When A Corrosion Survey Is Needed
A visual inspection can tell you that corrosion is present. It cannot tell you how much metal remains. A corrosion survey is needed whenever section loss could affect structural capacity or the choice of repair.
- A general or principal inspection reports heavy corrosion, pack rust or section loss
- A structural assessment needs measured rather than assumed section properties
- Repairs or strengthening are being designed and the extent of plating or replacement must be defined
- Weathering steel needs its residual thickness measured at the specified monitoring locations
- Hollow sections, such as parapet posts and gantry members, may be corroding from the inside
Methods We Use
We choose the method to suit the question being asked and the condition of the steel.
| Method | What it gives you | Best used for |
|---|---|---|
| Ultrasonic thickness testing (UT) | Spot measurements of remaining wall thickness | Grid surveys, weathering steel monitoring, hollow sections |
| Phased array corrosion mapping | A thickness map over an area, showing pitting and localised loss | Critical areas where the extent and depth of loss must be sized |
| Pit depth gauging | Direct depth of individual pits after cleaning | Accessible surfaces with isolated pitting |
| Visual and hammer testing | Location of delamination, pack rust and loose corrosion products | Initial screening of the whole structure |
Corrosion Hot-Spots On Steel Bridges
Section loss on bridges concentrates in predictable places. Our survey plans target these first.
- Girder ends and bearing areas beneath leaking deck joints
- Bottom flanges and web-to-flange junctions where debris and water collect
- Around drainage outlets and where drainage runs through the steelwork
- Crevices at bolted splices, riveted connections and between plates, where pack rust forms
- The interior of hollow sections, which can lose wall thickness unseen
- Splash zones next to carriageways exposed to de-icing salt spray
Weathering Steel
Weathering steel bridges rely on a stable rust patina rather than paint. CD 361 Weathering steel for highway structures requires structure records to specify locations for corrosion monitoring and residual thickness measurements, and CS 450 requires repeated thickness measurements of weathering steel at principal inspections. Joints, drainage, flat surfaces, drip details, bolted connections and sealant interfaces with concrete need particular attention.
Reporting
Results are reported by member and location, with thickness data, maps, photographs and the percentage of section loss compared with the original section. We highlight results that need engineering assessment, so the report can go straight to the structural engineer.