THESIS
2016
iii leaves, iv-xvi, 187 pages : illustrations ; 30 cm
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-ba...[
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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)
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