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A need exists to develop a non-destructive testing technique that can identify
the formation and propagation of diagonal tension cracks in conventionally
steel reinforced concrete deck girder (RCDG) highway bridges in the State of
Oregon. Such a technique could be included into a structural health
monitoring (SHM) system installed on specific bridges to automatically
monitor the current state of structural damage in primary load supporting
elements and provide notification of recent damage to bridge engineers in
nearly real time. This research investigates the practical application of AE
used to supplement a conventional SHM on vintage RCDG bridges.
Background work presented in the Appendices investigates stress wave
propagation in non-reinforced and steel reinforced concrete media. Based on
the characterization of stress wave speeds, amplitude attenuation, frequency
content and wave forms found in concrete media, testing methods are
developed and applied to 31 full sized RCDG test specimens that include
variations in loading, load capacity and structural detailing. Several different
AE test procedures are used to characterize the damage states of the test
beams as they are progressively loaded to failure. Four previously developed
AE parameters that characterize both damage progression and damage state
are applied which include the Felicity and Calm Ratios, Severity and the
Historic Index. Both Felicity and Calm Ratios were found to respond to the
damage state of the test beam as determined from more conventional
assessment methods such as crack width and load. For the practical in-service
loading ranges of 20 to 80% of ultimate capacity both the Felicity and Calm
Ratios were found to respond in a nearly linear manner with increasing
damage. Three categories of damage state are defined which are based on the
ODOT crack comparator tool which is used for in-service maintenance
inspections of these bridges. Felicity and Calm Ratio values are related to
these damage states for the specific type of bridge girders being tested and can
be used to estimate in-service damage states. The Severity and Historic Index
responses were found to be an effective means of identifying the formation
and extension of diagonal tension cracks as they developed. Threshold levels
for these two parameters are identified for specific AE sensor types when
applied to this class of bridge girder.
A preliminary set of AE testing and analysis procedures were developed that
were applied to three in-service bridges. These bridge tests used both
controlled and ambient loading protocols. The structural response to each load
case was quantified by using both crack width motion and reinforcing steel
strain range. These structural parameters were correlated with the AE data.
The Calm Ratio was found to be of practical importance and that the values
recorded were in reasonably good agreement with the laboratory data once the
imposed loads and current crack widths were considered. The Severity and
Historic Index were also found to be of practical importance to bridge testing
and structural health monitoring as they were found to be very sensitive to
increasing damage, yet exhibit good stability provided enough AE activity
was present. A recommend set of guidelines and practices for applying AE to
vintage RCDG bridges is developed and presented. |
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