Behaviour of frp confined normal and high strength concrete [autosaved]

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  • BEHAVIOUR OF FRP-CONFINED NORMAL

    AND HIGH STRENGTH CONCRETE UNDER

    CYCLIC AXIAL COMPRESSION

    BY TOGAY OZBAKKALOGLU1AND EMRE ALEIN2

    1SENIOR LECTURER,SCHOOL OF CIVIL

    ENGINEERING,UNIVERSITY OF ADELAIDE

    2RESEARCH FELLOW, DEPT OF CIVIL ENGINEERING,

    SELUCK UNIV.,KONYA , TURKEY

    JOURNAL OF COMPOSITES FOR CONSTRUCTION,ASCE,JULY

    2012

    BETHU PRAVEEN KUMAR(12CE65R11)

    STRUCTURAL ENGINEERING

    DEPARTMENT OF CIVIL ENGINEERING

    IIT KHARAGHPUR

  • OVERVIEW

    INTRODUCTION

    EXPERIMENTAL PROGRAMME

    TEST SETUP

    TEST RESULTS

    DISCUSSIONS

    REFERENCES

  • INTRODUCTION

    We know that concrete is strong in compression and

    fails due to tension.

    FRP(fiber reinforced polymers) are used as

    confinement for concrete

    Application of FRP as confining materials is used for

    retrofitting of existing columns and new column as EQ

    resistant

  • INTRODUCTION

    Monotonic stress-strain behavior of FRP confined has been studied past two decades

    HSC members are known to exhibit brittle behavior so their use in seismically active regions is restricted

    However by providing sufficient confinement we can increase their ductility

  • EXPERIMENTAL PROGRAMME

    24 FRP cylinders of dia 152.5mm and height 305mm are used

    TEST PARAMETERS : compressive strength ,type of FRP ,amount of confinement ,type of loading

    Mix consists of crushed blue stone of max size 10mm as coarse aggregate and 8% of binder by weight is replaced by silica

  • EXPERIMENTAL PROGRAMME

    28 days strength of NSC was found to be 39MPa and HSC is 103 MPa the stress(fIc0) and strain(co) at failure was recorded

    AFRP was used as confinement for both NSC and HSC where as CFRP are used for HSC

    FRP jackets of 22 specimens were formed by manually wrapping FRP sheets around concrete and other 2 are confined by formerly manufactured AFRP

  • TEST SETUP

    Axial deformations of the specimens were measured by LVDT as shown in fig

    Transverse shear strain were measured from 3 unidirectional strain gauges of gauge length 20mm that were bounded on FRP jackets

    For elastic loading the loading rate is 3KN/s and after softening displacement control is 0.01mm/s

  • TEST SETUP

    For 12 of the specimens the load was monotonically increasing for other 12 specimens cyclic compression involving unloading and loading at 0.15% axial strain

    Specimen are labeled as follows H-A-4L-M1 where H is HSC and A is aramid polymer ,4 layer of confinement , under Monotonic loading the final number 1 is to denoted a difference between 2 similar specimens

  • TEST RESULTS

    Failure Mode: All the specimens fail due to

    rupture of FRP jacket

    Concrete shear cones were formed in NSC

    specimens due to gradual crushing of concrete

    In HSC specimens the failure is highly localized

    around a major shear crack

  • FAILURE PATTERNS OBSERVED

  • TEST RESULTS

    Axial - behavior :The - curves of

    monotonically loaded specimens exhibit an

    ascending first branch that is followed by an

    ascending or almost flat second branch

    Where as HSC specimens experiences a

    sudden drop in strength starting right at the

    transition point this is due to initial softening

  • TEST RESULTS OF FRP CONFINED CONCRETE

    CYLINDERS Specimen fcc(MPa) cu(%) h,rup(%) flu/fco CU/CO flua/fco K1(avg) K2(avg) K3(avg)

    N-A-2L-M1 69.2 2.32 1.71 0.40 10.9 0.28 2.82 13.8 0.68

    N-A-2L-M2 67.1 2.30 1.56 0.40 0.25

    N-A-3L-M1 85.0 2.86 1.66 0.61 14.1 0.40 2.86 11.2

    N-A-3L-M2 87.6 3.11 1.84 0.61 0.45

    H-A-4L-M1 122.3 1.45 1.18 0.32 4.0 0.15 1.26 8.4 0.46

    H-A-4L-M2 118.7 1.29 1.29 0.31 0.16

    H-A-6L-M1 154.7 1.70 1.10 0.45 4.9 0.20 2.33 9.7

    H-A-6L-M2 153.2 1.70 1.07 0.45 0.19

    H-C-4L-M1 98.9 0.93 0.89 0.23 2.7 0.13 5.5 0.55

    H-C-4L-M2 103.3 0.96 0.81 0.21 0.11

    H-C-6L-M1 122.3 1.13 0.94 0.31 3.4 0.19 1.17 5.8

    H-C-6L-M2 124.4 1.16 0.78 0.37 0.18

    N-A-2L-C1 64.3 2.25 1.50 0.42 10.7 0.25 2.67 14.6 0.68

    N-A-2L-C2 64.3 2.25 1.56 0.40 0.25

    N-A-3L-C1 97.4 4.04 1.76 0.61 20.0 0.43 3.46 14.9

    N-A-3L-C2 104.5 4.43 2.02 0.61 0.49

    H-A-4L-C1 136.4 1.82 1.24 0.32 5.1 0.16 2.0 13.1 0.50

    H-A-4L-C2 125.4 1.63 1.10 0.31 0.14

    H-A-6L-C1 157.2 1.87 1.16 0.46 5.8 0.21 2.39 9.5

    H-A-6L-C2 170.9 2.13 1.45 0.45 0.26

    H-C-4L-C1 102.3 1.07 0.69 0.23 3.2 0.10 8.9 0.48

    H-C-4L-C2 96.0 1.06 0.81 0.23 0.12

    H-C-6L-C1 123.7 1.14 0.64 0.31 3.3 00.13 1.21 7.6

    H-C-6L-C1 129.9 1.16 0.81 0.33 0.17

  • TEST RESULTS

    At this point hoops strains recorded on FRP

    jacket increases rapidly but the confinement

    pressures generated by FRP are sufficient to

    confine concrete

  • DISCUSSIONS

    Envelope curve of concrete represents the

    upper boundary of the response under cyclic

    axial compression

    The envelope curve is drawn by connecting the

    initial unloading points on the - curve of

    cyclically loaded specimen

  • DISCUSSIONS

    Unloading Reloading and Plastic Strain :To define complete - of cyclically loaded specimen ,unloading and reloading paths are required in addition to envelope curve

    Unloading path intersects the strain axis at a value referred to as residual plastic strain

    The relationship between pl and un,enve is an important aspect of cyclic loading

  • DISCUSSIONS

    Lame et al(2006) demonstrated that the relationship between pl and un,enve is linear for CFRP confined NSC cylinders for cu>0.0035

    Trend lines as shown are drawn for every specimen and the following observations are made

    Trend lines of specimen with same concrete strength and confinement material coincide

  • DISCUSSIONS

    Trend lines of AFRP confined cylinders and CFRP confined HSC shows that they does not significantly depend on type of FRP confinement

    Comparison of trend lines of HSC and NSC indicates that it does not significantly depend on unconfined compressive strength

    Which was against to lames and tengs model which suggests reduction in plastic strain with increase in unconfined strength

  • DISCUSSIONS

    There is a relationship between pl and un,enve given by shao et al - model given by

    Comparison of experimental and that obtained by above equations are plotted in the fig below

    The fig shows that the pl of the present study is over estimated by shao et al model is due to Esec

    sec

    ,

    ,E

    enveun

    enveunpl

    1'

    0for 10

    ,sec c

    enveun

    c fE

    E

    2.5 '

    1for 44.1'

    004.0 cr,

    coco

    enveun

    ff

    5.2f'

    for 34.0co

    enveun,

  • DISCUSSIONS

    At any stage of loading history beyond initial elastic portion with increasing deformation the unloading stiffness of NSC and HSC specimens decreases much more significantly than predicted by shaos models

    The observations and discussions presented suggests the variation of unloading stiffness can be accurately predicted by using un,enev/cu while giving due consideration to unconfined concrete strength

  • DISCUSSIONS

    The ultimate condition of FRP is referred to as the ultimate strength and strain of concrete recorded just before failure

    The nominal confinement ratio flu/fIco is calculated

    from equations assuming uniform confinement distribution

    The value obtained above is a theoretical value and does not represent actual confining pressure developed in FRP at failure

    co

    fuff

    co

    lu

    Df

    t

    f

    f

    '

    E2

    '

  • DISCUSSIONS

    Which is because the ultimate hoop strain

    reached in FRP is much smaller than ultimate

    tensile strain in fiber which necessitates a

    strain reduction factor k for finding actual

    confining pressure at failure

  • DISCUSSIONS

    Effect of loading pattern:k values does not

    depend on the load cycles so is the hoops

    strain h,rupt

    But lam et al proposed an observed increase in

    h,rupt with increase in loading/unloading cycles

  • DISCUSSIONS

    Effect of unconfined concrete strength: By

    comparing samples of same fIlu/fIco ,indicate

    that the ultimate strength is lower for HSC than

    NSC under cyclic loading

    K values of HSC are consistently lower than

    that of NSC which suggests that it is strength

    dependent

  • DISCUSSIONS

    Stress and strain enhancement coefficients K1

    and k2 are calculated by using lame and tergs

    expression shown below

    K1 and k2 are lower for HSC when compared to

    NSC

    co

    alu

    co

    cc

    f

    fK

    f

    f

    '1

    '

    ' ,1

    45.0

    ,,

    2'

    75.1

    co

    ruph

    co

    alu

    co

    cu

    f

    fK

  • DISCUSSIONS

    By comparison of H-A-4L and H-C-6L layer with

    same concrete strength and same confining

    pressure shows that for AFRP confine is

    more then CFRP confinement

    Comparison of K for these 2 specimens

    suggest that it does not depend on type of FRP

    co

    cu

  • DISCUSSIONS

    It is well unde