Material Proportion Analysis of Unbonded PC Strand Sheath

The sheath of the unbonded prestressed steel strand has a good protection effect on the steel strand during use. The big problem in sheath production is the ratio of PE pure material and PE recycled material. The proportion of PE recycled material is too high or all PE recycled materials are used, which will greatly reduce the tensile yield stress and nominal tensile fracture strain of the sheath. This will adversely affect the protective performance of the sheath, resulting in the damage of the sheath in the project and the loss of the protective effect on the steel strand. In the experimental study of the sheath ratio, we conducted some experimental studies on the changes of the amount of PE pure material and PE recycled material and the tensile yield stress and tensile fracture nominal strain of the sheath.

1. Polyethylene resin

Polyethylene resin (PE pure material), referred to as PE, is a thermoplastic resin obtained by the polymerization of ethylene. Because polyethylene has the advantages of odorless, non-toxic, good corrosion resistance, and can withstand the erosion of most acids and alkalis, high-density polyethylene resin is generally used as the sheath for unbonded steel strands. The polyethylene resin used to make the unbonded steel strand sheath is solid color, no impurity particles, and the density is 0.942g/cm³ ~ 0.965g/cm³. It is processed by thermoplastic molding method. The polyethylene is extruded into a tube shape through extrusion equipment, wrapped on a steel strand coated with an anti-corrosion lubricating coating, and held tightly by the heat shrinkage of the polyethylene tube.

2. PE recycled material

PE recycled material is recycled polyethylene material, generally in granular form. The density is smaller than that of the pure polyethylene resin, and the strength, stiffness, hardness, and heat resistance are also lower than those of the pure material. PE recycled material has good fluidity and fast cooling speed and is suitable for thermoplastic processing, but its anti-aging performance is poor. Therefore, the proportion of PE recycled material in the unbonded prestressed steel strand sheath is reasonably controlled, so that it can meet the comprehensive performance requirements of the sheath, and at the same time, it can save resources, reduce solid waste pollution, and make full use of renewable resources.

3. Proportion test of the sheath

Using high-density polyethylene resin as the main material, the polyethylene sheath formulations of different ratios are made by mixing PE recycled materials with different mass fractions respectively. It is extruded and plasticized through a plastic extruder into a tube and wrapped on a steel strand. After the sheath is cooled and hardened, it is peeled off from the steel strand, and the comprehensive mechanical properties are tested and analyzed. Finally, a more reasonable ratio of polyethylene sheath raw materials is selected.

The average thickness of the sheaths with different ratios is basically about 1.2mm. The tensile yield stress did not change much, and at 13-16 MPa, the tensile yield stress decreased with the decrease of the pure material ratio. The values of the nominal strain at tensile fracture vary widely. The pure material sheath reaches 756%, and the nominal strain at break of 30% pure material and 50% pure material, and 70% pure material is relatively stable, ranging from 436 to 469%.