The potential of smart aggregate for seismic stress monitoring has been identified, but only moderate dynamic stress of relatively high frequency can be measured reliably due to the limitations of conventional charge amplifiers. To improve the performance of smart aggregate-based monitoring systems, a charge amplifier was designed especially for the demands of seismic stress monitoring. Its performance was tested in the low frequency range down to 0.05 Hz with high charge input up to 2 × 10⁷ pC representing stress up to 100 MPa by feeding it frequency-sweep charge signals. With the specially designed amplifier, the performance of the smart aggregate-based monitoring system was tested in both frequency-sweep and amplitude-sweep loading tests, and the influence of lead zirconate titanate depolarization was evaluated at static loads of 4.8, 14.4, and 24 MPa. The results show that the demands of seismic monitoring with a smart aggregate-based system can be satisfied, and that the sensitivity of the system remains constant under dynamic loads up to at least 45 MPa.

Piezoelectric-based seismic stress monitoring provides an innovative approach to assessing the health of concrete structures during earthquakes. In this research, we evaluate the application of piezoelectric-based smart aggregate (SA) sensors for monitoring the seismic stress on high-strength concrete columns. The principle behind using smart aggregates for seismic stress monitoring is based on the assumption that concrete stress can be reliably predicted by the average output voltages of a limited number of embedded smart aggregates within an acceptable margin of error. This experiment is designed to evaluate the effects of meso-scale randomness on high-strength concrete and the effects of different loading paths on the proposed smart aggregate. Loading–unloading loops of increasing amplitude at the nonlinear stage and monotonic loading to failure were carried out on four high-strength concrete cylinders. Each specimen had six smart aggregates embedded. A statistical analysis based on the test results determined the sensitivity curve during the loading–unloading and the full-range damage processes. Monitoring errors of concrete stress monitored by smart aggregate during the pre- and post-peak stages were also discussed. The research concludes that there is the potential for deploying smart aggregate in engineering applications to monitor seismic stress on high-strength concrete structures.

Ultrasound is widely used in nondestructive testing (NDT). It is the basis of many NDT techniques, such as acoustic emission, phased array and synthetic aperture focusing technique. The attenuation of ultrasound is a key factor that ensures the feasibility of these techniques. Conventional studies of attenuation are mostly conducted in specimens that have dimensions smaller than ultrasound wavelengths. In this type of study, reflections from media boundaries interfere with the wave, resulting in a geometry-dependent attenuation that cannot be applied in another specimen with a different geometry, even if the material is identical. Another factor that impacts on the viability of ultrasound NDT techniques is the spatial uniformity of waves in the concrete sample. To serve as a reference in testing defects, the degree of randomness caused by the material’s heterogeneity should be estimated in conditions free of defects. In this study, a large concrete specimen was fabricated to suppress boundary reflections of the wave, creating an environment less dependent on geometry. Twenty-two PZT (lead zirconate-titanate ceramics) transducers were arranged in a three-by-seven array to transmit and receive ultrasound waves. This experiment sought to measure attenuation over distance and by frequency, as well as analyzing attenuation factors such as geometrical spreading and material absorption. A quadratic trend with good consistency between attenuation and frequency was found, indicating minimal local attenuation. The degree of randomness in the wave was measured by three indices: amplitude, group velocity and the frequency spectrum cross-correlation factor. Frequency dispersion was also described using group velocity. The findings of this study can serve as a reference for other NDT research, disclosing geometry-independent attenuation and the variability of wave characteristics in concrete.

Piezoelectric-based sensors are low cost, highly stable, responsive, and changeable in shapes, allowing them to be easily embedded within concrete structures to perform various monitoring tasks. In this research, an in-built piezoelectric-based three-direction stress sensor is proposed, which could monitor the internal normal stresses in three mutually orthogonal directions. The sensor has a cubic shape with edges of about 25 mm. It consists of three PZT (lead zirconate titanate) patches, each of which is sandwiched between a steel base and a square steel plate with epoxy. To test the sensors’ characteristics, a finite element analysis was carried out to investigate stress transfer inside the sensor. Besides, a servo-hydraulic machine was used to conduct a cyclic loading test, delivering compressive stress up to 30 MPa on the three sensors. Both the numerical simulation and experiment results have shown that the cross-interference effect between different directions is negligible. Also observed was a high level of consistency in the sensitivity of measurements, regardless of the monitoring direction and sensor. With the advantages of a simple structure, low cross interference, and consistent sensitivity, the proposed three-direction normal stress sensor has a great potential to be mass produced and used in monitoring the health state of structural concrete.