The four concrete water tanks at a large NHS Hospital were in a extremely poor condition, with the old epoxy tank lining breaking down creating an ideal environment for the growth of aquatic microorganisms. Further to this when entered the first tank and thick layer of black mould was found to be present on the ceiling of the tank – a result of the ceilings not having been coated when the tanks were last relined.
As per the tender requirements we allowed for the removal of all existing coatings and the application of a DWI approved polyurethane coating, in the form of Acothane DW.
Initially the existing water tank lining was removed by method of blasting, exposing and preparing the concrete beneath. Following clearing of spent blast abrasive, all surfaces were high pressure washed to ensure that they were spotlessly clean in order to ensure optimum adhesion of the new tank lining.
Filling the blow holes
Following blasting the concrete was found to contain large quantities of blow holes. As experienced tank liners we know that blow holes cannot be filled with paint alone. Therefore a cementitious fairing coat was applied to the tank walls and floor in the form of Flexcrete 841SP. This material was allowed to cure and hydrate – tested using a moisture meter inline with the manufacturers recommendations.
Within the ceiling of the tank there were voids and seams which required filling to ensure a seamless tank lining application. These were filled using Flexcrete Monomix applied by trowel.
Applying the lining to the tanks
Once sufficiently dry the first coat of the new water tank lining was applied. When applying Acothane DW to concrete surfaces it is first necessary to apply Acothane LV sealer. This low viscosity primer penetrates the surface, ensuring excellent adhesion of the new tank lining.
Acothane DW is particularly suitable for application to concrete water tanks as the material remains flexible when cured and can elongate 25%. This means that minor cracking or movement in the structure should be accommodated without the loss of fluid tightness.
Acothane DW was applied in two coats of contrasting colour, with a separate stripe coat to all angles and corners. Each coat was applied to a wet film thickness of 500 microns – checked regularly during application using wet film thickness combs.
Checking the coating for pin holing
As per best water tank lining practice the cured coating was checked for pin holing and porosity using a DC Holiday spark tester. Where found these were marked prior to touching in using the same material applied by brush.