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Comparative behavior under

compression of concrete columns
repaired by fiber reinforced polymer
(FRP) jacketing and ultra high-
performance fiber reinforced concrete
(UHPFRC)
a a b a
B. Rabehi , Y. Ghernouti , Alex Li & K. Boumchedda
a
Research Unit: Materials, Processes and Environment, University
Click for updates M’Hamed Bougara of Boumerdes, Boumerdes, Algeria
b
Civil Engineering Laboratory, University of Reims, Reims, France
Published online: 07 Oct 2014.

To cite this article: B. Rabehi, Y. Ghernouti, Alex Li & K. Boumchedda (2014) Comparative behavior
under compression of concrete columns repaired by fiber reinforced polymer (FRP) jacketing and
ultra high-performance fiber reinforced concrete (UHPFRC), Journal of Adhesion Science and
Technology, 28:22-23, 2327-2346, DOI: 10.1080/01694243.2014.966885

To link to this article: http://dx.doi.org/10.1080/01694243.2014.966885

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 Journal of Adhesion Science and Technology, 2014
Vol. 28, Nos. 22–23, 2327–2346, http://dx.doi.org/10.1080/01694243.2014.966885

Comparative behavior under compression of concrete columns

repaired by fiber reinforced polymer (FRP) jacketing and ultra
high-performance fiber reinforced concrete (UHPFRC)
B. Rabehia*, Y. Ghernoutia, Alex Lib and K. Boumcheddaa
a
Research Unit: Materials, Processes and Environment, University M’Hamed Bougara of
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Boumerdes, Boumerdes, Algeria; bCivil Engineering Laboratory, University of Reims, Reims,

France
(Received 24 June 2014; final version received 5 September 2014; accepted 11 September 2014)

This paper summarizes the experimental results from a comprehensive research pro-
gram to study the fundamental stress–strain behavior of damaged concrete repaired
by two techniques: increased concrete section and bonding fiber reinforced polymer
(FRP). In this work, two types of FRP composite jackets were used, carbon fiber
reinforced polymer (CFRP) and glass fiber reinforced polymer, and two types of
concretes were used to repair the damaged concrete by increased concrete section:
ordinary concrete and ultra high-performance fiber reinforced concrete (UHPFRC).
Fifteen circular columns of concrete (110 × 220) cm3 were initially pre-damaged up
to intense cracking, repaired by increased concrete section and by bonding FRP, and
tested under uni-axial compression by loading up the damage. The impact of differ-
ent design parameters, including plain concrete strength, types of composites, and
type of concrete used for increasing section, was considered in this study. The
strength enhancement and ductility improvement of specimens are discussed. A
simple model is presented to predict the compressive strength of repaired damaged
concrete columns. A significant strength and an increase in ductility were achieved,
particularly when the columns were repaired by increasing section with UHPFRC
and by bonding CFRP. These preliminary tests indicate that the use of UHPFRC is
an effective technique for rehabilitating damaged concrete columns, highly competi-
tive with the repaired concrete by wrapping specimens with FRP composite jackets.
Keywords: concrete; composites; UHPFRC; repaired; damage; FRP; axial
compression; cracking

1. Introduction
Recently, repairs of a degraded civil infrastructure have attracted a great amount of
attention due to highly commercial benefits. Depending on the geometry of structure
and the type of damage, different techniques can be applied for repairing of concrete
structures.
The improved behavior of structures requires repair and strengthening, and several
factors will influence the choice of the repairing method. These factors can be techni-
cal, economical or practical.
Rehabilitation can be divided into two categories: repairing and strengthening.
Repairing is the rehabilitation of a damaged structure or a structural component with

*Corresponding author. Email: rabehibahia@yahoo.fr

© 2014 Taylor & Francis

 2328 B. Rabehi et al.

the aim of restoring the original capacity of the damaged structure. Strengthening, on
the other hand, is the process of increasing of the existing capacity of a non-damaged
structure (or a structural component) to a specified level. Several rehabilitation
techniques have been developed and used to achieve a desired improvement.[1]
In some cases, it is required to increase the capacity of the structural element to
meet the increased demands of new standards or higher live loads. Increasing the cross
section of a concrete member is a simple and frequently applied strengthening method.
Increasing the cross section of a reinforced concrete column is quite an efficient
strengthening measure. Placing a steel tube around a column and filling the gap with
concrete can strengthen the column very effectively. Also, to enhance the performance
of the repaired concrete structure, polymer composites are usually used as the reinforc-
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ing materials because composites provide the properties of high strength, high damage
tolerance, and better resistance to detrimental environment.[2]
Many reinforcement techniques have been tested experimentally over the last 15
years. The majority of these reinforcements were designed with one of the three
materials, i.e. concrete, steel, and composite materials.
All these techniques can be successfully used but have some limits. In particular,
the use of reinforced concrete jacket is possible by adding layers of concrete,[3] or
fiber reinforced concrete [4] is nowadays extensively used in applications where fiber
reinforcement is not essential for the structural safety that can excessively increase the
section geometry. The use of externally glued steel plates as well as of fiber reinforced
polymer (FRP) may have problems for fire resistance. Furthermore, the use of FRP
may not satisfy minimum requirements for Serviceability Limit States. Recently, a
new technique has been developed for reinforcing concrete.[5–7] This solution is
based on the application of a thin jacket in high performance fiber reinforced concrete
(HPFRC) with a high compressive strength and a hardening behavior in tension.[8]
The latter property allows using this material in substitution of the traditional steel
reinforcement.
The objectives of the research described in this paper are to experimentally evaluate
the fundamental compressive stress–strain behavior of damaged concrete repaired by
two techniques: carbon fiber reinforced polymer (CFRP) or glass fiber reinforced poly-
mer (GFRP) composite jackets and increasing concrete section with two types of con-
crete, ordinary concrete (OC) and ultra-high performance fiber reinforced concrete
(UHPFRC), in a unified approach, including the testing procedure and the manufacture
of the sample specimens.

2. Experimental program
In this work, 15 identical circular concrete specimens were cast to have a cross section
with 220 mm in height and 110 mm in diameter.
These specimens are divided as follows:

Three OC specimens, for determining the concrete compression strength;

Six OC specimens damaged and then repaired by increasing section with con-
crete, in which three specimens were repaired by ordinary concrete (SR-OC) and
three others were repaired by UHPFRC (SR-HPC);

Six concrete specimens damaged and then repaired by bonding FRP, in which
three specimens were repaired by carbon fiber (SR-CF) and three others were
repaired by glass fiber (SR-GF).
 Journal of Adhesion Science and Technology 2329

2.1. Materials
2.1.1. OC mixture
In this study, the 15 specimens of OC are composed of crushed sand, 0–5 mm; two
classes of gravel, 3–8 mm and 8–15 mm; and cement, CEM II 32.5. Specific gravities
of the sand and the coarse aggregate are 2.62 and 2.65 g/cm3, respectively. The sand
has a fineness modulus of 2.55. The compositions determined by the Dreux Gorisse
method [9] for the OC are given in Table 1.

2.1.2. UHPFRC mixture

The UHPFRC mix design differs greatly from the design of normal and high-strength
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concrete. The composition of UHPFRC is characterized by high cement content, a

superplasticiser high-range water reducer, and the presence of an ultrafine as silica
fume. The composition UHPFRC applied in the present study for rehabilitation of dam-
aged specimens based on Bonneau method 1997,[10] is given in Table 1. The cement
used in the mix was normal Portland cement type NA442-CEM I. The water-to-binder
ratio (w/b) was 0.2 and a large amount of it is equal to 25% of the cement weight of
calcined clay (Metakaolin) at a temperature of 750 °C for 5 h,[11,12] extracted from
Djebel Debbagh (Algeria) named DD3, which contains essentially 43.3% of Al2O3 and
40.47% of SiO2.
For the aggregate, a sand of 2 mm maximum particle size is used as fine aggregate
which has a siliceous mineralogical nature containing 95% of SiO2. The sand has a
specific gravity of 2.72 g/cm3, a fineness modulus of 2.23, and a water absorption value
of 0.8%.
To obtain sufficient strain-hardening behavior, steel fiber corresponding to a 2.5%
volume ratio was intermixed; the steel fiber is SIKA METAL FIBRE RC-80/50-BN
type, and it has a length of 15 mm, diameter of 0.6 mm, aspect ratio of 25, and density
of 7.85 g/cm3.
A superplasticiser ‘MEDAFLOW 30’ was used in this work; it is a high-range
water reducer of the third generation.
The average compressive and flexural strengths of the UHPFRC are 153.5 and
19.2 MPa, respectively.[12]

Table 1. Compositions of concrete mixtures.

Wight (kg/m3)
Components
OC UHPFRC
Cement (CEM II 32.5) 350 /
Cement (NA442-CEM I) / 800
Sand (0/5) 686 /
Sand (0/2) / 1144
Gravel (3/8) 140 /
Gravel (8/15) 934 /
Water 212 200
Metakaolin (DD3) / 200
Superplasticiser (MEDAFLOW 30) / 53
Ratio (W/b) 0.6 0.2
Steel fiber 350 194
 2330 B. Rabehi et al.

Table 2. Mechanical properties of composites materials and resin.[3]

GFRP CFRP Resin

Tensile strength (MPa) 69–86 345–690 >30 at two days and 20 °C
Elastic modulus in tension (GPa) 74 >165 4.5 at seven days and 23 °C
Ultimate elongation in tensile (%) 4,5 1.7 /
Compressive stress (MPa) / / >55 at two days and 20 °C

2.1.3. Composite material (FRP material)

In this study, for jacketing damaged concrete, two types of composite materials have
been selected: GFRP and CFRP. The GFRP is made of continuous glass fiber type E
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with a thickness of 0.25 mm and a weight of 300 g/m2. The CFRP is a carbon fiber
fabric (SIKA WRAP HEX 230C), with a thickness of 1.2 mm. A resin system that was
used to bond the CFRP and GFRP was Sikadur-330. The mechanical properties of FRP
materials and the resin used are given in Table 2.

2.2. Preparation of specimens

After mixing OC, the filling cylindrical molds with 110 mm in diameter and 220 mm
in length were carried out.
For each series, three specimens were prepared by using the same composition.
After demolding, the specimens were deposited in water at a temperature of 20 ± 2 °C
for 28 days. At this age, the specimens underwent an operation by using jacketing of
carbon fiber or glass fiber (Figure 1).

2.3. Damaged specimens

It is known that if the load exceeds 70% of the compression strength of concrete,
the deformation in the concrete increases very rapidly. The failure of the specimen
can be provoked by cracking parallel to the loading axis.[13] In this work, the speci-
mens were damaged up to 70% of ultimate strength in compression before the repair
(Figure 2).
After visual inspection, the specimens have a remarkable state of damage (intense
cracking). This state of damage was estimated by the ultrasonic pulse velocity testing.
This method makes it possible to quantitate the internal damage of concrete specimen.
The principle is to measure the propagation time (t) of longitudinal waves in concrete,
issued by the issuer probe. Knowing the distance (s) travelled by the waves, it is
possible to derive a velocity (v) equal to (l/t) in m/s. The procedure consists in placing
sensors on the concrete surface coated with a thin layer of petrolatum; this layer will
prevent unwanted waves, as described in ASTM C597-97.[14] Measurements are
recorded on digital screen at the time of stabilization (Figure 3).
The results of the sonar test before and after the cracking of the four types of
concrete are presented in the following histograms (Figure 4).
Figure 4 clearly shows the decrease in ultrasonic velocity after damage of the
specimens. The average value of decreases is from 9 to 18%. This decrease is due to
the loss of performance and the compactness of concrete during loading. The difference
of reduction between the specimens can be also explained by the heterogeneity and
fragility of concrete.
 Journal of Adhesion Science and Technology 2331
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Figure 1. OC specimens.

Figure 2. Damaged specimens.

Figure 3. Measuring the state of damage concrete by ultrasound.

 2332 B. Rabehi et al.
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Figure 4. Ultrasonic velocity of the specimens before and after damage.

2.4. Repair of damaged concrete specimens

2.4.1. Specimens repaired by increasing section
Before the repairing, the damaged surface of the specimens was cleaned. Any defective
concrete and the outer cover of concrete specimens were carefully removed to obtain a
rough and solid surface. Dust particles were also removed with pressurized water and a
wire brush. After repaired with a same composition of concrete of specimens damaged,
the initial section of concrete specimens of diameter 10 cm became 15 cm, while
keeping the same height of 20 cm. The mechanical tests were performed at 28 days of
curing (Figure 5).

Figure 5. Repair of damaged specimens by increasing section.

Figure 6. Repair of specimens by jacketing FRP.

 Journal of Adhesion Science and Technology 2333

2.4.2. Specimens repaired by CFRP and GFRP

Before the repairing of damaged specimens by composite fiber fabrics, the concrete surface
had been treated to remove any surface grease, laitance, and any heterogeneous part. The
specimen surface was then cleaned and polished with a wire brush and then dusted with an
aspirator. Composite material was carefully cut to the desired dimensions. For bonding of
the composite on the concrete specimen, a layer of 1 mm of the adhesive is applied on the
surface and then we placed manually one (1) layer of composite material on the support.
Finally, a pressure was exerted on the composite by using a squeegee (Figure 6).

2.5. Testing of specimens

Specimens were loaded under a monotonic uni-axial compressive load up to failure
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using a hydraulic machine with the capacity of 1500kN. The loading rate was approxi-
mately 2.5kN/s. At load intervals of 50 kN, with displacement control, the higher tray
was fixed and the lower support was mobile. Before testing, the square faces of the
specimen were suffered with a surfacing machine to ensure parallelism and flatness of
the faces of support. An extensometric comparator was fixed to the side face of the
specimen at mid-height. The values of the vertical force and the corresponding
displacement were recorded simultaneously (Figures 7 and 8).

Figure 7. Surfacing of specimens.

Figure 8. Test setup and instrumentation.

 2334 B. Rabehi et al.

3. Results and discussion

3.1. Compressive strength and deformation
The results of the compressive strength and corresponding strain of all specimens’
concrete, obtained by experimental testing, are represented in Table 3; we note that the
compression strength is from 16.2 to 37.5 MPa and the corresponding strain varies
Table 3. Compressive strength and deformation of specimens.

Compressive strength Axial strain Ultimate strength Ultimate axial strain

Specimens fcc (MPa) εcc (‰) fcu (MPa) εcu (‰)
OC 23.6 ± 1.5 1.68 ± 0.13 21.2 ± 1.3 2.73 ± 0.1
SR-OC 16.2 ± 1.5 3.89 ± 0.4 14.5 ± 1.4 4.54 ± 0.5
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SR-HPC 37.5 ± 1.6 2.52 ± 0.2 33.6 ± 1.3 3.16 ± 0.2

SR-CF 32.5 ± 0.8 10.7 ± 1.2 29 ± 0.8 12.07 ± 0.7
SR-GF 22.5 ± 1 3.22 ± 0.3 19.7 ± 0.4 8.1 ± 0.1

Figure 9. Stress–strain curves of OC and concrete repaired by increasing section.

Figure 10. Stress–strain curves of OC and concrete repaired by bonding FRP.

 Journal of Adhesion Science and Technology 2335

from 1.68 to 10.7‰. The maximum dispersion of the experimental values is about 9%
for mechanical resistance and 11% for the deformation.
Figures 9 and 10 show the comparative stress–strain curves of reference concrete
specimens with specimens concrete repaired by increasing section and by bonding FRP,
respectively.
Comparing these values with those obtained on OC specimens, the gains in strength
and ductility of all specimens repaired are illustrated in Table 4 and represented in
Figures 11 and 12.

Table 4. Values of experimental results.

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Reference specimens Repaired specimens

Specimens fco (MPa) εco (‰) εu (‰) fcc (MPa) εcc (‰) εcu (‰) fcc/fco εcc/εco
OC 23.6 1.68 2.73 / / / / /
SR-OC 18.2 3.89 4.54 0.77 2.31
SR-HPC 37.5 2.52 3.16 1.58 1.50
SR-CF 32.5 10.7 12.07 1.38 6.37
SR-GF 22.5 3.22 8.1 0.95 1.92

Figure 11. Gain or loss in strength of specimen repaired in comparison with OC.

Figure 12. Gain or loss in ductility of specimen repaired in comparison with OC.
 2336 B. Rabehi et al.

Figure 9 shows the variation in the stress as a function of the strain. It is can be
observed from this figure that there exist three regions between the stress and the strain
relation. The first region is linear and shows the rigidity of the specimen. The value of
the rigidity obviously increases from the specimen SR-OC to the specimen SR-HPC.
After cracking, the strain increases quickly up to the ultimate strength of the specimen.
In the last region or after the ultimate strength, the stress of the specimen decreases.
On the other hand, in comparison with the specimen OC, the value of the stress of SR-
OC is less important, whereas the value of the strain is more important.
Figure 11 shows that the gain of the ultimate strength is not obtained in the case of
the specimen SR-GF. However, this specimen is more ductile. The mechanic perfor-
mance of the specimen SR-CF is obvious both on the strength and on the strain.
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Figure 12 shows the evidence of the gain or loss on the ductility of specimen
repaired in comparison with OC. The positive effect of the repair by UHPFRC or
CFRP is confirmed.
The different technical of rehabilitation offers an elastic modulus and rigidity which
can modify various axial behaviors of reference concrete and improve its resistance and
ductility. The axial behavior obtained from different specimens can show that typically
all curves have an initial slope until a point of inflection, followed by a large zone of
plastic deformation. Depending on the effectiveness of each technique, the stress level
and the plastic zone vary considerably from one to another.
All specimens were subjected to a law of behavior which consists of three phases
in accordance with other researches.[15–19] These curves can be illustrated by the
following regions:
Region 1: A linear phase without micro-cracking of concrete.
Region 2: A second linear ascending part, during how the core concrete micro-
crack, and the repair concrete or the FRP are put under tension.
Region 3: A declining phase during which the effort is taken up by the fibers or the
repair concrete that assembles the cracked concrete until rupture.
The concrete specimens, SR-OC that were damaged and repaired by increasing sec-
tion with OC, cannot recover their initial strength; they lose till 38% of their initial
strength but their deformations increase. Note that the diameter of specimens is about
15 cm instead of 10 cm before the repairing; in this case of repairing concrete by
increasing section, the adhesion between new concrete and the substrate is not perfect.
The rehabilitation of damaged concrete specimens by increasing section with
UHPFRC (SR-HPC) can significantly increase the ultimate load and axial strain, as
shown previously in Figure 9. For all specimens, the ratios (fcc/fco) and (εcc/εco) are
always significant, presenting values superior to one, a gain in compressive strength
and axial strain of about 58 and 50%, respectively, comparatively with specimens of
OC (not damaged). It is clear that the method of repairing by increasing section with
UHPFRC is more effective and more attractive in terms of mechanical behavior.
This can be explained by the fact that this method offers high performance through
the use of UHPFRC containing a superplastizer that reduces the amount of water, ultra-
fine as metakaolin (DD3) that improves the compactness; and furthermore, the steel
fibers makes them very ductile.
The resistance of UHPFRC is a reference quality as what is significantly increased
compared to that of the OC. It is about 153.5 MPa in compression and 19 MPa in
tension.
During a compression test, the UHPFRC behaves almost linearly up to the
maximum stress; the metal fibers are then used to avoid brittle fracture.
 Journal of Adhesion Science and Technology 2337

The UHPFRC contains no coarse aggregate (enhanced compactness) and contains

high levels of steel fiber (2.5%); these fibers by their inking improve the adhesion
between the damaged concrete and the UHPC used to repair, and to prevent the primer
and subsequent crack propagation.
In the case of specimen’s concrete damaged and repaired by bonding FRP, the rein-
forcing efficiency varied depending on the type of composite used, with different
degrees, as is shown in Figures 11 and 12. The concrete cylinders repaired by carbon
fiber fabric (SR-CF) showed a gain in compressive strength of about 38% and a signifi-
cant gain in axial strain of about 537%. However, the specimen’s concrete damaged
and repaired by glass fiber (SR-GF) can recover their initial strength and increase the
ductility; the gain in axial strain is about 92%, The resistance drop is negligible if it is
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about 5%.
The interesting results provided by the carbon fabric are attributable to their
mechanical properties (high tensile strength and high Young’s modulus) relative to the
glass fiber fabric.
Finally, the composite materials will play several roles. The first role is to confine
the concrete, giving it a greater reserve of strength. The second role is to exert a radial
preload compression to the concrete, and finally, after crash, the concrete hard cores
remain imprisoned inside the composite fabric; the cores of the specimens remain
unbroken, which leads to progressive rupture of the element.
Figure 13 shows the stress–strain curves of concrete specimens repaired by increas-
ing section with UHPFRC and repaired using carbon fiber fabrics. It can be observed
that the mechanical behaviors of both the specimens are different. The rigidity of the
SR-HPC specimens is higher than that of other specimens SR-CF. The plastic zone of
this specimen is also lower than that of SR-CF specimens. It is very clear that improv-
ing of the maximum deformation is considerable in the case of SC-RF specimens.
The maximum value is 12,070 μm/m.

Figure 13. Comparative stress–strain curve of concrete repaired by CFRP and concrete repaired
by HPC.
 2338 B. Rabehi et al.

Although these two techniques are different from an implementation point of view,
the repair technique by composite materials is known for its effectiveness according to
several authors.[3,20–23] The repair technique by increasing the section with UHPFRC
proves a very effective method for the recovery and improvement of initial strength of
damaged concrete. As an indication in our study, the compressive strength of the samples
repaired by increasing the section with UHPFRC (SR-HPC) exceeds 15% of the samples
repaired by CFRP (SR-CF). So we can say that the repair of concrete columns by increas-
ing the section with UHPFRC is a promising technique in the field of repaired damaged
structures; and more efficient than the repair technique using composite materials.

3.2. Failure mode

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3.2.1. Specimen’s concrete repaired by bonding composites materials:

All specimens repaired by GFRP have suffered a vertical ruptured of fabric at the over-
lap; this is probably due to the bi-directional nature of this composite, which has the
effect of redistributing lateral strains over the full height of the specimen. This failure
mode was confirmed by other authors.[24] The concrete core was damaged; the cracks
are intense since the medium until top of specimen (Figure 14).

Figure 14. Failure mode of specimens repaired by GFRP (SR-GF).

Figure 15. Failure mode of specimens repaired by CFRP (SR-CF).

 Journal of Adhesion Science and Technology 2339

All specimens repaired by CFRP have suffered a rupture of the FRP jacket. The
repaired specimens ruptured in a sudden and explosive manner and were preceded
by some sounds due to the high energy stored by the CFRP material. Many hoop
sections of the ruptured CFRP are formed. These hoops were either concentrated in
the central zone of the specimen or distributed over the entire height, as can be seen
in Figure 15. None of the specimens suffered a rupture at the overlap location of the
jacket, which explains the large strength and deformation of specimens repaired by
CFRP. After the failure, the concrete hard cores remain imprisoned inside the com-
posite, and the cores of the specimens remain unbroken, which leads to progressive
rupture of the element. This failure mode was observed by other authors.[25]
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3.2.2. Specimen’s concrete repaired by increasing section of concrete

In the case of specimens repaired by OC, a rupture and a total collapse of the layer
repair concrete, in the form of large platelets, were recorded (Figure 16). This can be
explained by the poor adhesion between the concrete core and the repair concrete.
For all specimens repaired by UHPFRC, the cracking appeared on the entire height
of the specimen; these cracks intensify with the load gradually, from the top to the
median zone of specimens. The total rupture is preceded by difficult detachment of
repair concrete due to the presence of steel fibers which ensure a perfect bond between
repair concrete and the core concrete (Figure 17). The fibers delay the crack propaga-
tion and total collapse of the specimens.

Figure 16. Failure mode of specimens repaired by OC (SR-OC).

Figure 17. Failure mode of specimens repaired by UHPRFC (SR-HPC).

 2340 B. Rabehi et al.

In general, the failure of all repaired concrete specimens is as follows:

In the case of repair by bonding FRP, the failure begins in the composite. Its
trajectory is as a function of the characteristics and the type of FRP. The concrete
cores remains imprisoned inside the composite.

For the repaired concrete by increasing section, the failure occurs at the interface
of the two types of concrete with different degrees of degradation, which depend
on the type of repair concrete.

4. Analytical results
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4.1. Strength models for confined concrete

Various models for confinement of concrete with FRP have been developed. Most of
the existing strength models for FRP-confined concrete adopted the concept of Richart
et al.[26] in which the strength at failure for concrete confined (fcc) increased according
to the hydrostatic fluid pressure applied and takes the following form:
fcc ¼ fco þ Kfl (1)
where fcc and fco are the compressive strength of confined and unconfined concrete,
respectively. fl is the lateral confining pressure and k is the confinement effectiveness
coefficient.
For circular section of columns, the value of the confining pressure ðfl Þ can be esti-
mated by the following equation [27–29]:
2  tfrp
fl ¼ ffrp (2)
d
where ffrp is the tensile strength of FRP in the hoop direction, tfrp is the thickness of
FRP, and d is the diameter of the confined-concrete cylinder section.
In applying their model to steel-confined concrete, Richart et al.[26] assumed that k
is a constant which is equal to 4.1. However, several studies revealed that existing
models for the axial compressive strength of steel-confined concrete are un-conservative
and cannot be used for FRP-confined concrete.(see [18,30–32], among others)
However, a number of strength models have been proposed specifically for
FRP-confined concrete that employ Equation (1) with modified expressions for k, (e.g.
[15,17,29,31–42]). Most of these models used a constant value for k (between 2 and 3.5)
indicating that the experimental data available in the literature show a linear relationship
between the strength of confined concrete fcc and the lateral confining pressure fl.[34–42]
Other researchers expressed k in non-linear form in terms of fl/fco or fl.[31,34]

4.2. Proposed equation

In this work, the confined specimens were damaged and thereafter repaired. There are
no models proposed in the literature to estimate the ultimate strength fcc. Based on the
analysis of the experimental results, a simple equation has been proposed in order to
predict the ultimate strength of repaired damaged concrete columns.
Equation (1) becomes:
fcc fl
¼1þK (3)
fco fco
 Journal of Adhesion Science and Technology 2341

The strength of damaged concrete fcd is lower about 30% than fco, replacing in
Equation (3) the fco by fcd:
fcc fl
¼1þK (4)
fcd fcd

4.2.1. Case of the repaired concrete by FRP

Figure 18 shows the relation between the strength enhancement ratio (fcc/fcd) and the
actual confinement ratio (fl/fcd) of the present test data. A linear relationship clearly
exists. This figure indicates that the ultimate strength of FRP repaired concrete can
be related linearly to the actual confinement ratio. Based on regression of test data,
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the ultimate axial strength of FRP-repaired concrete can be approximated by the

following expression:

For the specimens repaired with carbon fiber (SR-CF):

fcc fl
¼2þ (5)
fcd fcd

For the specimens repaired with glass fiber (SR-GF):

fcc fl
¼ 4:2 þ (6)
fcd fcd
The comparison between the predicted compressive strength of the specimens and
the experimental values is shown in Table 5. It can be observed that the values predicted
by Equation (3) match well the test data; the relative errors don not reach 2.5%.

Figure 18. Evolution of confinement according to the lateral pressure for concrete repaired by
FRP jacketing.

Table 5. Comparison of experimental and analytical results.

fcd fcc (exp) d tfrp fl fcc (Equation fccðpredÞ  fccðexpÞ

Specimens MPa MPa mm mm MPa K 4) MPa ð%Þ
fccðexpÞ
SR-CF 16.5 32.5 110 1 8.2 2 33 +1.5
SR-GF 22.5 2 1.3 4.2 22 −2.2
 2342 B. Rabehi et al.

4.2.2. Case of the repaired concrete by increased concrete section

The value of the confining pressure ðfl Þ for repaired columns by increased concrete
section is obtained as follows:
2t
fl ¼ ft (7)
d
where fl is the lateral confining pressure, t is the thickness of concrete layer, and d is
the diameter of concrete cylinder section.
ft is the tensile strength of concrete in the hoop direction (Figure 19). In the case of
OC, it is given by the following equation [43]:
ft ¼ 0:6 þ 0:06fc28 (8)
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fc28 is the resistance of concrete at 28 days. In our study, this resistance is similar to
(fo).
Figure 20 shows the relation between the strength enhancement ratio (fcc/fcd) and
the actual confinement ratio (fl/fcd) of the present test data. A linear relationship clearly
exists. This figure indicates that the ultimate strength of damaged concrete repaired
with increasing section can be related linearly to the actual confinement ratio. Based on
the regression of test data, the ultimate axial strength of repaired concrete can be
approximated by the following expression:

Figure 19. Effective confined concrete repaired with concrete layer.

Figure 20. Evolution of confinement according to the lateral pressure for concrete repaired by
increasing section.
 Journal of Adhesion Science and Technology 2343

Table 6. Comparison of experimental and analytical results.

fcc fcc
f′cd (exp) d t ft fl (Equation 4) fccðpredÞ  fccðexpÞ
Specimens MPa MPa mm mm MPa MPa K MPa ð%Þ
fccðexpÞ
SR-OC 16.5 18.2 110 25 2 0.9 1.8 18 −1.1
SR-HPC 37.5 19.2 8.7 2.5 38 +1.3

For the specimens repaired with ultra UHPFRC (SR-HPC):

fcc fl
¼ 2:5 þ
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(9)
fcd fcd

For the specimens repaired with OC fiber (SR-OC):

fcc fl
¼ 1:8 þ (10)
fcd fcd
The comparison between the experimental and analytical results of the compressive
strength values is shown in Table 6.
The comparison between the test data and the predicted values shows that in
general, a good match between the experimental and predicted results is obtained. The
maximum deviation is about 1.3%.

5. Conclusion
This paper provides information on structural behavior of strengthened and repaired
columns by two techniques: increasing sections, using an OC and an UHPFRC, and
the bonding of composites materials (FRP).
On the basis of the experimental results discussed in the paper, the following
conclusions can be drawn:

The specimens repaired by increasing concrete section with the OC have a burst
fracture in compression of concrete cover due to a bad adhesion between the
substrate and new concrete.

The method of repairing by increasing section with UHPFRC is more effective
and more attractive in terms of mechanical behavior, a gain in compressive
strength, and axial strain of about 58 and 50%, respectively, comparatively with
specimens of OC (not damaged).

The concrete cylinders repaired by carbon fiber fabric (CFRP) showed a gain in
compressive strength about 38% and a significant gain in axial strain of about
537%. However, the specimen’s concrete damaged and repaired by glass fiber
(GFRP) can recover their initial strength and increase the ductility,

The compressive strength of the concrete specimens repaired by increasing the sec-
tion with UHPFRC exceeds 15% of the concrete specimens repaired by carbon fiber.

A good correlation has been obtained between the experimental results and the
proposed equations for the prediction of the ultimate strength of repaired concrete
columns. The maximum deviations were 2.2 and 1.3% for the specimens repaired by
FRP and repaired by increased concrete section, respectively.
 2344 B. Rabehi et al.

Finally, it can be said that the new repair technique of concrete columns by increas-
ing the section with UHPFRC is a promising technique to repair damaged structures
and competitive with the repair technique using composite materials. It is worth men-
tioning the possibility of increasing the durability of the structure by applying the
UHPFRC jacket, due to the reduced crack openings and to the compactness of the
UHPFRC matrix.

Acknowledgments
This work was supported by the General Directorate for Scientific Research and Technological
Development of Algeria. The authors gratefully acknowledge the generous assistance of Sika
Eljazair for the supply of the composite material used in this study.
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List of symbols
CFRP Carbon fiber reinforced polymer
GFRP Glass fiber reinforced polymer
fc Compressive strength of concrete
εc Axial strain of concrete
fu Ultimate strength of concrete
εu Ultimate axial strain of concrete
fco Compressive strength of strengthening concrete (unconfined concrete)
fcd Compressive strength of damaged concrete (unconfined concrete)
εco Axial strain in strength of strengthening concrete (unconfined concrete)
fcc Compressive strength of concrete confined by FRP or a layer of concrete
εcc Axial strain in strength of concrete confined by FRP or a layer of concrete
fcu Ultimate strength of concrete confined by FRP or a layer of concrete
εcu Ultimate axial strain of concrete confined by FRP or a layer of concrete

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