高温预处理对足尺胶合木梁力学性能的影响

Mechanical Properties of Full-Scale Glulam Beam Made of Thermally Treated Lamellas

【目的】研究高温预处理对足尺胶合木梁力学性能的影响,明确高温热改性和环境湿度对木材平衡含水率、木材顺纹抗剪强度和顺纹抗拉强度的影响规律,揭示高温预处理对胶合木梁抗弯性能影响的作用机制,为高温热改性技术在木结构领域中的应用提供参考。【方法】以高应力等级的兴安落叶松为研究对象,采用工业化热处理技术对落叶松木材进行高温热改性,以高温热改性后的落叶松木材为层板,制备12个足尺胶合木梁。基于EN 408标准四点弯曲方法,分析高温热改性和环境湿度对胶合木梁抗弯弹性模量、抗弯强度、破坏模式、跨中截面荷载-应变曲线和跨中截面应变分布规律等抗弯性能的影响。【结果】高温热改性会在一定程度上降低胶合木梁的抗弯强度,但可明显提高高湿度条件下胶合木梁的抗弯弹性模量,与90%环境湿度下未处理胶合木梁相比,高温热改性后,同湿度下胶合木梁的抗弯强度降低29.79%,抗弯弹性模量提高23.71%;高温热改性可降低胶合木梁抗弯弹性模量对环境湿度的敏感性,环境湿度从60%提高到90%,未处理胶合木梁的抗弯弹性模量降低23.27%,经高温热改性预处理的胶合木梁抗弯弹性模量降低7.55%;60%和90%环境湿度下的荷载-位移曲线和跨中截面应变分布曲线表明,胶合木梁在高湿环境中具有更明显的非线性特性,高温预处理后的胶合木梁表现为线弹性。环境湿度对胶合木梁的抗弯性能具有较为明显的劣化作用,90%湿度下胶合木梁抗弯强度和抗弯弹性模量分别为43.98 MPa和12.191 GPa,比60%湿度下未处理胶合木梁低17.07%和23.27%;环境湿度对木材平衡含水率影响明显,高温热改性是降低木材平衡含水率的有效措施,环境湿度从60%提高到90%,高温热改性处理后落叶松木材平衡含水率分别从10.74%和20.62%降至4.76%和11.18%。【结论】60%和90%环境湿度条件下,未处理胶合木梁为拉伸破坏,高温热改性构件为拉剪联合破坏,高温热改性后材料的顺纹抗剪强度和顺纹抗拉强度降低是材料破坏模式改变的根本原因。高温热改性后胶合木梁在高湿环境条件下抗弯弹性模量明显改善。

【Objective】The mechanical properties of full-scale glued laminated timber(glulam)beams made of thermal treated laminas were performed.The effects of thermal modification and humidity in the surrounding environment on wood moisture content,longitudinal shear strength and tensile strength were determined.The study can provide a reference for thermal treatment technology in the structural applications.【Method】The high-grade Larix gmelinii was used in the tests.A total of 12 full-scale glulam beams were fabricated with modified laminas using industrial thermal modification technology.The effects of humidity in the surrounding environment and industrial thermal treatment on bending properties were determined,including bending strength,modulus of elasticity(MOE),failure mode,the relationships between load and deformation and the strain profile of full-scale glulam beams.All beams were tested using the four-point bending method according to BS EN 408 standard.【Result】The results showed that,thermal modification improved MOE significantly in high humidity with a slight reduction in bending strength.In 90%relative humidity,bending strength of thermally treated glulam beams decreased by 29.79%than that of the untreated.MOE of thermally treated glulam beams increased by 23.71%than that of the untreated.MOE of the untreated glulam beams under the two humidity conditions was 23.27%difference,and the thermally treated glulam beams under the two humidity conditions was only 7.55%difference.The curves of bending load with displacement and strain profile curves of the glulam beams in different relative humidity showed that,the untreated glulam beams exhibited a more nonlinearity than the thermally treated glulam beams,especially at a high moisture in the surrounding environment.The relationships between moisture in the surrounding environment and mechanical properties were negatively correlated.Compared with the untreated control at 60%relative humidity,bending strength and modulus of elasticity of glulam beams at 90%relative humidity were 43.98 MPa and 12.191 GPa,and low ered by 17.07%and 23.27%,respectively.The effects of humidity in the surrounding environment on equilibrium moisture content of wood specimens were significant,and equilibrium moisture content of wood specimens can be deduced obviously by industrial thermal treatment.Thermal treatment decreased equilibrium moisture content from 10.74%to 4.76%and from 20.62%to 11.18%at 60%and 90%relative humidity,respectively.【Conclusion】The untreated glulam beams at 60%and 90%relative humidity both failed in tension mode,while the series of the thermally treated glulam beams all failed in combined flexure and shear mode.Longitudinal tensile strength and shear strength of wood specimens both decreased after thermally treated,and led to the decrease of bending strength of the glulam beams.Industrial thermal treatment technology could improve modulus of elasticity of glulam beams significantly in a high relative humidity with a reduction in bending strength,so thermally treated wood materials could be used in construction after checked in bending strength.