External Prestressing In Concrete
External prestressing concrete structures can be divided into two types: simple type and mixed type according to the arrangement of external prestressing tendons. The simple type is to arrange all the prestressing tendons outside the concrete. In the hybrid type, a part of the prestressing tendons is arranged outside the concrete, and another part of the prestressing tendons is arranged inside the concrete. Compared with the traditional prestressing concrete structure (all the prestressing tendons are arranged inside the concrete), external prestressing has significant advantages such as fast construction speed, labor-saving, and economical structure.
Most of the external prestressing tendons arranged on the concrete are anchored at the fulcrum of the bridge and on the transverse beam in the middle, and the direction of the prestressing tendons is changed by the turning block or the middle transverse beam. Therefore, the external prestressing tendons mostly adopt the method of polyline reinforcement.
The external prestressing tendons are generally composed of steel strand bundles, the outside is protected by plastic pipes or steel pipes, and the casing is filled with anti-corrosion materials. The prestressing tendons in the casing can be divided into two types: bonded and unbonded.
The tensioning and anchoring method of external prestressing tendons is the same as the general post-tensioning method, except that special anti-rust treatment is required for the steel strands and anchor parts.
In the use of external prestressing anchorages, the following issues need to be paid attention to:
1. Vibration of external prestressing tendons. The distance between the fixed points of the prestressing tendons is controlled within 7m-8m, and the natural period of the general prestressing tendons does not exceed 0.1 IS-0. 15s.
2. The external prestressing tendons are concentrated in the anchorage and steering positions, so reliable measures must be taken to disperse the stress and strengthen the local area.
3. The friction coefficient of external prestressing tendons. The external prestressing tendons only need to consider the friction between the prestressing pipe and the reserved pipe when passing through the turning part. Under normal circumstances, only considering the frictional effect of the angle change, u=0.3 can be taken.
4. Evaluation of the ultimate bearing capacity of the structure. In the external prestressing structure, the strain of the prestressing tendons is the same as that of unbonded tendons. It is not the strain of concrete on the section, but the average strain between two anchor points. Therefore, the external prestressing structure is about 20% to 30% lower than the bonded structure.