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(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..
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