We had previously locally sealed leaks on site using Specialty Products Incorporated Corrolastic HT which had been proved effective, so the decision was taken to treat all of the roof to roof joints and the circumferential wall to roof joint using the same polyurea technology.
The reason that this polyurea material was first employed is that it has been tested to withstand up to 6 bar of negative pressure – far in excess of the pressure of the gas contained within the digester. This polyurea is also highly chemical resistant and able to withstand the levels of hydrogen sulphide contained within biogas.
Initially where the roof overhangs the external walls the HDPE lining which extended from within the tank was removed using mechanical breakers. The reason for this is that HDPE is a difficult surface to adhere to and we wanted to ensure an excellent bond of the polyurea gas sealing to the anaerobic digester.
Once removed the area to be coated was treated using Corrolastic SG. This material not only made good the defects from where the HDPE lining had been removed, but also uniquely will adhere to any remaining HDPE anchor studs.
The areas of concrete to be coated on the anaerobic digester tank roof and vertical faces were prepared by method of vacuum controlled diamond grinding to remove contaminants and surface laitance to ensure excellent adhesion of the primer and thus the polyurea gas sealing.
All surfaces to be coated were vacuumed clean and as part of our standard polyurea application quality assurance the moisture content of the concrete was tested and recorded.
Before application of the damp tolerant epoxy primer again as part of our standard polyurea application quality assurance the climatic conditions were tested and recorded.
The damp tolerant epoxy primer was applied by method of brush and roller, with an aggregate dressing broadcast into the wet resin to provide an excellent mechanical anchor for the subsequent polyurea application. The secondary reason for this also was to act as an insurance policy against poor adhesion as a result of amine blush /blooming given the external location of the works.
Amine Blush/blooming occurs when epoxy coatings are exposed to moisture before they are fully cured. This forms a waxy by product at the surface which prevents subsequent coatings from adhering properly and is a serious concern, especially for a project such as this where the polyurea is going to be subjected to negative loads which will present further strain on the adhesion of the polyurea.
All joints were filled with backing rod prior to sealing using a polyurethane jointing compound. All pipes and items not to be coated were masked off to prevent overspray damage.
Finally SPI Corrolastic HT polyurea was applied to a thickness of 2mm as recommended by the manufacturer. The integrity of the polyurea was tested using a DC Holiday spark tester to check for pin holing which would allow gas leaks once the anaerobic digester was filled and returned to service.
To keep within our clients tight programme in order to minimise the time the anaerobic digester was out of service our team worked through the weekend, which the client was very pleased about
