Ultrasonic echo testing is a more and more frequently used technique in civil engineering to investigate concrete building elements, to measure thickness as well as to locate and characterise built-in components or inhomogeneities. Currently the Synthetic Aperture Focusing Technique (SAFT), which is closely related to Kirchhoff migration, is used in most cases for imaging. However, this method is known to have difficulties to image steeply dipping interfaces as well as lower boundaries of tubes, voids or similar objects. We have transferred a processing technique from geophysics, the Reverse Time Migration (RTM) method, to improve the imaging of complicated geometries. By using the information from wide angle reflections as well as from multiple events there are fewer limitations compared to SAFT. As a drawback the required computing power is significantly higher compared to the techniques currently used. Synthetic experiments have been performed on polyamide and concrete specimens to show the improvements compared to SAFT. We have been able to image vertical interfaces of step-like structures as well as the lower boundaries of circular objects. It has been shown that RTM is a step forward for ultrasonic testing in civil engineering. 1. Introduction Quality assurance and damage assessment of concrete structures as buildings, bridges, and dams are major tasks in civil engineering. On one side construction drawings may be missing or are considered to be unreliable, which is a drawback for maintenance and rehabilitation measures. On the other side parts of the public infrastructure, namely, bridges, suffer from aging, increased traffic, and increasing individual truck weight. In addition nondestructive inspection methods are required to provide a reliable quality assurance for new, repaired, or rebuilt structures. Since about twenty years, ultrasonic techniques are more and more frequently used to map the interior of structures, for example, to locate layers, voids, objects, or other features. The localisation and characterisation of tendon ducts is of major importance. These ducts are used to apply external forces to the concrete via prestressed steel wires to enhance its tension resistance. After prestretching these wires, the duct is grouted with mortar in order to couple the wires and the concrete as well as to avoid corrosion. Remaining air voids are a problem for the durability of the structure. In many cases the first step in an assessment is to locate the tendon ducts, because quite often they are not placed in accordance with the design plans.
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