NDT Division was asked to conduct a Phase 1 assessment of the concrete spillway. The engineers determined two locations to assess the different construction conditions and the concrete's condition. One test location was at the top of the spillway, and one was located downstream. The engineers required compressive strength estimates of the concrete, thickness of the concrete, reinforcing pattern present, and potential voiding information on the materials below the slab.
NDT Division collected Impact echo/pulse velocity (IE/PV) and ground penetrating radar (GPR) measurements to assess the area of the two test locations. The IE/PV data was collected in a grid pattern within the designated areas to determine the concrete compressive strength as well as the thickness of the slab. GPR data using a high-frequency antenna (1500 MHz) was collected along a similar grid to determine the reinforcing pattern and cover depth and verify the slab thickness. A low-frequency antenna (900 MHz and 400 MHz) was used to determine the condition of material placed below the slab and if voiding was present.
The IE/PV data results yielded compressive surface strength estimates between 3000 and 5000 PSI for the concrete and confirmed the thickness of the slab was between 14-18 inches. The GPR scans determined vertical and horizontal bar spacing for each area, depth of cover, and slab thickness verification. The results of this testing were combined into a report and submitted to the engineer to assist them in determining the differences in construction between the two areas in question. GPR showed variable conditions supporting the slab, including bedrock, fill material, and the presence of voiding below the spillway concrete. Small holes were drilled into voiding areas to allow for a borescope to confirm the GPR results.