Influence of Indented Wire Geometry and Concrete Parameters on the Transfer Length in Prestressed Concrete Crossties

Abstract

A study was conducted to determine the variation in the transfer length of prestressed concrete railroad ties with different indented wire geometries and different concrete properties, including slump and release strength. The study included 12 different reinforcement wire types that are used in concrete railroad ties worldwide. This paper presents the results from transfer length measurements on 96 pretensioned concrete members that were cast in the laboratory. In order to replicate the wire-to-concrete proportions commonly used in prestressed concrete railroad ties, small (3 1/2″ (88.9 mm) × 3 1/2″ (88.9 mm)) prestressed concrete prisms were fabricated and each contained four 5.32-mm-diameter indented wires. A special jacking arrangement was used to ensure that each of the wires was tensioned to the same jacking force. The wires were initially tensioned to 7000 pounds (31.14 kN) each, and the transfer of prestress force into the members was accomplished by a gradual release method replicating the one used in most prestressed concrete crosstie manufacturing plants. The study consisted of two phases. In the first phase, 36 concrete prisms were cast to investigate the effect of different wire indent geometry in a 6-inch (152.4mm) slump concrete mix with 4500 psi (31.03 MPa) release strength. In the second phase, a total of 60 prisms were used to investigate the effect of 4 different concrete parameters with a select group of 5 indented wire types. The second phase included concrete release strengths of 3500 psi (24.13 MPa) and 6000 psi (41.37 MPa), and concrete consistencies (slumps) of 3 (76.2) and 9 inches (228.6 mm). The results have shown that there is a significant variation in transfer lengths for the different indented wires at the same release strength. Additionally, the results show that the transfer lengths decreased significantly with modest increases in the concrete release strength. However, there was no correlation observed between transfer lengths and different concrete slumps for mixes having the same water-to-cementitious (w/c) ratio. For each concrete pour, the splitting tensile strength and modulus of elasticity were measured at the time of prestress transfer. All wire indents were measured according to ASTM A-881 [1] and the results of both phases are presented.