American Journal of Applied Science and Technology
82
https://theusajournals.com/index.php/ajast
VOLUME
Vol.05 Issue 07 2025
PAGE NO.
82-85
10.37547/ajast/Volume05Issue07-14
Bond Strength of GFRP Bar with Concrete
Kamoliddin Muminov
PhD student, Namangan State Technical University, Uzbekistan
Ravshanbek Mavlonov
PhD, Namangan State Technical University, Uzbekistan
Received:
31 May 2025;
Accepted:
29 June 2025;
Published:
31 July 2025
Abstract:
In this article, the bond strength between GFRP rebar and concrete is investigated, and the differences
compared to the bond between steel rebar and concrete are analyzed.
Keywords:
Concrete, FRP bar, steel bar, bond strength.
Introduction:
In order for reinforced concrete structures to perform
reliably, sufficient bonding between concrete and
reinforcement is required, as the transfer of stresses
between these two materials is a crucial factor [1
–
3].
Although the bond strength between steel
reinforcement and concrete is well-documented in
literature and design standards, the bond behavior of
fiber reinforced polymer (FRP) reinforcement with
concrete differs significantly [4
–
5]. This difference
arises from the unique characteristics of FRP
compared to conventional steel reinforcement, such
as surface texture, modulus of elasticity, shear
brittleness, and tensile strength. Therefore, a clear
understanding of the bond strength between
concrete and FRP is essential [6
–
7].
To determine the bond strength between FRP and
concrete, tests are conducted in accordance with the
requirements of GOST 31938-2012 [8]. According to
this standard, bond strength is assessed by
performing a pull-out test, where the rebar is
extracted from a concrete cube specimen (Fig. 1). This
method is based on evaluating the shear stresses
developed along the interface between the concrete
and the surface of the composite reinforcement. The
test is performed under the maximum load recorded
during pulling, regardless of the failure mode (either
along the reinforcement or at the bond interface
between the concrete and reinforcement).
If the diameter of the reinforcement is 10 mm or less,
a concrete cube with 100 mm edges is used; for
diameters of 12
–
18 mm, a 150 mm cube; and for
diameters above 18 mm, a 200 mm cube is required.
The bonded length of the FRP in the concrete should
be equal to 5 times the diameter of the
reinforcement. The unbonded portion of the rebar
inside the concrete must be protected with a
polyvinyl chloride (PVC) plug or sleeve. During testing,
a closed steel fixture with side dimensions of at least
200 mm and a minimum thickness of 20 mm is used.
This fixture must have a central hole for the rebar to
pass through.
The bond strength between FRP reinforcement and
concrete is determined using the following
expression:
fb
r
cL
P
=
(1)
where:
P
– tensile force, кН;
c
– nominal length of the outer circumference of the reinforcement,
d
c
=
, mm;
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American Journal of Applied Science and Technology (ISSN: 2771-2745)
L
fb
– embedded bond length of the reinforcement, мм.
Figure 1. Setup diagram for embedding
GFRP rebar in a concrete cube
Figure 2. Test setup diagram for pulling
GFRP rebar out of a concrete cube:
1 – measuring device; 2 – cube specimen; 3 –
support plate; 4 – soft plug; 5 – movable
crosshead of the testing machine; 6 – fixed
crosshead of the testing machine; 7 –
coupling.
Testing of the GFRP bar specimens was conducted in
accordance with GOST 31938
–
2012 [8], while testing
of the steel bar specimens followed the international
standard GOST R 57357
–
2016 [9], using a universal
hydraulic machine (Figure 3).
For this study, both types of reinforcement were used
GFRP and steel bars with a diameter of 10 mm. The
specimens were anchored into concrete cubes with
edge dimensions of 150 mm. To accurately evaluate
the bond between the reinforcement and concrete,
all concrete cube specimens were prepared with
identical dimensions and kept under standard
conditions for 28 days.
Since the surface texture of the GFRP bars was non-
uniform and there was a risk of slippage during
compression due to the serrated grips of the
hydraulic press, a special metal coupling (Figures 1
and 2) was installed for the GFRP specimens during
testing. This coupling ensured reliable performance of
the reinforcement during the pull-out process. In
contrast, due to the mechanical properties of the
steel reinforcement and the requirements specified
in the standard (GOST R 57357
–
2016), the use of
additional couplings was not required for this type of
reinforcement.
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American Journal of Applied Science and Technology (ISSN: 2771-2745)
a)
b)
Fig. 3. Test for determining the bond strength between concrete and GFRP (a) and steel (b)
reinforcement
To study the bond between concrete and
reinforcement, six specimens were prepared for each
type of reinforcement: GFRP and steel. In the case of
GFRP specimens, reliable results could be obtained
only if the rebar was firmly connected with the
coupling and behaved as a single unit. According to
the test results, there was no significant difference
observed in the bond performance between GFRP
and steel reinforcement. The average bond strength
of the GFRP bars was 18.63 MPa, while that of the
steel bars was 20.85 MPa (Table 1). If the ribs on the
surface of the GFRP bars are securely attached to the
bar div, they can provide high bond strength with
concrete, comparable to that of steel reinforcement.
Table 1. Test results of bond strength between GFRP and steel reinforcement with concrete
Series
Bar type
Concrete
strength R
n
,
MPa
Tensile
for P,
kN
Slip,
mm
Bond
strength,
МПа
Mean bond
strength,
MPa
G1-10
GFRP
30.2
29.2
17.8
18.59
18.63
G2-10
GFRP
30.2
29.9
17.1
19.03
G3-10
GFRP
30.2
28.6
17.4
18.21
G4-10
GFRP
30.2
30.1
17.6
19.16
G5-10
GFRP
30.2
28.2
16.8
17.95
G6-10
GFRP
30.2
29.6
17.2
18.84
S1-10
A-III (steel)
30.2
34.1
19.2
21.71
20.85
S2-10
A-III (steel)
30.2
28.7
16.9
18.27
S3-10
A-III (steel)
30.2
33.6
17.5
21.39
S4-10
A-III (steel)
30.2
34.2
18.3
21.77
S5-10
A-III (steel)
30.2
32.1
18.9
20.44
S6-10
A-III (steel)
30.2
33.8
19.0
21.52
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