Evaluation of Mechanical Properties of Reinforced Poplar Laminated Veneer Lumber

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1 Evlution of Mechnicl Properties of Reinforced Poplr Lminted Veneer Lumber Jie Wng, Xiolei Guo, Wei Zhong, b Huiyun Wng, nd Pingxing Co, * Three types of reinforcement mterils, crbon fiber-reinforced polymer (CFRP) sheet, glss fiber-reinforced polymer (GFRP) mesh, nd composite of the CFRP sheet nd GFRP mesh, were used to reinforce poplr lminted veneer lumber (LVL), nd the multi-step hotpressing method ws lso pplied. The mechnicl properties, i.e., modulus of rupture (MOR), modulus of elsticity (MOE), nd horizontl sher strength (HSS), of the reinforced LVL were investigted, s well s the effects of ly-up loction of the CFRP sheet/gfrp mesh composite. The results indicted tht pplying the multi-step hot-pressing method nd incorporting the CFRP sheet, GFRP mesh, nd the CFRP sheet/gfrp mesh composite noticebly improved the MOR nd MOE under horizontl nd verticl lodings. Only the multi-step hot-pressing method ws ble to gretly improve the HSS of reinforced LVL under both loding modes. The improved effect of the three kinds of reinforcing mterils on the mechnicl properties ws ordered s follows: CFRP sheet/gfrp mesh composite > CFRP sheet > GFRP mesh. Locting the CFRP sheet/gfrp mesh composite closer to the surfce veneer lyer yielded the best mechnicl properties for the reinforced poplr LVL. Keywords: Poplr lminted veneer lumber; Crbon fiber sheet; Glss fiber mesh; Multi-step hot pressing method; Mechnicl properties Contct informtion: : Fculty of Mteril Science nd Engineering, Nnjing Forestry University, Nnjing , Chin; b: Gungzhou Homebon Timber Mnufcturing Co., Ltd., Gungzhou , Chin; *Corresponding uthor: copx@njfu.com.cn INTRODUCTION In Chin, becuse of the serious shortge of high-qulity timber from nturl forests, poplr is widely plnted nd fst-growing species tht hs become one of the min resources in the wood industry (Xu 2014). However, its loose mteril nd low mechnicl properties limit its ppliction in structurl products. Developing the dvnced technology to improve the performnce of poplr products nd to expnd the scope of the ppliction of poplr products re two very importnt gols. Lminted veneer lumber (LVL) is wood product similr to plywood, but, unlike plywood, is prllel-lminted veneer (Lufenberg 1983; Wei nd Zhou 2012). Becuse of its excellent performnce properties, such s good dimensionl stbility, uniform strength, dptbility to engineering, free size design, nd suitbility for utomtic production, LVL is high-qulity product tht hs been used in both nonstructurl nd structurl pplictions, such s the flooring industry, the furniture industry, pckging, nd construction (Sski 2001; Anonymous 2009; Mei nd Zhou 2009; Tenorio et l. 2011; Wei nd Zhou 2012). However, becuse of the poor performnce properties of poplr, poplr LVL is rrely used s structurl timber. Wng et l. (2015). Mech. Prop. Reinf. LVL, BioResources 10(4),

2 Reinforcing technology is fesible nd effective method for improving the mechnicl properties of LVL mde of low-qulity, fst-growing wood. As result, reinforced LVL cn be n lterntive to high-qulity solid wood timber. Over mny yers, vrious studies hve been conducted regrding the reinforcement of LVL. Lufenberg et l. (1984) nlyzed the economic fesibility of reinforcing LVL with synthetic fibers. The evlution indicted tht there were gret dvntges in using glss fiber to reinforce LVL. Pirvu et l. (2004) investigted the interfce properties of crbon FRP-wood hybrid composite nd the mechnicl nd physicl properties retention of crbon/vinyl ester composite fter preservtive tretments. Wei et l. (2013) studied the modulus of rupture nd the modulus of elsticity of poplr LVL reinforced by crbonfiber-reinforced polymer (CFRP) in two different configurtions. The results showed tht poplr LVL reinforced by single lyer of CFRP on one side hd higher bending strength thn tht reinforced by single lyer of CFRP on ech side. Buell nd Sdtmnesh (2005) reserched the effects of crbon fiber fbric on the strengthening of timber bridge bems to increse their lod cpcity. The results indicted tht the stiffness, bending, nd sher strength of bems reinforced by crbon fiber fbric showed greter increses when compred with the un-reinforced bems nd concluded tht crbon fiber fbric could reduce the effects of wood defects on the mechnicl properties of the bems. Kim et l. (2010) investigted the influence of vrious CFRP properties on the behvior of the vrious timber species nd found tht the energy bsorption cpcity of the reinforced bems were greter thn those of the non-reinforced bems. Zhou et l. (2015) studied the influence of moisture on the performnce of the entire CFRP-wood composite system nd found tht the strength of the interfce determined the mechnicl properties of the CFRP-wood composite nd tht wter bsorption ws very importnt for the durbility of the entire CFRP-wood composite system. Bl (2014) studied the effects of woven glss fiber on the mechnicl properties of reinforced LVL. The study found tht mking use of woven glss fiber to reinforce LVL hd some positive influences nd some negtive influences on the mechnicl properties. Wng et l. (2011) introduced n investigtion to reinforce LVL with rmie fiber. The study indicted tht using rmie fiber to enhnce LVL could improve the mechnicl properties of LVL, especilly the level of sher strength. Zhng nd Hu (2010) studied the influence of the ly-up position of glss fiber mesh on improvements in the mechnicl properties, nd three useful methods to predict the MOE of reinforced LVL were tested. Rftery nd Hrte (2011) found tht the resonble usge of GFRP to enhnce low-grde glued lminted timber could improve the stiffness nd the ultimte moment cpcity, nd the utiliztion of compression strength ws incresed. Hu et l. (2010) investigted the effects of vrious fctors on the mechnicl properties of LVL reinforced by metl mesh, showing tht the ly-up position nd corseness of the metl mesh gretly effected MOE nd the type nd ly-up position of the metl mesh significntly influenced the MOR. In ddition, mny other methods of reinforcing LVL hve been studied, such s veneer densifiction using impregntion, nd dding bmboo veneer into LVL. All of these methods hve been shown to be effective t reinforcing the mechnicl properties of LVL (Zhu et l. 2005; Liu et l. 2007; Liu et l. 2009; Hller et l. 2015). Although mny studies hve been done to investigte the methods of reinforcing LVL, pplying the composite of the crbon sheet/glss fiber mesh nd multi-step hot pressing method to reinforce LVL hs not been widely reported. Furthermore, reserching the reinforcement effect of different reinforcement mterils on improving LVL mechnicl properties is very importnt from the perspective of designing high- Wng et l. (2015). Mech. Prop. Reinf. LVL, BioResources 10(4),

3 qulity LVL with low cost. The ims of the present study were s follows: 1) to investigte the effects of the inclusion of crbon fiber sheet/glss fiber mesh composite, crbon fiber sheet, nd glss fiber mesh on the mechnicl properties of reinforced poplr LVL; 2) to compre the reinforcement effects of these three reinforcement mterils on poplr LVL; 3) to study the influence of the multi-step hot-pressing method on the mechnicl properties of reinforced poplr LVL; nd 4) to study the effects of ly-up position of the reinforcement crbon fiber sheet/glss fiber mess composite on the mechnicl properties of reinforced poplr LVL. EXPERIMENTAL Mterils nd Specimens The poplr veneers were provided by Pece Wood Co., Ltd., locted in Xuzhou, Jingsu Province, Chin. The dimensions of the poplr veneers were 1220 mm (length) 1220 mm (width) 3.2 mm (thickness), the verge moisture content ws pproximtely 7.8%, nd the verge density ws g/cm 3. A phenol formldehyde dhesive (PF), provided by Dyne Gungdong Co., Ltd., ws chosen to glue the lminted veneer lumber (LVL); its solids content ws 43.52%. Alkli-free glss fiber (E-glss fiber) mesh ws provided by Hejin Deliyun Fiberglss Products Co., Ltd. The E-glss fiber mesh size ws 4 mm 4 mm, nd its unit weight ws pproximtely 150 g/m². A pn-crbon fiber sheet, whose specifiction ws 1 k, ws chosen s one of the reinforcement mterils; this ws provided by Liso Composite Mteril Technology Co., Ltd. A silne coupling gent of grde A-1100, whose mss frction ws 97%, ws selected for the surfce tretment of the crbon fiber sheet nd glss fiber mesh. All reinforced poplr LVLs were mnufctured on multi-lyer hot-pressing press mchine (BY214*8/6-15, Chin) t Gungzhou Homebon Timber Mnufcturing Co., Ltd. Methods Reinforced poplr LVL mnufcturing process The reinforced LVL ws fbricted s follows. First, crbon fiber sheets nd glss fiber meshes were soked in silne coupling gent queous solution for 15 min. The mss frction of the silne coupling gent queous solution ws 2%. Second, the crbon fiber sheets nd glss fiber meshes were dried in n electrotherml constnt temperture drying cbinet t 120 C for 20 min. Third, PF ws spred over the poplr veneer with roller. Fourth, the nine-lyer poplr veneers spred with PF, the crbon fiber sheets, nd the glss fiber meshes were lminted prllel to the grin direction. The poplr veneers were lminted in ccordnce with the rule of fce-to-fce nd bck-to-bck. Fifth, the composite ws pre-pressed in cold press for 30 min to llow prt of the PF to penetrte into the poplr veneer. The pre-pressing ws lso used to decrese the totl thickness of the composite, which would help position the composite in the hot-pressing mchine. Sixth, the composite ws hot-pressed t 140 C until the PF hd cured. For both processes, the veneer resin content of ech side ws 160 g/m 2. The mnufcturing prmeters used when reinforcing poplr LVL vi the multi-step hotpressing method re shown in Tble 1. For the reinforcement of poplr LVL with the glss fiber mesh nd crbon fiber sheet, the mnufcturing prmeters were estblished s follows: pre-pressing pressure of 1.0 MP, hot-pressing pressure of 1.2 MP, hotpressing time of 100 s/mm, nd hot-pressing temperture of 140 C±5 C. Wng et l. (2015). Mech. Prop. Reinf. LVL, BioResources 10(4),

4 The dimensions of the products were 1220 mm (length) 610 mm (width) 24 mm (thickness). A totl of six types of reinforced poplr LVL nd one type of nonreinforced poplr LVL ( control LVL) were produced. The control LVL nd LVL reinforced by reinforcing mterils were mnufctured with one-step hot pressing method. The dimensions of the specimens were in ccordnce with the Chinese Ntionl Stndrd GB/T (2006). Tble 1. Pressing Prmeters of Multi-Step Hot-pressing Method Pressing stge Pressing pressure (MP/m²) Pressing temperture ( C) Pressing time Pre-pressing 1.0 mbient temperture 30 min First stge of hotpressing Second stge of hotpressing ±5 100 s/mm ±5 80 s/mm The type of poplr LVL reinforced by multi-step hot pressing method (LVL-MH) just consisted of nine-lyer poplr veneers. The mnufcturing process when using the multi-step hot pressing method to reinforce the poplr LVL nd the structure of this type of reinforced LVL re shown in Fig. 1. Poplr veneer LVL-MH Fig. 1. The mnufcturing process for the multi-step hot pressing method All types of LVL reinforced by reinforcement mterils tht were used in the study re shown in Fig. 2. The first type just consisted of nine-lyer poplr veneers, nd this type of LVL ws mnufctured s control (LVL-CG) in this study (Fig. 2A). The second type (LVL-CS+GM) ws mde of nine-lyer poplr veneers, eight-lyer CFRP sheets, nd eight-lyer GFRP meshes, the CFRP sheet/gfrp mesh composite for LVL- CS+GM ws lid up between ll veneers symmetriclly (Fig. 2B). The third type (LVL- GM) ws composed of nine-lyer poplr veneers nd eight-lyer GFRP meshes, the GFRP mesh for LVL-GM ws lid up between ll veneers (Fig. 2C). The fourth type (LVL-CS) ws composed of nine-lyer poplr veneers nd eight-lyer CFRP sheets, the CFRP sheet for LVL-CS ws lid up between ll veneers (Fig. 2D). The fifth nd sixth types (LVL-SD nd LVL-CD) both consisted of nine-lyer poplr veneers, two-lyer CFRP sheets, nd two-lyer GFRP meshes; the reinforcement of the CFRP sheet/gfrp mesh composite for LVL-SD ws lid up ner to the surfce veneer symmetriclly (Fig. 2E); the reinforcement of the CFRP sheet/gfrp mesh composite for LVL-CD ws lid up ner to the core veneer symmetriclly (Fig. 2F). Wng et l. (2015). Mech. Prop. Reinf. LVL, BioResources 10(4),

5 b c c b (A) LVL-CG (B) LVL-CS+GM (C) LVL-GM b c b c (D) LVL-CS (E) LVL-SD (F) LVL-CD Fig. 2. Six types of poplr LVL for construction: (A), (B), (C), (D), (E), nd (F); () poplr veneer, (b) CFRP sheet, nd (c) GFRP mesh In ddition to the control group LVL-CG, six other types of reinforced LVL were worked s the experiment groups. LVL-MH ws mde for studying on the effect of multi-step hot pressing method on improving mechnicl properties of poplr LVL. LVL- CS+GM, LVL-GM, nd LVL-CS were mde for reserching on the improvement effect of different kinds of reinforcement mterils on mechnicl properties for reinforced poplr LVL. LVL-SD nd LVL-CD were mde in order to study the improvement effect of different ly-up loctions of reinforcement mterils on mechnicl properties for reinforced poplr LVL. Mesurement of mechnicl properties The mechnicl properties exmined in this reserch included the modulus of rupture (MOR), modulus of elsticity (MOE), nd horizontl sher strength (HSS). For ech mechnicl property, both the horizontl loding nd the verticl loding modes were tested. Thus, this reserch investigted the vlues of MOR, MOE, nd HSS under horizontl loding (MORH, MOEH, nd HSSH) nd the vlues of MOR, MOE, nd HSS under verticl loding (MORV, MOEV, nd HSSV). Three replictes were used, yielding totl of 21 LVL billets. Testing specimens were cut from ech billet ccording to the Chinese Ntionl Stndrd GB/T (2006). The dimension of specimens for MORH nd MOEH ws 600 mm 25 mm 25 mm (Length Width Thickness). The dimension of specimens for MORV nd MOEV ws 600 mm 90 mm 25 mm (Length Width Thickness). The dimension of specimens for HSSH ws 150 mm 25 mm 25 mm (Length Width Thickness). The dimension of specimens for HSSV ws 150 mm 40 mm 25 mm (Length Width Thickness). The mechnicl properties were mesured using wood electronic universl testing mchine (WDW-100D, Chin). The methods used to mesure the mechnicl properties were in ccordnce with the Chinese Ntionl Stndrd GB/T (2006). Wng et l. (2015). Mech. Prop. Reinf. LVL, BioResources 10(4),

6 RESULTS AND DISCUSSION Anlysis of the Effects of Multi-Step Hot-pressing Method on the Mechnicl Properties of Reinforced Poplr LVL Tble 2 shows the mesured vlues of MOR, MOE, nd HSS for the nonreinforced LVL (LVL-CG) nd for the poplr LVL tht hd been reinforced using the multi-step hot-pressing method (LVL-MH). One-wy ANOVA showed tht there ws significnt difference t 99% confidence mong the mechnicl properties. Tble 2. Mechnicl Properties of Poplr LVL Reinforced by the Multi-Step Hot-Pressing Method Reinforcement type MOR H MOR V MOE H MOE V HSS H HSS V LVL-CG LVL-MH Std Std Tble 2 shows tht the vlues of MORH, MOEH, HSSH, MORV, MOEV, nd HSSV of the LVL-MH were higher thn those of the control group, LVL-CG, by 37.23%, 38.04%, 20.01%, 37.73%, 30.93%, nd 32.11%, respectively. These results my hve occurred becuse six resin lyers, mong the totl eight lyers, were hot-pressed twice, such tht the resin ws ble to cure further. All the veneers of the LVL were hot-pressed twice, which led to n increse in the density of the veneers. Severl studies hve found tht, s long s the mount of compression pplied to wood veneer remins below its compression strength limit, greter compression vlues, nd consequently, higher vlues of veneer density, re ssocited with higher vlues of MOE nd MOR (Li et l. 2010; Zhng et l. 2012). Anlysis of the Effects of CFRP Sheet nd GFRP Mesh on the Mechnicl Properties of Poplr LVL Vlues of the mechnicl properties, MOR, MOE, nd HSS, for vrious types of poplr LVL tht were reinforced using CFRP sheet nd GFRP mesh re listed in Tble 3. Tble 3. Mesured Vlues of the Mechnicl Properties for Vrious Types of Reinforced Poplr LVL Reinforcement type MOR H MOR V MOE H MOE V HSS H HSS V LVL-CG LVL-GM LVL-CS LVL- CS+GM Wng et l. (2015). Mech. Prop. Reinf. LVL, BioResources 10(4),

7 One-wy ANOVA showed tht there ws significnt difference t 99% confidence mong MOEH, MOEV, MORH, MORV, nd HSSH. There ws no significnt difference mong HSSV. Figure 3 shows the percentges of improvement in mechnicl properties for the three types of reinforced poplr LVL. H V V Fig. 3. Percentge of improvement in mechnicl properties in comprison to control for three types of reinforced poplr LVL Figure 3 illustrtes the effects of reinforcement mterils on the MOR, MOE, nd HSS of reinforced poplr LVL. Mechnicl properties of three types of reinforced LVL were higher thn tht of the control group. The tensile strength (TS) nd MOE of glss fiber mesh nd crbon fiber sheet re higher thn for wood veneer, so using glss fiber mesh nd crbon fiber sheet could improve the mechnicl properties of LVL. Secondly, pplying the glss fiber mesh, crbon fiber sheet, nd the composite of the crbon fiber sheet/glss fiber mesh to reinforce LVL could increse its verge density or compression rtio when the thickness ws constnt. Incresing compression rtio resulted in incresing in mechnicl properties. Wng nd Di (2005) showed tht compression rtio (lower thn 10%) incresed by 1%, which led to n pproximtely 1% increse in spen LVL stiffness. Wei et l. (2013) found tht n increse in compression rtio of LVL would improve MOE of LVL. Among the three types of reinforced LVL, the LVL-CS+GM showed the highest degree of improvement in ll mechnicl properties except for HSSV. In ddition, LVL- CS showed greter improvement in mechnicl properties thn did LVL-GM. The results cn be ttributed to the following fctors: firstly, the vlues of TS nd MOE for E-glss fiber (TS: 3.4GP; MOE: 72.3GP) in this reserch were lower thn tht of Pn-crbon fiber (TS: 3.93GP; MOE: 221GP), so the reinforcement effect on LVL mechnicl properties of Pn-crbon fiber ws greter thn tht of E-glss fiber. Secondly, the composite of the crbon fiber sheet /glss fiber mesh hd better mechnicl properties compred to crbon fiber nd glss fiber, nd lso the composite hd the effect of compound enhncement between crbon fiber sheet nd glss fiber mesh, which led to incresing lod-crrying cpcity of LVL-CS+GM. So LVL-CS+GM hd the best Wng et l. (2015). Mech. Prop. Reinf. LVL, BioResources 10(4),

8 mechnicl properties. Thirdly, due to the low surfce ctivity of glss fiber sheet, bonding strength of LVL-GM nd LVL-CS+GM ws lower thn LVL-CG, so HSSV of LVL-GM nd LVL-CS+GM decresed compred to LVL-CG. Therefore, the degrees of improvement in mechnicl properties, excluding HSSV, for the three reinforcement types were ordered s follows: LVL-CS+GM > LVL-CS > LVL-GM. Anlysis of the Effects of Ly-up Loction of Reinforcing Mterils on the Mechnicl Properties of Poplr LVL To study the effect of the ly-up loction of the reinforcing composite mteril on the mechnicl properties of the poplr LVL, two types LVL-SD nd LVL-CD of reinforced poplr LVL were designed, s presented in Fig. 2 (E) nd (F). The mesured vlues of the mechnicl properties for LVL-SD nd LVL-CD re shown in Tble 4. One-wy ANOVA showed tht there ws significnt difference t 99% confidence mong MOEH, MOEV, MORH, nd MORV. There ws significnt difference t 95% confidence mong HSSH, nd HSSV. The percentges of improvement in mechnicl properties for the LVL-SD nd LVL-CD re shown in Fig. 4. Tble 4. Mesured Vlues of Mechnicl Properties of Reinforced Poplr LVL with Vrious Ly-Up Loctions of Reinforcement Mteril Ly-up loction MOR H MOR V MOE H MOE V HSS H HSS V Std LVL-CG LVL-SD LVL-CD ` Fig. 4. Percentges of improvement in mechnicl properties in comprison to control for LVL- SD nd LVL-CD As cn be seen in Fig. 4, LVL-SD could be used to obtin better mechnicl properties thn could LVL-CD. However, HSSV did not improve with the use of these Wng et l. (2015). Mech. Prop. Reinf. LVL, BioResources 10(4),

9 forms of reinforced poplr LVL. According to the mechnics theory of composite mterils (Shen et l. 2006), mximum stress would occur on the outermost lyer of LVL specimens during bending. When reinforcement mteril ws lid closer to the outer lyer veneer, the influence of the reinforcement mteril on improving mechnicl properties ws more significnt, nd the lrger lod-crrying cpcity would be obtined. In other words, locting the composite of CFRP nd GFRP closer to the surfce veneer lyer yielded better mechnicl properties for poplr LVL, which grees with the reserch conclusions of Mei nd Zhou (2009). CONCLUSIONS 1. Under both horizontl nd verticl loding modes of poplr LVL, ll tested mechnicl properties, i.e., MOR, MOE, nd HSS, were gretly improved by the use of the multi-step hot-pressing method for the reinforcement of LVL. 2. When the mechnicl properties of LVLs reinforced by different mterils (CFRP sheet/gfrp mesh composite, CFRP sheet, nd GFRP mesh) were compred, it ws possible to conclude tht for ll types of reinforced LVL, in comprison to the control LVL, MOR nd MOE were improved under horizontl loding nd verticl loding, nd HSS ws improved under horizontl loding, but becme less desirble under verticl loding. 3. The poplr LVL reinforced by the CFRP sheet/gfrp mesh composite exhibited the highest MOR nd MOE. The poplr LVL reinforced by the CFRP sheet showed the second highest MOR nd MOE. The poplr LVL reinforced by GFRP mesh exhibited the lowest MOR nd MOE of the three types of reinforced LVL. 4. Better mechnicl properties were found for the poplr LVL when symmetricl lyup loction ws used for the reinforcing mteril nd when the CFRP sheet/gfrp mesh composite ws locted closer to the surfce veneer lyer. ACKNOWLEDGMENTS The uthors re grteful for the support from the Science nd Technology Project of Jingsu Province (No. BY ) nd the Ntionl Science nd Technology Support Pln of Chin (No. 2012BAD24B01) nd the Priority Acdemic Progrm Development of the Jingsu Higher Eduction Institutions (PAPD). REFERENCES CITED United Ntions Economic Commission for Europe nd Food nd Agriculture Orgniztion of the United Ntions (2005). Forest products nnul mrket review , United Ntions Genev, Switzerlnd. Buell, T. W., nd Sdtmnesh, H. (2005). Strengthening timber bridge bems using crbon fiber, J. Struct. Eng. 131(1), DOI: /(ASCE) (2005)131:1(173) Wng et l. (2015). Mech. Prop. Reinf. LVL, BioResources 10(4),

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