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.