Fibre Reinforced Polymer (FRP) is one of the most popular material in the field
of strengthening reinforced concrete beams due to its superior mechanical and
physical properties compared to steel, particularly with respect to tensile
and fatigue strengths. There are several Fibre Reinforced Polymer (FRP) systems
now commercially available for external strengthening of concrete structures.
Amongst these, Carbon Fibre Reinforced Polymer (CFRP) laminate is a popular
choice due to its high strength. With the development of structurally effective
adhesives and plating methods, strengthening using CFRP laminates have increased
tremendously in recent years. However, the plate bonding methods often have
serious premature failure problems due to separation of plates and concrete
rip off along the tensile reinforcing bars (end peeling) or premature shear
failure before reaching their ultimate capacities (Bencardino
et al., 2007; Kim et al., 2007). Investigation
on the failure mechanism of end peeling has been conducted by a number of researchers.
It is reported by Gerco et al. (2007), Yao
and Teng (2007) and El-Mihilmy and Tedesco (2007) that shear and normal
(peeling) stress concentrations at the cut-off point or around the flexural
cracks are the main reason for local failures. Due to these stress concentrations
at plate end, a shear crack can be initiated at the plate end which induces
a horizontal crack at the level of the tension reinforcement which propagates
rapidly towards the load point and eventually causes separation of the plates
(Xiong et al., 2007). However, researchers would
find a solution to minimize end peeling using appropriate end anchors. Unfortunately,
although lot of research studies had been carried out on strengthening r.c.
beams using CFRP laminates, not much informations can be gathered on the effects
of intermediate anchors on end anchored CFRP laminate strengthened beams to
prevent premature shear failure completely. The main goal of the research is
to study the effects of intermediate anchors using steel plate on end anchored
CFRP laminate flexural strengthened reinforced concrete beams in view of failure
loads, failure modes, strain characteristics, deflections and cracking patterns.
MATERIALS AND METHODS
Description of specimens: Three r.c. beams of rectangular cross-sections
were tested in this study. These beams are designated as beams A1, C3
and C4. Beam A1 was left as the un-strengthened control beams specimen
and beams C3 and C4 were strengthened by CFRP laminate (1.2x80×1900 mm).
Both strengthened beams, C3 and C4 were end anchored using L shape anchoring
plates. The anchorage lengths used was 200 mm. From the strengthened beams,
C4 was intermediate anchored in the shear span of the beam using the same
shape of anchoring plates. The anchorage lengths used was 40 mm. Both
of the end and intermediate anchors were of steel plates, 2 mm in thickness
with the vertical dimension of 250 mm (the full height of the beam) and
horizontal dimension of 125 mm (the full width of the beam). The test
variables are shown in Table 1.
|| Beam details
|| Test specimens
Fabrication of specimens: All beam specimens were of 2,300 mm
long, 125 mm wide and 250 mm deep as shown in Fig. 1.
These beams were reinforced with two 12 mm diameter high yield steel bars
in the tension zone. Ten millimeter mild steel bars were used as hanger
bars and 6 mm bars were used for shear reinforcements which were symmetrically
placed as shown in Fig. 1. The spacing of the shear
reinforcements was 75 mm.
Strengthening and anchoring: For all beams, the length of the
bonded plate was maintained at 1900 mm, which covered almost the full-span
length of the beams (Fig. 2). The main reason for the
full span-length strengthening with CFRP laminates was to maximize the
The concrete surface treatment prior to plating works was very important
to guarantee the perfect bonding between concrete and strengthening plates.
Concrete was ground with a diamond cutter to expose the coarse aggregates.
Dusts were then blown out by compressed air. Colma cleaner was used to
remove carbon dusts form the bonding face of the CFRP laminate. The well
mixed sikadur adhesive was then trawled on to the surface of the concrete
specimens to form a thin layer. The adhesive was applied with a special
dome shaped spatula onto the CFRP (Sika CarbaDur) laminates. The plates
were positioned on the prepared surface. Using a rubber roller, the plates
were gently pressed into the adhesive until the material was forced out
on both sides of the laminates. The surplus adhesive was then removed.
L shape end anchors were placed at the end of both of the strengthened
beams (C3 and C4). The intermediate anchors were only placed in the shear
span of the strengthened beam C4 (Fig. 2). It was hope
that the beam can carry the maximum load before failure by either plate
or laminate rupture or concrete compression failure rather than premature
shear failure. The spacing of the intermediate anchorages was 110 mm,
which was equal to half of effective depth (d/2) so that every shear crack
will cross the plate. The plate was sand blasted and the surface preparation
and application methods were similar to that of the plating method. Before
placing the end and intermediate anchors, the adhesive was applied on
the prepared bonding face of the beams and an inner face of the anchors.
The anchor-plates were fixed on to the beam and then pressed by a rubber
roller. After fixing, they were clamped for 3 days for setting.
Materials: Ordinary Portland Cement (OPC) was used in casting
the beams. The maximum size of coarse aggregate used was 20 mm. The concrete
mix was designed with a targeted strength of 30 MPa. The mix proportion
adopted is as shown in Table 2. The compressive strengths
of the concrete were obtained from three cubes after 28 days curing according
to British Standard (BS 1881).
Two 12 mm diameter of high yield deformed bars were used as the tensile
reinforcement. The measured yield and tensile strength of these bars were
551 and 641 MPa, respectively. Ten millimeter diameter mild steel bars
were used as hanger bars in shear span zone. Six millimeter diameter bars
were used for stirrups. The measured yield and tensile strength of the
stirrups were 520 and 572 MPa, respectively. The modulus of elasticity
of all steel bars was 200 GPa. For beam strengthening, CFRP laminates
(Sika CarboDur, S812) were used. The tensile strength and modulus of elasticity
of CFRP laminates were 2800 MPa and 165 GPa, respectively. The design
and ultimate strain of CFRP laminates were 0.0085 and 0.017 according
to the manufacturers instruction.
||Strengthening and anchoring details
|| Beam instrumentations
|| Results of proportions mix design
Instrumentation and test procedure: Figure 3
shows the location of the different instruments used to record data during
testing. Electrical resistance strain gauges were used to measure the
strains in the steel bar, CFRP laminate and the top of the r.c. beams.
Demac gauges were attached along the height of beam at the mid-span region
to measure the horizontal strains. Three linear variable displacement
transducers (LVDTs) were used to measure the vertical deflection of the
beam at mid-span and under the two load points (Fig. 3).
The load was applied incrementally under a load control procedures up
to failure using the Instron 8505 Universal Testing Machine.
RESULTS AND DISCUSSION
Mode of failure: Figure 4 shows the failure modes
of control beam (A1), end anchored CFRP laminate strengthened beam (C3) and
end with intermediate anchored CFRP laminate strengthened beam (C4). The control
beam failed in the conventional flexural manner of steel yielding followed by
crushing of concrete. The end anchored CFRP laminate strengthened beam failed
by premature shear failure. The end with intermediate anchored CFRP laminate
strengthened beam failed by crushing of concrete in ductile manner. It is investigated
that the steel plate intermediate anchors completely prevented the premature
shear failure. Whereas, Bencardino et al. (2007)
and Kim et al. (2007) used CFRP wrap in FRP laminate
strengthened beams to prevent premature shear and they examined that using of
CFRP wrap premature shear failure could not be prevented completely. In this
research mild steel plate was used as intermediate anchors with the spacing
of half of the effective depth of the beam, which had significant effect to
prevent premature shear failure completely. Further, the intermediate anchors
enhanced the shear resisting capacity of the beam. Normally, when the shear
resisting capacity is increased, then the beam would fail by either flexure
or crushing of concrete. In this research, as the beam (C4) was over reinforced
due to high strength of CFRP laminate, it failed by crushing of concrete rather
than by flexure.
|| Mode of failures
Though the beam C4 failed by crushing of concrete, the failure mode had some
ductile characteristics. This was due to yield of the tensile bar before the
failure of the beam that can be seen from the Fig. 5. Such
type of failure has also been reported by a number of researchers (Ei-Mihilmy
and Tedesco, 2001; Smith and Teng, 2002).
Failure load: The experimental failure loads of all the beams
are shown in Table 3. The results showed that the failure
loads of all the strengthened beams (C3 and C4) were higher compared to
the control beam (A1).
In comparison, the failure load of the beam C4 was 8.2% higher than the
beam C3. This is due to the usage of intermediate anchors in the shear
span of beam C4. Because of the intermediate anchors, the beam failed
by crushing of concrete rather than the premature shear failure. Since
the beam C3 failed by premature shear and the beam C4 failed by crushing
of concrete, the failure load of beam C4 was higher. Table
3 shows the comparisons between the measured and theoretical flexural
failure loads. It is seen that the measured failure loads of beams C3
and beam C4 are higher than the theoretical flexural failure load.
|| Load versus bar strain
Bar strain: The load versus bar strains of beams A1, C3 and
C4 are shown in Fig. 5. The strengthened beams indicated
smaller value of bar strains compared to the control beam. However, the
bar strain of beam C4 was lower than the bar strain of beam C3. This was
because of the higher stiffness of beam C4. It is noted that the stiffness
of beam C4 increased due to the use of intermediate anchors.
Figure 5 also shows that, the strains of the control
beam (A1) and strengthened beams (C3 and C4) increased suddenly after
around 10 and 15 kN load, respectively. This would due to the occurring
of first crack (invisible) in the concrete section. Figure
5 also shows that the tensile steel of both strengthened beams (C3
and C4) yielded at around 120 kN load which lead to fail the beams with
ductile manner although the beams were failed by shear and crushing of
Laminate strain: Figure 6 shows that the laminate
strain of the beam C4 was less compared to the beam C3 for its higher stiffness.
The Fig. 5 also shows that the laminate strain increasing
rate of beam C3 and C4 is linear up to 20 kN. After 20 kN load this rate is
suddenly increases due to first crack (as explained before).
|| Test results of failure loads of all beams
|| Load versus laminate strain
The CFRP laminate
strain of beam C3 and C4 is also linear up to 120 kN load (bar yield point).
It is seen from the Fig. 6 that the strain increment of CFRP
laminate was changed after 120 kN load though the CFRP laminate doesnt have
any yield point. This would because of the yield of steel bar.
Concrete compressive strain: The concrete compressive strains
of strengthened beams C3 and C4 were less than the control beam due to
their higher stiffness. At failure the concrete compressive strains of
strengthened beams were also higher than the control beam due to the higher
ailure loads. It is seen from the Fig. 7 that at failure
the concrete strain of beam C4 exceeded 0.0035 which indicates to fail
the beam by crushing of the concrete.
Strain variation on beam depth: Figure 8 shows
that the strain variation and neutral axis depth of all strengthened beams
is similar. This trend is as expected since the materials and the stiffness
of strengthened beams are equal. It was also found that intermediate anchorages
did not have any effect on strain variation.
Deflection: Figure 9 shows the load versus mid-span
deflection curves for all the beams. All the beams indicated linear, elastic
portions of the curves at the initial stages. Both of the strengthened
beams showed smaller deflection compared to the control beam due to their
higher stiffness. The beam C4 showed less deflection than beam C3 because
of its higher stiffness. Figure 8 also shows that the
deflection of beam C3 and C4 suddenly increased after around 120 kN. This
might due to steel bar yielding. When the bar was yielding, the strain
of the bar increased suddenly and would deflect the beam further.
|| Load versus concrete strain
Cracking load: The strengthened beams in general showed higher
cracking loads compared to the control beam (Table 3).
Since first crack load depends on the modulus of rupture of the concrete
and the stiffness of strengthening materials, the first crack loads of
both the CFRP laminate strengthened beams (end anchored and end with intermediate)
were found to be similar.
Crack spacing: The total crack number of beams A1, C3 and C4 was
11, 18 and 18, respectively. The average crack spacing of beams A1, C3
and C4 was 182, 111 and 111 mm, respectively. The strengthened beams showed
less crack spacing than the control beam. However the crack spacing of
beams C3 and C4 was similar. It was noticed that in the case of beam C4,
most of the crack had occurred near every anchoring plate. This would
due to the stress concentration near the anchoring plates.
Crack width: It is seen in the Fig. 10 that
the crack width of all strengthened beams were less than the control beam.
It is also seen that the crack width of beam C4 is lower than the crack
width of beam C3 though the number of crack of both beams are same. This
may due to a slightly stiffer behaviour of beam C4 because of the usage
of intermediate anchors in the shear span zone.
|| Strain variations of beam, (a) Strain at 30 kN load
and (b) Strain at 70 kN load
|| Load versus deflection
|| Load versus crack width
Further, due to the stress concentration near the anchoring plate, most
of the shear crack in the shear span zone had occurred from the end of
the intermediate anchors. Normally shear crack would propagate in an inclined
manner. Since, due to the intermediate anchorage, this shear crack was
not propagated in an inclined manner, the crack width of beam C4 in the
shear span zone was less.
The following conclusions can be drawn from the present study:
||The end and intermediate anchored CFRP laminate flexural
strengthened beams gave higher failure loads than the control beam.
The intermediate anchored strengthened beam showed higher failure
load compared to the beam without intermediate anchors
||The control beam failed in a conventional manner, i.e., flexural
failure. End anchored CFRP laminate strengthened beam failed in premature
shear. End with intermediate anchored CFRP laminate strengthened beam
showed a concrete compression failure tendency with a ductile mode.
Steel plate intermediate anchors had completely prevented the premature
shear failure of CFRP laminate flexurally strengthened r.c. beams
||The reinforcement strains of the strengthened beams were found to
be less than the reinforcement strain of the control beam. At the
same load, the intermediate anchored strengthened beam showed the
least bar strain
||The maximum concrete compressive strain at the top of the mid-span
of the control beam was found to be the least. Since the control beam
and end anchored CFRP laminate strengthened beam failed by the conventional
flexural and shear failure mode respectively, at failure, the concrete
compressive strains were found to be less than 0.0035. For the case
of end with intermediate anchored CFRP laminate strengthened beam,
it failed in a compression failure mode and thus the concrete compressive
strain at failure load was found to be higher than 0.0035
||All strengthened beams showed lesser deflections than the control
beam. At the same load, end with intermediate anchored CFRP laminate
strengthened beam gave slightly lower deflection compared to the end
anchored strengthened beam
||The cracking load of the control beam was found to be less than
the strengthened beams. The crack width of all strengthened beams
was less than the control beam. End with intermediate anchored strengthened
beam showed less crack width compared to end anchored strengthened
beam though the number of cracks of both beams were same
The authors would like to thank the Majlis Penyelidikan Kebangsaan Sains
Negera under the E-science Fund 13-02-03-3022 for providing the fund to
carry out the research reported in this study. Thanks are also due to
Sika Kimia and their staff for providing the technical and materials supports
for this study. The authors would also like to express their gratitude
to whomsoever had contributed to this work either directly or indirectly.