The concrete sludge tank was suffering from significant degradation as a result of exposure to hydrogen sulphide and biogenic sulphuric acid. The tank had not been lined when new and the extraction system had degraded, resulting in higher than expected H2S levels.
The first stage of refurbishing concrete assets in line with BS EN 1504 is to determine the extent of the concrete degradation. We appointed concrete condition experts Martech to produce a concrete condition report.
A key part of this when refurbishing concrete assets which have been exposed to hydrogen sulphide is by determining the extent of the concrete contamination with Sulphate by the taking of dust samples as per the results shown.
Once the extent of the concrete contamination has been determined a plan can be instigated as to what depth of concrete must be removed so that only concrete with a sulphate content of less than 4% remains in place.
As part of the project because of the extent of concrete that required removal a propping system was required. This was incorporated into our works package to simplify contractor interfaces, with our own teams installing the Ischebeck system.
Once access had been installed the full extent of the concrete degradation and steel reinforcing corrosion was apparent.
The degraded concrete was then removed to the prescribed depths by hyrdodemolition using one of own Hammelmann ultra high pressure water jetting units. To expediate the concrete removal where possible a robotic hydro demolition device was used.
Following Hydrodemolition further sulphate dust tests were taken throughout the tank to confirm that the concrete removal had been sufficient.
The concrete was reinstated by method of dry spraying by our trusted subcontractor GSSL, spraying Weber concrete repair materials. This material was installed in a panel arrangement and the curing of the material monitored using a rebound hammer in order to inform when the propping arrangement could be dropped in order to allow the completion of the soffit.
Because of the access arrangements the works had to be completed in two visits. Following the concrete repairs to the top section heating and dehumidification was introduced to properly dry out the cementitious materials and ensure the best climatic conditions for coating application.
All surfaces to be coated were double primed, with the first primer coat being diluted 50% to aid penetration into the substate for maximum adhesion. The second coat was applied without dilution to apply a thicker film to better seal the surface. The reason for double priming is that when applying heated plural sprayed tank linings such as Raven Lining 405 this reduces the outgassing and potential for pin holing from the concrete as concrete itself contains lots of entrapped air pockets which when heated force air through the uncured coating film.
At the termination point from the works at the top section an anchor rebate was cut to ‘tuck’ the tank lining into as per best practice and the manufacturers recommendations.
Raven Lining 405 tank lining was then spray applied using our own plural spray equipment to a nominal thickness of 3mm, confirmed during application with frequent wet film thickness readings.
Prior to removing the access system to allow lower section to be completed the applied Raven 405 tank lining was tested for porosity using a DC holiday spark test and any pin holes marked and made good by filling using hand mixed material and filling blades due to the high viscosity of the material.
We removed the access system and a conventional scaffold deck was installed to the conical floor of the sludge tank with access to high level provided by means of PASMA tower.
Because the degradation to the lower sections of the concrete tank was not as severe (verified by the concrete condition report and sulphate dust testing) the concrete only required preparing and the a tank lining applied.
The concrete was prepare by method of 500 bar water jetting to remove surface laitance
This exposed a lot of porosity in the concrete and to provide a suitable surface for coating application a cementitious fairing coat was applied, which was sponge float finished to provide an excellent mechanical profile for the tank lining to adhere to. Raven Lining is quite unique in that due to its ultra high build nature it can be applied to very rough and pitted concrete substrates where speed and fast turnaround is required, however this does use more material and is significantly harder work than making good with cementitious materials.
This was allowed to hydrate and again heating and dehumidification introduced to cerate the best conditions for tank lining application. Again as per our standard tank lining quality assurance the moisture content of the substrate and the climatic conditions were tested and recorded.
The tank lining application process as per the top section of the tank was repeated for the bottom section. The only different was that where the tank lining overlapped on to the previously coated section it was necessary to abrade the previously applied Raven Lining 405 tank lining as this had exceed its recoat window. For 100% confidence at this interface a second chase abutting the previously ‘toed in’ material was cut, as well as abrading the overlap section.
Raven Lining 405 has a design life of 50 years in waste water environments, providing a long lasting tank lining in demanding environments such as this. As per their catch phrase ‘Proven, True & Blue!’.
