Unbonded Post-Tensioning System Technology in Building Construction

Introduction

During the last three decades, unbonded post-tensioning has progressively become the predominant construction choice for commercial concrete buildings in the United States. The utilization of unbonded tendons is now standard practice for concrete structures. Because of their excellent performance record and their economical and versatile application, the total use of unbonded tendons increased from approximately 7500 tons in 1976 to over 180,000 tons of prestressing steel in 2005, adding up to over 1 billion square feet of slab constructed with unbonded post-tensioning.

Early applications of unbonded tendons and their limitations in design and construction were soon succeeded by well-established analysis, design, detailing, and field procedures. Driven by durability considerations, post-tensioning manufacturers have implemented product improvements for corrosive environments. The most mean- ingful testimony for unbonded post-tensioned construction is the successful performance of over 1 billion square feet of concrete construction.

The four common types of tendon systems are monostrand tendons, single-bar tendons, multistrand tendons, and multiwire tendons. During the 1960s and early 1970s, several proprietary tendon systems were used for concrete building structures. Unbonded single-strand or monostrand (1/2-in.-diameter, seven-wire) tendons with various types of sheathing applications were used predominantly over construction using unbonded single-bar tendons in plastic (or metal) ducts and unbonded button-head wire tendon.

Unbonded Post-Tensioning System Technology

In the United States, the first unbonded monostrand tendons were used in the mid-1950s for building construction using greased and paper-wrapped seven-wire strand. The spirally applied continuous paper strip was intended to be a bond breaker between the strand and concrete, and the grease coating assumed the role of corrosion protection. Plastic sheathing introduced during the mid- to late 1960s assumed the roles of

(1) bond breaker,

(2) protection against damage by mechanical handling, and

(3) providing a barrier against intrusion of moisture and chemicals.

Durability of Unbonded Tendons

Post-tensioned concrete members inherently provide enhanced durability due to the limitation of cracks that provide access to reinforcement for corrosive agents. The use of post-tensioning typically eliminates most slab joints, which, if not eliminated, may give corrosive agents access to beams and columns.

The excellent durability performance of existing concrete structures in corrosive environments reflects the potential of unbonded monostrand tendons. Most of these structures were built without specific durability design and construction considerations. The visual inspection of a 15-year-old parking structure in Baltimore, Maryland, following demolition  confirmed that no significant corrosion had occurred on the unbonded tendons over the 15-year service life (Suarez and Posten, 1990).

The distinguishing characteristic of an unbonded tendon is that, by design, it does not form a bond along its length with the surrounding concrete. The axial force in the stressed tendon is transferred to the concrete primarily by the anchors provided at each end. Because the force of an unbonded tendon is primarily resisted by the anchors at each end, the long-term integrity of tendons and anchors throughout the service life of an unbonded tendon are of concern.

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