Investigation of mechanical wave technique in materials characterization and damage detection

Abstract

In this study, the mechanical wave technique has been investigated for the cement-based materials characterization and damage detection. In the investigation, cement-based piezoelectric composites are employed as the sensing elements and the ultrasonic array as well as acoustic emission technique the basic sensing methods. The piezoelectricity of the cement-based piezoelectric sensors enables powerful and accurate real-time detection of the stress and stain change. To facilitate the mechanical-electrical conversion between stress and the electrical output of the piezoelectric sensors, a mathematical model is established based on Duhamel's integral, the constitutive law and the charge-leakage characteristics of the piezoelectric composite. The results indicate that the cement-based piezoelectric sensors and associated measurement setup have good capability of mechanical wave detection. An active acoustic method using cement-based piezoelectric sensors is employed to monitor the setting and hardening of early age concrete, by recording and analyzing the wave velocity and attenuation coefficient. The effects of water-to-cement ratio, pozzolanic materials (fly ash and silica fume) and internal sulfate attack are examined. The central frequency of the acoustic excitation is 6 kHz, which is much lower than that of ultrasound and can enhance the signal-to-noise ratio when applied to very early age concrete. The longitudinal wave velocity measurement can reveal clearly three stages in the hydration process of early age concrete. Sulfate attack and corrosion of steel reinforcement are major problems of the durability of concrete structures. Ultrasonic technique and acoustic emission are investigated for detecting the concrete deterioration and reinforcement corrosion. A 2-D diffusion model of external sulfate attack has been established to describe the evolution process of pore structure and elastic modulus of reinforced concrete beam under loading conditions. Cement-based piezoelectric sensors are employed to investigate the corrosion-induced deterioration of steel-reinforced ordinary Portland cement and magnesia-phosphate cement concrete structures under the coupling effect of loading and accelerated ions diffusion with the help of frequency-entropy analysis of AE signals. The change of frequency-entropy curve corresponds well to the revolution of damage degree of reinforced concrete structures, from micro-cracking nucleation to localized macro-cracks propagation. The major contributions of the thesis are: 1. The mechanical-electrical conversion model of cement-based piezoelectric sensors has been established. Cement-based piezoelectric sensors have been proved as good candidates for mechanical wave detection in civil engineering. (Chapter 3) 2. The active acoustic method has been employed to replace the traditional ultrasonic testing for setting and hardening monitoring of early age concrete. Due to the lower frequency adopted, it is demonstrated that acoustic method performs better than the ultrasonic method and more suitable for cement-based materials. (Chapter 4 and 5) 3. A 2-D diffusion model of external sulfate attack is established for the first time to describe the evolution process of pore structure and elastic modulus of reinforced concrete beam under loading conditions. The simulated results agree well with the detected elastic modulus change by ultrasonic array testing method. (Chapter 6) 4. The corrosion-induced deterioration processes of the reinforced MPC concrete beams under the coupling effect of loading and accelerated chloride penetration has been preliminary investigated using AE technique and Shannon entropy analysis for the first time. It is found that the distinguished time-frequency characteristics of AE signals can reflect the degree of corrosion-induced damage of the reinforced MPC concrete beams, and the average frequency shift is a good indicator of damage levels. The change of frequency-entropy curve corresponds to the revolution of damage degree of reinforced concrete structures, from micro-cracking nucleation to localized macro-cracks propagation. (Chapter 7)

Date of Award	2016
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology