Hidden defect: Pre-stressing wires / strands / anchorages

Problem: Breakages of post-tensioning tendons meant that residual strength testing was required to monitor the effect of failures on the structure.

Solution: JF Strainstall undertook programmes of in-situ stress measurement in the post-tensioned concrete structure. This was part of the assessment of the structure’s residual strength following well publicised breakages of post-tensioning tendons, as detected by acoustic emission monitoring. James Fisher Testing Services also set up instrumentation for monitoring the structure during re-stressing operations.

Hidden defect: Missing / inadequate rebar / pre-stressing

Problem: Risk of sudden bridge collapse.

Solution: JF Strainstall ascertained the dead load condition within a segmental post-tensioned concrete bridge deck. A monitoring system was fitted to the bridge, including acoustic emission sensors to detect pre-stressing wire breaks. Strain and displacement sensors were employed to monitor potential crack development.

Hidden defect: Half joints

Problem: The renewal of approach viaducts required strength testing to be carried out on existing half joints.

Solution: As part of a renewal of approach viaducts, a test to destruction of a half joint was undertaken, with monitoring by JF Strainstall. This was used to provide information about any hidden strength and to give confidence about the likely condition of the dozens of other similar joints within the junction complex.

Hidden defect: Corrosion of embedded reinforcement

Problem: During resurfacing work, concerns were raised as to the condition of the exposed reinforced concrete decks.

Solution: JF Strainstall conducted extensive half-cell potential measurements across the full deck areas. Colour contour plotting of the results highlighted several areas where high levels of corrosion activity were evident. Laboratory testing of incremental concrete samples extracted from these areas showed significantly high chloride values, also indicating probable corrosion activity. Removal of the surface concrete confirmed the problem with several reinforcing bars showing major loss of steel section leading to a reduction in strength.

All chloride contaminated concrete was removed and additional or new reinforcement was installed prior to concreting and re-waterproofing.

Hidden defect: Concrete delamination and corroded reinforcement

Problem: General condition survey found large areas of delaminated concrete.

Solution: JF Strainstall completed a comprehensive tap hammer survey to determine the full extent of the problem. Where concrete fell away the underlying reinforcement was seen to exhibit heavy surface corrosion. A full suite of testing was then carried out, including; cover depth, half-cell potential, resistivity, chloride sampling and carbonation depth testing. The cause of the problem was determined to be carbonated concrete down to the depth of the reinforcement, where reduced alkalinity had led to corrosion of the bars and subsequent concrete breakdown. All affected concrete was removed, the bars were cleaned, and spray concrete repairs were applied.