Detection of Voids in Grouted Ducts for Prestressed Concrete Bridges using Thermal
Imaging and Ground-Penetrating Radar
David G. Pollock, Ph.D., P.E.
Associate Professor
Department of Civil & Environmental Engineering
Washington State University (WSU)
Transportation Research Center (TRAC)
The potential effectiveness of thermal imaging and ground-penetrating radar (GPR) detection of
tendons and voids in prestressed concrete members was demonstrated in a recently completed
FHWA-sponsored research project entitled Nondestructive Evaluation of Prestressed Concrete
Box Girders for the I-90/4th Avenue On-Ramp Bridge. In particular, thermal imaging with an
infrared (IR) camera was successfully used as a global nondestructive inspection technique to
locate tendons and detect the presence of simulated voids (expanded polystyrene) as small as 1.5
in. (thick) adjacent to post-tensioning strands in grouted steel ducts and grouted high density
polyethylene (HDPE) ducts embedded in 8 in. thick and 12 in. thick concrete slabs with concrete
cover of 2 in. or less (Pearson, 2003; Musgrove, 2006). GPR was successfully used as a local
inspection technique to precisely locate simulated voids as small as 1.5 in. (thick) adjacent to
post-tensioning strands in grouted HDPE ducts embedded in concrete slabs at depths of 2 in. to 6
in. (Conner, 2004). Furthermore, during the dissection and inspection of several tendon
anchorage specimens retrieved from the demolition of the I-90/4th Avenue on-ramp bridge in
Seattle, a void (½ in. thick) in the grout was observed at the duct-to-anchorage interface for one
of the specimens (Pollock, 2005). Based on recent discussions with Washington State
Department of Transportation (WSDOT) engineers, additional research is needed to:
1) detect and precisely locate smaller voids (approximately ½ in. thick) that often occur near
tendon harp points due to grout segregation during pumping operations.
2) investigate the detection of voids in grouted ducts at depths of 6 in. to 12 in. in concrete
members.
3) demonstrate and document the effectiveness of nondestructive thermal imaging and GPR
inspection through field inspection of a concrete bridge from the WSDOT inventory.
This research will include the construction and nondestructive evaluation of concrete specimens
containing small simulated voids (½ in. thick) in grouted steel and HDPE ducts at varying
depths, similar to the voids that occur at harp points in prestressed concrete bridge girders.
Inspection techniques will be developed/improved for detecting the small simulated voids using
commercial thermal imaging and GPR equipment. A concrete bridge (to be selected by WSDOT
engineering staff) will also be inspected using thermal imaging and GPR equipment to evaluate
the effectiveness and practicality of incorporating these nondestructive evaluation techniques in
WSDOT's bridge maintenance and inspection program. Finally, thermal imaging and GPR
inspection guidelines will be developed for dissemination to bridge inspection and maintenance
staff in various state Departments of Transportation. The project will have a duration of 18
months, with an overall budget of $120,000.