Mechanical and Bond Properties of 18-mm- (0.7-in.-) Diameter Prestressing Strands

For several years, 18-mm- (0.7 in.-) diameter strands have been successfully used in cable bridges and for mining applications. The use of these large diameter strands in pretensioned concrete girders could allow approximately 35% increase in the prestressing force compared to the same number of 15-mm- (0.6 in.-) diameter strands and 92% increase compared to 13-mm- (0.5 in.-) diameter strands. Consequently, this process will allow for longer spans, shallower structural depth, and/or wider girder spacing in bridge construction. For the same prestressing force, the use of 18-mm- (0.7 in.-) diameter strands results in fewer strands to jack and release, fewer chucks, and greater flexural capacity due to lowering the center of gravity of the strands. Despite the advantages of using large diameter strands in pretensioned concrete girders, the lack of data on their mechanical and bond properties hinder their wide use in bridge construction. In this paper, the mechanical and bond properties of 18-mm- (0.7 in.-) diameter strands are evaluated. One hundred and two strand specimens were obtained from different strand producers and production cycles to evaluate the ultimate strength, yield strength, modulus of elasticity, and elongation at two different laboratories. Test results indicated that all strands adequately met the requirements of the ASTM standard A416-06, with the exception of the minimum yield strength requirements (90% of the specified ultimate strength). The power formula for stress-strain relationship was used to provide an accurate predictor of the behavior of strands. Also, 58 strand specimens were tested for their bond in mortar and concrete using the North America Strand Producers (NASP) test method. Test results demonstrated that the bond of 18-mm- (0.7 in.-) diameter strands is proportional to the concrete strength. A formula for predicting the NASP pull-out test value as a function of concrete strength was also developed. In addition, NASP test results for clean and rusted strands were measured and compared at different slip values.