Detecting voids inside ducts of bonded steel tendons using impulse thermography: Fid-Bau Portal

Detecting voids inside ducts of bonded steel tendons using impulse thermography

Rieck, C. / Hillemeier, B.

Ducts of tendons are filled with grout of cementitious materials to protect the prestressing steel against corrosion and to ensure adequate bond between concrete and prestressing steel. Poorly grouted ducts reduce the durability of prestressed concrete structures. In this paper an NDT-method is presented to detect voids in the ducts of prestressed concrete using impulse thermography. Impulse thermography is developed and applied to detect subsurface defects as for example in concrete and masonry structures. For detection, heat is induced by either an internal or external heat source. The transient heat transfer to the surface is then observed using an infrared camera. Compared to defect-free regions, defects appear as areas of different temperatures. Also, deeper defects are observable at a later time and with reduced contrast. To investigate applicability of impulse thermography towards detection of voids in ducts, three concrete specimens were constructed, using two different types of tendons at various depths (5 cm to 10 cm). Cylindrical, 40 cm-long voids are located inside the ducts. The specimens were heated by different internal and external heat sources. Due to the heat of hydration of the cement lime inside the ducts, voids with a concrete cover of up to 10 cm could be detected at an early stage. After hydration of the lime, voids can be detected by heating the specimens from the surface with an infrared array. Alternatively, heat from a welding apparatus may be induced through the prestressing steel, using the electric resistance of the prestressing wire. Preliminary investigations using the FEM-program ANSYS helped in the design of specimens as well as in the experimental work. This project is sponsored by the Deutsche Forschungsgemeinschaft (DFG).