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Causes Analysis of Prestressed Tensioning SlippageⅡ
（Anchor Block Reasons）

During the prestressed construction, tensioning slippage is a headache problem, which seriously affects the construction period and quality. It is also a common problem in the tensioning construction process. The reasons for slippage are complex and diverse. Here we only discuss the slippage analysis caused by wedges, anchor block, anchor plate, pc strands, prestressed concrete and tensioning methods, etc.
1. The manufacturing precision error of the anchor block taper hole is large (or the wrong anchor block is used), which causes the taper of the anchor block to be mismatched with the taper of the wedges, then resulting in slippage.
2. The front and back planes of the anchor block are not parallel, the axis of the wedges is severely deviated from the axis of the anchor block, or the error is large, the wedges is unilaterally forced, then resulting in slippage.
3. The heat treatment hardness of the anchor block does not meet the requirements, the anchor block is too soft, and the wedge is completely plunged into the anchor block taper hole; as the tension continues to increase, the anchor block taper hole cannot support the ideal shape of the large end of the wedges, then causing the wedges to lose its ideal Wrapped state, then causing slippage.

To be continued

4. Some suppliers blindly pursued the low cost and continuously reduced the outer diameter and thickness of the anchor block. As a result, the anchor block cannot effectively support the wedges in the axial and radial directions, then resulting in slippage.
5. The diameter error of the cone hole of the anchor block is relatively large. In an anchor block, there is only one cone hole or two cone holes with a diameter of 2.5mm, which reflects the height of 10mm in the axial direction; the restraint plate directly touches this error hole, and unable to achieve effective limit for all taper holes, so the wedges slippage is 100%; the reduced taper hole is caused by the fatigue operation of the worker who misread the machine scale; 100% inspection of the anchor block taper hole is the only preventive method.

6. The cone hole of the anchor block is severely corroded, causing severe friction with the outer surface of the wedges. When the friction force is greater than the tensile force, the clip cannot effectively follow up, that is, the pc strand continues to move, but the wedges stops moving and only relying on the wedges tooth to resist tensile force. Experiments have shown that the wedges tooth can only resist less than 1% of the tensile force. This kind of reason causes the wedges slippage is the most violent. Sudden loss of wedges stressing force, then the wedges will fly out quickly in the opposite direction of the stressing force, hitting all objects that can be hit; of course, the first choice is the operating workers and the management personnel wandering around the site; including supervisors and safety personnel, Therefore, it is strictly forbidden for non-related personnel to hang out and stand by at the stressing site.
7. A major mistake in the using material of the anchor block is that a small amount of low carbon steel is mixed into the material. The book "Metallic Materials and Treatment Materials" clearly points out that low-carbon steel cannot effectively increase the hardness during the conventional heat treatment process. ; The occurrence of this situation is often caused by the simultaneous processing of multiple products with similar shapes on site, and the mixing of processing and turnover processes. Due to the material reasons, the actual hardness of the anchor block is far from the design hardness. The wedges is trapped into the anchor block cone during the tensioning process. The too soft anchor block cannot form an effective support for the wedges, then resulting in slippage; Using the wrong anchor block material is an extremely hidden quality hazard that is extremely difficult to find. During the processing, only 100% hardness testing can be found. Strengthening on-site management of production and strict control of outsourcing units are effective method to control errors in using materials.

8. After the anchor block is heat-treated, there is no tempering treatment, the hardness of the anchor block far exceeds the ideal hardness of the material, which will cause circumferential cracks and collapse in the center of the anchor block during the tensioning process, then resulting in wedges slippage; This kind of situation is extremely dangerous. The circumferential anchor block that has not collapsed will fly out from the opposite direction of the pulling force after the stress is suddenly lost, and hit all objects that can be hit. Taking No. 45 as an example, the hardness reaches HRC55 before tempering, and the hardness is controlled at HRC24-30 after tempering. If tempering is neglected, the probability of cracking and collapse after stressing will greatly increase.
9. Anchor block cracks have two situations: raw material cracks and heat treatment cracks; no matter what kind of cracks, it may cause the anchor block to collapse during use and cause the wedges to slip. Therefore, the ultrasonic flaw detection of raw materials and the magnetic particle flaw detection of the finished product in the production process of the anchor block are particularly important. All cracks have a tendency to continue to extend and expand during the stress application process; even if there is no collapse at the time, they will slowly corrode, extend and deteriorate over time, eventually leading to irreparable major quality accidents
10. The looseness of the raw materials can also cause the anchor block core to be fragile, then cause the anchor block to be collapsed and cause slippage. During the cooling process of hot-rolled steel, the external cooling is fast, and the loose parts will be effectively compensated by the still flowing molten iron, while the core cools slowly, but there is no flowing melted iron to replenish, resulting in looseness; the effective way to detect looseness is low Power detection.

Analysis of the Causes of Post Tensioning Slippage
(Wedges Reasons)

In prestressing construction, tensioning slippage is a headache problem, which seriously affects the construction period and project quality. It is a common problem in the tensioning process also. The reasons for slippage are complex and varied. For this series, we only discuss the slippage analysis caused by wedge, anchor block, anchor plate, pc strands, prestressed concrete and tensioning methods, etc.:
1.The actual taper of the wedges and the designed taper produce a large error; it will cause slippage; when the wedges is under force, the wedges is not stressed in accordance with the designed situation, which causes the wedges to be partially stressed and causes slippage;
2.The tooth surface and tapered surface of the wedges are severely corroded, which causes the wedges does not move axially to the taper hole in the direction of the pc strand tension during the tensioning process. At this time, the wedges strain part is only affected on the teeth, when the tensile force exceeds the limit of tooth strength, the tooth collapses, then causing slippage;

To be continued...

3.The large end diameter of the wedges produces a large error during the manufacturing process! That is to say, the outer diameter of the wedge is not controlled within the tolerance range of the drawing design, the larger is too large, and the smaller is too small; when the group anchor is tensioned, due to the control of the limit depth of the recess plate, the larger outer diameter wedges has been severely scratched with the pc strand due to the limit of the recess plate, and the scraped iron filings fill the tooth gap of the wedges, this causes the tooth shape of the wedges to lose the ability to bite and hold, resulting in the wedges slippage; If the outer diameter of the wedges is too large, it will directly damage the pc strands. While if the outer diameter of the wedges is too small, it will cause wedges slippage due to the loss of the limit ability of the recess plate;

4. Too low or too high heat treatment hardness of the wedges will also cause slippage. The wedges hardness is usually about HRC60, and the hardness of the pc strand is about HRC45. If the wedges hardness is lower than HRC55, the probability of slippage will be large increased, HRC55 wedges teeth to bite HRC45 pc strands, the probability of occlusion failure is very high. If the hardness of the wedges is too high, greater than HRC65, the plasticity of the wedges will deteriorate and the brittleness will increase. In the process of tensioning and stressing, the probability of tooth chipping is very high. At the same time, due to the brittleness of the wedges, cracks, breaks and collapses will occur when subjected to force, resulting in slippage.

5. Incomplete teeth of the wedges, thin teeth, or insufficient teeth, or the teeth are not in the ideal shape, which directly causes the tooth strength to be greatly reduced, unable to withstand the tensioning force then it caused slippage; For example, a large area of the wedges has no teeth or a large error between the tooth height and the design height, and a large area of flat teeth or thin teeth

6.The stress releasing cone at the small end of the inner cone of the wedges is too short, the small end pressure is too large, and the pc strand is bitten by the wedges, then causing slippage.

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