Bond Characterization of 17.8-mm (0.7-in.) Diameter Prestressing Strand

Abstract

Seven-wire prestressing strands are commonly used to pretension concrete bridge members. While the industry standard is 12.7-mm (0.5-in.) or 15.2-mm (0.6-in.) diameter strand, interest in developing greater pretensioning forces has resulted in the proposed use of 17.8-mm (0.7-in.) strands. This paper reports an experimental program aimed at characterizing the bond properties of 17.8-mm seven-wire prestressing strand and contrasting these with 9.5-mm (3/8-in.), 12.7-mm (1/2-in.), and 15.2-mm (−0.6-in) strands. The geometric and material properties of the strands are reported, and the evaluation of bond performance is assessed by standard test methods. Beam-end bond tests of both straight and 90° hooked strands are presented. The results of in situ transfer length determined from full-scale girders are also presented. Finally, finite-element analyses aimed at evaluating the influence of the Hoyer effect and strand spacing are presented. In all the tests, bond performance of 17.8-mm strand was found to be adequate and appropriately represented by existing AASHTO transfer and development length equations. The potential for utilizing the embedment of hooked strands into cast-in-place end diaphragms to increase the strand stress that may be developed near girder ends is proposed. From the in situ determination of transfer length, the mean and fifth percentile transfer lengths (determined with 95% confidence) of 0.7-in. strands were determined to be 30.4db (db is the strand diameter) and 53.4db, respectively, both less than the 60db prescribed by AASHTO. Additionally, the conventional 51-mm (2-in.) minimum center-to-center strand spacing appears to be adequate for 17.8-mm strands in terms of release stresses. No local cracking or other deleterious effects associated with strand spacing were observed. It is shown numerically that the lower Hoyer effect strand dilation and larger circumference result in marginally lower crack-inducing circumferential stresses. Although the current AASHTO determination of transfer and development length appears to overestimate these values for all strand diameters including 17.8-mm strands, the overestimation of transfer length underestimates concrete tensile stresses at prestress transfer in the area affected by the transfer length. This may result in unanticipated cracking. To address this, a two-tier approach is proposed: (1) using a reduced transfer length—40db is proposed—to check tensile stresses at prestress release; and (2) using the longer development length—using a transfer length component of 60db—to determine the load-carrying capacity of a prestressed concrete member.