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An overview of ACI, PTI, and ASTM documents
The first use of unbonded post-tensioning in the United States was for roof slabs constructed in the late 1950s by the lift slab method where the slabs were cast on forms on the ground and then lifted into place; the tendons consisted of buttonheaded wires wrapped in paper. Since that time, the amount of unbonded post-tensioning used in building construction has increased dramatically and there have been considerable changes in both construction methods and post-tensioning systems. Almost all unbonded post-tensioning is now done with cast-in-place concrete and uses plastic-sheathed, seven-wire strand.
The Post-Tensioning Institute (PTI) published its first specification covering post-tensioning materials and installation in 1985.1 A second edition2 was published in October 2000 and essentially the same document was published in October 2001 by ACI as "Specification for Unbonded Single Strand Tendons and Commentary (ACI 423.6-01/ACI 423.6R-01)."3 ACI 423.6-01/ACI 423.6R-01 was subsequently revised to include just materials, and was renumbered to ACI 423.74·5; installation requirements for post-tensioning are covered in Section 9 of ACI 301, "Specifications for Structural Concrete."6·7
Unbonded Post-Tensioning Tendons
Unbonded post-tensioning tendons complying with ACI 423.7 consist of seven-wire strand coated with a corrosion-inhibiting lubricant and covered with plastic sheathing. Tendons are plant-fabricated by a process that involves applying the coating to the strand, extruding the sheathing, and cutting the strand to the required length (Fig. 1). Unless the tendon will need to be stressed at both ends, one anchor (referred to as the fixed-end anchor) is attached at the plant. Stressing-end anchors and wedges, including those for the intermediate anchorages at construction joints, are shipped loose with each delivery. Strand is typically extruded in full coils, then cut to the lengths required for a project. Tendons are identified for placement by attaching tags and/or painting colored stripes (color codes) on the fixed-end anchors or sheathing (Fig. 2). The tag numbers or color codes are then referenced on the post-tensioning installation drawings.
Strand
ACI 423.7 requires that strand conform to ASTM A416/416M, "Standard Specification for Low-Relaxation, Seven-Wire Steel Strand for Prestressed Concrete." ASTM A416/416M includes both 250 and 270 ksi (1725 and 1860 MPa) strand, but virtually all strand used for post-tensioning is 270 ksi (Grade 270); 250 ksi (Grade 250) strand...