Although misaligned or misplaced dowel bars can adversely affect the performance of concrete pavements, engineers have had few practical options for verifying the position and orientation of dowel bars. This article describes a new state-of-the-art, nondestructive testing device known as MIT Scan-2 that offers a faster, easier and more accurate means for measuring the three-dimensional position of dowel bars embedded in concrete. The MIT Scan-2 system consists of a sensor unit, an onboard computer that controls the testing process and records data, and a special plastic rail system that guides the unit along the joint to be scanned. During testing, the device emits a weak, pulsating magnetic signal and detects a transient magnetic response signal induced in the metal dowel bars. Employing the methods of tomography, the user is able to determine the position of a bar by taking a cross section mathematically to obtain useful information such as the bar's orientation in the vertical and horizontal planes. Field tests using the MIT Scan-2 system have shown that many in-service pavements may contain at least a few bars that do not satisfy the current specification requirements, even on well-performing pavements with no signs of distress. The MIT Scan-2 system was also used successfully to study the relative effectiveness of using dowel baskets versus dowel bar inserter systems during construction.
Abstract