Report on Corrosion and Repair of Unbonded Single-Strand Tendons (ACI 423.4R-14)

Description

This report provides general information on the evalua-tion of known or suspected corrosion problems in unbonded single-strand tendons, historical information on the types and components of unbonded tendons and on the durability and corrosion protection provisions in the building code, and describes typical repair methods in use today. Expertise in design, construction, evaluation, and repair of structures using single-strand unbonded tendons is strongly recom-mended for a team undertaking evaluation and repair of corrosion problems. Historically, there have been corrosion problems with other types of pre- and post-tensioning systems (Nehil 1991); however, certain aspects of corrosion of unbonded single-strand tendons are unique.

The causes and effects of corrosion of unbonded single-strand tendons are, in several respects, different from those of bonded conventional reinforcing or other post-tensioning (PT) systems, so the methods for evaluating and repairing corrosion of single-strand tendons are also different. For example, because the tendons are largely isolated from the surrounding concrete, they may not be affected by deleterious materials such as chlorides and moisture in the concrete.

The surrounding concrete does not stop corrosion alone because, if water gains access to the inside of the sheathing, PT coating is rendered ineffective. Measures taken to repair and protect the surrounding concrete may not repair or reduce deteriora-tion of the prestressing steel where corrosion has been initi-ated. The tendons usually require separate evaluation and repair.