PTI DC80.3 PDF

Post-tensioning design, evaluation, and repair requires specialized knowledge. This guide is intended to provide an introduction to the techniques and procedures used to evaluate and repair unbonded post-tensioned structures. While the procedures outlined are effective on many projects, the requirements for each project vary due to different factors. Although there are many ways to effectively repair existing structures, this guide cannot address all of them. The principles addressed are general. The repair procedures discussed vary considerably in their value, expense, and disruption to the normal activities within the structure. Each project should be evaluated individually to assess the cost-effectiveness of the procedures described.

While this guide provides practical ways to assess the existing condition and extent of deterioration for the posttensioning system in a structure, identifying all existing conditions and predicting future tendon failure is typically not possible (ICRI 120.1; ICRI 210.2). Although failure of tendons manufactured with extruded sheathing is uncommon, older post-tensioning systems, non-encapsulated anchorages with poor-quality grout pockets and strand tail protection, and structures with deteriorated or poor-quality concrete are of concern. The rate of deterioration depends on many factors, including the environment, quality of original construction, structure use, concrete durability, degree of concrete deterioration, and maintenance.

An evaluation of unbonded post-tensioned systems requires an experienced investigator that can rely on previous experience and knowledge gained from working with similar structures. The evaluation should consider that all conditions of deterioration have not been identified and that the scope of the investigation and the presence of finish materials may limit the findings. The investigator should clearly identify the type of post-tensioning sheathing used on the project. Examples of post-tensioning sheathing include paper-wrapped, push-through, heat-sealed, or extruded. The investigator should be familiar with the various durability limitations and advantages of each sheathing type.

The experienced investigator should consider the type and age of the structure and the standards and practices used at the time of construction. Original construction code requirements and design of post-tensioned systems may vary from current code requirements and design. The evaluation should consider these differences in assessing the integrity of the existing post-tensioned system.

There is no substitute for experience and knowledge in working with post-tensioned structures. An experienced contractor and licensed design professional team is essential when working with post-tensioned structures. It is not enough for the firms to be experienced; the actual personnel performing the work should be experienced. It is recommended that all firms and on-site supervisory personnel have a minimum of 5 years of experience working with posttensioned structures, including certification in PTI training programs (such as the PTI Level 1 Unbonded PT – Field Installation program).

Strand that might become damaged by corrosion, accidental impact, brittle wire cracking, or coring and drilling after construction can lead to tendon failure. The failure of a tendon reduces the load-carrying capacity and may result in a sudden release of energy. If the highly stressed concrete behind the anchorage assembly fails, shattered concrete could be projected from the anchorage zone and high, low, or both high and low points of a tendon profile. Tendons can also erupt from the slab edge, the top, the bottom, or both the top and bottom of the surface.

Although dealing with high forces, the force in the tendon at stressing in almost all cases will be the largest experienced in the life of the structure. This is an important concept of proof testing, which is unique to posttensioned structures.