Ultrasonic measurements of grouted tendon ducts in the concrete structure using Elop Insight Scanner
Authors:   ELOP Technology, Chapagain Kamal Raj ,   ELOP Technology, Bjerke Werner ,   ELOP Technology, Wagle Sanat 
Published: January 2023

Infrastructures such as bridges, tunnels, dams, retaining walls, docks, parking garages and buildings are constructed from reinforced concrete. These structures deteriorate due to cyclic temperature and loading variations, aging, as well as physical causes such as accidents and aggressive chemical attack from the environment. Post tension tendon ducts commonly used in bridges are filled with grout and steel tendons. The voids in the grout inside the tendon ducts can occur due various reasons such as improper filling, material mixing, blockage due to tendons etc. The water can penetrate in these voids inside the tendon ducts which can initiate corrosion. It is therefore vital to detect voids/ unfilled or defective portions of the tendon ducts at an early stage of the construction phase to ensure the strength and safety of the structure. Different types of NDT (Non-Destructive Testing) methods are frequently used for monitoring such infrastructures during regular operation to prevent failure, increase life span and improve safety. One of these NDT techniques, ultrasonic testing, is relatively simple in operation and gives more accurate description of air pockets, voids, cracks, and delaminations in the concrete structures compared to other NDT methods.

ELOP has developed a rolling ultrasonic scanner COBRI (COncrete and BRidge Inspection) together with a complete software solution for asset management of structures for faster and better inspection and monitoring of the internal condition of the concrete. It will provide asset owners with information about when, where and how to inspect, maintain and more accurately assess damages and overall condition of their structures. The COBRI scanner contains an array of transducers assembled in two separate rollers used in pitch-catch mode. It uses a patented elastomer solution to provide dry coupling to the concrete surface during scanner movement, without the need for any added fluid couplant. The system uses SAFT (Synthetic Aperture Focusing Technique) algorithm for image reconstruction to increase image quality and signal to noise ratio. It will provide users with improved tomographic (B, C and D-scan) image quality and 3D imaging.

In the current work, we have used the COBRI scanner to explore its ability to image grouting defects inside tendon ducts used in concrete structures. The concrete specimen used in the evaluation has two tendon ducts positioned at 8 cm from the top surface with dimensions of 70 cm in length, 55 cm in width and 25 cm in thickness with average aggregate size of 22 mm. One of the tendon ducts is completely empty. The second one is half-length fully filled with grouting along the prestressed steel and other half only three quarters filled, leaving an air cavity on the top. The scanner is rolled approximately 50 cm over the concrete surface and the tomographic (B, C and D-scan) SAFT images are generated at different locations of the concrete specimen together with a 3D volume image. The results showed that the COBRI scanner is able to differentiate the filled and partially filled tendon ducts in the concrete structure. As COBRI is a rolling ultrasound scanner, it will help the bridge owners to scan and assess the grouting defects in the tendon ducts much more efficiently than other scanners available in the market.