Research On The Damping Properties Of Cementitious Materials Based On The Regulation Of Coconut Fiber-matrix Interface

Dynamic loads generated by earthquakes,wind,rail transit and other factors can cause the vibration response of the building structure,thereby endangering the usefulness,safety and durability of the overall building structure.According to vibration theory,the harm caused by vibration can be reduced by improving the damping performance of building structures.The damping performance of the building structure can be improved by designing the corresponding damping structure or improving the damping performance of the building structure material itself.As the main material in the building structure,the enhancement of the damping properties cementitious materials has become a hot topic in research.The damping properties of cementitious materials are difficult to be improved through conventional component optimization,but can be effectively improved by adding low-dosage synthetic fiber materials.On this basis,coconut fiber was used as an alternative material for synthetic fiber to improve the damping properties of cementitious materials.To further improve the effect of coconut fiber on the damping properties of cementitious materials,based on different interfacial energy consumption mechanisms,the modification process of fiber was designed,and the fiber-matrix interface structures that can effectively dissipate vibration energy were constructed through the surface modification of coconut fiber.The main research contents of this paper are as follows:Based on the interfacial shear-hysteresis model,the relationship between stress,strain and energy dissipation was analyzed to explore the influence of the weakening of the coconut fiber-cement matrix interface on the damping properties of cementitious materials.The analysis results show that the shear slip movement between the fiber and the matrix was promoted due to the reduction of the critical shear stress,and the vibration energy can be effectively dissipated through the friction slip of fiber-matrix interface.Based on this,to weaken the interfacial bonding between the fiber and the cement matrix,different methods were used to chemically modify the surface of the coconut fiber.Among them,the local debonding interface can be constructed by modifying coconut fiber with the oxygen-alkali method.The vibration energy can be dissipated through the friction of the fiber-matrix local debonding interface and the elastic-plastic deformation of the debonding part of fiber,thereby improving the damping performance of coconut fiber on cement-based materials.The loss tangent of cement mortar increased by 16.50%and 25.45%,respectively,by adding 0.75 vol.%of unmodified coconut fiber and chemically modified coconut fiber with the oxygen-alkali method.The damping enhancement mechanism of the local debonding fiber-matrix interface can be limited by the sustainability of elastic-plastic deformation.Considering this situation,to avoid the plastic damage of fiber,the surface modification process of coconut fiber was optimized to increase the fiber-matrix interface where friction movement can occur.The laccase degradation process was used to biologically modify the surface of the coconut fiber in a targeted manner,and the deboned fiber-matrix interface was constructed.On this basis,a friction-slip model of the debonded interface was established to explain the enhancement mechanism of the damping properties of cementitious materials derived from the friction of the fiber-matrix interface under dynamic loads.When the interfacial shear stress was greater than the critical shear stress,the effective dissipation of vibration energy can be achieved by the friction of the fiber-matrix interface.The test results show that the loss tangent of cement mortar increased by 7.51%and 26.82%,respectively,by adding 0.5 vol.%unmodified coconut fiber and biologically modified coconut fiber with laccase.Based on the constrained damping structure model,the relationship between stress,strain and displacement of each layer was analyzed,and the influence of constrained damping structure on the damping properties of cementitious materials was explored.The analysis results show that in the entire dynamic load process,the effective dissipation of vibration energy can be achieved through the stretching,bending and shear deformation of the damping layer.Based on this,Na IO₄ and Na BH₄were used to selectively oxidize and re-reduce the surface of the coconut fiber to build the viscoelastic layer on the surface of the fiber,which can be used as the damping layer to build the constrained damping structure in the cementitious materials to improve the damping performance of the cementitious materials.Compared with ordinary cement mortar,the loss tangent of cement mortar can be increased by up to30.30%due to the construction of the viscoelastic layer on the fiber surface.The energy consumption mechanism of the fiber-matrix interface structure at different response stages was analyzed and used to explore the mechanism of the coupling between the weakening of the fiber-matrix and the transition zone of the viscoelastic on the damping properties of cementitious materials.Due to the weakening of the fiber-matrix interface,the energy consumption of tensile and bending deformation of the constrained layer in the constrained damping structure and the energy consumption of shear deformation of the damping layer were reduced,but at the same time the energy consumption of interfacial friction was increased.The free radical polymerization method was used to graft the flexible molecular layer of methyl methacrylate on the surface of coconut fiber,which can realize the weakening of the fiber-matrix interface while constructing the viscoelastic transition zone.Compared with ordinary cement mortar,the loss factor of cement mortar is increased by 39.12%by adding 0.5 vol.%coconut fiber grafted with methyl methacrylate on the surface.