Upon opening the anaerobic digester to remove grit our client found that the existing tank lining was in a very poor condition, with extensive blistering and breaks that had allowed the concrete to be attacked and degraded because of exposure to hydrogen sulphide/biogenic sulphuric acid.
We proposed the removal of the existing failed tank lining and application of Sika CR7000 tank lining which is suitable for use in an anaerobic digester environment.
Initially the failed lining and degraded concrete was removed by method of ultra high-pressure water jetting.
SikaCem -133 S Gunite was then spray applied by our sprayed concrete subcontractor GSSL. This was sponge float finished in order to provide a mechanical profile for the subsequent tank lining to adhere to.
As per the manufacturers recommendations the concrete repair materials were protected with polythene sheeting to ensure proper curing.
Once the concrete curing regime had been achieved the sheeting was removed and the concrete repairs cleaned by method of high pressure washing to remove any dust contamination.
As part of our standard tank lining quality assurance the moisture content of the concrete was tested and recorded.
This is not strictly required when using the Sika CR7000 system as the primer Sikagard P770 can be applied to extremely damp substrates and has no substrate moisture content limitations. This was the reason that this tank lining system was selected for this application as the client had an extremely tight programme of works and could not afford any delays whilst waiting for the concrete to hydrate.
Again, as part of our standard tank relining process the climatic conditions were tested and recorded prior to and during tank lining application.
All surfaces were primed using Sikagard P770 applied by method of brush and roller at a practical coverage rate of 0.4Kg/m2.
Wet film thickness readings were taken throughout the application process to ensure that the correct thicknesses were achieved.
Once cured the applied tank lining was tested for porosity using a DC Holiday spark tester to check for pin holes. In an aggressive hydrogen sulphide rich environment such an anaerobic digester a single pin hole is all that is required for attack of the concrete substrate to recommence.
