Load tests were conducted on pretensioned members made with five different strands (three 7-wire strands and two 3-wire strands) to determine the effect of concrete release strength on the development length and flexural capacity of members. Strands named generically SA, SC, SD, SE and SF and they were all indented except SA (no surface indentation). All strands had diameter of 3/8″ (9.52 mm) except SC which had diameter of 5/16″ (7.94 mm). Among all types of strands used in manufacturing of test prisms, SC and SF were 3-wire strands, while SA, SD and SE were 7-wire strands. A consistent concrete mixture was used for the manufacture of all test specimens, and the different release strengths were obtained by allowing the specimens to cure for different amounts of time prior to de-tensioning. For SA, SD, SE and SF strands, each prismatic specimen (prism) had a 5.5″ (139.7 mm) × 5.5″ (139.7 mm) square cross section with four strands arranged symmetrically. However, prisms made with SC strand had 4.5″ (114.3 mm) × 4.5″ (114.3 mm) square cross section with four strands arranged symmetrically. The prisms were identical except for the strand type and the compressive strength at the time of de-tensioning. All four strands were pulled and de-tensioned gradually when the concrete compressive strength reached 3500 (24.13 MPa), 4500 (31.03 MPa) and 6000 (41.37 MPa) psi. Precise de-tensioning strengths were ensured by testing 4-in.-diameter (101.6 mm) × 8-in.-long (203.2 mm) compression strength cylinders that were temperature match-cured.
The prisms were loaded in 3-point-bending to determine the ultimate bond characteristics of each reinforcement type for the different concrete release strengths. A loading rate of 900 lb/min (4003 N/min) for 5.5″ (139.7 mm) × 5.5″ (139.7 mm) prisms was applied at mid-span and the maximum sustained moment was calculated for each. Same procedure with loading rate of 500 lb/min (2224 N/min) was applied to 4.5″ (114.3 mm) × 4.5″ (114.3 mm) prisms. Three 69-in.-long (175.26 cm) prisms, each having different concrete release strength, were tested with each of the 5 strand types. Two out of three testing prisms were tested at only one end and one was tested at its both ends. Thus, for each strand type and concrete release strength evaluated, a total of 4 tests were conducted for a total of 60 tests (5 strand types × 3 release strengths × 4 tested embedment lengths). Test results indicate that the concrete compressive strength at de-tensioning can have a direct impact on the ultimate flexural capacity of the members, and this has significant design implications for pretensioned concrete railroad ties. Results are discussed and recommendations made.