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(fI

c0) and strain(єco) 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 σ-Є behavior :The σ-Є 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 f’cc(MPa) Єcu(%) Єh,rup(%) flu/f’co ЄCU/єCO flua/f’co 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 σ-Є curve of

cyclically loaded specimen

DISCUSSIONS

Unloading Reloading and Plastic Strain :To define complete σ-Є 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 єpl and єun,enve is an important aspect of cyclic loading

DISCUSSIONS

Lame et al(2006) demonstrated that the relationship between єpl and єun,enve is linear for CFRP confined NSC cylinders for єcu>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 teng’s model which suggests reduction in plastic strain with increase in unconfined strength

DISCUSSIONS

There is a relationship between єpl and єun,enve given by shao et al σ-Є model given by

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

The fig shows that the єpl 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 shao’s models

The observations and discussions presented suggests the variation of unloading stiffness can be accurately predicted by using єun,enev/єcu 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 kξ for finding actual

confining pressure at failure

DISCUSSIONS

Effect of loading pattern:kξ values does not

depend on the load cycles so is the hoops

strain єh,rupt

But lam et al proposed an observed increase in

єh,rupt with increase in loading/unloading cycles

DISCUSSIONS

Effect of unconfined concrete strength: By

comparing samples of same fIlu/fI

co ,indicate

that the ultimate strength is lower for HSC than

NSC under cyclic loading

Kξ 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 terg’s

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 Kξ for these 2 specimens

suggest that it does not depend on type of FRP

co

cu

DISCUSSIONS

It is well understood that by increasing confinement both strength and strain enhancement ratios increases

However closer inspection of relation between fIcc/fI

co and flu,a/fI

co suggests that it is not linear for HSC specimens

This can be explained by tread of σ-є curve shown, there is a descending second branch of H-C-4L specimen suggests that confinement provided was not sufficient to provide enhancement

STRESS-STRAIN CURVE OF H-C-4L-C1 AND H-C-

6L-C1

DISCUSSIONS

When CFRP layers are increased to 6L the second branch has an ascending or almost flat trend

From this we can say the strength enhancement is observed when concrete is confined by a certain minimum confinement which is known as Threshold Confinement

Threshold Confinement is sensitive to unconfined concrete

DISCUSSIONS

The better prediction of ultimate strength of

FRP confined HSC has to be developed which

can accurately predict Threshold confinement

as a function of unconfined compressive

strength

COMPARISON WITH EXSISITNG STRESS-STRAIN

MODELS

Both envelope curves of shao’s and lam and teng’s was in accordance with the results obtained

Fig illustrates that lam and teng’s model is highly accurate in predicting both loading and unloading curves of FRP confined NSC

As we have already discussed shao’s model over predicts so it deviates from unloading and reloading curves obtained

COMPARISON WITH EXSISITNG STRESS-STRAIN

MODELS

Further more lam and teng’s model accurately predicting the reloading curve which is linear initial and becomes parabolic as it moves to envelope stress

Where as shao’s model is fully linear which is not happening in practical

Application of both models to FRP confined HSC leads to large errors in estimation σ-є curve which can be seen from above mentioned graphs

COMPARISON WITH EXSISITNG STRESS-STRAIN

MODELS

In case of lam and teng’s the deviation is due to inaccuracies in predicting plastic strain due to limited load cycle data

But shao’s model performance does not degrade for HSC when compared with lams but even does it improve

Finally it is not possible to accurately predict σ-є of FRP confined HSC under cyclic compressive loading

CONCLUSIONS

The envelope curve of cyclically loaded FRP confined concrete closely follows the σ-є curve of same concrete under monotonic loading

The residual plastic strain єpl of FRP confined is linearly related to unloading strain and this relation does not depend on amount of confinement, type of FRP used ,unconfined strength of concrete

CONCLUSIONS

For a given confinement ratio fIcc/fI

co both strength

and strain enhancement ratio decreases with

increase in unconfined concrete strength

Concrete experiencing similar level of confinement

when confined with AFRP and CFRP jackets

provides same confinement pressures but єcu of

concrete with AFRP is significantly high

REFERENCES

Lam, L., and Teng, J. G. (2009). “Stress-strain

model for FRP-confined concrete under cyclic

axial compression.” Eng. Struct., 31(2), 308–

321

Shao, Y., Zhu, Z., and Mirmiran, A. (2006).

“Cyclic modeling of FRP confined concrete with

improved ductility.” Cem. Concr.

Compos.,28(10), 959–968..

THANK YOU