Mechanical Properties of Concrete with Admixed Recycled Aggregate

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Mechanical Properties of Concrete with Admixed Recycled Aggregate

Shin Elizabeth Shaji

Department of Civil Engineering

Mar Baselios College of Engineering and Technology Thiruvananthapuram, India

Mr.Jithin J S

Department of Civil Engineering

Mar Baselios College of Engineering and Technology Thiruvananthapuram, India

Abstract The concrete industry alone uses 20 billion tons of aggregates, 1.5 billion tons of cement and 800 million tons of water. The overexploitation of natural aggregate may reduce the resources to a crucial level, compromising the needs of future generation. Thus the necessity of conservation of natural aggregate leads to the use of recycled aggregate and this may provide sustainable development. But the use of recycled aggregate will cause reduction in strength and durability due to the adhered cement mortar. This strength can be improved by the addition of mineral admixture such as metakaolin. In this study, metakaolin is added as a partial replacement of recycled aggregate. The fresh and hardened properties of four mixes such as M20, M30, M40 and M50 mixes with admixed recycled aggregate is studied.

Keywords: Natural aggregate, Recycled aggregate, Sustainable development, Metakaolin.

  1. INTRODUCTION

    1. General

      Concrete is a major construction material and plays a crucial role in the improvement of infrastructures such as highways, bridges, buildings etc. It is estimated that the total annual concrete production over the world is more than 10 billion tons[1]. The concrete industry alone uses 20 billion tons of aggregates, 1.5 billion tons of cement and 800 million tons of water. The idea of the conservation of natural aggregate (NA) has been largely ignored despite the fact that coarse aggregates make up 4050% of a concrete mix by volume while cement makes up about 10%[2]. Volume of cement and concrete production probably will continue to increase. The overexploitation of NA may reduce the resources to a crucial level, compromising the needs of future generation. Thus the necessity of conservation of NA leads to the use of recycled aggregates (RA) and this may provide sustainable development. Sustainable development means development that satisfies the present needs without limiting the possibilities of fulfilling the needs in the future. The use of aggregate obtained from crushed concrete is an example of recycling and conservation of raw materials. It is estimated that about 450 million tons of construction waste is generated every year with only 28% recycled and 72% disposal, which has caused environmental pollution and hazard of humans health. Hence, maximizing the use of RA in construction sites is becoming economically important and environmentally necessary[3].

    2. Recycled Aggregate (RA)

      RA mainly differs from NA as it is composed by two different materials: NA and residue old cement mortar attached. Old cement mortar is the origin of the worse properties of RA. Recycled coarse aggregate (RCA) tends to have decreased specific gravity, increased water absorption, and increased abrasion loss compared to NA, since they contain the mortar paste from the original concrete. Because of the increased water absorption, RCA also has greater water demand which can be resolved by increasing the amount of mix water, pre-soaking the aggregates or by using mineral admixtures. Generally, RAC has lower compressive strength, less stiffness, increased creep, and increased shrinkage as compared to NA concrete. At full aggregate replacement, the concrete losses its compressive strength from 10% to 20%. In comparison, the effect of RCA on the stiffness of concrete is greater with losses up to 33% at full replacement. There are various methods to manufacture RCA with better properties, such as removal of adhered mortar paste via microwave treatment, or acetic acid treatment, or treating with polymer emulsions[3,4].

    3. Admixed Recycled Aggregate

      Mineral admixtures can be successfully used as partial replacement of cement in order to mitigate the poor performance of the RCA, thus obtaining admixed recycled aggregate. Mineral admixtures such as fly ash (FA), silica fume (SF), metakaolin (MK) and ground granulated blast slag (GGBS) have been utilized for many years either as supplementary cementitious materials in Portland cement concrete or as a component in blended cement. Generally, due to their high pozzolanic activity, the mineral admixture improves the mechanical and durability properties of the concrete [5]. Here the mineral admixture metakaolin is added to the RCA and mixed well. Then the admixed coarse aggregate is put for two stage mixing (TSM). TSM divides the mixing process into two parts and proportionally splits the required water into two, which are added at different timing.

  2. OBJECTIVE The objective of this study is:

      • To study the mechanical properties of concrete with admixed recycled aggregate

    1. General

  3. METHODOLOGY

    on Indian standards and foreign standards. Their properties and specifications are given in the following section

    In order to accomplish the objective of the study, the following methodologies are to be adopted.

    1. Procurement of Materials

    2. Testing of Materials

    3. Mix Design

    4. Determination of Fresh and hardened properties

      1. Procurement of Materials

        The following materials are procured

        1. Portland Pozzolana Cement

        2. Natural Aggregate

        3. Super Plasticizer

        4. Metakaolin

        5. Recycled Coarse Aggregate

      2. Testing of Materials

        1. Cement

          Specific gravity, fineness, initial and final setting time as per IS 4031(part 5) 1988, consistency and compressive strength tests were conducted as per IS 1489 (part 1) 1991.

        2. Aggregate

          Bulk density, water absorption, fineness modulus, sieve analysis as per IS 383-1970 and specific gravity tests were conducted as per IS 2386 (part III)-1963

      3. Mix Design

        In this study, M20, M30, M40 and M50 grades of concrete are used. The mix was designed using IS 10262:2009. Slump is fixed as 90mm. The fresh properties of the mix were evaluated by measuring the slump according to IS 1199-1959 and the hardened properties were evaluated according to IS 516-1959

      4. Determination of Fresh and Hardened properties

      The strength and behaviour of concrete containing admixed recycled aggregate were studied in M20, M30, M40 and M50 mixes. These mixes were prepared using normal coarse aggregate. The coarse aggregate is then replaced with recycled coarse aggregate, and obviously the strength goes down. Mineral admixture, metakaolin, is used as partial replacement of recycled aggregate. The fresh properties were evaluated by measuring slump according to IS 1199-1959. Super plasticizer was added for keeping the workability constant. The hardened properties were evaluated according to IS 516-1959 and IS 5816-1999 and the optimum dosage of mineral admixture was found from the hardened properties.

  4. EXPERIMENTAL INVESTIGATION

    1. Material Properties

      The materials used for the study were cement, manufactured sand as fine aggregate, coarse aggregate, recycled coarse aggregate, metakaolin, water and superplasticizer. The first step in the experimental investigation is the analysis of the properties of the above ingredients, and was carried out based

      1. Cement

        Cement paste is the bindr that holds the aggregate together and reacts with mineral materials in hardened mass. Portland Pozzolana Cement is used for the entire experimental work.

        TABLE I.PROPERTIES OF CEMENT

        Test

        Test Results

        IS code specification

        Fineness

        4

        < 10

        Specific gravity

        3

        3.15

        Standard consistency

        34%

        Setting time

        Initial

        180 min

        >30 min

        Final

        250 min

        <600 min

        28 days

        54

        Not less than 53 MPa

      2. Fine Aggregate

        Manufactured sand passing through IS sieve 4.75 conforming to zone II is used. The different results were tabulated in table II

        Properties

        Values

        Bulk density (gm/cc)

        1.958

        Specific gravity

        2.75

        Sieve analysis

        Confirming to Zone II

        Water absorption

        6.01%

        TABLE II. PROPERTIES OF FINE AGGREGATE

        .

        Fig 1.Grading curve of fine aggregate

      3. Coarse Aggregate

        Various tests were conducted as per IS specifications and results are provided in Table III.

        TABLE III. PROPERTIES OF COARSE AGGREGATE

        Properties

        Values

        Bulk density (gm/cc)

        1.65

        Specific gravity

        2.87

        Sieve Analysis

        Confirms to IS specification

        of IS 383-1970

        Water Absorption (%)

        0.63

        SAMPLE

        20

        0

        1

        10

        Sieve Size (mm)

        100

        40

        UPPER LIMIT

        LOWER LIMIT

        120

        100

        80

        60

        % Finer

        Fig 2.Grading curve of coarse aggregate

        f) Superplasticizer

        TABLE VI. PROPERTIES OF SUPERPLASTICIZER

        Properties

        Values

        Specific gravity

        1.06 to 1.12 at 300oC

        Chloride content

        Nil

        Appearance

        Liquid

        Colour

        Beige

        Chemical Composition

        Poly carboxylate ether

        pH

        6 to 8

    2. Mix Design

      The mix design of M20, M30, M40 and M50 are shown in table VII

      1. Recycled Aggregate

        The properties of recycled aggregate is shown in table IV

        TABLE IV.PROPERTIES OF RECYCLED AGGREGATE

        Properties

        Values

        Bulk density (gm/cc)

        1.53

        Specific gravity

        2.69

        Water absorption (%)

        2.1

        Fineness modulus

        8.166

        Sieve analysis

        Conforms to IS specification 383:1970

        Fig 3. Grading curve of recycled aggregate

      2. Metakaolin

      Physical properties of metakaolin is shown in table V

      TABLE VII. MIX DESIGN

      Trial mix

      Cement content (kg/m3)

      Mix proportion

      Water cement ratio

      Compressive strength (N/mm²)

      M20

      340

      1: 1.97: 3.81

      0.45

      28.10

      M30

      360

      1: 1.93: 3.58

      0.45

      38.89

      M40

      390

      1: 1.81: 3.36

      0.4

      48.65

      M50

      420

      1:1.71:3.29

      0.35

      58.10

    3. Hardened Properties

      1. Cube Compressive strength

        The cube strength was evaluated according to IS 516-1959 (Reaffirmed 2004). The test was conducted on a 2000 kN compression testing machine.

      2. Cylinder Compressive Strength

        The cylinder strength was evaluated according to IS 516- 1959 (Reaffirmed 2004). The test was conducted on a 1000 kN universal testing machine.

      3. Modulus of Elasticity

        The modulus of elasticity was evaluated according to IS 516- 1959 (Reaffirmed 2004). The test was conducted on a 1000 kN universal testing machine.

      4. Splitting Tensile Strength

        The splitting tensile strength was evaluated according to IS 5816-1999 (Reaffirmed 2004). The test was conducted in a 2000 kN compression testing machine. The splitting tensile strength is calculated using Eqn.1

        TABLE V. PHYSICAL PROPERTIES OF METAKAOLIN

        Property

        Test result of metakaolin

        Blaine value (cm2/g)

        22000-25000

        Specific gravity

        2.6

        where,

        2

        =

        (1)

        = splitting tensile strength in N/mm P = maximum load in N

        d = diameter of the specimen in mm l = length of specimen in mm

      5. Modulus of Rupture

    The modulus of rupture was evaluated according to IS 516- 1959 (Reaffirmed 2004). The test is conducted in a 100 kN flexure testing machine. Modulus of rupture is calculated by Eqn.2

    TABLE IX. HARDENED PROPERTIES OF M20 MIX

    Where,

    =

    2

    Mix

    Cylinder compressive strength (N/mm²)

    Split tensile strength (N/mm²)

    Modulus of elasticity (N/mm²)

    Modulus of rupture (N/mm²)

    M20

    23.65

    3.11

    2.32 X 104

    4.05

    M20R100

    19.3

    2.46

    1.9 X 104

    3.24

    M20R100-10

    28.92

    3.89

    2.86 X 104

    5.15

    (2)

    = modulus of rupture P = maximum load

    l = length of specimen between supports b = width of the specimen

    d = depth of the specimen

  5. RESULTS AND DISCUSSIONS

    1. Fresh Properties

      The fresh properties were evaluated by measuring slump according to IS 1199-1959 (Reaffirmed 2004). Since the slump was observed to be low, superplasticizer was added to maintain the slump between 80-90mm.

    2. Hardened Properties

      By using recycled aggregate by 100%, the strength of mix reduces. To overcome this, mineral admixture such as metakaolin is added to it. The optimum percentage of metakaolin was found out from cube compressive strength and then the hardened properties of control mix, mix with 100% recycled aggregate and mix with 100% aggregate & optimum percentage of metakaolin was found out.

      1. M20 Mix

        The optimum percentage of metakaolin in M20 mix is shown in table VIII.

        TABLE VIII.OPTIMUM PERCENTAGE OF METAKAOLIN IN M20 MIX

        .

        From Table VIII, it is clear that the optimum percentage of metakaolin in M20 mix was found out to be 10% by weight of recycled aggregate. The hardened property of other mixes is shown in table IX.

      2. M30 mix

        The optimum percentage of metakaolin in M30 mix is shown in table X

        TABLE X. OPTIMUM PERCENTAGE OF METAKAOLIN IN M30 MIX

        Mix

        Percentage of metakaolin

        ompressive strength (N/mm2)

        M30

        38.89

        M30R100

        32.52

        M30R100-2.5

        2.5

        33.98

        M30R100-5

        5

        34.54

        M30R100-7.5

        7.5

        37.20

        M30R100-10

        10

        43.17

        M30R100-12.5

        12.5

        38.25

        The optimum percentage of metakaolin was found out to be 10% by weight of recycled aggregate. The hardened properties of M30 mix is shown in table XI

        Mix

        Percentage of metakaolin

        Compressive strength (N/mm2)

        Mix

        Cylinder compressive strength (N/mm²)

        Split tensile strength (N/mm²)

        Modulus of elasticity (N/mm²)

        Modulus of rupture (N/mm²)

        M20

        28.10

        M20R100

        23.43

        M20R100-2.5

        2.5

        24.63

        M20R100-5

        5

        25.13

        M30

        31.50

        3.26

        2.73 X 104

        4.3

        M20R100-7.5

        7.5

        26.46

        M30R100

        25.88

        2.62

        2.29 X 104

        3.57

        M20R100-10

        10

        32.33

        M20R100- 12.5

        12.5

        30.51

        M30R100-10

        39.08

        4.16

        3.38 X 104

        5.21

        TABLE XI.HARDENED PROPERTIES OF M30

      3. M40 Mix

        The optimum percentage of metakaolin in M40 mix is shown in table XII.

        Mix

        Percentage of metakaolin

        Compressive strength (N/mm2)

        M40

        48.65

        M40R100

        40.09

        M40R100-2.5

        2.5

        43.86

        M40R100-5

        5

        45.23

        M40R100-7.5

        7.5

        47.98

        M40R100-10

        10

        51.05

        M40R100-12.5

        12.5

        48.53

        TABLE XII. OPTIMUM PERCENTAGE OF METAKAOLIN IN M40

        The optimum percentage of metakaolin in M40 mix was found out to be 10% by weight of recycled aggregate. The hardened properties of M40 mix is shown in table XIII

        Mix

        Cylinder compressive strength (N/mm²)

        Split tensile strength (N/mm²)

        Modulus of elasticity (N/mm²)

        Modulus of rupture (N/mm²)

        M40

        39.83

        3.76

        3.16 X 104

        5.26

        M40R100

        32.95

        3.04

        2.59 X 104

        4.23

        M40R100-10

        48.28

        4.66

        3.98 X 104

        6.52

        TABLE XIII.HARDENED PROPERTIES OF M40

      4. M50 Mix

    The optimum percentage of metakaolin in M50 mix is shown in table XIV.

    Mix

    Percentage of metakaolin

    Compressive strength (N/mm2)

    M50

    58.10

    M50R100

    50.55

    M50R100-2.5

    2.5

    53.48

    M50R100-5

    5

    55.48

    M50R100-7.5

    7.5

    63.28

    M50R100-10

    10

    57.23

    M50R100- 12.5

    12.5

    54.89

    TABLE XIV.OPTIMUM PERCENTAGE OF METAKAOLIN IN M50 MIX

    The optimum percentage of metakaolin in M50 mix was found out to be 7.5% by weight of recycled aggregate. The hardened properties of M0 mix is shown in table XV

    TABLE XV. HARDENED PROPERTIES OF M50 MIX

    Mix

    Cylinder compressive strength (N/mm²)

    Split tensile strength (N/mm²)

    Modulus of elasticity (N/mm²)

    Modulus of rupture (N/mm²)

    M50

    47.06

    5.21

    3.57 X 104

    6.2

    M50R100

    38.82

    4.16

    3.02 X 104

    5.05

    M50R100-7.5

    55.76

    6.72

    4.32 X 104

    7.87

  6. CONCLUSIONS

The following are the conclusions obtained from this study.

  • By replacing coarse aggregate by 100% recycled coarse aggregate, the hardened properties were reduced by about 15-20% in every mix.

  • By the addition of metakaolin, the hardened properties were increased by 20-30% in each mix.

  • The optimum percentage of metakaolin was found to be 10% in M20,M30 and M40 & 7.5% by weight of recycled aggregate in M50 mix on the basis of compressive strength

ACKNOWLEDGEMENT

The authors would like to express their sincere gratitude to the Principal and Department of Civil Engineering, MBCET, Thiruvananthapuram for the support and facilities provided for the completion of this work.

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