| Timber transportation structures are generally exposed to varying and frequently harsh conditions. Over time, this exposure can lead to deterioration resulting from decay, insect attack, weathering, and mechanical damage. In turn, this deterioration may lead to a loss of structural integrity that is detrimental to the structure and its users. Nondestructive evaluation (NDE) can be used to monitor a bridge's condition and maintain structural safety.; A variety of NDE techniques were reviewed for their ability to identify and quantify decay in large bridge timbers. Ultrasonic and x-ray inspection techniques appeared to provide the best results in the literature. Ultrasonic inspection was selected for further development since it presented less health risks than x-ray inspection.; Ultrasonic inspection techniques were developed while inspecting two large Douglas fir timbers in a laboratory setting. One timber contained very little decay while the other timber contained severe decay at one end. The timbers were inspected over a closely spaced grid with the through-transmission technique. Moderate and more advanced decay was easily identified via changes in wave velocity. However, it was not ascertained if ultrasonic inspection could identify incipient decay.; Small Douglas fir specimens were employed to investigate the effects of incipient decay on ultrasonic waves. First, small compression perpendicular-to-the-grain coupons were removed from the severely decayed beam in the region expected to contain incipient decay. Second, small coupons were inoculated with the brown rot fungi, Postia placenta. Matched control coupons were kept free from decay. Each coupon was subjected to nondestructive inspection followed by a compression perpendicular-to-the-grain test. Incipient decay significantly affected ultrasonic parameters and compression perpendicular-to-the-grain behavior. Ultrasonic wave velocity and peak voltage were the two parameters most sensitive to incipient decay.; The developed ultrasonic inspection techniques were employed implemented in a field study and a validation test. Decay was identified in large timber pile caps of a Washington State highway bridge. The pile caps have subsequently been removed. Ultrasonic inspection was also used to successfully identify voids in a glulam beam that was used as a validation test by the Federal Highway Administration. |
