Ultrasonic non-destructive testing methods such as Impact-echo are often conducted under different conditions. Such results might be distorted by ambient temperature or by water content of the tested sample. Tested mortar sample displayed shift of the fundamental frequency by 3.7 %. This article confirms necessity of standardized conditions during ultrasonic testing for both laboratory and in-situ testing.

Most concrete structures are subjected to a range of temperature corresponding to normal environmental temperatures. However, there are important cases where concrete structures may be exposed to much higher temperatures (e.g., jet aircraft engine blasts, building fires, chemical and metallurgical industrial applications in which the concrete is in close proximity to furnaces, and some nuclear power-related postulated accident conditions). Exposure of concrete to high temperatures affects its mechanical properties. In this paper we examine the dependence of the fundamental frequency on temperature to which the concrete beams were heated. Fundamental frequencies were obtained by an innovative method used Pseudorandom Binary Sequence of Maximum Length as a perturbation signal. For the verification of the results the Ultrasonic Pulse Velocity in concrete were also measured and flexural bending strengths were determined. The results show method with Pseudorandom Binary Sequence of Maximum Length as a perturbation signal as a very promising for non-destructive testing of thermally damaged concrete.

Behaviour of concrete under elevated temperatures is very complex. There is a change of mechanical and physical parameters with temperature. In this paper we study the relations of thermal damage processes in concrete and parameters obtained by different ultrasonic methods. The concrete specimens were heated in programmable laboratory furnace. Selected temperature (200°C, 400°C, 600°C, 800°C, 1000°C and 1200°C) were maintained for 60 minutes. The first ultrasonic measurement technique in this paper was Ultrasonic Pulse Velocity method. The pulse velocity in a concrete depends on its density and its elastic properties. Therefore, it is possible to deduce the quality and the compressive strength of the concrete from the ultrasonic pulse velocity. The second ultrasonic measurement technique in this paper uses broadband pulse-compression signal, with variable amplitude to measure the change of fundamental frequency. This method is based on Nonlinear Elastic Wave Spectroscopy. Nonlinear Elastic Wave Spectroscopy methods takes advantage of the fact, that nonlinearities in material manifest themselves as a resonant frequency shifts and harmonics or dumping coefficients changes. The progress of nondestructive testing parameters was confirmed by results from the destructive tests.

No abstract

Distribution function of relaxation time mathematically describes frequency dependence of the complex permittivity. Empirical function describing the complex permittivity in the frequency domain needs to be broken down to real and imaginary part, and thus separated. The permittivity is a function of the frequency of the electric field and describes behavior of technical dielectric in an alternating electric field. In this case the dielectric is a building material. The paper describes application of simulation models with variable distribution parameters and relaxation time.