World Journal of Engineering
Experimental studies and theoretical models for concrete
columns confined with FRP composites: A review
Journal:
World Journal of Engineering
Manuscript ID
WJE-01-2018-0026.R4
Manuscript Type:
Review Paper
Keywords:
FRP composites, circular and non-circular columns, confinement, Stress-
strain model
World Journal of Engineering
World Journal of Engineering
Experimental studies and theoretical models for concrete
columns confined with FRP composites: A review
Abstract
Purpose - Advanced fibre reinforced polymer (FRP) composites have been increasingly used over
the last two decades for strengthening, upgrading, and restoring degraded civil engineering
infrastructure. Substantial experimental investigations have been conducted in recent years to
understand the compressive behaviour of FRP-confined concrete columns. A considerable number of
confinement models to predict the compressive behaviour of FRP strengthened concrete columns
have been developed from the results of these experimental investigations. The purpose of this paper
is to present a comprehensive review of experimental investigations and theoretical models of
circular and non-circular concrete columns confined with FRP reinforcement.
Design/methodology/approach – The paper reviews previous experimental test results on circular
and non-circular concrete columns confined with FRP reinforcement under concentric and eccentric
loading conditions and highlights the behaviour and mechanics of FRP confinement in these
columns. The paper also reviews existing confinement models for concrete columns confined with
FRP composites in both circular and non-circular sections.
Findings - This paper demonstrates that the performance and effectiveness of FRP confinement in
concrete columns have been extensively investigated and proven effective in enhancing the structural
performance and ductility of strengthened columns. The strength and ductility enhancement depend
on the number of FRP layers, concrete compressive strength, corner radius for non-circular columns,
and intensity of load eccentricity for eccentrically loaded columns. The impact of existing theoretical
models and directions for future research are also presented.
Originality/value – Potential researchers will gain insight into existing experimental and theoretical
studies and future research directions.
Keywords FRP composites, circular and non-circular columns, confinement, stress-strain model
Paper type Literature review
1. INTRODUCTION
Over the last three decades, the application of advanced composite materials as external
reinforcement for strengthening and retrofitting existing civil engineering infrastructure has received
significant research attention. Conventional retrofitting techniques, including concrete and steel
jacketing, have been used extensively for the repair and rehabilitation of reinforced concrete (RC)
structures. Several researchers have investigated the influence of these jacketing methods on the
compressive behaviour of RC columns and found that they are useful in enhancing the performance of
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World Journal of Engineering
these columns (Bousias et al., 2006; Julio and Branco, 2008; Bousias et al., 2007; Vandoros and
Dritsos, 2006; Choi et al., 2013; Xiao and Wu, 2003; Campione, 2012; Kaish et al., 2012; Garzón-
Roca et al., 2011; Lai and Ho, 2015; Vandoros and Dritsos, 2008; Kaish et al., 2013). Bousias et al.
(2006) investigated the seismic behaviour of rectangular concrete columns retrofitted with RC
jacketing and fibre reinforced polymer (FRP) wrapping under cyclic loadings. The results of the
investigation show that concrete columns retrofitted with RC external jacketing experience better
performance in cyclic deformation capacity than FRP-wrapped concrete columns. Vandoros and
Dritsos (2006) studied the influence of interface treatment methods of concrete jacketing, such as
roughening the surface of the original column, using steel dowels, and roughening the surface
combined with steel dowels, on the behaviour of strengthened concrete columns under displacement-
controlled earthquake simulation loading. The results confirm that the different interface treatment
procedures for connecting the jacket to the original column could significantly affect the modes of
failure and crack patterns of strengthened columns. The results also show that strengthened RC
columns with combined roughening and dowels experience the best performance in strength,
deformation capacity, and energy dissipation capacity. Choi et al. (2013) investigated the effects of
steel jacketing on the bond strength of concrete and the lateral effectiveness of circular RC columns.
Their study indicated that steel jacketing could convert splitting bond failure to pull-out bond failure
as well as enhancing concrete bond strength. Moreover, they found that the steel jackets can delay
yielding of longitudinal steel reinforcement, hence preventing spalling of concrete, which resulted in
increased ultimate drift and displacement ductility of the confined RC columns. Regardless of their
significant advantages in regard to strength and ductility enhancement, these jacketing systems also
have some inherent shortcomings, including that they are labour intensive and time-consuming and
could possibly increase the cross-sectional area of structural members. FRP composites have been
used effectively in recent years as alternative materials for rehabilitating, strengthening, and upgrading
damaged RC structures due to their superior tensile strength, corrosion resistance, durability, and light
weight compared to steel jacketing (Hollaway and Teng, 2008; Teng et al., 2002; Hollaway, 2010).
Several experimental studies have proven that FRP wrapping of RC columns is an effective means of
enhancing their strength and ductility as it provides confinement to the concrete core (Ozbakkaloglu
and Oehlers, 2008; Moshiri et al., 2015; Vincent and Ozbakkaloglu, 2014; Mirmiran et al., 1998;
Pessiki et al., 2001; Chaallal et al., 2003; Chaallal et al., 2000; Sezen and Miller, 2011; Ilki et al.,
2008; Alecci et al., 2014; Ilki and Kumbasar, 2003; Wang et al., 2017; Sheikh et al., 2007; Silva and
Rodrigues, 2006; Rodrigues and Silva, 2001; Rodrigues and Silva, 2001; Cui and Sheikh, 2010b;
Masia et al., 2004; Lam and Teng, 2004; Matthys, 2000; Parghi and Alam, 2018).
This paper presents a comprehensive review of previous experimental investigations into and
confinement models of FRP-confined concrete columns in both circular and non-circular cross-
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World Journal of Engineering
sections subjected to a concentric and eccentric load. The study begins by reviewing the available
experimental research studies on such columns and subsequently highlights the behaviour and
mechanics of FRP confinement in circular and non-circular concrete columns. Finally, the paper
reviews stress-strain models that have been proposed to predict the strength and axial strain of
concrete columns confined by FRP composites in circular and non-circular cross-sections.
2. EXPERIMENTAL INVESTIGATIONS
2.1 Axially Loaded FRP-Confined Circular Concrete Columns
Researchers have conducted numerous empirical studies to assess the performance of circular
concrete columns confined by FRP composites under axial loads. Mirmiran et al. (1998) investigated
the influence of different test parameters on the behaviour of FRP-confined concrete columns
subjected to axial loading. These test parameters included the type of concrete cross-section, length-to-
diameter ratio, and adhesive bond. To examine the influence of concrete cross-sectional shape, the
researchers tested a series of 12 square concrete columns with a 152.5mm × 152.5mm cross-section
and height of 305mm, and thirty 152.5mm × 305mm cylindrical specimens. The square columns had a
corner radius of 6.35mm. Unidirectional E-glass fibre tubes and polyester resin were used to confine
all the concrete column specimens. The FRP tubes had a varying thickness of 1.45, 2.21, and 2.97mm.
The results showed that the glass fibre reinforced polymer (GFRP) confinement in the non-circular
section was insufficient in restraining the concrete in the core compared to the uniformly confined
circular concrete columns. Concerning the effect of GFRP confinement in non-circular concrete
columns, the authors introduced a modified confinement ratio (MCR) given by:
=
(1a)
=
(1b)
where D is the internal dimension of the tube, R is the corner radius, f
l
is the confinement pressure, f
frp
and t
frp
are the hoop strength of FRP tube and jacket thickness, and
is the confinement ratio for the
equivalent circular section. Moreover, the results confirmed that no strengthening is expected for an
MCR<15% because of the insufficient FRP confinement of the concrete core in non-circular sections.
The authors suggested that rounding sharp corners could improve the effectiveness of the GFRP jacket
and concluded that the gain in axial stress and strain of the strengthened columns depends on FRP
jacket strength and stiffness.
Berthet et al. (2005) investigated the compressive behaviour of axially loaded short cylindrical
concrete columns confined with carbon and glass fabrics. The parameters investigated include
compressive strength of concrete, number of FRP layers, and type of FRP reinforcement. In this study,
five grades of concrete were used to prepare the concrete cylinders (20MPa, 40MPa, 50MPa, 100MPa,
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World Journal of Engineering
and 200MPa). Three concrete grade specimens were 160mm in diameter and 320mm in height, and
two were 70mm in diameter and 140mm in height. The cylindrical concrete columns were wrapped
with high tensile strength carbon fibre reinforced polymer (CFRP) and GFRP reinforcements. The
results demonstrated a significant increase in ultimate axial strength and strain due to an increased
number of FRP layers. The ultimate capacity of strengthened concrete cylinders was also found to be
dependent on confinement pressure and concrete strength. Furthermore, the study concluded that the
mechanical confinement efficiency of FRP wraps decreased to about 15% and 25% due to the increase
in the strength of the concrete core.
Matthys et al. (2006) studied the behaviour of large-scale FRP-confined cylindrical concrete
columns subjected to axial loads. They prepared a series of six large-scale RC cylinders with a
diameter of 400mm, height of 2000mm, and a concrete compressive strength of 36.1MPa. The RC
columns were strengthened with CFRP sheets, GFRP fabrics, and hybrid fibre reinforced polymer
(HFRP) fabrics. The results showed that FRP confinement is an efficient means of enhancing the
strength and ductility of RC columns. However, strength and ductility gain depend on the stiffness and
tensile strength of the FRP fabric material. The authors also reported that the tensile strain of FRP
reinforcement was much higher than the effective hoop failure strain considering the linear elastic
behaviour of the FRP composites. Figure-1 illustrates the mechanism of FRP rupture experienced by
the strengthened columns.
Almusallam (2007) studied the performance of axially loaded concrete cylinders confined with
E-glass fabrics material having a tensile strength of 540MPa and elastic modulus of 27GPa. The plain
concrete columns had a dimension of 150mm × 300mm and concrete strength ranging from 40 to
100MPa. The findings show that GFRP laminates, when used as external reinforcement to concrete
cylinders, could enhance the axial and lateral strength of concrete cylinders up to 110% and provide
ductility enhancement. Moreover, the author confirmed that the strengthened cylindrical columns with
normal concrete strength experienced a significant percentage gain in ultimate strength compared to
the wrapped cylinders with high-strength concrete, as shown in Figure-2.
Sheikh et al. (2007) investigated the behaviour of large-scale concrete-filled prefabricated glass
FRP shells subjected to a concentric load. A series of 17 cylindrical concrete columns with a diameter
of 356mm and height of 1524mm were fabricated and tested. The column specimens were prepared
with concrete cured for 28 days and compressive strength of 30MPa. The effect of test variables,
including number of GFRP layers, fibre orientation, and amount of longitudinal and lateral steel
hoops, was examined. It was found that the prefabricated GFRP shells could be used as permanent
formwork as well as an effective confinement reinforcement for concrete columns. Moreover, the
results also revealed that the cylindrical columns with inclined GFRP shells experienced more ductile
behaviour compared to columns with longitudinal or lateral GFRP shells.
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