Prestressed Anchorage for Bridges: Failure Mechanisms and Site-Based Condition Grading

Problem Boundary: A Last‑Minute Hold Before Tensioning

At a municipal prestressed concrete continuous‑beam bridge, the day before tensioning was scheduled, the laboratory ran a routine spot check on the anchorages that had just been delivered. The hardness tester gave three readings on one wedge that fell so far apart that the entire batch was rejected on the spot. Tracing the heat number later showed that improperly tempered parts had been mixed in. The project chief engineer summed it up: “You can measure anchorage dimensions with callipers, but who can see hardness and internal stress with the naked eye?” For a material-by-material comparison across bridge span classes, see our prestressed anchorage comparison for bridge span conditions page.

This is exactly the hardest‑to‑detect risk in prestressed bridges: anchorage failure offers no warning, and when it occurs it means strand pull‑in or brittle fracture. This paper is written for material managers, testing engineers, and site technicians on bridge projects. It won’t pile on definitions; it will solve three problems:

1. Why do anchorages fail? – Only by clarifying the mechanism can you interpret the signals that test data give you.
2. What severity level does my bridge’s service condition fall into? – No abstract classification; use observable field characteristics to decide.
3. What to inspect upon delivery, and what to control during installation? – From material selection to acceptance, each step is derived with justification.

By the end you will know how to identify the dominant failure mechanism for your bridge anchorage and be able to specify the appropriate hardness combination and inspection checklist.

No specific project names, suppliers, or unsourced figures are given. All reasoning is drawn from current highway bridge codes and fundamental materials science.

How Corrosion, Hardness, and Stress Interact

A prestressed anchorage works at a high‑stress three‑body interface: strand–wedge–anchor head (bearing plate), which stays in contact under sustained load for decades. The three drivers of failure are environmental corrosion, hardness mismatch, and overload‑related deformation.

Stress Corrosion Cracking

After tensioning, the anchor head and wedges are subject to tensile, compressive and shear stresses transferred by the strands. If the bridge is within about 10 km of the coast or exposed to de‑icing salt in winter, a micro‑environment can easily form inside crevices, lowering pH and accumulating Cl⁻, triggering stress corrosion cracking (SCC). Such cracks initiate at microscopic corrosion pits, propagate along grain boundaries, and show a dendritic fracture surface, all without any visible deformation beforehand. The risk is especially high where residual stresses from machining or heat treatment are elevated, because SCC preferentially attacks high‑hardness regions with tensile stress.

Hardness Mismatch Leading to Slippage or Brittle Fracture

Wedges grip the strand through the wedge effect between their outer conical surface and the tapered bore of the anchor head. If the wedge hardness is too low relative to the anchor head, the tooth tips will be flattened under axial tension, causing strand pull‑in to exceed the allowed limit, slippage. Conversely, if wedge hardness is too high while the anchor head is softer, the wedges will cut into the tapered bore, creating plastic deformation of the bore and again causing prestress loss. The most dangerous situation: an excessively hard wedge with insufficient toughness can fracture in a brittle manner at low temperature or under impact, instantaneously releasing the prestress.

The logic behind hardness matching is not arbitrary. According to the anchorage performance test requirements in the annex of Technical Specification for Construction of Highway Bridges and Culverts (JTG 3362‑2018), the anchorage assembly must pass repeated loading cycles while achieving a minimum anchoring efficiency (typically 95 %) prescribed by the code or project specification. Hardness matching is a prerequisite for that efficiency.

Bearing Plate Deformation and Bursting

The bearing plate transmits the strand force directly to the concrete end face. If its hardness is too low, the plate will undergo compressive plastic flow during tensioning, imposing uneven pressure on the concrete beneath the anchorage and causing local cracking. Conversely, if the plate is too hard but insufficiently thick, brittle cracks may develop in the plate itself.

These three mechanisms are not isolated: on coastal bridges, SCC cracks tend to initiate preferentially from regions with higher hardness and residual stress; hardness mismatch aggravates fretting wear, causing corrosion products to build up and wedge cracks open further. A related failure mode, hydrogen embrittlement, can affect high‑strength fasteners in aggressive environments, for a case example, see Bay Bridge Bolt Hydrogen Embrittlement Lesson. Understanding “why” is the key to knowing what to watch for during inspection.

Condition Grading: Field-Observable Signals

Engineers on site do not need complex corrosion charts. The following five observable features directly correspond to three practical condition severity levels:

• Distance from the bridge to the coastline or saline soil boundary
  Measure with GPS. If the bridge is within roughly 10 km of the sea and the prevailing wind blows toward the bridge, atmospheric chloride deposition is likely high enough to pose a serious corrosion risk.
• Winter road maintenance practice
  If the historical de‑icing salt application rate and the presence of white salt efflorescence near drainage

From Failure Mechanism to Material Grade

This chapter explains the “why” behind each choice. The decision chain for each component links the failure mechanism to the material and specification.

From Stress Corrosion Cracking to Material Upgrade

Mechanism → Failure mode → Countermeasure

If the bridge is classified as severe or special due to high chloride exposure, the most likely failure mode is SCC in the wedges or anchor head. The first line of defense is material selection. Wedges can be made of mature Cr-Mo alloy steel, quenched and tempered to provide the required hardness while retaining sufficient toughness; the microstructure should avoid continuous carbide films along grain boundaries (which would form SCC-sensitive paths). For extreme marine environments, case-hardened steel with a hard surface and tough core may be considered, but any new material must pass the full type test according to the project specification or JTG 3362-2018.
The second line of defense is post-installation duct grouting (see installation section below), which cuts off the corrosive environment.

From Hardness Mismatch to Specified Hardness Window

Mechanism → Failure mode → Hardness combination

If the wedge hardness is too low relative to the anchor head, the tapered bore cannot maintain sufficient gripping force, and the assembly will exceed the pull-in limit specified in the static load test of JTG 3362-2018 Appendix A (typically 6 mm, but always refer to the design documents and the code version cited in the contract). This requires a reasonable hardness difference between wedge and anchor head. The code-recommended range—for example, wedge HRC 40-44, anchor head HB 220-260—provides sufficient gripping force without damaging the anchor head.
Normal condition selection: wedge 40Cr or equivalent, anchor head 45# steel or Q345B (HB 220-260). Severe condition: keep the same hardness combination; do not arbitrarily increase hardness to avoid embrittlement. SCC protection relies on alloy composition and environmental barriers, not on making parts harder.

From Bearing Plate Deformation to Steel Grade and Thickness

If the bearing plate hardness is too low, plastic flow occurs under full tensioning force, causing uneven concrete bearing. The steel grade and thickness of the plate must be selected based on the design tensioning force and the local concrete bearing check. Quenched and tempered steel HB 180-220 is a typical range, but specific values should be taken from the project anchorage design drawings or the manufacturer’s type test report. A plate with hardness above the specified upper limit also cannot transfer the load uniformly into the concrete.

Recommended combination for normal conditions: wedge 40Cr / HRC 40-44, anchor head 45# steel / HB 220-260, bearing plate Q345B / HB 180-220. When manufactured and installed according to applicable codes, this combination has been verified by extensive engineering practice to meet anchoring efficiency and pull-in limits. Under severe conditions, increase the frequency of non-destructive testing and the grouting density requirements, rather than switching to untested steel grades.

Delivery Inspection: Five Mandatory Checks

Upon arrival of anchorages, the material, testing, and supervision parties must conduct a joint acceptance. The following five items require hands-on measurement or witnessed testing—not just reviewing reports.

Installation Control: Key Points

During installation, the following points must be controlled to prevent premature failure:

• Cleanliness: Ensure the strand, wedge, and anchor head contact surfaces are free of oil, rust, and debris.
• Alignment: The strand must be properly centered in the anchor head to avoid eccentric loading.
• Tensioning sequence: Follow the specified sequence and load steps; do not exceed the design tensioning force.
• Grouting: After tensioning, perform duct grouting promptly and ensure fullness to protect against corrosion.

Summary of Decision Points

• For high chloride exposure, select Cr-Mo alloy steel wedges with tempered martensite microstructure and ensure full grouting.
• For hardness matching, maintain wedge HRC 40-44 and anchor head HB 220-260 as a starting point; adjust based on project-specific type tests.

Next Steps

To prepare for your anchorage procurement and inspection, gather the following:

1. Project design documents specifying anchorage type, steel strand grade, and tensioning force.
2. Environmental data: distance to coast, de-icing salt usage, and humidity records.
3. Applicable code version (e.g., JTG 3362-2018) and any supplementary project specifications.
4. Manufacturer’s type test reports and material certificates for the proposed anchorage system.

For further assistance on fastener and anchorage selection, visit our capabilities page or contact us.