Post Tension Procedures And Requirements
1. Inspection of concrete components and ducts
Before the tensioning equipment is stretched, it is important to check whether the position of the bearing plate is accurate, whether the surface is flat, and whether it is perpendicular to the end of the anchor hole. If the anchor plate is not perpendicular to the end of the hole, stress will be lost on the one hand, and on the other hand, the tension may fail due to the stress concentration crushing the anchor concrete, resulting in immeasurable losses. There are many examples of such lessons. For tunnels that are bundled before pouring, check whether the steel strands can slide freely in the tunnels before tensioning.
2. Tension steps and tension method
The steps of tensioning should conform to the design regulations. The tensioning method generally adopts two-end tensioning. When the channel is short, single-ended tensioning can be used. When the conditions do not allow two-end tensioning, single-ended tensioning can also be used. However, the stress loss of prestressed anchorage is relatively large when single-ended tensioning, and measures to reduce stress loss must be adopted. For example, in the Qianqiu Bridge construction, the main span is a three-hole 60m reinforced concrete prestressed tied arch, only the first hole can be tensioned at both ends, and the remaining two holes can only be tensioned at one end, but due to the long channel, single-ended tensioning has a large frictional resistance, and whether the stress can be smoothly transmitted to the other end and how much can be transmitted. This has no successful precedent nationwide. In order to obtain scientific data, under the condition of a hole with two ends of tension, a single-ended tension test is adopted, and the channel friction test is mainly carried out. It is concluded that without any measures, the tensile force transmission of one end tensioned linear beam is 73.34%, and the curve beam tensile force is 52.4%, which is far from the tensile stress requirements. After the test of adding soapy water to reduce the friction resistance of the tunnel, it is concluded that the stress transmission is 83.85%, which is an increase of 16.48%, which can basically meet the tensile requirements. Because the single-ended tension test of the first hole provides scientific data, the last two single-ended holes use soapy water to reduce the friction resistance of the tunnel, and the tension and elongation meet the requirements, and the single-ended tension is solved. In the contradiction of the transmission of pulling force, the tension is successful at one time.
3. Tension procedure
The general tensioning procedure is: 0→initial stress σ0→105σK→σK (anchoring)
Among them: σK is the control stress during tension, including the predicted prestress loss value.
Note: When both ends are tensioned, the jacks at both ends should work in the same way as lifting and lowering, marking, and measuring elongation. “Dual control” is used during tensioning, that is, the tension and elongation are controlled. The elongation should be within ±6% of the design elongation, otherwise, the cause should be found out.