 7.40 8.60

6 FA 25 (fly ash 25%) 2.40 4.40 7.60 9.20

7 FA 30 (fly ash 30%) 2.20 4.40 7.20 8.00

Table 4.5 Bond strength of concrete containing various FA percentages

S.

No.Identification

7 Days

N/mm2

28 Days

N/mm2

56 Days

N/mm2

90 Days

N/mm2

1CM - Control mix

(without fly ash)2.85 4.51 4.95 5.10

2 FA 5 (fly ash 5%) 2.76 4.45 4.94 5.12

3 FA 10 (fly ash 10%) 2.53 4.43 4.97 5.16

4 FA 15 (fly ash 15%) 2.39 4.35 4.99 5.18

5 FA 20 (fly ash 20%) 2.20 4.30 5.06 5.19

6 FA 25 (fly ash 25%) 2.11 4.22 5.06 5.19

7 FA 30 (fly ash 30%) 2.09 4.31 5.00 5.15

68

Figure 4.7 Graphical representation of compressive strength gain

Figure 4.8 Representation of relative compressive strengths at 90 days

69

Figure 4.9 Graphical comparison of the compressive strength gain

between control and 25% fly ash blended specimens

Figure 4.10 Graphical representation of split tensile strength gain

70

Figure 4.11 Representation of relative split tensile strengths at 90 days

Figure 4.12 Graphical comparison of the split tensile strength gain

between control and 25% fly ash blended specimens

71

Figure 4.13 Graphical representation of flexural strength gain

Figure 4.14 Representation of relative flexural strengths at 90 days

72

Figure 4.15 Graphical comparison of the flexural strength gain between

control and 25% fly ash blended specimens

Figure 4.16 Graphical representation of bond strength gain

73

Figure 4.17 Representation of relative bond strengths at 90 days

Figure 4.18 Graphical comparison of the bond strength gain between

control and 25% fly ash blended specimens

74

4.5.1 Discussion on Test Results

Fly ash, when used in concrete upto certain proportions contributes

to the mechanical strength of concrete due to its pozzolanic reactivity.

However, since the pozzolanic reaction proceeds slowly, the initial

mechanical strengths of fly ash concrete tends to be lower than that of the

concrete without fly ash.

This phenomenon was observed in all the mechanical strength tests

performed until the 28 days strength. But later on, when the next set of

specimens where tested on the 56th day of casting, there was a change in the

trend. There was slight increase in compressive, flexural and bond strengths

particularly for 20 % blended fly ash concrete. The split tensile strengths of

the cylinders cast with 20 % fly ash almost attained the same values as that of

the control specimens. This change in trend continued and ultimately the 90

days strength of all the specimens in all the specified tests clearly over took

the strength values attained by normal concrete without fly ash. The effect

was more pronounced in 20 % replacement of fly ash concrete. This is due to

the continued pozzolanic reactivity taking place in the blended cement

hydration development at the curing cessation time. Hence concrete

developed greater strength at later age exceeding that of concrete without fly

ash. Pozzolanic reaction can only proceed in the presence of water or enough

moisture should be available for long time. Therefore fly ash concrete require

longer curing period. Hence to achieve this, the specimens were continuously

cured by complete immersion in water.

4.6 TESTS FOR EVALUATING MICRO STRUCTURAL

PROPERTIES

4.6.1 Water Absorption Test

Concrete cubes of 150mm size were cast along with each

proportion of mix. Three cubes were cast for the control mix and for each

75

percentage of fly ash added exclusively to determine the absorption of water.

Special care was taken to see that the cubes were free from observable cracks,

fissures and shattered edges.

The cube specimens after 90 days of water curing were dried in an

oven at temperature of around 100 to 110 C for 24 hours. The specimens

were allowed to cool in dry air and weighed. This procedure was repeated

until the difference between the two successive readings did not exceed 0.5%

of the lowest weight obtained. This weight was designated as A.

The weighed cube specimens were then immersed in water

approximately at about 20-25 C for 48 hours. The saturated weights of the

specimens were taken. Again the specimens were soaked in water for 24

hours and the weights of the specimens were taken once again to check if the

weights taken after 48 hours and 72 hours were not greater than 0.5% of the

heavier weight. The final surface dry weight was designated as B.

Percentage of water absorption: (B – A) /A x 100

A : Weight of the oven dried concrete specimen.

B : Weight of the surface dried concrete specimen after immersion.

The coefficient of water absorption as per ASTM C 642-97 was

also determined since it is a measure of water permeability

Coefficient of water absorption, Ka = {(Q/A)2}x1/t

Ka = Coefficient of water absorption

Q = Quantity of water absorbed by the oven dried specimen in

time t

A = Total surface area of concrete through which water penetrates

76

Figure 4.19 Concrete specimens cast for various tests are being cured in

water

4.6.2 Determination of Voids Percentage

Similar to that of the water absorption test, three concrete cubes of

150mm size were also cast along with the mix for the control and for each

percentage of fly ash added exclusively to determine the percentage of voids.

Special care was taken again to see that these cubes also were free from

observable cracks, fissures and shattered edges.

The cube specimens after 90 days of water curing were dried in an

oven at a temperature of about 100 to 110 C for 24 hours. The specimens

were allowed to cool in dry air and weighed. This procedure was repeated

until the difference between the two successive readings did not exceed 0.5%

of the lowest weight obtained. This weight was designated as A.

The specimens were then placed in a container filled with tap water

and boiled for about 5 hours and were allowed to cool by natural loss of heat

for not less than 14 hours to a final temperature of about 20 to 25 C. The

surface moisture was removed with a towel and the specimens were weighed.

The soaked, boiled, surface dried weight was designated as C. The specimens

77

were then suspended by a wire in water and the weights were taken. This

weight was designated as D.

Percentage of voids = (C – A)/(C – D) x 100.

where A - Weight of the oven dried concrete sample.

C - Weight of the surface dried concrete sample in air after

immersion and boiling

D - Weight of the concrete sample in water after immersion and

boiling.

4.6.3 Effective Porosity and Bulk Density

For determination of effective porosity of 90 days water cured

concrete samples, specimens of size 83mm diameter and 50mm thick were

cast and used. The specimens after curing were surface dried, weighed and

then kept in an oven at 105 C for 48 hours in order to completely evaporate

the moisture content present in the concrete. After weighing the oven dried

specimen and determining its bulk volume, the effective porosity was

determined by the relation

Effective porosity (%) = {(B-A)/V}x100

where B - saturated mass of the surface dry specimen in air after

immersion

A - mass of oven dried specimen in air

V - Bulk volume of the specimen

Bulk density values of the control concrete and for each percentage

of fly ash added were also determined.

78

4.7 RESULTS OF THE MICRO STRUCTURAL PROPERTIES

TESTS

The values obtained from the various micro structural properties

tests performed on the hardened concrete specimens are tabulated below

Table 4.6 Test values of micro structural properties

S. No. Identification

Coefficient of

water

absorption

x 10-10

m2/sec

Percentage

of voids

(%)

Effective

porosity

%

Bulk density

Kg/m3

1CM - Control mix

(without fly ash)3.50 6.61 14.74 2264

2 FA 5 (fly ash 5%) 2.86 4.96 14.08 2302

3 FA 10 (fly ash 10%) 1.92 3.30 13.44 2340

4 FA 15 (fly ash 15%) 1.78 1.36 12.45 2387

5 FA 20 (fly ash 20%) 1.23 1.07 12.02 2394

6 FA 25 (fly ash 25%) 0.96 0.66 10.06 2404

7 FA 30 (fly ash 30%) 0.78 0.58 10.02 2406

Figure 4.20 Representation of relative coefficient of water absorption

79

Figure 4.21 Representation of relative percentage of voids

Figure 4.22 Representation of relative percentage of porosity

80

Figure 4.23 Representation of relative bulk density values obtained for

various mixes

4.7.1 Discussion on Test Results

The test values obtained on the micro structural properties of fly

ash blended concrete specimens cast with various percentages of fly ash on

the 90th

day of casting reveal that with increase in fly ash percentages, the

pozzolanic reaction is also enhanced and this contributed to making of the

texture of concrete dense, resulting in decrease in water absorption,

percentage of voids and porosity.

At 28 days of curing, at which time only little pozzolanic activity

had taken place, fly ash concrete specimens were found to be more permeable

than plain or normal concrete specimens without fly ash. At 90 days the trend

reversed due to continued pozzolanic reaction between cement paste and fly

ash substantially reduced the permeability in the cementitious matrix by

transforming the larger pores to fine pores.

81

On the other hand, obviously the bulk density of the fly ash blended

concrete increased with increase in incremental percentages of fly ash.

4.8 TESTS FOR EVALUATING DURABILITY

4.8.1 Open Circuit Potential (OCP) Test

The probability of steel reinforcement to corrode was assessed by

measuring the Open Circuit Potential (OCP) of embedded steel with respect

to standard reference electrode. In reinforced concrete structures, concrete act

as electrolyte and the reinforcement develop a potential depending on the

concrete environment which may vary from place to place or from time to

time in the structure. In the vicinity of corrosion of rebar in a structure, the

value of corrosion potential will become increasingly negative. Accordingly,

potential measurements made between a single half cell and the reinforcement

indicates probabilities of corrosion risk in reinforced concrete structures. The

reference electrode used for potential monitoring of steel in concrete was

saturated calomel electrode as it offered good stability.

Table 4.7 OCP measurements in mV for various mixes

Days Control 5% FA 10% FA 15% FA 20% FA 25% FA 30% FA

0 -95 -90 -87 -82 -76 -72 -70

15 -175 -171 -166 -164 -161 -158 -155

30 -286 -280 -272 -264 -250 -230 -225

45 -329 -318 -301 -293 -287 -275 -267

60 -382 -375 -364 -350 -330 -310 -300

75 -459 -435 -410 -388 -370 -346 -331

90 -512 -489 -461 -448 -422 -398 -382

105 -560 -541 -519 -488 -458 -420 -403

82

Figure 4.24 Graphical representation of OCP values observed for

various percentages of fly ash

Figure 4.25 Comparative graphical representation of OCP values

observed between the control and 25% fly ash blended concrete

83

4.8.2 Impressed Voltage Test

As explained in the previous chapter, to determine the corrosion

rate by diffusion of chloride, cylindrical concrete specimens with pre-weighed

rod centrally placed were used. The concrete specimen was immersed in 3.5%

NaCl solution and the rod was made anode (connected to +ve terminal) with

respect to an external stainless steel electrode (connected to –ve terminal)

serving as cathode. On applying a constant impressed voltage from a D.C.

source, the variation of current was recorded with respect to time. A sharp rise

in current indicated the onset of corrosion and cracking of the concrete was

usually visible thereafter. The time taken for initiation of first crack was

considered as a measure of their relative resistance against chloride

permeability and reinforcement corrosion.

Figure 4.26 Impressed Voltage test under progress

84

Table 4.8 Corrosion initiation time observed for various mix specimens

Time in

hours

Current in mA

Control 5% FA 10% FA 15% FA 20% FA 25% FA 30% FA

0 6 6 6 6 6 6 6

280 6 6 6 6 6 6 6

282 7 6 6 6 6 6 6

287 7.4 6 6 6 6 6 6

288 7.5 6 6 6 6 6 6

288 8 6 6 6 6 6 6

290 8 6 6 6 6 6 6

296 10.5 6 6 6 6 6 6

298 10.5 6 6 6 6 6 6

300 13 8 6 6 6 6 6

304 13 8 6 6 6 6 6

308 16 9.5 6 6 6 6 6

310 16 9.5 6 6 6 6 6

315 20 9.5 6 6 6 6 6

325 28 11 8.5 6 6 6 6

338 32 11 8.5 6 6 6 6

348 33 15 10 9 6 6 6

360 34 15 10 9 8 6 6

365 35 15 10 13 10 6.5 6

367 35 15 11 13 12 7 6

369 35 24 11 15 12 8 6

371 35 24 16 15 12 9 6

373 35 24 16.5 18 13 9.5 6

375 35 26 19 21 15 9.5 6

378 35 26 22 22 16 12 6

382 35 30 23 22 22 16 8

386 35 31 28 29 28 20 8.5

390 35 31 29 29 31 20 8.5

394 35 33 32 33 31 23 11

398 35 33 35 33 35 23 15

402 35 33 35 37 38 25 20

410 34 37 37 39 25 21

415 37 38 40 25 21

420 39.5 39 41 25 22.5

85

Figure 4.27 Graphical representation of corrosion initiation time

observed for various mix specimens

Figure 4.28 Comparative graphical representation of corrosion

initiation time between control and 25% fly ash blended

concrete specimens

86

Figure 4.29 Tested specimens of Impressed Voltage test

4.8.3 Linear Polarization Resistance (LPR) Technique

As explained under methodology, it is the best known

electrochemical technique for evaluation of instantaneous corrosion rate in the

laboratory. The calomel electrode, metal cathode and rebar embedded in

concrete were connected to the LPR meter. 20 mV of direct voltage was

applied to the rebar and direct current was measured. The polarization

resistance Rp was obtained by dividing the direct voltage by direct current.

From Rp values, the corrosion currents icorr were calculated and the

corresponding corrosion rates in mmpy were obtained.

87

Table 4.9 Corrosion rates from Linear Polarization Resistance

S. No. Identification

Polarization

resistance

Rp

in Ohms

Corrosion

current

icorr

A/cm2

Corrosion

rate

“mmpy”

1CM - Control mix

(without fly ash)6.695 x 106 7.766 x 10-6 0.0932

2 FA 5 (fly ash 5%) 7.332 x 106 7.092 x 10-6 0.0851

3 FA 10 (fly ash 10%) 7.770 x 106 6.692 x 10

-60.0803

4 FA 15 (fly ash 15%) 8.489 x 106 6.125 x 10

-60.0735

5 FA 20 (fly ash 20%) 9.150 x 106 5.683 x 10-6 0.0682

6 FA 25 (fly ash 25%) 12.093 x 106 4.300 x 10

-60.0516

7 FA 30 (fly ash 30%) 13.055 x 106 3.983 x 10-6 0.0478

4.8.4 Gravimetric Weight Loss Method

The 16mm diameter bars that were cut to specified lengths,

embedded in concrete and utilized for the above three tests were initially

immersed in pickling solution (water + HCl of equal quantity), cleaned and

weighed. At the end of the tests, the specimens were broken and the corroded

rebars were again immersed in pickling solution, cleaned and weighed. The

difference between the initial and final weights were taken as the weight loss

of the specimen and it was converted into reduction in thickness and

expressed as loss in thickness in mm per year (mmpy).

88

Table 4.10 Corrosion rates from Weight Loss method

S. No. IdentificationCorrosion rate

mmpy

1 CM - Control mix (without fly ash) 0.0904

2 FA 5 (fly ash 5%) 0.0816

3 FA 10 (fly ash 10%) 0.0775

4 FA 15 (fly ash 15%) 0.0712

5 FA 20 (fly ash 20%) 0.0656

6 FA 25 (fly ash 25%) 0.0542

7 FA 30 (fly ash 30%) 0.0505

4.9 DISCUSSION ON TEST RESULTS

All the above durability tests performed reveal that due the

decrease in the permeability and porosity of the fly ash blended concrete with

increase in percentages of fly ash, the possibility of penetration of harmful

corroding agents like chlorides in adverse environments is much reduced.

Since the ingress of chlorides through the dense fly ash blended concrete

medium takes more time, the corrosion initiation time also was increased.

With increase in percentage of fly ash, the corrosion initiation time

accordingly increased in the impressed voltage test. The same trend was

observed in OCP and LPR techniques. The corroded rods which were initially

weighed before testing were again weighed after corrosion test by breaking

open the concrete specimen and after thorough cleaning of the cement paste

stuck over the embedded surface of the rods. The weight loss measurements

observed was converted to its equivalent loss in mmpy and this result also

validated the earlier concept. It was determined that 25% replacement of

cement by the fly ash obtained from the Thermal Power Plant not only

enhances the corrosion initiation time by 30% but also improves the

workability and mechanical and micro-structural properties to the optimum.