PC strands（prestressed concrete strands）are stranded steel cables consisting of high-strength steel wires that are stress-relieved (stabilized) for the production of prestressed concrete. While there are different types of PC strands, including bare strands, stainless steel strands, epoxy-coated strands, and carbon fiber strands, here we'll focus on the most common - bare strands.
The raw material used is 1080 carbon wire. It is a hot-rolled coiled bar product with an alloy to achieve the required stretch and ductility. Typically, bars used to make PC strands are between 3/8” – 1/2” in diameter and 1.6 NT - 2.8 NT in weight.
Once the raw materials have been sourced, the production process can begin. The first step in the process is wire descaling and precoating. Iron oxide scale (iron oxide) exists on the surface of the bar, which needs to be removed mechanically or chemically before production.
Pickling is a chemical descaling method, a process in which the rod is placed in an acid bath and the chemical reaction causes the mill scale to fall off. After the bar has been pickled, it is coated with a textured washcoat that promotes lubricating adhesion during the upcoming drawing process.
PC strands are available in a variety of diameters, so the next step in the process is to draw the wire to the desired diameter through a series of 8 or 9 progressive carbide dies. This is a cold working process, so great care must be taken to avoid premature die wear and steel damage. In the process, the wire cross-sectional area was reduced by approximately 85%. The tensile strength of the wire when it is taken up is generally about 172ksi. Through this wire drawing process, the steel is work-hardened to the required tensile strength of >270ksi. The diameter of the wires is tightly controlled to maintain very tight tolerances.
Once the seven spools are drawn to the desired diameter, they are loaded into the stranding machine. According to ASTM 416, six outer wires and a single wire of a slightly larger diameter are required. The stranding machine pulls the wire from the spool while maintaining the specified wrap rate. The twist rate controls the twist of the strands to comply with ASTM 416. After the conductor is stranded, the strand is subjected to high temperatures while releasing residual drawing stress under tension, permanently elongating the strand, increasing yield strength, and reducing relaxation losses. The combination of these steps gives strands with very consistent elastic moduli up to and beyond 80% of the strand's ultimate strength. Finally, the strands are placed in a water bath to freeze the steel in a permanently elongated state and rinse any residual drawing lubricant from its surface.
At this point, the strand is present on-site, but it is in an unmanageable 20-27 net ton main coil. It is now rewound to the desired package size, typically 3.0 to 3.5 net tons. Samples will be cut for testing, and materials will be bundled and packaged for shipping.
The samples cut in the last step are sent to the laboratory for testing in accordance with ASTM 416. These tests include ultimate tensile strength, total elongation, yield point at 1% elongation under load, strand diameter, minimum diameter difference between center and outer wires, lay length and strand area, and modulus of elasticity.
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