DESIGN GUIDELINES FOR TRANSFER, DEVELOPMENT AND DEBONDING OF LARGE DIAMETER SEVEN WIRE STRANDS IN PRETENSIONED CONCRETE GIRDERS. FINAL REPORT

Abstract

Recently, a new and larger seven-wire strand was offered by industry for use in pretensioned concrete. The new strand size, 0.6 in. in diameter, has 40% greater area and 40% greater capacity than the current industry standard, 0.5-in.-diameter strand. Larger strand sizes can lead to improved efficiency of pretensioned structures; however, larger strands require greater bond forces to anchor the strands. In October of 1988, the FHWA issued a moratorium suspending the use of 0.6-in. strand in pretensioned applications. Recent studies had indicated that current design provisions were inadequate. Additional restrictions were placed on smaller sizes of strands. The limitations were adopted on an interim basis until additional research could substantiate or restructure current industry standards. One objective of this investigation is to determine the transfer and development length of 0.5-in.-diameter and 0.6-in.-diameter prestressing strands. The debonding, or blanketing, of strands is an alternative to draping strands in order to control the maximum concrete stresses. Debonding strands can simplify girder construction; draping strand is more difficult and more dangerous. Likewise, debonded strands enjoy economical advantages compared to draped strands. The second objective of this research is to develop design guidelines for the use of debonded strands in pretensioned concrete beams. A testing program was conducted that included measurement of transfer lengths, measurement of development lengths, and testing the behavior and performance of beams made with debonded strands. A simple analytical model was developed to predict the behavior of pretensioned bond. Bond failure is predicted based on the distress caused by cracks when they propagate through the anchorage zone of prestressing strands. Tests showed that the model accurately predicts strand anchorage, or, conversely, bond failure. Based on the experimental data, it was determined that bond failure would be prevented if no cracking occurred in the anchorage zone of a pretensioned strand. Design recommendations are made for transfer length, development length, and the use of debonded strands.