Hybrid Fibre Reinforced Geopolymer Concrete Beam under Ambient Curing


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Hybrid Fibre Reinforced Geopolymer Concrete Beam under Ambient Curing

Devika C P

M. Tech Student Dept. of Civil Engineering

YCET, Kollam, India

Mrs. Deepthi R Nath

Asst. Professor Department of Civil Engineering

YCET Kollam, Kerala.India

Abstract Concrete is the most common building material in the world and its use has been increasing during the last century as the need for construction projects has escalated. The environmental problems caused by cement production can be reduced by finding an alternate material. One of the potential materials to substitute for conventional concrete is geopolymer concrete (GPC). GPC is an inorganic alumino- silicate polymer synthesized from predominantly silicon, aluminum and by-product materials such as fly ash. Utilization of fly ash and Ground Granulated Blast Slag as an alternative material in concrete reduces the use of OPC in concrete. Evolution of geopolymer concrete cured at ambient temperature broadens its suitability and applicability to concrete based structures. When steel fibres are added to this special concrete it improves the ductile behavior and energy absorption capacity to a great extent. The main objective of this study is to investigate the impact of steel fibres and hybrid polypropylene- steel fibres on the mechanical, shear and flexural behavior of GPC. Crimped steel fibre with varying percentages (0%, 0.25%, 0.5%, 0.75% & 1%) is adopted in this study. And then polypropylene fibre is added to the optimum steel fibre mix with varying percentages (0%, 10%, 20%, 30% & 40%). The addition of bres changes its brittle behavior to ductile with signicant improvement in tensile strength, tensile strain, toughness and energy absorption capacities. For curing, temperature was fixed at room temperature for 24 hours. The concrete specimens were tested for mechanical properties of concrete namely cube compressive strength, split tensile strength, flexural strength and other tests were conducted for cement, chemical admixture, coarse aggregate & fine aggregate. The results of the experimental program reveal that mechanical properties of GPC are improved with the addition of fibres and also improve the load carrying capacity of beams.

Keywords Geopolymer concrete, Steel fibre reinforced concrete, Hybrid fibre reinforced concrete, Steel fibre, Polypropylene fibre

  1. INTRODUCTION

    Portland cement concrete is a mixture of Portland cement, aggregates, and water. Concrete is the most often- used construction material throughout the world because of its mouldability, durability, and resistance to fire and energy efficiency. Demand for concrete as construction material is on the increase and so is the production of cement. The production of one tonne of cement liberates about one tonne of CO2 to the atmosphere.

    In order to address environmental effects associated with Portland cement, there is need to develop alternative binders to make concrete. The recent environmental awareness in construction industry promotes the use of alternative binders to partially or fully replace the cement.

    One of the efforts to produce more environmentally friendly concrete is to replace the amount of Portland cement in concrete with by-product materials such as fly ash. Another effort to make environmentally friendly concrete is the development of inorganic alumina-silicate polymer, called Geopolymer, synthesized from materials of geological origin or by-product materials such as fly ash that are rich in silicon and aluminium. In the future, fly ash production will increase, especially in countries such as China and India. Accordingly, efforts to utilize this by- product material in concrete manufacture are important to make concrete more environmentally friendly.

    Concrete exhibits brittle behaviour due to its low tensile strength. The addition of bres, either short or continuous, changes its brittle behaviour to ductile with signicant improvement in tensile strength, tensile strain, toughness and energy absorption capacities. The binder in the Fibre Reinforced Cement Composites (FRCCs) is mainly Portland cement. Efforts have been made to replace the cement based binder in the current FRCC with geopolymeric binder resulting in Fibre Reinforced Geopolymer Composites (FRGCs), which is greener than the former one.

    The objectives of the work can be summarized as follows:

    • To develop the proper mix proportion for geopolymer concrete.

    • To study the effect of steel fibres on the mechanical properties of fly ash based GPC and find out its optimum.

    • To study the effect of hybrid fibres on the flexural and shear behavior of steel fibre reinforced GPC.

    • To compare the load deflection behavior, first crack load, crack pattern and failure mode, ductility index, energy absorption capacity and ultimate load of HFRGPC beams with GPC beams.

  2. PRELIMINARY INVESTIGATION

    It includes the material characterization, mix design, properties of fresh concrete and properties of hardened concrete. The main objective of the study was to obtain the mix proportions for GPC and find out the optimum percentage of steel fibre. For the same purpose the material properties of the constituent materials were first determined.

    1. Test on constituent materials

      Flyash : Class F flyash of specific gravity 2.36 obtained from Mettur thermal power plant was used for the experiments.

      Ground Granulated Blast Furnace Slag : Obtained from steel plant, Karnataka of specific gravity 3.08 was used for the study. 50% of flyash was replaced with GGBS in the study.

      Fig.1 GGBS

      Fine aggregate : M sand is used as fine aggregate. M sand passing through 4.75mm IS sieve conforming to grading zone II of IS 383:1970 was used. Specific gravity and fineness modulus of Sand used were 2.38 and 2.84 respectively.

      Coarse aggregate : Coarse aggregate of maximum size 20 mm from local source was used.

      Alkaline liquid: A combination of sodium hydroxide and sodium silicate solutions was used as the alkaline liquid to activate fly ash. A sodium hydroxide solution was prepared by dissolving sodium hydroxide flakes in water. It was obtained from venad lab equipments and chemicals, kollam.

      Fig 2 Sodium hydroxide flakes

      Steel Fibre : Crimped steel fibres having diameter 0.5 mm and length 25 mm were used for the present study. fibre was purchased from STEWOLS INDIA (P) Ltd.

      Fig.3 Crimped steel fibre

      Polypropylene Fibre: Locally available Polypropylene fibres of length 12mm and aspect ratio 318 was used for the study.

      Fig.4 Polypropylene fibre

      Superplasticizer: Naphthalene based superplasticizer Conplast SP-430, supplied by M/s Fosroc Chemical (India) Pvt. Ltd. was used as superplasticizer. Water: Potable water is generally considered as being acceptable. Hence water available in the college water supply system was used for casting. Reinforcing bars : Main reinforcement consists of 10mm and 8mm HYSD steel bars of Fe 415 grade. 6mm steel bars were used as stirrups.

    2. Mix Design

    Inorder to arrive the mix proportion for the present study, the optimum values of different parameters were adopted from previous literature. The previous studies on Geopolymer concrete. (M I Abdul Aleem and P D Arumairaj, 2012) used a mix proportion of fly ash: Fine Aggregate: Coarse Aggregate are 1:1.5:3.3 with a solution (NaOH & Na2SiO3 combined together) to fly ash ratio of

      1. Trail mixes were arrived by slightly modifying the amount of solution. The ratio of activator solutio-to flyash was selected 0.7 as the obtained mix with 0.7 was well

        workable. The details of the mix proportion for 1m3 of concrete which has finalized is given in Table 1

        TABLE 1 DETAILS OF MIX

        Materials

        Quantity (kg/m3)

        Coarse aggregate

        1260

        Fine aggregate

        540

        Fly ash

        171.43

        GGBS

        171.43

        NaOH

        24.02

        Na2SiO3

        183.67

        Water

        49.45

  3. MIX DESIGNATION OF DIFFERENT MIXES WITH VARYING PERCENTAGE OF STEEL

    FIBRE

    The concrete specimens were prepared by varying the proportion of steel fibres in the geopolymer concrete mix by adding the fibres from 0.25% to 1% by volume of concrete. Mix designation is presented in Table 2. Workability test were also carried out for each mix. The mechanical properties studied were cube compressive strength test, splitting tensile strength and flexural strength.

    TABLE 2. MIX DESIGNATION

    Sl. No.

    Designation

    Steel Fibre (%)

    1

    GPC

    0

    2

    SFRGPC 1

    0.25

    3

    SFRGPC 2

    0.50

    4

    SFRGPC 3

    0.75

    5

    SFRGPC 4

    1

    Fig.5 Casting of specimens

    Fig. 6 Cured Specimens

    From critical evaluation of hardened properties of SFRGPC it can be seen that the addition of steel fibre increased the compressive strength, splitting tensile strength and flexural strength.

    TABLE 3 CRITICAL EVALUATION ON STRENGTH OF

    SFRGPC MIX

    Mix designation

    Cube compressive strength (N/mm2)

    28 days

    Flexural strength (N/mm2) 28 days

    Splitting tensile strength (N/mm2) 28 days

    GPC

    43.51

    4.61

    2.44

    SFRGPC1

    45.62

    5.23

    2.78

    SFRGPC2

    51.24

    6.84

    3.84

    SFRGPC3

    53.21

    7.26

    3.92

    SFRGPC4

    54.32

    7.28

    4.14

    The increase was significant for mix with 0.5% steel fibre. And also the further increase in the amount of steel fibre resulted in poor workability, which mainly affects the further increase in flexural strength. Thus from the above discussion, SFRGPC2 can be selected as optimum steel fibre reinforced GPC mix for the further study.

  4. EXPERIMENTAL INVESTIGATION

    The main aim of the experimental investigation was to study the ductility and energy absorption capacity of fibre reinforced geopolymer concrete beams. The influence of steel and polypropylene fibre on first crack load, load deflection behavior, cracking pattern, energy absorption capacity, ultimate load and failure mode were studied. In the present study the effect of steel and polypropylene fibre in the flexural and shear behavior of GPC beams were studied.

        1. Mix Proportion Of Hybrid Fibre Reinforced Geopolymer Concrete (HFRGPC)

          Hybrid fibre composites were prepared by replacing crimped steel fibre having diameter 0.45mm, length 30mm and aspect ratio 66 with polypropylene fibre of length 12mm in volume fractions of 10%, 20%, 30%, 40% in the optimum steel fibre mix.

          TABLE 4 MIX DESIGNATION OF HFRGPC MIX

          Sl.

          No.

          Designation

          Steel Fibre (%)

          Polypropylene Fibre (%)

          1

          HFRGPC 1

          90

          10

          2

          HFRGPC 2

          80

          20

          3

          HFRGPC 3

          70

          30

          4

          HFRGPC 4

          60

          40

        2. Details of Specimens

          The specimens are standard cubes of 150mm side, cylinders of diameter 150mm and 300mm height, beams of size 500x100x100mm and 1200x100x150mm. Details of number of specimens are given in Table 5.

          TABLE 5 DETAILS OF SPECIMEN

          Sl.

          No.

          Specimen

          Property

          Size(mm

          )

          Number s

          1

          Cube

          Compressiv e strength

          150 x150 x150

          54

          2

          Cylinder

          Splitting tensile strength

          150mm diameter and 300mm height

          27

          3

          Beam

          Flexural strength

          500 x100 x100

          27

          4

          Large beam

          Flexural and shear crack pattern

          1200×10

          0x150m m

          24

          Total

          132

        3. Preparation and casting of specimens

          For each mix of GPC, SFRGPC and HFRGPC mix six concrete cubes of size 150x150x150mm were casted for compressive strength test, three cylinders of 150mm diameter and 300mm height for splitting tensile strength test and three beams of size 500x100x100mm for flexural strength test were casted.

          To study the flexural crack pattern and shear crack pattern total of 24 reinforced concrete beams of size 1200x100x150mm long were casted.

          Concrete was mixed in the laboratory. All specimens were vibrated with a mechanical vibrator. They were demoulded after 24 hour and were cured under room temperature curing. After 28 days, large beams were white washed for easy detection of crack.

          Fig.7 Casting of beam specimen

          Fig.8 Beams after white washing

        4. Tests on Specimens

          1. Study on workability

          2. Study on strength

            • Compressive strength

            • Splitting tensile strength

            • Flexural strength

          3. Study on flexural crack pattern

          4. Study on shear crack pattern

        5. Test setup

          The beams were tested under two point loading with simply supported end condition. Specimens are tested in a loading frame of 2000 kN (200 t) capacity with an effective span of 1100 mm. Load cell of 200 kN capacity with a least count of 1 kN is used to measure the applied load. The load is increased in stages till the failure of the specimen in the case of monotonic loading and at each stage of loading deflection at mid span is found out using a dial gauge. The test setup is shown in Fig 4.8.

          Following observations were made

          1. First crack load

          2. Displacement at mid span

          3. Ultimate load

          4. Crack pattern and failure mode

    Fig.9 Test setup

  5. RESULTS AND DISCUSSION

    The test result covers the fresh properties, mechanical properties and flexural and shear properties of HFRGPC mix. The detailed investigation on the effect of hybrid fibre on the flexural and shear behavior was carried out. The influence of hybrid fibre addition on the workability of GPC mix was studied by conducting the compacting factor test.

    1. Test on workability of HFRGPC specimens

      The influence of hybrid fibre addition on the workability of GPC mix was studied by conducting the compacting factor test. The test results are shown in Table 6

      TABLE 6 WORKABILITY OF HFRGPC SPECIMENS

      Sl. No.

      Mix Designation

      Compacting Factor

      1

      GPC

      0.99

      2

      SFRGPC

      0.96

      3

      HFRGPC 1

      0.94

      4

      HFRGPC 2

      0.92

      5

      HFRGPC 3

      0.89

      6

      HFRGPC 4

      0.88

    2. Test on hardened properties of HFRGPC specimens

      TABLE 7 SUMMARY ON HARDENED PROPERTIES OF HFRGPC MIXES

      Mix designation

      Cube compressive strength (N/mm2)

      28 days

      Flexural strength (N/mm2) 28 days

      Splitting tensile strength (N/mm2) 28 days

      GPC

      43.51

      4.61

      2.44

      SFRGPC

      51.24

      6.84

      3.84

      HFRGPC 1

      50.11

      7.32

      3.42

      HFRGPC 2

      48.16

      7.48

      3.02

      HFRGPC 3

      46.46

      8.08

      2.91

      HFRGPC 4

      40.39

      8.02

      2.80

      The replacement of steel fibre with polypropylene fibre resulted in a decrease of compressive strength. But it is greater than normal GPC mix upto 30% replacement and after that it decrease below normal GPC mix. The addition of polypropylene fibre plays an important role in flexural strength. On the addition of polypropylene fibre, there was a considerable difference in flexural strength from 20% to 30%. As the percentage replacement increased from 30 to

      40 there was a decrease in result. So the optimum replacement of steel fibre is fixed as 30% ie, 70% steel and 30% polypropylene fibre.

    3. Test results on Beams

      1. First crack load and ultimate load

        From the test results it can be seen that the addition of fibres increases the load carrying capacity for all the beams. The appearance of first crack was lower when compared to SFRGPC. But the ultimate flexural strength is higher for HFRGPC. This is due to the finer polypropylene fibres bridging the micro cracks more effectively than the steel fibres. The results of the first crack load and ultimate load for flexural beam specimens are tabulated in Table 7 and that for shear beam specimens are shown in Table 8.

        TABLE 8 TEST RESULT FOR FIRST CRACK LOAD AND ULTIMATE LOAD OF FLEXURAL BEAM SPECIMEN

        Sl. No

        Beam Designation

        First Crack Load (kN)

        Ultimate Load (kN)

        Percentage gain of Ultimate Load

        1

        GPC

        20

        47

        2

        SFRGPC

        25

        52

        13.04

        3

        HFRGPC 1

        24

        54

        17.39

        4

        HFRGPC 2

        22

        56

        21.73

        5

        HFRGPC 3

        20

        60

        30.43

        6

        HFRGPC 4

        14

        42

        -8.69

        TABLE 9 TEST RESULT FOR FIRST CRACK LOAD AND ULTIMATE LOAD OF SHEAR BEAM SPECIMEN

        Sl. No

        Beam Designation

        First Crack Load (kN)

        Ultimate Load (kN)

        Percentage gain of Ultimate Load

        1

        GPC

        19.68

        43

        2

        SFRGPC

        24.47

        46

        6.97

        3

        HFRGPC 1

        23.60

        50

        16.27

        4

        HFRGPC 2

        21.04

        53

        23.25

        5

        HFRGPC 3

        18.91

        57

        32.56

        6

        HFRGPC 4

        14.27

        41

        -4.65

        1. Load deflection behavior

    30

    30

    Load (kN)

    Load (kN)

    The recorded values of load and deflection were used to draw the load deflection plots. Mid span deflection was noted at every 2kN load increment. Deformation corresponding to each increment of load for all specimens was noted. The load deflection graph for all shear and flexural specimen is shown in Fig 6 and 7

    70

    60

    GPC

    70

    60

    GPC

    50

    SFRGPC

    50

    SFRGPC

    40

    HFRGPC1

    40

    HFRGPC1

    HFRGPC2

    HFRGPC2

    HFRGPC3

    0 HFRGPC4

    0 5 10 15

    HFRGPC3

    0 HFRGPC4

    0 5 10 15

    Deflection (mm)

    Deflection (mm)

    20

    10

    20

    10

    Fig.10 Influence of fibre on the flexural behavior of beam under loading

    60

    50

    Load (kN)

    Load (kN)

    40

    30

    20

    10

    0

    0 5 10

    Deflection (mm)

    GPC SFRGPC

    HFRGPC 1

    HFRGPC 2

    HFRGPC 3

    HFRGPC 4

    0.4

    0.3

    0.2

    0.1

    0

    0.4

    0.3

    0.2

    0.1

    0

    Energy absorption capacity(kNm)

    Energy absorption capacity(kNm)

    Fig.13 Energy absorption capacity for all shear beam specimens

    Fig.11 Influence of fibre on the shear behavior of beam under loading

    1. Energy absorption capacity

      Energy absorption capacity(kNm)

      Energy absorption capacity(kNm)

      In general, the energy absorption capacity of a given material could be obtained from area under the load deflection plot of the specimen. Similarly toughness of the specimen can be measured using the load deflection curve obtained. Concrete will be effective in resisting the load until the formation of the first crack. At this stage concrete is relieved of its tensile stress and steel becomes effective at the cracked section. Energy absorbed at ultimate load can be obtained by calculating the area under load deflection curve upto the ultimate load.

      0.5

      0.4

      0.3

      0.2

      0.1

      0

      0.5

      0.4

      0.3

      0.2

      0.1

      0

      Fig.12 Energy absorption capacity for all flexural beam specimens

      From the results it can be seen that energy absorption capacity was maximum for 30% replacement of steel fibre with polypropylene fibre.

    2. Displacement ductility

    Ductility Index

    Ductility Index

    The term ductilty in reinforced concrete beam may be defined as the ability of the beam to undergo large plastic deformation after the yielding of tensile reinforcement. An attempt was made in the present investigation to obtain the ductility factor for all the beams tested. The ductility factor is defined as the ratio of the ultimate deflection (u) to the deflection at yield (y). Due to inherent limitations of the testing machine, full load deflection curve could not be obtained. Therefore load deflection behaviour up to 90 % of peak load was noted. Displacement ductility was calculated as the ratio of deflection at yield load and deflection at 90% peak load.

    7

    6

    5

    4

    3

    2

    1

    0

    Mixes

    7

    6

    5

    4

    3

    2

    1

    0

    Mixes

    Fig. 14 Displacement ductility factor for all flexural specimens

  6. CONCLUSION AND SCOPE

  1. Conclusion

    Based on the experimental results, the following conclusions are drawn:

    • GPC mix i.e. geopolymer concrete withoutfibres showed maximum workability. The workability of concrete had been found to decrease with increase of

    fibre content in concrete. It might be due to viscous nature of geopolymer concrete and uneven distribution of fibres in the mix.

    • Addition of steel fibres increases the split tensile strength. The increase was significant for mix with 0.5% steel fibre.

    • Addition of steel fibre increased the flexural strength of concrete. The optimum mix percentage obtained was for 0.50% steel by volume. Thereafter there is no remarkable increase in flexural due to decreased workability and lumping of fibres.

    • Compressive strength of HFRGPC increases upto 30% replacement of steel fibre with polypropylene and then decreases. Hence the optimum replacement percentage of steel fibre is 30% by volume.

    • Hybrid fibre reinforced concrete showed an increase in flexural strength. The appearance of first crack was lower when compared to SFRGPC. But the ultimate flexural strength is higher for HFRGPC. This is due to the finer polypropylene fibres bridging the micro cracks more effectively than the steel fibres.

    • The First crack load, Ultimate load, Energy Absorption capacity and ductile behaviour of HFRGPC is higher than SFRGPC with same volume fraction of steel fibres, both for flexure and shear.

  2. Scope

  • The work can be extended by changing type of fibre.

  • Study shear behaviour of FRGPC beams for various shear span to depth ratios.

  • Study the flexural behaviour of FRGPC beams for various reinforcement ratios and various cover.

  • This work can also be extended by applying cyclic and reverse cyclic loading.

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