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Mechanical behaviour of slender

RC walls under seismic loading
strengthened with externally bonded
CFRP
a a a
S. Qazi , L. Michel & E. Ferrier
a
Department Of CIVIL Engineering , Université Lyon 1-INSA LYON ,
Villeurbanne , France
Published online: 30 Apr 2013.

To cite this article: S. Qazi , L. Michel & E. Ferrier (2013) Mechanical behaviour of slender RC
walls under seismic loading strengthened with externally bonded CFRP, European Journal of
Environmental and Civil Engineering, 17:6, 496-506, DOI: 10.1080/19648189.2013.791076

To link to this article: http://dx.doi.org/10.1080/19648189.2013.791076

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 European Journal of Environmental and Civil Engineering, 2013
Vol. 17, No. 6, 496–506, http://dx.doi.org/10.1080/19648189.2013.791076

Mechanical behaviour of slender RC walls under seismic loading

strengthened with externally bonded CFRP
S. Qazi, L. Michel and E. Ferrier*

Department Of CIVIL Engineering, Université Lyon 1-INSA LYON, Villeurbanne, France

Recent post-earthquake surveys have highlighted the excellent performance of rein-

forced concrete (RC) wall-type structures compared to frame-type structures. Any
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damage observed in RC walls was primarily due to design and construction work
flaws. To overcome these defects, strengthening of existing RC walls is mandatory.
In this article, experimental results for six RC shear walls are discussed. The walls
were designed to fail in flexure. Four out of the six specimens were strengthened
externally with Carbon Fiber Reinforced Polymer (CFRP) strips bonded to the wall
panel, and mesh anchors were introduced at the wall foundation joint to limit CFRP
debonding. Two specimens, one RC alone and one RC strengthened with CFRP,
were subjected to a static load test, and four specimens, one RC alone and three RC
strengthened with CFRP, were subjected to cyclic load tests. The test results discus-
sion includes load response, cracking pattern, strength, ultimate displacement and
energy dissipation. The CFRP strengthening technique adopted worked well with
respect to improving specimen strength, reducing deformity and dissipating energy.
Keywords: RC walls; FRP strengthening; seismic loading

Introduction
Reinforced concrete (RC) walls are used in structures for more than just vertical load
support; the walls assure lateral stability, maintain the lateral drift within reasonable
limits and dissipate seismic-induced energy. The superb seismic performance of RC wall
structures was observed in earthquake surveys conducted in recent years (Fintel 1995;
Wyllie, Abrahamson, Bolt, Castro, & Durkin, 1986). Though RC walls are used in build-
ings to dissipate seismic-induced energy, they also are vulnerable to seismic damage.
The main causes of damage are occurrences of unpredictably high seismic activity and
improper design or construction flaws (Inoue, Yang, & Shibata, 1997; Oh, Han, & Lee,
2002; Brun, Reynouard, & Jezequel, 2004). An RC wall’s load-response behaviour
depends to a great extent on its height-to-length (H/L) ratio. An RC wall that has an H/L
ratio greater than 2 is considered to be a slender or long wall, and a wall that has an H/L
ratio less than 2 is considered to be a short wall (Brun, Reynouard, & Jezequel, 2003).
Slender walls are sensitive to bending loads and encounter failure either by concrete toe
crushing, yielding of vertical reinforcement or a combination of both, and, in some cases,
shear slipping occurs between the wall and its foundation (Lopes, 2001; Greifenhagen &
Lestuzzi, 2005). The pathology of a slender wall is almost analogous to that of a column.
Therefore, reinforcement procedures should be relatively similar between the two, but
need to be studied more precisely (Paterson & Michell, 2003). This study investigates

*Corresponding author. Email: emmanuel.ferrier@univ-lyon1.fr

Ó 2013 Taylor & Francis

 European Journal of Environmental and Civil Engineering 497

the influence of external Carbon Fiber Reinforced Polymer (CFRP) reinforcement on the
load-displacement response of slender RC walls. In total, six RC walls were designed and
fabricated to be under-reinforced so that they failed in flexure. Four out of these six were
subsequently strengthened with CFRP. The test results discussion includes load deflection
curves, failure modes, strengths, ultimate displacements and energy dissipation.

Experimental programme
Specimen detail
The basic details of the slender RC wall (geometry and reinforcement) and the test
setup in the present study were derived from the research of Greifenhagen (2006). The
RC wall details are shown in Figure 1.
The test specimens modelled the RC wall at the ground floor of a two-storey
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building constructed according to the 1960–1970 Swiss construction style at a 1:3 scale.
The vertical reinforcement of the test specimens followed the Eurocode2 design specifi-
cation (min .004
cross-sectional area), and a clear cover of 2 cm was kept. To take into
account the construction joint phenomenon in the wall’s performance, test specimens
were fabricated non-monolithically. The test specimen head and foundation blocks were

Figure 1. RC slender wall detail.

 498 S. Qazi et al.

fabricated from one initial batch of concrete. They were cured for 28 days and afterwards
aligned on the floor with a wall panel mould. The wall panels were fabricated with a sec-
ond batch of concrete. The concrete used had 35 ± 2 MPa of compressive strength. After
curing the wall panels for 28 days, both sides of each wall were made smooth by sand-
blasting, and CFRP strengthening was applied to four specimens.

CFRP strengthening
The choice of a reinforcement pattern is always difficult, due to antagonistic parameters
(e.g. maximum load and energy dissipation). The CFRP strengthening technique
adopted in this research work was chosen with these goals and limitations: (a) Limit the
crack propagation observed in the RC wall. (b) Apply strengthening only on wall
surfaces, because the wall edges are normally connected to other structure elements. (c)
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Allow concrete cracking in a controlled manner to utilise friction for energy dissipation.
(RC elements dissipate energy due to steel yielding and friction between cracked
concrete surfaces). The RC walls with no external CFRP reinforcement are labelled as
SL1 and SL3, and the remaining four that were strengthened with CFRP are labelled as
SLR2, SLR4, SLR5 and SLR6. The evaluated properties of the material used are
tabulated in Table 1.
The CFRP reinforcement arrangement applied on both faces of the wall is shown in
Figure 2. The mesh anchor installed at the wall foundation joint had a diameter equiva-
lent to 12 mm. The mesh anchors were made by winding CFRP fibre tow around two
nails that were fixed apart at a distance of 80 cm. The winded fibre tows were then
released from the nails and folded in the middle. At the folded end, a CFRP rod or steel
wire was attached to ease mesh anchor insertion into the hole. The other end was cut
with scissors to splay the fibre tows over the bonded CFRP strip. The tensile strength
of a mesh anchor of 26 fibre tows (based on experimental results) was 22 kN. Each
anchor was embedded in a hole drilled into the foundation block up to a depth of
150 mm, and its remaining length was splayed over the vertically bonded FRP strips.
This was then over-bonded by a horizontally bonded CFRP strip. Table 2 summarises
the CFRP reinforcement arrangements made on the test specimens. Two types of CFRP
strips were used. Unidirectional strips had the fibre aligned along the strip’s longitudinal
axis while bidirectional strips had the fibre arranged along both longitudinal and
transversal directions. In terms of performance, the two are identical; however, the
bidirectional version is easier to handle. In specimens SLR5 and SLR6, mesh anchors
were placed in holes drilled within the wall panel to limit the debonding of the CFRP
strips. Holes were drilled at the CFRP strips’ intersection point. The anchor installed
within the wall panel in specimen SLR5 consisted of 4 fibre tows, whereas the anchor
in SLR6 consisted of 12 fibre tows (Figure 2).

Table 1. Material properties.

Type Compressive strength Tensile strength Modulus

Concrete 35 ± 2 MPa – –
Steel rebar – 570 MPa 200 GPa
CFRP strip – 1300 MPa 105 GPa
Epoxy – 24 ± 1.5 MPa 1960 MPa
 European Journal of Environmental and Civil Engineering 499
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Figure 2. CFRP reinforcement description.

Table 2. External CFRP reinforcement detail.

CFRP Strip Anchor fibre tows number

Width (mm) Wall foundation
Specimen Type Centre Ends Centre Ends Wall Panel
SL1 – – – – – –
SLR2 Bidirectional 50 50 26 26 –
SL3 – – – – – –
SLR4 Bidirectional 50 44 44 –
SLR5 Bidirectional 50 44 44 4
SLR6 Unidirectional 50 75 44 60 4 and 12

Test setup
The RC-wall test setup is depicted in Figure 3(a). Test specimens were subjected to
displacement-controlled lateral loading, with the walls acting as cantilevers. In all
specimens, a constant axial compression load equal to 90 kN (0.075⁄f 0c ⁄Ag) was
maintained with the help of a pressure gauge. With regard to lateral loading, the first
two specimens of each type (SL1 and SLR2) were subjected to quasi-static loading to
measure specimen performance. In this case, the lateral displacement was provided at a
speed of 0.01 mm/s. Four LVDT’s were placed along the wall’s height at its free end to
check its deflection pattern (Figure 3(b)). One LVDT was positioned at the centre of the
head beam; the second was placed at the top of the wall panel; the third was placed in
the vicinity of the bottom of the wall panel; and the fourth was placed at the centre of
 500 S. Qazi et al.
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Figure 3. (a)Test setup (b) Measurement arrangement.

the foundation block to measure the slippage of the wall foundation. The recorded value
of the foundation block slippage was deducted from the displacement value measured
at the wall’s head to obtain the actual value of the horizontally induced displacement.

Experimental results
RC-wall response under monotonic loading
The behaviour of specimens SL1 and SLR2 under a static load is depicted in load-
deflection curves in Figure 4. The curves represent the evolution of the load sustained
by the sample vs. the induced displacement. The CFRP strengthening technique applied
on specimen SLR2 increased its strength and ductility. In specimen SL1, the test was
stopped for safety reasons when the crack at the wall foundation joint exceeded 80% of
the wall length. Conversely, the test was stopped on specimen SLR2 when the sustained
load value dropped by up to 80% of the observed ultimate load values observed in this
case. The pragmatic ultimate strength of specimens SL1 and SLR2 were 25 and 41 kN,
respectively, at a respective wall top deflection of 14.3 and 11.75 mm. The two curves
are almost identical up to an induced displacement of 2.02 mm, at which point cracks

Figure 4. Load-displacement curve of specimens SL1 & SLR2.

 European Journal of Environmental and Civil Engineering 501

Figure 5. Failure pattern of specimens SL1 & SLR2 subjected to monotonic loading.
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were observed. Past this displacement, specimen SLR2 behaved in a manner different
from that of SL1. This result is observed because CFRP reinforcement initiates its
contribution to load distribution after the development of initial cracks. The CFRP
external reinforcement enhanced the specimen strength by almost 52%.
Specimen SL1 exhibited a failure mode, characteristic of an under-reinforced slender
wall with insufficient reinforcement at the wall’s foundation joint area (Figure 5).
During the load test, horizontal cracks formed within the wall panel in the lower half
section of its load end, and a much wider crack formed at the joint area. As the induced
displacement load reached the 14.2 mm level (an equivalent load of 25 kN), the latter
crack spread up to almost 53 cm (more than 3/4 of the wall length), and a number of
vertical cracks formed within the bottom of the wall panel at the load end.
The CFRP retrofitting arrangement in specimen SLR2 changed the specimen crack
pattern from that of the unmodified wall. The CFRP bands bonded at the wall’s bottom
hindered the visibility of crack formation in this area. In this case, the first crack
appeared at a wall height of 25 cm when the induced displacement reached a level of
1.74 mm (an equivalent load of 20 kN). The crack initiated in the horizontal direction
and afterwards deflected diagonally downwards towards the wall’s bottom. In this case,
the CFRP bonded strip bridged the cracks and limited crack widening and propagation.
However, debonding of the CFRP strips was observed after the test near the lower half
of the free end of the wall because the CFRP strips cannot withstand the compression
load very well.

RC-wall response under cyclic load

The hysteresis curves of specimens SL3, SLR4, SLR5 and SLR6 are shown in Figure 6.
Specimens were subjected to reverse-static cyclic load tests to simulate seismic actions.
Following the recommendations of the ACI (ACI T1.101, 2001), specimens were
subjected to three full cycles at each level to take into account the effect of smoothen-
ing of crack surfaces. Due to the variations made in the external CFRP reinforcement
configurations among specimens, the lateral displacement load levels were based on
drift instead of ductility to allow easy comparison. The drifts were [.1, .2, .3 … .8, 1
… 1.8%]. As in the static case, a constant axial compression load of 90 kN was
sustained at the head beam.
The hysteresis curves of specimens SLR4, SLR5 and SLR6 depict improvements
in ultimate load capacity and ultimate displacement over those shown by SL3. The
 502 S. Qazi et al.
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Figure 6. Hysteresis curves.

Figure 7. Hysteresis envelope curves.

hysteresis curves depict an elastic plastic failure in specimens SL3 and SLR4 and an
abrupt failure in specimens SLR5 and SLR6 due to the failure of the anchors installed
within the wall panels. The larger area of the hysteresis curves for strengthened speci-
mens translates to higher energy dissipation, although the ratio of energy dissipated to
total energy was observed to be in the range of .4. Therefore, the CFRP strengthening
technique did not deteriorate the wall’s energy dissipation capacity, thanks to the adop-
tion of a partial strengthening technique. The CFRP material has a characteristic elastic
behaviour; therefore, it tends to increase specimen strength and reduce its dissipation
capacity. In our case, the RC elements dissipate the induced energy through the relative
friction in concrete cracks and steel rebar yielding. Figure 7 shows the hysteresis enve-
lope curves of all four specimens. The hysteresis envelope curves show that the CFRP
strengthening arrangement modified the RC wall behaviour to a great extent. Table 3
lists the ultimate load values recorded for specimens subjected to the cyclic load test.

Strain distribution within bonded CFRP strip

The arrangement to measure the strain in specimen SLR2’s CFRP reinforcement is
depicted in Figure 8. The letter symbols A, B and C are used to label the vertically
bonded CFRP strips, and SG labels represent strain gauges.
 European Journal of Environmental and Civil Engineering 503

Table 3. Observed ultimate strength of specimen under cyclic loading.

Specimen SL3 SLR4 SLR5 SLR

Load (kN) 27.75 43.44 45.69 56.63
Increase (%) – 57 65 104
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Figure 8. Specimen SLR2 strain gauge detail.

Figure 9. Longitudinal strain distribution in CFRP strip A.

Figure 9 shows the longitudinal strain distribution (LSD) curves of CFRP strip A.
Each curve represents a load level and is prepared by plotting the recorded data from
strain gauges SG1, SG2 and SG3. The slightly negative strain values observed in the
LSD curve at the 6 kN load level occurred under the effect of a vertical compression load
induced over the head beam. Up to a load level of 24 kN, the maximum strain was
recorded by SG1, as the flexural load effect is highest at the wall’s base. However, as
the induced load exceeds the 24 kN threshold level, the strain in SG1 increases but the
maximum strain is observed in SG3. This shift in maximum strain location occurred
 504 S. Qazi et al.
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Figure 10. Longitudinal strain distribution in CFRP strip C.

Figure 11. Strain distribution in CFRP strips along the wall’s length.

because a crack developed in the wall panel at a height of 31 cm, when the induced load
exceeded the 27 kN level. The CFRP strip across the crack tended to bridge it and, in
turn, experienced the strain itself. Additionally, the mesh anchor arrangements made in
this case were kept almost identical to those of SR2. Therefore, the CFRP reinforcement
had only one layer instead of three layers above a wall height of 25 cm. This resulted in
a higher strain development in SG3, as the CFRP reinforcement was thin. On the other
hand, the gradual increase in recorded strain of SG1 and SG2 signifies the profitable
contribution of the mesh anchor arrangement made at the wall’s foundation joint.
 European Journal of Environmental and Civil Engineering 505

To analyse the compression load effect on the CFRP reinforcement, the LSD curves
of CFRP strip C are shown in Figure 10. Before the 24 kN load level, the highest strain
developed in the vicinity of the wall’s bottom. After that point, the LSD curve pattern
changes as the maximum strain was recorded by SG8. This variation is attributed to the
debonding and buckling of the CFRP strip, as shown in Figure 8. Figure 11 shows the
strain pattern along the wall’s length from the load end to the free end. The curve is
comprised of strain gauges SG1, SG4 and SG7, which were bonded in an orientation
along the wall’s length. Therefore, the curves depict the variation in flexural strain
along the wall’s length. The positive strain values recorded by SG4 at the higher load
level reflect the propagation of tensile cracks across three-fourths of the wall length at
the point of failure.

Conclusions
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This research work highlights the positive influence of external CFRP reinforcement on
slender RC walls. The CFRP strips bonded to the RC-wall panel did improve their
ultimate load capacity and ductility, and the strips limited the crack propagation to a
certain extent. The mesh anchor placement at the wall’s foundation joint remedied the
joint failure due to improper arrangement of reinforcement in this region. The mesh also
limited the problem of CFRP strip debonding, which is a major issue in external FRP
reinforcement, by transferring load effects from the bonded strips to the lower
foundation block. The partial FRP strengthening technique adopted here proved to be
successful, as it did not deteriorate the capacity of the RC wall to dissipate energy. This
arrangement ensured concrete cracking within the wall panel to a certain extent, which,
in turn, resulted in energy dissipation because the RC structures can dissipate energy
through relative friction in the concrete crack and rebar yielding.

Acknowledgements
The authors would like to thank the French National Research Agency (ANR) for their financial
support for this research programme through INPERMISE.

References
American Concrete Institute. (2001). ACI T1.101: Acceptance criteria for moment frames based
on structural testing.
Brun, M., Reynouard, J. M., & Jezequel, L. (2003). A simple shear wall model taking into
account stiffness degradation. Engineering Structures, 25(1), 1–9.
Brun, M., Reynouard, J. M., & Jezequel, L. (2004). Damaging potential of low-magnitude near-
field earthquakes on low-rise shear walls. Soil Dynamics and Earthquake Engineering, 24,
587–603.
Eurocode2. (2004). Design of concrete structures – Part 1-1: General rules and rules for buildings,
BS EN 1992-1-1.
Fintel, M. (1995). Performance of Buildings with Shear Walls in Earthquakes of the Last Thirty
Years. PCI Journal, 40, 62–80.
Greifenhagen, C., & Lestuzzi, P. (2005). Static-cyclic tests on low reinforced concrete shear walls.
Engineering Structures, 27, 1703–1712.
Greifenhagen, C. (2006). Sesimicbehavior of lightly reinforced concrete squat shear walls (doc-
toral dissertation). ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE, Switzerland.
Inoue, N., Yang, K., & Shibata, A. (1997). Dynamic non-linear analysis of reinforced concrete
shear wall by finite element method with explicit analytical procedure. Earthquake Engineer-
ing & Structural Dynamics, 26, 967–986.
Lopes, M. S. (2001). Experimental shear-dominated response of RC walls Part 1: Objectives,
methodology and results. Engineering Structures, 23, 229–239.
 506 S. Qazi et al.

Oh, Y. H., Han, S. W., & Lee, L. H. (2002). Effect of boundary element details on the seismic
deformation capacity of structural walls. Earthquake Engineering & Structural Dynamics, 31,
1583–1602.
Paterson, J., & Michell, D. (2003). Seismic retrofit of shear walls with headed bars and carbon
fiber warp. Journal of Structural Engineering, 129, 606–614.
Wyllie, L. A., Abrahamson, N., Bolt, B., Castro, G., & Durkin, M. E. (1986). The Chile earth-
quake of March 3, 1985 – performance of structures. Earthquake Spectra, 2, 293–371.
Downloaded by [McMaster University] at 06:26 28 December 2014