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Washington State Transportation Center (TRAC)


Thermal imaging and ground-penetrating radar was conducted on concrete specimens with simulated air voids. For the thermal imaging inspections, six concrete specimens were constructed during the month of June 2007 to simulate the walls of post-tensioned box girder bridges. The objective was to detect simulated air voids within grouted post-tensioning ducts, thus locating areas where the post-tensioning steel strands are vulnerable to corrosion. The most important deduction taken from these inspections was that PT-ducts and simulated voids were more detectable in the 20 cm (8 in.) thick specimens than in the 30 cm (12 in.) thick specimens. While inspections of the 20 cm (8 in.) thick specimens revealed the majority of their simulated voids, only one thicker specimen inspection (12c) indicated the presence of simulated voids (four voids in two ducts). Also, PT-ducts were much clearer and visible in the thermal images of the thinner specimens. Ground-penetrating radar (GPR) inspection was conducted on fourteen concrete specimens between August and October 2007. Based on the GPR surveys conducted in this study, it is apparent that the detection of post-tensioning strands and simulated voids within grouted ducts embedded in concrete is possible with a 1.5 GHz GPR system. The layout of the top layer of steel reinforcement in each concrete specimen was evident in the GPR images, but the bottom layer of reinforcement was not clearly detected since it was effectively “hidden” beneath the top layer of rebar. Although none of the post-tensioning strands and simulated air voids within the grouted steel ducts was detectable, simulated voids within plastic ducts were generally detectable in GPR images. The high dielectric constant of the steel ducts did not allow the microwaves to transmit through the surface of the duct and reach the simulated voids. However, the general location of the duct, its orientation and its depth in the concrete were accurately determined using GPR. Thus it can be inferred that the void orientation is critical for detection in GPR images.


Bridge inspection, thermal imaging, GPR


Originally published in Washington State Transportation Center (TRAC), Research Report for Project DTJH61-05-C-00008, Task No.8.