SHEAR CAPACITY OF REINFORCED CONCRETE
COLUMNS RETROFITTED WITH VERY FLEXIBLEFIBER REINFORCED POLYMER WITH VERY LOWYOUNG’S MODULUS
Hu Shaoqing
MEE08165

Supervisor: Susumu KONO**

ABSTRACTFRP with low Young’s modulus (SoftFRP) was developed in the late 1990’s and early 2000’s.
Polyketone fiber is a kind of new material of FRP with low Young’s modulus and made from carbon
monoxide and ethylene. And compared with aramid fiber, its material cost is lower. To investigate theimprovement of shear strength of column wrapped with this kind of FRP sheets, four specimens
among which one was unstrengthened and three specimens were wrapped by different amount of FRP
sheets which provided the equivalent shear reinforcement ratio was 0.04%, 0.08% and 0.12% were
tested. All specimens were tested under lateral reversal cyclic loading and a constant axial load. It isfound that the shear strength of the RC column effectively increased with strengthening FRP sheet.
And it prevents the sudden brittle fracture of the column because of the confinement of FRP sheetswith large deformation capacity. The shear resistance of the FRP sheet becomes effective after theyield of shear reinforcement, and the strain of FRP sheet becomes large. The shear contribution of the
FRP sheets could be explained by truss-arch shear resisting mechanism.
Keywords: FRP with low Young’s modulus (SoftFRP), shear strength, truss-arch mechanism.

1. INTRODUCTION
FRP with low Young’s modulus (SoftFRP) was developed in the late 1990’s and early 2000’s. It wasmade from relatively economic material like polyacetar [Iihoshi 2005] or polyester [Igarashi 2000].
Compared to CFRP, FRP with low young’s modulus has low cost, easy construction, few dust andnoise. So it is applicable to be used as a retrofitting material [Kono 2008]. Polyketone fiber is a kind of
new material of FRP with low Young’s modulus and made from carbon monoxide and ethylene. And
compared with aramid fiber, its material cost is relative low. To investigate the improvement of shear
strength of column wrapped with polyketone fiber sheets, four specimens of columns, one is
un-retrofitted and three other specimens retrofitted, were tested under constant axial load and lateralcyclic load.

2. TEST SETUP
2.1. Specimen
The specimen was a vertical column fixed at the top and bottom, as shown in Figure 1. The stubs of
top and bottom are strong enough as fixed ends for the column. The column had a rectangular

Institute of Engineering Mechanics (IEM), China Earthquake Administration (CEA) , China
Assoc. Prof., Department of Architecture and Architectural Engineering, Kyoto University, Japan**1

cross-section 400mm × 400mm and a total length of 1200mm. All specimens had the same
reinforcement detailing. The steel reinforcement was twelve D16 (SD 295A) for longitudinal
reinforcement and D6 (SD 295A) for transverse reinforcement at 100mm spacing. The shear(span/depth) ratio, 1.5 and the axial force ratio, 0.3 were adopted in the specimen design.
For the strengthened test specimens, the unidirectional FRP sheet strips made from polyketone
were impregnated with epoxy resin and wrapped around the column with different spaces. And theequivalent reinforcement ratios of specimen D15F04, D15F08 and D15F12 provided by FRP were
0.04%, 0.08% and 0.12%, respectively. The width of the FRP strip was 75mm with the thickness of
0.333mm.
The specimen parameters are listed in the Table 1. For Specimens D15F00 and D15F12, thecompressive strength is 18.8MPa. And for Specimens D15F04 and D15F08, the compressive strengthis 14.6MPa. The material properties of steel and FRP material are shown in Table 2.

300

75
F1 150150

300

F2 150150

300

F3 150150

150

F4 150
75

150

1200

(a) D-15-F00(b) D-15-F04

F1
F2

75
F1

225

F2

F3 200F4200

F5F6 200F7 225F8 75(c) D-15-F08

F3Strain Gauge

F4

1 layerF5

2 layers

F6( Unit: mm )(d) D-15-F12

Figure.1 Four specimens and strain gauges on FRP sheet

Specimen
D-15-F00
D-15-F04
D-15-F08
D-15-F12

Materials
Steel
FRP

Table 1 Details of Specimens
Axial
Concrete Young’s
FRP sheetsforce
modulus (MPa)(75mm of width)
ratio
0.3
2275
No
0.3
1942
4 strips×1 layer
0.3
1942
8 strips×1 layer
0.3
2275
6 strips×2 layers

The equivalent
reinforcement
ratio of FRP
0%
0.04%
0.08%
0.12%

Table 2 Material properties of steel bar and FRP sheets
Yield
TensileYoung’sType
Strength (MPa) Strength (MPa) modulus (GPa)
D6
407
520
173
D16
366
527
185
Polyketon fiber
1709
37.6

Yield strain
(%)
0.235
0.198
4.54

Compressive
strength of concreteFc (MPa)
18.8
14.6
14.6
18.8

2.2. Loading and Measuring System
Cyclic reversed horizontal loads were statically applied to the specimens by two 2000kN hydraulicjacks in both positive and negative directions. During the cyclic horizontal loading, vertical axial loads
were also applied by two 8000kN hydraulic jacks. The vertical axial load levels were determined inaccordance with the axial force ratio designed to be 0.3 which was first applied and then maintained
constant during the test. Loading was mainly controlled by measured displacement in terms of themember drift angle which was computed by averaging lateral displacements measured at north andsouth ends of column over the effective length of 1200mm. The first cycle of loading was performed
up to 50kN with load controlled. Subsequently two cycles of repeated loading were applied for eachdrift angle: ±0.1%, ±0.25%, ±0.5%, ±0.75%, ±1.0%, ±2.0%, ±4.0%, ±6.0% by using displacement
controlled. The test was terminated when the specimen could not sustain the axial force.

2

Two displacement transducers were attached to the central section of the top stub and the bottom
stub separately to measure the relative displacement of the specimen. The strains of the longitudinal
reinforcement, shear reinforcement and FRP sheets in the test region were measured by strain gauges
attached on the surface of the steel bars and FRP sheets, respectively.

3. EXPERIMENTAL RESULT
3.1. Test Observation
Figure 2 presented damage of specimens at the drift angle R=4.0%. Shear crack occurred at drift angle
R=0.50%, R=-0.375%, R=0.25%, R=0.25% for Specimen D15F00, Specimen D15F04, Specimen
D15F08, Specimen D15F12, respectively. With the increase of drift angle, more shear cracks formed
and the width of shear cracks increased. For the retrofitted specimens, it was observed that the FRP
sheets and concrete all dilated. For Specimen D15F00, at the first loading cycle at R=4.0%, cover
spalling in the middle of column was observed and the longitudinal bar buckled, then the specimen
could not sustain the axial force. For Specimens D15F04 at the first loading cycle R=4.0%, Specimen
D15F08 at second loading cycle R=4.0%, Specimen D15F12 at first loading cycle R=6.0%, and FRP
sheets fractured, cover spalled off and the longitudinal bar buckled then the axial force could notsustained.

(a) D-15-F00(b) D-15-F04(c) D-15-F08(d) D-15-F12
Figure 2 Damage of specimens (at the drift angel R=4.0%)

Lateral Load (KN)

The maximum load capacity and drift angle at the positive direction and negative direction andthe initial stiffness are presented in Table 3. Compared to Specimen D15F00, Specimen D15F12
increased the ultimate capacity Qmax from 352.0kN to 413.1kN. And the drift angle corresponding to
the maximum lateral load changed from 0.488% to 0.758%. Compared to Specimen D15F04,
Specimen D15F08 increased the ultimate capacity Qmax by 11.5%. The effect retrofitted by FRP on
improving the ultimate capacity is rather obvious The difference of initial stiffness of four specimens
is small. So it can be considered that the FRP did not affect the initial stiffness but the shear strength.
From the envelope of lateral load-drift angle curves, the degradation of shear strength ofSpecimens D15F08 and D15F12 is gentler than that of Specimen D15F00. It may be said that theconfinement of FRP increased the ductility of column.
D15F00400
Table 3 Summary of test results
D15F04
InitialMaximum lateral load capacity
D15F08200
D15F12
stiffness
Negative
Positive
Specimen(105
R (%) Qmax (kN)0
kN/rad) R (%) Qmax (kN)
D15F00
1.70
0.488
352.0
-0.481-359.4-200
D15F12
1.82
0.758
413.1
-0.498-361.4
D15F04
1.74
0.475
334.3
-0.498-311.2-400-330.8
D15F08
1.75
0.513
372.9
-0.401
-6
-4
-2
0
2
4

6

Drift Angle (%)

Figure 3 Envelop of the lateral loaddrift angle relation
33.2. Strain Development in FRP Sheets
The shear contribution provided by the wrapped FRP sheet was studied in detail by the strain gauges
pasted on it. The strain distributions of the FRP sheet are shown in Figure 4. It was observed that thestrain of the FRP sheet was very small before the shear cracks occurred and began to increase very
quickly after shear cracking. And the strain of middle FRP strips is larger than that of end FRP strips.
For D-15-F04 and D-15-F08 at the drift angle R=0.5%, and for D-15-F12 at the drift angle R=0.75%,
the strain increased dramatically. It agrees with the yield of shear reinforcement. And the strain of FRP
sheet at the maximum lateral load did not attain its fracture strain.
1.6

1.6
F1

F2

F3

F1
F2

1.2

F3
F4

F5
F6

F7
F8

F1
F2

0.8

0.4

0

0.8

-2

0

2

4

0.4

6

0.4

(a) D-15-F04

0-4

Drift Angle (%)

F5
F6

0.8

0
-4

F3
F4

1.2

Strain (%)

1.2

Strain (%)

Strain (%)

1.6

F4

-2

0

2

4

6

Drift Angle (%)

-4

-2

0

2

4

6

Drift Angle (%)(b) D-15-F08
Figure 4 Strain developments in FRP Sheets(c) D-15-F12

4. ANALYTICAL SIMULATION
4.1. Shear Strength Calculation Based on FRP Specifications
According to the design and construction guideline of continuous fiber reinforced concrete [AIJ 2001],
the ultimate shear strength of column and beam retrofitted by continuous fiber sheet can be calculated
by Eq. (1).
Qsu  b jt
( p w w ) cot   tan  (1   )bDv B / 2(1)( p  )  p 
w

w

ws

wys

 p w f E fd  fd

p ws ,  wys are the shear reinforcement ratio and the yield strength of shear reinforcement; p wf : theequivalent shear reinforcement ratio of continuous fiber sheet with effective thickness t wf ;p wf  2t wf / b . E fd : The Young’s Modulus of continuous fiber sheet;  fd : Effective strain ofcontinuous fiber sheet.
The effective strain of continuous fiber sheet can be calculated by Eq. (2). And the value E fd  fd
can not exceed 2/3 of the tensile strength  f of continuous fiber sheet.

 fd  0.009  0.0002
If

p wf E fd

B

p wf E fd

B(2)>20, then  fd =0.005;

The effective factor of concrete is given in Eq.(3)v  0.7   B / 200(3)
For Specimen D15F00, the shear strength calculated by using the effective factor of concrete
compressive strength shown in Eq.(3) is lower than the experiment value 352kN. The objective of theexperiment is to find the relation between the amount of retrofitting material and the increase of shear4

capacity. So for Specimen D15F00, the effective factor of concrete compressive strength wasrecalculated according to the calculated shear strength equaling to the experiment data. And thefollowing Eq.(4) was used. The calculated shear strength is consistent with the experiment value
according to the modified effective compressive strength of concrete (Table 4).
v  1.0   B / 200(4)4.2. Shear Resisting Mechanism of Column Retrofitted by FRP Sheets
To further understand shear resisting mechanism of column retrofitted by FRP sheets, the shear
resistant forces of shear reinforcement and FRP are calculated according to the strain obtained fromexperiment respectively. Figure 5 presented the total shear force measured during test versus
calculated shear force carried by the stirrups. For four specimens, there is no quite difference until thedrift angle R=0.25%. After 0.25%, shear force carried by stirrups for Specimen D15F00 is lower thanthat of Specimen D15F12 at the same drift angle. And the shear force carried by stirrups of Specimen
D15F04 is larger than that of Specimen D15F08. Figure 6 showed the calculated shear force carried by
FRP sheets for Specimen D15F04, D15F08 and D15F12. For three retrofitted specimens the calculated
shear force carried by FRP sheets is almost equal to 0.0kN until drift angle R=0.25%. After 0.25%, theshear force carried by FRP sheets increased quickly for Specimen D15F04 and Specimen D15F12, but
increased quite slowly for Specimen D15F08.
160

120
100

D15F04
D15F08
D15F12

VFRP (kN)

140

Vstirrups(kN)

30

D15F00
D15F04
D15F08
D15F12

80
60

20

10

40
20

0

0
0

100

200
300
Total shear force (kN)

0400

Figure 5 Shear force carried by stirrups

100

200
300
400
Total shear force (kN)

500

Figure 6 Shear force carried by FRP sheets

4.3. Analytical Results Compared to Experimental Results.
Table 4 presented the calculated shear strength, calculated flexural strength and test results. The
ultimate flexural strength of column can be accurately predicted based on fiber model using the strainor stress-compatibility theories.
Shear failures occur when the shear capacity of column exceeds prior to the load level reaching
the flexural strength. From this point, it can be said the failure mode of Specimen D15F00 and
D15F04 is shear failure. Shear strength calculated by design equation using the effective factor Eq.(3)is lower than experimental results. And the ratios of the experimental figures to the calculated figuresare 1.28 and 1.18 separately. However, for the design equation using modified effective factor, v theshear strength agreed well on the experiment value. So it maybe said that the design equation
underestimate the ultimate shear strength.
The load level reached the flexural strength at first for the Specimen D15F12 and its behavior
flexural failure mode. The failure mode of Specimen D15F08 is flexural failure due to the longitudinalbar yield when the lateral load reaches the maximum load. Shear strength calculated by design
equation using the effective factor Eq.(3) is underestimated. And the ratios of the experimental figures
to the calculated figures are 1.19 and 1.10 separately.
So if using the design equation to get the ultimate shear strength, some modification should be5considered.
Specimens
D15F00
D15F12
D15F04
D15F08

Table 4 Calculated results compared to test results
Calculated Results(kN)
Test Result (kN)
Flexuralv Eq.(3)v Eq.(4)Strength(kN) Positive Negative
277.4 (1.28) 350.6 (1.00) 384.9 (0.91)
352.0-359.4352.2 (1.10) 425.5 (0.97) 384.9 (1.07)
413.1-361.4274.0 (1.18) 330.9 (1.01) 340.2 (0.98)
334.3-311.2298.6 (1.19) 355.5 (1.05) 340.2 (1.09)
372.9-334.8

Failure mode
Shear
FlexuralShear
Flexural

5. CONCLUSIONS
Polyketone fiber is a kind of new material of FRP with low Young’s modulus and made from carbon
monoxide and ethylene. This paper analyzes the test results of four specimens among which three
specimens wrapped with FRP strips and calculated the shear force contributed by FRP strips tounderstand how it worked after the shear reinforcement yield. And the following results were obtained.
The strains in FRP sheets were higher than those in shear reinforcement after the yield of shear
reinforcement. The shear resistance of the FRP sheet becomes effective after the yield of shear
reinforcement. It can be said that the confining action of column distributed moved from the shear
reinforcement to the FRP sheets. It prevents the sudden brittle fracture of the column because of theconfinement of FRP sheets with large deformation capacity.
For Specimens D15F00 and D15F04, shear failure occurred before bending yield. Shear strength
calculated by design equation using the effective factor Eq.(3) is lower than experiment results.
However, the shear strength using modified effective factor, v agreed well on the experiment value.
So it maybe said that the design equation underestimate the ultimate shear strength. Specimens
D15F08 and D15F12 are flexural yielding precedence. It is observed that the ultimate shear strength
calculation based on FRP specifications is underestimated. It means that the assessment is made in asafe side. So the ultimate shear strength calculation based on FRP specifications is used, somemodification should be considered.

ACKNOWLEDGEMENT
I would like to express my sincere gratitude to Assoc. Prof. S. KONO and the advisor Dr. T. MUKAI
for their continuous support, valuable suggestion and guidance during my study.
REFERENCE
Architectural Institute of Japan (2001), Design and Construction Guideline of Continuous Fiber
Reinforced Concrete, AIJ, (In Japanese), pp.289
Igarashi, S., 2000, Proceedings of the third Japan-Turkey workshop on earthquake engineering,
February 22, Istanbul.
Iihoshi, C., Fukuyama, H. and Tanaka, H., 2005, International Symposium on Earthquake
Engineering Commemorating Tenth Anniversary of the 1995 Kobe, Kobe/Awaji, Japan.
James G. MacGregor, James K. Wight, 2005, Reinforced Concrete Mechanics and Design (Fourth
Edition), Prentice Hall
Kono, S., Doi, M., Lee, J., Tanaka, H., 2008. 14th World Conference on Earthquake Engineering.
Michael P.Collins, 1991, Denis Mitchell, Prestressed concrete structure, Prentice-Hall, Inc.
Website: http://findarticles.com/p/articles/mi_qa5358/is_200503/ai_n21369170/.

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