Abstract

Unbonded post-tension (PT) tendons have been used for many years to reinforce concrete structures. Generally, these structures have performed well except where unbonded PT tendons have suffered from corrosion due to moisture penetration or protective grease deficiencies.

Like other technologies, unbonded PT systems have evolved and improved over the years from "paper-wrap" to "push-through" to "heat sealed" to "extruded" to "fully encapsulated" systems. Consideration should be given to the type of system when selecting the appropriate evaluation and corrosion mitigation methods for these systems.

Evaluation of unbonded post-tensioned structures is important to determine the current condition and to determine if corrosion and deterioration is likely to occur. If broken tendons or corrosive conditions are identified, a suitable mitigation strategy should be implemented such that the structural integrity can be maintained, and the service life of the structure can be met or extended.

This paper will discuss the types of unbonded PT structures which exist, evaluation techniques which can be used to identify the presence of corrosion, and mitigation methods which have been developed to mitigate corrosion of unbonded PT tendons. Project case studies will be presented to demonstrate the applicability and effectiveness of these techniques.

Introduction

Unbonded post-tensioned reinforcement in concrete structures has been used for many years in elevated slabs (parking garages and residential or commercial buildings), residential foundations, walls, and columns and more recently in bridge structures. The use of unbonded post-tensioned reinforcing allows for unique and cost-effective design and construction that include: thinner concrete sections, longer spans between supports, stiffer walls to resist lateral loads, and stiffer foundations to resist the effects of shrinking and swelling soils.

Unbonded post-tensioned reinforcing is accomplished by placing high strength steel tendons or bars into a sheathing or duct, allowing it to move as the tensioning force is applied after the concrete cures. The steel elongates as it is tensioned, and it is locked into place using an anchoring component that forms a mechanical connection and maintains the force in the strand for the life of the structure.

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