The prevalence of concrete and cement-based materials in the civil infrastructure
plus the risk of failure makes structural health monitoring an important issue in the
understanding of the complete life cycle of civil structures. Correspondingly, the
field of nondestructive evaluation (NDE) has been maturing and now concentrates
on the detection of flaws and defects, as well as material damage in early stages of
degradation.
This defect detection is typically usually done by looking at the impulse response
of the medium in question such as a cement-based material. The impulse response
of a solid can be used to image a complex medium. Classically, the waveform is
obtained by an active setup: an ultrasonic signal is generated at one location and
recorded at another location. The waveform obtained from imaging can be used to
quantitatively characterize the medium, for example by calculating the material's
diffusivity coefficient or dissipation rate.
In recent years, a different monitoring technique has been developed in seismology
to measure the velocity of different kinds of waves, the Coda Wave Interferometry
(CWI). In this CWI technique, the main focus is given to the late part of the recorded
waveform, the coda. CWI is now successfully used in seismology and acoustics.
In the current research, CWI is applied on concrete in different damage states
to develop basic knowledge of the behavior of the wave velocity, and how it can be
used to characterize cement-based materials. By comparing two impulse responses,
the relative velocity change between the two impulse responses is used to characterize
damage. Because of the stress-dependency of the velocity change, the calculations can also be used to directly calculate the Murnaghan's and Lam´e's coefficients. The
newer technique of CWI is applied - the Stretching Technique (ST) [27]. The first goal
of this research is to establish the viability of using CWI in cement-based materials.
Next, we use the ST in the application of stress as we compress concrete samples for
the detection of thermal damage, ASR-damage and mechanical softening.
| Identifer | oai:union.ndltd.org:GATECH/oai:smartech.gatech.edu:1853/37237 |
| Date | 30 August 2010 |
| Creators | Schurr, Dennis Patrick |
| Publisher | Georgia Institute of Technology |
| Source Sets | Georgia Tech Electronic Thesis and Dissertation Archive |
| Detected Language | English |
| Type | Thesis |
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