Effect of Strand Indentation Types on the Development Length and Flexural Capacity of Concrete Railroad Ties Made With Different Prestressing Strands

Pretensioned concrete prisms made with five different prestressing strand types (four 7-wire strands and one 3-wire strand) were load tested to failure to understand the effect of strand indentation types on the development length and bonding performance of these different reinforcements. The prestressing strands were denoted SA, SB, SD, SE and SF. SA was a smooth strand while the other four were indented strands. All strands utilized in manufacturing ofprisms had diameter of 3/8″ (9.52 mm). Among all types of strands, SF was the only 3-wire strand and the remaining strands were all 7-wire strands. For all types of strands, four straight strands were embedded into each concrete prism, which had a 5.5″ (139.7 mm) × 5.5″ (139.7 mm) square cross section. The strands were tensioned to 75 percent of ultimate tensile strength of strands and gradually de-tensioned when the concrete compressive strength reached 4500 psi (31.03 Mpa). A consistent concrete mixture with type III cement, water-cement ratio of 0.32 and a 6-in. slump was used for all prisms. Prisms were load tested in 3-point-bending at different embedment lengths to obtain estimations of the development length of each type of strand. Two out of three identical 69-in.-long (175.26 cm) prisms were load tested at one end and one was tested at both ends for each reinforcement type evaluated. First prisms were tested at 28-in. (71.12 cm) from the end, while second prisms were tested at 20-in. (33.02 cm) from the end. Third prisms were loaded at 16.5-in. (41.9 cm) from one end and 13-in. (33.02 cm) from the other end. Thus, a total of 20 load tests (5 strand types × 4 tests each) were conducted in this study. During each test, a concentrated load with the rate of 900 lb/min (4003 N/min) was applied at mid-span until failure occurred. Values of load, mid-span deflection, and strand endslip were continuously monitored and recorded during each test. Plots of load-vs-deflection were then compared for prisms with each strand type and span, and the maximum sustained moment was also calculated for each test. The load tests revealed that there is a large difference in the development length of the strands based on their indentation type.