SQUARE REINFORCED CONCRETE COLUMNS · inelastic region (that is the region after severe concrete...
8
Page1of8 SQUARE REINFORCED CONCRETE COLUMNS STRENGTHENED THROUGH FIBER REINFORCED POLYMER (FRP) SHEET STRAPS DimitraAchillopoulou PhD Candidate Democritus University of Thrace Laboratory of Reinforced Concrete, Department of Civil Engineering, Democritus University of Thrace (DUTh), 67100 Xanthi, Greece [email protected] Theodoros Rousakis Lecturer Democritus University of Thrace Laboratory of Reinforced Concrete, Department of Civil Engineering, Democritus University of Thrace (DUTh), 67100 Xanthi, Greece [email protected]* AthanasiosKarabinis Professor Democritus University of Thrace Laboratory of Reinforced Concrete, Department of Civil Engineering, Democritus University of Thrace (DUTh), 67100 Xanthi, Greece [email protected] Abstract The paper presents the performance of the existing stress – strain models that are proposed for FRP confinement of concrete columns through partial wrapping. The analytical models are compared against the experimental results of an investigation concerning square columns (150 mm side and 750 mm height) with slender longitudinal bars of different quality. The columns were wrapped partially by light glass FRP sheet confinement. A total of 16 specimens of low concrete strength were tested. Six of them were plain concrete columns while five columns contained four longitudinal bars of 220 MPa nominal yield strength and the remaining columns included four bars of 500 MPa nominal yield strength. Four different levels of strap glass FRP confinement were examined. The columns were subjected to monotonic loading up to failure. Partial light glass FRP confinement can enhance significantly reinforced concrete column strength and deformability. The effect of the partial wrapping is higher in columns with internal steel reinforcement. The study investigates the performance of four existing models. The model by Barros & Ferreira provides fairly accurate prediction of the whole stress-strain behaviour of the partially FRP confined reinforced concrete columns. Keywords: confinement, concrete columns, frp straps, axial compression 1. Introduction FRP strengthening of columns through jacketing is widely applied during last decades. Full FRP confinement has been proven an efficient method to enhance the axial load capacity and strain at failure of concrete columns, to prevent premature bars’ buckling, to reduce required lap length of bars or to provide enhanced resistance against bars corrosion among else. On the other hand only a few studies concern partial wrapping [1-3 among else]. Partial FRP wrapping in between existing steel stirrups leads to better utilization of the existing steel reinforcement while may provide higher axial strain of concrete at failure and higher strain at failure of the straps in some cases of circular columns [2] than full confinement. That, in turn
Transcript of SQUARE REINFORCED CONCRETE COLUMNS · inelastic region (that is the region after severe concrete...
![Page 1: SQUARE REINFORCED CONCRETE COLUMNS · inelastic region (that is the region after severe concrete cracking). The fib bulletin 14 [4] model utilizes the approach by Mander et al 1988](https://reader030.fdocuments.in/reader030/viewer/2022041003/5ea6a1e39fa7d95619220b12/html5/thumbnails/1.jpg)
Page1of8
SQUARE REINFORCED CONCRETE COLUMNS STRENGTHENED
THROUGH FIBER REINFORCED POLYMER (FRP) SHEET STRAPS
DimitraAchillopoulou
PhD Candidate
Democritus University of Thrace
Laboratory of Reinforced Concrete, Department of Civil Engineering, Democritus University of Thrace (DUTh),
67100 Xanthi, Greece
Theodoros Rousakis
Lecturer
Democritus University of Thrace
Laboratory of Reinforced Concrete, Department of Civil Engineering, Democritus University of Thrace (DUTh),
67100 Xanthi, Greece [email protected]*
AthanasiosKarabinis
Professor
Democritus University of Thrace
Laboratory of Reinforced Concrete, Department of Civil Engineering, Democritus University of Thrace (DUTh),
67100 Xanthi, Greece
Abstract
The paper presents the performance of the existing stress – strain models that are proposed for
FRP confinement of concrete columns through partial wrapping. The analytical models are
compared against the experimental results of an investigation concerning square columns
(150 mm side and 750 mm height) with slender longitudinal bars of different quality. The
columns were wrapped partially by light glass FRP sheet confinement. A total of 16
specimens of low concrete strength were tested. Six of them were plain concrete columns
while five columns contained four longitudinal bars of 220 MPa nominal yield strength and
the remaining columns included four bars of 500 MPa nominal yield strength. Four different
levels of strap glass FRP confinement were examined. The columns were subjected to
monotonic loading up to failure. Partial light glass FRP confinement can enhance significantly
reinforced concrete column strength and deformability. The effect of the partial wrapping is
higher in columns with internal steel reinforcement. The study investigates the performance of
four existing models. The model by Barros & Ferreira provides fairly accurate prediction of
the whole stress-strain behaviour of the partially FRP confined reinforced concrete columns.
Keywords: confinement, concrete columns, frp straps, axial compression
1. Introduction
FRP strengthening of columns through jacketing is widely applied during last decades. Full
FRP confinement has been proven an efficient method to enhance the axial load capacity and
strain at failure of concrete columns, to prevent premature bars’ buckling, to reduce required
lap length of bars or to provide enhanced resistance against bars corrosion among else. On the
other hand only a few studies concern partial wrapping [1-3 among else]. Partial FRP
wrapping in between existing steel stirrups leads to better utilization of the existing steel
reinforcement while may provide higher axial strain of concrete at failure and higher strain at
failure of the straps in some cases of circular columns [2] than full confinement. That, in turn
![Page 2: SQUARE REINFORCED CONCRETE COLUMNS · inelastic region (that is the region after severe concrete cracking). The fib bulletin 14 [4] model utilizes the approach by Mander et al 1988](https://reader030.fdocuments.in/reader030/viewer/2022041003/5ea6a1e39fa7d95619220b12/html5/thumbnails/2.jpg)
Page2of8
leads to an economy in materials, while partial wrapping is faster to apply. The fib bulletin 14
[3] and the Italian Code CNR-DT 200/2004 [5] provide suitable design tools. Analytical
prediction of the axial stress-strain behavior of partially FRP confined concrete is based on
proposals of semi-empirical or empirical models. The present study aims at the investigation
of the accuracy of the prediction of the whole stress-strain response provided by fib bulletin
14 [4], CNR-DT 200/2004 [5], ACI 440.2R-08 [6], and Barros & Ferreira 2008 [2] models.
They are compared against partially wrapped reinforced concrete columns of square section
tested at the Reinforced Concrete Lab of D.U.Th.
2. Confinement models for partial wrapping
The models used in the investigation are presented in table 1. The whole stress-strain
analytical behaviour of the FRP confined columns is of concern. Different approaches are
followed by the models in the generation of the sequential stress-strain values up to the
ultimate stress and strain. The analytical models by CNR-DT 200/2004 [5], ACI 440.2R-08
[6] and Barros & Ferreira 2008 [2] propose a simple linear stress-strain relation in the
inelastic region (that is the region after severe concrete cracking). The fib bulletin 14 [4]
model utilizes the approach by Mander et al 1988 [7]. The models also vary by their relations
to provide strength or axial strain at failure for confined concrete. The basic confinement
efficiency coefficients referring to square sections are identical. The ACI [6] acknowledges
further reduction of the confining effects in rectangular sections (non square) by the
introduction of additional effectiveness coefficients. To apply ACI [6] model in partially
wrapped columns the approach by fib [4] is adopted.
3. Validation of models for square RC columns
The experimental results concern 16 columns of square section (150x150x750mm), 1:2 scale.
They were subjected to axial compression up to failure. The research included six plain
concrete columns (group SG), 5 columns containing 4 smooth bars of 8 mm diameter with
low yield stress (quality S220, nominal fy = 220MPa) labeled as RCS1 and 5 columns varying
by the yield stress of their bars (quality B500C, nominal fy = 500MPa) symbolized as RCS2.
All reinforced columns had stirrups of 5.5 mm diameter spaced at 100 mm (figure 1a). Thus
longitudinal bars are considered slender. Four out of five columns in each group were
confined by glass FRP uni-directional sheets. The first one had full confinement followed by
columns with 40 mm width straps (2 layers), 50 mm (1 layer) and 1 layer of 65 mm width
straps (figure 1b). The fifth specimen in each group was left unwrapped for comparison.
The plain concrete strength during experiments was 13.4 MPa at ε0=2.2‰. The glass sheet
was E-glass FRP sheets S&PG90/10 (S&P – Sintecno, Scherer 1999) with the following
mechanical properties: modulus of elasticity Ef=73GPa, structural thickness tf=0.154mm,
strain at failure of cured GFRP sheet εju=2.8%. A two-component S2W-type primer resin
(Sintecno S.A.) was applied on the concrete surface. Then the glass sheet was impregnated
using a two-component S2WV-type resin (Sintecno S.A.) while pushing the FRP on the
concrete surface.
3.1.Comparison of columns experimental behaviour versus analytical predictions
All columns failed after fracture of the FRP straps. Figure 1c shows a typical column after
failure (specimen RCS2W40). Plain concrete column SG1 was fully wrapped by 1 layer of
GFRP sheet, presenting failure stress fcu= 17.6 MPa and corresponding strain εccu= 0.0154.
That is 1.24 times the strength of plain concrete square columns (strength of 14.24 MPa) and
4.74 higher strain (strain of plain concrete column 0.00325). Figure 2a shows the comparison
between experimental and predicted behaviour of plain concrete columns according to
different models. The model by fib is closer to the experimental curve. However the predicted
![Page 3: SQUARE REINFORCED CONCRETE COLUMNS · inelastic region (that is the region after severe concrete cracking). The fib bulletin 14 [4] model utilizes the approach by Mander et al 1988](https://reader030.fdocuments.in/reader030/viewer/2022041003/5ea6a1e39fa7d95619220b12/html5/thumbnails/3.jpg)
Pag
e3of8
Ta
ble
1.
Des
ign
Gu
idel
ines
fo
r F
RP
Co
nfi
nem
en
t o
f C
ross
Sec
tio
ns.
GU
IDE
LIN
E
EF
FE
CT
IVE
CO
NF
INE
ME
NT
CO
MP
RE
SS
IVE
ST
RE
NG
TH
(D
ES
IGN
)fcc
’ (M
Pa)
UL
TIM
AT
E A
XIA
LC
OM
PR
ES
SIV
E
SR
EN
GT
H O
F C
ON
FIN
ED
CO
NC
RE
TE
ε ccu
EF
FE
CT
IVE
C
ON
FIN
EM
EN
T
PR
ES
SU
RE
fl (
MP
a)
CO
EF
FIC
IEN
TS
CN
R-D
T
200
/20
04
[2]
(
)
⁄
√
(
)
,
(
)
AC
I 4
40
.2R
-08
[3]
{
(
)
(
(
)
)
Fo
r th
e in
flu
ence
of
pa
rtia
l
wra
pp
ing*
:
( )
( )
[( )(
)
(
)(
)
]
FIB
-
CE
B-F
IP
[4]
( √
)
[
(
)]
non
-cir
cula
r:
,
(
)
,
(
)
(
)
no
n-c
ircu
lar:
∑(
)
,
(
)
BA
RR
OS
MO
DE
L
[5]
1rs
t b
ran
ch:
2nd
bra
nch
:
[
(
)]
(
)
(
)
(
)
,
(
⁄
) (
⁄
)
*F
or
the
infl
uen
ce o
f p
arti
al w
rap
pin
g t
he
term
s o
f b
f an
d p
f o
f C
NR
co
de
are
use
d.
![Page 4: SQUARE REINFORCED CONCRETE COLUMNS · inelastic region (that is the region after severe concrete cracking). The fib bulletin 14 [4] model utilizes the approach by Mander et al 1988](https://reader030.fdocuments.in/reader030/viewer/2022041003/5ea6a1e39fa7d95619220b12/html5/thumbnails/4.jpg)
Page4of8
Figure 1.Dimensions and steel reinforcement detailing of columns of RC groups (a).Layout of FRP wrapping for SG1W65 column (b).Column RCS2W40 after failure (c). failure values are very high: ccu
*=0.024 , fccu*=19.00 Pa. The model by ACI provides a
fairly accurate concrete strength while axial strain is underestimated (1/2 of ccu). The CNR model also underestimates maximum experimental strains and overestimates strength. On the other hand B&F model (Barros & Ferreira 2008) provides fairly accurate strain at failure, though it overestimates strength. Plain concrete column SG2W40 partially wrapped by two layers of 40 mm width straps, presented a maximum stress fcc= 14.64 MPa and ultimate strain ccu= 0.01236 at a stress of fccu=12.40 MPa (Figure 2b). The model by fib captures fairly accurately the whole stress – strain behaviour overestimating strains. The models by ACI, CNR and B&F overestimate bearing load. The B&F and ACI model provides fair accurate strain predictions. Columns SG1W50 and SG1W65 (figures2c& 2d) present a rather “plastic” behaviour. Specimen SG1W50 reached the failure stress fcu= 13.5 MPa, slightly lower than the maximum one, at a strain ccu= 0.00865. Specimen SG1W65 had a similar behaviour with fcu= 14.11 MPa and ccu= 0.00953. ACI, B&F and CNR models overestimate strength. However they are close to failure strain. Fib model predicts a clearly softening behaviour, yet it overestimates strain. All specimens of RCS1 group having bars of 220 MPa nominal yield strength, presented a hardening stress-strain behaviour of increased stress and strain at failure compared to the plain ones (figures 3). The column RCS1W50 with only 50 mm width straps spaced at 100 mm, presented 1.28 times higher failure load than plain concrete column and 4.06 times higher deformability. That behaviour resulted despite the existence of slender bars. Barros & Ferreira model presents the highest convergence with the experimental results. ACI model underestimates strain at failure, while failure load is also underestimated in most columns. The model by fib shows a temporary softening behaviour- hardening for reinforced concrete columns with straps (figures 3a to d). Then, an increasing bearing load behaviour follows. The CNR model presents the highest divergence again. The columns with bars of 500 MPa nominal yield strength (group RCS2), presented a clear hardening stress-strain behaviour of increased stress and strain at failure compared to the plain ones (figures 4). Also, strain at failure of these specimens is even higher than the corresponding values of the identical ones of RCS1 group. Again, Barros & Ferreira model presents the highest convergence with the experimental results. The performance of ACI, fib and CNR model is similar to that for RCS1 group.
(a) (b) (c)
![Page 5: SQUARE REINFORCED CONCRETE COLUMNS · inelastic region (that is the region after severe concrete cracking). The fib bulletin 14 [4] model utilizes the approach by Mander et al 1988](https://reader030.fdocuments.in/reader030/viewer/2022041003/5ea6a1e39fa7d95619220b12/html5/thumbnails/5.jpg)
Pag
e5of8
a.
1 L
ay
er-
Fu
ll j
ack
et.
b
.
2 L
ay
ers-
40
mm
.
c.
1 L
ay
er-
50
mm
.
d.
1 L
ay
er-
65
mm
.
Fig
ure
2.C
om
pa
riso
n o
f ex
per
imen
tal
an
d a
na
lyti
cal
res
ult
s fo
r p
lain
co
ncr
ete
pri
sma
tic
colu
mn
s o
f S
G g
rou
p.
05
10
15
20
25
00,0
05
0,0
10,0
15
0,0
20,0
25
0,0
3
Axial Stress (MPa)
axia
l st
ra
in
SG
-fu
ll j
ack
eti
ng
_1
lay
er
FIB
CN
R
AC
I
B&
F
SG
_fu
ll
05
10
15
20
25
00,0
05
0,0
10,0
15
0,0
20,0
25
0,0
3
Axial Stress (MPa)
axia
l st
ra
in
SG
W40_2 l
ay
ers
FIB
CN
R
AC
I-44
0
B&
F
SW
40
05
10
15
20
25
00,0
05
0,0
10,0
15
0,0
20,0
25
0,0
3
Axial Stress (MPa)
axia
l st
rain
SG
W5
0_
1 l
ay
er
FIB
CN
R
AC
I
B&
F
SG
W5
0
05
10
15
20
25
00,0
05
0,0
10,0
15
0,0
20,0
25
0,0
3
Axial Strees (MPa) a
xia
l st
ra
in
SG
W65_1 l
ay
er
FIB
CN
R
AC
I
B&
F
SW
65
![Page 6: SQUARE REINFORCED CONCRETE COLUMNS · inelastic region (that is the region after severe concrete cracking). The fib bulletin 14 [4] model utilizes the approach by Mander et al 1988](https://reader030.fdocuments.in/reader030/viewer/2022041003/5ea6a1e39fa7d95619220b12/html5/thumbnails/6.jpg)
Pag
e6of8
a.
1 L
ay
er-
Fu
ll j
ack
et.
b
.
2 L
ay
ers-
40
mm
.
c.
1 L
ay
er-
50
mm
.
d.
1 L
ay
er-
65
mm
.
Fig
ure
3.
Co
mp
ari
son
of
exp
erim
enta
l a
nd
an
aly
tica
l res
ult
s fo
r p
rism
ati
c co
lum
ns
wit
h l
ow
qu
ali
ty b
ars
of
RC
S1
gro
up
.
05
10
15
20
25
30
00,0
05
0,0
10,0
15
0,0
20,0
25
0,0
30,0
35
Axial Stress (MPa)
axia
l st
ra
in
RC
S-1
full
ja
ck
eti
ng
_1
la
yer
FIB
CN
R
AC
I
B&
F
RC
S1_
full
05
10
15
20
25
30
00,0
05
0,0
10,0
15
0,0
20,0
25
0,0
30,0
35
Axial Stress (MPa)
axia
l st
ra
in
RC
S1W
40_
2 l
ay
ers
FIB
CN
R
AC
I
B&
F1
B&
F
RC
S1W
40
05
10
15
20
25
30
00,0
05
0,0
10,0
15
0,0
20,0
25
0,0
30,0
35
Axial Stress (MPa)
axia
l st
ra
in
RC
S1
W5
0_
1 l
ay
er
FIB
CN
R
AC
I
B&
F
RC
S1W
50
05
10
15
20
25
30
00,0
05
0,0
10,0
15
0,0
20,0
25
0,0
30,0
35
Axial Stress (MPa)
axia
l st
rain
RC
S1W
65_
1 l
ay
er
FIB
CN
R
AC
I
B&
F
RC
S1W
65
![Page 7: SQUARE REINFORCED CONCRETE COLUMNS · inelastic region (that is the region after severe concrete cracking). The fib bulletin 14 [4] model utilizes the approach by Mander et al 1988](https://reader030.fdocuments.in/reader030/viewer/2022041003/5ea6a1e39fa7d95619220b12/html5/thumbnails/7.jpg)
Pag
e7of8
a.
1 L
ay
er-
Fu
ll j
ack
et.
b
.
2 L
ay
ers-
40
mm
.
c.
1 L
ay
er-
50
mm
.
d.
1 L
ay
er-
65
mm
.
Fig
ure
4.
Co
mp
ari
son
of
exp
erim
enta
l a
nd
an
aly
tica
l res
ult
s fo
r p
rism
ati
c co
lum
ns
wit
h h
igh
qu
ali
ty b
ars
of
RC
S2
gro
up
.
05
10
15
20
25
30
00,0
05
0,0
10,0
15
0,0
20,0
25
0,0
30,0
35
0,0
4
Axial Stress (MPa)
axia
l st
ra
in
RC
S2
-fu
ll j
ack
eti
ng
_1
la
yer
FIB
CN
R
AC
I
B&
F
RC
S2-f
ull
05
10
15
20
25
30
00,0
05
0,0
10,0
15
0,0
20,0
25
0,0
30,0
35
0,0
4
Axial Stress (MPa)
axia
l st
ra
in
RC
S2
W4
0_2
la
yers
FIB
CN
R
AC
I
B&
F
RC
S2W
40
05
10
15
20
25
30
00,0
05
0,0
10,0
15
0,0
20,0
25
0,0
30,0
35
0,0
4
Axial Stress (MPa)
axia
l st
ra
in
RC
S2
W5
0_
1 l
ay
er
FIB
CN
R
AC
I
B&
F
RC
S2W
50
05
10
15
20
25
30
00,0
05
0,0
10,0
15
0,0
20,0
25
0,0
30,0
35
0,0
4
Axial Stress (MPa)
axia
l st
ra
in
RC
S2W
65_1
lay
er
FIB
CN
R
AC
I
B&
F
RC
S2W
65
![Page 8: SQUARE REINFORCED CONCRETE COLUMNS · inelastic region (that is the region after severe concrete cracking). The fib bulletin 14 [4] model utilizes the approach by Mander et al 1988](https://reader030.fdocuments.in/reader030/viewer/2022041003/5ea6a1e39fa7d95619220b12/html5/thumbnails/8.jpg)
Page8of8
In all series of columns the CNR stress-strain predictions tend to severely overestimate experimental load and underestimate strain. ACI model underestimates failure strains in most of the specimens, especially in reinforced concrete columns. Here it should be mentioned that the ACI recommendations do not propose a relation for partial wrapping and thus the common fib approach was applied to take into account reduced confinement effectiveness. The model by Barros & Ferreira yields fair accurate predictions in most of the cases of reinforced concrete FRP strengthened columns. The predictions of the strain at failure of the B&F model in most of the columns is satisfactory.
4. Conclusions
The study presents the performance of four existing models in predicting the whole stress-strain behaviour of partially FRP wrapped reinforced concrete columns. The models are compared against the experimental behaviour of 12 FRP strengthened square section columns tested in RC lab in DUTh. Experiments show that suitably designed light partial wrapping can lead to acceptable slightly softening stress-strain behaviour and axial strains around 1% for plain columns. In reinforced concrete columns including slender bars, the effect of the same partial wrapping in between steel stirrups is even higher. It leads to a clear hardening behaviour.
The model by Barros & Ferreira captures fairly well the general stress-strain response of most of the reinforced concrete FRP strengthened columns. Also it provides fairly accurate predictions of strain at failure in most of the examined columns. In cases of plain concrete columns it overestimates failure load. The model by ACI underestimates failure strain in most of the cases and also stress for full confinement. In plain concrete columns partially wrapped by FRP it overestimates concrete strength. The CNR model clearly provides very conservative strain predictions. Barros & Ferreira, ACI and CNR model cannot capture softening stress – strain behaviour occurring in the partially wrapped plain concrete columns of the study. On the other hand fib model while capable of describing softening stress-strain behaviour overestimates significantly failure strains.
5. Acknowledgements
The authors wish to thank S&P and Sintecno S.A. for providing the FRP sheets and the resins,
Skarlatos S.A. for providing concrete and Hellenic Halyvourgia for steel reinforcements.
6. References
[1] SAADATMANESH H., EHSANI M.R., LI M.W. Strength and Ductility of Concrete Columns Externally
Reinforced with Fiber Composite Straps. ACI Structural Journal, 91(4), 1994, pp. 434-447.
[2] BARROS, J., FERREIRA, D., “Assessing the Efficiency of CFRP Discrete Confinement Systems for
Concrete Cylinders”, Journal of Composites for Construction, Vol. 12, No. 2, March/ April 2008.
[3] EL MAADDAWY T. Strengthening of Eccentrically Loaded Reinforced Concrete Columns with Fiber-
Reinforced Polymer Wrapping System: Experimental Investigation and Analytical Modeling. Journal of
Composites for Construction, Vol. 13, No. 1, February 1, 2009. pp: 13–24.
[4] fib-CEB-FIP, Bulletin 14, “Externally Bonded FRP reinforcement for RC Structures,Design and use of
externally bonded fibre reinforced polymer reinforcement (FRP EBR) for reinforced concrete structures”,
July 2001.
[5] NATIONAL RESEARCH COUNCIL, ADVISORY COMMITTEE ON TECHNICAL
RECOMMENDATIONS FOR CONSTRUCTIONS, “Guide for the design and construction of externally
Bonded FRP systems for Strengthening Existing Structures”, CNR-DT 200/2004, Rome, July 13th
, 2004,
pp. 62-68&131-132.
[6] AMERICAN CONCRETE INSTITUTE, “Guide for the Design and Construction of Externally Bonded
FRP Systems for Strengthening Concrete Structures”, ACI 440.2R-08, July 2008, pp. 34-37.
[7] MANDER J.B., PRIESTLEY M.J.N., PARK R. (1998): “Theoretical stress-strain model for confined
concrete”. Journal of Structural Division, ASCE, V. 107, No ST11: 2227-2244.
[8] SCHERER J. (1999). “S&P – Sintecno, FRP – Polymer fibers in strengthening.”User guide, Brunnen.
![3 All] V.M.Singh S/O Mander Singh V. Feroze Varun …In para 29, 30 3 All] V.M.Singh S/O Mander Singh V. Feroze Varun Gandhi 1133 and 38 of the election petition, it has been pleaded](https://static.fdocuments.in/doc/165x107/5eb38a368f634e7183260aa3/3-all-vmsingh-so-mander-singh-v-feroze-varun-in-para-29-30-3-all-vmsingh.jpg)
