EffectsofWaterImmersionontheAdhesionbetweenAdhesive ...present study were provided by Dagong...

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

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

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

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.


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

)


(b)

0 15 30 45 60 750.0

0.8

1.6

2.4

3.2

Frac

ture

elon

gatio

n (%

)


(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.


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.


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.


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.


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.


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.

References

[1] C. Li, G. Xian, and H. Li, “Tension-tension fatigue perfor-mance of a large-diameter pultruded carbon/glass hybridrod,” International Journal of Fatigue, vol. 120, pp. 141–149,2019.

[2] J. F. Chen and J. G. Teng, “Anchorage strengthmodels for FRPand steel plates bonded to concrete,” Journal of StructuralEngineering, vol. 127, no. 7, pp. 784–791, 2001.

[3] Y.Wang, X. Li, J. Li, Q.Wang, B. Xu, and J. Deng, “Debondingdamage detection of the CFRP-concrete interface based on

0.0

0.2

0.4

0.6

0.8

1.0

Redu

ctio

n co

effic

ient

of f

ract

ure e

nerg

y

Mode IMode II_Shrestha et al. [15]_EMode II_Shrestha et al. [15]_FMode II_Au et al. [5]

Figure 15: Comparisons of the mode I (peel) fracture energy andmode II (shear) fracture energy.


piezoelectric ceramics by the wave-based method,” Con-struction and Building Materials, vol. 210, pp. 514–524, 2019.

[4] J. Shrestha, D. W. Zhang, and T. Ueda, “Durability perfor-mances of carbon fiber–reinforced polymer and concrete-bonded systems under moisture conditions,” Journal ofComposites for Construction, vol. 20, no. 5, Article ID04016023, 2016.

[5] C. Au and O. Büyüköztürk, “Peel and shear fracture char-acterization of debonding in FRP plated concrete affected bymoisture,” Journal of Composites for Construction, vol. 10,no. 1, pp. 35–47, 2006.

[6] F. Al-Mahmoud, J.-M. Mechling, and M. Shaban, “Bondstrength of different strengthening systems—concrete ele-ments under freeze-thaw cycles and salt water immersionexposure,” Construction and Building Materials, vol. 70,pp. 399–409, 2014.

[7] H. Liang, S. Li, Y. Lu, and T. Yang, “Reliability analysis ofbond behaviour of CFRP-concrete interface under wet-drycycles,” Materials, vol. 11, no. 5, p. 741, 2018.

[8] M.-W. Wei, J.-H. Xie, H. Zhang, and J.-L. Li, “Bond-slipbehaviors of BFRP-to-concrete interfaces exposed to wet/drycycles in chloride environment,” Composite Structures,vol. 219, pp. 185–193, 2019.

[9] J. Li, J. Xie, F. Liu, and Z. Lu, “A critical review and assessmentfor FRP-concrete bond systems with epoxy resin exposed tochloride environments,” Composite Structures, vol. 229,p. 111372, 2019.

[10] Y. Tao and J.-F. Chen, “Concrete damage plasticity model formodeling FRP-to-concrete bond behavior,” Journal of Com-posites for Construction, vol. 19, no. 1, Article ID 04014026,2014.

[11] C. Li, G. Xian, and H. Li, “Water absorption and distributionin a pultruded unidirectional carbon/glass hybrid rod underhydraulic pressure and elevated temperatures,” Polymers,vol. 10, no. 6, p. 627, 2018.

[12] Z. Lu, J. Xie, H. Zhang, and J. Li, “Long-term durability of basaltfiber-reinforced polymer (BFRP) sheets and the epoxy resinmatrix under a wet-dry cyclic condition in a chloride-con-taining environment,” Polymers, vol. 9, no. 12, p. 652, 2017.

[13] Y. Pan, J. Shi, and G. Xian, “Experimental and numericalstudy of the CFRP-to-concrete bonded joints after waterimmersion,” Composite Structures, vol. 218, pp. 95–106, 2019.

[14] B. Hong and G. Xian, “Ageing of a thermosetting poly-urethane and its pultruded carbon fiber plates subjected toseawater immersion,” Construction and Building Materials,vol. 165, pp. 514–522, 2018.

[15] J. Shrestha, T. Ueda, and D. W. Zhang, “Durability of FRPconcrete bonds and its constituent properties under the in-fluence of moisture conditions,” Journal of Materials in CivilEngineering, vol. 27, no. 2, Article ID A4014009, 2014.

[16] L.-h. Tam and D. Lau, “Moisture effect on the mechanical andinterfacial properties of epoxy-bonded material system: anatomistic and experimental investigation,” Polymer, vol. 57,pp. 132–142, 2015.

[17] Y. Pan, G. Xian, and M. A. G. Silva, “Effects of water im-mersion on the bond behavior between CFRP plates andconcrete substrate,” Construction and Building Materials,vol. 101, pp. 326–337, 2015.

[18] A. Zhou, O. Büyüköztürk, and D. Lau, “Debonding of con-crete-epoxy interface under the coupled effect of moisture andsustained load,” Cement and Concrete Composites, vol. 80,pp. 287–297, 2017.

[19] Z. Ouyang and B. Wan, “Modeling of moisture diffusion inFRP strengthened concrete specimens,” Journal of Compositesfor Construction, vol. 12, no. 4, pp. 425–434, 2008.

[20] S. Amidi and J. Wang, “Subcritical debonding of FRP-to-concrete bonded interface under synergistic effect of load,moisture, and temperature,” Mechanics of Materials, vol. 92,pp. 80–93, 2016.

[21] J. Pan and C. K. Y. Leung, “Debonding along the FRP-con-crete interface under combined pulling/peeling effects,” En-gineering Fracture Mechanics, vol. 74, no. 1-2, pp. 132–150,2007.

[22] O. R. Mata and R. A. Atadero, “Evaluation of pull-off tests as aFRP-concrete bond testing method in the laboratory andfield,” Practice Periodical on Structural Design and Con-struction, vol. 19, no. 2, Article ID 04014001, 2014.

[23] ACI, Guide Test Methods for Fiberreinforced Polymers (FRPs)for Reinforcing or Strengthening Concrete Structures, Vol. 440,ACI Committee, Farmington Hills, MI, USA, 2012.

[24] J.-G. Dai, H. Yokota, M. Iwanami, and E. Kato, “Experimentalinvestigation of the influence of moisture on the bond be-havior of FRP to concrete interfaces,” Journal of Compositesfor Construction, vol. 14, no. 6, pp. 834–844, 2010.

[25] D. Lau and O. Büyüköztürk, “Fracture characterization ofconcrete/epoxy interface affected by moisture,” Mechanics ofMaterials, vol. 42, no. 12, pp. 1031–1042, 2010.

[26] Z. Suo and J. W. Hutchinson, “Sandwich test specimens formeasuring interface crack toughness,” Materials Science andEngineering: A, vol. 107, pp. 135–143, 1989.

[27] ASTM, Standard Test Method for Tensile Properties of Plastics,ASTM International, West Conshohocken, PA, USA, 2014.

[28] Z. Lu, G. Xian, and H. Li, “Effects of thermal aging on thewater uptake behavior of pultruded BFRP plates,” PolymerDegradation and Stability, vol. 110, pp. 216–224, 2014.

[29] Y. Pan and G. Xian, “Influence of long-term outdoor exposurein a frigid zone on the CFRP-to-concrete bond behavior,”Construction and Building Materials, vol. 215, pp. 462–474,2019.

[30] X. Xu, Q. Huang, Y. Ren, D.-Y. Zhao, J. Yang, andD.-Y. Zhang, “Modeling and separation of thermal effectsfrom cable-stayed bridge response,” Journal of Bridge Engi-neering, vol. 24, no. 5, Article ID 04019028, 2019.

[31] S. Yoon, B. Han, and Z. Wang, “On moisture diffusionmodeling using thermal-moisture analogy,” Journal of Elec-tronic Packaging, vol. 129, no. 4, pp. 421–426, 2007.

[32] T. K. Tsotsis and Y.Weitsman, “Energy release rates for crackscaused by moisture absorption in graphite/epoxy compos-ites,” Journal of Composite Materials, vol. 24, no. 5,pp. 483–496, 1990.

[33] G. Xian and V. M. Karbhari, “DMTA based investigation ofhygrothermal ageing of an epoxy system used in re-habilitation,” Journal of Applied Polymer Science, vol. 104,no. 2, pp. 1084–1094, 2007.

[34] M.-Y. He and J. W. Hutchinson, “Kinking of a crack out of aninterface,” Journal of Applied Mechanics, vol. 56, no. 2,pp. 270–278, 1989.

[35] O. Büyüköztürk, M. J. Buehler, D. Lau, and C. Tuakta,“Structural solution using molecular dynamics: fundamentalsand a case study of epoxy-silica interface,” InternationalJournal of Solids and Structures, vol. 48, no. 14-15, pp. 2131–2140, 2011.


International Journal of

AerospaceEngineeringHindawiwww.hindawi.com Volume 2018

RoboticsJournal of

Hindawiwww.hindawi.com Volume 2018


Active and Passive Electronic Components

VLSI Design



Shock and Vibration


Civil EngineeringAdvances in

Acoustics and VibrationAdvances in



Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawiwww.hindawi.com

Volume 2018

Hindawi Publishing Corporation http://www.hindawi.com Volume 2013Hindawiwww.hindawi.com

The Scientific World Journal

Volume 2018

Control Scienceand Engineering

Journal of


Hindawiwww.hindawi.com

Journal ofEngineeringVolume 2018

SensorsJournal of



RotatingMachinery


Modelling &Simulationin EngineeringHindawiwww.hindawi.com Volume 2018


Chemical EngineeringInternational Journal of Antennas and

Propagation




Navigation and Observation


Hindawi

www.hindawi.com Volume 2018

Advances in

Multimedia

Submit your manuscripts atwww.hindawi.com
https://www.hindawi.com/journals/ijae/https://www.hindawi.com/journals/jr/https://www.hindawi.com/journals/apec/https://www.hindawi.com/journals/vlsi/https://www.hindawi.com/journals/sv/https://www.hindawi.com/journals/ace/https://www.hindawi.com/journals/aav/https://www.hindawi.com/journals/jece/https://www.hindawi.com/journals/aoe/https://www.hindawi.com/journals/tswj/https://www.hindawi.com/journals/jcse/https://www.hindawi.com/journals/je/https://www.hindawi.com/journals/js/https://www.hindawi.com/journals/ijrm/https://www.hindawi.com/journals/mse/https://www.hindawi.com/journals/ijce/https://www.hindawi.com/journals/ijap/https://www.hindawi.com/journals/ijno/https://www.hindawi.com/journals/am/https://www.hindawi.com/https://www.hindawi.com/

Date post:	31-Jan-2021
Category:	Documents
Upload:	others
View:	0 times
Download:	0 times

EffectsofWaterImmersionontheAdhesionbetweenAdhesive ...present study were provided by Dagong...

Documents