The Use of New Ultrasonic Pulse Echo Tomography in Historic Structure Applications

Nondestructive evaluation methods involving ultrasonic pressure waves have been used in modern and historic masonry investigations for many years. However, these methods (impact-echo and ultrasonic pulse velocity) have generally been limited to gathering information along a single ray path at a time. Tomographic software has been available for many years to post-process ultrasonic data, but this process was generally not automated, and it did not provide immediate feedback. With the advent of inexpensive and accessible computer processing power, several companies have introduced ultrasonic pulse echo arrays that collect and process tomographic images of subsurface conditions instantaneously. These arrays are often advertised for use in the location of subsurface conditions such as plastic electrical conduit and prestressing tendon grout in modern concrete structures. However, due to the use of ultrasonic waves and the ability to perform testing with access to one side of a structure only, this tool provides several unique capabilities in the investigation of historic structures. This paper describes some of the distinctive types of investigation available with new ultrasonic pulse echo array technology for historic structures. Several case studies will be included that illustrate the advantages and limitations of this equipment in the context of historic structures. Case studies include investigation of stone roof elements, cracking due to anchor corrosion, mortar joint conditions, and stone delamination.