Download pdf - Study on Durability of Fly Ash Concrete

STUDY ON DURABILITY OF FLYASH CONCRETEBy

A.L.DEEPAK [email protected] [email protected]

ABSTRACT

Concrete, typically composed of gravel, sand, water, and Portland cement, is an

extremely versatile building material that is used extensively worldwide. Reinforced

concrete is very strong and can be cast in nearly any desired shape. Unfortunately,

significant environmental problems result from the manufacture of Portland cement.

Worldwide, the manufacture of Portland cement accounts for 6-7% of the total carbon

dioxide (CO2) produced by humans, adding the greenhouse gas equivalent of 330 million

cars driving 12,500 miles per year.

Fortunately, a waste product can be substituted for large portions of Portland

cement, significantly improving concrete’s environmental characteristics. Fly ash,

consisting mostly of silica, alumina, and iron, forms a compound similar to Portland

cement. High volume of fly ash are used in concrete creates a stronger, more durable

product and reduces concrete’s environmental impact considerably. Due to its strength

and lower water content, cracking is reduced.

When the study deals with the durability of concrete like Acid attack, Alkaline

attack and Sulphate attack which results in volume change, cracking of concrete and the

deterioration of concrete has to be considered. The study reveals that high volume fly ash

concrete show that higher resistance against acid attack, alkaline attack, and sulphate

attack compared with conventional concrete.

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1.0 INTRODUCTION

Fly ash is a byproduct of coal burning power plants - the ash that flies up their

smokestacks and gets captured in giant filters. It can be used to produce stronger, more

durable and more environmentally friendly concrete. When used in concrete, fly ash acts like

cement, and actually replaces a percentage of the Portland cement normally used. Concrete

with high percentages of fly ash looks and finishes the same as regular concrete, with a few

minor adjustments. Fly ash already replaces around 15% of cement in much of the concrete

used today, but we can do much better by using it to replace 50% or more.

2.0 EXPERIMENTAL PROGRAMME

MATERIALS USED

Ordinary Portland Cement (OPC) available in the market conforming to IS 12269 – 1987

was used for casting the specimens. Locally available river sand in dry condition was

used for the preparation of specimens. The grading of sand conforms to Zone-II as per IS

383 – 1970. The specific gravity sand was 2.61 and fineness modulus value was 2.642.

The loose and compacted bulk density values of sand were 1633 and 1765 kg/m3,

respectively.

Crushed granite aggregate conforming to IS: 383–1970 was used for the

preparation of concrete. Coarse aggregate of size 20 mm down, having the specific

gravity value of 2.80 and fineness modulus of 6.59 were used. The loose and compacted

bulk density values of coarse aggregates were 1598 and 1775 kg/m3, respectively. Clean

potable water was used for mixing concrete.

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Fly ash (Class F Type) having specific gravity of 2.15 obtained from Ennore

Thermal Power Plant at Chennai was used for the replacement of cement.

MIX PROPORTIONING

It is the process of selecting suitable ingredients of concrete and determining their

relative quantities with the purpose of producing economical concrete. The mix

proportion of 1: 1.49: 3.26 [W/C = 0.50] and 1: 1.36: 2.09 [W/C = 0.42], respectively

for the controlled concrete of M20 and M40 grades were arrived as per the specifications

and used throughout the study. Fly ash concrete mixtures were then proportioned by

replacing 10%,20%,30% and 40% of cement with Ennore fly ash ( Class F ) and the

volume of concrete was adjusted by reducing the sand content

PREPARATION OF THE TEST SPECIMENS

A tilting type mixture machine was used to mix the ingredients of concrete mixtures.

Steel moulds were used to cast the specimens and table vibrator was employed to

compact the concrete in the moulds. Curing was started immediately after top surface of

concrete in the moulds became hard and devoid of free water, by covering the moulds

with wet gunny clothes. At the age of about 24 hours after casting, the specimens were

demoulded and kept submerged in water tank for curing till testing. 100mm cubes were

prepared for evaluating compressive strength, water absorption, acid attack, alkaline

attack, and sulfate attack and temperature effect on compressive strength. The cylinder

specimens of the size 100mm diameter and 200mm high. for split tensile strength,

150mm. diameter and 300mm. high for young’s modulus and 100 x 100 x 500mm

prisms for flexural strength specimens were cast.

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3.0 RESULTS AND DISCUSSIONS

3.1 ACID RESISTANCE OF CONCRETE

The tests results of the M20 and M40 have summarized in Tables 3.1.1 and 3.1.2

respectively. The loss of compressive strength in all cases ahs been expressed as a

percent of the strength of concrete at 60 days immersion in the Hydrochloric acid (HCl)

solution. It was found that under HCl attack, the percentage loss of compressive strength

was 20.9%, 20.5%8, 16.7%, 12.7% and 11.8% at the flyash content of 0, 10, 20, 30 and

40% respectively for the M20 mix without superplasticizers, 20.0%, 14.3%, 13.1%,

7.69% and 5.90% respectively. For M20 mix with naphthalene based superplasticizers

and 25.0%, 24.6%, 19.5%, 17.5% and 14.6% respectively for the mix with melamine-

based superplasticizers.

Table 3.1.1 Effect of Acid Attack on Compressive strength of M20 Concrete mixtures

Mix Flyash

content

Dosage of

SP

(by weight

of binder)

Loss in Compressive strength at 60 days curing in acid

solution( % )

Without

SP

With Naphthalene

based SP

With Melamine based

SP

A 0% 0.9% 20.9 20.0 25.0

AFA10 10% 1.1% 20.5 14.3 24.6

AFA20 20% 1.3% 16.7 13.1 19.5

AFA30 30% 1.5% 12.7 7.69 17.5

AFA40 40% 1.7% 11.8 5.90 14.6

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Note: * All the values are the average of the three identical specimens

** SP - Superplasticizers

Table 3.1.2 Effect of Acid Attack on Compressive strength of M40 Concrete mixtures

Mix

Flyash

content

Dosage of

SP

(By weight

of binder)

Loss in Compressive strength at 60 days curing in

acid solution (%)

Without

SP

With Naphthalene

based SP

With Melamine

based SP

B 0% 0.9% 41.0 29.8 23.5

BFA10 10% 1.1% 40.7 27.9 20.4

BFA20 20% 1.3% 38.9 24.0 13.3

BFA30 30% 1.5% 38.6 23.1 13.9

BFA40 40% 1.7% 35.0 15.4 4.2


** SP - Superplasticizer

In M40 concrete mixtures the loss of compressive strength at 0, 10, 20, 30 and 40 percent

fly ash are 41.0%, 40.7%, 38.9%, 38.6% and 356.0% respectively for the mix without

superplasticizers, 29.8%, 27.9%, 24.0%, 23.1% and 15.4% respectively for naphthalene

based mix and 23.5%, 20.4%, 13.3%, 13.9% and 4.2% respectively. For M40 concrete

mixtures with melamine based superplasticizers. The loss of compressive strength

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attributed to the presence of greater amount of Calcium hydroxide, Ca (OH) 2, in both the

mixes without and with lower quantity of fly ash content in concrete mixtures.

3.2.ALKALINE RESISTANCE OF CONCRETE

The results of alkaline resistance of concrete are in graphs 3.2.1 and 3.2.2.

0

5

10

15

20

25

30

35

A AFA10 AFA20 AFA30 AFA40

Flyash content (%)

Los

sin

Com

pres

sive

stre

ngt

h(%

)

WSP NSP MSP

Fig. 3.2.1 Effect of Alkaline Attack on Compressive strength of M20 Concrete mixtures

0

10

20

30

40

B BFA10 BFA20 BFA30 BFA40

Flyash content (%)

Loss

inC

ompr

essi

vest

reng

th(%

)

WSP NSP MSP

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The addition of fly ash content of 10, 20, 30 and 40 percent to the concrete mix of M20

shows the percentage loss of compressive strength of 20.9%, 21.8%, 22.2%, 12.7% and

2.9%, respectively for the mix without superplasticizers. The addition of both types of

superplasticizers not showing much resistance against the strength loss of concrete cubes.

But, higher amount of fly ash content shows higher resistance against alkaline attack than

the smaller quantity of fly ash.

In M40 concrete mix without superplasticizers shows the percentage loss of

compressive strength are 18.0%, 23.7%, 25.2%, 11.4% and 0% at 0, 10, 20, 30 and 40

percent of fly ash content, respectively. The additions of melamine-based mixtures are

showing the percentage of loss of strength of 28.3%, 20.4%, 16.8%, 8.3% and 0%,

respectively. The naphthalene-based superplasticizers shows nearly same performance of

melamine based mix up to 20% of fly ash content.

3.3 SULFATE RESISTANCE OF CONCRETE

The effect of fly ash content on the sulfate resistance of concrete was studied using fly

ashes at 10% to 40% replacement of cement. Percentage loss in strength decreases with

increase of fly ash contents.

In M40 concrete mixtures also the increase in fly ash content shows the decrease in

percentage loss of compressive strength. In the case of alternate wetting and drying of

specimen (cyclic attack) the 30% and 40% of fly ash content show the full strength of

M20 specimens without any losses. In M40 concrete mixtures the same trend was

observed at 40% fly ash content.

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The percentage loss in compressive strength at 0, 10, 20, 30 and 40 percent fly ash

content are 6.9%, 5.1%, 2.8%, 2.5% and 0% for M20 concrete mixtures without

superplasticizers. From the tables, it is observed that the additions of superplasticizers

show the good resistance against sulfate attack for both the concrete mixtures. The

naphthalene based superplasticized M20 concrete showed the greater loss of compressive

strength compared with the concrete mixtures with and without melamine based

superplasticizer.

Table 3.3.1 Effect of Sulfate Attack in cyclic on Compressive strength of M20

Concrete mixtures

Mix Flyash

content

Dosage of

SP

(by weight

of binder)

Loss in Compressive strength at 60 days curing ( % )

Without

SP

With

Naphthalene

based SP

With Melamine

based SP

A 0% 0.9% 12.5 7.8 15.4

AFA10 10% 1.1% 10.1 5.6 8.7

AFA20 20% 1.3% 5.1 5.5 6.8

AFA30 30% 1.5% 0 0 0

AFA40 40% 1.7% 0 0 0



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Table3.3.2 Effect of Sulfate Attack in cyclic on Compressive strength of M40

Concrete mixtures

Mix Flyash

content

Dosage of

SP

(By weight

of binder)

Loss in Compressive strength at 60 days curing ( % )

Without

SP

With

Naphthalene

based SP

With Melamine

based SP

B 0% 0.9% 16.4 10.5 9.7

BFA10 10% 1.1% 7.6 7.3 4.4

BFA20 20% 1.3% 3.7 1.5 0

BFA30 30% 1.5% 2.3 0 0

BFA40 40% 1.7% 0 0 0



In M40 concrete mixtures the addition of melamine based superplasticizer with

30% and 40% fly ash content show the zero percentage of loss of compressive strength.

Due to the alternate wetting and drying of specimens the percentage loss of compressive

strength is more than the continuous sulfate attack. When fly ash content increases the

losses are decreases. Melamine based superplasticized concrete is showing higher

resistance in M40 concrete mixtures for both the cases. But, in M20 concrete mixtures

without superplasticizer performing well compared with superplasticized concrete. In

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cyclic test, the concrete mixtures of M20 with naphthalene base show higher resistance to

sulfate attack.

CONCLUSIONS

1. In M20 mix both the types of superplasticizers are not showing much

resistance against the strength loss compared with M20 mix without

superplasticizers. Between the two types of superplasticizers used the

melamine based M40 mix shows good resistance even at 30% and 40% of fly

ash content.

2. The replacement of cement with fly ash content of would render both concrete

concrete mixtures more durable. This observation is more than the maximum

limit of 25 percentage of class F fly ash in concrete mixes recommended by

the ACI committee.

References

1. Shikoku Island Concrete Research Association: Report by Self-CompactingConcrete Research Committee, "Self-Compacting Concrete in Shikoku Island"2000 to 2002, 2002

2. “MASTER BUILDER” – MAGAZINE

3. INDIAN CONCRETE JOURNAL

4. Websites referred:

i. www.thecivilengineer.com

ii. www.concrete.comFaaDoOEngineers.com