Behaviour of CFST members under compression externally reinforced by CFRP composites.
Sundarraja, M.C. ; Prabhu, G. Ganesh
1. Introduction
Composite construction may be considered as a reliable choice of
attaining proper balance between the advantages it offers and the cost.
An extensive variety of composite columns are available nowadays, but
the concrete filled steel tubular (CFST) sections are most commonly
used. CFST members are used in diversity of applications due to their
excellent earthquake resistant properties such as high ductility, large
energy absorption capacity and high-strength capacity. Steel tube lies
in the outer limits can serve as formwork for the concrete infill during
construction in addition local inward buckling commonly observed in bare
steel tube columns is effectively prevented. In the meanwhile, aging of
infrastructures concerned with the metallic structures and member
deterioration of steel structures due to corrosion are often reported.
Various strengthening or rehabilitation techniques such as section
enlargement, external bonding of steel plates and fibers, etc. has been
proposed to overcome these problems. In the past, section enlargement
method has been proved as a suitable method for the rehabilitation of
reinforced concrete columns. It was also expected that this method will
be effective in repairing of CFST columns. However, it resulted in a
significant increase in the column cross-section and the construction
time is too long. Compared to the above methods, plate bonding technique
provides a practical and cost effective solution. The earliest
investigators utilized steel plates for external strengthening. Though
the technique was successful in practice, it posed some harms such as
adding self-weight, required heavy lifting equipment to place the plates
in position, difficulty in shaping and fitting in complex profiles and
complication in bonding/welding and furthermore added plates are
susceptible to corrosion which leads to an increase in future
maintenance costs. In contrast, rehabilitation using fibre reinforced
polymer (FRP) composites do not exhibit any of these drawbacks.
Though the composite materials were introduced in the year 1909,
the composite industry began to bloom only after 1930s (Balazs,
Borosnyoi 2001). Glass fibre reinforced polymers (GFRP) were first used
in aircraft radar covers at the end of 1930s (Hollaway 1993) and FRP
boat hulls and car bodies were developed with glass fibres as the major
reinforcement (Lam, Teng 2002). As a non-conductive material, glass was
used as an insulator to prevent galvanic corrosion of metals. However,
under certain conditions of exposure, glass fibres proved to be
sensitive to alkaline environments and moisture attack (Peters 1998). At
the end of 1960s, Royal Aircraft Establishment had developed the carbon
fibre reinforced polymer for special applications (Hollaway 1993).
Unlike glass, carbon is an electrical conductor and hence galvanic
corrosion could take place if carbon fibres are placed in direct contact
with metals (Miller et al. 2001) but such fibres behave very well
against creep deformation and relaxation (Balazs, Borosnyoi 2001). After
introduction of advanced composite materials in the construction
industry, the second generation utilized those materials in external
strengthening technique. The application of carbon fibre reinforced
polymer (CFRP) with reinforced concrete structures has been widely
carried out and reported in the past few decades. However, research
related to FRP applications to steel structures has started quite
recently and there are few applications still in practice due to
uncertainties concerning the long term behaviour of these applications
and the bonding between the composite materials and steel (Hollaway
1994).
One of the first known studies on this topic involved, the use of
CFRP laminates to repair steel structures conducted by Sen and Liby
(1994). Six composite beams were tested under four-point loading. An
epoxy adhesive was used to bond the CFRP laminates to the tension flange
of the steel beam in different configurations. High strength steel bolts
were also used in an attempt to transfer the load to CFRP laminates. The
results indicated that even though significant ultimate strength was
gained but more modest improvement in the elastic response are required.
In another investigation, Jiao and Zhao (2004) studied the performance
of butt-welded very high strength (VHS) steel tubes strengthened with
CFRP under axial tension. Three types of epoxy resins with different lap
shear strength were used. Three kinds of failure modes such as adhesive
failure, fiber tear and mixed failure were observed. The above
investigation concluded that a significant strength can be achieved
using CFRP-epoxy strengthening technique and also they recommended
suitable epoxy adhesive for strengthening of VHS steel tubes.
Photiou et al. (2006) investigated the effectiveness of an
ultra-high modulus, and high modulus CFRP prepreg in strengthening the
artificially degraded steel beam of rectangular cross-section under
four-point loading by using two different wrapping configurations. The
beam containing the ultra-high modulus CFRP was failed when the ultimate
strain of the carbon fibre was reached in the pure moment region. The
failure load exceeded the plastic collapse load of the undamaged beam.
The beams strengthened by using the high modulus CFRP exhibited ductile
response leading to very high deflections even after higher ultimate
load was reached and also neither fibre breakage nor adhesive failure
was observed.
Seica and Packer (2007) investigated the FRP materials for the
rehabilitation of tubular steel structures for underwater applications.
Six tubes were wrapped with CFRP composites. In that two specimens were
prepared under in-air conditions and remaining four were prepared under
seawater curing conditions. Specimens were tested under four point
loading. It was observed that the ultimate strength of the tubes wrapped
under in-air and seawater curing conditions having 16-27% and 8-21% more
than that of bare steel beam respectively. Tao and Han (2007) presented
the results of axial compression and bending tests of fire-damaged
concrete-filled steel tubes repaired using unidirectional CFRP
composites. Both circular and square specimens were tested to
investigate the repair effects of CFRP composites on them. The test
results showed that the load-carrying capacity and the longitudinal
stiffness of CFRP-repaired CFST stub columns increased while their
ductility decreased with the increasing number of CFRP layers. In
another study, Tao et al. (2006) repaired the fire-exposed CFST beams
and columns by unidirectional CFRP composites. The test results showed
that the load-bearing capacity was enhanced by the fibre jackets to some
extent, while the influence of CFRP repair on stiffness was not
apparent. Choi and Xiao (2010) presented a simplified analytical model
of the CFST member confined by CFRP jackets with different parameters in
order to strengthen the traditional CFST column system. The accuracy of
the analytical model results was compared to the experimental results
conducted by the traditional method.
From the past research, it can be observed that there have been
investigations done with the use of CFRP as a strengthening material for
metallic members and also presence of CFRP significantly enhance the
behavior of steel tubular members. However, research related to
strengthening of CFST members using fibre are not widespread and also
more tests are required to derive an optimal combination of fibre
orientation, number of layers and sequence in applying CFRP layers. The
main focus of the study is to experimentally investigate the suitability
of carbon fibre reinforced polymer for strengthening of CFST column
members and also compare the effectiveness of geometric shapes of the
upgrading material (i.e. wrapping scheme). Finally, suitable wrapping
scheme that can be used to repair CFST members was recommended.
Furthermore, to eliminate the galvanic corrosion between steel tube and
CFRP, a thin layer of glass fibre mat was introduced between steel and
CFRP.
2. Materials
2.1. Concrete
The concrete mix proportion designed by IS method to achieve the
strength of 30 N/[mm.sup.2] and was 1:1.39:2.77 by weight. The designed
water cement ratio was 0.35. Three cube specimens of size 150 x 150 x
150 mm were cast and tested at the age of 28 days to determine the
compressive strength of concrete. The test result of concrete is given
in Table 1.
2.2. Carbon fibre
The unidirectional carbon fibre called MBrace 240, fabricated by
BASF India Inc. was used in this study. It is a low modulus CFRP fibre
having modulus of elasticity of 240 kN/[mm.sup.2] and the tensile
strength was 3800 N/[mm.sup.2]. The thickness and width of the fibre was
0.234 mm and 600 mm, respectively. It is fabric type and can be tailored
into any desired shape. The properties of CFRP supplied by the
manufacturer are given in Table 2.
2.3. Adhesive
The MBrace saturant supplied by BASF India Inc. was used in this
study to get sufficient bonding between steel tube and carbon fibre. It
is a two part systems, a resin and a hardener and the mixing ratio was
100:40 (B:H). The properties of saturant supplied by the manufacturer
are summarized in Table 3.
2.4. Steel tube
The square hollow steel tube confirming to IS 4923: 1997 and IS
10262: 1987 having a dimension of 91.5 x 91.5 mm was used in this study.
The thickness and height of the square hollow steel tube were 3.6 mm and
600 mm, respectively. The yield strength provided by the manufacturer
was 250 N/[mm.sup.2].
3. Experimental study
3.1. Specimen fabrication
The 600 mm height square hollow tubes were cut from 6 m length
hollow tubes. To get the flat surface, both ends of the steel tube were
surfaced by the surface grinding machine. Inside portion of the hollow
steel tubes were thoroughly wire brushed to remove the rust and loose
debris presented. Then the hollow steel tube specimens were filled with
concrete and compacted by a steel rod to avoid any flaws or air gaps
that occur inside the specimen. To eliminate the leakage of slurry
during compaction, a steel plate was placed at the bottom prior to
filling concrete. The concrete was cured for 28 days. Surface
preparation of the metal substrate is very important to achieve good
bonding between steel tube and CFRP fabrics. The strength of the
adhesive bond is directly proportional to the quality of the surfaces to
which it is bonded. So the exposed surface of the tubular specimen was
blasted by the coarse sand to remove the rust and also to make the
surface rough one. The entire sand blasted surface was cleaned by using
acetone to remove all contaminant materials before retrofitted with the
fibres. Prior to the columns strengthened by carbon fibre, the glass
fibre fabric was introduced between the steel surface and CFRP
composites to eliminate the galvanic corrosion. Finally, the carbon
fibres were bonded to the exterior surface of the CFST members with the
different wrapping schemes and thicknesses. During wrapping of fibre
fabrics, the resin and hardener are correctly proportioned and
thoroughly mixed together and the excess epoxy and air were removed
using a ribbed roller moving in the direction of the fibre.
3.2. Description of specimens
Among twenty one specimens, eighteen were externally bonded by CFRP
strips having a constant width of 50 mm wrapped with the spacing of 20
mm and 40 mm and remaining three specimens were unbonded. The wrapping
schemes are shown in Fig. 1. The size and length of the columns used
were 91.5 x 91.5 x 3.6 mm and 600 mm, respectively. To identify the
specimen easily, the columns were designated with the names such as
HS-50-20-T1, HS-50-20-T2, HS-50-20-T3, HS-50-40-T1, HS-50-40-T2 and
HS-50-40-T3. For example, the specimen HS-50-20-T3 specifies that it was
strengthened by three (3) layers of 50 mm width horizontal strip (HS) of
CFRP fabrics in transverse direction (T) with the spacing of 20 mm. The
control columns are specified as CC1, CC1 and CC3.
3.3. Experimental setup
The CFST columns were tested in compression testing machine of
capacity 2000 kN. Each member was positioned on the supports taking care
to ensure that its centerline was exactly in line with the axis of the
machine. The verticality of the specimens was checked using plumb bob
and sprit level. The specimens were instrumented to measure longitudinal
axial compression. The load was applied to the column by hydraulic jack
and monitored by using 1000 kN capacity load cell. Axial deformation of
the column was measured by using linear voltage displacement transducer
(LVDT) which was kept at top of the jack. The load cell and LVDT were
connected with the 16-Channel Data Acquisition System to store the
respective data. At the beginning, a small load of 20 kN was applied
slowly, so that the columns settle properly on its supports. Then the
load was removed after checking the proper functioning of the
instrumentation. The trial load was applied again slowly and the column
was then tested to failure by applying the compressive load in small
increments and the observations such as axial deformation and ultimate
load were carefully recorded. The load at which the CFRP starts
rupturing and the nature of failure were also noted for each column. The
experimental setup is shown in the Fig. 2.
[FIGURE 1 OMITTED]
4. Results and discussion
4.1. Failure modes
The columns were loaded to until failure to understand the
influence of carbon fibre fabrics on the axial behavior of CFST members.
Until reach a load of 850 kN 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 934 kN, 928 kN and 923 kN, respectively, which is shown in Fig. 3.
Crushing of concrete was not occurred in order that the applied load was
decreased slowly after the failure load but favourable enhancement in
ductility performance was noticed.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Failure of specimens HS-50-20-T1(1), HS-50-20-T1(2) and
HS-50-20-T1(3) were occurred at the load of 969 kN, 983 kN and 989 kN,
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 columns and thereafter delamination of fibre due to
outward buckling of steel tube was observed on the sides of the CFST
members is shown in Fig. 4. 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 may be attributed to immediate absence of confinement provided by
the CFRP and resulted outward buckling of tubes. The similar failure
mode was observed in the case of specimens strengthened with two layers
of CFRP fabrics [HS-50-20T2(1), HS-50-20-T2(2) and HS-50-20-T2(3)] and
the rupture of fibre was occurred at 250 mm below the top of the column
as shown in Fig. 5. 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 further loading, until reaching a load of 1000 kN
on jack, all three columns were exhibited linear elastic behavior and
thereafter nonlinear response was observed. At the respective failure
load of control column (CC1), no obvious changes in the specimens were
noticed and observed an axial deformation of 6.9 mm.
Among these, the 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 Fig. 6. And the specimen HS-20-T3(3) failed by rupture
of fibre and it was observed at mid height of the specimen at the load
of 1165 N.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
The specimens HS-50-40-T1(1) and HS-50-40T1(2) failed by local
buckling of steel tube observed in unbonded region at the mid height and
at 50 mm from the bottom of the column respectively, and at the load of
956 kN and 972 kN, respectively. In addition no rupture of fibre was
identified which is shown in Fig. 7. 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 989 kN 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-50-40-T2(3) failed by local buckling
of steel tube which was observed at 140 mm from the bottom of the column
and it was observed at mid height in the case of column HS-50-40-T2(2)
which are shown in Figs 8 and 9.
[FIGURE 6 OMITTED]
Until reach a failure load of control column (CC1), there was no
obvious change was 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
8.87 mm, 6.2 mm and 6.72 mm, respectively. After further loading,
initial rupture of fibre was observed at the load of 962 kN, 976 kN and
933 kN, respectively. Among the three columns, HS-50-40-T3(1) and
HS-50-40-T3(3) exhibited local buckling of steel tube without any
rupture of fibre and was occurred at 1033 kN and 1032 kN, respectively
as shown in Fig. 10. 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 1022 kN. In the overview, when increasing the
number of layers, there may be possible failure of local buckling of
steel tube alone rather than fibre rupture.
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
[FIGURE 10 OMITTED]
4.2. Axial stress-strain behavior
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 4.
The axial stress strain behavior of CFST members confined by CFRP
fabrics with respect to control specimen is shown in Figs 11 to 13. From
that it was observed that the control and confined columns exhibited the
elastic behavior at initial stage followed by in-elastic response when
increasing the load further and in addition 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. And also, it was
noticed that columns confined with three layers of CFRP have more
ability to control the axial deformation compared to columns confined by
one and two layers of CFRP. Comparing the behavior of columns
HS-50-20-T1(2), HS-50-20-T2(2) and HS-50-20-T3(1) to that of control
column (CC1), all three columns showed significant control in axial
deformation and enhancement in stiffness, especially, the behavior of
HS-50-20-T3(1) was outperformed and is shown in Fig. 14. Until reach
failure load of CC1, the columns HS-50-20-T1(2), HS-50-20-T2(2) and
HS-50-20-T3(1) displayed linear elastic behaviour and thereafter
nonlinear elastic behaviour was observed as shown in Fig. 11. At the
corresponding failure load of CC1, mid span deflection of specimens
HS-50-20-T1(2), HS-50-20-T2(2) and HS-50-20-T3(1) observed was lesser
than that of CC1.
[FIGURE 11 OMITTED]
The enhancement in axial deformation and its control of above
specimens was 12.2%, 56.66% and 89.05% more than that of control column,
respectively. Comparing the behavior of HS-50-20-T2(2) to that of
HS-50-20-T3(1), axial stress-strain behavior of column HS-50-20-T2(2)
followed the same path of HS-50-20 T3(3) until reach the load of 670 kN
as shown in Fig. 13 and at the same time the enhancement in axial
deformation control of HS-50-20-T3(3) was much better than that of
column HS-50-20-T2(3) which is shown in Fig. 14. From the Fig. 14, 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, in addition, the enhancement in
axial deformation control due to increase in number of layers was also
not proportional. The above nonlinearity in axial deformation control
when 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 FRP strips having a spacing of 40 mm effectively reduce the
axial deformation and also increase the stiffness of the columns as
shown in Fig. 12. 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 FRP layers, the columns HS-50-40-T2(1) and HS-5040-T3(2)
showed better control in axial deformation compared to columns CC1 and
HS-50-40-T1(2). It 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 specimen and
their mid-span deflection at corresponding failure load of control
column was 8.87 mm, 6.2 mm and 6.72 mm, respectively, as shown in Fig.
15. Until reaching a failure load of 510 kN, 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 993 kN onwards
which is shown in Fig. 12. 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 8.75 compared to column HS-50-40-T3(2) which
has a axial deformation of 7.64 mm. The column HS-50-40-T3(2) enhanced
their axial deformation control by 44.76% and 14.90% when compared to
columns HS-50-40-T1(3) and HS-50-40-T2(1), respectively, as shown in
Fig. 15.
[FIGURE 12 OMITTED]
In the case of columns strengthened by CFRP strips having spacing
of 20 mm and 40 mm, the columns with three layers of fibre fabrics in
both the wrapping schemes showed significant control in axial
deformation, especially, the columns having 20 mm spacing of CFRP strips
was outperformed which is shown in Figs 13 and 16. The enhancement in
axial deformation control may be because of more confining pressure
uniformly exerted by the CFRP strips.
Compared to column HS-50-40-T1(3), column HS-50-20-T1(2) followed
the same path of HS-50-40-T1(3) until failure but the load carrying
capacity of column HS-50-20-T1(2) was high which is shown in Figs 13 and
16. The column HS-50-40-T1(3) has axial deformation of 9.92 mm little
higher than that of column HS-50-20-T1(2) which has a axial deformation
of 9.94 mm and is shown in Fig. 16. And also, the column HS-50-20-T2(2)
tends to have more capability of controlling axial deformation compared
to column HS-50-40-T2(1).
[FIGURE 13 OMITTED]
[FIGURE 14 OMITTED]
[FIGURE 15 OMITTED]
Fig. 16 also illustrates that the column HS-50-40-T3(2) has more
axial deformation (10.8 mm) than that of column HS-50-20-T3(1) (9.32 mm)
and furthermore which is 15.87% more than that of HS-50-20T3(1). In
overall, columns strengthened by 50 mm width CFRP strips having spacing
of 20 mm effectively control the axial deformation compared to the
column strengthened by same width of CFRP strips having a spacing of 40
mm.
[FIGURE 16 OMITTED]
4.3. Axial load carrying capacity
Table 4 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 enhance the load carrying capacity of the columns,
especially the columns strengthened by three layers of CFRP strips in
both the spacing were outperformed.
The enhancement in axial load carrying capacity of columns
HS-50-20-T1(3), HS-50-20-T2(2) and HS-50-20-T3(1) was found to be
10.72%, 17.80%, and 30.21% more than that of control column (CC1)
respectively, and is shown in Fig. 17. In similar manner, the columns
HS-50-40-T1(3), HS-50-40 T2(1) and HS-50-40-T3(2) showed 8.68%, 13.51%,
22.19% more load carrying capacity than the control column respectively
which is shown in Fig. 18. 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 were proved. It can be seen from Figs 17 and 18 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 column HS-50-20-T1(3), has higher axial load
carrying capacity of 1008 kN compared to column HS-5040-T1(3) which has
a load carrying capacity of 989 kN is shown in Fig. 19. The enhancement
in load carrying capacity of columns HS-50-20-T2(2) and HS-50-20 T3(1)
is 3.77% and 6.56% respectively more than that of columns HS-50-40-T2(1)
and HS-50-40-T3(2) respectively as shown in fig. 19. The difference in
load carrying capacity is due to drop in confirming pressure exerted by
the CFRP strips when increasing the spacing between the CFRP strips.
[FIGURE 17 OMITTED]
[FIGURE 18 OMITTED]
[FIGURE 19 OMITTED]
Significant enhancement in load carrying capacity was not observed
in the case of columns confined by single layer of CFRP strips in both
the spacing which is due to insufficient generation of confinement
pressure. From the Fig. 19, it can be seen that the axial load carrying
capacity of the confined columns increases as the number of CFRP layers
increases and also enhancement in axial load carrying capacity was not
proportional. Column HS-50-20-T3(1) enhanced their axial load carrying
capacity by 17.54% and 10.55% more than that of columns HS-50-20-T1(3)
and HS-50-20-T2(2) respectively. Similarly, the column HS-50-40-T3(2)
which is having 12.44% and 7.64% more load carrying capacity than that
of 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 significantly enhance axial load carrying capacity and delaying
the buckling of CFST column and also it is suggested that both the
wrapping schemes used in this research work are suitable for
strengthening of columns subjected to axial compression.
4.4. Effect on ductility response
Ductility is defined as the ability of material to plastically
deform without any breaking. Ductility index of the control and CFRP
strengthened beams were found as per Tao et al. (2006) and shown in Figs
20 to 22.
[FIGURE 20 OMITTED]
[FIGURE 21 OMITTED]
[FIGURE 22 OMITTED]
It has been found that the control specimen exhibited more ductile
nature compared to CFRP strengthened beams and also the ductility of the
strengthened specimens decreased when the number of FRP layers
increased. This decrease in ductility is due to the sudden rupture of
CFRP fabrics. It can be seen from the Fig. 22 that ductility of the
confined columns increases as the spacing of the CFRP layers increases.
This increase in ductility may due to reduction of CFRP layers.
5. Conclusions
Horizontal wrapping style of narrow strip of CFRP fabrics is
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 eighteen specimens wrapped with CFRP strips
with different spacing, the following conclusions can be made:
--The delamination of fibre due to outward buckling of steel tube
was observed on the sides of the CFST members only after the rupture of
fibre in the case of specimens with one and two layer of CFRP strips.
Therefore, it can be understood that a good composite action exist
between the two components were confirmed;
--The control and confined columns exhibited the elastic behavior
at initial stage followed by inelastic response when increasing the load
further and in addition 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;
--The enhancement in axial deformation and its control of specimens
HS-50-20-T1(2), HS-50-20T2(2) and HS-50-20-T3(1) was 12.2%, 56.66% and
89.05%, respectively, but it was 6.2%, 42.23% and 36.24%, respectively,
for HS-50-40-T1(3), HS-50-40-T2(1) and HS-50-40-T3(2) more than that of
control column, respectively;
--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, in addition, the enhancement in axial
deformation control due to increase in number of layers was not
proportional;
--In general, 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 enhancement in axial
load carrying capacity of columns HS-50-20-T1(3), HS-50-20-T2(2) and
HS-50-20-T3(1) was found to be 10.72%, 17.80%, and 30.21% more than that
of control column, respectively. In similar manner, the columns
HS-50-40-T1(3), HS-50-40-T2(1) and HS-5040-T3(2) showed 8.68%, 13.51%,
22.19% of more load carrying capacity than the control column,
respectively;
--It was also found that the ductility of the strengthened
specimens decreased when the number of fibre layers increased and also
ductility of the confined columns increases as the spacing of the CFRP
layers increases;
--From the above observations, it can be concluded that external
bonding of CFRP strips significantly enhance axial load carrying
capacity and delaying the buckling of CFST column and also it is
suggested that both the wrapping schemes used in this research work are
suitable for strengthening of columns subjected to axial compression.
doi: 10.3846/13923730.2012.743925
Acknowledgement
This research work has been carried out through the research
funding [Grant No. SR/FT/ET-019/2009] received from SERC-DST, New Delhi,
India under Fast Track Project for Young Scientists.
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M. C. Sundarraja (1), G. Ganesh Prabhu (2)
Thiagarajar College of Engineering, Madurai, Tamilnadu, India
E-mail: 1 mcsciv@tce.edu (corresponding author)
Received 01 Jun. 2011; accepted 08 Jul. 2011
M. C. SUNDARRAJA. Dr, Assistant Professor in School of Civil
Engineering at Thiagarajar College of Engineering, Madurai, India. He is
a member of Institution of Engineers (India). He contributed much to FRP
strengthening in concrete structures through his PhD research work by
publishing papers in International and National Journals and also
through his Post-Doctoral Fellowship carried at Queensland University of
Technology, Australia under Endeavour Awards in FRP strengthening of
high performance steel structures. He extended his research on FRP
strengthening of CFST members by receiving research funding projects
from UGC and DST, India at a total cost of Rs.21.54 lakhs. And also he
is guiding Post Graduate and PhD research students in the field of
strengthening of hollow and concrete filled steel tubular structures
using advanced FRP composites.
G. GANESH PRABHU. PhD research student in Structural Engineering at
Thiagarajar College of Engineering, Madurai, India. He has completed
research work on effect of hole on ultimate strength of steel plates
using ANSYS in his Post Graduate degree. He is working as a Research
Associate in Major Research Project received from UGC, India by Dr M. C.
Sundarraja and also carrying his research work on FRP strengthening of
CFST members under flexure and compression. He has acquired working
knowledge on ANSYS modelling software. Now he is at the stage of
completing his research work and started publishing his work in
International Journals and Conferences.
Table 1. Details of concrete mixes
Dimension of Average Average cube
concrete cube ultimate strength at 28
load (kN) days (N/[mm.sup.2])
150 x 150 x 150 mm 871.66 38.75
Table 2. Properties of carbon fiber
S. No Properties Value
1 Modulus of elasticity 240 kN/[mm.sup.2]
2 Tensile strength 3800 N/[mm.sup.2]
3 Density 1.7 g/[cm.sup.2]
4 Thickness 0.234 mm
Table 3. Properties of saturant
S. No Properties Value
1 Mixed density (kg/litre) 1.13 [+ or -] 0.03
2 Mixing ratio (B:H) 100:40
3 Mixed viscosity at 25[degrees]C 4000 [+ or -] 500
4 Setting time <3 hours at 25 [degrees]C
Table 4. Failure load and Load carrying capacity of all specimens
Load at Maximum
Designation of Failure initial axial
columns load (kN) rupture of deformation
FRP (kN) (mm)
CC1 934 11.98
CC2 928 -- 12.28
CC3 923 -- 11.99
HS-50-20-T1(1) 969 831 8.66
HS-50-20-T1(2) 983 826 7.95
HS-50-20-T1(3) 1008 843 8.33
HS-50-20-T2(1) 1072 923 10.96
HS-50-20-T2(2) 1052 915 10.09
HS-50-20-T2(3) 1043 921 11.27
HS-50-20-T3(1) 1125 943 13.17
HS-50-20-T3(2) 1120 904 11.70
HS-50-20-T3(3) 1185 932 10.29
HS-50-40-T1(1) 956 836 9.73
HS-50-40-T1(2) 972 834 9.76
HS-50-40-T1(3) 989 846 9.98
HS-50-40-T2(1) 1033 912 10.87
HS-50-40-T2(2) 1032 927 11.12
HS-50-40-T2(3) 1022 951 10.76
HS-50-40-T3(1) 1084 962 11.18
HS-50-40-T3(2) 1112 976 11.07
HS-50-40-T3(3) 1099 933 11.23
% of reduction
in axial % of increase
Designation of deformation in axial
columns compared to load carrying
CC1 (kN) capacity (kN)
CC1 -- --
CC2 -- --
CC3 -- --
HS-50-20-T1(1) 14.58 3.75
HS-50-20-T1(2) 12.12 5.25
HS-50-20-T1(3) 16.72 7.92
HS-50-20-T2(1) 56.65 14.78
HS-50-20-T2(2) 62.32 12.63
HS-50-20-T2(3) 60.15 11.67
HS-50-20-T3(1) 92.05 20.45
HS-50-20-T3(2) 91.50 19.91
HS-50-20-T3(3) 91.12 26.87
HS-50-40-T1(1) 5.88 5.05
HS-50-40-T1(2) 7.21 6.81
HS-50-40-T1(3) 6.21 8.68
HS-50-40-T2(1) 42.23 13.52
HS-50-40-T2(2) 31.22 13.41
HS-50-40-T2(3) 39.63 12.31
HS-50-40-T3(1) 50.15 19.12
HS-50-40-T3(2) 36.24 22.20
HS-50-40-T3(3) 49.23 20.77
