PSI - Issue 10

M. Petrov et al. / Procedia Structural Integrity 10 (2018) 303–310 M. Petrov et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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As can be seen from Fig.6, it can be observed that at the loads preceding the crack formation stage, there is a significant decrease in the maximum correlation coefficient and a step-like change in the magnitude of the frequency shift at which it is observed. These two parameters can be used as diagnostic criteria for the evaluation of a fracture of reinforced concrete under uniaxial compression. To quantify the signal spectrum shift at various stages of deformation of the spectrum, the spectrum gravity center has been used. Changes in the gravity center give information on the shift of the signal spectrum (as a whole) towards high or low frequencies. Fig.7 shows the dependence of the spectrum gravity center on the mechanical stresses caused by compression. As can be seen from Fig.7, a gradual shift of the spectrum gravity center toward lower frequencies is observed during the process of increasing the external compressive stress. The most significant shift toward low frequencies is observed at the pre-destruction stage. The maximum change in the spectrum gravity center is 8 kHz (40%).

Fig. 7. The relation between the spectrum gravity center and the external compressive stress for concrete reinforced with two steel rebars.

Cracks begin to form in the concrete sample subjected to uniaxial compression. The scattering of elastic waves upon cracks reflects the process of attenuation with time. To determine the nature of the changes in the attenuation coefficient of the electric signals' energy as affected by the magnitude of the external load, time-frequency analysis has been used, as described by Quiviger et al. (2012). This technique allows for monitoring the attenuation of the signal energy as a function of time in the region of any chosen frequency range. The experimental data were processed in the LabView based procedure. We have chosen there a 1 millisecond window sliding size, and the total energy of frequency response ranged from 1 to 50 kHz was calculated for this sliding window. Afterwards the sliding window was shifted to 10% of its duration in order to provide a 90% of overlapping for each of the following windows and the spectrum was calculated again. Fig.8 shows the changing of energy attenuation coefficient of electric responses, during uniaxial compression of reinforced concrete.

Fig. 8. Dependence of energy attenuation coefficient of electric responses registered from concrete reinforced with a steel reinforcing cage on external pressure.