Inspection and Evaluation of Post-tensioned Structures
I-88 and I-355 interchanges in Downers Grove, Illinois are composed of four precast segmental bridges under the management of Illinois State Toll Highway Authority (ISTHA). All four structures are approximately 15 years old and are prestressed with external and internal DYWIDAG Post-tensioning Systems.
Following the new Florida Department of Transportation standards for inspection and evaluation of post-tensioned structures, the Illinois Division of the Federal Highway Administration made recommendations to the Illinois State Toll Highway Authority to inspect and evaluate tendons in high-risk post-tensioned concrete bridges. The Repair and Strengthening Business Unit of DYWIDAG-Systems International USA inc. made a proposal to ISTHA to inspect this set of four segmental bridges and was awarded the project in early 2003.
DYWIDAG scope of work consisted of the preparation of the inspection plan and its implementation. The inspection plan was specific and tailored to the different types of post-tensioning tendons present in the structures. There were five phases of the project. A thorough visual inspection of post-tensioning anchorages and tendon paths was first performed. A Non-Destructive Test (NDT)using Ground Penetrating Radar (GPR) and Impact Echo (IE) scanning were performed to locate ducts and detect any grout voids that might be present in the ducts. Invasive inspection of a sample population of post-tensioning anchorages and tendon paths was performed.
A vibration technique was used on critical external tendon ducts to determine the status of the post-tensioning force and to check for any loss of prestress force. DYWIDAG volumeter machines were used to measure the volume of voids and vacuum pumps were used to verify vacuum environment inside the voids. The project started in March, 2003 and ended in July 2003.
During the visual inspection of all four bridges, careful examination was performed along the paths of embedded tendons as well as locations of their anchorages where various anomalies varying from concrete deterioration to water leaks and heavy efflorescence were observed. Cracks of HDPE pipes of external longitudinal tendons were also found.
The Ground Penetrating Radar was useful in detecting or confirming the location of embedded ducts.44 steel ducts were located with GPR.60 plastic ducts were located off the as-built drawings as GPR could not locate them. The Impact Echo Scanning indicated signs of no voids in 20 locations and debonding in 76 locations. The results of IE in 8 locations were inconclusive.
Preliminary results of visual inspections and Non-Destructive Testing were the basis for further invasive inspections. Altogether,204 locations were examined. Small to relatively significant voids were found in 64 locations with some tendons having exposed post-tensioning cables.
The vibration technique was useful and relatively accurate in determining the actual force in the external longitudinal tendons. 96 segments of external longitudinal tendons were tested by connecting an accelerometer to the HDPE pipe and exciting the pipe. The data was collected and processed. The actual force in the tendon was computed and compared to available effective design forces.
The project was a success for DYWIDAG. The prepared Final Report recommends specific course of action and is now being reviewed by ISTHA. The Repair and Strengthening Business Unit of DYWIDAG established a comprehensive Inspection Plan for precast segmental bridges and is now well positioned in the industry for similar projects.
It is important to mention that the ISTHA bridges are an example of well built structures despite the relative weakness of corrosion protection and grouting specifications in use at the time.The bridges were generally in good condition.