Construction and Building Materials 40 (2013) 899–907

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Construction and Building Materials

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Curing effects on steel/CFRP double strap joints under combined mechanicalload, temperature and humidity

Tien-Cuong Nguyen a, Yu Bai a,⇑, Xiao-Ling Zhao a, Riadh Al-Mahaidi b

a Department of Civil Engineering, Monash University, Clayton, VIC 3800, Australiab Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, VIC 3122, Australia

h i g h l i g h t s

" Detailed comparative study on environmental, loading and curing effects." Harsh environments represented by cyclic temperature and humidity, combined with loading." Curing effects on durability of the adhesively-bonded system is quantified." Creep response under cyclic temperature and humidity received up to 270 h." Time-to-failure in a harsh environment quantified as a function of load level.

a r t i c l e i n f o

Article history:Received 5 August 2012Received in revised form 20 November 2012Accepted 22 November 2012Available online 28 December 2012

Keywords:CFRPSteelDouble strap jointDurabilityEnvironmental effectsCuring

0950-0618/$ - see front matter � 2012 Published byhttp://dx.doi.org/10.1016/j.conbuildmat.2012.11.035

⇑ Corresponding author. Tel.: +61 3 9905 4987.E-mail address: yu.bai@monash.edu (Y. Bai).

a b s t r a c t

This paper examines the effects of curing conditions on the performance of steel/CFRP double strap jointssubjected to combined loading, cyclic temperature and humidity. A series of joints, cured respectively atan elevated temperature (120 �C) and room temperature (about 23 �C) were exposed to different com-bined environmental and loading conditions. It was found that curing at an elevated temperature hadno effects on the joint ultimate strength tested at room temperature. However, at 50 �C, the joints curedat 120 �C showed almost no strength reduction, in comparison to those cured at room temperature with a50% strength reduction. Curing at elevated temperature also helped to improve the time-to-failure ofjoints: joints cured at room temperature failed within less than 2 h when exposed to the environmentalconditions, while joints cured at 120 �C could survive up to 270 h, dependent on the applied load levels.Partial safety factors proposed in design guidelines for FRP composites under environmental effects wereused to evaluate the experimental results obtained, suggesting a very critical degradation of jointmechanical properties due to the environmental effects investigated in this study.

� 2012 Published by Elsevier Ltd.

1. Introduction

Carbon fibre reinforced polymer (CFRP) strengthening of struc-tures traditionally applied to concrete structures, and more re-cently applied to steel structures has been gaining increasinginterest. Research on the strengthening of steel structures withFRP, such as by Teng and Hu [1] and Song et al. [2] in axial com-pression, Haedir et al. [3] in bending, Jiao and Zhao [4] in tension,and Xiao et al. [5] on concrete filled CHS, have shown significantbenefits in strength and stiffness of steel members with externallybonded CFRP. However, as CFRP composites are externally bondedto steel members, the bond behaviour under service environmentsis, as a result, of great concerns.

Elsevier Ltd.

Significant reduction in strength and stiffness has been reportedfor FRP composites, when temperature goes over the glass transi-tion temperature (Tg) [6,7]. Furthermore, the mechanical proper-ties of structural adhesive between steel and CFRP compositessignificantly decrease when it is exposed to elevated temperatureseven below 60 �C [8]. Such a mechanical degradation of structuraladhesives always results in the failure of adhesively bonded steel/CFRP joints in this critical temperature range, as evidenced in [9]that strength decreased by 80% when temperature reaches only60 �C.

However in practice, 50–60 �C can be easily reached on struc-tural surface directly exposed to sunlight in a typical summer cli-mate, therefore to enhance the strength of adhesively-bondedsteel/CFRP systems at this elevated temperature range becomes acrucial task for potential engineering applications. Curing a struc-tural adhesive at a higher temperature than room temperature isone of the simple methods to improve the mechanical performance

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of adhesively bond joints, thanks to the enhancement of Tg of thestructural adhesive.

The improvement of Tg as a result of curing at an elevated tem-perature has been subjected to extensive studies. As reported in[10], the Tg of a neat epoxy resin, that had been cured at differenttemperatures and for different times at each temperature, wasexamined by means of dynamical mechanical analysis (DMA). It re-vealed that the Tg could be increased by 100 �C by increasing thecure temperature to 230 �C [10]. The glass transition temperature(Tg) of epoxy resin systems has been demonstrated as a functionof cure time and cure temperature [11–14]. In general, Tg increaseswith cure time toward an asymptote, and the rate at which the ap-proach occurs is temperature-dependent. This difference in Tg canbe explained by the effects of curing temperature on the curingreactions. Those effects directly influence the conversion (extentof curing), the average molecular weight and the cross-link density,which further dominate the final epoxy network structure and thethermal and mechanical properties, including Tg [11,15]. Duringthe curing process, chemical reactions occur, which transformslow molecular weight liquid epoxy resin (reactant) into a cross-linked polymeric product, and the corresponding material charac-teristic including Tg changes as well. It is well-known that at ahigher temperature, the reaction rate becomes higher. In addition,the average molecular weight of the polymer chains increases withcuring temperature due to the increase in the extent of reaction,therefore Tg is expected to increase [11]. Finally, curing at highertemperature will increase the crosslink density [11], which in turncontributes to the increase of Tg of the system. As a result, an epoxycured at higher temperature may achieve a high curing degreetherefore possess a higher Tg than that cured at a lowertemperature.

In addition, studies have also been done to investigate thestrength development of structural adhesives as a function of cur-ing temperature and curing time. Increasing in mechanical proper-ties was observed through curing at elevated temperatures. Forexample, curing at 60 �C for 1 h led to flexural strength and flexuralmodulus of glass fibre/epoxy composites higher than those curingat room temperature for 20 days [16]. An increase of bond strengthof a structural epoxy with a higher curing temperature at a con-stant curing duration was observed by Tu and Kruger [17] frompull-off tests. Matsui studied the effects of curing conditions onthe development of shear strength of an adhesive [18] and foundthat curing time was clearly related to the curing temperature toobtain the maximum shear strength. Although various studiesshowed strength development is dependent on curing time andtemperature, no quantitative relationship between them were pro-posed [19]. It seems that curing at a higher temperature leads to ahigher (or at least equal to) strength obtained with curing at roomtemperature. The implications of the results were that a fasterstrength evolution can be obtained if the epoxy is cured at a highertemperature. On the other hand, the epoxy cured at room temper-ature can achieve the same mechanical properties as cured at high-er temperature. This is likely a preferred curing method for CFRPcomposites used in civil engineering construction, as it reducescost and requires no sophisticated curing equipment.

It appears that an appropriate curing condition may also im-prove the long-term performance of steel/CFRP double strap jointssubjected to harsh environments such as combination of tempera-ture, humidity and loading. To date, however, information is verylimited in literature. Previous studies have addressed the effectsof elevated temperature, cyclic temperatures, ultraviolet, andmoisture on steel/CFRP double strap joints [20–22]. This includesa long term exposure to seawater at an accelerated temperatureof 50 �C. However, all the investigated joints were cured at roomtemperature. A relevant study on the environmental durability ofCFRP system for strengthening steel structures was reported by

Dawood and Rizkalla [23]. In this study steel/CFRP double strapjoints using unmodified SP Spabond 345 two part epoxy adhesivesubjected to 35% of their ultimate load could survive between 4and 17 weeks of exposure to 1 week wet/1 week dry cycling at atemperature of 38 �C. The midpoint glass transition temperatureof the cured adhesive was 62 �C but no detailed curing processwas provided and the effects of different curing processes werenot investigated. Recent studies in 2012 by Moussa et al. [24,25]examined the effects of curing conditions on the physical andmechanical properties of structural adhesives, and showed thatexposing the adhesive to temperatures above Tg did not result inany degradation of the material but, on the other hand, helped toimprove the mechanical properties of the material. These studiesfocused on the structural adhesive itself and the results may advicethat the improvement of mechanical properties of the adhesivemay result in better mechanical performance of adhesively-bondedjoints made using the same adhesive under a combination of envi-ronmental effects including moisture.

This paper bridges this gap of knowledge by examining the curingeffects on the mechanical performance of steel/CFRP double strapjoints exposed to harsh environments of cyclic temperature andhumidity. Those joints, after separately cured at two differentconditions including at normal room temperature and at anelevated temperature of 120 �C, were subsequently sustained at dif-ferent load levels in tension from 15% to 35% of the joint ultimateload measured at room condition, and exposed simultaneously todifferent harsh environments. The time-to-failure of the joints wererecorded during the environmental exposure. The joints, which didnot fail during the exposure, were tested to failure to determine theirresidual strength and failure modes. Therefore, the effects of loadingon joint mechanical performance can be evaluated and the enhance-ment through elevated temperature curing can be addressed.

2. Experimental investigations

2.1. Material properties

Tensile coupon CFRP specimens were prepared and tested according to ASTMD3039-08 to determine the tensile strength of the CFRP. The specimens were pre-pared by the wet lay-up method. The composites were formed by impregnating lay-ers of carbon fibre fabrics with epoxy resin Araldite 420. Each coupon had an overalllength of 250 mm and width of 15 mm. Dry CF 130 fibres were cut to precise sizeand brushed with adhesive Araldite 420. The thickness of the composite was con-trolled by pressing it using a heavy steel plate. Aluminium plates with dimensionsof 50 mm � 15 mm were sand blasted and cleaned with acetone prior to attachingat the two ends of the CFRP specimens for gripping purposes and to apply uniformload to the specimens. The prepared specimens were then cured at room tempera-ture for 7 days as recommended by the manufacturer. Tension tests were carriedout by Instron testing machine. Two strain gauges were attached to the centre ofeach side of the specimens to determine the strain (and therefore thus modulus)of the CFRP. The average tensile stress and ultimate strain were found as2300 MPa and 1.02% for CFRP laminates using CF 130 fibres. The average measuredelastic modulus was approximately 256 GPa.

Carbon fibre sheet (CF 130), manufactured by BASF, was bonded with epoxy tosteel plates used in the fabrication of the double strap steel joints. The steel elasticmodulus was 200 GPa, and the yield stress and ultimate stress were found as359 MPa and 430 MPa, respectively, from standard tension coupon tests accordingto AS1391 [26]. The epoxy adhesive used was two-part Araldite 420, manufacturedby Huntsman. This adhesives is suitable for bonding both steel and polymeric com-posites [27]. The epoxy adhesive has average tensile strength, elastic modulus andultimate strain of 32 MPa, 1.9 GPa and 0.024 respectively [28], whereas its averageshear strength and shear modulus are 25 MPa [27] and 1000 MPa [29], respectively.Tg of the epoxy was determined by the temperature at the intersect point of twotangent lines of the first and second stages of the storage modulus from DMA test.In this way, Tg was determined as 42 �C [20].

2.2. Test specimens

The steel plates with a dimension of 5 mm � 50 mm � 180 mm are prepared,prior to the fabrication of adhesive-bonded steel/CFRP joints. These preparations in-volved the processes of sand-blasting, as well as cleaning the area with acetone, inan attempt to remove oil, grease and rust. The removal of the latter three ensured

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better mechanical interlocking. CFRP was used to join two steel plates together andusing wet lay-up method. The steel/CFRP double strap joints were fabricated asshown in Fig. 1a. In other words, the epoxy was applied on each surface of the steel,followed by three layers of CFRP, and the same epoxy was applied between CFRPlayers to form the polymer matrices of the CFRP composite. The bond length (L)of the joint was chosen as 100 mm (see Fig. 1a). This bond length is larger thanthe joint effective bond length of 50 mm measured at room temperature [9] witha consideration of its potential increment with temperature. Some finished speci-mens were illustrated in Fig. 1b. Three layers of CFRP was chosen based on a previ-ous study conducted by Fawzia [28] because additional layers would notsignificantly increase the load carrying capacity of the joint [28].

Two different curing temperatures were employed for the joints: room temper-ature and an elevated temperature (120 �C). Specimens cured at room temperaturewere left at room temperature for 14 days before the environmental exposure (acuring time of 7 days at room temperature was suggested in the manufacturer’sspecification). Specimens cured at the elevated temperature were cured at 120 �Cfor 1 h based on the manufacturer’s recommendation, and then were left at roomtemperature for 14 days before the environmental exposure. It should be noted thatthe room temperature test results in [9] and [20–23] and ones in the current paperwere very similar. All specimens showed the ultimate strength as about 90 kN.

2.3. Test scenarios and environmental conditioning

A total of 28 specimens, comprising 14 different conditions, were tested (i.e. 2specimens per condition) and categorized into two scenarios, as follows (seeTable 1):

� Scenario 1 (S1): joints cured under two conditions were subjected to 0% (i.e.unloaded) and 35% of their ultimate load and simultaneously exposed to twodifferent temperatures of 20 �C and 50 �C (representing room temperatureand an elevated temperature that can likely be the extreme temperature insummers). In this scenario, RH was assumed to be in a range between 10%and 30%.� Scenario 2 (S2): joints cured under two conditions were sustained to different

load levels including 15%, 25% and 35% of their ultimate load, and simulta-neously exposed to cyclic temperature between 20 �C and 50 �C at constant rel-ative humidity of 90%. In each 60-min cycle, temperature was kept constant at20 �C for 25 min; it was then ramped up to 50 �C at a heating rate of 6 �C/min,and followed by 25 min of thermal soak at 50 �C; temperature was then cooledto 20 �C, at rate of �6 �C/min, to accomplish one thermal cycle.

For specimens that did not fail under sustained loading, they were left at roomtemperature for 7 days in a lab environment before the mechanical test. The tem-perature and RH corresponded to those (23 �C and RH in a range between 10%and 30%) in an indoor environment.

Fig. 1. (a) Schematic view (not to scale) and (b) finished specimens of steel/CFRPdouble strap joints.

2.4. Thermal and mechanical loading program

S1 specimens were preloaded to a certain load level as specified in Table 1 andmaintained constantly during the thermal loading program. Thermal loading proce-dure was applied on the joints through an environmental chamber (manufacturedby Instron with a capacity of 600 �C) that was fitted into a Shimadzu UniversalHydraulic Testing Machine (see Fig. 2) to allow the specimens to undergo a thermalsoaking. The temperature of the environmental chamber and the CFRP surface ofthe joints were measured using two thermal couples, with accuracy of ±1 �C. An-other thermal couple was inserted into the adhesive layer of a dummy specimen(which was identical to test specimens). Measurement of the temperature of thisadhesive layer was assumed to be the temperature of the adhesive layer of the test-ing specimens.

For S2 specimens, subjected to cyclic temperature and constant humidity, theload was applied through compression of springs in a steel frame (for the specimenswithout preload in conditions C7 and C8, their residual strength after the exposurewere measured as benchmark). The springs were specially designed and manufac-tured for this study by Whelan Springs Australia. Each spring can carry up to 35 kNin compression capacity, with the dimensions of 30 mm wire, 90 mm internaldiameter, 8.25 total coils and 350 mm long. The steel frame was designed and madefrom rigid box steel section (50 mm � 50 mm � 2.5 mm). The connections betweendifferent steel components were achieved through welding. The load levels wereprecisely measured by the displacement of the calibrated springs – a 0.7 mm short-ening of each spring result in a compression force of 1 kN. The joint specimens wereinstalled into the frame and loaded in tension by tightening the top nut. This actioncompressed the spring which in-turn imposed tension forces to the steel loadingrod. The tension forces from the steel loading rod were then transferred to the jointspecimens as the steel rods were connected to the joint specimens by bolts throughconnection holes (40 mm in diameter) at one end of each joint specimen, while theother end was held fixed in position also by bolts (see Figs. 1 and 3a).

Such a loaded system was then moved into a Weiss C1500 environmentalchamber as shown in Fig. 3b. A relative humidity (RH) of 90% was provided bythe environmental chamber, in which the temperature and relative humidity areprecisely controllable (Fig. 3b). The chosen thermal cycle between 20 �C and 50 �Ccould represent the lowest and highest temperatures within a summer’s day insome hot climate countries such as Australia. In colder climates, freeze thaw cyclingshould be considered. However, simulation of such an environment is not withinthe scope of the current research.

Each S2 specimen was equipped with two strain gauges attached to the centreof the joint on each face (see Fig. 1a and b). The strain gauges were protected frommoisture and temperature effects by 3 layers of protective chemical coating (M-Coat D) and 2 layers of protective wax coating (see Fig. 1). The strain gauges wereconnected to a data taker to record the joint strain during the exposure.

In both scenarios, some specimens failed during the exposure. The time-to-fail-ure of the failed specimens was, therefore, defined as the time duration from thestart of the exposure to when the specimens failed.

3. Experimental results and discussion

3.1. Curing effects on ultimate strength of joints (C1–C4)

Fig. 4a presents the ultimate load of the steel/CFRP double strapjoints cured at two different curing schemes: at 20 �C for 14 daysand 120 �C for 1 h and then 14 days at room temperature (see con-ditions C1 and C2 in Table 1). The joints were then tested to failureto determine their ultimate strength. The figure clearly indicatesthat for both curing schemes, the ultimate loads of the joints(which were tested at room temperature) were about the same(i.e. around 90 kN). Curing at an elevated temperature did not im-prove the ultimate strength of the joints (tested at room tempera-ture). A similar result was observed by Lapique and Redford [30] inwhich the mechanical properties of a commercial epoxy adhesive(Araldite 2014) such as viscosity, strength and stiffness increasedwith time when cured at room temperature. It was found thatspecimens cured at 23 �C for 28 days and specimens cured at64 �C for 4 h had the same tensile strength.

Fig. 4b presents the ultimate load of the joints tested at 50 �C(C3 and C4 in Table 1). There was a clear difference between jointscured at room temperature and those cured at the elevated tem-perature. The latter maintained 100% of their strength at 50 �C(i.e. 90 kN) in comparison to the strength of joints tested at roomtemperature (i.e. 90 kN), while the former maintained only about50% (i.e. 43 kN). Curing at elevated temperature therefore im-proved the joint load carrying capacity at elevated temperatures

Table 1Test matrix of steel/CFRP double strap joints subjected to different combinations of elevated temperatures, humidity and loading.

Scenario Condition Curing conditiona Applied load (%) Temperature (�C) RH Exposure or failure time (h)b

S1 C1 RT 0 20 N/A N/AC2 ET 0 20 N/A N/AC3 RT 0 50 N/A N/AC4 ET 0 50 N/A N/AC5 RT 35 50 N/A 0.5 (failed)C6 ET 35 50 N/A 2

S2 C7 RT 0 20–50 90% 300C8 ET 0 20–50 90% 300C9 RT 15 20–50 90% <2 (failed)C10 ET 15 20–50 90% 270 (failed)C11 RT 25 20–50 90% <2 (failed)C12 ET 25 20–50 90% 44 (failed)C13 RT 35 20–50 90% <2 (failed)C14 ET 35 20–50 90% 12 (failed)

a RT means room temperature and ET means elevated temperature.b Exposure time for C6–C8, and failure time for C5 and C9–C14.

Fig. 2. Specimen setup and instrumentation for Scenario S1.

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(i.e. 50 �C in this case). The increase in performance of the joints at50 �C can be attributed to a higher glass transition temperature re-sulted from curing at a higher temperature.

Fig. 3. (a) Loading frame and (b) environmental chamber for C7–C14 in Scenario S2.

3.2. Curing effects on time-to-failure of loaded joints (C5 and C6)

Structural adhesives or FRP composites demonstrate a time-dependent degradation when exposed to a constant temperaturein an elevated range, which may induce failure if the exposure timeis sufficiently long. Particularly, the strength of steel/CFRP doublestrap joints, having the identical joint configuration and the adhe-sive as in the present study, demonstrated to be time-dependent[20]. In that study, the steel/CFRP joints, cured at room tempera-ture only, were tensioned to 20% of their ultimate load and thenexposed to increasing temperature (from room temperature tothe target of 50 �C with a heating rate of 2 �C per minute). It wasfound that the joints failed after 32 min, only 2 min after the adhe-sive layer reached 50 �C. In the present study, to examine the ef-fects of curing at an elevated temperature on the time-to-failureof the joints, joints cured at room temperature and 120 �C weretensioned to 35% of their ultimate load (i.e. 31.5 kN) while sub-jected to increasing temperature from room temperature to thetarget 50 �C. It was found that those joints, subjected to conditionC5, failed after 29 min of exposure. However, the joints subjectedto condition C6 did not fail after 120 min of exposure. More inter-estingly, the residual strengths of the joints that survived after C6

exposure were found as 100% (i.e. 87 kN and 91 kN) of the ultimateload compared with the control specimens in C1 and C2. This resultagain demonstrates that curing at a higher temperature enhancedthe performance of the joints at elevated temperatures.

3.3. Curing effects on ultimate strength of unloaded joints withenvironmental impacts (C7 and C8)

The joints subjected to conditions C7 and C8 in Table 1 weretested at room condition after the exposure of 300 h (i.e. 300 ther-mal loading circles). The results obtained were presented in Fig. 4c.For both of those cured at room and elevated temperatures, thejoint ultimate load decreased, after the exposure time, by about

Fig. 4. Ultimate loads of unloaded joints cured at different temperatures, tested at (a) 20 �C (i.e. conditions C1 and C2 in Table 1) and (b) 50 �C (i.e. conditions C3 and C4 inTable 1) and (c) residual strength of unloaded joints cured at different temperatures, tested at room temperature after exposure to cyclic temperature and humidity (i.e.conditions C7 and C8 in Table 1).

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10–15% compared with the unconditioned specimens (conditionsC1 and C2). A similar result for both cases demonstrates that thecuring has no significant enhancement on the residual strengthof the joints after exposure to cyclic temperature between 20 �Cand 50 �C at a constant 90% relative humidity. In general, the lossof joint strength was due to either the degradation of the adhesiveor the weakening of the adhesive/adherend interface or both. How-ever, in this particular study, by examination of the failure mode ofthe joints (see Fig. 5a) subjected to conditions C7 and C8 (i.e. withno load was applied), the hypothesis of degradation of the adhe-sive/adherend interface was eliminated because the joints failedby CFRP delimitation rather than interface failure. Visual inspec-tion indicates that there was no corrosion of the steel in thebonded area (Fig. 5b), and the fibres still remained on the steelsurface.

3.4. Effects of curing and load level on time-to-failure of loaded jointswith environmental impacts (C9–C14)

All specimens from conditions C9–C14 (see Table 1) subjectedto mechanical loading (15–35%) failed within a relatively shortperiod during conditioning – ranging from 2 to 270 h, dependingon the load level and curing condition of the joints (see Fig. 6). Thismay be due to the polymers (both adhesive and resin of CFRP)being subjected to high strains. These stretched polymer moleculesfacilitate a higher degree of temperature and moisture penetration.This high degree of humidity and temperature penetration will

Fig. 5. (a) Typical failure mode of unloaded steel/CFRP double strap joints subjectedto combined loading, thermal cycling and constant humidity and (b) enlargedfailure area.

split polymer molecules from cross-linked structure causing a sud-den failure [31].

Visual inspection on the failure mode revealed that the failureoccurred in the adhesive layer of the joints with partial fibresremaining on the steel (see Fig. 6a and b). The failure mode chan-ged from CFRP delimitation (for unloaded joints in C7 and C8, seeFig. 5) to adhesive/adherend interface failure (for loaded joints inC9–C14, see Fig. 6). Such a change of failure modes demonstratedthat the moisture was accumulated and accelerated by the appliedloads. Theory of adhesion by physical adsorption is further justifiedin explaining the failure in the exposed joints. After the failure oc-curred, the steel surface was quickly corroded due to a highlymoisturised environment as shown in Fig. 6.

Fig. 7 presents the time-to-failure (as defined in Section 2) ofthe joints from C9–C14 under different combined load levels andcuring conditions. As can be seen from the figure, in general,

Fig. 6. Typical failure mode of loaded steel/CFRP double strap joints subjected tocombined loading, thermal cycling and constant humidity: (a) failure duringexposure and (b) a closer look at failure area.

Fig. 7. Time-to-failure of steel/CFRP double strap joints subjected to mechanical load, thermal load and humidity from (a) C9, C11, C13 and (b) C10, C12, C14.

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specimens sustained at a higher load level failed in a shorter timethan those at lower load levels. It is worth pointing out that thejoints, either cured at room temperature or elevated temperature,failed after a number of thermal cycles, with an applied load rang-ing from 15% to 35% of the load capacity of the unconditioned spec-imen. However, the residual joint capacity after the environmentaleffects alone (i.e. without applied load during exposure in C7 andC8) remained to be 90% of the initial strength (see Fig. 4). There-fore, the effect of the sustained load was of great significance interms of degradation of joint capacity in a combined temperatureand humidity environment, and a higher level of sustained loadconsiderably reduced the joint time-to-failure.

The curing effect was clearly shown by comparing Fig. 7a and b.The joints cured at room temperature, regardless of the level of ap-plied load, failed shortly (less than 2 h) during the environmentalexposure. On the other hand, the joints cured at elevated temper-ature could sustain 15% of the unconditioned specimen’s ultimateload for 270 h. With a higher applied load of 25% and 35%, thejoints could survive for 44 and 12 h, respectively. These durationswere much longer than those of the joints cured at room temper-ature, showing the benefits of curing at the elevated temperature.In addition, the applied load versus time-to-failure of joints curedat the elevated temperature extended more significantly when theapplied load was lower. On the other hand, time-to-failure of jointscured at room temperature appeared to almost linearly decreasewith increasing load, although the failure occurred in a relativelyshort time (less than 2 h).

Fig. 8. Creep behaviour of time-dependent strain curves during exposure in Scen

3.5. Curing effects on creep behaviour of joints with environmentalimpacts

Fig. 8 shows the strain measured at centre of the joint over timefor the joints from C10 and C14 (during exposure). The strains werefound to increase over time until sudden failures occurred for allthese joints. The obvious trend of the creep behaviour from C10was illustrated in Fig. 8a for the joints cured at elevated tempera-ture. This strain versus time curve is consistent with the generalshape of the creep curve of most metals, polymers, ceramics andcomposites. Such a general shape was composed of three stages:primary creep, steady-state creep and tertiary creep. The primarycreep strain occurred fairly quickly; after that the strain increasedsteadily with time and this stage is normally called steady-statecreep. The relationship of steady creep rate and applied stress nor-mally follows power law while that of steady creep rate and tem-perature generally complies with Arrhenius kinetics [32]. Duringcreep, damage, in the form of internal cavities accumulated. Whenthe damage first appeared, tertiary stage of the creep curve startedand grew at a sudden increase of rate [32].

Within each thermal cycle, there was a small fluctuation instrain due to the cyclic thermal loading (not visible in Fig. 8a be-cause of a long time scale). The fluctuation in strain within eachthermal cycle is clearly shown in Fig. 8b for the joints from C14.The time intervals between two peak strain values were approxi-mately 60 min, which corresponds to the time to complete onethermal circle from 20 �C to 50 �C and back to 20 �C again (see

ario S2 for joints cured at an elevated temperature from (a) C10 and (b) C14.

Fig. 9. Creep behaviour of time-dependent strain curves during exposure in Scenario S2 for joints cured at an elevated temperature from (a) C9, (b) C11 and (c) C13.

Table 2Safety factors from British guideline [33].

Safety factor Parameter value Partialfactor

Environmental factor, cme Adhesive properties determined for theenvironmental conditions in service

1.0

Adhesive properties determined forenvironmental conditions differentfrom service conditions

2.0

Time-related factor, cmt Long-term loading 2.0Short-term loading 1.0

Table 3Safety factors from Italian guideline [34].

Safety factor Parameter value Partialfactor

Environmental factor, ga Interior exposure conditions 0.95Exterior conditions 0.85Aggressive environmental conditions 0.85

Long-term factor, gl Continuous (creep and relaxation) 0.80Fatigue 0.50

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Section 2.3). A three-stage development of strain over time is notobvious in Fig. 8b, because the exposure was relatively short(12 h) and thus the general strain versus time curve was greatly af-fected by the thermal cycles. It is noted that due to some technicalproblems the strain versus time curves for C12 specimens with atime-to-failure of 44 h was not retrieved.

In contrast to the creep behaviour of joints cured at elevatedtemperature in Fig. 8, the effect of loading was not as clearly shownfor the joints cured at room temperature as presented in Fig. 9. Allroom temperature cured joints showed similar creep behaviours asthey undergone the first two thermal cycles within 2 h. Due to lim-ited number of thermal cycles and time, distinguished effects (ifany) of different applied load levels could not be identified. Itshould be noted that all joints showed an initial constant strainand a decrease in strain at 0.7 h of the start of the exposure. Thiswas due to the initial time needed by the environmental chamberto stabilise the initial temperature at around 20 �C to 23 �C to beginthe first thermal cycle.

4. Safety factors considering mechanical degradation due toenvironmental effects

Although a design standard has not been established yet forsteel structural strengthening using adhesively-bond CFRP com-posites, a few design guidelines have been published to providethe partial safety factors considered for material resistance of lam-inated FRP composites in strengthening steel structures [33,34] orlaminated composites fabricated manually by wet lay-up method[35], although the latter was not specifically developed for steelstructures strengthening using composites. The partial factors forenvironmental durability from the British guideline [33], Italianguideline [34] and EuroComp [34] are given in Tables 2–4 (notethat safety factors which are greater than unity indicate they aredivisors, while those less than unity indicate they are multiplier).Despite of a limited number of tested joints, the results of jointultimate strength obtained from this study could be used for an

exploratory evaluation of the partial safety coefficients proposedin those guidelines.

According to the British guideline [34], for the steel/CFRP sys-tem which has the properties of the adhesive determined fromthe laboratory condition different from service conditions (as inthe present study), the environmental factor, cme, is 2.0. Long termcmt of 2.0 should be used because the specimens were placed insustained load during the long exposure. Therefore, the corre-sponding residual strength after long term degradation from thisguideline is 25%. This prediction is fortunate to ensure a relativelysafe design value of the elevated temperature cured joint strengthoperating at 50 �C without the presence of humidity, because thejoints in C6 did not fail at 50 �C while sustaining 35% (i.e. greaterthan the threshold safety load level of 25% of the ultimate load sug-gested by this guideline) and their residual strength was 100% ofthe unconditioned specimens. However, with the presence ofhumidity, British guideline safety threshold was not sufficient forthe steel/CFRP joints. Although it suggested 25% of the uncondi-tioned specimen’s ultimate load as threshold safety load level, alljoints in C9–C14 failed while sustaining only 15% ultimate load le-vel (less than 25%).

According to Italian guideline [35], the environmental safetyfactor, ga, of 0.85 and long-term factor, gl, of 0.8 should be used be-cause of the aggressive environments as presented in the presentstudy. This safety factor corresponds to a degradation to 68% whichis insufficient for all the joints exposured to different load andenvironmental conditions in S2 and the joints cured at room tem-perature in S1. It remained inconclusive for the joints cured at theelevated temperature in S1 due to a limited number of tests.

According to EuroComp manual [35], because the specimens inthe current study were wet lay-up fabricated and not fully cured atroom temperature, cM,2 should be adopted as 2.0; cM,3 was taken as3.0 for long-term loading. The effects of cM,1 are not included sinceonly experimental (not design) values are involved in. The result-ing partial safety factors (cM,2 � cM,3) devoting to manual fabrica-tion and environmental effects is 6.0. This corresponds to astrength degradation of 16.67%. Similar to British and Italian stan-dards, this prediction is not prediction for all joints in S2 (which

Table 4Safety factors from EuroComp [35].

Partial factor parameter Parameter value Partial factor

Characteristics of materials or components, cM,1 If the target characteristic is obtained from tests 1.15If the target property is derived from theory 2.25

Uncertainty of the materials and the production process, cM,2 If the composite is fabricated by wet lay-up method and fully cured at work, or 1.4Not fully cured at work 2.0

Environmental effects and duration of loading Short-term 1.2Long-term 3.0

906 T.-C. Nguyen et al. / Construction and Building Materials 40 (2013) 899–907

failed at load level as low as 15%) while remained inconclusive forjoints in S1 since there were no load levels below 16.67% employedin the experimental program in the study.

It should be noted that this result was obtained based on thesteel/CFRP bonded system using two-part epoxy adhesive Araldite420. It may vary from case to case if different adhesives, specimenpreparation procedures, or different environmental conditionswere applied. For example, as mentioned earlier, the time-to-fail-ure of steel/CFRP double strap joints using an epoxy adhesive (withtensile modulus and ultimate strength of 2980 MPa and 38 MPa,respectively) survived between 4 and 17 weeks of exposure to1 week wet/1 week dry cycling at temperature of 38 �C in a studyconducted by Dawood and Rizkalla [23], while in the present studywith an epoxy adhesive (with tensile modulus and ultimatestrength of 1900 MPa and 32 MPa), the steel/CFRP joints failedwithin between 2 h and 270 h.

A structural adhesive with better performance in terms of tem-perature and moisture resistance is expected for such steel/CFRPapplications under harsh environments. To improve the thermalresistance of epoxy based system, stable epoxy co-reactants orhigh temperature curing agents may be incorporated into theadhesive. It was reported that the excellent thermal stability ofthe phenolic resins coupled with the adhesion properties of epoxycan provide an adhesive capable of withstanding 177 �C long termoperation [31]. Similarly, curing agents, such as phthalic anhy-dride, pyromellitic, dianhydride, and chloriendic anhydride, giveepoxy adhesives greater thermal stability which can operate attemperatures more than 149 �C [31]. However, as it is evident fromthis study, a general guideline may not be universally applicablefor all types of adhesives. Understanding the bonding performanceunder aggressive environments to which the steel/CFRP structuresare to be exposed remains a crucial task of designers.

5. Conclusions

Steel/CFRP double strap joints were subjected to two differentcuring conditions: cured at room temperature and cured at ele-vated temperature (120 �C) and then exposed to different combi-nations of mechanical load, thermal load and humidity toexamine their mechanical performance. The following conclusionscan be drawn:

(1) Curing at elevated temperature did not help to improve thestrength of steel/CFRP joints which were tested at room con-dition. However, when tested at an elevated temperature of50 �C, the elevated temperature cured joints showed signif-icant strength improvement, attaining almost twice strengthof those cured at room temperature.

(2) Mechanical loading caused significant effects on the jointstrength degradation under an aggressive environment ofcombined cyclic temperature (between 20 �C and 50 �C)and a constant 90% relative humidity. Without mechanicalload that was applied to the joints, the joint strengthremained about 90% of their initial strength after the expo-

sure to this harsh environmental condition. When the jointscured at elevated temperature were subjected to 15% of theirultimate and exposed to the same condition, failure occurredafter 270 h; and this duration time was shortened to 44 hand 12 h when the applied load level increased to 25% and35% respectively. Meanwhile, all joints cured at room tem-perature failed within 2 h of exposure when a load (15%,25% or 35%) was applied.

(3) It was therefore evidenced that the time-to-failure of thejoints under sustained load and exposed to environmentalcondition were dependent on the load level and curing con-dition. Lower load level or higher curing temperatureresulted in a longer time-to-failure. The curing effectsbecame more significant in terms of the improvement oftime-to-failure for a lower applied load level. A decrease ofthe applied load from 35% (of the joint capacity) to 15% couldincrease the time-to-failure from 12 h to 270 h for jointscured at elevated temperature, while the time-to-failureremained almost the same for joints cured at roomtemperature.

(4) The failure mode changed from CFRP delimitation to a com-bination of adhesive layer failure and CFRP delimitationwhen mechanical loading was applied to the joints whileexposed to cyclic temperature and humidity. Such a changewas possibly due to the stretch of adhesive molecules, whenthe load was applied, facilitating a higher degree of temper-ature and moisture penetration to the adhesive layer.

(5) The British guideline, Italian guideline, and EuroCompdesign manual were found to be insufficient in safety predic-tion of the joints in the current study, suggesting that a gen-eral guideline may not be universally applicable for all typesof adhesives. Enhancement of structural adhesives underaggressive environments to which the steel/CFRP structuresare exposed to and understanding the corresponding bondperformance remain critical tasks.

Acknowledgments

The authors wish to acknowledge the financial support pro-vided by the Australian Research Council through the ARC Discov-ery Scheme and Monash University through Monash GraduateScholarship. The tests were conducted in the Civil Engineering Lab-oratory at Monash University. Thanks are also given to Mr. LongGoh and Mr. Kevin Nievaart for their assistance. The second authorDr. Yu Bai is the recipient of the Australian Research Council Dis-covery Early Career Researcher Award.

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