InTrans / Aug 14, 2018

Ultrasonic Inspection of Large Bridge Timbers

Timber bridges are susceptible to attack from wood decay fungi. Significant losses in wood material properties are associated with increasing decay. Therefore, timber bridges must be monitored for decay in order to maintain structural safety. Historically, decay has been identifiable in large timber structural elements only after significant losses in structural performance have occurred. Recent developments in nondestructive evaluation techniques have focused on identifying decay before large strength losses occur. For this study, an ultrasonic inspection technique was developed for identifying incipient decay in large bridge timbers. Two large bridge timbers were nondestructively evaluated via ultrasonic inspection. Relative ultrasonic wave velocity measurements were used to identify locations of moderate to advanced decay. The timbers were then dissected and the predicted internal condition compared favorably to the actual internal condition. Specimens suspected of containing incipient decay were removed from one of the bridge timbers and
subjected to further ultrasonic testing followed by physical testing. The objective was to develop an ultrasonic inspection technique that can be used to identify the presence of decay before significant losses in physical properties occur. The developed technique involved transmitting ultrasonic waves through the specimen and measuring characteristics of the received signal. The characteristics of the received signal were then correlated with the physical properties of the specimen. Two main signal parameters can be used to identify decay. Wave velocity can be used to identify the presence of moderate to advanced decay but cannot be used to identify incipient
decay. Relative ultrasonic wave attenuation can be used to identify incipient through advanced decay. The ability to identify incipient decay will allow for corrective action to take place before structural safety is compromised and before expensive structural repairs are required.