THE PROBLEM
Wood Reservoir has a full water capacity of 5,566,000 cubic metres covering a site of 280 acres. The reservoir was built over the period 1951-54, a process which involved damming the valley of the River Medway. A daily average of 3.08 million imperial gallons (14,000 m3) is extracted through Weirwood Water Treatment works.
As part of a water quality improvement and asset resilience scheme the concrete storage tank within the works was to be refurbished in order to ensure the continued operation of the works and ensure the water produce by it achieved Southern Water’s high standards.
SOLUTION TO THE CONCRETE STORAGE TANK REFURBISHMENT
The scope of this project involved the following;
- Mechanical and electrical strip out
- Removal of failed polyurea coatings applied by others
- Removal of the existing sectional concrete wall
- Construction of a new reinforced concrete wall within the concrete storage tank
- Concrete repairs to the tank structure
- Water stopping injection works
- Resurfacing of the concrete using cementitious fairing coats
- Application of a regulation 31 approved tank lining
- Third Party testing
- Sterilisation works
Prior to our refurbishment works taking place it was first necessary to remove the high lift pumps in order to gain access to some of the compartments within the concrete storage tank. Electrical Isolations were undertaken for ourselves by Econotech and mechanical works by ARC fabrications. The mechanical works included the removal of the existing steel supporting frame from the sectional concrete wall.
Once the mechanical works had been completed we were able to begin our refurbishment works of the concrete contact tank. The old polyurea coating which had been poorly applied to improperly prepared concrete was removed mechanically using low vibration breakers. This was followed by the removal of the sectional concrete wall by Coleman Utility Services, the clients preferred civil engineering contractor.
In place of the sectional wall a site cast reinforced concrete wall was constructed by Coleman Utility services to a design provided by the client. This formed part of our works package which enabled us to liaise effectively with the civil engineering contractor to integrate these works into the site program.
Due to the compartmentalized nature of the concrete storage tank we were able to progress with the concrete repair, resin injection and surface preparation works.
All concrete repairs we isolated by saw cutting and broken out using low vibration breakers. These works were undertaken prior to preparation of the water tanks internals by method of abrasive blasting to allow the steel reinforcing to be prepared to the optimum preparation standard of SA2.5 as per ISO8501-1 using the same method.
A significant part of this contract was the resolution of ground water ingress into the tank. This was entering the water tank through both cracks in the soffit and through construction joints in the slab and the walls.
The extent of this water ingress was far beyond what had been anticipated prior to the project starting and was a continual challenge throughout the project. The water ingress into the water tank was resolved using a combination of hydrophilic water stopping resins and Sika Combiflex bandage.
The concrete was prepared by method of abrasive blasting to remove material which had been softened by the soft water that is present in the area. This exposed the aggregates in the concrete, which some places was as deep as 10mm.
Repairs to the structure were made using Flexcrete Monomix as specified. To reinstate the lost concrete cover and create a surface suitable for the application of the tank lining Flexcrete Monolevel was applied, in some places in several layers due to the thickness required.
To ensure the correct application of the tank lining material extensive dehumidification and heating was put in place to dry the newly applied fairing coat to the correct moisture content. This was extensively tested as per the manufacturers recommendations.
All construction joints were first sealed using Siakflex 11FC and then over banded using Sika Combiflex as per the specification.
The specified tank lining for the concrete storage tank was Acothane DW. This is applied in three coats, consisting of a clear sealer and two high build coats of contrasting colour. This was applied by method of brush and roller to a thickness of 1000 microns.
To ensure the correct application of the tank lining a stringent quality assurance regime was instigated, involving wet film thickness readings, climatic monitoring during application, temperature logging during cure, in house porosity testing and third party porosity testing.
A particular area of attention was the overlaps between sections of the tank due to the relatively short recoat window of Acothane DW. These areas were carefully abraded to ensure excellent adhesion.
Following the completion of the refurbishment works we were responsible for the washing and sterilisation of the tank internals prior to returning to service.
In addition to these main concrete storage tank works we were also involved in several other emergent projects on site with relation to stopping water ingress into the water tank. These will be documented in other case studies.
