Construction technology and characteristics of prestressed anchor

The post-tensioning method of anchor means that after reaching more than 75% of the design strength, the cement concrete is poured first, and then the prestressed steel is tensioned to form the construction method of prestressed concrete members.

First, the member is made and the corresponding holes are left in the member body according to the prestressed tendons. When the concrete strength of the member reaches the specified strength (generally not less than 75% of the standard value of the design strength), the prestressed tendons are inserted into the reserved holes for tension, and the prestressed tendons are anchored at the end of the member through the anchor, and the pretension of the prestressed tendons is transmitted to the concrete through the anchor at the end of the member, thus generating the precompressive stress. Finally, the cement slurry is poured into the hole so that the prestressed tendons and concrete members form a whole.

Post-tensioning method:

① with bonded prestressed concrete

The concrete is poured first, and when the concrete reaches more than 75% of the design strength, then the steel bar (steel bar bundle) is tensioned. The main tensioning procedure is: bury the pipe hole → pour the concrete → pump the pipe → maintain the tendon tensioning → anchor → grouting (to stop the steel bar rusting). The method of force transfer relies on anchorage to prevent the elastic rebound of steel bars, so that the section concrete can obtain pre-compressive stress. This method makes the steel bar and the concrete integrated, called bonded prestressed concrete. Due to the action of the bonding force (resistance), the tensile stress of the prestressed steel bar is reduced, and the compressive stress of the concrete is reduced, so this bonding should be minimized. This method has the advantages of simple equipment, no need for bench, flexible production, and is suitable for on-site construction of large components.

② Prestressed concrete is not bonded

The main tensioning procedure is to apply asphalt and other lubricating anti-corrosion materials along the full length surface of the prestressed steel bar. → Packing plastic paper or sleeve (prestressed steel does not establish a bond with concrete)→ pouring concrete curing → tensioning steel → anchoring. Like ordinary concrete, in the construction process, the steel bar can be placed in the design position directly poured concrete, without reserving holes, reinforcement, grouting, simplifying the construction process. Due to the increase of the effective precompression stress and the decrease of the cost of unbonded prestressed concrete, it is suitable for the beam body of curved steel bars with large span.

Construction precautions

(1) Cutting of prestressed tendons should be done with grinding wheel saw rather than arc cutting;

(2) When the steel strand is braided, it should be straightened one by one, bundled into bundles, and shall not be disordered. The extruded anchor or embossed anchor at the fixed end of the steel strand should be assembled with the pressure plate and spiral bar in advance;

(3) Mechanical equipment and instruments using prestress should be regularly maintained and calibrated;

(4) Before the prestressed tendons are tensioned, the concrete strength test report should be provided. Prestress can only be applied when the compressive strength of concrete meets the design requirements and is not less than 75% of the design strength grade.

(5) Before the prestressed tendons are stretched, the surface of the pressure plate should be cleaned and the quality of the concrete behind the pressure plate should be checked. If there are holes in the concrete here, it should be repaired with epoxy mortar before tensioning;

(6) When the anchor is installed, the anchor plate should be handled correctly, the clip should be tightened, and the piece position should be uniform: but do not knock too heavy when the clip is tightened, and use the rabbit to break the clip; (7) Before the large tonnage prestressed tendon is formally stretched, it should be tested and stretched with the operator. Confirm that the drawing process is reasonable, the drawing length is normal, and there are no harmful cracks, and then the batch stretching. If necessary, measure the actual hole friction loss. The curved prestressed beam shall not be stretched single by a small jack. Avoid unnecessary loss of prestress. When stretching, the operator must stand in the protected area, take protective measures, and pay attention to the operator not to stand in front of the stretch.

(7) When the prestressed tendon is tensioned, it should first load from zero to the initial tensile force at the starting point of the measured elongation value, and then load to the required tensile force in stages;

(8) Prestressed tendon tension management, the use of stress control, tensile check. The allowable deviation between the actual drawing value and the calculated drawing value is -5% ~ +10%. If this value is exceeded, the tension should be suspended; Measures should be taken to adjust the tension before continuing; If the elongation value is too small, overtension measures can be taken, but the tension limit should not be greater than 0.8fptk value. ; In order to increase the tension stress at the inner support and reduce the tension stress at the lower mouth of the anchor after tension, the over-tension relaxation technique can be used in the multi-wave curved prestressed tendons.

(9) Channel grouting is required to be dense, and the strength level of cement slurry shall not be lower than C30. Before grouting, the channel should be wet and clean, the grouting should be slow and uniform, without interruption, and the exhaust should be smooth. If the passage is blocked, the grout outlet must be replaced, but the cement slurry that has been injected once must be drained to avoid the presence of gas between the grout that has been injected twice. After filling the channel and closing the vent hole, continue pressurizing to 0.5-0.6Mpa, and then close the grout hole. The grouting pressure of vertical channel is determined according to the grouting height.

(10) The hole grouting of multi-span continuous prestressed tendons connected by connectors should be reperfusion after the completion of one span of tension, and should not be grouting after the completion of all spans of tension;

(11) The exposed length of the prestressed tendon after anchoring should not be less than 30 mm, and the anchor should be protected by sealing concrete. When prolonged exposure is required, measures should be taken to stop corrosion; When the steel strand appears rust, please remove the rust and dirt of the anchor clamping section and the outer end of the steel strand, so that the rabbit appears rust and dirt during installation and stretching, and fill the red slot of the clip, resulting in sliding wire;

(12) The tool clip is three pieces, and the working clip is two pieces, and the two cannot be mixed. The working anchor cannot be used as a tool anchor and cannot be reused;

(13) The anchor should be properly kept, and no rust, water or pollution of other debris should be allowed during use. The working clip can be used to remove the foam in the packaging box, but when the prestressed bundle is long, it is recommended to apply a small amount of lubricant (such as anchor release spirit) in the taper hole of the anchor plate, which is not only conducive to the follow-up and unanchoring of the working clip, but also conducive to multiple anchoring of the anchor. Before using the outer surface of the tool clip and the inner surface of the anchor plate cone hole, lubricant should be applied, and the debris on the surface of the clip should be removed frequently to make the anchor flexible. However, when the clip cracks or the tooth surface is damaged, it should be replaced and should not be used again;

(14) Protective measures should be taken when tensioning, and people cannot stand after the tensioning jack.

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