| From the perspective of material properties,whether the pouring operation can be completed under the specified setting state has a key impact on the formation of compactness and bonding performance of dam concrete.Therefore,the setting time of fresh concrete is crucial to the quality control of dam concrete construction.The traditional method for measuring the setting time of concrete is the penetration resistance method,which is detrimental to monitoring and is difficult to achieve continuous and real-time monitoring of the concrete setting process.The emerging ultrasonic nondestructive monitoring mainly adopts single transmission method or reflection method,and uses the characteristic value of wave velocity to determine the setting time.However,the reflection method only represents the properties of concrete at the interface.The transmission method has the problem that the calculation of wave velocity requires manual accurate determination of the first wave occurrence time,resulting in low measurement accuracy.Therefore,the accurate test of a non-destructive monitoring method of cement-based material setting time has important guiding significance for water conservancy projects.In this regard,this paper focuses on the combination of theory and experiment,and puts forward the ultrasonic ’ transmissionreflection ’ method to study the setting time of dam concrete.The main work is as follows :1.An experimental device was designed to monitor the setting process of dam concrete.This device can simultaneously transmit and receive the ultrasonic signals of transmission method and reflection method,and the change of ultrasonic characteristics in the setting process of dam concrete was studied.Based on this,the three stages of the change of the ultrasonic signal energy of the transmission method and the reflection method with the physical structure and chemical products in the concrete condensation process are analyzed,and the process of hydration reaction is explained by the combined method.The law of the development of ultrasonic energy with the dam concrete condensation process is revealed,which lays a theoretical foundation for the ’transmission-reflection ’ combined method based on ultrasonic energy.2.Aiming at the problem that ultrasonic signal acquisition is susceptible to environmental noise in the test process,this paper compares and analyzes four common filtering methods : fixed filter,Wiener filter,spectral subtraction and wavelet denoising.Combined with signal characteristics and noise reduction requirements,a denoising method combining band-pass filter with wavelet denoising in fixed filter is designed.This method can effectively remove the noise in the signal,further improve the accuracy of the ultrasonic data set,and provide reliable support for the subsequent analysis of the change of ultrasonic signal in the process of dam concrete condensation.3.A ’ transmission-reflection ’ joint method based on ultrasonic energy is proposed,and neural network is used to solve the nonlinear problem of transmission and reflection results.BP neural network model with high precision and strong generalization ability is trained based on L-M algorithm,and it is used to calculate the setting time of concrete in the next joint method.4.Compared with the reflection method based on energy analysis,the transmission method and the transmission method based on wave velocity analysis,the maximum relative errors of the condensation time calculated by the transmission-reflection method are reduced by 2.5% –18.7%,respectively,and are not limited to the calculation of the first wave time,taking into account the simplicity and accuracy of the test.Compared with the penetration resistance method,this method can continuously monitor the condensation state of concrete,and the test is convenient,fast and nondestructive.Through theoretical analysis and experimental verification,it is proved that the ’ transmission-reflection ’ joint method proposed in this paper can effectively monitor the condensation process of dam concrete and provide strong support for the quality control of dam concrete construction. |
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