Research On Transmission Properties Of Stress Waves In Concrete And Monitoring Of Structural Characteristic Parameters

Reinforced concrete structures are currently the most widely used form of construction.Due to the durability of concrete,it is necessary to evaluate its safety after it has experienced a ce rtain service life.With the increasing number of structural health monitoring methods,non-destructive testing methods based on the wave propagation method are widely used for their convenience and operability.The principle is that the propagation properties of stress waves in the medium can reflect changes in the internal properties of the material.Taking this as a starting point,in this paper,the transmission characteristics of stress waves in cement-based materials were deeply discussed,and the influence of porosity,water seepage height,cracks,stress level and temperature effects on stress wave propagation in concrete were studied with piezoelectric based smart aggregate(SA)as the main sensor system.The influence of wave propagation law in concrete provides a theoretical basis for the application of piezoelectric sensors in the health monitoring of concrete structures.The main work is as follows:The propagation characteristics of stress waves in homogeneous media and inhomogeneous media were revealed.Targeted on concrete,which consists multiphase materials with initial defects,the propagation of stress waves at the interfaces of different materials was discussed.The mesoscopic finite element model of piezoelectric based SA and concrete were established,and the numerical simulation of stress wave propagation in concrete was carried out.The effects of the elastic modulus and area ratio of coarse aggregate on t he propagation of stress waves were systematically studied.The results show that the stress wave will reflect and refract at the interface between two media.The amplitude of the reflected wave and the refracted wave are independent of the frequency,and only depend on the amplitude of the incident wave,the incident angle and the Poisson's ratio of the material.When the elastic modulus of the aggregate increases,the amplitude decreases and the wave velocity increases,and the higher the excitation frequency,the smaller the amplitude and the wave velocity;as the aggregate area ratio increases,the stress wave velocity increases.The effect of porosity on stress wave transmission was studied.Three different cementation blocks of different porosity were poured.The relationship between the porosity and stress wave of cement paste was experimentally studied by establishing a piezoelectric sensor monitoring platform.At the same time,the actual pore structure and distribution of the pores inside the specimen were obtained by X-ray computed tomography(CT scan).The three-dimensional model of the cement paste was established.The stress wave propagation in the three-dimensional models of the pore specimens was simulated.The established three-dimensional model can more accurately reflect the propagation law of stress waves in cement-based materials.It also provides a new idea for testing the porosity of cement-based materials.The effect of internal humidity of concrete on stress wave transmission was studied.The short concrete columns embedded with piezoelectric smart aggregates were designed,and the influence of water seepage height on the characteristic parameters of stress waves was studied by placing them in different water seepage environments.The results verify the feasibility of using P-wave amplitude and equivalent P-wave velocity as parameters to reflect concrete seepage conditions.The method of determining the arrival time of stress wave is very important to the accuracy of the results.When the method of determining the total propagation time of stress wave is selected in the actual structural health monitoring project,it is necessary to comprehensively consider the environmental noise situation of the project sites and accuracy requirements.This study provides a new method for the internal humidity monitoring of concrete structures using smart aggregates.The influence of crack on stress wave transmission was discussed.A method for monitoring the damage distribution of concrete structures was proposed.The effectiveness of this method was verified by the health monitoring of an scaled reinforced concrete beam in the laboratory.The results show that in the effective monitoring system,stress wave characteristic parameters,i.e.,stress wave transmission time difference(TTD)and amplitude ratio can be used to reveal the damage distribution of concrete structures.This method has great engineering application prospects for the crack distribution monitoring of the actual bridge structure that has been built.Based on the monitoring system,the combination of digital image correlation method(DIC)and linear variable differential transformer(LVDT)was utilized to innovatively explore the influence of crack width on t he stress wave transmission path.The influence of stress on the characteristics of stress wave spectrum considering the temperature effect was studied.The influence of stress on the piezoelectric based smart aggregate itself and the monitoring system embedded in the concrete were investigated.The effects of axial stress on the monitoring results of the hardening and maturity monitoring of concrete columns with SA-based monitoring system were carried out.Considering the temperature effects,the influence of axial stress on t he monitoring of mature concrete using SA-based monitoring system was studied.The results show that the wave velocity of stress wave propagating in concrete shows a good linear law with the development of stress in the elastic range of concrete.This study predicts a new approach to structural axial stress monitoring in the elastic range of concrete.When the temperature difference is not large,the influence of the ambient temperature on the wave velocity of the stress wave in the concrete can be neglected.