The strength of post-tensioned members of pre-stressed concrete structures is basically related to tensile strength of post-tensioned steel. For Protection against rusting, this steel is usually put in PVC or corrugated steel dust then ducts are filled with cement mortar. Nevertheless, due to incorrect method of filling, steel may be rusted. Besides, imperfect design of used mortar may lead to bleeding casing longovoides, sometimes to the extent of the whole duct. Bleed voids are the main cause of steel rusting, because they provide paths for penetration and movement of salt solutions and other harmful chemical solutions hrough the ends of anchorage and existing cracks. This investigation presents a nondestructive test called “impact —echo” for studying the position of mortar inside these ducts and determining the location and size of voids. Theoretical and experimental studies indicate the high accuracy of the method in tracing and size determination of these voides. It has been shown in this study that information about existence and size of these voids can be obtained using studying the amount of reflected dominating frequency resulted from a mechanical impact on the surface of the concrete member.
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