Transfer of prestress by pretensioned wire tendons
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Key words: End zone, prestress transfer, wire tendon, transmission length, pull-in, plain wire, indented wire, concrete strength, size of wire, gradual release, sudden release, shock release, time dependent effects.An empirical investigation into the transfer of prestress force from wire tendons to concrete in the end zones of pretensioned prestressed concrete beams was accomplished in this project. The experimental tests featured 56 small scale prestressed concrete beams.Some of the factors influencing prestress transfer which were considered in the current tests are as follows:(a) type of release - gradual, sudden or shock(b) surface condition of the wire - plain or indented(c) size of the wire(d) concrete compressive strength at the time of transfer(e) time dependent effectsMost of the tests involved gradual release of steel tendons with the prestressing force transferred in approximately ten equal increments. Sudden release in a single step was achieved by allowing the supporting abutments to retract rapidly. Shock release was implemented in some beams by angle grinding the wires. The type of release which gave the best quality of prestress transfer was gradual release. This was followed by sudden and shock releases respectively.There were four types of wires used in the laboratory tests: namely the 5 mm dia. Plain, 5 mm dia. Chevron indented, 7 mm dia. Plain and 7 mm dia. Belgian indented wires. Transmission lengths were determined from strain distributions for these wires. Pull-ins of the wire tendons at the ends of the beams were also measured.There was significant scatter in the experimental data. Different ranges of transmission lengths and pull-ins were obtained for the various types of wires used.Three equations were derived for the 5 mm dia. Plain, 5 mm dia. Chevron and 7 mm dia. Plain wires, which linearly correlated pull-ins to the transmission lengths. These relationships provide a qualitative and quantitative method of indirectly monitoring for the transmission lengths through the measurements of pull-in.Statistical inference tests proved that indented wires were superior in performance compared to plain wires, but the differences were more apparent for the pull-ins than for the transmission lengths.Comparisons on the influence of tendon size substantiated that greater pull-ins occurred for larger wires but the differences were not significant for the transmission lengths.For concrete strength at the time of transfer of less than 32 MPa, the transmission lengths and pull-ins were significantly larger than those for higher strengths. It is recommended that concrete strength at transfer be at least 32 MPa for pretensioned prestressed concrete.Apart from the maturity and strength of concrete, the quality of a mix also influenced the transmission length and there was limited data to suggest that a better grade mix despite having lower strength at a more tender age could outperform a lower grade mix with greater strength released after a longer curing period.Formulae for plain and indented wires were found by dimensional analysis which correlated the transmission length to the diameter of wire tendon and the stress/strength ratio of the prestressed beams.Pull-ins increased significantly over 6 months but the changes in the transmission lengths were small. Normalised longitudinal strain distributions did not indicate that transmission lengths would remain unchanged over time.
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