Experimental assessment of advanced thermal imaging for detecting voids in ducts

Abstract

This research will evaluate the ability of a transient based Infrared Thermography (IRT) technology called the Infrared Ultra Time Domain (IR-UTD) system to detect defects in Post Tensioned (PT) ducts. In recent years, there has been an increasing number of bridges in the National Bridge Inventory (NBI) constructed using PT concrete design. Post Tensioning is a design concept in which steel tendons are placed inside plastic or galvanized steel ducts that are embedded within the concrete itself. If not constructed properly, voids are created during the construction process. These voids allow water and chlorides to ingress into the ducts and cause the steel tendons to corrode. A PT specimen will be cast with embedded defects at known locations and exposed to diurnal heating and cooling cycles. The IR system will collect data while the specimen is fully exposed to the environment. Data will also be collected after an air controlled enclosure is constructed around the specimen to simulate the temperature gradient that the outer wall of a box girder bridge is subjected to. This research shows that in direct sunlight, the IR-UTD processing produced indications from implanted defects of varying sizes inside of a galvanized steel duct. Additionally, the effect from having air control on the inside of the structure didn't significantly affect the results. To obtain a full scope of the abilities of the IRT system, a procedure to properly insulate the specimen to prevent uneven thermal gradients will be necessary. Testing during the summer months where the sun is in the sky for a longer period would also be beneficial.