CHAPTER 6 AXIAL STRENGTHENING USING CFRP...

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CHAPTER 6

AXIAL STRENGTHENING USING CFRP FABRICS

6.1 AIM OF INVESTIGATION

External reinforcements using steel plates have been used in earlier

attempts to rehabilitate the structures, since 40 years. The most important

problem that imitated is corrosion, difficulties in handling of steel plates, and

connections. Recently, the fiber reinforced polymers composites have been

adding great advantages to construction industry. These composites were

originally developed for aerospace industry. This shows high strength-to-

weight ratio, resistance to corrosion, good creep strain, good fatigue strength,

potential for decreasing cost of installation and repairs. It shows less weight

compared with steel plates, and nonmetallic, non-magnetic properties of FRP

offer a viable alternative to bonding to steel plates.

The main objective of this investigation is to study the compressive

behaviour of the short CFST columns retrofitted using carbon fiber reinforced

polymer fabrics. Its also focuses on the aspect of the structural behavior such

as enhancement in load carrying capacity and stiffness of CFST Columns

strengthened with externally bonded carbon fiber reinforced polymer fabrics

in different patterns. Improvement in axial load carrying capacity and

deformation of FRP jacketed CFST members over un-jacketed members are

reported. The failure modes, efficiency, strength gain and deformability of

strengthened columns are also addressed. Factors influencing the axial stress-

strain behaviour of FRP confined CFST column, such as effectively confined

69

regions and their relationship to jacket properties are identified and discussed.

Further, this work presents a simple comparative study between the

compression members strengthened with carbon fiber reinforced polymer

fabrics as well as the control members.

6.2 DESCRIPTION OF SPECIMENS

The CFST columns were externally bonded by CFRP strips having

a width of 30mm and 50mm wrapped with different spacing. The size and

length of the columns used were 91.5x91.5x3.6mm and 600 mm respectively.

To identify the specimen easily, the columns were designated with the names

such as HS-30-20-T2, HS-30-20-T3, HS-30-40-T2, HS-30-40-T3, HS-30-60-

T2, HS-30-60-T3, HS-50-20-T1, HS-50-20-T2, HS-50-20-T3, HS-50-30-T1,

HS-50-30-T2, HS-50-30-T3, HS-50-40-T1, HS-50-40-T2 and HS-50-40-T3.

For example, the specimen HS-50-20-T3(2) specifies that it was strengthened

by three (3) layers of 50-mm wide horizontal strip (HS) of CFRP fabrics in

transverse direction (T) with the spacing of 20 mm and the numeral within the

brackets indicates the number of specimen. The control or reference

(unbonded) columns were designated as CC1, CC2 and CC3. Table 6.1

summarizes the detailed description of all specimens.

Table 6.1 Detailed descriptions of all specimens

S.No Column designation Column designation description

1 HS-30-20-T2

CFST column confined with horizontal CFRP strips (HS) having a width of 30mm (30) with the spacing of 20mm (20) by two layers in transverse direction (T2).

2 HS-30-20-T3

CFST column confined with horizontal CFRP strips (HS) having a width of 30mm (30) with the spacing of 20mm (20) by three layers in transverse direction (T3).

70

3 HS-30-40-T2


4 HS-30-40-T3


5 HS-30-60-T2


6 HS-30-60-T3


7 HS-50-20-T1

CFST column confined with horizontal CFRP strips (HS) having a width of 50mm (50) with the spacing of 20mm (20) by one layer in transverse direction (T1).

8 HS-50-20-T2


9 HS-50-20-T3


10 HS-50-30-T1


11 HS-50-30-T2


12 HS-50-30-T3


13 HS-50-40-T1


71

14 HS-50-40-T2


15 HS-50-40-T3


6.3 RESULTS AND DISCUSSION - COLUMNS SRENGTHENED

BY 30mm WIDE CFRP STRIPS

This research work intended to examine several aspects related to

the use of CFRP fabrics for strengthening square short columns subjected to

axial compression. The objectives of the study are as follows: (i) to evaluate

the effectiveness of the external CFRP strengthening of short CFST columns,

(ii) to evaluate the effect of wrapping patterns of CFRP layers on the ultimate

strength and ductility of the confined CFST columns (iii) to evaluate the axial

stress-strain behaviour of FRP confined CFST columns (iv) to evaluate the

efficiency of the FRP fabrics in terms of utilization of the strength and

deformation capacity of the FRP materials. Among the seven columns, six

columns were externally bonded by CFRP strip having a constant width of

30mm with spacing varies from 20mm, 40mm and 60mm, and the remaining

column was reference column. The load carrying capacity of the CFRP

strengthened CFST columns improved when increase the number of layers

and decrease the spacing of the strips.

6.3.1 Failure Modes

The failure load and failure modes of all specimens are summarized

in Table 6.2. During the testing of control specimen, there is a huge sound

occurred due to the crushing of in-filled concrete and also buckling observed

72

at the bottom of the specimen as received by J.G.Teng [4] when reached its

ultimate load as shown in Figure 6.1.

Figure 6.1 Failure mode of control column (CC)

In the case of columns strengthened by using two and three layers

of CFRP strips with 20mm spacing (HS-30-20-T2, HS-30-20-T3), the

crushing of concrete was observed initially at the bottom and as a result, the

concrete core expanded laterally, thus producing larger lateral strains. In the

meantime, the CFRP lie in the outer limits started to resist lateral expansion

consequently then they are subjected to tension in lateral direction. The

rupture of fibre was occurred at top edge of the column and thereafter

delamination of fibre due to outward buckling of steel tube was observed on

the sides of the specimen which is shown in Figure 6.2. Therefore, it can be

understood that a good composite action exist between the two components

were confirmed. After rupture of CFRP, the load gets suddenly reduced. The

abrupt reduction in load leads to immediate absence of confinement provided

by the CFRP and resulted outward buckling of tubes.

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Table 6.2 Experimental results of all specimens (30mm wide strips)

Designation of columns

Failure load (kN)

Load at initial

rupture of FRP

(kN)

Maximum axial

deformation (mm)

% of reduction in

axial deformation

compared to CC

% increase in axial

load carrying capacity

CC 850 -- 17.60 -- --

HS-30-20-T2 988 923 13.78 35.98 16.23

HS-30-20-T3 1080 915 12.12 96.95 27.05

HS-30-40-T2 940 843 13.82 28.09 10.58

HS-30-40-T3 990 832 12.43 55.66 16.47

HS-30-60-T2 900 811 16.78 3.45 5.88

HS-30-60-T3 930 792 17.55 11.94 9.41

Figure 6.2 Failure mode of column HS-30-20-T3

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In the case of columns confined by two and three layers of CFRP

strips with 40mm spacing (HS-30-40-T2, HS-30-40-T3), they were not able to

develop more confinement pressure due to more spacing between the CFRP

strips. Finally, they were failed by local buckling of steel tube observed in

unbonded region at the load of 940kN and 990kN respectively. In addition, no

rupture of fibre was identified which is shown in Figure 6.3. Similar failure

was also occurred in the case of columns confined by two and three layers of

CFRP strips with 60mm spacing (HS-30-60-T2, HS-30-60-T3) as shown in

Figure 6.4. From the above observations, it can be noted that when increasing

the spacing of CFRP strips, the unwrapped area will become more and

subjected to maximum strain during loading and the buckling of steel tube

was occurred in the unwrapped zone due to insufficient confining pressure

provided by the steel tubes.


75


6.3.2 Axial Stress-Strain Behaviour

Table 6.2 shows the summary of test results such as maximum axial

deformation and percentage of control in axial deformation with respect to

reference column. The axial stress-strain behavior of CFST members confined

by CFRP with respect to control specimen is shown in Figures 6.5 to 6.7. The

CFST members confined by CFRP fabrics sustained higher ultimate load and

larger axial deformation compared to control column. And also, it was noticed

that the columns confined with three layers of CFRP have more ability to

control the axial deformation compared to columns confined by two layers of

CFRP. Comparing the behavior of columns HS-30-20-T2 and HS-30-20-T3 to

that of control column (CC), both the columns showed significant control in

axial deformation and enhancement in stiffness, especially, the behavior of

HS-30-20-T3 was outperformed as shown in Figures 6.5 and 6.6.

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0

20

40

60

80

100

120

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035

Axial strain

Axi

al st

ress

(N/m

m2 )

CC HS-30-20-T2

HS-30-40-T2 HS-30-60-T2

Figure 6.5 Axial stress strain behaviour of columns with two layers of

CFRP strips – Comparison

At the respective failure load of CC, axial deformation of

specimens HS-30-20-T2 and HS-30-20-T3 observed was 12.34mm and

8.52mm respectively and this was 30% to 90% lesser than that of CC. The

enhancement in axial deformation and its control of above specimens was

35.98% and 96.94% more than that of control column respectively. According

to the Figures 6.6 and 6.8, the columns HS-30-40-T2 and HS-30-40-T3

enhanced their axial deformation control by 28.09% and 55.65% respectively

compared to the control column (CC) and their axial deformation at the

respective failure load of control column was 13.10mm and 10.78mm

respectively. Compared to control column, the enhancement in deformation

control of HS-30-60-T2 and HS-30-60-T3 was not obvious, and they

enhanced their axial deformation control by 3.45% and 11.94% respectively

as shown in Figures 6.6 and 6.8.

77

0

20

40

60

80

100

120

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035

Axial strain

Axi

al st

ress

(N/m

m2 )

CC HS-30-20-T3

HS-30-40-T3 HS-30-60-T3

Figure 6.6 Axial stress strain behaviour of columns with three layers of

CFRP strips – Comparison

0

20

40

60

80

100

120

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035

Axial strain

Axi

al st

ress

(N/m

m2 )

CC HS-30-20-T2HS-30-20-T3 HS-30-40-T2HS-30-40-T3 HS-30-60-T2HS-30-60-T3

Figure 6.7 Axial stress - strain behaviour of all columns - Comparison

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In the case of columns strengthened by three layers of CFRP strips

having spacing of 20mm, 40mm and 60mm, showed significant control on

axial deformation, especially, the columns having 20mm spacing of CFRP

strips was outperformed which is shown in Figure 6.7and 6.8. The

enhancement in axial deformation control may be due to more confining

pressure uniformly exerted by the CFRP strips. The column HS-30-60-T3 has

higher axial deformation of 17.55mm compared to columns HS-30-20-T3 and

HS-30-40-T3 which has an axial deformation of 9.42mm and 11.98mm

respectively and is shown in Figure 6.7.

0

4

8

12

16

20

Axi

al d

efor

mat

ion

(mm

)

Two layers Three layersNumber of FRP layers

20mm Spacing 40mm Spacing 60mm Spacing

Figure 6.8 Axial deformation with respect to number of CFRP layers –

Comparison

The enhancement in deformation control of column HS-30-20-T3

at the respective failure load of columns HS-30-40-T3 and HS-30-60-T3 was

15.86% and 86.30% respectively as shown in Figure 6.8. And also, the

column HS-30-20-T2 tends to have more capability of controlling axial

deformation compared to columns HS-30-40-T2 and HS-30-60-T2. Figure 6.8

also illustrates that the column HS-30-60-T2 has more axial deformation

(16.78mm) than that of columns HS-30-20-T2 (12.98mm) and HS-30-40-T2

79

(14.13mm). In overall, the columns strengthened by 30mm CFRP strips

having a spacing of 20mm, effectively control the axial deformation

compared to the column strengthened by same width of CFRP strips having a

spacing of 40mm and 60mm.

6.3.3 Load Carrying Capacity

Table 6.2 summarizes the maximum load-carrying capacity and

percentage increase in it of all CFRP-strengthened columns compared with

the control column. The experiments aimed at raising the axial strength of

columns and also to advance the lateral confinement pressure by means of

providing external CFRP strips in the form of horizontal lateral external ties.

As expected, the external bonding of CFRP strips considerably enhances the

load-carrying capacity of the columns, especially the columns strengthened by

three layers of CFRP strips was outperformed. The enhancement in axial

load-carrying capacity of columns HS-30-20-T2, HS-30-40-T2 and HS-30-

60-T2 were found to be 16.23%, 10.58%, and 5.88% respectively, more than

that of control column and are shown in Figures 6.9 and 6.10.

0

200

400

600

800

1000

Ulti

mat

e L

oad

(kN

)

CC HS-30-20-T2 HS-30-40-T2 HS-30-60-T2

Designation of Columns

Figure 6.9 Ultimate load of columns with two layers of CFRP strips -

Comparison

80

0

200

400

600

800

1000

Ulti

mat

e L

oad

(kN

)

CC HS-30-20-T3 HS-30-40-T3 HS-30-60-T3


Figure 6.10 Ultimate load of columns with three layers of CFRP strips -

Comparison

Similarly, the columns HS-30-20-T3, HS-30-40-T3 and HS-30-60-

T3 showed 27.05%, 16.47%, and 9.41% respectively, more load-carrying

capacity than the control column, which is shown in Figures 6.9 and 6.10. As

a result, there is a good bonding action exist between the CFRP strips and

steel tube and also external bonding of CFRP strips considerably provided the

confining pressure to the column was proved. It can be seen from Figure 6.11

that, the specimens strengthened by CFRP strips having smaller spacing had

more axial load-carrying capacity than that of columns having larger spacing

of CFRP strips. The enhancements in load-carrying capacity of columns HS-

30-20-T2 and HS-30-20-T3 are 5.10% and 9.10%, respectively, more than

that of columns HS-30-40-T2 and HS-30-40-T3.

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0

200

400

600

800

1000

Ulti

mat

e L

oad

(kN

)

Two layers Three layers

Number of FRP layers20mm spacing 40mm spacing 60mm spacing

Figure 6.11 Ultimate load with respect to number of CFRP layers –

Comparison

The difference in load-carrying capacity is due to drop in confining

pressure exerted by the CFRP strips when increasing the spacing between the

CFRP strips. Significant enhancement in load-carrying capacity was not

observed in the case of columns confined by CFRP strips having a spacing of

60mm which is due to non-development of confinement pressure. From the

Figure 6.11, it can be seen that the enhancement in axial load-carrying

capacity is observed when the number of CFRP layers increased. The column

HS-30-20-T3 enhanced their axial load-carrying capacity by 9.31% more than

the column HS-30-20-T2. Similarly, the column HS-30-40-T3 has 5.26%

more load-carrying capacity than the column HS-30-40-T2. From the above

observations, it can be concluded that external bonding of CFRP strips

significantly enhances the axial load-carrying capacity and delaying of the

buckling of CFST column and also it is suggested that CFRP strips with

smaller spacing is suitable for strengthening of columns subjected to axial

compression.

82

6.4 RESULTS AND DISCUSSION- COLUMN STRENGTHENED

BY 50mm WIDE CFRP STRIPS

Out of 30 columns, 27 columns were externally bonded by CFRP

strips having a constant width of 50mm with the spacing of 20mm, 30mm and

40mm and the remaining three specimens were reference columns. The test

results of the specimens strengthened by 50mm strips are as follows.

6.4.1 Failure Modes

The columns were tested until failure to understand the influence of

carbon fibre fabrics on the axial behavior of CFST members. The summary of

test results such as failure load and load at initial rupture are given in Table

6.3. Until reach a load of 850kN on jack, linear response was observed in all

unwrapped specimens and thereafter non-linear response was observed.

Outward buckling at the top on all four sides of the steel tube was observed in

the case of control specimens CC1, CC2 and CC3 at the load of 934kN,

928kN and 923kN respectively and the failure mode of a control specimen is

shown in Figure 6.12. After reaching the failure load, the applied load was

gradually reduced and enhancement in ductility performance was noticed in

reference column. The failure of specimens such as HS-50-20-T1(1), HS-50-

20-T1(2) and HS-50-20-T1(3) were observed at the load of 969kN, 983kN

and 989kN respectively and at the same time the axial deformation of the

specimens exceeded their permissible limit. The rupture of fibre was occurred

at top edge of the column and thereafter delamination of fibre due to outward

buckling of steel tube was observed on the sides of the specimen which is

shown in Figure 6.13 to 6.15. Therefore, it can be understood that a good

composite action exist between two components were confirmed. After

rupture of CFRP, the load gets suddenly reduced. The abrupt reduction in load

leads to immediate absence of confinement provided by the CFRP and

resulted outward buckling of tubes.

83

Figure 6.12 Failure mode of column CC2

Figure 6.13 Failure mode of column HS-50-20-T1(1)

84



The similar failure mode was also observed in the case of

specimens strengthened with two layers of CFRP fabrics [HS-50-20-T2(1),

85

HS-50-20-T2(2) and HS-50-20-T2(3)] and the rupture of fibre was occurred

at 250mm below the top of the column as shown in Figure 6.16 to 6.18.

Figure 6.16 Failure mode of column HS-50-20- T2(1)


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At the initial stage, crushing sound of resin was observed in the

case of columns HS-50-20-T3(1), HS-50-20-T3(2) and HS-50-20-T3(3).

After reaching the utlimate load, the specimens were further loaded and until

reaching a load of 1000kN on jack, all three columns were exhibited linear

elastic behavior and thereafter non linear response was observed. At the

respective failure load of control column (CC1), no obvious changes in the

specimens were noticed and an axial deformation of 6.9mm was observed.

Among above specimens, HS-50-20-T3(1) and HS-50-20-T3(2) exhibited a

sudden failure which result in rupture of CFRP jackets occurred at the bottom

of specimen after they attained their peak loads and is shown in Figure 6.19

and 6.20. And the specimen HS-50-20-T3(3) failed by rupture of fibre and it

was observed at mid height of the specimen at the load of 1165kN which is

shown in Figure 6.21.

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88


Table 6.3 Experimental results of all specimens (50mm wide strips)


Failure load (kN)

Load at initial

rupture of FRP

(kN)

Maximum axial

deformation (mm)

% of reduction in

axial deformation compared

to CC1

% of increase in axial

load carrying capacity

CC1 934 -- 11.98 -- --

CC2 928 -- 12.28 -- --

CC3 923 -- 11.99 -- --

HS-50-20-T1(1) 969 831 8.66 14.58 3.75

HS-50-20-T1(2) 983 826 7.95 12.12 5.25

HS-50-20-T1(3) 1008 843 8.33 23.68 7.92

HS-50-20-T2(1) 1125 923 10.96 56.65 20.44

HS-50-20-T2(2) 1052 915 10.09 62.32 12.63

HS-50-20-T2(3) 1043 921 11.27 60.15 11.67

89

HS-50-20-T3(1) 1145 943 13.17 92.05 22.51

HS-50-20-T3(2) 1160 904 11.70 91.50 24.19

HS-50-20-T3(3) 1202 932 10.29 85.05 28.69

HS-50-30-T1(1) 965 823 9.94 22.11 3.32

HS-50-30-T1(2) 991 820 8.79 19.58 6.10

HS-50-30-T1(3) 1001 882 10.01 25.12 7.17

HS-50-30-T2(1) 1070 904 11.60 34.11 14.56

HS-50-30-T2(2) 1022 934 11.89 42.12 9.42

HS-50-30-T2(3) 1066 941 12.14 41.15 14.13

HS-50-30-T3(1) 1122 928 11.23 50.01 20.12

HS-50-30-T3(2) 1200 934 11.79 66.24 28.48

HS-50-30-T3(3) 1105 918 12.12 50.12 18.31

HS-50-40-T1(1) 956 836 9.73 5.88 2.43

HS-50-40-T1(2) 972 834 9.76 7.21 4.12

HS-50-40-T1(3) 989 846 9.98 13.08 5.88

HS-50-40-T2(1) 1033 912 10.87 50.16 10.52

HS-50-40-T2(2) 1032 927 11.12 31.22 10.49

HS-50-40-T2(3) 1022 951 10.76 39.63 9.42

HS-50-40-T3(1) 1084 962 11.18 50.15 16.05

HS-50-40-T3(2) 1112 976 11.07 35.90 19.05

HS-50-40-T3(3) 1099 933 11.23 49.23 17.66

In the case of columns wrapped using 50mm width of fibre strips

with the spacing of 30mm [HS-50-30-T1(1), HS-50-30-T1(2) and HS-50-30-

T1(3)], the initial rupture of fibre was observed at the load of 900kN, 930kN

and 933kN respectively. Among these columns, the specimen HS-50-30-

T1(1) failed by local buckling observed at the mid height at the load of 955kN

and, in addition, no rupture of fibre was observed which is shown in Figure

6.22. The failure mode of columns HS-50-30-T1(2) and HS-50-30-T1(3) were

outward buckling of steel tube cum rupture of fibre occurred at top of the

90

column at the load of 1008kN and 1027kn respectively which is shown in

Figure 6.23 and 6.24. From the above observations, it can be noted that when

increasing the spacing of CFRP strips, the unwrapped area will become more

and resulted a maximum strain during loading and also the sufficient

confining pressure was not provided by the FRP composites and due to that

the buckling of steel tube was occurred in the unwrapped zone. The similar

behaviour as same as that of columns HS-50-30-T1(2) and HS-50-30-T1(3)

was occurred in the specimens HS-50-30-T2(1), HS-50-30-T2(2) and HS-50-

30-T2(3) but the rupture of fibre was observed at the bottom of the column as

shown in Figure 6.25 to 6.27 and also the load carrying capacity was higher.

In the case of specimens HS-50-30-T3(1), HS-50-30-T3(2) and HS-50-30-

T3(3), the local buckling of steel tube followed by rupture of fibre was

noticed at the bottom at the load of 1122kN, 1200kN and 1202kN

respectively and finally the fibres were delaminated which is shown in Figure

6.28 to 6.30.


91



92



93



94



95

The specimens HS-50-40-T1(1) and HS-50-40-T1(2) failed by local

buckling of steel tube observed in unbonded region at the mid height and at

50mm from the bottom of the column respectively and at the load of 956kN

and 972kN respectively. In addition, no rupture of fibre was identified which

is shown in Figure 6.31. But the column HS-50-40-T1(3) failed by local

buckling of steel tube followed by rupture of fibre occurred at the top of the

column and at the load of 989kN and, furthermore, rupture of fibre was

observed only at face of the column. From the above observations, it can be

noted that when increasing the spacing of CFRP strips, the unwrapped area

will become more and subjected to maximum strain during loading and the

buckling of steel tube was occurred in the unwrapped zone due to insufficient

confining pressure provided by the FRP composites.


The similar behaviour was occurred in the case of specimens HS-

50-40-T2(1), HS-50-40-T2(2) and HS-50-40-T2(3) but the load carrying

capacity was higher. Among these, the columns HS-50-40-T2(1) and HS-50-

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40-T2(3) failed by local buckling of steel tube which was observed at 140mm

from the bottom of the column but it was observed at mid height in the case of

column HS-50-40-T2(2) which are shown in Figure 6.32 and 6.33. Until reach

a failure load of control column (CC1), there was no obvious change observed

in the columns HS-50-40-T3(1), HS-50-40-T3(2) and HS-50-40-T3(3) and

also their axial deformation was 8.87mm, 6.2mm and 6.72mm respectively.

After further loading, initial rupture of fibre was observed at the load of

962kN, 976kN and 933kN respectively. Among the three columns, HS-50-40-

T3(1) and HS-50-40-T3(3) exhibited local buckling of steel tube with out any

rupture of fibre and was occurred at 1033kN and 1032kN respectively as

shown in Figure 6.34. The column HS-50-40-T3(2) failed by rupture of fibre

occurred at top edge of the columns due to outward buckling of steel tube at

the load of 1022kN is shown in Figure 6.35. In overall, when increasing the

number of layers, there may be possible failure of local buckling of steel tube

alone rather than fibre rupture.


97



98


From the above observations, it can be seen that external bonding

of CFRP strips provides external confinement pressure effectively and

intended to delay the local buckling of steel tube and also the delamination of

fibre was occurred when increasing the number of layers. In all cases, it was

observed that rupture of fibre occurred at the sides of columns in addition no

rupture of fibre at the corner was observed.

6.4.2 Axial Stress- Strain Behaviour

Test results of the columns such as maximum axial deformation

and percentage of control in axial deformation with respect to reference

column are summarized in Table 6.3.The axial stress-strain behavior of the

control columns is shown in Figure 6.36. Figure 6.37, 6.39 and 6.41 show the

axial stress-strain behavior of CFST members confined by CFRP fabrics with

99

respect to control specimen. From that, it was observed that at the initial

stage, the control specimens and CFRP confined columns exhibited the linear

elastic behavior followed by in-elastic response when increasing the load

further. Furthermore, a significant fall in curve was observed at the peak stage

due to sudden rupture of CFRP. The CFST members confined by CFRP

fabrics sustained higher ultimate load and larger axial deformation compared

to control column. Compared to control column (CC1), the specimens HS-50-

20-T1(2), HS-50-20-T2(2) and HS-50-20-T3(1) showed significant control in

axial deformation and enhancement in stiffness, especially, the behavior of

HS-50-20-T3(1) was outperformed which is shown in Figure 6.37 and 6.38.

0

20

40

60

80

100

120

0.000 0.005 0.010 0.015 0.020 0.025

Axial strain

Axi

al st

ress

(N/m

m2 )

CC1 CC2 CC3

Figure 6.36 Axial stress strain behaviour of control columns -

Comparison

100

0

20

40

60

80

100

120

140

0.000 0.005 0.010 0.015 0.020 0.025

Axial strain

Axi

al st

ress

(N/m

m2 )

CC1 HS-50-20-T1(2)

HS-50-20-T2(2) HS-50-20-T3(1)

Figure 6.37 Axial stress strain behaviour of columns HS-50-20 –

Comparison

0

2

4

6

8

10

Axi

al d

efor

mat

ion

(mm

)

CC1 HS-50-20-T1(2) HS-50-20-T2(2) HS-50-20-T3(1)


Figure 6.38 Axial deformation of columns HS-50-20 - Comparison

101

At the failure load of CC1, the mid span deflection of specimens

HS-50-20-T1(2), HS-50-20-T2(2) and HS-50-20-T3(1) observed was

7.43mm, 5.92mm and 4.95mm respectively and their enhancement in axial

deformation control compared to control column was 23.28%, 52.66% and

85.05% respectively. The axial deformation control of column HS-50-20-

T1(2) was very small which is due to insufficient amount of confining

pressure generated by FRP fabrics. The axial stress strain behavior of column

HS-50-20-T3(1) followed the same path of HS-50-20-T2(2) until reach the

load of 670kN but the enhancement in axial deformation control of HS-50-20-

T3(1) was much better than that of HS-50-20-T2(2) which is shown in Figure

6.36. From the Figure 6.38, it can be seen that the columns confined with

three layers of CFRP tend to have more ability to control axial deformation

compared to those columns confined by one and two layers of CFRP.

According to the Figure 6.40, the columns HS-50-30-T1(2), HS-50-

30-T2(1) and HS-50-30-T3(2) enhanced their axial deformation control by

19.58%, 34.11% and 66.24% respectively compared to the control column

(CC1) and their axial deformation at failure load of control column (CC1) was

7.66mm, 6.83mm and 5.51mm respectively. The axial stress strain behavior

of column HS-50-30-T1(2) followed the same path of HS-50-30-T2(1) until

reach the load of 810kN, and thereafter, relaxation in axial deformation

control was observed and, in addition, the column HS-50-30-T2(2) sustained

higher ultimate load and larger axial deformation control which is shown in

Figure 6.39. The mid-span deflection of column HS-50-30-T3(2) at the

respective failure load of columns HS-50-30-T1(2) and HS-50-30-T2(1) was

5.85mm and 7.23mm and the percentage of enhancement in axial deformation

control was 51.11% and 43.70% respectively as shown in Figure 6.40.

102

0

20

40

60

80

100

120

140

0.000 0.005 0.010 0.015 0.020 0.025

Axial strain

Axi

al st

ress

(N/m

m2 )

CC1 HS-50-30-T1(2)

HS-50-30-T2(1) HS-50-30-T3(2)

Figure 6.39 Axial stress strain behaviour of columns HS-50-30 -

Comparison

0

2

4

6

8

10

Axi

al d

efor

mat

ion

(mm

)

CC1 HS-50-30-T1(2) HS-50-30-T2(1) HS-50-30-T3(2)



103

The FRP strips having a spacing of 40mm effectively reduce the

axial deformation and also increase the stiffness of the columns as shown in

Figure 6.41. Since sufficient amount of confining pressure was not generated

by FRP fabrics in the case of column HS-50-40-T1(3), axial deformation

control of column was very small. And at the same time, due to more number

of FRP layers, the columns HS-50-40-T2(1) and HS-50-40-T3(2) showed

better control in axial deformation compared to columns CC1 and HS-50-40-

T1(2).

0

20

40

60

80

100

120

140

0.000 0.005 0.010 0.015 0.020 0.025 0.030

Axial strain

Axi

al st

ress

(N/m

m2 )

CC1 HS-50-40-T1(3)

HS-50-40-T2(1) HS-50-40-T3(2)

Figure 6.41 Axial stress strain behaviour of columns HS-50-40 –

Comparison

From the Figure 6.42, 1t was found that the specimens HS-50-40-

T1(3), HS-50-40-T2(1) and HS-50-40-T3(2) enhanced their axial deformation

control by 6.2%, 42.23% and 36.24% respectively compared to control

104

specimen and their mid-span deflection at corresponding failure load of

control column was 8.87mm, 6.2mm and 6.72mm respectively.

0

2

4

6

8

10

Axi

al d

efor

mat

ion

(mm

)

CC1 HS-50-40-T1(3) HS-50-40-T2(1) HS-50-40-T3(2)



Until reaching a failure load of 510kN, the column HS-50-40-T3(3)

followed the same path of column HS-50-40-T2(3), and thereafter meager

relaxation in deformation control was observed but better control in axial

deformation was observed only after the load of 993kN onwards which is

shown in Figure 6.41. This meager relaxation in deformation control

attributed to the failure of the resin at the interface between the steel tube

substrate and the CFRP fabrics. The column HS-50-40-T2(1) has higher axial

deformation of 8.75mm compared to column HS-50-40-T3(2) as shown in

Figure 6.42. The column HS-50-40-T3(2) enhanced their axial deformation

control by 44.76% and 14.90% respectively when compared to the columns

HS-50-40-T1(3) and HS-50-40-T2(1) as shown in Figure 6.42.

As expected, the columns confined by CFRP in all three spacing,

the axial deformation control of the confined columns increases as the number

of layers increases but the enhancement in axial deformation control was also

not proportional. The above nonlinearity in axial deformation control when

105

increasing the number of layers of fibre may be attributed to crushing of resin

lying in between the fibres. When the resin started to crush, a sudden drop in

substantial load transfer was occurred. As a result, non linearity in axial

deformation control was observed. Furthermore, by increasing the number of

layers of fibre fabrics, the number of resin layers also increased so that more

nonlinearity in axial deformation control was observed. The axial stress-strain

behavior of columns having 20mm spacing of CFRP strips was outperformed

when compared with that of columns strengthened by CFRP strips having

spacing of 30mm and 40mm which is shown in Figure 6.43 and 6.44.

0

20

40

60

80

100

120

140

0.000 0.005 0.010 0.015 0.020 0.025 0.030

Axial strain

Axi

al st

ress

(N/m

m2 )

CC1 HS-50-20-T1(2)HS-50-20-T2(2) HS-50-20-T3(1)HS-50-30-T1(2) HS-50-30-T2(1)HS-50-30-T3(2)

Figure 6.43 Axial stress strain behaviour of all columns – Comparison

It can also be seen that the axial deformation control of the

confined columns increases as the spacing of the CFRP strips decreases. The

columns HS-50-30-T1(2) and HS-50-40-T1(3) have higher axial deformation

106

of 7.49mm and 7.66mm compared to column HS-50-20-T1(2) which has a

axial deformation of 7.41mm is shown in Figure 6.45. The column HS-50-20-

T2(2) enhanced their deformation control by 17.13% and 24.64% compared

to columns HS-50-30-T2(1) and HS-50-40-T2(1) respectively. Figure 6.43,

6.44 and 6.45 also illustrates that the columns HS-50-30-T3(2) and HS-50-

40-T3(2) has more axial deformation (11.7mm and 11.07mm respectively)

than that of column HS-50-20-T3(1) and furthermore which is 12.6% and

64.08% more than that of HS-50-30-T3(2) and HS-50-40-T3(2).

0

20

40

60

80

100

120

140

0.000 0.005 0.010 0.015 0.020 0.025 0.030

Axial strain

Axi

al st

ress

(N/m

m2 )

CC1 HS-50-20-T1(2)HS-50-20-T2(2) HS-50-20-T3(1)HS-50-40-T1(3) HS-50-40-T2(1)HS-50-40-T3(2)

Figure 6.44 Axial stress strain behaviour of all columns - Comparison

107

0

2

4

6

8

10

Axi

al d

efor

mat

ion

(mm

)

One Layer Two Layer Three LayerNumber of FRP layers


Figure 6.45 Axial deformations with respect to number of CFRP layers

6.4.3 Axial Load Carrying Capacity

Table 6.3 summarizes the maximum load carrying capacity and

percentage increase in it of all CFRP strengthened columns compared with

the control column. The main objective of this investigation is to increase the

axial strength of columns and also to advance the lateral confinement pressure

by means of providing external CFRP strips in the form of horizontal lateral

external ties. As expected, the external bonding of CFRP strips considerably

enhance the load carrying capacity of the columns, especially the columns

strengthened by three layers of CFRP strips in all spacing were outperformed.

Compared to control column (CC1), the enhancement in axial load

carrying capacity of columns HS-50-20-T1(3), HS-50-20-T2(1) and HS-50-

20-T3(3) was found to be 7.92%, 20.44%, and 28.69% and also columns HS-

50-30-T1(2), HS-50-30-T2(1) and HS-50-30-T3(2) showed 6.10%, 14.56%,

28.47% of more load carrying capacity than the control column respectively

which is shown in Figure 6.46 and 6.47. In similar manner, column having

40mm spacing of CFRP strips such as HS-50-40-T1(3), HS-50-40-T2(1) and

HS-50-40-T3(2) having 5.88%, 10.59% and 19.05% respectively more load

108

carrying capacity than the control column as shown in Figure 6.48. From the

above observations, it can be known that there is a good bonding action exist

between the CFRP strips and steel tube and also external bonding of CFRP

strips can be able to provide necessary confining pressure to the column were

proved.

0

200

400

600

800

1000

1200

Ulti

mat

e loa

d (k

N)

CC1 HS-50-20-T1(3) HS-50-20-T2(2) HS-50-20-T3(1)


Figure 6.46 Ultimate load for columns HS-50-20 - Comparison

0

200

400

600

800

1000

1200

Ulti

mat

e loa

d (k

N)

CC1 HS-50-30-T1(2) HS-50-30-T2(1) HS-50-30-T3(2)



109

From the Figure 6.49, it can also be seen that the specimens

strengthened by CFRP strips with smaller spacing have more axial load

carrying capacity and the increase in axial load mainly depends upon proper

designed spacing of CFRP strips. When compared with the columns HS-50-

30-T1(2) and HS-50-40-T1(3), the column HS-50-20-T1(3) has more load

carrying capacity which is shown in Figure 6.49. When compared with the

columns HS-50-30-T2(1) and HS-50-40-T2(1), the column HS-50-20-T2(1) is

having 5.1% and 8.90% more load carrying capacity respectively.

Furthermore, the increase in load of column HS-50-20-T3(3) is 2.11% and

8.69% more than that of columns HS-50-30-T3(2) and HS-50-40-T3(2)

respectively. From that, it can be understood that when increasing the spacing

between the CFRP strips, there is a sudden drop in confining pressure exerted

by the CFRP strips. In similar manner, significant enhancement in load

carrying capacity was not observed in the case of columns confined by single

layer of CFRP strips with 40mm spacing which is due to insufficient

generation of confinement pressure.

0

200

400

600

800

1000

1200

Ulti

mat

e L

oad

(kN

)

CC1 HS-50-40-T1(3) HS-50-40-T2(1) HS-50-40-T3(2)



110

From the Figure 6.46 to 6.48, it can be seen that the axial load

carrying capacity of confined columns increases as the number of CFRP

layers increases but the enhancement in axial load carrying capacity was not

proportional. The column HS-50-20-T3(3) enhanced its axial load carrying

capacity by 19.24% and 6.80% more than that of columns HS-50-20-T1(3)

and HS-50-20-T2(1) respectively and also the column HS-50-30-T3(2) which

is having 21.12% and 12.64% more load carrying capacity than that of

columns HS-50-30-T1(2) and HS-50-30-T2(1) respectively. Similarly, the

column HS-50-40-T3(2) enhanced its load carrying capacity by 12.44% and

7.64% compared to the columns HS-50-40-T1(3) and HS-50-40-T2(1)

respectively. From the above observations, it can be concluded that external

bonding of CFRP strips delayed the buckling of CFST column and

significantly enhanced the axial load carrying capacity and also it can be

suggested that CFRP strips having spacing of 20mm and 30mm used in this

research work are suitable for strengthening of columns subjected to axial

compression.

0

200

400

600

800

1000

1200

Ulti

mat

e L

oad

(kN

)

One layer Two layer Three layerNumber of FRP layers


Figure 6.49 Ultimate load for all columns - Comparison

111

6.4.4 Ductility Responses

Ductility is defined as the material can be able to plastically deform

without any breaking. Ductility has generally been measured by a ratio called

the ductility index or factor (μ). The ductility index is usually expressed as a

ratio of rotation (θ), curvature (φ), deflection (displacement) (Δ), and

absorbed energy (E) at failure (peak load) divided by the corresponding

property at the yield load. In the present study, ductility was obtained based

on displacement method. The ductility index of the specimen was calculated

by the following equation.

y

u

(1)

Where u is the deformation at the peak load and y is the

deformation at the yield point.

0

0.5

1

1.5

2

2.5

Duc

tility

Inde

x (D

I)

CC1 HS-50-20-T1(3) HS-50-20-T2(1) HS-50-20-T3(3)


Figure 6.50 Ductility index of columns HS-50-20 - Comparison

112

The failure modes of the all specimens strengthened by CFRP

fabrics were outward buckling of the steel tube followed by the rupture of

FRP fabrics. From the above observations, it was confirmed that, the steel

tube lies in the outer limits significantly contributes to the strength of the

section. The ductility index (DI) of the all strengthened beams compared to

control columns is shown in Figure 6.50 to 6.52.

0

0.5

1

1.5

2

2.5

Duc

tility

Inde

x (D

I)

CC1 HS-50-30-T1(2) HS-50-30-T2(1) HS-50-30-T3(2)



0

0.5

1

1.5

2

2.5

Duc

tility

Inde

x (D

I)

CC1 HS-50-40-T1(3) HS-50-40-T2(1) HS-50-40-T3(2)



113

Form the Figure 6.50 to 6.52, it was observed that, though the

strengthened beams exhibited explosive failure, but the external bonding of

CFRP fabrics did not much affect the ductility of the CFST section and the

ductility of the CFST section increases as the number of CFRP layers

increases. The reason may be attributed to low yield strain value of the steel

tube and confinement pressure exerted by the CFRP fabrics. As a result of

low yield strain value of the steel tube, steel tube may be started to yield

initially, at the same time, CFRP lies in the outer limits provide required

confinement pressure to the steel tube and delay the local buckling of the steel

tube. Finally as a result of outward buckling of the steel tube, the CFRP

stretch to relatively high strain values before providing their full strength and

the rupture of fibre occurred. So when increasing the number of layers, the

confinement pressure exerted by the CFRP fabrics increased as a result the

ductility of the strengthened specimen is increased.

0

0.5

1

1.5

2

2.5

Duc

tility

Inde

x (D

I)

One layer Two layer Three layerNumber of FRP layers


Figure 6.53 Ductility index of all columns - Comparison

Compared to CC1, the columns HS-50-20-T1(3), HS-50-20-T2(1)

and HS-50-20-T3(3) enhanced their ductility index by 17.10%, 29.56%,

36.60% respectively as shown in Figure 6.50 . and also columns HS-50-30-

114

T1(2), HS-50-30-T2(1) and HS-50-30-T3(2) showed 12.33%, 18.10%,

32.32% respectively of more ductility than the control column which is shown

in Figure 6.51. In similar manner, the columns having 40mm spacing of

CFRP strips such as HS-50-40-T1(3), HS-50-40-T2(1) and HS-50-40-T3(2)

having 9.25%, 17.39% and 24.10% respectively increase in ductility than the

control column as shown in Figure 6.52. From the Figure 6.53 it was also

observed that, the effect of increase in spacing between the CFRP strips on

ductility enhancement was not obvious.

6.5 ANALYTICAL STUDY

6.5.1 Prediction of the Axial Strength of FRP Confined CFST

Column

6.5.1.1 Existing confinement models

Jerome F. Hajjar and Brett C. Gourley (1995) proposed the

following equation for predicting the load carrying capacity (Po) of the CFST

column under axial compression.

cconcreteysteelo fAfAP (2)

Where yf = yield stress of steel tube, cf = cylinder compressive

strength of concrete and steelA and concreteA are cross sectional area of steel tube

and filled concrete. The axial load carrying capacity ( RdplN , ) of the CFT

columns according to EC4 can be determined by summing up the strengths of

the steel tube and the concrete core as in Equation 3. The application of EC4

is restricted to composite columns with concrete cylinder strength and steel

yield stress not greater than 50 and 355 MPa, respectively.

115

ckcysRdpl fAfAN , (3)

e

Rdpl

NN , (4)

Where eN = Elastic buckling load of the member (Euler critical load)

22 )(

e

ee l

EIN (5)

Where eEI)( = Effective stiffness of the composite column

ccdaae IEIEEI 8.0)( (6)

Where Ia and Ic are the moments of inertia of the cross sectional area of

the steel tube and the concrete respectively. Ea and Ecd are the young’s

modulus of the steel tube and the concrete. In the case of square columns, it is

necessary to consider the capacity reduction due to local buckling of the steel

tube wall of the column with large (B/t) ratio rather than the confinement

effect of the steel tube. For predicting the axial load of CFT column ( uN ), by

taking in to account the large (B/t) ratio, the modified equation given by Kenji

Sakino et al (2004) as follows:

cucscrsu fAAN (7)

),min( sysycr s (8)

97.600.4128.0698.01 2

s

sy

EtB

S

(9)

Where sy = Yield strength of steel tube.

When a FRP confined concrete column is subjected to axial

compression, as a result of concrete core expand laterally, thus this lateral

116

expansion is resisted by FRP lies in the outer limits as they are subjected to

tension in lateral direction. Lam and Teng (2002) proposed the equation for

calculating the confining pressure exerted by the FRP.

D

tff frpfrp

l

2 (10)

Where D = Diagonal length of square cross section, frpf = tensile strength of

FRP in the hoop direction and frpt = thickness of the FRP confinement.

6.5.1.2 Proposed approach model

Based on the above confinement models, new models were

proposed herein for predicting the axial load capacity of CFRP confined

CFST column. These models are simple one; in addition, additional

developments are required to take into account the effect of concrete strength,

yield strength of steel tube and height of columns. When a concentric axial

load is applied to a CFRP bonded CFST column (assuming the load is applied

uniformly across both materials), the steel tube lies in the outer limits and the

concrete core will both begin to expand laterally, in the meanwhile CFRP lies

in the outer limits started to resist that lateral expansion by providing

confinement pressure as they are subjected to tension in the lateral direction.

When the CFRP reached its ultimate lateral confining pressure, failure of

CFRP will be occurred. For the general case of a CFT column with the

number of CFRP layers corresponding to 1 ≤ n ≤ 4, the lateral confinement

pressure provided by the CFRP can be calculated by the following equations.

116.013

nDntf

f frpfrplcon (11)

123.015

4 n

nff

CFRP

frpfrplcon

(12)

117

Gross

CFRPfrp A

A (13)

Where lconf is lateral confinement pressure exerted by the CFRP

strips having spacing of 20mm, 30mm and 40mm. n and CFRP are the number

of CFRP layers (n = 1, 2,3,4) and static design safety for CFRP ( CFRP =1.2)

respectively. frp is the FRP volumetric ratio. CFRPA and GrossA are the cross

sectional area of the CFRP and CFST column member. For predicting the

compressive strength of CFRP confined CFST square column cconf ' , the

following formula was proposed.

unconuncon

lconccon f

ffkf

1' (14)

Where unconf and k are unconfined compressive strength of CFST

column and effective confinement coefficient. The proposed effective

confinement coefficient ( k ) value for the column confined by CFRP strips

having a spacing of 20mm, 30mm and 40mm are 3, 2.5 and 2 respectively.

The following equation was proposed to determine the axial load carrying

capacity of unconfined CFST member.

ckcysuncon fAfAf (15)

The calculated axial load carrying capacity CFRP confined CFST

columns are listed in Table 6.4 along with the failure load obtained from the

experiments. The average percentage of difference between calculated and

experimental value is ±5%.

118

Table 6.4 Experimental and analytical results - Comparison


Ultimate load (kN) Ptheo/Pexp

Difference ∆= 100

exp

exp XP

PP theo

(%) Experimental

Pexp Theoretical

Ptheo CC1 934 -- -- -- CC2 928 -- -- -- CC3 923 -- -- --

HS-50-20-T1(1) 969 991

1.021 -2.167 HS-50-20-T1(2) 983 1.007 -0.712 HS-50-20-T1(3) 1008 0.982 1.786 HS-50-20-T2(1) 1125

1073 0.952 4.800

HS-50-20-T2(2) 1052 1.011 -1.8061 HS-50-20-T2(3) 1043 1.026 -2.685 HS-50-20-T3(1) 1145

1174 1.024 -2.445

HS-50-20-T3(2) 1160 1.011 -1.121 HS-50-20-T3(3) 1202 0.975 2.413 HS-50-30-T1(1) 965

982 1.015 -1.554

HS-50-30-T1(2) 991 0.988 1.110 HS-50-30-T1(3) 1001 0.979 2.098 HS-50-30-T2(1) 1070

1049 0.979 2.150

HS-50-30-T2(2) 1022 1.024 -2.446 HS-50-30-T2(3) 1066 0.982 1.782 HS-50-30-T3(1) 1122

1133 1.008 -0.891

HS-50-30-T3(2) 1200 0.943 5.667 HS-50-30-T3(3) 1105 1.024 -2.443 HS-50-40-T1(1) 956

971 1.014 -1.464

HS-50-40-T1(2) 972 0.997 0.206 HS-50-40-T1(3) 989 0.988 1.921 HS-50-40-T2(1) 1033

1025 0.990 0.968

HS-50-40-T2(2) 1032 0.991 0.872 HS-50-40-T2(3) 1022 1.001 -0.098 HS-50-40-T3(1) 1084

1093 1.007 -0.738

HS-50-40-T3(2) 1112 0.982 1.799 HS-50-40-T3(3) 1099 0.993 0.637

119

6.6 CONCLUSION

Horizontal wrapping style of narrow strip of CFRP fabrics was

proposed in this study for improving the confinement pressure of concrete

filled steel tubular members externally. From the experimental data obtained,

the failure modes, axial stress-stain behaviour, ultimate load carrying capacity

and the contribution of FRP fabrics on CFT columns were discussed. And

also analytical model was developed for predicting the axial load capacity of

CFRP confined CFST columns. Based on the compressive tests on specimens

wrapped with CFRP strips with different spacing, the following conclusions

can be made

6.6.1 Columns Strengthened By 30mm Wide CFRP Strips

The columns strengthened by 30mm CFRP strips having a

spacing of 20mm, effectively control the axial deformation

compared to the columns strengthened by same width of CFRP

strips having a spacing of 40mm and 60mm.

The external bonding of CFRP strips considerably enhances the

load-carrying capacity of the columns, especially the columns

strengthened by three layers of CFRP strips was outperformed.

The columns HS-30-20-T3, HS-30-40-T3 and HS-30-60-T3

showed 27.05%, 16.47%, and 9.41% respectively more load-

carrying capacity than the control column.

The external bonding of CFRP strips significantly enhances the

axial load-carrying capacity and delaying of the buckling of

CFST column and also it is suggested that CFRP strips with

smaller spacing is suitable for strengthening of columns

subjected to axial compression.

120

6.6.2 Columns Strengthened By 50mm Wide CFRP Strips

Until reach a load of 850kN on jack, a linear response was

observed in all unwrapped specimens and thereafter non-linear

response was observed. Outward buckling at the top on all four

sides of the steel tube was occurred in the case of control

specimens CC1, CC2 and CC3 at the load of 934kN, 928kN and

923kN respectively.

The columns strengthened by 50mm CFRP strips having a

spacing of 20mm were failed by rupture of fibre which was

occurred at top edge of the columns and thereafter delamination

of fibre due to outward buckling of steel tube was observed on

the sides of the CFST.

When the spacing between the CFRP strips is increased, the

unwrapped area will become more and resulted in maximum

strain during loading and also the sufficient confining pressure

was not provided by the FRP composites and due to that the

buckling of steel tube was occurred in the unwrapped zone.

The columns confined with three layers of CFRP tend to have

more ability to control axial deformation compared to those

columns confined by one and two layers of CFRP. The CFST

members confined by CFRP fabrics sustained higher ultimate

load and larger axial deformation compared to control column.

The enhancement in axial deformation control of columns with

smaller spacing of CFRP strips such as HS-50-20-T1(2), HS-50-

20-T2(2) and HS-50-20-T3(1) compared to control column

121

observed at the failure load of control specimen was 23.28%,

52.66% and 85.05% respectively.

The columns HS-50-30-T1(2), HS-50-30-T2(1) and HS-50-30-

T3(2) enhanced their axial deformation control by 19.58%,

34.11% and 66.24% respectively compared to the control

column (CC1) and their axial deformation at the respective

failure load of control column (CC1) was 7.66mm, 6.83mm and

5.51mm respectively.

When compared to columns HS-50-30-T2(1) and HS-50-30-

T3(2), the axial deformation control of column HS-50-30-T1(2)

was very small which is due to insufficient amount of confining

pressure generated by FRP fabrics.

Until reach the load of 350kN, similar axial stress strain

behavior was observed in the case of columns HS-50-40-T1(3),

HS-50-40-T2(1) and HS-50-40-T3(2) and thereafter due to more

number of FRP layers, the columns HS-50-40-T2(1) and HS-50-

40-T3(2) showed better control in axial deformation compared

to columns CC1 and HS-50-40-T1(3).

The specimens HS-50-40-T1(3), HS-50-40-T2(1) and HS-50-

40-T3(2) enhanced their axial deformation control by 13.08%,

50.16% and 35.90% respectively compared to control specimen

and their mid-span deflection at the respective failure load of

control column was 7.66mm, 5.99mm and 6.74mm respectively.

The axial deformation control of confined columns increases as

the number of layers increases, but the enhancement in axial

deformation control was also not proportional. The above

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nonlinearity in axial deformation control may be attributed to

crushing of resin lying in between the fibres when increasing

the number of layers of fibre.

The axial stress-strain behavior of columns having 20mm

spacing of CFRP strips was outperformed when compared with

that of columns strengthened by CFRP strips having spacing of

30mm and 40mm.

The column HS-50-20-T2(2) enhanced its deformation control

by 17.13% and 24.64% compared to columns HS-50-30-T2(1)

and HS-50-40-T2(1) respectively.

The columns HS-50-30-T3(2) and HS-50-40-T3(2) have more

axial deformation (11.7mm and 11.07mm respectively) than that

of column HS-50-20-T3(1) and furthermore which is 12.6% and

64.08% more than that HS-50-30-T3(2) and HS-50-40-T3(2).

The external bonding of CFRP strips considerably enhance the

load carrying capacity of the columns, especially the columns

strengthened by three layers of CFRP strips in all spacing were

outperformed.

When compared to control column (CC1), the enhancement in

axial load carrying capacity of columns HS-50-20-T1(3), HS-

50-20-T2(1) and HS-50-20-T3(3) was found to be 7.92%,

20.44%, and 28.69% respectively.

The specimens HS-50-30-T1(2), HS-50-30-T2(1) and HS-50-

30-T3(2) enhanced their axial load carrying capacity by 6.10%,

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14.56% and 28.47% respectively compared to reference

column.

In similar manner, the columns having 40mm spacing of CFRP

strips such as HS-50-40-T1(3), HS-50-40-T2(1) and HS-50-40-

T3(2) having 5.88%, 10.59% and 19.05% respectively more

load carrying capacity than the control column.

The axial load carrying capacity of the confined columns

increases as the number of CFRP layers increases but the

enhancement in axial load carrying capacity was not

proportional

The specimens strengthened by CFRP strips with smaller

spacing have more axial load carrying capacity and the increase

in axial load mainly depends upon proper spacing of CFRP

strips.

When compared with the columns HS-50-30-T2(1) and HS-50-

40-T2(1), the column HS-50-20-T2(1) is having 5.1% and

8.90% more load carrying capacity respectively.

The increase in load of column HS-50-20-T3(3) is 2.11% and

8.69% more than that of columns HS-50-30-T3(2) and HS-50-

40-T3(2) respectively.

From the above, it can be seen that the CFRP strips with

smaller spacing delayed the buckling of CFST column and

enhanced axial load carrying capacity and also the increase in

axial load mainly depends upon proper spacing of CFRP strips.

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External bonding of CFRP fabrics did not much affect the

ductility of the CFST section in addition ductility of the CFST

section increases as the number of CFRP layers increases and

also effect of increase in spacing between the CFRP strips on

ductility enhancement was not obvious.

When compared to CC1, the columns HS-50-20-T1(3), HS-50-

20-T2(1) and HS-50-20-T3(3) enhanced their ductility index by

17.10%, 29.56%, 36.60% respectively.

The columns having 40mm spacing of CFRP strips such as HS-

50-40-T1(3), HS-50-40-T2(1) and HS-50-40-T3(2) having

9.25%, 17.39% and 24.10% respectively increase in ductility

than the control column.

CHAPTER 6 AXIAL STRENGTHENING USING CFRP...

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Transcript of CHAPTER 6 AXIAL STRENGTHENING USING CFRP...