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Research ArticleEffects of Water Immersion on the Adhesion
between AdhesiveLayer and Concrete Block
Jiajun Shi, Yunfeng Pan , Hedong Li , and Jun Fu
School of Civil Engineering and Architecture, Zhejiang Sci-Tech
University, Hangzhou, China
Correspondence should be addressed to Yunfeng Pan;
[email protected]
Received 17 July 2019; Revised 11 September 2019; Accepted 30
September 2019; Published 30 October 2019
Academic Editor: Chiara Bedon
Copyright © 2019 Jiajun Shi et al. +is is an open access article
distributed under the Creative Commons Attribution License,which
permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly cited.
+e effectiveness of load transfer in the CFRP-adhesive-concrete
system highly relies on the integrity of the interfacial
bondbetween adhesive layer and concrete. In the present paper, the
effects of water immersion on the mode I fracture energy of
theadhesion between CFRP adhesive and concrete were investigated
experimentally and numerically. Four-point bending test
wasconducted to measure the mode I fracture energy of the
interfacial layer between adhesive and concrete. +e moisture
contentdistribution and the hygrothermal stress were determined by
using the finite element method (FEM). +e mode I fracture energywas
found decreasing with increasing immersion time.+e difference
between themode I fracture energy at 2 weeks and 4 weeks israre. +e
failure mode of the four-point bending test specimen shifts from
concrete failure to interfacial debonding. +e moisturecontent at
the adhesive/concrete interface reaches equilibrium after 2 weeks
of water immersion.+e hygrothermal stress betweenadhesive and
concrete is smaller than the tensile strength of concrete.
Deterioration of the physical bond leads to the degradationof
bonding strength. +e reduction of the mode I fracture energy is
more severe than that of the mode II fracture energy.
1. Introduction
Strengthening structural members with a carbon fibre-reinforced
polymer (CFRP) sheet or plate is becoming moreand more popular
[1–3]. Debonding failure caused byformation and propagation of the
flexural-shear crack at theinterface between adhesive and concrete
is a common failuremode for the CFRP-adhesive-concrete system. +e
localstress at the crack tip is composed of a peeling stress
(ModeI) and a shear stress (Mode II), as shown in Figure 1.
+eeffectiveness of strengthening with CFRP highly relies on
theintegrity of the interfacial layer between CFRP adhesive
andconcrete [2], which may degrade under humid conditionsand
hygrothermal conditions [4–9]. Lots of studies haveinvestigated the
durability of the interfacial layer, in terms ofthe shear bond
stress (Mode II), experimentally [5] andnumerically [10]. However,
the durability of the interfaciallayer in terms of the mode I
fracture energy was rarelyinvestigated. +e degradation mechanism of
the adhesive-concrete interfacial layer needs to be further
studied.
Durability of the CFRP-adhesive-concrete system isaffected by
the performance of concrete, adhesive, CFRP,
and the interfacial layers between them [4, 11, 12].Compared
with concrete, the performance of the adhesiveand the interfacial
layer between adhesive and concrete ismore easily affected by water
[4, 5, 13]. It was reported thatthe compressive strength of
concrete varies slightly afterimmersion in water for 2 years [4].
However, after im-mersion in water for 1 year, 17% reduction was
found forthe tensile strength of adhesive [14]; for the
interfaciallayer between concrete and adhesive, its mode I
fractureenergy was reported to be reduced by 60% after exposureto
water for only 2 weeks [5]. Previous publications alsoreported that
the degradation of the interfacial layercaused by different
conditioning conditions is different[15, 16].
+e performance of the interfacial bonding between theadhesive
layer and concrete depends on the strength ofphysical and
interlocking bonding [15]. +e water moleculesdeteriorate themode I
fracture energy of the interfacial layer,as a result of the
degradation of the physical bonding, i.e.,disruption of the
hydrogen bond and the van der Waalsforce [13, 16–18]. +e water
molecules at the interfacial zoneare mainly from the concrete
substrate [19].
HindawiAdvances in Civil EngineeringVolume 2019, Article ID
7069757, 11 pageshttps://doi.org/10.1155/2019/7069757
mailto:[email protected]://orcid.org/0000-0002-5813-9801https://orcid.org/0000-0002-0911-1976https://orcid.org/0000-0002-3205-9600https://creativecommons.org/licenses/by/4.0/https://doi.org/10.1155/2019/7069757
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Water immersion changes the failure mode of the CFRP-to-concrete
system from thin concrete failure to debondingat the interface
between adhesive layer and concrete [5].Under dry condition, the
failure usually occurs in concretebeneath the adhesive. But, under
wet condition, the adhe-sive-concrete interfacial debonding always
occurs owing tothe moisture presence at the interface. is is
because theadhesive layer-concrete interfacial bond generally
de-teriorates more seriously than concrete with
moisturepresence.
Previous studies showed that the deterioration of
theCFRP-adhesive-concrete system depends on the
adhesivelayer-concrete bond [5, 20]. To evaluate the peel
perfor-mance, a mixed-mode test was proposed [21]. A movablebottom
portion was adopted to control the peel eects on thebond
performance. A modied double cantilever beam(MDCB) is a single
shear lap-like test setup [5].e load wasapplied perpendicular to
the FRP. e failure of the bondbetween FRP and concrete dominates by
the combination ofpeel and shear. e direct tensile test was
proposed toevaluate the peel FRP-concrete bonded joint
performanceunder direct tension [22, 23]. e interfacial bond of
thestrength-based approach is evaluated by the tensile
strength.Interfacial fracture energy is a better indicator of the
extentof bond degradation than that of the strength-based ap-proach
[24]. e eects of material stiness and sampledimensions are
excluded. A four-point bending specimenwith sandwiched epoxy layer
was chosen to study the ModelI of the adhesive layer-concrete bond
[25]. e de-termination of the mode I fracture energy in the
sandwichedspecimens has been widely carried out [26]. In the
presentstudy, the four-point bending specimens were adopted
toinvestigate the involvement of the properties of the
adhesivelayer-concrete bonded joints.
Previous studies reported that the integrity of the
CFRPstrengthening concrete structure depends on the bondbetween the
adhesive layer and concrete block in water[5, 15, 25]. us, adhesive
layer-concrete bonded jointswere chosen to evaluate the property
evolution of the CFRPstrengthening concrete structure. e major
objective ofthe present study was to investigate the eects of
water
immersion on the peel behavior of the adhesive layer-concrete
interfacial layer. e heat transfer module andstatic analysis module
of ABAQUS were, respectively,adopted to determine the moisture
distribution andhygrothermal stress at the adhesive layer-concrete
in-terfacial zone. e present paper was expected to shed lighton the
eects of water molecules on the mechanisms of theadhesive
layer-concrete bonded joint at the interface regioninvolved by the
physical and interlocking bonds.
2. Experimental
2.1. Raw Materials. e primer and adhesive used in thepresent
study were provided by Dagong Composite Corp.e properties of the
adhesive were determined with so-called dog-bone-shaped samples
according to ASTM D638[27]. e elastic modulus, tensile strength,
and ultimatestrain of the adhesive were measured to be 3.2
GPa,57.1MPa, and 1.9%, respectively. e glass transition
tem-perature (Tg) of the adhesive was measured with DMA(three-point
bending mode), and Tg was set as the peak oftan delta [28]. Tg of
the adhesive was measured to be 80.0°C.e primer was made of the
same epoxy with the adhesive.
e weight proportion of the concrete employed was1.00 :1.29 :
2.75 : 0.52 (cement: sand : gravel : water). emaximum size of the
gravel used is approximately 5mm.Prepared concrete blocks (40mm×
40mm× 40mm cubesfor compressive strength measurement and40mm× 40mm×
160mm prisms for the bending test) werecured at 95% relative
humidity (RH) for one month. ecompressive strength of concrete was
measured to be31.8MPa.
2.2. Four-Point Bending Test. Figure 2 shows the schematicsketch
of the sandwiched four-point bending test specimen.In order to
prepare the specimen, 40mm× 40mm× 160mmconcrete prisms were cut
into halves along their depth. ecut surface was cleaned with
acetone. Low viscosity epoxyprimer was then brushed onto the
cleaned cut surface, withthe pores in the concrete surface lled. e
adhesive wasbrushed onto a rectangular area of 40mm× 25mm.e
steelstrips with 1-mm-thickness were placed between concreteblocks
to accurately control the thickness of the adhesivelayer.
Subsequently, the two concrete blocks of40mm× 40mm× 80mm size were
attached to each otheralong the longitudinal direction of the
concrete blocks. esandwiched four-point bending specimens were
stored forone month in laboratory.
e four-point bending test setup is depicted in Figure 2,in which
l is 160mm. Both b and d are 40mm. e test wasconducted under
displacement control as a rate of 0.1mm/min [29].
2.3. Exposure Conditions and Absorption of the Adhesive.e
adhesive samples were immersed in water at 20°C.e adhesive
specimens of water immersion are25 mm × 25mm × 3mm. e water uptake
of the adhesivewas weighted periodically. e sample was taken from
the
Concrete Concrete
Concrete
Crack
FRP
Shear
Peel
Figure 1: Peeling and shear stress in actual
debondingconguration.
2 Advances in Civil Engineering
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immersion at interval, following by drying the samplesurfaces by
using a tissue paper. e mass of the sampleswas weighted by an
electronic balance with an accuracy of±0.01 mg. e exposure
conditions of the sandwichedfour-point bending specimens were
similar to those of theadhesive. e samples were taken out and
tested at weeklyintervals of 0, 2, and 4.
2.4. Finite Element Model. e moisture distribution in
theadhesive layer-concrete bonded zone is unavailable tomeasure.
erefore, the nite element method (FEM) wasapplied to model the
moisture diusion in the sandwichedfour-point bending specimens [30,
31].
Figure 3 shows the 3-dimensional geometrical model of
thesandwiched four-point bending specimen.e element type ofDC3D8
was used for the transient moisture diusion. eelement size for the
concrete block and adhesive layer were1mm and 0.1mm, respectively.
To simulate the specimens inwater, 100%moisture concentration was
specied on the outersurface of the sandwiched four-point bending
specimen.
e commercial software ABAQUS is widely used for thetransient
moisture diusion. However, the results of themoisture diusion by
the mass diusion module in ABA-QUS are di£cult to set as the
initial led for the next step ofstatic analysis. Data transfer from
the moisture diusion tostatic analysis was achieved by the heat
transfer module.Fick’s law for mass diusion is analogous to
Fourier’s law forheat transfer. e analogy is established as
follows:
e normalized concentration analogy can be expressedas [31]
temperature (T) � normalized concentration (Φ),k � DS,
ρCp � S,(1)
where T is the temperature, D is the moisture diusivity, k isthe
thermal conductivity, Cp is the specic heat, and ϕ isreferred to as
the “activity” of the diusing material anddened as
ϕ �C
S. (2)
e result of the moisture distribution is set as theboundary
conditions of specimens for static analysis. ethermal-hygro analogy
is developed as
α � β · S, (3)
where α is the thermal expansion coe£cient and β is
thehygroscopic expansion coe£cient. Table 1 shows the ma-terial
properties used in the simulations.
3. Results and Discussion
3.1.Water Absorption by the Adhesive. e water uptake canbe
expressed as a function of the square root of the time.eincremental
mass of the adhesive proportionally increaseswith the square root
of immersion in Figure 4, following byreaching the equilibrium
moisture uptake. Fick’s law isextensively adopted to model process
of the moisture dif-fusion [28]. e experimental results are tted by
thesimplied Fick’s law equation. e simplied form is givenas
follows:
Mt �M∞ 1 − exp − 7.3Dt
h2a( )
0.75
, (4)
where Mt is the moisture uptake at time t, M∞ is theequilibrium
moisture uptake, which is equal to S. D is thediusivity coe£cient,
and ha is the thickness of the weightedsample (3mm for present
adhesive specimens). D andM∞are obtained by the tting with equation
(4). D andM∞ aredetermined to be 63×10− 9mm2/s and 3.08% (wt.%),
re-spectively. Compared to D� 78×10− 9mm2/s andM∞ � 2.52% (wt.%)
from the reference [17], D decreases by
h
a
d
s
L
d
b
P
Steel plate
Adhesive layer1mm
Unbonded zone
20 30 30 20
Figure 2: Schematic sketch of the sandwiched four-point bending
specimen (all units in mm).
Concrete
Adhesive
Figure 3: 3-dimensional geometrical model of the
sandwichedfour-point bending specimen.
Advances in Civil Engineering 3
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19%, while M∞ increases by 17%. It was reported that theextent
of the equilibrium moisture content is in§uenced bythe chemical
structures of the epoxy system, while does notvary by the
environmental temperatures and the immersionduration [33]. e eects
of the hydrolysis on the adhesiveare signicant in water from 300
days to 450 days. ehydrolysis causes a microcrack in the adhesive,
and waterquickly penetrates into the pores of the adhesive. It
results inthe larger diusivity coe£cient and equilibrium
moisturecontent.
3.2. Eects of the Water Uptake on the Properties of theAdhesive.
Figure 5 shows the relationship between theimmersion duration and
the properties of the adhesive. ewater immersion insignicantly
in§uences the properties ofthe adhesive. e elastic modulus and
tensile strength of theadhesive reduce by 1% and 6% after 2 months,
respectively.e elongation at break of the adhesive increases
withimmersion duration from 0 day to 30 days.e elongation atbreak
of the adhesive increases by 29%.
e varied properties of the adhesive depend on thewater uptake
[14].e water uptake of the tension specimensdiers from the
specimens of the water immersion. It resultsfrom the size dierence
of specimens between absorptionand the tension of specimens. e
longitudinal direction oftension specimens is one order larger than
that of thethickness and width. us, it is assumed that the
watermolecules only diuse along the thickness and width of
tension specimens. e tension specimens in thickness andwidth are
15.0mm and 3.3mm, respectively. e relation-ship between immersion
duration and the water uptake canbe determined by (4). Figure 6
shows the relationship be-tween the water uptake and the properties
of the adhesive. Itindicates that the short-term water immersion
(M∞ < 1.4%)insignicantly in§uences the tensile strength and
elasticmodulus.
Tg with 2 months of water immersion only decreases by9%.Water
plays the plasticization role in the adhesive within2 months of
water immersion.
3.3. Failure Modes. e failure modes of the sandwichedfour-point
bending specimen are shown in Figure 7. A thinconcrete laminate
beneath the adhesive was pulled o for thecontrol specimens. e
tensile strength of the adhesive andconcrete are 57.1MPa and
1.9MPa, respectively. e tensilestrength of the adhesive is 30 times
larger than that of theconcrete. e failure modes shifted from a
thin concretefailure to adhesive layer-concrete interface
separation for theaged specimens. It is attributed to water uptake
in the ad-hesive layer-concrete bonded zone, following the
reductionin the bond strength. Compared to the concrete and
ad-hesive laminate, the adhesive layer-concrete bond is theweakest
laminate. us, the precrack at the unbonded zonecannot kink into the
concrete block, and the crack propa-gates along the adhesive
layer-concrete interface.
e sandwiched four-point bending specimen is de-formed in
four-point (pure) bending. e interface bond islocated in the pure
bending region. e bond stress betweenconcrete and adhesive only
involves the normal stress. emode I fracture energy is adopted to
evaluate the bondperformance. e mode I fracture energy of the
sandwichedfour-point bending specimens can be determined by
[25]
G �f21σ2rπaE1
, (5)
f1 � 1.122 − 1.4a
d( ) + 7.33
a
d( )
2− 13.08
a
d( )
3+ 14.0
a
d( )
4,
(6)
σr �6Mbd2
, (7)
where G is mode I fracture energy, f1 is a correction factorfor
four-point pure bending, and M is the moment at theinterfacial
bond, which is equal to 15P. a, d, and h are shownin Figure 1. a is
15mm, and h is 25mm.
e mode I interfacial fracture energy of the sandwichedfour-point
bending specimens was determined by equation(5). Figure 8 shows the
relationship between the exposure
Table 1: Material properties used in the simulations.
Materials Diusivity coe£cient (mm2/s) Equilibrium moisture
content (%) Hygroscopic expansion coe£cient (H2O%)a
Adhesive 63×10− 9 3.08 3.24×10− 3Concrete 1.7×10− 5 7.10 5×10−
3ae value of the hygroscopic expansion coe£cient was referred to
Ref. [32].
0 2000 4000 6000 80000
1
2
3
4
Experimental resultsFitting curve
Wat
er u
ptak
e (%
)
Time (s1/2)
Figure 4: Water uptake curves of the adhesive samples immersedin
water.
4 Advances in Civil Engineering
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duration and themode I fracture energy.emode I fractureenergy
reduced by 54% after 2 weeks. e cracking directioncan be predicted
by [5, 34]
ΓiΓc≤ 1, (8)
where Γi and Γc are the interface and concrete fractureenergy
release rate, respectively. If equation (8) is satised,
the crack propagates along the interface between adhesiveand
concrete. e concrete fracture energy was reported tobe about 25
J/m2 [5], while the mode I fracture energy of thesandwiched
four-point bending specimen reduced to 360 J/m2 and 340 J/m2 after
2 weeks and 4 weeks, respectively. Inthe case of the sandwiched
four-point bending specimenafter 2 weeks and 4 weeks, it seems to
satisfy equation (8).us, the crack should propagate into concrete.
In fact,
0 15 30 45 60 750
15
30
45
60
75
Tens
ile st
reng
th (M
Pa)
Exposure duration (day)
(a)
0 15 30 45 60 750.0
0.8
1.6
2.4
3.2
4.0
Elas
ticity
mod
ulus
(GPa
)
Exposure duration (day)
(b)
0 15 30 45 60 750.0
0.8
1.6
2.4
3.2
Frac
ture
elon
gatio
n (%
)
Exposure duration (day)
(c)
Figure 5: Eects of immersion duration on the tensile strength
(a), elastic modulus (b), and fracture elongation (c) of the
adhesive.
0.0 0.5 1.0 1.5 2.00
15
30
45
60
75
Tens
ile st
reng
th (M
Pa)
Water uptake (%)
2 months
(a)
0.0 0.5 1.0 1.5 2.00
1
2
3
4
Elas
ticity
mod
ulus
(GPa
)
Water uptake (%)
2 months
(b)
Figure 6: Eects of the water uptake on the tensile strength (a)
and elastic modulus (b) of the adhesive.
Advances in Civil Engineering 5
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Figures 7(b) and 7(c) show the failure mode of the
interfacialdebonding for specimens in water after 2 weeks and 4
weeks.e crack stayed at the adhesive layer-concrete interface.
Itmeans the unsatisfaction of equation (8). It is attributed tothe
enhancement of the strengthened layer. e strength-ened layer
involves the penetration of the primer into thepore at the concrete
surface. e enhancement results fromthe fracture energy of the
strengthened layer after waterimmersion, owing to the swelling of
the primer. In the caseof the aged specimens, the interface
fracture energy (Γc)
involved in equation (8) is replaced by that of thestrengthened
layer.us, the strengthened layer prevents thecrack into
concrete.
3.4. Load-Deformation Behavior of the Sandwiched Four-Point
Bending Specimen. Figure 9 shows the typical load-deformation
behavior of the four-point bending for controlspecimens. It shows
that the load linearly increases with thedeformation. Less dierence
of the curve of load-de-formation is found for control and aged
specimens. Allspecimens of the ultimate load and corresponding
de-formation are shown in Table 2. e dierences in the
load-deformation curves between the aged specimens and
controlspecimens are the ultimate load capacity, initial
elasticmodulus, and the ultimate deformation. Table 2 shows thatthe
ultimate load capacity and corresponding ultimate de-formation
rapidly reduce by 31% and 55% after 2 weeks,respectively. e
penetration of water uptake at the adhe-sive-concrete interface
causes the degradation of the ulti-mate load capacity and ultimate
deformation. e moisturemolecules cause the degradation of the
chemical bond andthe internal stress, following by the microcrack
at the in-terface. e ultimate load capacity and ultimate
deformationinsignicantly vary between 2 weeks and 4 weeks of
waterimmersion. It is attributed to the similar failure mode
forboth specimens exposed to water after 2 weeks and 4 weeks.e
initial stiness (α) of the load-deformation behavior isdened in
Figure 9. Table 2 shows that the initial stinessincreases with
exposure duration. It means that the ductilityof the sandwiched
four-point bending specimen decreasesowing to the water
immersion.
Unbonded zone Concrete
(a) (b)
(c)
Figure 7: Failure modes of the sandwiched four-point bending
specimen: (a) control, (b) after 2 weeks of water immersion, and
(c) after 4weeks of water immersion.
0 1 2 3 4 50.0
0.3
0.6
0.9
1.2
Inte
rfaci
al fr
actu
re en
ergy
(J/m
2 )
Time (week)
Figure 8: Degradation of the interfacial fracture energy with
ex-posure duration.
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3.5.DistributionofMoistureContentandHygroscopicStress atthe
Adhesive Layer-Concrete Bonded Zone. Figure 10 showstypical FEM
results in the bond zone of the 3D model of thesandwiched
four-point bending specimen. Figure 11 showsthe moisture content
distribution at the adhesive layer-concrete interface after 2
weeks, 4 weeks, and 6 weeks.Compared to the moisture content close
to the center zone,the bond zone on the edge aborts more moisture
content.e moisture content on the edge rises simultaneously.
ecenter zone of the water content gradually increases to thewater
content of the edge zone with the exposure time. emoisture content
at the interface mainly migrates from theconcrete blocks rather
than the adhesive. e diusivitycoe£cient of the concrete is two
orders of magnitude largerthan that of adhesive. e dierences in
moisture contentdistribution at the interface do not vary from 2
weeks to 4weeks. In the case of 2 weeks, most of the moisture
contentreaches to 2.99%, which is approximately equal to
theequilibrium moisture content (e.g., 3.08%).
e water uptake in the adhesive and concrete involvesthe
interfacial internal hygroscopic stress owing to themismatch of the
hygrothermal expansion coe£cient be-tween adhesive and concrete.
Figure 12 shows the typicalFEM results of the hygroscopic stress.
In the case of 2 weeks,the hygroscopic stress on the edge is larger
than that at thecenter zone. e shape of the stress distribution is
similar to
that of the moisture content distribution in Figure 11.
Asdiscussed, the hygroscopic stress depends on the moisturecontent.
Most of the hygroscopic stress reaches to 1.1MPaexcept the
interface at the edge.e hygroscopic stress at theedge of the
interface is 1.3MPa. It is attributed to the stressconcentration.
It is worth noting that the hygroscopic stressis smaller than that
of the tensile strength of concrete(1.9MPa). It means that the
there is no microcrack at theinterface. It is believed that the
reduction in the physical
Table 2: Experimental program and main test result.
Specimensa Exposure duration (week) Fracture energy (mJ/mm2)
Ultimate load (kN) Ultimate slip (mm) Initial stiness (N/mm)P0W-1 0
0.46 2.27 0.57 4.1P0W-2 0 0.91 3.19 0.82 3.8P0W-3 0 0.94 3.25 0.92
3.7P2W-1 2 0.37 2.04 0.37 5.3P2W-2 2 0.28 1.78 0.26 6.1P2W-3 2 0.42
2.16 0.41 3.2P4W-1 4 0.50 2.38 0.28 8.0P4W-2 4 0.26 1.71 0.23
6.0P4W-3 4 0.39 2.10 0.25 7.1a0W, 2W, and 4W denote exposure to 0
week, 2 weeks, and 4 weeks of water immersion, respectively. 1, 2,
and 3 denote the number of the specimens.
Concrete
Adhesive
Y
X
+ 1.000e + 00+ 9.684e – 01+ 9.368e – 01+ 9.052e – 01+ 8.736e –
01+ 8.419e – 01+ 8.103e – 01+ 7.787e – 01+ 7.471e – 01+ 7.155e –
01+ 6.839e – 01+ 6.523e – 01+ 6.207e – 01
NT11
Figure 10: Typical FEM results of moisture content
distribution.
0.0 0.2 0.4 0.6 0.8 1.00
1
2
3
4
Load
(kN
)
Slip (mm)
α
Figure 9: Typical load-deformation behavior of the four-point
bending for control specimens.
Advances in Civil Engineering 7
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AdhesiveYX
Concrete
3.08
3.04
3.00
2.9625
2015
105
0 010
2030
40
X-axis (m
m)
Y-axis (mm
)
Moi
sture
cont
ent (
%)
(a)
3.0800
3.0793
3.0786
3.077925
2015
105
00
1020
3040
X-axis (mm
)
Y-axis (mm)
Moi
sture
cont
ent (
%)
(b)
3.0800
3.0793
3.0786
3.077925
2015
105
00
1020
3040
X-axis (mm
)
Y-axis (mm)
Moi
sture
cont
ent (
%)
(c)
Figure 11: Typical FEM results of the moisture content
distribution: (a) 2 weeks, (b) 4 weeks, and (c) 6 weeks.
AdhesiveY
X
Concrete
2.0
1.6
1.2
0.8
0.425
2015
105
0 010
2030
40
X-axis (mm
)
Y-axis (mm)
Stre
ss (G
Pa)
(a)
2.0
1.6
1.2
0.8
0.425
2015
105
0 010
2030
40
X-axis (m
m)
Y-axis (mm)
Stre
ss (G
Pa)
(b)
Figure 12: Continued.
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bond mainly causes the degradation of the adhesive
layer-concrete bond.
e physical bond is related to the hydrogen bond andthe van der
Waals force. e water molecules disrupt thehydrogen bond between
DGEBA chain (adhesive) and SiO2(concrete) [17, 35]. In addition,
the moisture moleculesenlarge the distance between center of mass
of the DGEBAchain (epoxy) and SiO2 (concrete). e molecular
simula-tion shows that the binding energy under wet conditions
isonly one-third of the value attained under dry conditions[35].
Figure 13 shows the disruption of the hydrogen bond.
3.6. Evolution ofMode I Fracture Energy inWater. e
moreliterature results [5, 25] are introduced to evaluate the
eectsof the water immersion on the mode I fracture energy owingto
lack of the su£cient long exposure duration in the presentstudy.
Figure 14 shows the comparisons of the literature andpresent
results. e water immersion signicantly reducesthe mode I fracture
energy. It is obviously indicated that thenormalized fracture
energy reduces to be a similar value afterfour weeks. e residual
normalized mode I fracture energydoes not vary between four weeks
and ten weeks. edegradation in the bond is related to the moisture
content atthe adhesive layer-concrete interface. e moisture
content
reaches a threshold value. us, the failure modes shift
fromconcrete cohesive failure to interfacial debonding. echange of
the failure mode mainly results in the reduction ofthe mode I
fracture energy. As discussed above, 2 weeks of
2.0
1.6
1.2
0.8
0.425
2015
105
0 010
2030
40
X-axis (mm
)
Y-axis (mm)
Stre
ss (G
Pa)
(c)
Figure 12: Typical FEM results of the hygroscopic stress: (a) 2
weeks, (b) 4 weeks, and (c) 6 weeks.
H3C CH CH2 O C O CH2 CH CH3
OHOH
H
O
Si
H
O
Si
CH3
CH3n
O
H HO
H H
OH
H
O H
H
Figure 13: Disruption of the hydrogen bond.
0 3 6 9 120.0
0.3
0.6
0.9
1.2
1.5
Present studyAu et al. [5]
Lau et al. [25]Reduction coefficient
Nor
mal
ized
frac
ture
ener
gy
Exposure duration (week)
Figure 14: Comparisons of the literature and present
results.
Advances in Civil Engineering 9
-
water immersion in the present study involves an averagerelative
humidity of 98%. e cross section of the concreteblock in the
present study is similar to that of the literature inRef. [25]. It
is believed that the normalized fracture energyreduced to be a
steady value after two weeks. us, theaverage of all the normalized
mode I fracture energies aftertwo weeks is 0.486. e reduction
coe£cient of mode Ifracture energy is assumed to be 0.486.
In the application, the deterioration of either mode Ifracture
energy or mode II fracture energy causes thefailure of the FRP
strengthening the concrete. e liter-ature results of the evolution
of the mode II (shear)fracture energy in water are adopted. It is
di£cult to di-rectly compare the present samples with the
literaturesamples owing to the dierences in the dimension of
thesamples. As discussed above and in Ref. [5], the degree ofthe
deterioration of the bond performance mainly dependson the water
content at the adhesive layer-concrete in-terface. us, the residual
mode II fracture energy of thepull-out samples involves the e£cient
long exposure du-ration and the similar concrete compressive
strength.Figure 15 shows the comparisons of the mode I
(peel)fracture energy and mode II (shear) fracture energy.
ereduction of the mode I fracture energy is more severe thanthat of
the mode II fracture energy. It means that the modeI fracture
energy is more susceptible than mode II fractureenergy to the water
immersion. e reduction coe£cientof the mode I fracture energy is
about 50% of that of themode II fracture energy.
4. Conclusions
e major objective of this paper investigated the eect ofwater
immersion on the normal stress using the sandwichedfour-point
bending specimen experimentally and numeri-cally, and the following
conclusions can be drawn:
(1) After 2 weeks of exposure in water, the sandwichedfour-point
bending specimen is damaged in theseparation of the adhesive
layer-concrete interfacebond. e reduction in ductility and initial
stinessof the sandwiched four-point bending specimenresults in the
change of the failure mode fromconcrete cohesive failure to
interfacial debonding.
(2) e water molecules signicantly reduce the mode Ifracture
energy. e degradation in the mode Ifracture energy involves the
moisture content at theadhesive layer-concrete interface.
(3) e evolution of the interfacial bond strength inwater seems
to be independent of the interlockingbond. e deterioration of the
physical bond causessignicant reduction of the interface bond
strength.
(4) Compared to the reduction in the mode II fractureenergy, the
mode I fracture energy in water reducesslightly. e residual mode I
fracture energy in waterreduces to 0.486 of the mode I fracture
energy withunaged specimens.
Data Availability
e data used to support the ndings of this study areavailable
from the corresponding author upon request.
Conflicts of Interest
e authors declare no con§icts of interest.
Authors’ Contributions
Y. P. and H. L. carried out conceptualization. J. F. and Y.
P.contributed to methodology. J. S., Y. P. F. J., and Y. Yperformed
formal analysis and investigation. J. S. and Y. Ywrote the original
draft. Y. P. H. L., and J. F. reviewed andedited the manuscript. Y.
P was responsible for fundingacquisition.
Acknowledgments
is research was funded by the Zhejiang Provincial NaturalScience
Foundation of China (Project no. LY19E080029),Production and
Construction Group’s Programs for Scienceand Technology Development
(Project no. 2019AB016), theZhejiang Basic Public Welfare Research
Project (Project no.LGF8E080016), and the First-Class Disciplines
Project ofCivil Engineering in Zhejiang Province.
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