Prestressing strand manufacturing

 Prestressing strands is high tensile strength steel. The tensile strength of prestressing strands can reach up to 1860 Mpa. the process of manufacturing prestressing steel differs significantly from the conventional reinforcement. 

Figure 1

The process of manufacturing strands will begin by de-scaling the raw material (wire rod). The purpose of de-scaling is to remove the iron oxides (mill scale) from the surface of the wire rod. after removing the mill scale, the wire rod's surface is coated with a textured coating to enhance the adherence of lubricants to the raw material surface. The most commonly used coating is zinc phosphate. 

Figure 2

The wire rod then passed through a series of dies. The process of reducing the wire rod diameter is known as cold-drawing. In this process, the wire rod is drawn through a die with the required diameter. To prevent the wearing of dies, the wire rod will be drawn through a lubricant box. the lubricant will coat the wire with a thin layer that prevents the premature wearing and damaging of the die. 

The wire rod's cross-section will be reduced by 85%, and the tensile strength will increase by almost 50%. The tensile strength of the wire rod is approximately is 1180 Mpa. after drawing, the wire tensile strength increase to 1860 Mpa. after the completion of the drawing of 7 wires, the stranding process will start. The strand will consist of 7 wires, one wire will be at the center, and 6 wires will be wrapped around the center wire. 

Figure 3

After completing the wounding of wires into strands, the strands will be thermo-mechanically treated by subjecting the strands to continuous heating of 375 C while under tension at 40% of the ultimate tensile strength. This process will relieve the residual drawing stresses and reduce the relaxation of strand losses.