Semester

Summer

Date of Graduation

2001

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Civil and Environmental Engineering

Committee Chair

Roger H. L. Chen.

Abstract

The main objective of this research is to develop both passive and active nondestructive testing techniques for reinforced and prestressed concrete structures using acoustic waveguides. The first technique introduces a two-dimensional surface waveguide to enlarge the acoustic emission (AE) monitoring area of a reinforced concrete structure. A two-dimensional steel wire system is developed as an acoustic waveguide to study the AE behavior of a reinforced concrete floor slab. AE sensors are mounted to the end of the waveguides to detect AE signals, which are generated by pencil-lead breaks at various locations on concrete surface. Results show that the use of the surface waveguides can significantly enlarge the AE monitoring area. In order to identify locations of AE sources, a neural network system is employed. Four data sets of AE parameters and their corresponding locations are used to train the neural network system. Satisfactory results in predicting the AE source locations are obtained when using the trained neural network system to identify AE source locations of a testing data set.;The second technique involves the measurement of tensile forces in a prestressing strand in prestressed concrete structures using ultrasonic stress waves. The commonly used 1/2-inch diameter seven-wire prestressing strands are studied. In this study, both experimental measurements and theoretical analysis are conducted. A stress wave is generated at one end of a prestressing strand and the wave is detected at the other end using an ultrasonic transducer. The stress waves due to tensile stresses of the strand up to 77% of its ultimate strength are investigated. The theoretical analysis is conducted by accounting for acoustoelasticity effect and the dispersion of waves. This analysis is used to calculate the traveling times of different frequency components of the wave propagating in the strand, which is subjected to prestress forces. The analysis provides a successful description of the behavior of a longitudinal transient wave traveling through a long, prestressed, circular strand. The Wigner-Ville Transform is used as a signal-processing tool in order to identify the arrival times of different frequency components of the detected waveforms. The analytical and experimental results correlate well, and good measurement accuracy is observed. Both experimental and analytical results indicate that the velocity (or traveling time) of each frequency component of the traveling wave can be related to tensile force level in the strand. This technique can effectively be used for measuring tensile forces in ungrouted post-tensioning strands of a prestressed concrete structure.

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