EFFECTS is no doubt that steel has been the

EFFECTS
OF CURVATURE ON FLEXURAL PERFORMANCE OF GFRP REINFORCED CONCRETE BEAM

 

S.Kajanan

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Department of Civil Engineering, University
of Moratuwa, Katubedda , Moratuwa, Sri Lanka.

[email protected]

Supervisor: Dr.J.C.P.H.Gamage

Department of Civil Engineering, University of
Moratuwa, Katubedda , Moratuwa, Sri Lanka.

 

Abstract: There is no doubt that steel has been the most widely used material by
mankind, however it must be realized that steel is not always the best material
to use. A main problem in
deteriorating the life of reinforced concrete structure is corrosion of the
reinforcing steel due to this the industry is moving towards the fibre
reinforcement methods. Fibre Reinforced Polymer (FRP) bar, a non-corrosive
material which is a viable substitute for steel in preventing the corrosion. Carbon
Fibre Reinforced Polymer and Glass Fibre Reinforced Polymer bars are emerged as
promising alternative to traditional steel with excellent result in terms of
corrosion resistance. In case of GFRP, they have high tensile strength and
light in weight but they are brittle in compression and have lower stiffness
than steel reinforcement. Although GFRP technology has been use for several
years, some countries do not have adequate knowledge to use this technology
effectively. Many
studies are going on using GFRP bars as alternative material for steel. GFRP
bars behave linearly elastic up to failure and concrete beam reinforced with
GFRP bars exhibit a lower post-cracking bending stiffness comparing with steel
and steel reinforcement cannot be used at all situations because of its high
elastic modulus. Although the flexural performance of concrete beam using GFRP
bars has been extensively studied, there have not been much studies so far
addressing the effects of curvature on flexural performance of concrete beam
reinforced with GFRP bars. Because of its
flexibility, it can be preferred in concrete beams of curved structures. Therefore the present study addresses on gaining an insight in to
the flexural behaviour performance of GFRP Reinforced curved concrete beam.

Keywords:  GFRP; curved beam; flexural performance;
Reinforcement

 

1.   
Introduction

Glass
Fibre Reinforced Polymer (GFRP) is a composite material made of polymer
reinforced with fibres. GFRPs are commonly used in harsh environment on
Bridges, outside garages, off shore structures, aerospace, automotive marine
and construction industries. Corrosion of steel is and the oxidation of iron in
moisture condition leads to the spalling of concrete which intern reduce the
strength of the structure. Reducing this corrosion effects GFRP is usable due
to its non-corrosive nature. The repair and maintenance of the infrastructure
such as bridges are costly and require a lot of time to complete, and often the
end result is not satisfactory. Damages to steel reinforcement exposed to
de-icing salts is the main factor affecting the life of bridge structure. These GFRP bars have high tensile strength, low
weight to strength ratio, good fatigue resistance and good non-magnetic
properties. Although the use of GFRP has become a popular reinforcement
material in the recent construction which is cheaper
than other FRP bars and hence its transportation cost also getting reduced .Designing
with GFRP does not required any special kind of knowledge, it is similar as
reinforcement concrete structure.

There are
considerable number of studies that have been carried out on flexural
performance of concrete beam using GFRP. An experimental study has been carried
out by G.Naveen kumar to investigate the effect of reinforcement ratio, cracks
patterns, deflection nature and other parameters related to flexural behaviour
of GFRP concrete beam. It was observed that the GFRP reinforced beam undergoes
more deflection for small loads. (G Naveen Kumar and Karthik Sundaravadivelu, 2017) .There are some
other observation has been made in studies done by S.Yamini roja which
indicated that the crack forming load was found early in GFRP reinforced beam due
to its low modulus of elasticity and mainly they fail in flexural zone due to
cracks. (S. Yamini Roja1, P. Gandhi2, DM. Pukazhendhi2 and R.
Elangovan3, 2014)
Further studies carried out by M.B.Varma, stated that GFRP bars having roving
shows more flexural strength than plain GFRP bars. (M.B.Varma, 2011) Although there have
been many studies conducted on GFRP bars but due to its limitation of
serviceability criteria the usage of GFRP bars limited and nowadays it’s very
important to go for curved structure specially in bridge structures using GFRP.

2.      
Background

2.1    
Nature of GFRP  bars

GFRP bars are generally manufactured by
the pultrusion process using thermoset polymeric resins with 75% glass fibre
composition. GFRP fibres are anisotropic and have high tensile strength and
also have linear stress-strain behaviour. (Imjai, 2007)

Figure 1: Tensile properties of steel
and various FRP bars (Pilakoutas.K, 2002)

Generally the mechanical properties of
glass fibre reinforced polymer reinforcements are influenced by the
characteristics, orientation and shape of the particular fibre, fibre/matrix
volumetric ratio, on the manufacturing processing of the fibre and the bonding
at the interface between fibres and matrix. The placement of  the fibres in place, transferring  and distributing stresses through fibres,
bringing a lateral support against buckling under compression and protect  fibres from abrasion and surrounding
environment are done by the resin matrix.

When considering the usage of fibre in
civil engineering applications, the durability and the capacity because it is
very important to maintain the structural performance in the severe changing
environmental conditions where they are in use. There are three different
phases of material such as fibre, resin and interface. The durability of FRP
reinforcing rods is related to these phases. As the constructions will be
exposed to various environments which leads to the deteriorations and
degradations. This will lead to the reduction in the long term durability and
performance.

Due to non-corrosive nature of GFRP it
is advantageous for civil infrastructures especially in marine and salt
environment. Increasing the GFRP reinforcement ratio is key factor for
enhancing load carrying capacity and controlling deflection (Ashour AF). Due to low elastic
modulus, GFRP concrete beam shows higher deflection and larger crack widths
comparing to the steel reinforced structure. (Toutanji HA).GFRP bars have
relatively low stiffness in comparison with steel, which results in large
deflections. They show a brittle behaviour than the traditional steel
reinforcements. This often makes the limit of deflection and crack width at
service loads the governing criteria in design of members. (Chidananda S. H,
2017)

2.2    
Flexural Performance

Flexural strength of beam reinforced with GFRP is
more than normal RC structure. Two factors decides the flexural strength of
concrete beam reinforced with GFRP.

1.       Number of Roving

2.       Percentage of fibre (M.B.Varma, 2011)

 

Figure 2: Different type of GFRP bars

The flexure behaviour of GFRP reinforced beams
depends on the low modulus of elasticity and the rupture strain of the beams.
In a balanced reinforced section, when the compression concrete reaches its
maximum, the tensile reinforcement reaches its ultimate strength.  The ultimate load of 23% for beam prototype
reduced and failure of beam is under flexure by using GFRP as the main
reinforcement. When replacing GFRP as the main and shear reinforcement, it
showed 33% reduction in ultimate load and the beam showed shear failure. The
GFRP reinforced beam failures occurs due to the bond failure between GFRP rods
and the concrete and reduced post cracking stiffness. The shear strength of
beam changes in a significant level due to the effect of using GFRP rods in
transverse direction. Even if we increase the strength of concrete, there won’t
be any significant change in the strength of beam. The change in the ratio of
longitudinal reinforcement decides the mode of failure. (G Naveen Kumar and Karthik
Sundaravadivelu, 2017)

The flexural strength is determined by the formula

f = Pf x L/b d2                                                    (1)

f is Flexure strength (MPa), Pf is Central point
load through two point loading system (KN), b is Width of beam in mm and d is
Depth of beam in mm. (M.B.Varma, 2011)

2.3 Tests on GFRP concrete beam

The direct pull out test between GFRP and concrete
shows the failure of GFRP rod layers. Also it reveals that the increment in
diameter of the bar decreases the strength of the bond. The concrete beams
reinforced with the GFRP bars shows more strain and deflection values. When
observing the behaviour of stress strain graphs for this, the curve is linear
before cracks and beam behaves linearly with the reduced stiffness after
cracks. (G Naveen Kumar and Karthik Sundaravadivelu,
2017)

The bond strength is defined as,

Bond strength = P/?dL                                            (2)

Where P is the applied load; d is the bar diameter;
and L is the embedment length. (Xingyu Gu, 2016)

An investigation was carried out using rectangular
GFRP beams under static loading and impact loading to investigate load-deflection
behaviour, energy absorption capacity, crack pattern and failure mode. It was
observed pre-and post-cracking behaviour under four point bending. However, post-cracking,
the bending stiffness was significantly lower as a result of the low elastic modulus
of the GFRP reinforcement bars. (Matthew Goldston, 2016)

Another experimental study was conducted to check
strain distribution and it was noticed that for the high value of strain GFRP
were ruptured and for low value GFRP did not rupture at the beam failure and
also it was concluded that neutral axis increases with reinforcement ratio and
having higher compressive strength possessed higher neutral axis depth. (Ali S. Shanour, Ahmed A. Mahmoud, Maher A.
Adam, Mohamed Said, 2014)

The sectional analysis used for traditional RC beams
can be used to predict the failure mode of GFRP RC beams accurately. The
failure mode of the GFRP RC beams can be indicated by using the ratio of the
beam reinforcement to the calculated balanced reinforcement. Concrete crushing
on the top surface occurred for GFRP RC beams reinforced with more than the
balanced reinforcement. The rupture of the GFRP reinforcement bars governed, while,
for the GFRP RC beams reinforced with lower than the balanced reinforcement.
Before beam flexural behaviour starts contributing to resisting the impact
load, the resistance of GFRP RC beams under impact loading is observed to be
controlled by inertia forces at first contact. (M. Goldston, 2016)

The major factors of resisting dynamic forces are
geometrical properties of the beam and the total mass. The over-reinforced
beams were  observed to experience minor
inclined shear cracking and crushing of concrete cover around the impact zone at
approximately 45 angles regardless of the shear capacity of the GFRP RC beams,
resulting in a ”shear plug” type of failure under impact loading.  Whereas, it was showed that the GFRP RC beams
under static loading were flexural critical. Therefore, it is important to
consider the shear behaviour of flexure-critical GFRP RC beams in dynamic
modelling or in designing beams for impact loads. (M. Goldston, 2016)

Strength of concrete whether it is higher or lower
shows bilinear response for load-deflection behaviour under static loading. The first part
of the curve up to cracking represents the behaviour of the un-cracked beams.
The second part represents the behaviour of the cracked beams with reduced stiffness.
GFRP RC beams designed as over-reinforced with 1.0% and 2.0% reinforcement
ratio showed signs of reserve capacity or “ductility” prior to total failure. (Matthew Goldston, 2016)

2.4
Past studies

Researches carried out mainly about the
flexural performance of concrete beam using GFRP bars but is not focused on the
effects of curvature. The uses of GFRP in curved structure like bridges have
considerably reduced the total cost and increased the life time. So this case
study would be very helpful and give some ideas about the curvature effects.

 

 

 

 

 

 

 

 

3.      
Conclusion

When the past studies in the matter are
observed and the literature review was conducted as above, it could be
concluded that a research gap exists.

4.      
Acknowledgements

The author wish to acknowledge the research
supervisor Dr (Mrs) J.C.P.H. Gamage for her guidance through the research.

5.      
References

440, A. C. (n.d.).
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FRP Bars.

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