Assessing ability of ground-penetrating radar to detect internal moisture and fungal decay in Douglas-fir beams

This report describes the testing plan and preliminary results in an ongoing project to assess the efficacy of using ground-penetrating radar (GPR) to detect internal moisture and fungal decay within Douglas-fir beams. Initially, the test beams were assessed using a variety of physical, mechanical, and nondestructive evaluation (NDE) test methods including micro-resistance drilling, Janka hardness, stress wave transmission, and GPR. After initial baseline assessment, test beams were inoculated with brown-rot fungus, Fomitopsis pinicola, and exposed to just above-ground conditions 22 miles (35.4 km) north of Gulfport, Mississippi, USA. Test beam specimens were inspected in the field at 6, 12, and 17 months of exposure using GPR, moisture measurements, and stress wave transmission. Select specimens were removed from the field during each site visit and returned to USDA Forest Service, Forest Products Laboratory, where the NDE tests were repeated. In addition, computer tomography (CT) scans were taken of a subset of the returned specimens. Finally, destructive Janka hardness testing provided insight into mechanical property change during exposure. The GPR scans were compared with the NDE, CT scans, and hardness results. It was found that the GPR scan was sensitive to the presence of internal moisture, which will be useful in locating areas of potential decay.