ACI 423.4R-14 Report on Corrosion and Repair of Unbonded Single-Strand Tendons.
In the early 1960s, the convenience of placing unbonded single-strand tendons was realized and the number of suppliers increased. The marketplace found the competi- tiveness of this system favorable, and the use of unbonded single-strand tendons increased signifcantly. Anchorage hardware varied considerably and included high-strength spirals, barrels, or castings with wedges and fttings that were swedged (mechanically attached) to the prestressing steel. The type of anchor that has become the most common is a casting that contained a recess to house a two-piece wedge for use with a single strand (Fig. 4.1b). This is currently the predominant system for unbonded single-strand tendons. By the late 1960s, plastic began replacing paper sheathing. Three different processes were used (Fig. 4.1c and 4.1d). In one system, the strand was covered by a preformed push- through plastic tube. In a second system, the strand was wrapped longitudinally with a heat-sealed strip of sheathing. Currently, the coated strand is covered by molten plastic that is continuously extruded around it (Schupack 1991b). Initially, the thickness and composition of the sheathing material were left to the supplier and were not uniform in the industry. The Post-Tensioning Institute (1985) regulated the type of sheathing material, the types of coatings used, and the dimensional tolerances of the system as it is applied to the strand. ACI 423.7 summarizes the more comprehensive specifcations for both the sheathing and the coating.
Advances in the sheathing systems were generally implemented in direct response to short-comings of the systems, difficulties in manufacturing, or improvements in the manufacturing systems. One of the earliest forms of sheathing was the paper-wrapped systems. These systems used a paraffin- or wax-impregnated paper that was typically helically wound around the tendon with little to no attention given to the start or termination of the wrap.
As a result, the paper wrapping would not necessarily be sealed at the overlaps in the wrapping. thereby allowing any moisture or chlorides that entered the concrete a path to reach the tendon wires. Furthermore, at cracks that crossed the tendon, water could infiltrate and erode the paper-wrapping, leaving gaps in the sheathing at locations of moisture infiltration. In addition, the paper-wrapping could be easily damaged during shipping or installation. To address these issues and provide a more durable sheathing material, the industry changed to plastic-based sheathing systems such as the push-through, heat-sealed, and extruded systems.
The push-through system required that the sheathing be sufficiently oversizod to allow the coated strand to he inserted without too much difficulty. This system ofien resulted in larger-than-desirable gaps between the strand and the sheathing, and the coating systems could not be relied on to completely occupy this interstitial space. This resulted in a tendon with many air voids inside the sheathing and allowed infiltration of water during storage. shipping. installation, and in service.ACI 423.4R pdf download.