During our last project, five prestressed concrete bridges were investigated in refer to the condition evaluation of the prestressing tendon ducts.
After the completion of the visual inspection of each structure, it was concluded that evaluation of injection grout into tendon ducts should be performed.
It is a common problem in prestressed concrete structures to observe that some tendon ducts are partially or even totally voided.
This defect has crucial meaning for structural and functional safety reasons of the bridge structure. Existence of partially injected ducts causes continuous durability threat to the prestressing system (corrosion damages/defects are already visible even so bridge is rather new) and to the overall structure long-term safe use.
Below, the main services provided during investigation of tendons at a bridge are described:
  • Firstly, locating of tendons at several positions of the web of the box girder is performed by performing scans with GPR equipment. Mainly, 2D scans were executed at several testing areas in order to ensure that tendons are positioned properly. Since, tendons are located through GPR scans then evaluation of ducts can be performed. Each tendon is marked with a line on concrete surface (length 1,5m-2,0m) and a testing grid per 100mm is defined.
  • Then Ultrasound 3D tomography measurements are performed in order to locate suspicious areas (voided) around and inside the duct. Also, by ultrasound measurements (B-scans), the shear velocities of concrete are estimated at several testing positions and the depth of the tendon’s duct.

Following the tomography evaluation, at certain suspicious testing positions, Impact-echo tests are performed in order to estimate the depth and the extent of possible voids around or/and into the duct. The red colored area shown in picture below, indicates a suspicious zone where probably tendon duct is partially or totally empty.

Finally, microdrills and core drillings are executed at the most suspicious testing points in order to evaluate the condition of the ducts. Then, opening of the corrugated metallic ducts is performed by using special tools and injection grout sampling is performed. Also, condition of strands by visual observation is performed and the corrosion class of the strands is evaluated. The aforementioned process is repeated for each duct and according to the insitu findings.