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Hydrodemolition and Concrete Repairs, Margate STW

THE PROBLEM

Margate Sewage treatment works sits nestled in the cliffs more like a bond villains lair than a waste water treatment plant. Unfortunately there was a dark force lurking within in the form of severe degradation of the concrete within the inlet chamber and Archimedes screw chambers.

This had occurred as a result of exposure to Hydrogen Sulphide and the subsequent biogenic sulphuric acid that is produced when Hydrogen Sulphide is processed by aerobic bacteria in the gas space.

This was identified for repair along with replacement of the Archimedes screws as part of a major resilience project.

The works sit in a challenging subterranean environment, restricted in space and with access challenges.

THE SOLUTION

Prior to our tendering for the works as per EN 1504 a concrete condition survey had been conducted which had critically identified the extent of sulphate contamination of the concrete. This identified to which depths existing concrete had to be removed to in order for the remaining concretes sulphate content to be acceptable.

Initially access was gained by appointing approved scaffolders TONE Scaffolding Services who we appointed and erected designed scaffolds.

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A particular challenge of this project was that the inlet chamber would remain live throughout this element of the works. This prevented the use of a silt buster to treat waste water from the preparation, so in order to prevent debris entering the treatment process screening was installed on the lowest level of the scaffold to catch aggregates released from the hyrdodemolition works.

Using our own Hammelman Ultra High Pressure water jetting units we removed the concrete to the prescribed depths. The effectiveness of this was verified by the taking of and laboratory analysis of dust samples for sulphate content.

Cores were also taken from the central wall between the two screw chambers once access had been obtained to confirm the volume of degraded concrete which could be removed at any one time.

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We employed robotic hyrdodemolition equipment for both safety improvements and program efficiency where practicable.

A major benefit of concrete removal by hyrdodemolition is that it does not damage the existing steel reinforcing and prepares the steel ready for concrete reinstatement

Once the degraded concrete had been removed we were able to identify steel reinforcing that had been corroded to such an extent that replacement was required

Defective steels were removed using hydraulic cutters and new steels reinstated to match the original design and with the requisite overlaps.

The specified concrete repair materials were dry spray repair mortars from Natural Cement. These were applied by our trusted subcontractor GSSL with climatic monitoring undertaken throughout to ensure the correct conditions as per the manufacturers recommendations.

Post application the curing and strength of these concrete repair materials was monitored using rebound hammers and by the taking of and laboratory analysis of core samples from spray outs.

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Once sufficiently cured Flexcrete E942 Epoxy modified cementitious coating was applied in two coats of contrasting colour by method of airless spray. This material was selected for its moisture tolerance, especially given the live conditions within the inlet chamber.

Wet film thickness readings were taken throughout the application process to ensure that the required thicknesses were achieved.

These works were conducted in three separate mobilisations over three years, including within the Covid 19 pandemic. The challenging working environment with frequent gas alarms and stand downs due to high flows added to the complexity. Fortunately the quality of our team meant that we were able to overcome and adapt to issues as they arose.