Study on Durability of Fly Ash Concrete

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Durability of concrete

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  • STUDY ON DURABILITY OF FLYASH CONCRETEBy

    A.L.DEEPAK K.J.V.S.PRAVEENdeepak_amerineni@yahoo.com Praveen_satyakommuri@yahoo.com

    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 concretes 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 concretes 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: 3831970 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 youngs 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

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

    ** 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

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

    ** SP - Superplasticizers

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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

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

    ** SP - Superplasticizers

    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.comFaaDoO

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