Experimental Identification of the Fractional Parameter for a Viscoelastic Model of Concrete at Different Ages Based on the Impulse Excitation Technique

The experimental procedure for identifying a fractional parameter for concrete has been suggested by utilizing the experimental device RFDA Basic, which allowed the authors to measure internal friction, as well as the components of the complex elastic modulus. The main principle of the instrument is based on the Impulse Excitation Technique. Slight impact applied to a specimen of a special shape with specific support conditions initiates vibrations of the sample. The signal is recorded by a microphone and is transferred to a PC. RFDA software calculates resonant frequencies and corresponding values of damping, or internal friction. Using the internal friction test data, the vector diagrams have been constructed. The fractional parameter has been found directly from the vector diagrams. According to the experimental results, within the concrete age interval between 3 and 182 days, the average value of the fractional parameter has decreased by 276.33%, i.e., by more than two times. This phenomenon could be explained by the microstructural changes of concrete during its hardening: viscosity is substantially decreasing, while the elastic properties take on the dominating position.