Load tests were conducted on pretensioned concrete prisms cast with 13 different 5.32-mm-diameter prestressing wire types that are used in the manufacture of pretensioned concrete railroad ties worldwide. The tests were specifically designed to evaluate the bond performance of wires with different indentation type under the cyclic loading. The prestressing wires were denoted “WA” through “WM” and indentation types included smooth, spiral, chevron, diamond, 2-dot and 4-dot. Four wires were embedded into each concrete prism, which had a 3.5″ (88.9 mm) × 3.5″ (88.9 mm) square cross section. The wires were initially pulled to 7000 pounds (31.14 KN) 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.
For each type of wire one 69 in-long (175.26 cm) prism was tested in 4-point-bending under the cyclic load and one under static load at 20-in (50.8 cm) embedment length. For cyclic load tests, the prisms were supported by two rollers spanning 45″ (114.3 cm) and load was applied on a spreading beam set on the top of the test prism. The prism setup and loading configuration were symmetric and load was applied to the prism from spreading beam to two bearings spaced 15″ (38.1 cm) apart. During each test, a concentrated load with the rate of 250 lb/min (1112 N/min) was applied until two cracks were observed at the maximum moment region on the test prisms (region between two bearings).
Once cracks were observed, the load was held constant for 3 minutes at the cracking load. After holding load constant for three minutes, load started to cycle between 400 lb (1779 N) to 4000 lb (17790 N) with the frequency of 3 Hz. The test was designed to go through 200,000 cycles and interlock limits were assigned to the program to stop the test in case of prism failure under the cyclic loading. For prisms able to finish 200,000 cycles of load, the procedure was to unload to zero and start loading the prism with the rate of 250 lb/min (1112 N/min) until prism failed. Values of load, mid-span deflection, and wires end-slip were continuously monitored and recorded during each test. Load-vs-deflection graphs were then plotted and compared for prisms with each wire type, and the maximum sustained moment was also calculated for each test. Also, a set of statically load tests were conducted on the identical pretensioned prisms to compare the results of statically and cyclically load tests. The cyclic load tests revealed that there is a significant difference in the bond performance of different wire types under the cyclic loading and they behave differently under cycles of loadings and unloadings.
