One, foreword
In recent years because of the unbonded prestressed incomparable advantages in the body, such as section size is small, simple construction, easy to guarantee the quality, it is more important in vitro, can replace others. External prestressing is widely used in many countries such as America, Germany's new bridge construction, and applied to the reconstruction of the old concrete structure, reinforcement and maintenance. While developing the study of external prestressing technology in China is relatively small, mainly applied in the reinforcement of old Bridges. In a word, with the development of prestressed concrete Bridges and high strength concrete, the application of external prestressing technology will be the important trend of modern prestressed technology.
Second, the application of externally prestressed beam in bridge reinforcement
External prestressing method has the following advantages: (1) less weight increase, but can significantly improve the bearing capacity; (2) the increased weight of upper is little, and little effects on the bottom; (3) the construction is simple, short time limit, the economic benefit is obvious; (4) the construction process is not interrupt or less interrupted traffic; (5) of the original structure of small injury, will not affect the clearance under the bridge; 6. Stress can be adjusted, prestressed beam can be replaced.
Three, externally prestressed beam in the construction of new bridge applications
The first is a step longbridge precast segmental construction. The type represented by Long Key Bridges, in vitro pre-stressed beam using the same more than ordinary shares as the body prestressed steel strand and anchorage, also USES the cement grouting. Thus prestressed lower cost. This kind of externally prestressed structure is usually adopted in precast segmental see dry joints and double shear keys. When a whole across all precast segmental after installed on the support structure in place, continuous and external prestressing, the overall structure of the form a cross. Within the cross beam of deviators bending, the in vitro using polyethylene pipe or tube bundle coat, pipe near the place with the main beam steering pouring into a whole, this kind of beam can only be removed cannot be replaced in vitro.
The second is the cantilever construction or jacking construction of prestressed concrete continuous girder bridge, usually adopt mixed arrangement of in vivo and in vitro. The form used in external prestressing cables instead of original configuration within the web of a large number of prestressed tendons, simplifies the web structure, reduce its thickness. Using cantilever construction, prestressed cantilever beam with straight body, into the bridge after tensioning of externally prestressed continuous beam with large tonnage, which eliminated a lot of wear and grouting technology, easy to control construction quality.
Four, prestressed construction technology: (a) end anchorage of beam and cross to cross rib, the construction of the pier top groove, (2) the steel strand blanking and the wear, (3) the steel strand tensioning, the bridge reinforcement tensioning by two cases of symmetric on both ends of the joint at the same time, divided into two parts: the tensile preloaded and high stress and tension. (4) grouting
Five, the main technical key points in construction: the original bridge construction situation is different with the original design of the bridge, such as the thickness of the beam flange, the dimension of the pier top horizontal beam insulation, at the top of the block should be fully considered in the design of the position of groove and the pier top cross beam, the relationship between the insulation in the bridge reinforcement, due to the pier top horizontal beam width is greater than the original design of many, the pier top groove can't construction, so as to strengthen the design change. From epoxy coated steel strand exposure in box girder, steel strand is especially important on the nest, so the next and wear in the process of the beam, it is forbidden to drag on the concrete ground steel strand, wear with the erection of steel beam with cotton silk, not damaged PE protective layer, if you have accidentally, individual lesions, with PE coating a homogenous tape winding. Pier top groove and steering gear processing should be performed in the factory, it is strictly prohibited to processing at the scene, on-site installation, we will strictly according to drawings, in the process of transportation and welding, measures should be taken to prevent welding deformation, wear beam should stay determined before installation. Preloaded process, must arrange personnel in box girder, the steel strand taut, make the necessary adjustments; High tension stress, the observation in the guide slot loaded on top and turned to cross rib steel strand to whether have break phenomenon, the phenomenon, if any, should be protective treatment. Grouting is close-grained degree will directly affect the bonding effect, so in the grouting should strictly control the water cement ratio, and keep the pressure evenly.
Four, conclusion: the engineering practice shows that using external prestressed strengthening old bridge and the construction of the bridge is a very effective measure, strengthening, unloading and reduce the role of the internal force of the structure, is more widely applied.
