How to distinguish between unbonded and bonded steel strands

　　Unbonded steel strands are not covered with rubber on the outside. Unbonded steel strands are mainly used in post-tensioning systems. The difference from bonded steel strands lies in the fact that the prestress is not bonded to the surrounding concrete; during its working period, longitudinal relative sliding between the prestress and the surrounding concrete is always allowed. The prestress is entirely transferred to the concrete via anchor bolts.

　　Unbonded steel strands are mainly used in civil construction with large static loads, large-scale factories, or high-rise buildings. Advantages include fast construction speed, reduced beam and slab thickness, and increased structural performance and economic benefits.

　　Bridge engineering and large-scale garage prestressed engineering projects, due to large dynamic loads, require bonded steel strands in the design, i.e., ordinary steel strands. Due to the direct bonding of grouting construction with concrete, a stronger gripping force is generated, unlike the unbonded steel strand construction where fixing both ends is very difficult, thus enhancing the safety factor.

　　What is the difference between epoxy-coated steel strands and unbonded steel strands?

　　When applying epoxy coating to steel strands, sometimes single-wire coated steel strands and epoxy steel strands are filled. The difference between these two materials can be roughly seen from their names. Single-wire coating is where epoxy powder is applied to the surface of each steel strand separately using electrostatic spraying technology, forming a dense epoxy coating protective film on its surface, and 7 single prestressed steel strands are combined; however, filled steel strands are where dry, heat-curable epoxy resin powder is evenly coated onto the surface of the entire steel strand via electrostatic coating, and then fused into the gaps inside, forming a sealed whole of 7 prestressed steel strands.

　　Secondly, for unbonded steel strands, in the usual explanation, by applying grease and a plastic coating (PE material) to the surface of bonded steel strands (or bare wires), in subsequent engineering projects, the unbonded steel strands acting on the concrete form a layer of grease and the steel strands and concrete are isolated, allowing for free relaxation and maintaining a state of non-bonding with the concrete, hence the name unbonded steel strands.

　　Then, the epoxy coating thickness on the surface of the single-wire coated steel strand is relatively thin, 0.13mm at the thinnest point and 0.20mm at the thickest point, averaging about 0.17mm; however, the epoxy coating thickness on the surface of the filled epoxy steel strand is 0.64mm at the thinnest point and 0.85mm at the thickest point, averaging about 0.76mm. Thus, from the data, the effect of the filled epoxy steel strand is about 4.5 times that of the single-wire coated surface epoxy coating. Then, regardless of which steel strand is chosen, the main purpose is to eliminate stress. Each steel strand must be free of bends or knots. If there are bends or knots, they must be cut off. In daily use, according to strict specifications, each wire must pass through the length, and connections are strictly prohibited.

　　In addition, when calculating the length of the raw materials, the length of the overall component and the length of the jack should be fully considered. In addition, the pre-elongation of the end cap in the tensioning, the value of the elastic rebound variable, the tensile elongation value of the prestress, and various factors affecting the original material and the tensile force during construction are also considered. However, the basic principle remains the same, and the specific calculation method is similar to that of bonded prestressed steel strands.

　　However, one point to note about unbonded steel strands is that during dismantling, try to use a cutting wheel saw or water-grinding cutting machine for cutting, but arc cutting is strictly prohibited. When each steel strand is cut to length, the equal-length cutting method should be used, and the length should be measured with a tape measure before cutting. When inserting the steel strand downwards, it is recommended to use 18# or 20# metal wire to tie it on both sides of the cut. After cutting, do not loosen it to avoid injury to people or objects.