Classification and Precautions for Use of Prestressed Anchorage
Regarding the relevant knowledge of prestressed anchorages, today I will take you to understand the two major classifications of prestressed anchorages and what are the precautions for the use of them.
Categories of prestressed anchorages
Prestressed anchorages can be divided into two main categories:
1. Stressing anchorage: An anchor that is installed at the end of the prestressed tendons and can be tensioned is called a stressing anchorage. The so-called anchorage is a permanent anchoring device used in post-tensioned structures or components to maintain the tensile force of the prestressed tendons and transfer it to the concrete.
2. Dead-end anchorage: Anchorage installed at the end of prestressed tendons, usually buried in concrete without tension.
Precautions for the use of prestressed anchorages
1. The prestressed tendons should be cut off with a grinding wheel saw or a cutting machine and arc cutting is not allowed;
2. When the steel strands are braided, they should be straightened out one by one and tied into a bundle without the disorder. The extruded anchor or bond head anchor at the dead-end of the steel strand shall be assembled with the bearing plate and spiral reinforcement in advance;
3. The prestressing tensioning equipment and instruments should be maintained and calibrated regularly;
4. Before the prestressed tendons are tensioned, the concrete strength test report shall be provided. When the compressive strength of the concrete meets the design requirements and is not less than 75% of the design strength, the prestress can be applied;
5. Before tensioning the prestressed tendons, the surface of the bearing plate should be cleaned and the quality of the concrete behind the bearing plate should be checked. If there is a cavity in the concrete, it should be repaired with epoxy mortar before tensioning;
6. When installing the prestressed anchorage, the anchor head should be straight, the wedge should be tightened, and the position of the wedge should be even: but the wedge should not be hit too hard when tightening the wedge to avoid the damage;
7. Before the formal tensioning of large-tonnage prestressed tendons, professional personnel should perform trial tensioning. After confirming that the tensioning process is reasonable, the length of the tension is normal, and there are no harmful cracks, formal tensioning can be performed. If necessary, measure the actual channel friction loss. For curved prestressing beams, small jacks shall not be used for single tensioning, so as to avoid unnecessary loss of prestressing force. During tensioning, the operator must stand in a safe area and take protective measures. It is strictly forbidden for the operator to stand at the front end of the tensioned prestressed tendons during tensioning;
8. When the prestressed tendons are tensioned, they should be loaded from zero to the initial tensile force at the starting point of the measured elongation value, and then loaded to the required tensile force in stages;
9. The management of prestressed tendon tension adopts stress control and elongation check. The allowable deviation between the actual elongation and the calculated elongation value is -5%~+10%. If this value is exceeded, the tension should be suspended, and the tension can be continued after taking measures to adjust; if the elongation value is too small, over-tension measures can be taken, but the tension limit shall not be greater than 0.8fptk. In the multi-wave curved prestressed tendons, in order to increase the tensile stress at the inner support and reduce the tensile stress at the lower mouth of the anchor after the tension, the over-tensioned loosening technology can be adopted;
10. The duct grouting is required to be dense, and the strength grade of cement slurry should not be lower than C40. The ducts before grouting should be moist and clean, the grouting should be carried out slowly and evenly without interruption, and the exhaust should be smooth. If the ducts are blocked, the grouting port must be replaced, but the grout poured in for the first time must be discharged to avoid the presence of gas between the grout poured in twice. After filling the channel and sealing the vent hole, it is advisable to continue to pressurize to 0.5-0.6Mpa, and then close the grouting hole later. The grouting pressure of the vertical channel should be determined according to the grouting height;
11. The grouting of multi-span continuous prestressed tendons connected by couplers should be tensioned for one span and then poured into another span. It is not allowed to grout at one time after all spans are fully tensioned;
12. The exposed length of the prestressed tendons after anchoring should not be less than 30mm, and the prestressed anchors should be protected by end-capped concrete. When long-term exposure is required, measures should be taken to prevent rust; when the steel strand has rust, please remove the floating rust and dirt, so as not to cause the wedge to slip due to the rust and dirt filling the chute during installation and tensioning;
13. The tool wedge is three-piece, and the working wedge is two-piece. The two cannot be mixed. Working anchors cannot be used as tool anchors and cannot be reused;
14. The prestressed anchorages should be kept properly, and there should be no rust, water, and other sundries during use. When the prestressed beam is long and needs to be repeatedly tensioned, it is recommended to apply a small amount of lubricant to the hole of the anchor head, which is beneficial to the unanchoring of the working wedge and the multiple anchoring of the anchor. Apply paraffin to the tool wedge and clean the surface dust before use to make the wedges removed easily after tensioning, but if the wedge is cracked or the tooth surface is damaged, it needs to be replaced and must not be used again;
15. There should be safety measures when tensioning, and it is forbidden to stand behind the tensioning jack;
16. The post-tensioning anchor system should be used as a complete set and cannot be mixed with other systems. If you want to do a static load test, please clean the wedge with an organic solvent (such as gasoline) and wipe the anti-rust oil in the anchor head hole, otherwise, it will affect the anchoring performance.
Henan Zhongjiao Road&Bridge Engineering Materials Co., Ltd specializes in producing the post-tensioning system for over 10 years. If you want to know more prestressed anchorages, welcome to consult firstname.lastname@example.org.