Prestressed anchor head use for 12.7mm and 15.24mm pc strand

Problems encountered in the operation of prestressed anchorage and solutions.

1. Broken Wire Causes and Treatments
The occurrence of broken prestressed anchor head wires in steel strands is caused by non-uniform or severe corrosion of steel strands. The steel strands encounter electric welding sparks or used as electric welding wires. The mechanical properties of the steel strands change; the strands near the anchor holes cross severely.

The anchorage deviates from the anchor plate mouth, the strand is in the middle, or the flare tube end and the bellows have a large angle, resulting in individual wire stress concentration; clamp hardness and strand does not match; tensioning equipment is not pressed The specified calibration or failure of the oil pressure gauge results in excessive tension.

After wire breakage, the commonly used treatment method is to increase the control tension of other steel strands as compensation; however, it should be noted that the maximum tensile stress in any case must not exceed 0.8 times the standard value of ultimate tensile strength of steel strands.
The second is to change the beam, that is, unloading, withdrawing the anchor, changing the bundle, and re-tensioning the broken wire to the design stress value. Third, enable spare bundles if necessary. For large-span prestressed concrete girder bridges, spare holes are often reserved in the design, and spare bundles can be activated when the broken wires are severe.

2. Causes and Treatments of Negative Deviation of Extension Value
The elongation value of prestressed tendons depends on the size of tensile force. Under certain conditions, there is a certain tensile force corresponding to a certain value of tensile strength. There is a certain effective prestress in a section of the beam body. In the large-span prestressed concrete bridge segmental construction process, the deviation of the actual elongation value tends to change from a positive value to a negative value as the length of the steel strand increases or the curvature radius of the steel beam decreases.
The longer the prestressed tendons, the greater the negative deviation, even exceeding the allowable negative deviation of -6%. Once the negative deviation of the strand elongation exceeds -6%, the negative deviation of the average elongation value of the prestressed strand in the same section may lead to the situation of insufficient prestressing and reduce the stress reserve of the structure.