THE PROBLEM
The concrete sludge tank had been constructed without a tank lining applied, relying solely on extraction and chemical dosing to control the hydrogen sulphide levels within the tank. This strategy had not performed and as a result the tank structure had been severely degraded as a result of attack from biogenic sulphuric acid – the consequence of Hydrogen Sulphide mixing with moisture inside the tank. This chemical attack was then causing aggregates to be released into the sludge, which was subsequently damaging the centrifuges further along the treatment process.
SOLUTION
We were asked initially to commission a concrete condition survey to assess the extent of the degradation. We employed the services of concrete condition experts Martech Technical Services Ltd who undertook a full concrete condition survey and subsequent report. This identified that Sulphate levels were above those permitted up to a depth of 30mm into the concrete matrix from the surface.
To remain in service and not continue to degrade beneath the new tank lining Sulphate levels must be below 4%.
A proposal was developed to remove the defective concrete and make the necessary concrete repairs prior to the application of a new tank lining.
Initially all loose aggregates and sulphur deposits were removed by method of pressure washing to minimise debris and contamination during the preparation works.
We proposed the removal of the defective concrete material to the required depths by method of hydro demolition using ultra high pressure water. This methodology offers the following benefits;
- Hydro demolition does not introduce cracking and micro fractures in the concrete as vibration tools can do
- Reduced Hand Arm Vibration exposure to site operatives
- Dust free
- Cleans and prepares steel reinforcing without damaging, reducing steel replacement works required
Following the hydro demolition works sulphate dust tests were taken throughout the tank to confirm that the hydro demolition works had removed sufficient material to ensure that all remaining concrete had a sulphate content less than 4%.
Waste water was processed through our Siltbuster hydro demolition unit to remove suspended solids and reduce the pH before disposing of in the permitted drainage.
Following the hydro demolition works any steel reinforcing corroded beyond permissible limits was replaced and all steelwork treated using Corroless EPF rust stabilising epoxy primer. Corroless EPF contains a mined pigment that converts corrosion present to stable iron oxides such as magnetite. Because of these properties it can be applied to steel prepared to standards as low as ST2 as per ISO8501-1, making it ideal for water jetted surfaces.
The concrete was reinstated by our trusted sprayed concrete subcontractor GSSL – Gunite and Shotcrete Services Ltd. The material applied was Reader Spraycrete 40. Because of the concerns about the structural integrity of the tank roof, this application had to be conducted in two phases working around the propping system installed.
To ensure all surfaces were laitance and dust free for the best adhesion of the new tank lining all surfaces were cleaned by method of high pressure spinning jet (5,000 PSI). The dry sprayed concrete was allowed to hydrate and once the required strength had been achieved as tested using a rebound hammer heating and air movement was introduced to reduce the moisture content below 6% in preparation for the new tank lining.
Dehumidification was introduced in order to ensure the optimal climatic conditions for tank lining application. Even though the proposed tank lining is highly moisture tolerant and does not have any climatic restrictions for application, we are of the opinion that if the best conditions can be achieved they should as this will ensure the longest service life possible from the new tank lining.
The tank lining to be applied was Raven Lining 405, an extremely durable fibre reinforced ultra high build epoxy tank lining specifically designed for demanding waste water applications.
The primer for Raven Lining (Raven 175) was applied in two coats to minimise outgassing during the tank lining installation. Raven Lining 405 is hot spray applied, so it is always preferable (although not necessary) to double prime where possible to fully seal the pores of the concrete matrix.
The first coat of epoxy primer was diluted 50% as per the tank lining manufacturers recommendations. By reducing the viscosity this increases penetration into the concrete matrix increases both adhesion and sealing. A second coat was then applied diluted only 10% to provide more of a film for better sealing properties.
Following priming Raven Lining 405 Ultra High Build Epoxy Tank Lining was plural spray applied to a thickness of 3mm. At this thickness this tank lining has a design life of 50 years from completion.
To ensure a pin hole free finish which is essential in demanding waste water tank lining applications where high levels of Hydrogen Sulphide are present a DC holiday spark test was conducted as per our standard tank lining quality assurance. Any pin holes found were marked and made good using the same material.
Because of the fast curing properties of Raven 405 tank lining any repairs if not conducted within the short overcoat window must be mechanically abraded. The fast curing properties of this tank lining mean that the tank lining can if required be returned to service in under 24 hours where required.
The blue colour of this tank lining always looks particularly good and very transformative given the condition of this tank before tank relining works commenced.
