Research ArticleBond Behavior of Wet-Bonded Carbon Fiber-ReinforcedPolymer-Concrete Interface Subjected to Moisture

Yiyan Lu , Tao Zhu , Shan Li , and Zhenzhen Liu

School of Civil Engineering, Wuhan University, Wuhan 430072, China

Correspondence should be addressed to Shan Li; lishan@whu.edu.cn

Received 10 October 2017; Revised 9 January 2018; Accepted 13 February 2018; Published 14 March 2018

Academic Editor: Young Hoon Kim

Copyright © 2018 Yiyan Lu et al.This is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The use of carbon fiber-reinforced polymer (CFRP) composite materials to strengthen concrete structures has become popular incoastal regions with high humidity levels. However, many concrete structures in these places remain wet as a result of tides andwave-splashing, so they cannot be completely dried before repair. Therefore, it is vital to investigate the effects of moisture on theinitial and long-term bond behavior between CFRP and wet concrete.This research assesses the effects of moisture (i) during CFRPapplication and (ii) throughout the service life. Before CFRP bonding, the concrete blocks are preconditioned with a water contentof 4.73% (termed “wet-bonding”). Three different epoxy resins are applied to study the bond performance of the CFRP-concreteinterface when subjected tomoisture (95% relative humidity). A total of 45 double-lap shear specimens were tested at the beginningof exposure and again after 1, 3, 6, and 12 months. All specimens with normal epoxy resins exhibited adhesive failure. The failuremode of specimens with hydrophobic epoxy resin changed from cohesive failure tomixed cohesive/adhesive failure and to adhesivefailure according to the duration of exposure. Under moisture conditioning, the maximum shear stress (𝜏max) and correspondingslip (𝑠max) of the bond-slip curve first increased and then decreased or fluctuated over time. The same tendency was seen in theultimate strain transmitted to the CFRP sheet, the interfacial fracture energy (𝐺𝑓), and the ultimate load (𝑃𝑢). Analytical models of𝐺𝑓 and 𝑃𝑢 for the CFRP-concrete interface under moisture conditioning are presented.

1. Introduction

The use of fiber-reinforced polymer (FRP) composites hasemerged as one of the most promising techniques in thefield of concrete structural strengthening [1–4] due to theirwell-established advantages, which include a high strength-weight ratio, fatigue resistance, ease of installation, and cost-effectiveness [5–9]. Of the various types of FRP composites,carbon fiber-reinforced polymer (CFRP) is used extensivelyfor rehabilitation of concrete structures. It has shown out-standing performance when used for strengthening concretestructures, providing improvement of load carrying capacity,stiffness, or ductility. Although CFRP composites providean effective method to strengthen concrete structures, theeffectiveness of strengthening system mainly depends on thedurability of adhesive bond between CFRP and concretesubstrate [10–12].

In coastal regions such as southern China, Hong Kong,many concrete structures are wet at the time of repair and

the CFRP-strengthened systems are subjected to moisture,under the influence of tides and wave-splashing. Therefore,the performance of CFRP in strengthening wet concreteshould bewell recognized for further application, and variousmethods have been studied. Zhou andLucas [13] revealed thatwater sorption could modify the mechanical properties ofepoxy resins. Tatar and Hamilton [14] reported that moisturehad a negative effect on the adhesion properties of epoxyresin. Choi et al. [15] studied the CFRP bond capacity underhygrothermal exposure using a new three-point bend beamtest. The results suggested that CFRP-concrete bond systemsusing different epoxies showed significant differences indurability, although the same type of fibers and the same typeof concrete specimens were used. Tuakta and Buyukozturk[16] conducted cyclic moisture conditioning tests and foundthat the adhesive bond cannot regain its original bondstrength after successivewet-dry cycles at both roomandhightemperatures.Wan et al. [17] usedmodified double cantileverbeam (MDCB) specimens to study the effects of water on

HindawiInternational Journal of Polymer ScienceVolume 2018, Article ID 3120545, 11 pageshttps://doi.org/10.1155/2018/3120545

2 International Journal of Polymer Science

Steel rod CFRP sheetConcrete

block

Unbonded zone Bond length

Anchorage Strain gauges 7@20 mm

(a)

Specimen Strain gauges

(b)

Figure 1: Test arrangement: (a) details of specimen; (b) photograph of test setup.

the CFRP-concrete bond during CFRP application and afterCFRP curing.The results indicated that the presence of waterduring primer application caused a significant decrease inthe bond quality. When a water-tolerant primer was applied,the bond strength increased, but it was not comparable tothat of a dry-bonded specimen using the normal primer.The bond performance continued to degrade with exposure.The presence of water also changed the mode of failurefrom cohesive within the concrete to adhesive along theconcrete-to-primer interface. Dai et al. [18] studied the effectsof moisture on the initial and long-term bonding behav-ior of the CFRP-concrete interface using various primersand concluded that moisture strongly influenced the bondperformance and that the application of an appropriateprimer can eliminate the effects of concrete surface moistureon short-term interfacial bond performance. The failuremode changed from cohesive failure to interfacial failure,and the bond strength continued to degrade with moistureexposure. However, previous research studies have focusedon the performance and duration of concrete membersstrengthened with conventional CFRP composites. Recently,a few studies have been conducted on the use of highmodulusCFRP to strengthen concrete structures. Richardson[19] investigated the use of high modulus CFRP plate tostrengthen damaged reinforced concrete beams. The resultsshowed that the 400GPa CFRP of reinforcement ratio 0.17%increased the ultimate strength of concrete beam by up to51% and enhanced flexural stiffness in the elastic and plasticranges by 8% and 12%. He also developed an analyticalmodel to predict the performance of high modulus FRP-strengthened slender RC columns. Sadeghian and Fam [20]introduced a technique that aims at controlling second-orderlateral deflection using longitudinal high modulus bondedreinforcement. Therefore it was encouraging to investigatethe duration of high modulus CFRP-strengthened concretestructures.

This paper presents the experimental results from aseries of high modulus CFRP-concrete double-lap specimens

with the following purposes: (i) characterize the debondingprocess using mechanical parameters after moisture con-ditioning and (ii) provide reference for the establishmentof relative strengthening codes. Three ambient-cured epoxyresins obtained from different suppliers, two conventionaltypes (WSX [represented by A] and MA [represented byB]) and a hydrophobic type (CH-3 [represented by C]), areused in this study. Epoxy resin WSX is popularly used fordry concrete substrate and epoxy resin MA can be usedfor dry and wet concrete substrate, whereas epoxy resinCH-3 is particularly formulated for use in wet, damp, ormoist concrete substrates and is used to verify whetherthe hydrophobic epoxy resin can better cope with moistureat the CFRP-concrete interface over both short and longterms. Comparisons are presented on failure modes, bond-slip relationships, interfacial fracture energy, and ultimateloads.

2. Experimental Program

2.1. Test Specimen. The 45 conditioned specimens were me-chanically tested to failure in standard laboratory conditions.The duration of exposure to moisture was the only variablethat was changed during the experiment. Because three dif-ferent epoxy resins were used, these specimens were dividedinto three main groups. Each group was conditioned for 0,1, 3, 6, and 12 months. All double-lap shear specimens arenamed in the form of RH-𝑥-𝑦-𝑧. For example, RH-A-6-2 isspecimen number 2 with epoxy resin A subjected to 95%relative humidity (RH) for 6 months before testing.

2.2. Preparation of Specimen. Figure 1(a) shows a schematicview of the CFRP-concrete double-lap shear specimen. The25mm CFRP sheet is centered along the longitudinal axis ofthe 75mm concrete block, leaving two 25mm shoulders onthe side of the bond to remove edge effects. The bond lengthis 150mm, and a 35mm unbonded zone is left to avoid theloss of a concrete chunk by shearing near the loaded end

International Journal of Polymer Science 3

Table 1: Material properties of CFRP coupon and epoxy resin.

Material Young’s modulus(GPa)

Ultimate strength(MPa)

Shear strength(MPa)

Failure strain(%)

CFRP 245 3870 — 1.74CFRP-A 266 3933 — 1.48CFRP-B 268 3968 — 1.48CFRP-C 292 4428 — 1.52Epoxy A 2.4 33 21 1.40Epoxy B 2.7 45 22 1.84Epoxy C 4.5 48 26 1.65Note. CFRP-A: CFRP coupon using epoxy resin A; CFRP-B: CFRP coupon using epoxy resin B; CFRP-C: CFRP coupon using epoxy resin C.

[21, 22]. The concrete blocks are cast using wooden molds,demolded after 24 hours, and cured for 28 days. Afterwards,all concrete blocks are submerged in salt water for 1 yearso that the concrete’s compressive strength remains stableduring moisture exposure. The bonding sides of the concreteblocks are then sandblasted with a hand grinder to provideappropriate rough surfaces for bonding, and any debris,grease, laitance, and loose material at the surface are cleanedwith acetone using a cotton cloth. To investigate the effectsof the presence of water before bonding the CFRP sheets, theconcrete substrate surfaces are preconditioned to simulate theconstruction conditions that may be encountered in a moistenvironment. All concrete blocks are submerged in deionizedwater for 3 days, removed, andwiped tomeasure the absorbedwater. The difference in the weight of the concrete beforeand after conditioning represents the moisture mass uptake.In this process, the measured average moisture content ofthe concrete is 4.73%. After measurement of moisture, thefully saturated CFRP sheet is carefully placed on the designedregion of the concrete block upon which epoxy has beenapplied. A roller is used to squeeze out excessive epoxy untila uniform bonding layer is formed. In a similar manner,two 30mm-wide CFRP hoops with the fibers oriented inthe transverse direction are wrapped to provide sufficientanchorage so that failure takes place at the bonded zone. Allspecimens are then cured in the laboratory for 1 week atambient temperatures. It should be noted that, in the wet-layup system, epoxy resin is used to bond the CFRP sheet toconcrete and at the same time serves as saturation resin forthe CFRP sheet.

2.3. Moisture Conditioning. All specimens are conditionedat 20∘C ± 1∘C and 95% ± 3% RH inside a conditioningchamber in a laboratory with moisture control. At eachtarget exposure duration (i.e., 0, 1, 3, 6, and 12 months),three randomly chosen specimens for each epoxy resin areremoved periodically for testing.

2.4. Material Properties. The concrete blocks are made fromgrade 42.5 Portland cement and aggregates with a maximumsize of 15mm. The blocks are cast and cured followingGB50010-2010 [23]. The average concrete cube strength after1 year of immersion is 33.5MPa, determined with three150 × 150 × 150mm concrete cubes. The CFRP sheet with

a thickness of 0.167mm is “NG” brand and supplied byWuda Jucheng, a company in China. Table 1 presents itsmaterial properties according to the product data sheet. Theproperties of flat coupons made with different epoxy resinsare determined from tensile tests in accordance with ASTMD3039 [24] and are shown in Table 1. CFRP coupons aremade using the wet lay-up process. First, the CFRP sheetsare brushed with mixed epoxy resin. Second, the CFRPsheets are cut into 15mm wide × 250mm long sectionsafter 7-day curing at ambient temperature. Third, CFRPtabs and aluminum tabs are bonded at the two ends. Eachcoupon is tested using a universal testing machine at aspeed of 1mm/min, with a strain gauge adopted on thecenter. The three commercial two-part epoxy resins chosenfor the current program, both as CFRP matrices and asadhesives, are (1) WSX epoxy resin (represented by A), (2)MAepoxy resin (represented by B), and (3)CH-3 epoxy resin(represented by C). Table 1 summarizes their material prop-erties according to the material data sheet supplied by theirmanufacturers.

2.5. Test Setup and Instrumentation. Before testing, all speci-mens are fittedwith nine quarter-bridge strain gauges bondedto theCFRP’s outer surface. Seven strain gauges in the bondedzone with a spacing of 20mm are used to measure thestrain distribution along the CFRP sheet at various loadinglevels, and the other two in the unbonded zone are usedto record the tensile load. Strain measurement is recordedby an electronic acquisition system. The double-lap sheartests are carried out in standard laboratory conditions witha universal hydraulic testing machine (capacity of 100 kN)displacement controlled at 0.5mm/min. After the specimenis rigidly clamped by steel rods at both ends, preliminarytesting is conducted to carefully adjust the alignment oftwo symmetric CFPR sheets and clamps so the load can betransferred uniformly into the CFRP-concrete interface. Thespecimen is then tested individually in direct tension until theCFRP sheets are entirely pulled off (as shown in Figure 1).It should be noted that, for this double-lap specimen, theCFRP sheets of two sides would be peeled off in sequence.After one side is peeled off, the total load is suddenly appliedeccentrically on the opposite side. Therefore, only the strainmeasurements of the first failure side are reported in thisstudy.

4 International Journal of Polymer Science

Inact adhesive layer

(a)

Aggregates attached

(b)

Island-like aggregates

(c)

Figure 2: Typical failure modes: (a) adhesive failure; (b) cohesive failure; (c) mixed cohesive/adhesive failure.

3. Experimental Results

3.1. Failure Mode. Three predominant failure modes wereobserved: adhesive failure, cohesive failure, and mixed cohe-sive/adhesive failure. Adhesive failure occurs between theepoxy and the concrete substrate, with or without looseconcrete remaining on the CFRP sheet. Cohesive failureoccurs inside the concrete block near the epoxy-concretecontact surface, and many particles of coarse and fineaggregate are attached to the surface that failed. Mixedcohesive/adhesive failure is a combination of cohesive failureand adhesive failure. Table 2 summarizes the failure modes ofall specimens, and Figure 2 presents selected images of failedspecimens (the left part shows substrates after peeling, andthe right part shows peeled CFRP sheets).

When the concrete substrates are wet, specimens RH-A-0-z and RH-B-0-z are representative of adhesive failure(Figure 2(a)). The peeled part consists of the CFRP sheet andan almost-intact adhesive layer without adhering aggregates.This failure mode differs from that observed in other studieswhen concrete is dry [25, 26], which indicates that the initialwet concrete substrate has a negative effect on the bondbetween epoxy resin and concrete because the hydrogenbonds between the epoxy and the concrete substrate maybe disturbed by the presence of water molecules at theinterface [27]. With the increase in moisture exposure, allseries RH-A-𝑦-𝑧 and RH-B-𝑦-𝑧 specimens show adhesivefailure. This means that the effectiveness of mechanical bondwasweakened. And it can be contributed to the residual waterwithin concrete surface and the degradation properties ofepoxy resins [28, 29].

In contrast, most samples with hydrophobic epoxy speci-mens exhibit various degrees of cohesive failure (Figure 2(b)).The failure begins near the loaded end and then crackskinks into the concrete substrate and propagates parallelto the interface until CFRP sheets are pulled off. Manyaggregates are attached to the CFRP sheet. Nevertheless, after3 months of exposure, one conditioned specimen displays

a mixed cohesive/adhesive failure in the concrete, with arelatively small amount of island-like residual aggregatesremaining (Figure 2(c)). Fewer aggregates are attached onthe CFRP sheet as the duration of exposure increases. Whenconditioning up to 12 months, the failure of specimen RH-C-12 is characteristic of adhesive failure. The negligible amountof mortar or fine aggregate bonded on the CFRP sheet isindicative of the poor bond between the epoxy and the con-crete.This result indicates that, after long exposures, moisturecould greatly influence the durability of the epoxy-concreteinterface. The failure shift is thought to be due to (i) thepossibility of resin degradation because of moisture reactionor chain scission and (ii) the reduction of interface bondingcaused by water presence via water absorption or moisturediffusion. Therefore, for long exposures, this deteriorationrenders the epoxy-concrete interface the weakest link ofthe CFRP-strengthened system, resulting in adhesive failurebetween the epoxy and the concrete.

3.2. Bond-Slip Relationship. Based on the linear elastic as-sumption and Hook’s law, the average bond shear stressbetween two adjacent strain gauges (𝜏av) can be obtained viastrain measurement along the CFRP sheet by the followingequation:

𝜏av = 𝐸𝑓𝑡𝑓 (𝜀𝑖 − 𝜀𝑖−1)Δ𝑥 𝑖 ∈ (2, . . . , 7) , (1)

where 𝐸𝑓 and 𝑡𝑓 are the elastic modulus and thickness ofthe CFRP sheet and 𝜀𝑖 − 𝜀𝑖−1 is the axial strain differencebetween adjacent CFRP strains at locations 𝑖 and 𝑖 − 1, whichare separated by a distance Δ𝑥.

Neglecting the concrete strain, the relative slip (𝑠𝑖) atgauge location 𝑖 can be written as

𝑠𝑖 = Δ𝑥2 (𝜀1 + 2𝑖−1∑𝑗=2

𝜀𝑗 + 𝜀𝑖) 𝑖 ∈ (2, . . . , 7) , (2)

International Journal of Polymer Science 5

Table 2: Test results of all specimens.

SpecimenID

Exposure(month)

𝜏max(MPa)

Average𝜏max

𝑆max(mm)

Average𝑆max

𝐺𝑓(N⋅mm/mm2)

Average𝐺𝑓 𝑃𝑢(kN)

Average𝑃𝑢 Failuremode

RH-A-0-10

5.4645.670

0.0340.038

0.2290.277

3.6194.364

ADRH-A-0-2 5.875 0.041 0.348 4.508 ADRH-A-0-3 5.670 0.039 0.254 4.964 ADRH-A-1-1

17.422

6.0210.057

0.0430.625

0.4065.255

4.786AD

RH-A-1-2 6.337 0.033 0.375 4.462 ADRH-A-1-3 4.304 0.038 0.219 4.641 ADRH-A-3-1

34.999

6.0360.066

0.0510.363

0.3953.931

4.468AD

RH-A-3-2 7.475 0.048 0.493 4.657 ADRH-A-3-3 5.636 0.037 0.328 4.817 ADRH-A-6-1

64.640

5.1030.045

0.0380.262

0.2743.639

4.023AD

RH-A-6-2 5.709 0.040 0.320 4.324 ADRH-A-6-3 4.960 0.027 0.240 4.105 ADRH-A-12-1

125.057

4.1920.039

0.0350.253

0.2063.887

3.898AD

RH-A-12-2 3.315 0.036 0.164 4.311 ADRH-A-12-3 4.204 0.031 0.200 3.497 ADRH-B-0-1

05.521

5.6850.039

0.0420.252

0.2873.820

4.179AD

RH-B-0-2 6.179 0.041 0.324 4.587 ADRH-B-0-3 5.355 0.044 0.285 4.130 ADRH-B-1-1

15.215

5.4910.049

0.0430.404

0.3504.666

4.453AD

RH-B-1-2 5.649 0.038 0.336 4.111 ADRH-B-1-3 5.609 0.041 0.311 4.582 ADRH-B-3-1

36.144

6.2490.039

0.0400.397

0.3764.734

4.667AD

RH-B-3-2 5.708 0.036 0.267 4.417 ADRH-B-3-3 6.894 0.044 0.463 4.850 ADRH-B-6-1

65.084

5.5980.052

0.0460.366

0.3654.671

4.590AD

RH-B-6-2 4.913 0.035 0.286 4.451 ADRH-B-6-3 6.797 0.051 0.443 4.649 ADRH-B-12-1

123.560

4.5190.036

0.0370.224

0.2434.127

4.134AD

RH-B-12-2 5.478 0.037 0.263 4.141 ADRH-B-12-3 — — — ADRH-C-0-1

07.819

8.8420.048

0.0500.499

0.5725.307

5.662CO

RH-C-0-2 9.841 0.053 0.710 6.532 CORH-C-0-3 8.865 0.050 0.507 5.146 CORH-C-1-1

18.172

9.6000.056

0.0580.850

0.8877.148

6.725CO

RH-C-1-2 10.495 0.056 0.864 6.562 CORH-C-1-3 10.132 0.062 0.948 6.464 CORH-C-3-1

38.067

7.1630.057

0.0630.517

0.5545.345

5.747CO

RH-C-3-2 5.895 0.049 0.368 4.997 C/ARH-C-3-3 7.528 0.082 0.776 6.900 CORH-C-6-1

65.868

7.4920.051

0.0640.563

0.6115.941

6.212AD

RH-C-6-2 7.778 0.088 0.671 6.145 C/ARH-C-6-3 8.830 0.054 0.597 6.551 C/ARH-C-12-1

129.332

8.1820.050

0.0510.471

0.4875.601

5.586AD

RH-C-12-2 — — — — ADRH-C-12-3 7.032 0.052 0.504 5.571 ADNote. AD: adhesive failure; CO: cohesive failure; C/A: mixed cohesive/adhesive failure.

6 International Journal of Polymer Science

0.1 0.2 0.30.0

Slip (mm)

RH-A-0-2RH-A-1-2RH-A-3-2

RH-A-6-2RH-A-12-3

0

2

4

6

8

Shea

r stre

ss (M

Pa)

(a)

0.1 0.2 0.30.0

Slip (mm)

RH-B-0-3RH-B-1-2RH-B-3-3

RH-B-6-2RH-B-12-1

0

2

4

6

8

Shea

r stre

ss (M

Pa)

(b)

0.1 0.2 0.30.0

Slip (mm)

RH-C-0-3RH-C-1-2RH-C-3-1

RH-C-6-3RH-C-12-3

0

2

4

6

8

10

12

Shea

r stre

ss (M

Pa)

(c)

Figure 3: Representative bond-slip curves at the loaded end: (a) RH-A-𝑦-𝑧 specimens; (b) RH-B-𝑦-𝑧 specimens; (c) RH-C-𝑦-𝑧 specimens.

0

3

6

9

12

Ｇ；Ｒ

(MPa

)

3 6 9 120

Time (month)

RH-A-0RH-A-1RH-A-3

RH-A-6RH-A-12

(a)

0

3

6

9

12

Ｇ；Ｒ

(MPa

)

3 6 9 120

Time (month)

RH-B-0RH-B-1RH-B-3

RH-B-6RH-B-12

(b)

0

3

6

9

12

Ｇ；Ｒ

(MPa

)3 6 9 120

Time (month)

RH-C-0RH-C-1RH-C-3

RH-C-6RH-C-12

(c)

Figure 4: Comparison of 𝜏max for specimens: (a) RH-A-𝑦-𝑧 specimens; (b) RH-B-𝑦-𝑧 specimens; (c) RH-C-𝑦-𝑧 specimens.

where 𝜀1 is the strain at the free end of the CFRP sheet; 𝜀𝑗 isthe strain of 𝑗th gauge attached to theCFRP sheet; and 𝜀𝑖 is thestrain at the loaded end of theCFRP sheet. Combining (1) and(2) yields the 𝜏-𝑠 curve along the CFRP-concrete interface.

Plotting all of the curves in one figure may result inoverlapping, so it is difficult to characterize the effects ofmoisture. For simplicity, therefore, only one representativecurve for specimens at each target exposure time is presentedin Figure 3, in which all 𝜏-s curves exhibit a bilinear shape.It is found that the exposure has little effect on the initialstiffness of the upward section, but it significantly influencesthe maximum shear stress (𝜏max), the corresponding slip(𝑠max), and the area under the curve. To characterize theeffects of moisture exposure, all values of 𝜏max and 𝑠maxare plotted in Figures 4 and 5, respectively. For the RH-A-𝑦-𝑧 specimens, a synchronous change of 𝜏max and 𝑠max isobserved, increasing in the first 3 months and decreasingwith longer exposure. For the RH-B-𝑦-𝑧 specimens, after aslight inverse variation at 1 month of exposure, 𝜏max and 𝑠maxincrease at 3 months and then decrease gradually. For theRH-C-y-z specimens, 𝜏max reaches a maximum at 1 month ofexposure and then fluctuates over time, whereas 𝑠max shows

progressive growth up to 6 months with a final drop at 12months. The values increase in the first few months becausethe postcuring effect of adhesive layer enhances the interfacialbonding even in a moist environment. However, due to thedifferent properties of these three epoxy resins, the highest𝜏max and 𝑠max values are achieved after different exposuredurations, because the mechanical properties of CFRP sheetremain almost unaffected under accelerated conditioning[30, 31] and the concrete compressive strength is almostunchanged. One possible reason for the decreasing trend isthat the absorbedwater, acting as a plasticizer, usually reducesboth mechanical and adhesion properties of epoxy resinduring conditioning [17]. This phenomenon would increasethe porosity and flaws within the epoxy resin, which inturn would have a detrimental effect on the interfacial shearstrength.

Figure 6 compares the CFRP strain distribution along thebond length with some representative examples. In general,all specimens behave similarly, showing a more or lessexponential trend in the strain distribution profile.The straininitially transfers within 20mm of the loaded end and thentransfers towards the free end. Generally, the strain of the

International Journal of Polymer Science 7

3 6 9 120

Time (month)

RH-A-0RH-A-1RH-A-3

RH-A-6RH-A-12

0.00

0.02

0.04

0.06

0.08

0.10

s Ｇ；Ｒ

(mm

)

(a)

3 6 9 120

Time (month)

RH-B-0RH-B-1RH-B-3

RH-B-6RH-B-12

0.00

0.02

0.04

0.06

0.08

0.10

s Ｇ；Ｒ

(mm

)

(b)

3 6 9 120

Time (month)

RH-C-0RH-C-1RH-C-3

RH-C-6RH-C-12

0.00

0.02

0.04

0.06

0.08

0.10

s Ｇ；Ｒ

(mm

)

(c)

Figure 5: Comparison of 𝑠max for specimens: (a) RH-A-𝑦-𝑧 specimens; (b) RH-B-𝑦-𝑧 specimens; (c) RH-C-𝑦-𝑧 specimens.

CFRP sheet decreases with an increase in the distance fromthe loaded end.

With the increase in the applied load, the longitudinalstrain of the CFRP sheet increases. However, when theapplied load reaches the failure load, the ultimate strain ofthe CFRP sheet is lower than 6000 𝜇𝜀, which is less than halfof the elongation of CFRP sheet. This means that the tensilecapacity of the CFRP sheet in the CFRP-to-concrete jointcannot be fully used.

With the increase in the exposure time, almost all thelongitudinal strains of the CFRP sheet beyond 80mm fromthe loaded end are very limited, implying that the exposuretime on the transfer length is limited. However, the spec-imens’ ultimate strains show various trends. The ultimatestrain of specimens RH-A-𝑦-𝑧 achieves the highest value at1 month and then decreases over time. The ultimate strain ofspecimens RH-B-𝑦-𝑧 reaches the maximum at 3 months andthen drops.Theultimate strain of RH-C-𝑦-𝑧fluctuateswithina range of 3500 𝜇𝜀 to 5500 𝜇𝜀 and does not show the trend ofmonotonic change.

3.3. Interfacial Fracture Energy. Figure 7 presents the valuesof interfacial fracture energy (𝐺𝑓) in a moist environment.For RH-A-𝑦-𝑧 specimens, 𝐺𝑓 increases rapidly in the firstmonth and remains almost unchanged until 3 months andthen decreases sharply over time. For RH-B-𝑦-𝑧 specimens,𝐺𝑓 increases until 6months and then decreases. For RH-C-𝑦-𝑧 specimens,𝐺𝑓 peaks at 1 month, and the general trend overthe next few months is decreasing-increasing-decreasing.With respect to the maximum value of 𝐺𝑓, +31.77%, 0.00%,−2.71%, −32.51%, and −49.26% variation for RH-A-𝑦-𝑧 speci-mens; +23.67%, +6.91%, +0.00%, −2.93%, and −35.37% varia-tion for RH-B-𝑦-𝑧 specimens; and +35.51%, 0.00%, −37.54%,−31.12%, and −45.10% variation for RH-C-𝑦-𝑧 specimens areobserved. This phenomenon could be explained by (i) thesolidification of epoxy resins in the first few months and (ii)the reduction of the mechanical and adhesion properties ofepoxy resin caused bywater infiltration anddiffusion throughmicrocracks and voids with longer exposure. According tothe variation trend shown in Figure 7, a Gaussian function

that relates the interfacial fracture energy after conditioning(𝐺𝑓,moisture) to the exposure time (𝑡, months) is obtained bycurve fitting:

𝐺𝑓,moisture = (𝐴 + 𝐵𝐶√𝜋/2𝑒−2((𝑡−𝑡𝑐)2/𝐶2))𝐺𝑓, (3)

where 𝐴 is the residual factor, B and C are constants, 𝑡𝑐 isthe time when the highest fracture energy is attained, and 𝐺𝑓is the interfacial fracture energy of the 0-month specimens.For the RH-A-𝑦-𝑧 specimens, 𝐴, 𝐵, 𝐶, and 𝑡𝑐 achieved byregression analysis are 0.84, 2.70, 3.37, and 3.19, respectively.For the RH-B-𝑦-𝑧 specimens, 𝐴, 𝐵, 𝐶, and 𝑡𝑐 are 0.83, 3.81,5.78, and 4.24, respectively. For the RH-C-𝑦-𝑧 specimens, 𝐴,𝐵, 𝐶, and 𝑡𝑐 are 0.85, 3.23, 4.32, and 2.56, respectively.

3.4. Ultimate Load. Figure 8 shows the values of the ultimateload (𝑃𝑢) under moisture conditioning. The average 𝑃𝑢 ofthe RH-A-𝑦-𝑧 specimens increases in the first month andthen decreases progressively. The average values of RH-A-0-𝑧 (4.364 kN), RH-A-3-𝑧 (4.468 kN), RH-A-6-𝑧 (4.023 kN),and RH-A-12-𝑧 (3.898 kN) vary by +8.82%, −6.64%, −15.94%,and −18.55%, respectively, from that of RH-A-1-𝑧 (4.786 kN).For the RH-B-𝑦-𝑧 specimens, the 𝑃𝑢 values show similarvariation, and the maximum value occurs at 3 months(4.677 kN). With the duration of exposure, 𝑃𝑢 of RH-B-6-z (4.590 kN) and RH-B-12-z (4.134 kN) decrease by1.65% and 11.42%, respectively. For the RH-C-𝑦-𝑧 speci-mens, cohesive failure inside the concrete substrate resultsin the highest 𝑃𝑢. However, 𝑃𝑢 shows a different trend,attaining its maximum value after 1 month and then fluc-tuating over time. The decrease in strength is associatedwith a change in the failure mode from cohesive failure tomixed cohesive/adhesive failure or adhesive failure. Whenthe duration of exposure reaches 12 months, about 16.94%loss of the ultimate load associated with adhesive failure isobserved.

8 International Journal of Polymer Science

(a1)

(a2)

(a3)

(a4)

(b1)

(b2)

(b3)

(b4)

(c1)

(c2)

(c3)

(c4)

0

1000

2000

3000

4000

5000

CFRP

stra

in (

)

100 12060 8020 400

Distance from loaded end (mm)100 12060 8020 400

Distance from loaded end (mm)

0

1000

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3000

4000

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6000

CFRP

stra

in (

)

0

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CFRP

stra

in (

)

100 12060 8020 400

Distance from loaded end (mm)

0

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CFRP

stra

in (

)

100 12060 8020 400

Distance from loaded end (mm)

0

1000

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CFRP

stra

in (

)

100 12060 8020 400

Distance from loaded end (mm)

0

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CFRP

stra

in (

)

100 12060 8020 400

Distance from loaded end (mm)

0

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CFRP

stra

in (

)

100 12060 8020 400

Distance from loaded end (mm)

0

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CFRP

stra

in (

)

100 12060 8020 400

Distance from loaded end (mm)

0

1000

2000

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CFRP

stra

in (

)

100 12060 8020 400

Distance from loaded end (mm)

100 12060 8020 400

Distance from loaded end (mm)

0

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CFRP

stra

in (

)

100 12060 8020 400

Distance from loaded end (mm)

0

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CFRP

stra

in (

)

100 12060 8020 400

Distance from loaded end (mm)

0

1000

2000

3000

4000

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6000

CFRP

stra

in (

)

1 kN2 kN3 kN

4 kN4.508 kN

1 kN2 kN3 kN

4 kN4.130 kN

1 kN2 kN3 kN

4 kN5 kN6.551 kN

1 kN2 kN3 kN

4 kN4.451 kN

1 kN2 kN

3 kN4.324 kN

1 kN2 kN3 kN

4 kN4.657 kN

1 kN2 kN3 kN

4 kN4.850 kN

1 kN2 kN3 kN

4 kN5.345 kN

1 kN2 kN3 kN

4 kN5 kN6.562 kN

1 kN2 kN3 kN

4 kN4.111 kN

1 kN2 kN3 kN

4 kN4.462 kN

1 kN2 kN3 kN

4 kN5.164 kN

Figure 6: Continued.

International Journal of Polymer Science 9

(a5) (b5) (c5)

0

1000

2000

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CFRP

stra

in (

)

100 12060 8020 400

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0

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stra

in (

)

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Distance from loaded end (mm)

0

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CFRP

stra

in (

)

1 kN2 kN3 kN

4 kN5.571 kN

1 kN2 kN

3 kN4.127 kN

1 kN2 kN

3 kN3.497 kN

Figure 6: Representative CFRP strain distribution along the bonded length: (a1) RH-A-0-2; (b1) RH-B-0-3; (c1) RH-C-0-3; (a2) RH-A-1-2;(b2) RH-B-1-2; (c2) RH-C-1-2; (a3) RH-A-3-2; (b3) RH-B-3-1; (c3) RH-C-3-1; (a4) RH-A-6-3; (b4) RH-B-6-2; (c4) RH-C-6-3; (a5) RH-A-12-1;(b5) RH-B-12-1; (c5) RH-C-12-3.

3 6 9 120

Time (months)

RH-A-0RH-A-1RH-A-3

RH-A-6RH-A-12

0.0

0.2

0.4

0.6

0.8

1.0

Gf

(N·m

m/m

Ｇ2)

(a)

3 6 9 120

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RH-B-0RH-B-1RH-B-3

RH-B-6RH-B-12

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0.2

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Gf

(N·m

m/m

Ｇ2)

(b)

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RH-C-0RH-C-1RH-C-3

RH-C-6RH-C-12

0.0

0.2

0.4

0.6

0.8

1.0

Gf

(N·m

m/m

Ｇ2)

(c)

Figure 7: Comparison of Gf for specimens: (a) RH-A-𝑦-𝑧 specimens; (b) RH-B-𝑦-𝑧 specimens; (c) RH-C-𝑦-𝑧 specimens.

3 6 9 120

Time (months)

RH-A-0RH-A-1RH-A-3

RH-A-6RH-A-12

0

3

6

9

Pu

(kN

)

(a)

3 6 9 120

Time (months)

RH-B-0RH-B-1RH-B-3

RH-B-6RH-B-12

0

3

6

9

Pu

(kN

)

(b)

3 6 9 120

Time (months)

RH-C-0RH-C-1RH-C-3

RH-C-6RH-C-12

0

3

6

9

Pu

(kN

)

(c)

Figure 8: Comparison of 𝑃𝑢 for specimens: (a) RH-A-𝑦-𝑧 specimens; (b) RH-B-𝑦-𝑧 specimens; (c) RH-C-𝑦-𝑧 specimens.

10 International Journal of Polymer Science

+25%

−25%

RH-A-yRH-B-yRH-C-y

0.0

2.5

5.0

7.5

10.0

Pred

icte

d ul

timat

e loa

d (k

N)

2.5 5.0 7.5 10.00.0

Experimental ultimate load (kN)

Figure 9: Comparison of 𝑃𝑢 between prediction and experiments.

This phenomenon could be explained by the relationshipbetween 𝑃𝑢 and 𝐺𝑓. According to mode in [32, 33], theultimate load is given by

𝑃𝑢 = 𝑏𝑓√2𝐸𝑓𝑡𝑓𝐺𝑓. (4)

Because 𝐸𝑓 of the CFRP sheet is nearly unchanged aftermoisture conditioning in this study (as well as 𝑡𝑓 and 𝑏𝑓),the variation of 𝑃𝑢 is similar to that of 𝐺𝑓, as discussed inSection 3.3.

Figure 9 plots a comparison of the ultimate load deter-mined by (4) and the average value obtained experimentallyfor each exposure duration.The results demonstrate satisfac-tory agreement, with deviations of less than 25%.

4. Conclusions

An experimental study was conducted with 45 wet-bondedspecimens fabricated by three different epoxy resins toinvestigate the effects ofmoisture on the initial and long-terminterface behavior. The following conclusions are reachedwithin the scope of this study.

(1) All RH-A-𝑦-𝑧 and RH-B-𝑦-𝑧 specimens exhibited anadhesive failure mode, but, for RH-C-𝑦-𝑧 specimens,the failure mode was obviously affected by moistureexposure. It likely changed over time from cohesivefailure to mixed cohesive/adhesive failure and toadhesive failure.

(2) A bond-slip relationship was obtained from strainreading.TheCFRP-concrete interface undermoistureconditioning increased during the first few monthsand then decreased or fluctuated over time in termsof 𝜏max and 𝑠max.

(3) Moisture exposure tended to increase the ultimatestrain of specimens initially, followed by a progressivedecrease or fluctuation.

(4) The interfacial fracture energy increased first andthen decreased gradually or fluctuated over time.Theultimate load exhibited a similar trend of variation.Models are proposed to predict the interfacial fractureenergy and ultimate load for specimens.

Conflicts of Interest

The authors declare that there are no conflicts of interest.

Acknowledgments

The tests reported herein were made possible by the financialsupport from the National Natural Science Foundation ofChina (51578428) and Special Project on Technical Innova-tion of Hubei (no. 2016AAA025). The authors wish to thankthe technicians of School of Civil Engineering of WuhanUniversity for their help throughout the test process.

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