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AXIAL BEHAVIOR OF REINFORCED CONCRETE COLUMNS JACKETED WITH FRP
COMPOSITES
A. ILKI, E. KARAMUK, O. PEKER AND N. KUMBASAR
Istanbul Technical University, Istanbul, Turkey
1. Introduction
All over the world, lack of adequate ductility is one of the main deficiencies of existing structures, which
were constructed before recent codes were published. For countries like Turkey, where structural codes are
often neglected, this is a serious concern even for relatively newer structures. Consequently, improvement of
structural members in terms of ductility has been one of the most common types of structural retrofitting
recently in many parts of the world. By improving ductility of structural members, it may be possible to
withstand against earthquakes without increasing the strength of the members. It is clear that the structures
retrofitted in terms of ductility may experience significant damage during severe earthquakes, however, it
should be kept in mind that for most of the existing structures, earthquake resistance at the level of life safety
may be sufficient. On the other hand, since retrofitting in terms of ductility do not change the stiffness of the
structure, the seismic demand does not increase.
In this study, 16 reinforced concrete columns were tested under uniaxial compression. 11 of these
specimens were jacketed with carbon fiber reinforced polymer (CFRP) sheets, while 5 reference specimens
were tested without retrofit. 10 of the specimens were cast by medium strength concrete, while 6 specimens
were cast with low strength concrete. Other test parameters include the cross-section shape (circular, square,
rectangular) and thickness of CFRP jackets. At the end of the experimental work, it was concluded that
significant enhancement was possible for ductility and axial strength of the reinforced concrete columns
jacketed with CFRP composites.
An analytical work was also carried out for determining the contributions of internal transverse steel
reinforcement and external CFRP sheets on the improvement of the behavior of the specimens. Using the
utilized analytical approach, ultimate compressive deformations and axial strengths of the specimens could be
predicted with a reasonable accuracy.
2. Experimental Work
The general characteristics of the specimens tested under uniaxial compression are outlined in Table 1. The
general appearance of the specimens and photographs taken during the construction of the specimens are
presented in Fig. 1. The testing setup for uniaxial loading tests is shown in Fig. 2. The axial stress-axial strain
curves for the reinforced concrete members with circular cross section, and medium concrete strength (NS-C-
145-0, NS-C-145-3, NS-C-145-5) and low concrete strength (LS-C-145-0, LS-C-145-3, LS-C-145-5) are
presented in Fig. 3, for demonstrating a small part of the test results. Other results are planned to be presented
in the full paper.
TABLE 1. General characteristics of the specimens
Specimen f′co (MPa) As Asw Shape FRP
NS-C-145-0 23.4 6φ10 φ8/145 Circular 0
NS-C-145-3 23.4 6φ10 φ8/145 Circular 3 plies
NS-C-145-5 23.4 6φ10 φ8/145 Circular 5 plies
NS-R-1-200-0 23.4 4φ14 φ8/200 Square 0
NS-R-1-000-3 23.4 0 0 Square 3 plies
NS-R-1-200-3 23.4 4φ14 φ8/200 Square 3 plies
NS-R-1-200-5 23.4 4φ14 φ8/200 Square 5 plies
NS-R-2-175-0 23.4 4φ12 φ8/175 Rectangular 0
NS-R-2-175-3 23.4 4φ12 φ8/175 Rectangular 3 plies
NS-R-2-175-5 23.4 4φ12 φ8/175 Rectangular 5 plies
LS-C-145-0 12.8 6φ10 φ8/145 Circular 0
LS-C-145-3 12.8 6φ10 φ8/145 Circular 3 plies
LS-C-145-5 12.8 6φ10 φ8/145 Circular 5 plies
LS-R-1-200-0 13.5 4φ14 φ8/200 Square 0
LS-R-1-200-3 13.5 4φ14 φ8/200 Square 3 plies
LS-R-1-200-5 13.5 4φ14 φ8/200 Square 5 plies
Figure 1. Specimen production steps and general appearance of specimens
2x CDP10
500
2x
270
PL-60-11-1L
4x CDP50
4x CDP50
Figure 2. Test setup for uniaxial compression tests
8 8 8
No FRP
Axial Stress (σc/f'co)
Axial Stress (σc/f'co)
Ø2
6 3 plies 6 50
Ø1
5 plies
0
4 4 No FRP
3 plies
500
500
145
2 2
5 plies
0 0
0 20000 40000 60000 80000 100000 0 20000 40000 60000 80000 100000 250
Axial Strain (mm/mm) Axial Strain (mm/mm)
(a) (b)
Figure 3. Axial stress-axial strain relationships (a) for circular members with medium strength concrete, (b) for circular members
with low strength concrete
3. Analytical work
While predicting the analytical strengths and ultimate axial deformations for FRP jacketed members, the
model proposed by Ilki and Kumbasar (2004) was used for the contribution of FRP Jacket on the strength and
deformability enhancement, and the model proposed by Mander et al. (1988) was utilized for the contribution
of existing transverse reinforcement. The predicted axial strengths and deformations are presented in Fig. 4,
together with the experimental values.
100 0.10
80 0.08
εcc (analytical)
f'cc (analytical)
60 0.06
40 0.04
20 0.02
0 0.00
0 20 40 60 80 100 0.00 0.02 0.04 0.06 0.08 0.10
f'cc (experiment) εcc (experiment)
Figure 4. Comparison of experimental and analytical ultimate strengths and axial strains for FRP jacketed specimens
4. Conclusions
FRP jacketing of reinforced concrete columns improves the uniaxial compressive strength and ultimate axial
strain significantly, particularly for the specimens with relatively lower concrete strength. The method
utilized for predicting the ultimate strength and deformation of reinforced concrete members confined
internally by transverse reinforcement and also externally by FRP composite sheets gives reasonable results.
As all retrofitting techniques, this technique may be adequate for certain type of structures, while it might not
be adequate for some other cases.
Keywords: carbon; ductility; fibers; reinforced concrete; retrofitting
