Construction and Building Materials xxx (2016) xxx–xxx

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Construction and Building Materials

journal homepage: www.elsevier.com/locate/conbuildmat

Experimental study on concrete columns reinforced by hybrid steel-fiber

reinforced polymer (FRP) bars under horizontal cyclic loading
Zeyang Sun a, Gang Wu a,⇑, Jian Zhang b, Yihua Zeng a,c, Wenchao Xiao a
a
Southeast University, Key Laboratory of Concrete and Prestressed Concrete Structures of the Ministry of Education, Nanjing, China
b
Department of Civil and Environmental Engineering, University of California, Los Angeles, USA
c
Department of Structural Engineering, Ghent University, Ghent, Belgium

h i g h l i g h t s

We tested four concrete columns under cyclic loading.

The reinforcements including steel bars, hybrid steel/FRP bars, and SFCBs.

The failure modes and hysteretic behaviors were analyzed.

a r t i c l e i n f o a b s t r a c t

Article history: Effective post-yield stiffness of reinforced concrete (RC) columns can significantly contribute to the seis-
Received 10 February 2016 mic performance of RC structures. However, because of the elastoplastic properties of steel bars, the post-
Received in revised form 21 September yield stiffness of an ordinary RC column can be very slight or even negative. Fiber reinforced polymer
2016
(FRP) can provide a high degree of ultimate strength, light weight, and protection from corrosion. By com-
Accepted 3 October 2016
Available online xxxx
bining steel and FRP, a designable post-yield stiffness can be achieved for concrete structures reinforced
with steel-FRP composite bars (SFCBs) or hybrid steel/FRP bars. This paper conducted cyclic loading tests
on four concrete columns with different reinforcement types, including steel bars, hybrid steel/FRP bars,
Keywords:
Concrete column
and SFCBs. The test results showed that (1) the columns reinforced with different bars had similar strain
Hybrid reinforcement distributions from column base to cap prior to yielding. After yielding, the plastic deformation of the ordi-
Post-yield stiffness nary RC column concentrated at the column base and the loading capacity decreased with the increase of
Hysteretic behavior lateral drift because of the P-d effect. (2) Unlike the negative post-yield stiffness of an ordinary RC col-
Energy dissipation umn, the post-yield stiffness of a column with hybrid reinforcements was positive. As the post-yield stiff-
ness ratio of the longitudinal reinforcement increased by 27 percent, the post-yield stiffness of the
concrete column increased by 7.4 percent. Therefore, the corresponding displacement ductility could
reach 11—much greater than that of the RC column (6.28). (3) As a result of the more robust hysteretic
curve of the RC column, the equivalent viscous damping coefficients of the RC column were greater than
those of the hybrid column, whereas the hybrid reinforced concrete columns could dissipate earthquake
energy without a corresponding loss of strength.
Ó 2016 Published by Elsevier Ltd.

1. Introduction quake, and approximately 100 piers had to be demolished due to

their excessively large residual displacements [2]. Numerous stud-
Under the excitation of different near-fault ground motions, the ies had been conducted on the inelastic displacement demand and
performance of ordinary steel bar reinforced concrete (RC) col- hysteretic behavior of concrete columns under multi-directional
umns falters after yielding due to the rebar’s elastoplastic proper- seismic shakings [3]. Numerous statistical analyses have shown
ties [1], thus leading to poor structural post-earthquake that a stable post-yield stiffness of concrete columns (>5%) signif-
recoverability. Over 250 piers collapsed in the 1995 Kobe earth- icantly contributes to lower residual displacements and increased
stability of displacement responses compared with cases without
stable positive post-yield stiffness [4].
⇑ Corresponding author. Unfortunately, it is difficult to achieve stable post-yield stiffness
E-mail addresses: sunzeyang@seu.edu.cn (Z. Sun), g.wu@seu.edu.cn (G. Wu), for an ordinary RC column because of the elastoplastic properties
zhangj@ucla.edu (J. Zhang).

http://dx.doi.org/10.1016/j.conbuildmat.2016.10.001
0950-0618/Ó 2016 Published by Elsevier Ltd.

Please cite this article in press as: Z. Sun et al., Experimental study on concrete columns reinforced by hybrid steel-fiber reinforced polymer (FRP) bars
under horizontal cyclic loading, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.001
2 Z. Sun et al. / Construction and Building Materials xxx (2016) xxx–xxx

of the steel bar [5]. However, in hybrid reinforcing bars composed dles of 2400 tex basalt fibers produced in the same batch as
of steel and fiber reinforced polymer (FRP), a designable post-yield ‘S10B85.’ The reinforcement ratio of the ordinary RC column C-
stiffness can be achieved by adjusting the ratio of FRP to steel [6,7]. S12 is 1.09%. The equivalent longitudinal reinforcement ratio (qesf)
Fisher and Li [8] noted that the philosophy of ‘‘strong column, of the hybrid columns (C-S10B85, C-H) with respect to the elastic
weak beam” is easily implemented for frame structures when the modulus of steel is also 1.09%. Compared with C-S10B85, the qesf
columns exhibit high post-yield stiffness. In marine environments, of C-S10B49 is approximately 89% of that of column C-S10B85
an ordinary RC structure has a short lifespan due to the low anti- due to a smaller content of BFRP in S10B49.
corrosion properties of steel [9]. To improve the seismic perfor- The detailed dimensions of the columns and the loading pat-
mance and durability of RC structures, Wu et al. [10] proposed a terns are shown in Fig. 1. The shear span ratio is 5, i.e., the distance
novel hybrid reinforcement—a steel-FRP composite bar (SFCB)— between the loading point and the column’s base (L) is 1250 mm.
composed of an inner ribbed steel bar and an outer longitudinal The vertical load in this test was controlled by an electrohydraulic
FRP crafted by a pultrusion process. The authors conducted subse- servo test system to maintain stable axial compression force dur-
quent research with respect to the mechanical properties of the ing horizontal cyclic loading. A spherical hinge was placed at the
SFCB [11] and the corresponding behaviors of concrete beams/- column top ensure the direction of the vertical load. To minimize
columns that were strengthened/reinforced by SFCBs [12–14]. the unfavorable effect of the horizontal friction caused by the uni-
Based on this previous research, this paper presents horizontal cyc- axial compression at the column cap, tetrafluoroethylene plates
lic loading tests of four concrete columns with different hybrid and a pulley were set between the vertical loading actuator and
reinforcement, including steel bars, hybrid steel/FRP bars, and the reaction frame. The friction coefficient between the tetrafluo-
SFCBs. The test results, including hysteretic curves, strain distribu- roethylene plates and the reaction frame was approximately
tions, and the energy dissipation capacity, are analyzed and subse- 0.03, and the friction coefficient between the pulley and the
quently followed by application recommendations. tetrafluoroethylene plates was less than 0.03. Therefore, it is rea-
sonable to assume that a constant axial load was vertically applied
2. Experimental program to the cap center of each column during the test. The average
tested compressive strength of the concrete cubes
2.1. Specimen design and test setup (150  150  150 mm) at 28 days was 36.64 MPa, and the corre-
sponding cylinder compressive strength was 29.31 MPa. The axial
The post-yield stiffness ratio of an SFCB (rsf) can be defined by load (P) applied to the concrete column was 200 kN and the corre-
Eq. (1). For an ordinary RC column, the reinforcement ratio q is sponding axial compression ratio was 0.11.
defined by the total area of the steel bar over the gross section area.
For a hybrid reinforced concrete column, the equivalent longitudi- 2.2. Loading program and measurements
nal reinforcement ratio (qesf) with regard to an ordinary steel rein-
forced concrete column is defined by Eq. (2). The horizontal cyclic loading on the column cap was controlled
by lateral force prior to column yielding, with a loading gradient of
r sf ¼ Ef Af =ðEs As þ Ef Af Þ ð1Þ 10 kN for each step. After yielding, the loading was controlled by
yield displacement (7 mm in this paper), with each displacement
qesf ¼ Ef Af =rsf Es Ag ð2Þ cycled three times. The test measurements included the following:
where Es and As are the elastic modulus and cross-section area of (1) the column cap force versus lateral displacement curves; (2)
the inner steel bar, respectively; Ef and Af are the elastic modulus the crack formation and development; and (3) the strain distribu-
and cross-section area of the SFCB’s outer FRP, respectively; K1 tion of the longitudinal reinforcement, which was measured by
and K2 are the stiffness of the concrete column before and after seven strain gauges along the longitudinal bar. The surface of the
yielding, respectively; Ag is the gross cross-section area of the con- steel bar/FRP bar was rubbed with sandpaper before bonding the
crete column. Columns with the same qesf will have the same initial strain gauge, and the dimension of each strain gauge was
stiffness. 3  5 mm. The strain gauge and the detailed locations are shown
Four concrete columns were designed with a rectangular sec- in Fig. 2.
tion of 250  250 mm. The specimen numbers and the correspond-
ing mechanical properties of the reinforcements are presented in 3. Test results
Table 1. The notation ‘C-S12’ denotes that column C-S12 is longitu-
dinally reinforced by ordinary steel bars of 12 mm in diameter. The 3.1. Test phenomena and load displacement curves
notation ‘C-S10B49’ denotes that the concrete column is reinforced
by ‘S10B49,’ which is a type of SFCB made of a 10-mm diameter As for the hybrid reinforced concrete columns, the concrete
inner steel bar longitudinally compounded with 49 bundles of cover near the column base initially cracked, followed by yielding
2400 tex basalt fibers; ‘tex’ is the weight (g) of one fiber bundle of the longitudinal steel bars or the inner steel bars of the SFCBs.
per kilometer. The case is similar for C-S10B85, only with 85 bun- Spalling of the concrete cover in the column base subsequently
dles. The concrete column named ‘C-H’ is hybrid reinforced by steel occurred with the rupture or partial rupture of the FRP. The load-
bars and basalt FRP (BFRP) bars. The BFRP bar consists of 85 bun- lateral displacement curves (V-d curves) of the columns are shown

Table 1
Specimen numbers and mechanical properties of the longitudinal reinforcements.

Column Reinforcement Diameter (mm) Elasticity modulus (GPa) Post-yield stiffness ratio Yield strength (MPa) Ultimate strength (MPa) qesf
number
C-S12 12.00 200 / 400 529.60 1.09%
C-S10B49 16.16 111.3 0.189 208.2 691.42 0.96%
C-S10B85 18.00 94.6 0.266 189.2 544.08 1.09%
C-H Steel bar 10.00 200 / 450 621.00 1.09%
BFRP bar 13.00 45.38 / / 1075.60

Please cite this article in press as: Z. Sun et al., Experimental study on concrete columns reinforced by hybrid steel-fiber reinforced polymer (FRP) bars
under horizontal cyclic loading, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.001
 Z. Sun et al. / Construction and Building Materials xxx (2016) xxx–xxx 3

Fig. 1. Loading pattern and geometry of concrete column (unit: mm).

ent columns at similar drift ratios, the RCsf of the hybrid reinforced
Longitudinal columns is calculated at the same ductility level (l_S12 = 6.28).
Reinforcement Strain The test phenomena and experimental results of each concrete
gauges on column are provided as follows.
FRP bar
Ľ 3.1.1. C-S12
ļ The first flexural crack of C-S12 was observed at approximately
150 mm from the column foundation when the horizontal load
Ļ
was approximately 30 kN; the corresponding crack width was
ĺ approximately 0.06 mm. The load of the turning point in the hys-


Ĺ teretic curve (Fig. 3(a)) caused by the cracks was approximately

Anchorage zone

ĸ 50 kN; a total of six corresponding cracks were observed near the

column base, with a maximum crack width of 0.08 mm. The aver-
ķ


age lateral load of pushing and pulling was 72.5 kN when the lat-
eral displacement at the column cap reached 14 mm; the
subsequent lateral load began to decrease with the increase of lat-
eral displacement. When the displacement reached 42 mm, the
load capacity decreased to 85% of the peak load, i.e., the failure
Fig. 2. Schematic strain gauge location on the longitudinal reinforcement.
point. When the lateral displacement at the cap increased to
49 mm, a partial spalling of the concrete at column base was
in Fig. 3. Flexural failure occurred in all specimens; the failure observed in the second loading loop. As the loading continued,
modes are shown in Fig. 4. The characteristic values of the concrete the buckling of three longitudinal reinforcements at the A-side
columns are listed in Table 2, where Vcr, Vy, and Vu are the crack and one at the B-side occurred when the lateral displacement
load, yield load and ultimate load, respectively; dcr, dy, and du are was 56 mm (Fig. 4(a)).
the corresponding lateral displacements; and l is the displacement
ductility coefficient (du/dy). The yield loads and displacements were 3.1.2. C-S10B49 and C-S10B85
determined by the first yielding of the longitudinal steel bar. The The first flexural crack of C-S10B49 occurred at approximately
ultimate load displacement was the point at which the load capac- 250 mm from the column foundation when the lateral load was
ity decreased to 85% of the peak load. The post-yield stiffness ratios approximately 40 kN; the crack width was approximately
of the columns (RCsf) are also presented in Table 2 and can be 0.02 mm. The load at the turning point in the hysteretic curve
defined by Eq. (3): (Fig. 3(b)) caused by cracking was approximately 54 kN. A total
of fifteen cracks appeared on the two sides of C-S10B49, of which
the maximum crack was observed at 150 mm from the column
RC sf ¼ K 2 =K 1 ð3Þ
foundation with a width of 1 mm. The outer FRP of the SFCB at
the corner of the A-side began to rupture gradually after the lateral
where K1 = Vy/dy and K2 = (Vu  Vy)/(du  dy) are the initial and post- displacement reached 33 mm, with the maintained lateral load
yield stiffness of the concrete column. To compare the RCsf of differ- decreasing with the increasing lateral displacement (Fig. 4(b)).

Please cite this article in press as: Z. Sun et al., Experimental study on concrete columns reinforced by hybrid steel-fiber reinforced polymer (FRP) bars
under horizontal cyclic loading, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.001
4 Z. Sun et al. / Construction and Building Materials xxx (2016) xxx–xxx

Drift Ratio (%) Drift Ratio (%)

-8 -6 -4 -2 0 2 4 6 8 -8 -6 -4 -2 0 2 4 6 8

100 100 FRP ruptured, and concrete spalling

Negative post-yield stiffness of C-S12

50 50
Force (kN)

Force (kN)
0 0

-50 -50

-100 C-S12 -100 C-S10B49

-100 -75 -50 -25 0 25 50 75 100 -100 -75 -50 -25 0 25 50 75 100
Lateral Diplacement (mm) Lateral Diplacement (mm)
(a) C -S12 (b) C -S10B49
Drift Ratio (%) Drift Ratio (%)
-8 -6 -4 -2 0 2 4 6 8 -8 -6 -4 -2 0 2 4 6 8
Stable post-yield stiffness of C-H
100 Stable post-yield stiffness of C-S10B85 100

50 50
Force (kN)
Force (kN)

0 0

-50 -50

-100 C-S10B85 -100 C-H

-100 -75 -50 -25 0 25 50 75 100 -100 -75 -50 -25 0 25 50 75 100
Lateral Diplacement (mm) Lateral Diplacement (mm)

(c) C -S10B85 (d) C -H

Fig. 3. Hysteretic curves of concrete columns.

As for C-S10B85, the turning point of the hysteretic curve types of reinforcement configurations could share similar seismic
caused by cracking was approximately 56 kN and three cracks performances under potential earthquakes.
were observed on the column base after this loading step; the max-
imum crack was located at a 50-mm height from the column foun- 3.2. Comparison of cracks
dation with a 0.15-mm crack width. When the lateral displacement
of C-S10B85 reached 21 mm, a slight spalling of the concrete cover The cracking loads of all specimens ranged between 44 kN and
at the column base occurred. The concrete cover in the four corners 50 kN, and the corresponding lateral drifts at the column cap ran-
of the column base completely crumbled when the lateral dis- ged between 0.09% L and 0.17% L. The crack developments of the
placement reached 42 mm. After the column yielded, the load- four columns at the 21-mm lateral displacement (1.68% L) were
displacement of C-S10B85 remained stable until the lateral dis- shown in Fig. 5(a), the corresponding maximum crack width and
placement reached 69 mm, which indicated good ductility and crack amount of concrete columns were presented in Fig. 5(b). It
deformation ability (Fig. 4(c)). can be seen that (1) C-S12 exhibited the least amount of cracking,
although it displayed the largest crack width among the maximum
3.1.3. C-H cracks of all tested columns. (2) As for the SFCB columns, the
The cracking load in the V-d curve was approximately 44 kN amount of cracking in C-S10B85 was approximately 1.5 times that
(Fig. 3(d)), and five cracks were found at the A-side and B-side, of C-S10B49, indicating that the increase of FRP content increased
respectively (10 cracks in total), with an average crack width of the plastic strain propagation of the steel, thus resulting in
0.05 mm. Spalling of the concrete cover in the column base was increased cracking. (3) Columns C-H and C-S10B85 exhibited sim-
found at a 49-mm lateral displacement. A loud sound (possibly ilar amounts of cracking, although the maximum crack width of
the BFRP rupture) occurred at the B-side when the lateral displace- the C-H was smaller. This could be explained by superior bonding
ment reached 84 mm (Fig. 4(d)). The hysteretic curves of C-H and behavior between the ordinary steel bar and the concrete than
C-S10B85 were similar to each other, indicating that these two between the SFCB and the concrete [11].

Please cite this article in press as: Z. Sun et al., Experimental study on concrete columns reinforced by hybrid steel-fiber reinforced polymer (FRP) bars
under horizontal cyclic loading, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.001
 Z. Sun et al. / Construction and Building Materials xxx (2016) xxx–xxx 5

Bar buckling and SFCB´s

outer FRP ruptured

Steel bar
buckling

Ĭ 150mm
Ĭ 200mm
(a) C-S12 (b) C-S10B49

Steel bar ruptured

Bar buckling and SFCB´s

outer FRP ruptured

Ĭ 300mm
Ĭ 300mm

Cleavage of FRP bar

(c) C-S10B85 (d) C-H

Fig. 4. Failure modes of the concrete columns.

Table 2
Characteristic values of the concrete columns.

Column number dy (mm) Vy (kN) dp (mm) Vp (kN) du (mm) Vu (kN) RCsf_l_S12(%) l (du/dy)
C-S12 6.70 70 10.25 78.97 42.10 67.12 0.78 6.28
C-S10B49 6.57 67 35.36 79.14 46.70 67.27 2.85 7.11
C-S10B85 7.00 73 55.09 88.70 80.90 75.40 3.68 11.56
C-H 6.97 69 40.12 92.37 79.20 78.51 6.64 11.36

3.3. Strain distribution of longitudinal reinforcement two columns had not yielded at this drift level, the strain distribu-
tions along the longitudinal bar are similar to each other, with
Fig. 6 shows the A3/B4 strain-lateral drift relations (in the plas- maximal strains of approximately 2000 le, and the lateral dis-
tic hinge region) of the longitudinal reinforcements on C-S12 and placement of concrete column can be calculated by the integral
C-H. It can be seen that the strain of A3/B4 for C-S12 (Fig. 6(a)) of the curvature along the column height.
increased almost linearly with the increase of column lateral dis- The strain distributions of the steel bars and FRP bars at
placement before the column yielded. After the column yielded, three different drift levels are shown in Fig. 8. When the lateral
the peak tensile strain of A3/B4 remained almost constant while displacement at the column cap reached 14 mm, the FRP bar’s
the lateral drift kept increasing, meaning that the deformation maximum strain of the C-H column was approximately 2.7
increase was caused by the rotation of the significantly concen- times of that of the C-S12 column, and the maximum strain
trated plastic hinge. Fig. 6(b) presents the A3/B4 strain-column of the steel bar in C-H was 1.4 times that of the steel bar in
cap drift relation on the steel bars and the A3 strain on the FRP C-S12.
bar; it shows that the strain on both the steel and FRP bars At 28-mm lateral displacement, the maximum strain of the FRP
increased almost linearly with the development of lateral drift, bar in C-H was approximately 3.4 times that of C-S12, and the
indicating that the plastic strain of the steel bar was propagated maximum strain of the steel bar in C-H was 2.0 times that of the
by the elastic FRP. steel bar in C-S12. Within 240 mm of the column foundation (four
The peak strain development at different positions, from the strain gauges in this region), the strains of the steel bar in C-H were
anchorage region to the column base, was extracted to study the all over 7200 le (Fig. 8(b)), whereas the corresponding strains of
strain distribution along the reinforcement. The strain distribu- the steel bar (within 80 mm to 240 mm) in C-S12 were similar to
tions of the steel bar and the FRP bar of columns C-S12 and C-H those at a 14-mm lateral displacement (less than 3100 le), even
at 7-mm lateral displacement are shown in Fig. 7. Because the the strain gauge at the interface of column base and foundation

Please cite this article in press as: Z. Sun et al., Experimental study on concrete columns reinforced by hybrid steel-fiber reinforced polymer (FRP) bars
under horizontal cyclic loading, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.001
6 Z. Sun et al. / Construction and Building Materials xxx (2016) xxx–xxx

C-S12 C-S10B49 C-S10B85 C-H

(a) Photos of the cracked columns at 21-mm lateral displacement

Maximum Crack width

20 Crack amount 20

Crack amount
Crack width (mm)

15 15

10 10

5 5

0 0
C-S12 C-S10B49 C-S10B85 C-H
Concrete column
(b) The crack width and crack amount
Fig. 5. Comparison of crack development at 21-mm lateral displacement.

5000 12000
A3 A3_steel bar
Column yielding B4 B4_steel bar
4000 10000
A3_FRP bar
3000 8000
Rebar yielding
2000 6000
Strain ( )
)

1000 4000
Strain (

Rebar yielding

0 2000

-1000 0

-2000 -2000
C-S12
C-H
-3000 -4000
-40 -20 0 20 40 -40 -20 0 20 40
Lateral displacement (mm) Lateral displacement (mm)

(a) Steel bar in C-S12 (b) Steel bar and FRP bar in C-H

Fig. 6. Strain-lateral displacement relation of reinforcements in C-S12 and C-H.

of C-S12 was less than 5000 le. This implies that the plastic strain 3.4. Skeleton curve
development of C-S12 primarily concentrated at the column base,
which was inadvertently not captured by the strain gauge at the The average (push and pull) skeleton curves of the four columns
interface of column base and foundation. are shown in Fig. 9(a). It can be seen that the four columns share a
It can be seen in Fig. 8(c) that the strain of the FRP bar increased nearly identical initial stiffness and yielding point. The lateral dis-
dramatically with the increase of lateral displacement after C-H placements were approximately 6.7 mm at the first yielding of the
yielded. However, the increase in strain of C-H’s FRP bar in the steel bar, whereas the yielding displacements of the concrete col-
region of 240–320 mm (from the column foundation) was signifi- umns were approximately 10 mm. After C-S12 yielded, the load
cantly lower than that in the region of 0–240 mm. carrying capacity decreased with the increase of the column cap

Please cite this article in press as: Z. Sun et al., Experimental study on concrete columns reinforced by hybrid steel-fiber reinforced polymer (FRP) bars
under horizontal cyclic loading, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.001
 Z. Sun et al. / Construction and Building Materials xxx (2016) xxx–xxx 7

2500 caused by superior bonding behavior between the steel bar and
the concrete.
2000 με The overturning moment (MOT) of a concrete column caused by
2000 the axial gravidity (P-d effect) can be calculated by MOT = P  d, and
the corresponding section moment at column base (MCB) is calcu-
lated by MCB = v  L + P  d [16], in which v is the horizontal lateral
Strain ( )

1500
force, L is the column height, P is the axial compression load, and d
is the lateral displacement. The ratio between the overturning
1000 moment and column base section moment (MOT/MCB) was illus-
C-S12
Steel bar of C-H
trated in Fig. 9(b), it can be seen that, before the lateral displace-
FRP bar of C-H ment reached 30 mm, the four columns had similar P-d effects
500 because of the similar load carrying capacity. After the lateral dis-
placement reached 40 mm, the MOT/MCB of C-S12 or C-S10B49 was
0
much bigger portion than that of C-S10B85 or C-H, which demon-
-200 0 200 400 600 800 1000 1200 strated that, hybrid reinforced columns can have stronger capacity
Distance from column foundation (mm) to resist the P-d effect. A comparison of the post-yield stiffness
ratio at the same ductility coefficient between C-S12 and C-H (qsf -
e
Fig. 7. Strain distributions of longitudinal reinforcement at 7-mm lateral = 1.09%) shows that an increase of the post-yield stiffness ratio
displacement. (rsf) of longitudinal reinforcement of 27 percent leads to an
increase of approximately 7.4 percent for the corresponding
displacement, which implies a negative post-yield stiffness. The post-yield stiffness ratio (RCsf) of the concrete column. Pettinga
load carrying capacity of C-S10B49 remained stable prior to the et al. [5] noted that if a concrete or steel structure has a positive
rupture of the outer FRP in composite bar S10B49. After the gradual post-yield stiffness of approximately 5%, then the structure can
rupture of FRP, the load carrying capacity of C-S10B49 was similar exhibit a smaller residual drift response under earthquake excita-
to that of C-S12. The skeleton curve of C-S10B85 was found to be tion, whereas increases of post-yield stiffness ratios above 5–10%
almost identical to that of C-H. However, the bearing capacity of may not significantly further reduce the residual/maximum dis-
the latter was slightly better than that of the former. This may be placement response.

5000 C-S12, 7 mm 9000

C-H (steel bar), 7 mm
C-S12, 14 mm 8000 C-H (steel bar), 14 mm
C-S12, 28 mm
4000 7000 C-H (steel bar), 28 mm
Concentration of plastic strain
6000 Propagation of plastic strain
Strain ( )

3000
Strain ( )

5000
2000
4000
2000
3000
2000
1000 2000
1000
0 0
-1000
-200 0 200 400 600 800 1000 1200 -200 0 200 400 600 800 1000 1200
Distance from column foundation (mm) Distance from column foundation (mm)
(a) Steel bar in C-S12 (b) Steel bar in C-H

16000
C-H (FRP bar), 7 mm
14000 C-H (FRP bar), 14 mm
C-H (FRP bar), 28 mm
12000
10000
Strain ( )

The strain of FRP increased dramatically

8000 after SFCB's inner steel bar yielded
6000
4000
2000
2000
0
-2000
-200 0 200 400 600 800 1000 1200
Distance from column foundation (mm)
(c) FRP bar in C-H
Fig. 8. Strain distributions of longitudinal reinforcement in C-S12 and C-H.

Please cite this article in press as: Z. Sun et al., Experimental study on concrete columns reinforced by hybrid steel-fiber reinforced polymer (FRP) bars
under horizontal cyclic loading, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.001
8 Z. Sun et al. / Construction and Building Materials xxx (2016) xxx–xxx

100 20
The load carrying capacity can remail stable after yielding Hybrid reinforced columns can have stronger
stronger capacity to resist the P-δ effect
80 FRP ruptured gradually
15

MOT/MCB (%)
Force (kN)

60 Column yielding
10
First yielding of Negative post-yield
40 longitudinal reinforcement stiffness of C-S12

Stiffness degradation
C-S12 5 C-S12
caused by cracking
20 C-S10B49 C-S10B49
C-S10B85 C-S10B85
C-H C-H
0 0
0 20 40 60 0 20 40 60 80 100
Lateral Diplacement (mm) Lateral Diplacement (mm)
(a) Skeleton curves with P-δ effect (b) The ratio between overturning moment and
column base moment
Fig. 9. Skeleton curves.

3.5. Energy dissipation that of C-H, as shown in Fig. 11(a), which is caused by the more
robust hysteretic curve of C-S12 (Fig. 10(b)).
During cyclic loading, the input energy is dissipated by hys- The normalized Ecum is shown in Fig. 11(b), and the regression
teretic loops. A typical hysteretic loop of a concrete column is pre- equations with reference to Elmenshawi and Brown [15] are listed
sented in Fig. 10, where the area of triangle OAE (SOAE) represents in Eqs. (5) and (6). It can be seen that C-S12 has a higher normal-
the deformation energy (Ad) under a positive loading process, ized dissipated energy than that of the hybrid reinforced columns.
which includes the plastic deformation energy (Ap) and the elastic    2
deformation energy (Ae). The value Ad can be calculated using Eq. Ecum d d
¼ 1:82  0:72  1:30 ð5Þ
(4): Ey C-S12 dy dy

   2
Ad ¼ 0:5  V max  dmax ¼ Ae þ Ap ð4Þ Ecum d d
¼ 1:11 þ 1:59  3:35 ð6Þ
Ey Hybrid dy dy
where Vmax and dmax are the maximal lateral load and displacement
in the corresponding loading cycle, respectively. The equivalent viscous damping coefficient (fe) [17] can be used
In a hysteretic loop, the enclosed area (SABC + SOAE) is the dissi- to represent damping for concrete columns; this can be calculated
pated energy resulting from plastic deformation, and the summa- by Eq. (7), where the influence of the degeneration of strength and
tion of the areas of the hysteretic loops at different cycles is the stiffness can be eliminated:
cumulative dissipated energy (Ecum). The development of Ecum with 1 SBAC þ SCDB
the increase of loading cycles is shown in Fig. 11(a); the four col- fe ¼ ð7Þ
2p SOAE þ SODF
umns had similar Ecum values at the same drift ratio (the difference
was below 10%). It can be seen from the hysteretic curves in Fig. 3 where SBAC and SCDB are the areas enclosed by hysteretic curves BAC
that although the loading capacity of C-H is greater than that of C- and CDB, respectively, and SOAE and SODF are the areas of triangles
S12 at large drift ratios, the envelope area of C-S12 is very close to OAE and ODF, respectively.

150
C-12
C-S10B49
100
C-S10B85

50
Load (kN)

-50

-100
-60 -40 -20 0 20 40 60
Lateral Displacment (mm)
(a) Energy types (b) Hysteretic loops of concrete columns at
42 -mm lateral displacement
Fig. 10. Typical hysteretic loops and different energy types.

Please cite this article in press as: Z. Sun et al., Experimental study on concrete columns reinforced by hybrid steel-fiber reinforced polymer (FRP) bars
under horizontal cyclic loading, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.001
 Z. Sun et al. / Construction and Building Materials xxx (2016) xxx–xxx 9

Fig. 11. Energy dissipation capacity.

The equivalent viscous damping coefficients (fe) at different lat- The fe in different cycles at the same loading step were slightly
eral drifts are illustrated in Fig. 12(a); C-S12 had a greater fe than different, particularly for the first loading cycle. As shown in Fig. 12
the other columns. As for C-S10B49, the fe coefficient ranged (b), the beginning point of the first loading cycle at the 35-mm lat-
between that of C-S12 and C-H. C-S10B85’s fe was slightly greater eral displacement level was the residual displacement of the for-
than that of C-H. mer loading drift level (28-mm lateral displacement), whereas
The equivalent viscous damping coefficients of the four col- the beginning loading points of the second and third loading cycles
umns were regressed with reference to [15] as shown in Eqs. were the residual displacements of the same lateral drift level,
(8)–(10), in which the datum of C-S10B85 and C-H were regressed which resulted in a smaller fe for the first loading cycle.
together. By comparing the Ecum and fe of the ordinary RC column and the
 
d hybrid reinforced column, it can be seen that the advantages of the
fe C-S12 ¼ 0:169 ln þ 0:135 ð8Þ hybrid reinforced column are a more stable bearing capacity and a
dy
  greater deformation capacity. During an earthquake, hybrid rein-
d
fe C-S10B49 ¼ 0:111 ln þ 0:2 ð9Þ forced concrete columns can dissipate earthquake energy without
dy a corresponding loss of strength, which is favorable for preventing
  the collapse of concrete columns at a large drift ratio due to rapid
d
fe C-S10B85 & C-H ¼ 0:043 ln þ 0:244 ð10Þ strength degradation as well as the P-d effect.
dy

Fig. 12. Equivalent viscous damping coefficient and hysteretic loops at one loading gradient.

Please cite this article in press as: Z. Sun et al., Experimental study on concrete columns reinforced by hybrid steel-fiber reinforced polymer (FRP) bars
under horizontal cyclic loading, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.001
10 Z. Sun et al. / Construction and Building Materials xxx (2016) xxx–xxx

4. Conclusions (Nos. 51528802, 51408126), the Natural Science Foundation of

Jiangsu Province, China (No. BK20140631), and the Key Laboratory
This paper conducted horizontal cyclic loading tests on four of Concrete and Prestressed Concrete Structures of the Ministry of
concrete columns; the reinforcements included ordinary steel bars, Education, China (CPCSME2013-02).
steel-FRP composite bars, and hybrid steel/FRP bars. The following
conclusions can be drawn: References

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Please cite this article in press as: Z. Sun et al., Experimental study on concrete columns reinforced by hybrid steel-fiber reinforced polymer (FRP) bars
under horizontal cyclic loading, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.001