Evaluation of measured ultrasonic signals showed that ongoing deterioration

Evaluation of measured ultrasonic signals showed that ongoing deterioration of microstructure of mortar bars can be successively detected by both time and frequency domain analysis of recorded signals. The parameters recorded and computed (P-wave velocity, attenuation of ultrasonic signal, energy, etc.) during the experiment show different scales of changes that can reflect both normal development of concrete microstructure (hardening of concrete mixture) and/or degradation of microstructure due to developing ASR. Measured expansion of the test mortar bars of 0.5% in 40-days lasting experiment corresponded to a drop in P-wave velocity of about 400m/s (approx. 9% of maximum value) in the case of the specimens subjected to the effect of 1N NaOH. The amplitudes of the first onset and maximum signal amplitude exhibited similar trend but the first onset with 80% change of the value seems to be much more sensitive to the development of material’s microstructure change than the maximum signal amplitude with only 10% change. This can be explained by the fact that the amplitude of the first onset, as well as arrival time, corresponds directly to P-wave propagation, which is affected only by the actual composition and microstructure of the material tested. The amplitude of the first onset is also 10-times more sensitive than the velocity of P-wave propagation, which is given by the integral character NF-κB Signaling Library of the sensor (sensor area). Lower changes of ultrasonic sounding parameters were detected in mortar bars with the innocuous aggregate in comparison with mortar bars containing aggregate with higher ASR potential.

Acknowledgements
This study was financially supported through research project GAP104/12/0915 with funding provided by the Czech Science Foundation and by the Czech Academy of Sciences, project RVO 67985831. The technical help of Mr. Filler, Mrs. Erdingerová, and Mr. Nemejovský was very much appreciated.

Introduction
The purpose of this work was to improve the existing models that allow spatial averaging effects of piezoelectric hydrophones to be accounted for. Miniature hydrophones are widely used to detect the spatial and temporal characteristics of ultrasound fields [1]. The hydrophone is used to measure the averaged acoustic pressure over the active area of the element. This technique is recommended for the measurement of many field parameters that are considered important in the International Electrotechnical Commission (IEC) standards [2]. The effective radius of the hydrophone should ideally be comparable to or smaller than one quarter of the acoustic wavelength, as indicated in paragraph 5.1.6.1 of IEC 62127-1 [2]. This is to ensure that the phase and amplitude variations over the active NF-κB Signaling Library do not significantly contribute to the measurement uncertainties. In the far field of the ultrasonic transmitter, the criteria can be relaxed based on the dimensions of the transducers, the acoustic wavelength and the distance between the hydrophone and the transmitter surface [2,3]. The error caused by the finite aperture size is termed the spatial averaging error [4]. Umchid and Gopinath also noted that the effective diameter of the hydrophone should be on the order of the half-wavelength at the highest frequency to be measured to eliminate the effect of spatial averaging [5]. In water, this requires the use of an active aperture on the order of 50μm at 15MHz. However, although a special 50-μm-diameter hydrophone has been reported [6], the majority of commercially available piezoelectric hydrophones has nominal active apertures (or diameters) within 0.2–1mm, which is too large for measurement in an ultrasonic fields above a few MHz. Hydrophones that do not meet the requirements for a point detector produce spatially averaged values of the acoustic pressure.
For the applications which require sensors with high temporal and spatial resolution, the interferometric [7–9] and fiber-optic methods [10–13] are noteworthy. A fiber-optic hydrophone with a tip diameter of approximately 7μm has been reported by Lewin et al. [12]. The hydrophone enables extension of the working frequency to 100MHz without inducing spatial averaging effects. Although the fiber-optic hydrophones are promising in biomedical ultrasound applications, they are primarily found in well-equipped university- or testing houses laboratories and are relatively expensive. The spatial averaging effects due to the finite aperture size of piezoelectric hydrophones have attracted much attention from investigators. Boutkedjirt and Reibold [14,15] addressed this issue as an inverse problem and used numerical methods to deconvolve the averaging effects. Other methods for correcting the spatial averaging effects have been proposed although their discussions were restricted to idealized models. For the measurement arrangement in which the hydrophone is located along the acoustic axis of the transmitting transducer, Daly and Rao [16] derived a set of closed-form expressions based on the Lommel diffraction formulation for both planar and spherically focused transmitters. In addition, Preston et al. [17] and Smith [18] used a beam-plot method based on the theoretical model of the pressure distribution in the focal plane to estimate the spatial averaging effects. Zeqiri and Bond [19] and Radulescu et al. [20] numerically calculated the theoretical pressure distribution over the hydrophone in the focal plane, with the purpose of estimating the same effects. Harris [21,22] used the generalized spatial impulse response function of the velocity potential to evaluate the averaging effects in the transient field of planar transducers. Markiewicz and Chivers [23]used the spatial impulse response method [21,22] to investigate the typical errors involved in far field measurements. Furthermore, Beissner [24] derived an exact integral expression for the spatial averaging effects in the steady state of a planar transducer field when the acoustic axes of the transmitter and the hydrophone are coaxial. Goldstein et al. [25] estimated the spatial averaging effects of a hydrophone along the acoustic axis by determining the effective radius of a planar transducer used as a transmitter. Later Goldstein [26] determined the magnitudes of the axial and lateral pressures for the field characterization of a planar transducer in a steady-state field that contained no hydrophone.