I. Preface

　　In recent years, due to the unparalleled advantages of unbonded prestressing, such as small section size, simple construction, and easy quality assurance, and more importantly, the external tendons can be replaced and re-tensioned, external prestressing has been widely used in the construction of new bridges in many countries such as the United States and Germany, and is also used in the reconstruction, strengthening, and maintenance of existing concrete structures. However, the research work on external prestressing technology in China is relatively less developed, mainly applied to the strengthening of old bridges. In short, with the development of prestressed bridges and high-strength concrete, the application of external prestressing technology will be an important trend in the development of modern prestressing technology.

　　II. Application of External Prestressed Strands in Strengthening Old Bridges

　　In the methods of strengthening old bridges, for reinforced concrete and prestressed concrete beam and slab bridges, the method of strengthening by setting prestressed tie rods or prestressed strands at the lower edge of the beam to apply external prestressing to the tensile zone can offset the internal forces generated by self-weight and external loads, and greatly improve its bearing capacity.

　　The external prestressing method has the following advantages: ①Small increase in self-weight, but can significantly improve the bearing capacity; ②Due to the small increase in the upper self-weight, the impact on the lower part is small; ③Simple construction, short construction period, and significant economic benefits; ④The construction process does not interrupt or less interrupts traffic; ⑤Small damage to the original structure, does not affect the bridge's underclearance; ⑥Stress can be adjusted, and prestressed strands can be replaced.

　　The external prestressed strengthening system consists of horizontal reinforcement, inclined reinforcement, upper anchor points, sliders, supports, horizontal reinforcement fixed supports, etc. The prestressing tendon structure and construction method of the external strand bridge strengthening structure are quite different from the conventional internal bonded or unbonded prestressing tendons. Therefore, the calculation method of prestress loss is also different. Calculations show that compared with general prestressed concrete structures, the prestress loss of external strand strengthening structures is much smaller. In view of this, the control stress of prestressing steel should be appropriately reduced to avoid the external prestressing tendons being in a high-stress state for a long time, which is beneficial to improving the stress state of the external strand structure.

　　III. Application of External Prestressed Strands in New Bridge Construction

　　Since its development in the 1930s, the external prestressed structure has been constantly innovating and improving its structural system. Therefore, the application of external prestressed strands is also constantly changing and enriching. The application of external prestressed strands in new bridge construction can be mainly summarized into the following four types:

　　The first type is a long bridge constructed with prefabricated segments across spans. This type is represented by the Long Key Bridge, where the external prestressed strands use the same ordinary multi-strand steel wires and anchors as the internal prestressing, and also use cement grouting. Therefore, the prestressing cost is relatively low. This type of external prestressed structure usually uses dry joints and double shear keys in the prefabricated segments. After all the prefabricated segments of the entire span are installed in place on the support structure, external prestressing is added to form an integral structure of one span. The external prestressed strands are deflected at the turning blocks in the span, and the external strands are jacketed with polyethylene pipes or steel pipes. The pipes are cast integrally with the main beam at the turning blocks. This type of external strand can only be removed and cannot be replaced.

　　The second type is a prestressed concrete continuous beam bridge constructed using cantilever construction or top-launching construction, usually using a mixed arrangement of internal and external strands. In this form, external prestressing tendons replace a large number of prestressing tendons originally arranged in the web, simplifying the web structure and reducing its thickness. When using cantilever construction, the cantilever strands are straight internal prestressing, and the continuous strands tensioned after completion of the bridge use high-tonnage external prestressing, thus eliminating a large number of tendon threading and grouting processes, making it easy to control construction quality.

　　The third type is a derivative of the second type, characterized by changing the concrete box girder web into a concrete truss or using a steel structure. This type often combines innovative structural design with an aesthetically pleasing appearance, forming a representative work of external prestressed structures.

　　The fourth type is called the "Tanla" type external prestressed structure. It places the external tendons, whose eccentricity used to be controlled within the effective height of the main beam, above the effective height of the beam. Therefore, it has the dual characteristics of beam bridges and cable-stayed bridges, and can be regarded as a structural system between prestressed concrete box girder bridges and prestressed concrete cable-stayed bridges. It uses a partial cable structure to help the main beam bear the vertical load, thereby reducing the beam height.

　　IV. Prestressed Construction Process

　　(1) Construction of anchor end beams, mid-span turning ribs, and pier top guide grooves

　　These three parts determine the spatial position of the steel wires, and the size of the equivalent load is determined by the cable shape and tensioning stress. The steel wires at the mid-span turning ribs and pier top guide grooves are deflected and subjected to local compressive stress. This requires that the embedded position and direction of the anchor plate at the anchor end beam must be accurate. The fabrication of the mid-span turning ribs and pier top guide grooves should be carried out strictly according to the drawing requirements, ensuring both the radius of curvature at the bending point and polishing the ends to make them smooth to prevent the compression and slippage of the steel wires at the ends during tensioning.

　　(2) Steel wire cutting and threading

　　In bridge strengthening, since the anchor plate and steel pipe need to be grouted after tensioning to form a bonded section, the PE layer and grease of the steel wire in the bonded section should be cleaned during cutting. Controlling the length and position of this section is difficult because it is necessary to consider the influence of the steel wire sag during the threading process, ensuring that the PE protective layer enters the sealing cover in advance, and also consider the influence of tensioning elongation, ensuring that the elongated parts at both ends are consistent to ensure that the bonding forces of the two bonded sections are approximately equal. During the threading process, since the length of the steel wire is more than 150m, and it needs to pass through multiple pier top guide grooves and mid-span turning devices, it is impossible to thread 12 steel wires as a whole in the box girder. Therefore, the single-wire threading method is used. The winding of the steel wire will affect the establishment of effective prestress, so it is necessary to ensure that the steel wire is not wound in the entire bridge length range. In actual construction, the steel wire, working anchor plate holes, and sealing cover small holes are pre-numbered, and each bundle of 12 steel wires uses a single-wire threading method. Every section uses a rubber pad corresponding to the small hole of the sealing cover to limit the position of the steel wire. After tensioning, it is found that this method makes each bundle of steel wires straight and without winding.

　　(3) Steel wire tensioning

　　Bridge strengthening uses two symmetrical boxes, single-wire, and simultaneous tensioning at both ends. The tensioning process is divided into two parts: pre-tightening and high-stress tensioning.

　　1. Pre-tightening

　　To ensure the steel strands are straight and untangled after being tensioned from a loose state, pre-tensioning is required before formal tensioning. The quality of pre-tensioning determines the success of the entire reinforcement. Firstly, even with necessary measures, steel strands in a loose state still have significant sag due to their length. Therefore, to ensure the lengths of the bonded sections at both ends are approximately equal, pre-tensioning must be performed symmetrically at both ends; secondly, the magnitude of the pre-tensioning force must ensure that the steel strands are taut and untangled during pre-tensioning, and that the steel strands do not misalign during high-stress tensioning. Too much or too little pre-tensioning force will not achieve the desired effect. In reinforcement construction, the pre-tensioning force is 15% of the design tensioning force.

　　2 High-Stress Tensioning

　　Because the bridge reinforcement uses full-length epoxy-coated steel strands, multiple-stage continuous tensioning is required during tensioning, and the working grips need to be anchored multiple times. When the working grips are temporarily anchored, debris from the epoxy coating protective film will adhere to the teeth of the grips. As the working grips repeatedly clamp the steel strands, the accumulation of epoxy coating debris between the teeth will cause slippage, affecting the anchoring effect of the working grips. To address this, a temporary anchoring device has been developed. During intermediate stages, the temporary anchoring device's tool grips clamp the steel strands, avoiding intermediate temporary anchoring of the working grips and ensuring anchoring effectiveness.

　　(4) Grouting

　　The external tendon anchoring beams use a partially bonded form. To meet the requirements of the designer and owner for the bond strength of the partially bonded steel strands, grouting of the partially bonded section after tensioning is a crucial step: A 1:1 model test was conducted before construction. Under the condition of ensuring dense and full grouting, the bond strength of the partially bonded section can reach 108% of the design tensioning force, meeting the anchoring requirements: In the project, grouting is carried out within 24 hours after tensioning. Manual grouting machines are used to ensure the uniform stability and pressure requirements of the grouting process.

　　5. Main Technical Points in Construction

　　The actual construction situation of the original bridge differs from the original design, such as the thickness of the box girder flanges and the dimensions of the pier top cross beams. The design must fully consider the relationship between the position of the pier top guide slot and the pier top cross beam. In the reinforcement of this bridge, because the width of the pier top cross beam is much larger than the original design, the pier top guide slot could not be constructed, thus changing the reinforcement design.

　　Since the epoxy-coated steel strands are exposed in the box girder, preventing kinking is particularly important. Therefore, during material preparation and stranding, dragging the steel strands on the concrete floor is strictly prohibited. The reinforcement used for stranding must be wrapped with cotton thread to prevent damage to the PE protective layer. If accidental damage occurs, the damaged areas should be wrapped with tape of the same material as the PE protective layer.

　　The processing of the pier top guide slot and turning device should be done in the factory and is strictly prohibited on-site. During on-site installation, strict adherence to the drawings is required. During transportation and welding, measures should be taken to prevent welding deformation. Before stranding, a line should be pulled to determine whether the installation is appropriate.

　　During pre-tensioning, personnel must be arranged in the box girder to make necessary adjustments when the steel strands are tightened; during high-stress tensioning, observe whether the steel strands are squeezed or broken at each pier top guide slot and turning rib. If such a phenomenon occurs, protective measures should be taken.