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Development Length of 0.6-Inch Prestressing Strand in Standard I-Shaped Pretensioned Concrete Beams

Barnes, Robert W.

2000

The use of 0.6 in prestressing strand at a center-to-center spacing of 2 in allows for the optimal implementation of High Strength Concrete (HSC) in precast, prestressed concrete bridge superstructures. For this strand configuration, partial debonding of strands is a desirable alternative to the more traditional method of draping strands to alleviate extreme concrete stresses after prestress release. Recent experimental evidence suggests that existing code provisions addressing the anchorage of pretensioned strands do not adequately describe the behavior of these strands. In addition, the anchorage behavior of partially debonded strands is not fully understood. These uncertainties have combined to hinder the full exploitation of HSC in pretensioned concrete construction.

A research study was conducted to determine the anchorage behavior of 0.6 in strands at 2 in spacing in full-size bridge members. The experimental program consisted of assessing transfer and development lengths in plant-cast AASHTO Type I I-beams. The influence of concrete compressive strengths ranging from 5700 to 14,700 psi was examined. In order to consider the full range of strand surface conditions found in practice, the prestressing strand featured either a bright mill finish or a rusted surface condition. The anchorage behavior of partially debonded strands was investigated by using a variety of strand debonding configurations—including debonded strand percentages as high as 75 percent. A limited investigation of the effect of horizontal web reinforcement on anchorage behavior was performed. Pull-out tests were performed in an attempt to correlate results with the bond quality of the strands used in the study. The correlation between strand draw-in and the anchorage behavior of prestressing strands was also examined.

A review of the evolution and shortcomings of existing code provisions for the anchorage of prestressing strands is presented. Results of the experimental program are reported, along with recommended design procedures based on these results and those from other studies. The use of 0.6 in strand at 2 in spacing is concluded to be safe, and partial debonding of prestressing strands is shown to be an effective means of reducing stresses in the end regions of pretensioned girders.

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