International Journal of Engineering and Advanced Technology (IJEAT)
ISSN: 2249 – 8958, Volume-9 Issue-3, February, 2020
Quality Assessment of Strengthened Concrete by
FRP Laminates using Non-Destructive Testing
W. F. Tawhed, Yasmen R. Elsayed1
Abstract: A non-destructive testing program has been designed
to evaluate the integrity of the bond strength of plain concrete
beams strengthened by Glass Fiber Reinforced Polymer (GFRP)
Laminates. A series of concurrent static load and non-destructive
testing experiments were carried out in the materials and testing
laboratory at the college of engineering, Mataria, Helwan
University, Cairo, Egypt.
A total of 90 plain concrete standard beam specimens of
dimensions 150 mm x 150 mm x 750 mm were constructed in the
laboratory with three different design strength categories (38, 45,
and 50) MPa. The beam specimens were strengthened by
externally bonded GFRP laminates with various number of layers
namely (3, 5 and 7) layers. In addition, the effect of debonding of
the GFRP laminates was investigated by simulating it by variation
in voids between concrete and laminates namely, (0, 30 and 60%).
This study investigates the effectiveness of externally bonded
GFRP laminates on the flexural strength of plain concrete beams
by using Ultrasonic Pulse Velocity (UPV) device before and
during loading until failure and their effect on the p-wave
velocities. Four-point flexural tests were performed on the
concrete beams, strengthened with different layouts of GFRP
laminates and different percentage of voids at the
concrete-laminate interface. The capacity of the beams and
p-wave velocity were investigated.
It was found that as the percentage of voids decreased, the
capacity of strengthened concrete beams increased linearly. The
reduction in voids enhanced the beam flexural behavior and
controlled tension crack propagation. In addition, it was observed
that use of GFRP laminates were more effective with higher
concrete characteristic strength provided that debonding is not
present. Finally, it was evident that the UPV technique was
successful in detecting the variation in concrete p-wave velocity
with strength and laminate layers variation.
KEYWORDS: FRP Laminates; FRP strengthening; Laminates
debonding; Non-destructive testing; UPV testing
I. INTRODUCTION
Strengthening
of reinforced concrete structures by
externally bonded FRP systems is now a widely accepted
technique [1]. However, the use of bonding techniques always
implies following installation procedures as presented in the
Egyptian Code of Practice (ECP) [2] or the American
Concrete Institute (ACI) [3]. To ensure, both, long-term
performance and durability of FRP reinforcements,
application must be as per the standard installation
Revised Manuscript Received on February, 10 2020.
* Correspondence Author
Waleed F. Tawhed*, Civil Engineering Department, Helwan
University, Cairo, Egypt. Email: waleed.tawhed@m-eng.helwan.edu.eg
Yasmen R. Elsayed, Civil Engineering Department, Helwan University,
Cairo, Egypt. Email: yasmenramdan2008@yahoo.com
Retrieval Number: C5170029320/2020©BEIESP
DOI: 10.35940/ijeat.C5170.029320
procedures.
The presence of debonding can significantly affect the
structural performance and durability of the strengthening
systems. Defects should be detected, located and evaluated in
order to estimate if replacement is needed. In these
conditions, checking of the bonded overlays through in situ by
non-destructive evaluation (NDE) techniques is more
suitable. The quality-control program should involve a set of
specified inspections and tests.
In this research work, non-destructive testing is utilized to
evaluate the integrity of a total of ninety concrete beams
strengthened by GFRP laminates of these specimens, nine
were control beams. Load-testing (destructive) was
performed in conjunction with the non-destructive testing to
verify and correlate the results. In this study, the specimens
were evaluated in the laboratory using the ultrasonic-pulse
velocity method to evaluate their initial condition and assess
the progression of damage when subjected to load tests in the
laboratory. UPV tests were performed before and after the
loading sequence for each beam. Correlations between the
UPV and load-test results were established in a form of a
family of curves that were useful in tracking the progression
of damage due to increase in delamination. Finally, the results
obtained from the loading tests were used to determine the
energy absorption and the stiffness improvement index of the
specimens due to strengthening by GFRP sheets.
II. OBJECTIVES
This research article presents the results of an assessment
of the behavior of concrete beams strengthened by GFRP
laminates under four-point load concurrent with a
non-destructive test using UPV. The objectives set forth for
this research effort are:
1. to examine the physical and mechanical properties of
locally constructed GFRP laminates in the laboratory.
2. to investigate the bond strength of externally bonded
GFRP laminates to concrete by applying different %
delaminations (voids) at GFRP-concrete interface.
3. to evaluate the efficiency of strengthening by different
number of GFRP layer laminates.
4. to determine the sensitivity of the UPV technique to
changes in the composite structure due to increase in
stiffness by GFRP layers.
5. to evaluate the accuracy of the UPV technique in
detection of the extent of bond strength loss with
delaminations.
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Behavior of Hollow Core Slabs Reinforced with GFRP Prestressing Bars
III. RESEARCH SIGNIFICANCE
As strengthening and retrofit of concrete structures by
FRP composites in the form of externally bonded
reinforcement continue to gain acceptance, the need for
monitoring the performance of these structures increases.
The overall response of these structures, the used FRP
laminate condition and the integrity of the interfacial bond
between FRP and concrete all need to be evaluated. As
visual inspection might be convenient and effective for
external defects, but it is qualitative and subjective. On the
other hand, ultrasonic techniques provide a viable
methodology capable of assessing both surface and deep
subsurface discontinuities. Thus, applying non-destructive
methods will provide a comprehensive evaluation tool of the
structural condition and service life for concrete structures.
IV. BACKGROUND
A. FRP Basics
The quality of an FRP rehabilitation system is highly
affected by its integrity, which includes the composite and the
adhesive layer in the case of bonded strip, and the composite
and the impregnating polymeric resin in the case of dry sheets
and fabrics. The interface between the FRP composite and the
concrete substrate transfers the loads from the concrete to the
FRP composite. Potential defects in FRP composites could
also affect the quality of the rehabilitated structures. The
presence of such defects as voids, improper cure, debonds and
delaminations is almost common during the manufacture and
installation of the composite systems. The general effects of
potential defects on the rehabilitated concrete structures are
discussed by Kaiser and Karbhari [4].
B. FRP Physical and Mechanical Properties
The properties of prefabricated FRP system depend
primarily on the source material of FRP. Such properties must
be tested for acceptance before application. Physical and
mechanical properties should be determined experimentally
for in-place FRP system. In case of prefabricated FRP
systems the gross area including fiber and polymer should be
considered. Table-I presents some mechanical properties of
GFRP and Carbon Fiber Reinforced Polymers (CFRP) at a
fiber volume fraction of 50% as prescribed in the (ECP) [2].
Table-I: Mechanical Properties of FRP Laminates [2]
Type of Fibers/Polymer
Longitudinal Tensile
Strength (N/mm2)
Transverse Tensile
Strength (N/mm2)
Shear Strength (N/mm2)
Longitudinal Modulus of
Elasticity (N/mm2)
Transversal Modulus of
Elasticity (N/mm2)
Shear Modulus of
Elasticity (N/mm2)
Poisson's Ratio
Glass–E / Ester Vinyl
Carbon / Epoxy
610
1448
49
52
16
93
54
181
14
10
5
7
0.25
0.3
C. Ultrasonic Pulse Velocity Test
Ultrasonic testing is one of the ten NDT methods described
in ACI applicable for testing FRP-concrete bond condition.
Retrieval Number: C5170029320/2020©BEIESP
DOI: 10.35940/ijeat.C5170.029320
Different non-destructive methods have been developed to
evaluate the quality of the FRP/concrete adhesive bond, based
on ultrasonic waves, microwave, impact-echo, infrared
thermography or a coupling of these two latter techniques [5].
Kundu et. al. [6] reported the detection of delaminations
between concrete and glass FRP plates by the ultrasonic
method. Bastianini, et. al. [7] used the ultrasonic technique:
however, they utilized the amplitude of the reflection signal
rather than the time delay for detecting the bond defects. The
method was successful in assessment of the bond in FRP
strengthened concrete, and masonry. Luprano, et. al. [8]
employed the ultrasonic method for the characterization of the
interface between FRP composites and concrete. Their work
involved evaluation of the influence of carbon FRP materials
and the thickness of the reinforcement on the defects.
Ultrasonic test results were compared with the results
obtained by infrared thermography.
In the rehabilitation of a concrete bridge with carbon FRP
sheets, the acousto-ultrasonic NDT technology has shown the
ability to detect and image the delaminations between FRP
sheet and concrete substrate [9]. The defects, in the form of
delaminations, were intentionally formed at the FRP–concrete
interface during installation of the FRP sheet to investigate the
capability of this technique.
The propagation p-wave velocity (Cp) in concrete ranges
from 3000 to 5000 m/s depending on composition of
concrete, age, and condition, reduces to 2000 to 3000 m/s for
poor quality concrete [10].
Carino, et. al. [5] recommended that the wave velocity
obtained from ultrasonic pulse-velocity measurements was
more than this obtained from the impact-echo method and
recommended using a specific method for p-wave velocity
measurement using ultrasonic pulse-velocity. Martin and
Forde [11] found that there was no difference between the
velocity measurements from the two methods explaining that
the reason was due to the low ultrasonic excitation frequency
used compared to the commonly 100 kHz frequency used.
D. Correlation between Concrete Quality and Propagation
Velocity
The variation in wave velocity through testing a concrete
member, can be compared, to assess condition and quality of
concrete. The acceptable variation in p-wave velocity of
concrete was calculated to be about 2 to 2.5%. This is based
on an average variation in concrete member strength equal to
7.8% for laboratory conditions and 9.5% for field conditions
per ASTM standard [12].
The relation between age of concrete and wave velocity for
different grades of concrete has been studied by Amir (1988),
it is recommended that wave velocity increases with age for
all concrete grades. Billington, et. al. [13] used the
measurements of wave velocity to assess concrete quality and
found that crack propagation of measurement points, not
necessarily underneath it, degrade the general concrete
quality so reduces the wave velocity.
Pessiki and Johnson [14] experimentally verified the
difference between wave velocities in a beam versus a core as
shown in Fig. 1.
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International Journal of Engineering and Advanced Technology (IJEAT)
ISSN: 2249 – 8958, Volume-9 Issue-3, February, 2020
From Fig. 1(a) it is shown that at strength of 30 MPa the
corresponding beam velocity is 4000 m/s while in Fig. 1(b) at
the same strength, the velocity is 3800 m/s which is exactly
5% difference. Lee, et. al. [15] investigated that the
strength-velocity relationship of normal strength concrete is
noticeably different from that of high strength concrete.
a) Core Strength vs Beam
b) Core Strength vs Core
Velocity
Velocity
Fig. 1. Strength vs Velocity Relationships [14]
Materials
Glass Fibers
One roll of GFRP sheet was used, of 1.5 m wide by 50 m
long. The sheet was cut into layers of beams cross sectional
surface area 150 mm x 750 mm to cover each sample soffit as
shown in Fig. 2.
Fiber glass with uniform linear density of 15 N/m3 was
used to strengthen the beams. Layers of sheets were stacked at
different relative orientation angles. Varying the orientation
angle between every two GFRP sheets was between 30° and
45°. The layout of the fiber sheets was as follows:
3-layer sheets with angles of 90°/45°/90°.
5-layer sheets with angles of 60°/30°/0°/30°/60°.
7-layer sheets with angles of 90°/60°/30°/0°/30°/60°/90°.
E. Bond Quality at Internal Interfaces
In FRP-strengthened beams failure may occur due to beam
shear, flexural compression, FRP rupture, FRP debonding or
concrete cover ripping as presented by Ascione, et. al. [16],
and Bonacci and Maalej [17]. Based on experimental results
conducted by Chen and Teng [18], the most common failure
mode is due to debonding of FRP or ripping of the concrete
cover. These failure modes are undesirable because the FRP
cannot be fully utilized. In addition, such premature failures
are generally associated with the reduction in deformability of
the strengthened members. Premature failure modes are
caused by interfacial shear and normal stress concentration at
FRP cut off points and at flexural cracks along the beam [18].
Ultrasonic pulse-velocity studies of layered structures,
such as repair works made to concrete structures led to the
need to understand how bond quality at an interface between
two materials affect the response.
Sansalone and Streett [19] studied the quality of repairs
made to concrete box girders, repair done using concrete
identical to the parent concrete. Test was carried to determine
how well bonded these patches to concrete, P-wave velocity
in concrete was 4060 m/s.
V. EXPERIMENTAL PROGRAM
The experimental program carried out in this research work
is divided into three tasks; 1: GFRP laminates manufacture, 2:
Preparation of the test specimens and, finally, 3: Beam
load-testing and application of NDE test. Detailed description
of each task is presented in the following sections.
A. Manufacture of GFRP Laminates
The dry lay-up technique was adopted using two
directional GFRP sheets. These sheets are made of fibers
predominantly at an angle used to cover the structural
element. Installation on the concrete surface requires
saturating resin usually after a primer has been applied, dry
FRP sheets can be applied directly into the resin which has
been applied uniformly onto the concrete surface. Additional
resin applied onto the surface of FRP sheets in order to assure
full saturation of the fibers with the resin. The properties of
the materials used and the details of manufacture of the
laminates is presented in the following sub-sections.
Retrieval Number: C5170029320/2020©BEIESP
DOI: 10.35940/ijeat.C5170.029320
a)
Measuring the angles
b) Final Cut
Fig. 2. The Glass fiber roll
Resin
The resin used is polyester DAN-1000 NT mixed with
cobalt. The polyester comes in metallic drums weighting 20
kg. DAN-100 NT is a medium reactive unsaturated polyester
resin based on phthalic anhydride, with excellent and super
laminating properties. It impregnates the fiber glass quickly
and the laminate gives a smooth tack-free surface due to the
presence of wax. It is liquid if it is not mixed with peroxide.
The use of the polyester mixed with a small portion of
peroxide accelerates the process of hardening under normal
room temperature.
Determination of Tensile Strength and Young’s Modulus of
the GFRP Laminates
Three specimens were taken from every category with
average width of 30 mm. The total specimen length was about
300 mm including the length of the steel grips. Each sample
thickness was accurately measured by a vernier. Samples end
were supported by two side wooden supports to prevent
sample slippage. Fig. 3 shows the test setup for the GFRP
laminate samples.
All specimens were tested up to failure. The strains were
calculated by dividing the recorded extensions at uniform
intervals of load by the original specimen length. Thus, the
stress-strain relationship was plotted for each sample to
determine the modulus of elasticity for each specimen (Ef).
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and fifteen standard cylinders were cast for every concrete
strength group of beams for a total of forty-five cubes and
cylinders.
Table-III: Number of Test Specimens for Different
Concrete Strength
No. of
Laminate
s
3 Layers
5 Layers
7 Layers
% Voids
0
3
0
6
0
0
3
0
6
0
0
3
0
6
0
A
38 MPa
3
3
3
3
3
3
3
3
3
27
B
45 MPa
3
3
3
3
3
3
3
3
3
27
C
50 MPa
3
3
3
3
3
3
3
3
3
27
Grou
p No.
Fig. 3: Test Setup for GFRP Laminate Samples
The stress-strain curves of the used GFRP laminate
samples are shown in Fig. 4. It is shown from Fig. 4, that the
laminates exhibited linear behavior with no yielding up to
failure. From Fig. 4, it is shown that samples with greater
number of layers attained greater capacity and higher strain
level. The results of the stress–strain curves of GFRP
laminates are acceptable as per the ACI code [20] where it
was stated that the relationship is expected to follow a linear
trend.
Tota
l No.
Concrete Design Mixes
Three different concrete design mixes were used for each
group of specimens. The materials weight per one cubic meter
are listed for all groups in Table-IV.
Table-IV: Concrete Design Mixes for All Specimen
Groups
Grou
p
No.
A
Dolomite
(kN/m3)
Sand
(kN/m3)
Cement
(kN/m3)
13.25
7.85
3.90
Water
(Lit/m3
)
210
B
12.20
7.15
4.20
210
C
11.80
6.90
4.45
210
Determination of Concrete Compressive Strength
Fig. 4: Stress vs Strain Relationship for GFRP Laminates
with Different Number of Layers
Table-II presents a summary of the mechanical properties
obtained from the tensile tests performed on the samples.
Table-II: Mechanical Properties of GFRP Laminates
No.
Layers
Laminate
Thickness
(mm)
3
5
7
2.5
3.0
3.5
Average
Volume
Fraction
(Vf) %
50
52
53
Max.
Tensile
Strength
(MPa)
90.9
107.2
205.0
E
(MPa)
Max.
Strain
%
1901.5
1523.8
2154.2
5.7
9.0
10.8
The compressive strength of concrete was determined by
compression tests carried out on standard cubes and cylinders.
For all concrete beam groups 15 cubes and 15 cylinders were
taken from the batch. All cubes and cylinders were tested after
28 days to determine and verify the design strength for all
beam groups. Table-V shows a summary of the compression
test results. From Fig. 5 it is observed that the strength ratio is
almost constant for all concrete groups of different strength
and lies within the acceptable known range.
Table-V: Cube and Cylinder 28-Day Compressive
Strength for All Beam Groups
B. Preparation of Test Specimens
Three groups of beams constitute the experimental
program test specimens. The beams were plain concrete 750
mm long with a square cross section 150mm x150mm. The
beams were strengthened by GFRP laminates with three
different aspects: compressive strength, number of GFRP
laminate layers and percentage of voids at the interface
between FRP laminates and concrete. A total of ninety beam
specimens were tested, of which, nine control specimens were
constructed for each different concrete characteristic
compressive strength. Then, eighty-one specimens were
constructed to perform the parametric study which is
summarized in Table-III. In addition, fifteen standard cubes
Retrieval Number: C5170029320/2020©BEIESP
DOI: 10.35940/ijeat.C5170.029320
Group No.
Cube Strength
fcu (MPa)
Cylinder Strength
f’c (MPa)
Strength Ratio
f’c / fcu
A
B
C
38.8
45.7
50.7
34.3
39.8
44.7
0.88
0.87
0.88
Applying Intentional Delaminations (% Voids)
Several double-faced adhesive tapes, each, of area 45mm x
50mm were applied at the concrete beam soffit to simulate a
debonded area between first layer of GFRP laminate and
concrete. Thus, fifteen randomly dispersed tapes were used to
cover 30% of concrete beam soffit while, thirty were used to
cover 60% of the soffit. Fig. 5 presents the application of the
adhesive tapes for % void achievement.
Determination of Concrete Wave
Velocity
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ISSN: 2249 – 8958, Volume-9 Issue-3, February, 2020
As well as measuring compressive strength of standard
cubes and cylinders, the p-wave velocity (Cp) through the
standard specimens was measured to determine the expected
wave velocity through the beam specimens. In addition, the
measurement would serve to verify if the UPV equipment
would be sensitive to the change in specimen shape.
Fig. 7: Beam Testing Setup
a) Applying Adhesive Tape
b) Laminate Layup
Fig.5: Delamination Simulation by Adhesive Application
Fig. 6 presents examples of the wave velocity measurement
setup on the different samples. Further, a summary of the
obtained results is presented in Table-VI. From Table-VI it is
proven that the UPV equipment was sensitive enough to
differentiate between the different geometry of the specimens.
It is worth noting that the average values are within the known
range with respect to the compressive strength.
Fig. 8: Actual Load Testing Setup
NDE Testing
The system consisted of a calibrated ultrasonic pulse
velocity device used to measure ultrasonic pulse wave
velocity (UPV) during loading as shown from Fig. 8. The
UPV transmitter and receiver were located 450 mm apart on
the top face of the tested beam and centered around the
loading plate as shown in Fig. 7. The travel time of the UPV
pulses were recorded at each loading step from the testing
machine with an accuracy of 0.1 µs. The distance between the
UPV poles was then divided by the recorded travel time to
determine the velocity at each loading step.
VI. RESULTS AND DISCUSSION
a) Cube Cp Measurement b) Cylinder Cp Measurement
Fig.6: Measuring Ultrasonic Wave Velocity through
Cubes and Cylinders
Table-VI: Cube and Cylinder Ultrasonic Wave Velocity
for All Beam Groups
Group No.
Cube Velocity
Cpcu (m/s)
A
B
C
4508
4976
5059
Cylinder
Velocity
Cpc (m/s)
4022
4113
4796
C. Beam Test Setup for Load Test and NDE Test
Load Testing
All beams were tested under 4-point load concurrently with
the non-destructive test using the UPV, Fig. 7 shows a
schematic of the load test setup. Fig. 8 presents a complete
picture of the testing setup. Beam specimens were tested
under a monotonic load at a rate of 1 kN/min.
Retrieval Number: C5170029320/2020©BEIESP
DOI: 10.35940/ijeat.C5170.029320
All beam specimens were tested after 28 days of casting.
All specimens were loaded and the appearance of first crack,
cracking load, crack propagation, ultimate load and mode of
failure were observed. The recorded measurements are;
machine load, mid-span deflection and the UPV travel time
reading. The results of the monotonic loading tests performed
concurrently with UPV testing on the beam specimens are
presented and discussed in the following sections. The
discussion will focus on four major parameters in the
following sections which are:
Cracking and failure loads
Load vs mid-span deflection relationships
Strength vs UPV relationships
Stiffness improvement index
A. The Cracking and Failure Loads
For all tested beams, the first crack appeared in beam soffit
at the midspan between the two load heads coupled with an
excessive increase in deflection. Those flexural cracks
appeared when the total load of the beam reached between 35
and 40 kN for Group (A), between 40 and 45 kN for Group
(B) and between 45 and 50 kN for Group (C).
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Cracks initiated during the loading intervals and did not
propagate with loading. Two factors contributed to that;
existence of GFRP laminates used in strengthening the beams
and the beam span to depth ratio, which prevented the beam
failure in flexural mode.
Tables-VII through X show all test results of which
cracking loads, failure loads and maximum deflection for
each tested beam, it was observed that cracks started to
propagate through the 150 mm (distance between the two
loading line loads) until failure.
Table-VII: Summary of Test Results on 3-Layer Beams
Beam ID
% Voids
Pc (kN)
Pf (kN)
Δmax (mm)
A 31
A 32
A 33
B 31
B 32
B 33
C 31
C 32
C 33
0
30
60
0
30
60
0
30
60
38.5
41.0
25.3
37.9
45.8
43.0
45.0
43.0
40.5
55.0
39.2
30.0
82.4
76.0
52.5
81.0
79.0
50.0
3.9
2.4
2.4
6.6
5.6
3.9
6.0
5.8
3.4
numbers of tested beams, the cracks were vertical during early
stages of loading but propagated diagonally towards the
concentrated loads as the load was further increased. In
general, all tested beams experienced brittle failure.
B. Load vs Midspan Deflection Relationship
The beams strengthened with GFRP laminates showed
typical load vs deflection relationship. The relationship
considered of a bilinear curve: stiff up to the cracking load,
and then softens when concrete cracks, but becomes also
linear up to failure.
Fig. 9 presents an example of the load vs deflection for
Group A beams strengthened by 7 layers of GFRP laminates
with delaminations (% voids) 0, 30 and 60% respectively.
From Fig. 9 it is observed that the beam with 0% voids failed
at higher load and the first crack occurred at higher deflection.
All strength groups showed a similar response as that
followed by beams of Group A.
In the following subsections the effect delamination (%
voids) and the effect of number of laminate layers on load and
deflection of the beams will be presented for each concrete
strength group.
Table-VIII: Summary of Test Results on 5-Layer Beams
Beam ID
% Voids
Pc (kN)
Pf (kN)
Δmax (mm)
A 51
A 52
A 53
B 51
B 52
B 53
C 51
C 52
C 53
0
30
60
0
30
60
0
30
60
33.5
34.5
27.5
44.0
43.0
40.0
57.0
46.0
40.0
60.0
42.7
31.5
84.0
79.0
59.0
98.0
81.0
73.0
4.2
3.7
2.3
6.3
4.6
3.9
6.2
5.3
4.7
Table-IX: Summary of Test Results on 7-Layer Beams
Beam ID
% Voids
Pc (kN)
Pf (kN)
Δmax (mm)
A 71
A 72
A 73
B 71
B 72
B 73
C 71
C 72
C 73
0
30
60
0
30
60
0
30
60
48.0
50.5
43.2
41.5
50.8
40.7
55.0
53.0
48.0
87.5
74.1
51.5
114.0
86.0
60.0
121.0
102.5
87.5
4.7
4.9
3.4
5.2
5.3
3.3
5.5
5.8
5.1
Fig. 9: Load vs Deflection Relationship for 7-Layer
Strengthened Beams with Various % Delaminations
For Group A (38 MPa Strength)
Fig. 10 presents Group A comparisons between
load-carrying capacity with number of layers and % voids in
contrast to the control beam. From Fig. 10 it is shown that
strengthening using 3-layer GFRP laminates with 0%, 30%,
and 60% of voids increased the capacity by 2.08, 1.48, 1.13
times, respectively, from that of the control beam. While,
strengthening using 5-layer GFRP laminates with 0%, 30%,
and 60% of voids increased the capacity by 2.26, 1.61, 1.19
times, respectively. Finally, strengthening using 7-layer
GFRP laminates with 0%, 30%, and 60% of voids increased
the capacity by 3.30, 2.80, 1.94 times, respectively.
Table-X: Summary of Test Results on Control Beams
Beam ID
Pf (kN)
Δmax (mm)
A control
B control
C control
26.5
33.0
44.0
2.2
1.4
1.7
From all the previous tables it is observed that in all
specimens the deflections increased linearly with increasing
load prior to first cracking. First cracking during the test was
pointed in the load deflection curve by a marked reduction in
the stiffness. The initial crack occurred under the
concentrated line loads or within its close vicinity. New
flexural cracks were developed closer to the supports while
the existing cracks grew with increasing load. In a few
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Fig. 10: Load-Carrying Capacity for Group A Beams
with Different Number of Layers vs % Voids
Fig. 12: Load-Carrying Capacity for Group C Beams
with Different Number of Layers vs % Voids
For Group B (45 MPa Strength)
Fig. 11 presents Group B comparisons between
load-carrying capacity with number of layers and % voids in
contrast to the control beam. From Fig. 11 it is shown that
strengthening using 3-layer GFRP laminates with 0%, 30%,
and 60% of voids increased the capacity by 2.5, 2.3, 1.59
times, respectively, from that of the control beam. While,
strengthening using 5-layer GFRP laminates with 0%, 30%,
and 60% of voids increased the capacity by 2.55, 2.4, 1.79
times, respectively. Finally, strengthening using 7-layer
GFRP laminates with 0%, 30%, and 60% of voids increased
the capacity by 3.45, 2.6, 1.82 times, respectively.
C. Strength vs Ultrasonic Pulse Velocity Relationship
The results obtained from ultrasonic testing in correlation
with the concrete strength is presented for each beam group.
Table- XI presents wave velocity for each beam group.
Table- XI: Wave Velocity through Beam Groups
Group No.
%
Voids
Group (A)
Wave
Velocity (m/s)
Group (B)
Wave
Velocity (m/s)
Group (C)
Wave
Velocity (m/s)
Control
Beam
N/A
4116
4352
4450
Beam 31
Beam 32
Beam 33
Beam 51
Beam 52
Beam 53
Beam 71
Beam 72
Beam 73
0
30
60
0
30
60
0
30
60
4169
4046
3912
4313
3934
3724
4494
4028
4006
4401
4350
4299
4453
4390
4104
4707
4538
4219
4482
4367
4205
4601
4597
4208
4951
4763
4329
From Table-XI by comparing wave velocities for all beam
groups with different number of laminate layers it is observed
that:
Fig. 11: Load-Carrying Capacity for Group B Beams
with Different Number of Layers vs % Voids
For Group C (50 MPa Strength)
Fig. 12 presents Group C comparisons between
load-carrying capacity with number of layers and % voids in
contrast to the control beam. From Fig. 12 it is shown that
strengthening using 3-layer GFRP laminates with 0%, 30%,
and 60% of voids increased the capacity by 1.84, 1.8, 1.14
times, respectively, from that of the control beam. While,
strengthening using 5-layer GFRP laminates with 0%, 30%,
and 60% of voids increased the capacity by 2.23, 1.84, 1.66
times, respectively. Finally, strengthening using 7-layer
GFRP laminates with 0%, 30%, and 60% of voids increased
the capacity by 2.75 ,2.33 ,1.99 times, respectively.
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For Group A (38 MPa Strength)
Strengthening using 3-layer GFRP laminates with 0%
voids, the wave velocity through tested beams increased by
1.3% than that of the control beam, but for 30%, 60% it was
reduced by 1.7% and 4.9%, respectively. While for
strengthening using 5-layer GFRP laminates with 0% voids,
the wave velocity through tested beams increased by 4.8%
than that of the control beam, but for 30%, 60% it was
reduced by 4.4% and 9.5%, respectively. Finally,
strengthening using 7-layer GFRP laminates with 0% voids,
the wave velocity through tested beams increased by 9.2%
than that of the control beam, but for 30%, 60% it was
reduced by 2.1% and 2.7%, respectively.
For Group B (45 MPa Strength)
Strengthening using 3-layer GFRP laminates with 0% and
30% voids, the wave velocity increased by 1.2% and 0.1%,
respectively, than that of the
control beam, but for 60% it
was reduced by 1.2%. While
for strengthening using 5-layer
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GFRP laminates with 0% and 30% voids, the wave velocity
increased by 2.4% and 0.9%, respectively, that of the control
beam, but for 60% it was reduced by 5.6%. Finally,
strengthening using 7-layer GFRP laminates with 0% and
30% voids, the wave velocity increased by 8.2% and 4.3%,
respectively, than that of the control beam, but for 60% it was
reduced by 3.0%.
For Group C (50 MPa Strength)
Strengthening using 3-layer GFRP laminates with 0%
voids increases the wave velocity through tested beams 0.8%
than that of the control beam, but for 30%, 60% it was
reduced by 1.8%, and 5.5% respectively. While for
strengthening using 5-layer GFRP laminates with 0% and
30% voids, the wave velocity increased by 3.4% and 3.3%,
respectively, than that of the control beam, but for 60% it was
reduced by 5.4%. Finally, strengthening using 7-layer GFRP
laminates with 0% and 30% voids, the wave velocity
increased by 11.3% and 7.1%, respectively, that of the control
beam, but for 60% it was reduced by 2.7%.
For a better visual representation of the previous results,
Fig. 13 shows comparisons between wave velocities for all
beam groups with different number of laminate layers and
varying %voids (delaminations).
(a)
From all the previous results presented in Table-XI and
Fig. 13, it could be concluded that for the same strength and
number of layers, as the % delamination increased, the
velocity of the wave is reduced. This is an indication of the
degrading bond strength between concrete and the GFRP
layers even more than that of the control beam. This could be
explained by the fact that existence of the GFRP layers with
voids on the beams soffit result in distorting the p-wave inside
the beam and result in a reduction in the wave velocity.
On the other hand, for the same strength and %
delaminations, as the number of layers increased, the wave
velocity increased. This is an indication of the increased
system stiffness due to the bond action between concrete and
GFRP.
Finally, for the same number of layers and %
delaminations, as the concrete strength increased, the wave
velocity increased. This observation is more significant with
lower strength than that of lower strength.
By correlating the results obtained from the wave velocity
measurements to the load-carrying capacity of the beams it is
obvious that they are consistent. Where, for example, for the
same strength beams with the same number of laminate layers,
the increase in % delaminations reduce the load carrying
capacity and at the same time reduce the propagation wave
velocity. However, it is important to note that rate of decrease
in the capacity is not proportional to that of the wave velocity
decrease. The reason for this would be that improvement in
load-carrying capacity is due to the increase in flexural
strength due to the bond between the GFRP and the concrete.
While the increase in wave velocity is related to the increase
in the material stiffness due to the confinement from the
GFRP to the concrete at the interface.
D. Stiffness Improvement Index
From load-deflection curves the value of the stiffness of
each beam is calculated as the slope of the first straight part of
the load-deflection curve at value of load less than the
observed cracking load.
Stiffness Improvement Index (SII) may be defined as the
percentage of increase in deformation (from cracking stage to
failure stage) relative to deformation at cracking stage:
(1)
(b)
Where,
= Midspan deflection at ultimate load
= Midspan deflection at first cracking load
Fig. 14 presents the stiffness improvement index relations
for all beam groups. From Fig. 14 it is shown that the presence
of a larger number of GFRP layers on the beam enhances the
stiffness, thus the stiffness improvement index. In addition,
from Fig. 14, it is noted that GFRP laminates are more
effective with higher concrete strength as the fiber
reinforcement mechanisms increase with the increase of the
concrete strength, if fiber rupture is avoided.
(c)
Fig. 13: Comparison between Wave Velocities for All
Beam Groups; (a) 3-Layer, (b) 5-Layer, and (c) 7-Layer
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VII. SUMMARY & CONCLUSIONS
The current study experimentally investigates the capacity
of strengthened concrete beams with different number of
layers of GFRP laminates in with varying % delaminations
represented by % voids and contribution of both concrete and
GFRP laminates in carrying load. The different parameters
affecting the behavior of GFRP-strengthened plain concrete
beams, namely; concrete characteristic strength, the number
of laminate layers, and percentage of voids are investigated.
4.
5.
6.
7.
strengthened concrete beams increased with a semi
linear variation relation enhancing behavior of beams
and decreasing crack propagation.
The increase in number of GFRP laminate layers
enhances the stiffness improvement index.
GFRP laminates are more effective with higher concrete
strength provided that debonding is not present.
The UPV technique was sensitive to the variation in
concrete strength and GFRP laminate layers. Where the
p-wave velocity increased with concrete strength.
Further, existence of more laminate layers at beam soffit
increased the p-wave velocity at the interface.
The UPV technique showed changes in p-wave
propagation pattern and velocity between different
specimen shapes, (cubes, cylinders, and beams).
REFERENCES
(a)
(b)
(c)
Fig. 14: Comparison between Stiffness Improvement
Index for Tested Beams; (a) Group A, (b) Group B, and
(c) Group C
The experimental program included ninety plain concrete
beams. Three groups, each, of twenty-seven beams were
strengthened by 3-layer, 5-layer, and 7-layer GFRP laminates
with different % voids. The behavior of the beams under load
was studied along with a concurrent evaluation using UPV
tests. The inclusion of glass fibers in all beam groups resulted
in enhanced stiffness, stiffness improvement index and
absorbed energy.
From the herein presented discussions of the obtained
results, several conclusions could be drawn as follows:
1. The test results of the locally produced GFRP laminates
showed a product with accepted material properties and
high strength to weight ratio that could be applied
effectively to strengthen concrete beams.
2. It was found that interfacial debonding in most cases
initiates where a localized flexural crack formed in the
concrete, weak cracks appear at interface, Then, it
extends through the concrete adjacent to the bond
interface.
3. As the percentage of voids decreased, the capacity of
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DOI: 10.35940/ijeat.C5170.029320
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AUTHORS PROFILE
Author-2
Photo
Waleed F. Tawhed is an Assistant Professor in Civil
Engineering Department, Faculty of Engineering at
Mataria, Helwan University, Cairo, Egypt. He has
received his Ph.D. degree from the University of South
Carolina, Columbia, USA in 2005. He is specialized in
strength and properties of materials and Non-Destructive
testing. His research interests include; behavior and
construction of concrete materials, non-destructive
evaluation and non-destructive testing of concrete and
masonry materials, behavior and applications of modern
composite materials in repair and strengthening of
concrete structures, in addition to studying innovative
techniques for recycling organic materials to be used in
concrete structures. Dr. Tawhed is active in research and
academic supervision on numerous engineering
post-graduate students in the field of material science.
Yasmen R. Elsayed is a Teaching Assistant in Civil
Engineering Department, Faculty of Engineering at
Mataria, Helwan University, Cairo, Egypt. She is
specialized in strength and properties of materials and
Non-Destructive testing. Her research interests include;
behavior and construction of concrete materials, use of
advanced composite materials in repair and strengthening of concrete
structures, applications of non-destructive techniques in evaluation of
concrete structures.
Author-2
Photo
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