Vibration Characteristics and Use in Concrete Damage Assessment

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Title: Vibration Characteristics and Use in Concrete Damage Assessment
Author: Recalde, Juan Jose
Advisors: Dr. Michael L. Leming, Committee Chair
Dr. David W. Johnston, Committee Member
Dr. James M. Nau, Committee Member
Abstract: Simple, on-site techniques that can provide information on the extent of damage and the condition of concrete are useful forensic tools. In this investigation, electronic signals obtained from flexural dynamic response of thin concrete disks in free-free mode were analyzed to identify possible indicators of changes in mechanical properties and permeability of concrete. In Phase I, the signals were investigated to determine an appropriate descriptive model and to obtain the frequency and damping characteristics. In Phase II, the dynamic elastic modulus (Ed) and air permeability index (API) were measured on concrete disks of nominal and high strength mixtures before and after heat induced damage to associate with the signal characteristics of each disk. Finally, air permeability was compared with other penetrability tests to determine the extent of damage induced on each mixture to relate to their signal characteristics. A model with linear viscous damping was found to be a better descriptor of the signals than a model with friction damping. An appropriate method to obtain the frequency and damping ratio was found to be by curve-fitting the free vibration response of an equivalent single-degree-of-freedom system in the time domain. Differences between model and signal were compared using the root mean square (RMS) error, which was found to be lower on damaged disks. The contribution of the resonant frequency on a signal was measured using the signal-in-noise-and-distortion (SINAD), which was found to be a potential indicator of damage. Most damaged disks exhibited signals with high SINAD and low damping and were better described by the linear viscous damping model than signals from undamaged disks with higher RMS error and lower SINAD. A decrease in dynamic modulus of elasticity and an increase in air permeability on both mixtures after high temperature exposure was associated with high SINAD of the concrete disk signals. It seems possible to determine the exposure conditions and degree of damage on a single disk through API, SINAD and Ed. API was more sensitive to an increase in permeability after heat induced damage, although a high API may not necessarily suggest adverse service conditions since for damaged specimens, the high strength mixture had higher API than the conventional strength mixture, but maintained lower rates of absorption and liquid penetrability than conventional strength specimens.
Date: 2005-08-03
Degree: MS
Discipline: Civil Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/1969


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