Non-Destructive Testing (NDT) Inspection Tools to See What is Beyond the Surface

Abstract Only - Cost effective design/repair strategies continue to be a top priority for all owners of our nation’s infrastructure. Dams are at the forefront of this expectation and according to frequent conditional assessment, many are in critical need of repairs. Engineers are well schooled in visual observation techniques to determine and rate the condition of dams, spillways and penstocks. Often visual observation are not sufficient to fully assess the integrity of these concrete members. Looking "inside" the concrete, determining the in-situ concrete strength, assessing the rebar condition and quantifying cracking allows for a comprehensive inspection which produces a better repair concept for dam owners. Technological advancements using non-destructive testing (NDT) methods now give engineers more tools to assess infrastructure and owners gain more confidence with the planned rehabilitation.

Ground penetrating radar (GPR) is one valuable tool that can assess the entire concrete component. Different antennae frequencies allow for different depths of penetration to see “inside” the concrete to qualify the steel reinforcing spacing, depth of cover and overall thickness. However understanding the in-situ strength of the concrete can determine if surficial repairs are needed or if a more extensive concrete removal approach is warranted. GPR cannot accomplish that objective but a sonic/ultrasonic system can determine relative concrete strengths without the effort/cost of collecting concrete cores for analysis.

A Sonic/ultrasonic system acquires impact echo and pulse velocity data simultaneously. Impact echo data measures the concrete thickness (dimensions) and will determine if delamination of the rebar with the concrete exists. Pulse velocity data is used to determine the average compressional and shear wave velocity of the concrete. The concrete dimensions and in-situ strength are calculated by measuring the energy wave’s travel time through the concrete. This presentation will focus on 2 case studies using these NDT tools to assess existing dam components and how they bolstered confidence for the owners with each unique repair approach.

Case 1- A deteriorated spillway was suspected of have voids below the concrete base allowing for water to remove fine material. GPR was used to look through the concrete and determine voided areas refining quantities for pressure grouting bid items.

Case 2- Concrete penstock flow tubes were being relined with fabricated sections of metal liners. The distance between the concrete and metal was to be pressure grouted. Sonic ultrasonic testing equipment was used as a QA/QC tool to verify the ½” space/gap was properly filled with grout.