Experimental study on FRP-to-concrete bonded joints J. Yao a,b , J.G. Teng b , J.F. Chen c, * a Department of Civil Engineering, Zhejiang University, Hangzhou, 310027, P.R. China b Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hong Kong, China c Institute for Infrastructure and Environment, School of Engineering and Electronics, Edinburgh University, Alexander Graham Bell Building, The King’s Buildings, Edinburgh EH9 3JN, UK Received 16 January 2004; revised 2 June 2004; accepted 20 June 2004 Available online 7 August 2004 Abstract The behaviour of bond between FRP and concrete is a key factor controlling the behaviour of concrete structures strengthened with FRP composites. This article presents an experimental study on the bond shear strength between FRP and concrete using a near-end supported (NES) single-shear pull test. The test results are found to be in close agreement with the predictions of Chen and Teng’s [J. Struct. Eng. 127(2001) 784] bond strength model, which mutually verifies the reliability of both the test method and the Chen and Teng model in general. The NES single-shear pull test, given its simplicity and reliability, is therefore a good candidate as a standard bond test. The test results also showed that Chen and Teng’s [J. Struct. Eng. 127(2001) 784] bond strength model is slightly conservative when the FRP-to-concrete width ratios are at the two extremes, but this small weakness can be easily removed when more test results of good quality become available. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Polymer-matrix composites (PMCS); B. Debonding; B. Interface; Strength 1. Introduction External bonding of fibre reinforced polymer (FRP) composites has become a popular technique for strength- ening concrete structures all over the world [2]. An important issue in the strengthening of concrete structures using FRP composites is to design against various debonding failure modes, some of which were first studied for concrete beams bonded with a steel plate, including: (a) cover separation [3–5]; (b) plate end interfacial debonding [3,4,6]; (c) intermediate (flexural or flexural-shear) crack (IC) induced interfacial debonding [7] and (d) critical diagonal crack (CDC) induced interfacial debonding [8–10]. The bond strength between FRP and concrete is a key factor controlling debonding failures of various forms in FRP-strengthened structures. As a result, extensive research on this topic has been carried out, in addition to earlier work concerned with steel plates bonded to concrete which provided a useful initial basis. The existing work has included experimental studies conducted using single shear tests, e.g. [11–15], double shear tests, e.g. [16–23] and modified beam tests, e.g. [23–25], theoretical studies using fracture mechanics analysis [15,26–33] and finite element analysis [34,35], and the development of empirical models [1,23,36,37]. A review of these studies can be found in Ref. [1]. Existing studies suggest that the main failure mode of FRP-to-concrete joints in shear tests is cracking of concrete under shear, occurring commonly at a few millimetres from the adhesive-concrete interface [1]. The bond strength (i.e. the maximum transferable load) of the joint therefore depends significantly on concrete strength. In addition, the FRP-to-concrete member width ratio has a significant effect. A very important aspect of the behaviour of these bonded joints is that there exists an effective bond length beyond which an extension of the bond length cannot increase the ultimate load. This is the fundamental difference between externally bonded reinforcement and internal reinforcement for which a sufficiently long anchorage length can always be found that the full tensile strength of the reinforcement can 1359-8368/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2004.06.001 Composites: Part B 36 (2005) 99–113 www.elsevier.com/locate/compositesb * Corresponding author. Tel.: C44-131-650-6768; fax: C44-131-650- 6789. E-mail address: [email protected] (J.F. Chen).
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Experimental study on FRP-to-concrete bonded joints
J. Yaoa,b, J.G. Tengb, J.F. Chenc,*
aDepartment of Civil Engineering, Zhejiang University, Hangzhou, 310027, P.R. ChinabDepartment of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hong Kong, China
cInstitute for Infrastructure and Environment, School of Engineering and Electronics, Edinburgh University, Alexander Graham Bell Building,
The King’s Buildings, Edinburgh EH9 3JN, UK
Received 16 January 2004; revised 2 June 2004; accepted 20 June 2004
Available online 7 August 2004
Abstract
The behaviour of bond between FRP and concrete is a key factor controlling the behaviour of concrete structures strengthened with FRP
composites. This article presents an experimental study on the bond shear strength between FRP and concrete using a near-end supported
(NES) single-shear pull test. The test results are found to be in close agreement with the predictions of Chen and Teng’s [J. Struct. Eng.
127(2001) 784] bond strength model, which mutually verifies the reliability of both the test method and the Chen and Teng model in general.
The NES single-shear pull test, given its simplicity and reliability, is therefore a good candidate as a standard bond test. The test results also
showed that Chen and Teng’s [J. Struct. Eng. 127(2001) 784] bond strength model is slightly conservative when the FRP-to-concrete width
ratios are at the two extremes, but this small weakness can be easily removed when more test results of good quality become available.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: A. Polymer-matrix composites (PMCS); B. Debonding; B. Interface; Strength
1. Introduction
External bonding of fibre reinforced polymer (FRP)
composites has become a popular technique for strength-
ening concrete structures all over the world [2]. An
important issue in the strengthening of concrete structures
using FRP composites is to design against various
debonding failure modes, some of which were first studied
for concrete beams bonded with a steel plate, including: (a)
cover separation [3–5]; (b) plate end interfacial debonding
[3,4,6]; (c) intermediate (flexural or flexural-shear) crack
(IC) induced interfacial debonding [7] and (d) critical diagonal
crack (CDC) induced interfacial debonding [8–10].
The bond strength between FRP and concrete is a key
factor controlling debonding failures of various forms in
FRP-strengthened structures. As a result, extensive research
on this topic has been carried out, in addition to earlier work
concerned with steel plates bonded to concrete which
1359-8368/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
and (e) beam (or bending) tests. For better clarity, the first
four test methods are renamed here as: (a) far end supported
(FES) double-shear tests; (b) near end supported (NES)
double-shear tests; (c) far end supported (FES) single-shear
tests; and (d) near end supported (NES) single-shear tests
(Fig. 1). Collectively, all these four tests may also be
referred to as pull tests, as the plate is always directly pulled
by a tensile force.
FES double-shear pull tests and NES single-shear pull
tests have been the most popular test methods so far due to
Fig. 1. Classification of bond
their simplicity [33]. Both numerical [33] and experimental
[38] studies have shown that different test set-ups can lead to
significantly different test results. Within each test method,
small variations in the test set-up such as the height of the
support block in a NES single- or double-shear test may also
have significant effects based on a recent stress analysis
[33].
An FRP-to-concrete bond strength model is the key to the
accurate prediction of debonding failures in FRP-strength-
ened RC beams, including shear crack-induced debonding
failures [8,39] as well as intermediate flexural or flexural-
shear crack-induced debonding failures [7].
In debonding failures in FRP shear-strengthened RC
beams with transverse plates, the bond strength model
developed from pull tests is directly applicable [39]. Such a
model is also important in understanding the mechanism of
debonding induced by a critical diagonal crack near the end
of a longitudinal tension face plate for flexural strengthening
[8,10], where the longitudinal plate increases the concrete
component of the shear capacity and where the bond
strength developed from pull tests is also directly
applicable.
Furthermore, in intermediate crack-induced debonding
failures, the stress state in the critical region of the beam is
also closely similar to that of the concrete prism in a NES
single-shear pull test. The NES single-shear pull test
therefore appears to be a promising candidate as a standard
set-up for determining the FRP-to-concrete bond strength
and was therefore adopted in the present study. One of the
aims of the present experimental study is to examine the
effect of a number of small variations in this test set-up on
the resulting bond strength to aid in fine-tuning this test
method as a standard bond test method. Results from
previous NES single-shear pull tests also formed part of the
database on which Chen and Teng’s [1] recent bond strength
tests (Chen et al. 2001).
Fig. 2. Test specimen.
J. Yao et al. / Composites: Part B 36 (2005) 99–113 101
model was based, so the present test results also provide an
appropriate independent check of the validity of this bond
strength model.
Fig. 3. Relative vertical displacement between two sides of a flexural-shear
crack.
2.2. Specimen design
The NES single-shear pull test specimens consisted of a
concrete prism bonded with an FRP strip (Fig. 2). The
factors considered in the present test program include the
bond length Lfrp, the width ratio between the FRP strip and
the concrete prism bfrp/bc, the height of the concrete free
edge hc (Zheight of concrete prism hKheight of the
support block hb) (Fig. 2) and the offset in the load position
d. The first two factors have been identified to have a
significant effect on the bond strength but there have been
insufficient test data to rigorously verify the proposed
relationships [1]. The height of the concrete free edge hc
(Fig. 2b) has been shown to have a significant effect on the
stress distribution in the specimen [33], but its effect on the
ultimate bond strength is yet unclear. In practical pull tests,
there may be a small unintended offset d in the position of
the load (Fig. 2b). This offset may alternatively be expressed
as the initial loading angle q. The effect of this loading angle
needs to be understood if standardisation of the test set-up is
to be considered in the future. Furthermore, in flexurally
strengthened concrete structures, when debonding is
induced by the opening up of a flexural-shear crack, there
exists a relative vertical displacement between the two sides
of the crack, e.g. [40–42], so the FRP strip (or plate or sheet)
is loaded at a small positive (peeling) inclination angle to
the longitudinal axis on one side and at the same but
negative angle on the other side of the crack (Fig. 3). This is
thus another reason why the effect of a small loading angle
is worthy of some attention.
A total of 72 specimens in seven series were prepared
to investigate the effects of the above factors on the bond
strength (Table 1). The variables considered in Series I
(Specimens I-1–16) include the bond length Lfrp and the
support height hb (or height of the free concrete edge on
the loading side hcZhKhb). Series II (Specimens II-1–6)
and III (Specimens III-1–8) were designed to investigate
the effects of the loading offset and the FRP-to-concrete
width ratio respectively. Series IV–VII (Specimens IV-1–
14, V-1–12, VI-1–8 and VII-1–8) were designed following
the completion of the first three series to further explore
the effects of Lfrp, bfrp/bc and hc. Key parameters and test
results of all specimens are listed in Table 1. Specimens
II-1 and II-4 had a loading offset of dZ4 mm (equivalent
to an initial loading angle of 1.78) whilst Specimens II-3
and II-6 had a loading offset of dZK4 mm (equivalent to
an initial loading angle of K1.78). All other specimens
had no loading offset.
Concrete prisms of two different sizes were used. Half of
the specimens in Series III and V used 100!150!350 mm
concrete prisms so that a desired range of bfrp/bc ratios could
be achieved. All other specimens used 150!150!350 mm
concrete prisms. Concrete cubes and cylinders were tested
according to BS 1881 [43] to determine the material
properties at the time when the series of specimens made
from the same batch of concrete were tested.
GFRP was used in Specimens III-7 and III-8 while CFRP
was used in all others. The nominal thicknesses for the
CFRP and GFRP strips were 0.165 and 1.27 mm respect-
ively, the former being roughly the fibre sheet thickness
before resin impregnation with the latter being similar to the
thickness of the cured FRP strip. The FRP strips were
bonded to the concrete prisms following the manufacturer’s
instructions. The mechanical properties of the FRP
composites are shown in Table 2. The tensile strengths of
FRPs were determined according to ASTM D3039/
D3039M-95a [44] on the basis of the nominal thicknesses.
The nominal thicknesses were also used in all other
calculations of the present study. FRP composites were
Table 1
Details of specimens and test results
Test
specimen
Concrete
cylinder
strength f 0c(MPa)
Width of
concrete
prism bc
(mm)
FRP width
bfrp (mm)
FRP bond
length Lfrp
(mm)
Height of
free concrete
edge hc
(mm)
Test failure
load Ptest
(kN)
Test failure
mode
Predicted
failure load
Ppred (kN)
Ptest/Ppred
I-1 23.0 150 25 75 5 4.75 DB-C 5.72 0.83
I-2 23.0 150 25 85 5 5.69 DB-C 5.96 0.96
I-3 23.0 150 25 95 5 5.76 DB-C 6.02 0.96
I-4 23.0 150 25 95 5 5.76 DB-C 6.02 0.96
I-5 23.0 150 25 95 5 6.17 DB-C 6.02 1.02
I-6 23.0 150 25 115 5 5.96 DB-C 6.02 0.99
I-7 23.0 150 25 145 5 5.95 DB-C 6.02 0.99
I-8 23.0 150 25 190 5 6.68 DB-C 6.02 1.10
I-9 23.0 150 25 190 5 6.35 DB-C 6.02 1.05
I-10 23.0 150 25 95 75 6.17 DB-C 6.02 1.02
I-11 23.0 150 25 75 120 5.72 DB-C 5.72 1.00
I-12 23.0 150 25 85 120 6 DB-C 5.96 1.01
I-13 23.0 150 25 95 120 6.14 DB-C 6.02 1.02
I-14 23.0 150 25 115 120 6.19 DB-C 6.02 1.03
I-15 23.0 150 25 145 120 6.27 DB-C 6.02 1.04
I-16 23.0 150 25 190 120 7.03 DB-C 6.02 1.17
II-1 22.9 150 25 95 120 5.2 DB-C 6.02 0.86
II-2 22.9 150 25 95 120 6.75 DB-C 6.02 1.12
II-3 22.9 150 25 95 120 5.51 DB-C 6.02 0.92
II-4 22.9 150 25 190 120 7.02 DB-C 6.02 1.17
II-5 22.9 150 25 190 120 7.07 DB-C 6.02 1.17
II-6 22.9 150 25 190 120 6.98 DB-C 6.02 1.16
III-1 27.1 150 25 100 120 5.94 DB-C 6.27 0.95
III-2 27.1 150 50 100 120 11.66 DB-C 11.19 1.04
III-3 27.1 150 75 100 120 14.63 DB-C 15.02 0.97
III-4 27.1 150 100 100 120 19.07 DB-C 17.91 1.06
III-5 27.1 100 85 100 120 15.08 CPF 13.42 1.12
III-6 27.1 100 100 100 120 15.75 CPF 14.16 1.11
III-7 27.1 100 25.3 100 120 4.78 DB-C 4.92 0.97
III-8 27.1 100 50.6 100 120 8.02 DB-C 8.30 0.97
IV-1 18.9 150 25 95 5 5.86 DB-C 5.72 1.02
IV-2 18.9 150 25 95 5 5.9 DB-C 5.72 1.03
IV-3 19.8 150 25 95 5 5.43 DB-C 5.80 0.94
IV-4 19.8 150 25 95 5 5.76 DB-C 5.80 0.99
IV-5 18.9 150 25 95 15 5 DB-C 5.72 0.87
IV-6 19.8 150 25 95 15 7.08 DB-C 5.80 1.22
IV-7 18.9 150 25 95 30 5.5 DB-C 5.72 0.96
IV-8 19.8 150 25 95 30 5.93 DB-C 5.80 1.02
IV-9 18.9 150 25 95 45 5.38 DB-C 5.72 0.94
IV-10 19.8 150 25 95 45 6.6 DB-C 5.80 1.14
IV-11 18.9 150 25 95 60 5.51 DB-C 5.72 0.96
IV-12 19.8 150 25 95 60 5.67 DB-C 5.80 0.98
IV-13 18.9 150 25 95 90 6.31 DB-C 5.72 1.10
IV-14 19.8 150 25 95 90 6.19 DB-C 5.80 1.07
V-1 21.1 150 15 95 60 3.81 DB-C 3.71 1.03
V-2 21.1 150 15 95 60 4.41 DB-C 3.71 1.19
V-3 21.1 150 25 95 60 6.26 DB-C 5.89 1.06
V-4 21.1 150 50 95 60 12.22 DB-C 10.51 1.16
V-5 21.1 150 75 95 60 14.29 DB-C 14.10 1.01
V-6 21.1 150 100 95 60 15.58 DB-C 16.82 0.93
V-7 21.1 100 80 95 60 14.27 CPF 12.28 1.16
V-8 21.1 100 80 95 60 13.78 CPF 12.28 1.12
V-9 21.1 100 90 95 30 13.56 CPF 12.88 1.05
V-10 21.1 100 90 95 5 15.66 CPF 12.88 1.22
V-11 21.1 100 100 95 30 15.57 CPF 13.30 1.17
V-12 21.1 100 100 95 5 17.43 CPF 13.30 1.31
VI-1 21.9 150 25 95 60 6.01 DB-I 5.95 1.01
VI-2 21.9 150 25 95 60 5.85 DB-I 5.95 0.98
VI-3 21.9 150 25 145 60 5.76 DB-I 5.95 0.97
VI-4 21.9 150 25 145 60 5.73 DB-I 5.95 0.96
(continued on next page)
J. Yao et al. / Composites: Part B 36 (2005) 99–113102
Table 1 (continued)
Test
specimen
Concrete
cylinder
strength f 0c(MPa)
Width of
concrete
prism bc
(mm)
FRP width
bfrp (mm)
FRP bond
length Lfrp
(mm)
Height of
free concrete
edge hc
(mm)
Test failure
load Ptest
(kN)
Test failure
mode
Predicted
failure load
Ppred (kN)
Ptest/Ppred
VI-5 21.9 150 25 190 60 5.56 DB-I 5.95 0.93
VI-6 21.9 150 25 190 60 5.58 DB-I 5.95 0.94
VI-7 21.9 150 25 240 60 5.91 DB-I 5.95 0.99
VI-8 21.9 150 25 240 60 5.05 DB-I 5.95 0.85
VII-1 24.9 150 25 95 60 6.8 DB-C 6.14 1.11
VII-2 24.9 150 25 95 60 6.62 DB-C 6.14 1.08
VII-3 24.9 150 25 145 60 7.33 DB-C 6.14 1.19
VII-4 24.9 150 25 145 60 6.49 DB-C 6.14 1.06
VII-5 24.9 150 25 190 60 7.07 DB-C 6.14 1.15
VII-6 24.9 150 25 190 60 7.44 DB-C 6.14 1.21
VII-7 24.9 150 25 240 60 7.16 DB-C 6.14 1.17
VII-8 24.9 150 25 240 60 6.24 DB-C 6.14 1.02
Average 1.04
CoV 9.6%
Note: (a) CFRP was used in all specimens except III-7 and III-8 in which GFRP was used; (b) all concrete prisms had a height of 150 mm; (c) concrete cylinder
strength determined from cube strength according to fcLZ0.79 fcu
L (d) DB-C, debonding in concrete; DB-I, debonding at adhesive-concrete interface; CPF,
Concrete prism failure.
Table 2
Properties of FRPs
Type Thickness
(mm)
Tensile
strength ffrp(MPa)
Elastic mod-
ulus Efrp
(GPa)
Ultimate
tensile
strain 3frp
(%)
CFRP 0.165 4114 256 1.61
GFRP 1.27 351 22.5 1.56
J. Yao et al. / Composites: Part B 36 (2005) 99–113 103
bonded to the concrete prisms with epoxy resins. More
details of the material properties and specimen preparation
procedures are available in Ref. [45].
2.3. Test set-up
A steel rig for NES single-shear pull tests (Fig. 4a) was
carefully fabricated to carry out all the tests reported in this
article. In this rig, the load could be accurately positioned
vertically by adjusting the height of the bearing plate.
Different support blocks could be used to achieve the
required support heights on the loaded end (i.e. the end
nearer the applied load or the near end) of the concrete
prism. A positioning frame was used to prevent the far end
of the concrete prism from uplifting. The concrete prism
was separated from the positioning frame by a thin layer of
rubber to allow horizontal sliding of the concrete prism.
2.4. Instrumentation and loading procedure
Strain gauges and LVDTs were used to measure strains
in the FRP and displacements at various positions. Details of
these measurements are not given here, but are available
elsewhere [45], as the main concern of the present paper is
with the bond strength. Loading was applied through a
hydraulic jack at increments of about 5% of the ultimate
load predicted by Chen and Teng’s model [1]. Fig. 4b shows
a specimen during the test.
Fig. 4. Test rig.
3. Chen and Teng’s bond strength model
As the specimens were designed based on Chen and
Teng’s bond strength model [1] and the results are
Fig. 5. Debonding in concrete.
J. Yao et al. / Composites: Part B 36 (2005) 99–113104
compared with its predictions later in the article, it is
necessary to introduce this model before the test results are
presented. The bond strength expressed as per unit width of
the FRP strip, qu, is
q ZPu
bfrp
Z abwblLe
ffiffiffiffif 0c
p(1)
where Pu is the ultimate load in N, bfrp is the width of the
FRP strip in mm, bw and bl are dimensionless coefficients
reflecting the effects of the FRP-to-concrete width ratio bfrp/
bc and the bond length Lfrp respectively, Le is the effective
bond length in mm and f 0c is the cylinder compressive
strength of concrete in MPa. Based on the regression of test
data collected from the literature, Chen and Teng [1]
obtained the best fit value of aZ0.427. It was proposed to
use the 95th percentile of aZ0.315 as the lower bound for