Ultrasonic through-transmission method: Characterization and detection of the electromagnetic disturbances caused by pulse excitation observed in combined compression and shear wave measurements of cement paste

In the past, the determination of the mechanical properties, especially of viscoelastic materials, using the ultrasonic pulse transmission method based on piezo-electric elements has established itself as a widespread test technique. The associated digital data acquisition enables an almost continuous acquisition of signal data, which can map changes in material properties well. This method is specially valuable for cement paste in the early stage, where changes in material properties are most noticeable due to the underlying chemical processes. However, the quasi-continuous acquisition of signal data provides extensive signal series and large amounts of data that make automated signal analysis indispensable. Signal interference due to electromechanical side effects affects numerical analysis and limits performance. One observable side effect is the mapping of the electromagnetic field into the signal caused by high-voltage pulse excitation of the piezo-electric elements. When the amplitudes of compression or shear waves are small compared to those of electromagnetic interference, this side effect may disrupt automated signal analysis. This work describes a wavelet analysis approach which allows for narrowing down the time range in which electromagnetic interference occurs. That enables the targeted consideration of this disturbance in subsequent automated signal analysis. A notable feature is the targeted use of the disturbance characteristics for the wavelet design. First, electromagnetic interference became characterized qualitatively and quantitatively. For this purpose, signal data were used from ultrasonic pulse transmission experiments using the FreshCon device. Primarily cement pastes with different water-cement ratios at early stages were tested. Second, an analysis procedure was designed based on the characteristic properties of the disturbance and tested on the experimental data. Analysis results show that the time range in which the electromagnetic interference occurs is widely independent of the sample material. Comparative tests with water and air confirmed that observation. The results also do not show significant differences between compression and shear wave measurements. Furthermore, the amplitude distribution of the disturbance appears to consist of a superimposition of several damped oscillations, which depend, among other things, on the device settings of the pulse generator. This circumstance was taken into account in particular in the wavelet design. The presented approach seems suitable for delimiting the time range in which the disturbance occurs. Furthermore, it turned out that the use of device settings and the characteristics of interference stabilizes the analysis procedure. Since that, adaptation for different test devices seems to be necessary.