March 12, 2026
External post-tensioning is one of the most versatile and powerful techniques available for the structural rehabilitation of existing concrete bridges. By introducing a controlled compressive force into the structure through high-strength steel tendons located outside the concrete section, it is possible to reverse the effects of deterioration, increase the load-carrying capacity, and extend the service life of bridges that would otherwise require costly replacement. This guide explains the principles, applications, and design considerations for external post-tensioning in bridge strengthening.
External post-tensioning is particularly well-suited to the following scenarios. First, where the original internal prestress has been partially or fully lost due to corrosion of the prestressing tendons — a common problem in bridges built before the introduction of robust duct grouting specifications. Second, where increased traffic loads or changes in use have rendered the original design inadequate. Third, where the bridge has been damaged by overloading, impact, or seismic events. Fourth, where the bridge was originally designed as a reinforced concrete structure and needs to be upgraded to a higher load class.
External post-tensioning is most commonly applied to box girder bridges, where the tendons can be routed inside the box and anchored at the diaphragms. It can also be applied to T-beam and I-beam bridges, where the tendons are routed below the soffit and anchored at the end diaphragms or abutments. The technique is less suitable for slab bridges, where the geometry does not permit effective tendon routing.
Installation of external post-tensioning tendons inside a concrete box girder bridge. The tendons are threaded through orange HDPE deviator saddles that redirect the tendon profile to achieve the required load-balancing geometry.
An external post-tensioning system for bridge strengthening consists of four principal components.
Tendons: High-strength 7-wire strands (typically 15.2 mm or 15.7 mm diameter, with a characteristic tensile strength of 1,860 MPa) are the standard tendon type. Multiple strands are grouped into tendons of 4, 7, 12, or 19 strands, depending on the required prestressing force. The strands are protected by HDPE corrugated ducts filled with corrosion-inhibiting grease or wax.
Anchorages: Multi-strand wedge anchorages are used at both ends of each tendon. The anchorage consists of a steel anchor plate, a cast-iron trumpet, and individual wedge assemblies that grip each strand. The anchorage is embedded in or bolted to a concrete diaphragm or end block.
Deviators: Deviator saddles redirect the tendon profile from the straight line between anchorages to the required parabolic or harped profile. They are typically constructed from steel pipes or saddles embedded in concrete deviator blocks, which are cast onto the existing bridge web or soffit.
Stressing equipment: Hydraulic mono-strand or multi-strand jacks are used to stress the tendons to the required initial prestress force. The stressing operation is carefully monitored using load cells and elongation measurements to verify that the correct prestress has been achieved.
External post-tensioning anchorage block at the end of a bridge span. The tendons are anchored by multi-strand wedge systems and protected by HDPE corrugated ducts filled with corrosion-inhibiting grease.
The design of an external post-tensioning strengthening scheme involves several key considerations. Load balancing: The tendon profile is designed so that the upward forces exerted by the tendons at the deviator points balance a target proportion of the permanent load, typically 80–100%. This minimises the net bending moment in the structure under permanent load, reducing cracking and deflection. Stress analysis: The stresses in the concrete at transfer and under full service loading must be checked against the allowable limits for the concrete grade. Anchorage zone design: The local stresses in the concrete at the anchorage and deviator zones must be checked, and additional reinforcement provided if necessary. Second-order effects: For slender structures, the interaction between the prestress and the structural deformation (the P-delta effect) must be considered.
| Bridge Type | Typical Tendon Layout | Capacity Increase |
|---|---|---|
| Single-span box girder | 2–4 tendons, parabolic profile | 20–50% |
| Multi-span box girder | 4–8 tendons, continuous profile | 25–60% |
| T-beam bridge | 1–2 tendons per beam, harped profile | 15–40% |
| Reinforced concrete slab | Transverse and longitudinal tendons | 20–45% |
Bridgent supplies complete external post-tensioning systems for bridge strengthening, including 7-wire strands, multi-strand anchorages, HDPE ducts, deviator saddles, and hydraulic stressing jacks. Our engineering team provides full design support, from initial feasibility assessment through to detailed design, supply, and installation supervision. Contact us to discuss your bridge rehabilitation project.
Bridgent is your specialist partner for bridge construction and maintenance materials. From CFRP and post-tensioning systems to hydraulic jacks and noise barriers, we supply the complete range with full engineering support.
Tags:Bridge StrengtheningPost-TensioningBridge RehabilitationConcrete BridgeBridgent Products
Westcoast New Area, Qingdao City ,Shandong Province ,China
