Evaluation Of Bonding Technology And Mechanical Properties Of Bamboo-wood Composite Cross-laminated Timber

Cross-laminated timber（CLT）as an innovative mass timber construction material,is mainly fabricated with softwood,e.g.Spruce-pine-fir and Norway spruce.Many attempts were made to use other softwoods（e.g.Canadian hemlock,Japanese larch）,hardwood（e.g.Poplar,Eucalyptus and Birch）and even engineered wood products（EWPs,e.g.laminated veneer lumber,laminated strand lumber）for CLT manufacturing.According to the abundant bamboo resources and shortage of wood resources in China,the combination of bamboo and wood for CLT manufacturing could make full use of the advantages of bamboo and wood.Therefore,this paper mainly studied the bonding technology of bamboo-wood composite CLT and evaluated rolling shear properties of some bamboo products and mechanical properties of bamboo-wood composite CLT.Aiming to obtain better manufacturing parameters,L9（3⁴）orthogonal tests were adopted to evaluate the effects of adhesive,adhesive spread rate,pressing pressure and pressing time on the block shear strength（BSS）,percentage of wood failure,and rate of delamination（RD）of bamboo-wood composite CLT via block shear tests and cycle delamination tests.The results showed that the adhesive had a significant effect on the bonding qualities of bamboo-wood composite CLT.The optimal manufacturing parameters were A₂B₂C₂D₃ at dry condition,i.e.adhesive of EPI,adhesive spread rate of 270 g/m²,pressing pressure of 1.0 MPa and pressing time of 140 min.Meanwhile,the optimal manufacturing parameters were A₃B₁C₂D₂ at wet condition,i.e.adhesive of PRF,adhesive spread rate of 250 g/m²,pressing pressure of 1.0 MPa and pressing time of 420 min.The bonding qualities of bamboo-wood composite CLT were better than those of CLT fabricated with other alternative wood species.Both EPI and PRF adhesives met requirements of EN 16351-2015,GB/T 50329-2012 and GB/T 26899-2011.CLT is often used as load-bearing structures such as floor and roof.When CLT is subjected to out-of-plane loading,so-called planar（rolling）shear failure is prone to occur in the cross layer.The rolling shear property is a key in the design of CLT.In order to optimize an engineered bamboo product with better rolling shear properties,the rolling shear properties of recombined bamboo and bamboo plywood were tested by modified two-plate method,and the effect of width-to-depth ratio on the rolling shear properties of bamboo products was evaluated as well.The results showed that the rolling shear properties of bamboo products were obviously better than those of SPF.Meanwhile,the rolling shear properties of recombined bamboo products were obviously better than those of bamboo plywood.For example,the rolling shear modulusG_R,12 and strength?_R,12of RB（F）-4.5 specimens were 4.4 and 2.2 times than those of SPF-4.5 specimens,respectively,and 1.9 and 2.0 times than those of BP-4.5 specimens,respectively.The influence of width-to-depth ratio of transverse layer on rolling shear modulus and strength of different bamboo products was different.Based on the one-way analysis of variance（ANOVA）,it can be seen that the width-to-depth ratio of transverse layer had a significant influence on rolling shear modulus of bamboo products.And the width-to-depth ratio of transverse layer had no influence on rolling shear strength of bamboo products,except RB（Z）specimens.For each type of bamboo CLT,the width-to-depth ratio of transverse layer had no obvious influence on the main failure modes,but the failure modes of different bamboo CLT were slightly different,mainly due to the different bamboo units of each bamboo product.According to the influence of width-to-depth ratio on rolling shear properties of bamboo CLT,the recombined bamboo product,namely RB（F）-4.5,was selected,of which the rolling shear modulus and strength were 1048 MPa and 7.28 MPa,respectively.In order to further study the bending property of bamboo-wood composite CLT,two structural forms of bamboo-wood composite CLT were designed and manufactured based on the optimal bonding technological parameters,i.e.three-layer CLT with transverse layer of RB（F）（WBW-CLT）and five-layer CLT with outmost parallel layers and transverse layer of RB（F）（BWBWBWB-CLT）.In addition,three-layer CLT was fabricated with SPF as a control group（WWW-CLT）.The bending property of CLT specimens with three different structural forms was tested and compared according to the third-point bending test method.The results showed that the average apparent bending modulus and strength of WWW-CLT,WBW-CLT and BWBWB-CLT specimens were9374 MPa,40.36 MPa,9742 MPa,32.55 MPa,11595 MPa and 66.37 MPa,respectively.Compared with the common three-layer CLT specimens fabricated with SPF（WWW-CLT）,the apparent bending modulus of specimens with RB（F）cross layer increased by 3.92%,while the bending strength decreased by 19.35%.This indicated that the rolling shear properties of RB（F）could not be fully utilized in the bending test,and the mechanical properties of RB were not consistent with those of SPF.Compared with WWW-CLT,the apparent bending modulus and strength of BWBWB-CLT increased by 23.69%and 64.44%,respectively.The ductility of BWBWB-CLT was better than that of WWW-CLT and WBW-CLT.In the theoretical models for bending stiffness of CLT,shear analogy method is most widely used.In order to evaluate its applicability in bamboo-wood composite CLT,the mechanical properties of laminas and finished beams of the CLT were tested by dynamic test method.The applicability of dynamic test method and shear analogy method in bamboo-wood composite CLT was evaluated finally.The results showed that the apparent bending stiffness by dynamic tests and shear analogy method of WWW-CLT,WBW-CLT and BWBWB-CLT were 35.12×10⁹ N·mm²,34.51×10⁹ N·mm²,37.44×10⁹ N·mm²,36.56×10⁹ N·mm²,45.64×10⁹ N·mm²,and 40.05×10⁹N·mm²,respectively.Compared with the five-layer CLT,the apparent bending stiffness calculated by dynamic tests and shear analogy method of the three-layer CLT was closer to the static measured values,and both error ranges of three-and five-layer CLT were less than 10%.Meanwhile,compared with the shear analogy method,the apparent bending stiffness by dynamic tests of CLT was closer to the static measured value,and the error range was less than 4%.The apparent bending stiffness by dynamic and static tests met the following relationship,EI_app,s=0.855×EI_app,d+5.206（10⁹ N·mm²）,and the correlation between them was high,R²=0.99.For the three types of CLT specimens,the dynamic and shear analogy values of apparent bending stiffness were close to the static measured values,and the error range was less than 10%,indicating the dynamic test method and shear analogy method could be used to predict the apparent bending stiffness of bamboo-wood composite CLT.