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Journal ArticleDOI

Assessment of adhesive setting time in reinforced concrete beams strengthened with carbon fibre reinforced polymer laminates

01 May 2012-Materials & Design (Elsevier)-Vol. 37, pp 64-72

Abstract: The strengthened effectiveness and the performance capacity of repaired Reinforced Concrete (RC) structures with Carbon Fibre Reinforced Polymer (CFRP) sheets is dependent on the properties of the adhesive interface layer. Adhesive material requires a specific setting time to achieve the maximum design capacity. Adhesive producer provides technical data which demonstrates the increase with time of the capacity, up to the maximum. The aim of this study is to investigate the effect of the adhesive setting time on the modal parameters as an indication of the effectiveness of CFRP on repaired RC beams. Firstly, datum modal parameters were determined on the undamaged beam and subsequently the parameters were obtained when damaged was induced on the RC beam by application of load until the appearance of the first crack. Finally, the RC beam is repaired with externally bonded CFRP sheets, and modal parameters are once again applied after 0.5, 1, 2, 3, 5, 8, 11, 15 and 18 days. The comparison is made with the data based on half day results in order to monitor the change in the modal parameters corresponding to the adhesive setting time. The modal parameters where used as indicators for the effectiveness of CFRP are affected by the adhesive time as shown in this study. Results are compared with the adhesive technical data provided by the adhesive producer.
Topics: Adhesive (53%)

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Assessment of adhesive setting time in reinforced concrete beams
strengthened with carbon fibre reinforced polymer laminates
Moatasem M. Fayyadh , H. Abdul Razak
The dynamic parameters of structural elements have been an
area of interest over the last few years, especially with the
increased focus on health monitoring and damage detection, which
are dependent on the relation between the dynamic and physical
properties of structural elements. A great deal of research has
focused on the use of dynamic parameters to assess and monitor
the structural element performance and stiffness. Some have introduced
new damage detection algorithms, whilst others have discovered
existing phenomena related to dynamic parameters,
structural performance and stiffness [1,2]. A hybrid health monitoring
system was proposed to detect damage in PSC girder bridges
[3], where the occurrence of damage was alarmed by measuring
the change in frequency responses from accelerometers mounted
on the PSC girder. Structural health monitoring using statistical
time series methods was applied to damage diagnosis in a lightweight
truss structure [4], and both non-parametric and parametric
methods were shown to effectively tackle damage detection
and identification, with parametric methods achieving excellent
performance with zero false alarm, missed damage, and damage
misclassification rates. A new damage severity algorithm was proposed by [5], which based on the combination of both
natural frequencies and mode shape and it was proven to be better sensitive
than exist damage severity algorithms. Lower modes were
found to be more sensitive to the change in the support conditions
[6]. A new damage detection index based on the combination between
the mode shape vectors and their curvature was developed
and verified to have higher sensitivity than existing algorithms [7].
Carbon Fibre Reinforced Polymer (CFRP) is fast becoming an
extensively used material for externally bonded strengthening and
repair of Reinforced Concrete (RC) structures. Recent years have
seen many researchers using Fibre Reinforced Polymer (FRP) sheets
to strengthen and repair RC structures, accompanied by the examination
of the parameters affecting the performance of the RC structure
when strengthened and repaired with FRP material. Studies
have dealt with the configuration of the FRP sheets on the RC structural

elements and the failure modes of the FRP sheets [810]. In[11]
it was found that delamination initiate at the interface from the inner
free edges of the holes and with time, delamination from two
holes meet each other forming a big delamination area and it is evident
that kevlar/epoxygraphite/epoxy hybrid laminate is more
flexible compared to only graphite/epoxy laminates. The experimental
results carried in [12] demonstrated that composite wrapping
can enhance the structural performance of concrete columns
under axial loading and the number of layers of FRP materials and
the corner radius are the major parameters, having a significant
influence on the behaviour of specimens. In [13], a new model was proposed to predict the intermediate crack
debonding load and it was proven that the proposed model furnishes good predictions,
where the ratio between the IC debonding load predicted by the proposed
model and that experimental furnishes a mean value of
0.9619 and a standard deviation of 0.1062. The interfacial shear
stresses found to be influenced by the geometry parameters such
as thickness of the FRP plate and adhesive layer in range of the different
degrees [14], where the interfacial shear stress concentrations
and levels increase obviously with the increase of the thickness of
the FRP plate. The cross-sectional shape found to has a significant
influence on the effectiveness of the CFRP-confinement under concentric
loading [15], where member with the circular cross-section
benefited the most, followed by the member with the square
cross-section and the gain in load capacity of RC members with rectangular
cross-sections due to CFRP-confinement depends on the
aspect ratio of the cross-section.
The first dynamic assessment for the use of FRP plate as an
externally bonded strengthening system using modal testing is
conducted by [16]. Repairing a damaged RC beam with FRP plates
leads to a decrease in the natural frequencies for the first three
bending modes [16]. Dynamic parameters are used to assess the
use of FRP plates to repair damaged RC beams; the bending frequencies
of the repaired phase after exposure to load shows a small
increase when compared to the pre-repair damage phase [17]. The
fundamental mode shows lower sensitivity while the third and
fourth modes show higher sensitivity. The modal dampings are
sensitive to both the damaged and the repaired stage [17]. There
is variation in the sensitivity of the bending modes when repairing
the damaged beams with bonded CFRP sheets, depending on the
flexural steel ratio and the pre-repair damage level [18]. The use
of the MAC index shows a relatively smaller change in the damage

and repair compared to the natural frequencies. This is an inconsistency
which renders the MAC an unreliable index [18]. The fundamental
mode has higher sensitivity to damage and repair [19].
Modal testing conducted on an existing bridge repaired with externally
bonded CFRP sheets shows that both vertical and horizontal
excitation modes are affected, with all the modes experiencing
an increase in the frequencies at the repair phase when compared
to the damage phase [20].
For RC elements repaired with CFRP sheets, the performance and
the stiffness is affected by the properties of the adhesive interface
layer between the CFRP sheets and the RC structure. Some studies
have investigated the effect of the adhesive layer properties on
the structural and dynamic properties of the CFRP repaired RC
structures. The strength of the bonded adhesive reduces due to both
absorption of moisture and an increase in test temperature [21].
Thus, considerable drying may be required to obtain a good bond
[21]. In [22], a method is proposed to estimate the fatigue strength
of the adhesive bonded joint between CFRP sheets and the aluminium
surface. A 3D model to predict the behaviour of the bond between
the CFRP plate and concrete under elevated temperatures
is developed in [23]. The study finds that the adhesive properties
of epoxy are very sensitive to temperature variation. Investigations
have also been conducted with regards to the behaviour of the
adhesive bondage between CFRP sheets and concrete under an impact
loading [24]. Temperature cycles and moisture are associated
with failure in the concrete substrate, while slat fog cycles induce
failure at the concrete-adhesive interface [25]. Immersion in salt
water and salt fog considerably reduces the bond strength [25].
Bond strength increases with an increase in concrete strength and
decreases with an increase in the CFRP plate width [26]. The failure
mode is found to be significantly affected by elevation in temperature
and moisture [27]. In [28], significant effects are observed of
the FRP properties and epoxy curing condition on the interfacial
strength. In [29], the strain distribution is examined along the
bonding line and through the thickness of the adhesive layer and failure mechanisms. In [30], a decrease is observed
in the shear modulus due to creep dependence. It is also shown that beyond a
shear stress corresponding to 40% of the ultimate bonded strength,
creep is linear.
All of the aforementioned works clearly indicate that modal
parameters are affected by the properties and behaviour of RC
beams repaired with CFRP sheets, and mainly by the behaviour

of the adhesive interface between CFRP sheets and RC structures.
However, no research has been conducted on the use of modal
parameters to investigate the behaviour of the adhesive interface
and to study its properties. Thus, the main objective of this study
is to investigate the setting time behaviour of the adhesive interface
by using the modal parameters and to assess the results based
on the technical data sheets provided by the adhesive material producer.
In addition, this study will show the effect of the setting
time on the adhesive interface using modal parameters to assess
RC structures repaired with CFRP sheets.
Experimental work
In order to investigate the effect of the adhesive interface setting
time on the modal parameters in RC structures repaired with
CFRP sheets, one RC beam is cast. The RC beam is simply supported
and subjected to flexural loading in order to simulate loading on
actual RC structures such as bridge girders, beams and slabs. The
beam has a clear span length of 2.2 m, a width of 150 mm and a
depth of 250 mm in cross section. The dimensions are chosen to
facilitate laboratory testing. The beam details and test setup are
shown in Figs. 1 and 2, and Table 1 details the material properties.
After the beam is cast, it is left for one year to avoid any effect of
the environment or lifetime on the properties of the materials. It
is then tested under point load located at mid-span. Before application
of the load, dynamic properties are obtained using the modal
test and the results are used as data. Load is applied gradually at a
loading rate of 4 kN/min (i.e. loading and unloading) up to 12 kN,
at which point the first crack appears, before decreasing to zero.
Following this all that remains is to recover the deflection and
dynamic properties for the damage case. Roughness equipment is
used on the tension surface to obtain a suitable face and to have
as much friction as possible with the CFRP sheet. Figs. 3 and 4 show
the RC beams before fixing the CFRP sheets and the surface after
roughness. The surface is cleaned using air pressure to avoid any
residue or dust on the surface. This is because the substrate must
be sound, dry, clean and free from laitance, ice, standing water,
grease, oils, old surface treatments or coatings and all loosely adhering particles. The CFRP sheet is fixed using
adhesive material. The CFRP sheet is designed according to ACI 440.2R (2002) [31]
code requirements, as illustrated in the following section, and is
50 mm wide, and 1.2 mm thick while the length is along the clear
span of the beam. The CFRP properties are as shown in Table 2. The
dynamic data is collected half a day after fixing the CFRP sheets

and is gathered for 18 days.
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