Study On Band Gap Tuning And Vibration Reduction Characteristics Of Lightweight Concrete

The continuous expansion of the city scale has promoted the construction and development of urban rail transit.However,with the increase in the density of subway lines and the increase in operating frequency,the vibration and noise problems caused by train operation cannot be ignored.There are many types of existing subway vibration damping measures,but there are problems such as high cost,insufficient durability,later maintenance difficulties and low frequency vibration damping effect.Therefore,it is imperative to research and develop new subway vibration reduction materials.The proposed local resonance mechanism of phononic crystals can realize the goal of producing low frequency bandgap with small structure size,which makes it possible to reduce low frequency vibration.The research shows that lightweight concrete has certain energy absorption and vibration reduction effect,but the research on its vibration reduction mechanism is not deep enough.The difference of material composition and material parameters of lightweight concrete conforms to the structural characteristics of local resonant phononic crystal.Therefore,based on the local resonance mechanism,this paper discusses the vibration reduction mechanism of lightweight concrete,and studies its band gap regulation and vibration reduction characteristics.The main research contents and achievements are as follows:1.According to the structural characteristics of lightweight concrete,a generalized model for calculating the band gap of lightweight concrete is proposed.The improved plane wave expansion method for calculating the band gap of lightweight concrete two-dimensional structure is studied,which has the characteristics of fast convergence.The plane wave expansion method for calculating the bending vibration band gap of lightweight concrete thick plate structure is studied.2.The mechanism of light-weight concrete band gap and vibration damping mechanism are studied.Taking the XY mode as an example,the band gap starting frequency corresponds to the translational vibration mode of the stone scatterer,the band gap cutoff frequency corresponds to the translational vibration mode of the cement mortar matrix,and the flat band corresponds to the rotary vibration mode of the stone scatterer.When the band gap is opened,the vibration energy is localized in the unit structure,the propagation of the vibration wave is hindered,and the vibration wave is thus attenuated.3.The band gap of the two-dimensional structure of light concrete and the thick slab structure is calculated and analyzed,and the regulation law of the band gap is clarified.Under the condition that the equivalent stiffness k is constant,the initial frequency f₁ of the band gap is determined by the densityρ₁ of the scatterer,the cut-off frequency f₂ is mainly determined by the densityρ₃ of the matrix,and the equivalent stiffness k has the effect of amplifying the band gap widthΔf.The higher the symmetry of the shape of the scatterer,the more favorable it is to form a low-frequency wide band gap,and the rotation angle of the scatter body has no effect on the band gap.When the material ratio is considered,increasing the lattice constant a,increasing the filling rate f,and selecting a larger radius ratio of the scatterer to the cladding layer r₁/r₂ are beneficial to open the low-frequency wide band gap.4.This paper discusses the methods of adjusting and controlling the band gap of lightweight concrete,and studies the band gap characteristics of point defect structure and multi frequency resonance structure of lightweight concrete.The reduction of the size of the resonance unit of point defect structure makes the structure generate an additional high frequency band gap,which is conducive to widening the band gap,while the form of hole defect has no effect on widening the wide band gap;the multi frequency resonance structure can generate multiple band gaps,which can reduce the band gap frequency and expand the wide band gap.The band gap is related to the resonance characteristics of the unit.Adjusting the parameters of the unit can adjust the band gap of the lightweight concrete.5.The vibration attenuation test of the lightweight concrete track bed slab shows that the lightweight concrete has 6 vibration attenuation zones in the frequency range of 200 Hz,and its attenuation effect is 10～15 d B.Compared with plain concrete track bed,lightweight concrete track bed can effectively damp subway vibration.