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Journal ArticleDOI

Behavior of Large-Scale Rectangular Columns Confined with FRP Composites

01 Feb 2010-Journal of Composites for Construction (American Society of Civil Engineers (ASCE))-Vol. 14, Iss: 1, pp 62-71

Abstract: This paper focuses on axially loaded, large-scale rectangular RC columns confined with fiber-reinforced polymer (FRP) wrapping. Experimental tests are conducted to obtain the stress-strain response and ultimate load for three field-size columns having different aspect ratios and/or corner radii. Effective transverse FRP failure strain and the effect of increasing confining action on the stress-strain behavior are examined. Existing strength models, the majority of which were developed for small-scale specimens, are applied to predict the structural response. Since some of them fail to adequately characterize the test data and others are complex and require significant calculation, a simple design-oriented model is developed. The new model is based on the confinement effectiveness coefficient, an aspect ratio coefficient, and a corner radius coefficient. It accurately predicts the axial ultimate strength of the large-scale columns at hand and, when applied to the small-scale columns studied by other investigators, produces reasonable results.
Topics: Ultimate load (55%)

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Behavior of Large-Scale Rectangular Columns Confined with FRP
Composites
H. Toutanji
a
, M. Han
a
, J. Gilbert
b
and S. Matthys
c
a
Dept. of Civil and Env. Eng., University of Alabama in Huntsville, Huntsville, AL 35805, USA
b
Dept. of Mechanical and Aerospace Eng., University of Alabama in Huntsville, Huntsville, AL
35805, USA
c
Dept. of Structure Eng., Ghent University, Technologiepark-Zwijnaarde 9, B-9052, Gent,
Belgium
ABSTRACT
This paper focuses on axially loaded, large-scale rectangular reinforced concrete (RC) columns
confined with fiber reinforced polymer (FRP) wrapping. Experimental tests are conducted to
obtain the stress-strain response and ultimate load for three field size columns having different
aspect ratios and/or corner radii. Effective transverse FRP failure strain and the effect of
increasing confining action on the stress-strain behavior are examined. Existing strength models,
the majority of which were developed for small-scale specimens, are applied to predict the
structural response. Since some of them fail to adequately characterize the test data and others are
complex and require significant calculation, a simple design-oriented model is developed. The
new model is based on the confinement effectiveness coefficient, an aspect ratio coefficient, and
a corner radius coefficient. It accurately predicts the axial ultimate strength of the large-scale
columns at hand and, when applied to the small-scale columns studied by other investigators,
produces reasonable results.
Journal of Composites for Construction. Submitted June 10, 2008; accepted June 11, 2009;
posted ahead of print June 22, 2009. doi:10.1061/(ASCE)CC.1943-5614.0000051
Copyright 2009 by the American Society of Civil Engineers

Accepted
Manuscript
Not
Copyedited
2
Keywords: FRP; rectangular column; large-scale; axial strength; axial and lateral strain;
stress-strain response
INTRODUCTION
Confinement of concrete is an efficient technique to increase the load-carrying capacity and
ductility of RC concrete columns. Under the lateral confining pressure provided by the
confinement material, the concrete column is subjected to a tri-axial stress state, thereby
increasing the ultimate stress and strain.
Lateral confining action was initially accomplished by restraining the lateral expansion of
concrete columns with closely spaced steel stirrups. Since then, techniques have been developed
to upgrade and confine structures by means of FRP wrapping, independently, or in combination
with steel stirrups.
Investigators determined that the rectangular sections laterally confined using FRP were not
as effective as their circular counterparts. This was attributed to the higher stress concentration
found at the corners and the non-uniformity in confinement (Chaallal and Shahawy 2000).
Rounding a column’s corners has now become commonplace because it helps to reduce the
cutting edge effect on the confining sheets.
One early model used to predict the axial strength of rectangular columns was developed by
the International Conference of Building Officials (ICBO 1997). This model predicts the ultimate
axial strength of confined columns (f
cc
) and the ultimate axial strength of unconfined columns
(f
co
) for rectangular columns with aspect ratios (b/d) less than 1.5. Although other models have
been developed to predict the axial strength behavior of rectangular columns, the effects of aspect
ratio and section size on the ultimate load and stress-strain behavior have received limited
attention. Moreover, the majority of specimens tested to verify these models are relatively small
Journal of Composites for Construction. Submitted June 10, 2008; accepted June 11, 2009;
posted ahead of print June 22, 2009. doi:10.1061/(ASCE)CC.1943-5614.0000051
Copyright 2009 by the American Society of Civil Engineers

Accepted
Manuscript
Not
Copyedited
3
with cross sectional dimensions (d, b): d = 94mm (3.7in.), 108mm (4.25in.), 150mm (5.91in.),
152mm (5.98in.), and b = 108mm (4.25in.), 150mm (5.91in.), 152mm (5.98in.), 188mm (7.4in.),
203mm (7.99in.) (Lam and Teng 2003). It is therefore uncertain whether the existing models
developed to predict the axial strength characteristics of small-scale rectangular columns can be
applied to accurately characterize the behavior of their large-scale counterparts.
The current study focuses on two larger field-size columns 355x355mm (14x14in.) columns
with different radii, and one 250x500mm (10x20in.) column having the same radii as one of the
square samples confined with external FRP wrapping reinforcement. As far as the authors’
knowledge, these samples have the biggest size of all specimens tested by previous studies (Wang
and Restrepo 2001). The number of samples was limited due to the difficulty in testing these
larger structures, but the selections allow the effects of varying the aspect ratio (b/d), fiber
thickness, and corner radius to be examined. The effect of increasing confining strength and the
effective transverse FRP failure strain (defined as the transverse FRP strain at ultimate load ε
clu
over the FRP failure strain ε
fum
) were also investigated.
RESEARCH SIGNIFICANCE
This paper provides an evaluation of the previously published models that predict the
ultimate axial strength and the entire stress-strain response of FRP-confined concrete and assess
their reliability against the results obtained from large-scale columns. The effect of confinement
on the ultimate failure strain of the FRP composite sheets is quantified. This paper should provide
a better understanding of the behavior of fiber-wrapped or FRP confined rectangular concrete
columns. The results presented in this paper should be used to predict the ultimate strength of
actual-size columns in the current retrofitting projects in the field.
Journal of Composites for Construction. Submitted June 10, 2008; accepted June 11, 2009;
posted ahead of print June 22, 2009. doi:10.1061/(ASCE)CC.1943-5614.0000051
Copyright 2009 by the American Society of Civil Engineers

Accepted
Manuscript
Not
Copyedited
4
EXPERIMENTAL PROCEDURE
Test Specimens and Material Properties
This study concentrates on non-circular columns, and is a part of a previous study done by
Matthys et al. (2005, 2006). The three large-scale RC rectangular columns described herein are
referred to as K9, K10, and K11; columns K1 though K8 were circular with results reported
elsewhere (Matthys et al. 2006), and column K1 was unwrapped. Schematic diagrams of the
confined columns along with their wrapping configuration are shown in Figure. 1. Each column
has a total length of 2m (6ft-7in.), a longitudinal steel reinforcement ratio of approximately 0.98%,
and 8mm (0.31in.) diameter stirrups spaced every 140mm (5.51in.). An extra stirrup
reinforcement is included at the columns’ ends. Columns K9 and K10 are square; K11 is
rectangular. All three have approximately the same cross sectional area, A
g
= 125,000mm
2
(193.75in
2
).
The concrete used to construct the columns has a mean compressive strength at 28 days of
38.2MPa (5.5ksi). The corners of the columns are rounded with radii of 30mm (1.18in.) (K9 and
K11) and 15mm (0.59in.) (K10).
CFRP (graphite) fabrics are used to confine the specimens. The wet lay-up’ FRP type
reinforcement is impregnated and cured in-situ. The CFRP consists of a SyncoTape system,
comprised of quasi unidirectional fabric, TU600/25 (600g/m
2
(0.1229lb/ft
2
) fibers in the main
direction and 25g/m
2
(0.0051lb/ft
2
) in perpendicular direction), and PC 5800 epoxy. The fabric
has a width of 200mm (7.87in.) and a nominal thickness of 0.300mm (0.0118in.). The PC 5800 is
a solvent-free 2-component epoxy primer consisting of a resin (Component A) and a hardener
(Component B). The test parameters of the wrapped columns and the properties of the
reinforcement are given in Tables 1 and 2, respectively.
Journal of Composites for Construction. Submitted June 10, 2008; accepted June 11, 2009;
posted ahead of print June 22, 2009. doi:10.1061/(ASCE)CC.1943-5614.0000051
Copyright 2009 by the American Society of Civil Engineers

Accepted
Manuscript
Not
Copyedited
5
Specimen Preparation and Test Procedure
Test specimens and concrete quality control specimens were cast in the laboratory. The formwork
was removed after 1 day. Concrete curing took place under plastic foil during the first 7 days and
under laboratory conditions thereafter. Once the concrete columns were fully cured, they were
wrapped with FRP, following the manufacturer’s recommendations outlined in the next
paragraph.
Before the FRP was applied, the concrete surface was cleaned. The epoxy was prepared by
mixing 3 volumetric parts of component A (the resin) with 1 part of component B (the hardener).
This compound was deposited liberally on both surfaces of contact by using a paintbrush. A
uniform tensile force was applied to the fiber during application to ensure a tight wrap. Since the
tensile force was applied by hands, this force was kept as uniform as possible. Air was forced out
of the bonding layer using a customized roller. The FRP was applied a minimum of 7 and a
maximum of 9 days prior to the time that the columns were tested.
Each column was tested to failure in a displacement control mode; load was applied at a rate
of 0.5mm/min. The axial and transverse deformations of the columns were measured both
manually and electronically. Manual measurement relied on dial gauges having a gauge length of
1m (3.28ft) and mechanical extensometers with gauge lengths of 200mm (7.87in.) or 50mm
(1.97in.). Electronic measurements relied on strain gauges on the stirrups (with gauge lengths of
200mm (7.87in.) or 80mm (3.15in.)) and strain gauges on the vertical rebars.
TEST RESULTS
Behavior at Ultimate Load
Table 3 shows test results including the maximum load, Q
max
, the maximum stress, Q
max
/A
g
, and
the strength increase, Q
max
/Q
ref
, where Q
ref
is the maximum load of unwrapped circular column
Journal of Composites for Construction. Submitted June 10, 2008; accepted June 11, 2009;
posted ahead of print June 22, 2009. doi:10.1061/(ASCE)CC.1943-5614.0000051
Copyright 2009 by the American Society of Civil Engineers

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References
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Abstract: A stress‐strain model is developed for concrete subjected to uniaxial compressive loading and confined by transverse reinforcement. The concrete section may contain any general type of confining steel: either spiral or circular hoops; or rectangular hoops with or without supplementary cross ties. These cross ties can have either equal or unequal confining stresses along each of the transverse axes. A single equation is used for the stress‐strain equation. The model allows for cyclic loading and includes the effect of strain rate. The influence of various types of confinement is taken into account by defining an effective lateral confining stress, which is dependent on the configuration of the transverse and longitudinal reinforcement. An energy balance approach is used to predict the longitudinal compressive strain in the concrete corresponding to first fracture of the transverse reinforcement by equating the strain energy capacity of the transverse reinforcement to the strain energy stored in the concret...

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