THE PROBLEM
The existing polyurea tank lining had dramatically failed, blocking pumps and preventing the effective operation of the anaerobic digester. The failed polyurea lining had been collected in a skip on site.
SOLUTION
During our site survey we identified that the probable cause of the polyurea tank lining failure related to the interface between the primer and a cementitious fairing coat which had been applied. This could have been incorrect climatic conditions during application, to much moisture in the substrate or several other potential causes.
During our survey dust samples were collected to assess the sulphate content of the concrete. Fortunately these did not show elevated Sulphate levels, suggesting that the cementitious fairing coat which was still in place had afforded the concrete some protection against the aggressive environment found within an anaerobic digester. The gas space within a digester contains high concentrations of hydrogen sulphide, which when in contact with moisture forms hydrogen sulphide which is extremely aggressive to unprotected concrete surfaces.
A layer of struvite crystals had begun to build up on the lower section of the area to be coated (in the top of the liquid space). Initially this was removed by high pressure washing and the rest of the tank cleaned also.
Following cleaning all surfaces to be coted were prepared by method of abrasive blasting. We had expected that the cementitious fairing coat would remain in place, however it was not as well adhered as first impressions suggested. Therefore it was removed in its entirety, exposing the concrete substrate which was heavily profiled.
An anchor rebate was cut for the termination of the new epoxy tank lining as per then tank lining manufacturers recommendations.
During preparation works some cracks in the substrate were exposed. A structural engineer was consulted to determine if the cracks were dynamic or static – dynamic cracks would require a flexible treatment. Fortunately the cracks were assessed to be static, which due to the reinforced nature of the new epoxy tank lining can be accommodated without any additional treatment.
To provide a surface suitable for the application of the new epoxy tank lining we applied our own cementitious fairing coat in the form of Flexcrete 844SP. All surfaces were thoroughly cleaned and saturated with clean water following blasting and the Flexcrete material spray applied using our render pump. The fairing coat was sponge float finished to leave a surface suitable for the application of the epoxy primer.
The fairing coat was allowed to cure and hydrate naturally before the introduction of heating and dehumidification to create the best climatic conditions for coating application.
As part of our standard quality assurance the moisture content of the surfaces to be coated was tested and recorded using a moisture meter to ensure compliance with the tank lining manufacturers recommendations.
All surfaces to be coated were primed using Raven Lining 175 damp tolerant epoxy primer. As per the tank lining manufacturers recommendations the first coat of primer was thinned 50% to aid penetration into the substrate matrix to ensure excellent adhesion of the new tank lining.
A second coat of primer without the addition of thinning agents was applied to ensure the surface was fully sealed and further minimis pin holing and outgassing from the substrate. Because the proposed tank lining is applied heated this can cause outgassing in porous substrates such as concrete, although it can be still applied directly it is preferable to prime or further still double prime.
Raven Lining 405 was then spray applied to a thickness of 3mm. This fibre and ceramic reinforced epoxy tank lining really is the perfect solution for waste water anaerobic digester tank linings. The material is immensely strong, giving stability to aged and degraded concrete assets. Further to this the material has a design life of 50 years in aggressive environments, supported by SWAT testing – Severe Waste Water Accelerated Testing.
This is supported by a 20 years material guarantee, giving the client piece of mind on their investment.
As part of our standard quality assurance the new epoxy tank lining was tested for pin holing using a DC holiday spark tester. Due to the extensive preparation and priming regime pin holing was minimal. Where found pin holes were rectified using the same material.
Although not relevant to this application due to the extensive scaffold access required, Raven lining 405 can be returned to service extremely quickly following application – within 24 hours.
We believe the photos of the project demonstrate the care and attention given to this application, something we call the “Corroless Eastern Way”
