The Casino reservoir is an 11ML reservoir with an internal diameter of 35.7m and wall height of 11.695m. It was originally constructed in 1977 to hold potable water for the township of Casino in New South Wales. The walls were constructed of wire wrapped (4.3mm) prestressed reinforced concrete. The spacing varied from 60 wires per 300mm at the bottom, to 8 wires per 300mm at the top, covered by 40mm of Gunite mortar.
Local authorities noticed cracking and spalling along the exterior of the reservoir, exposing the prestressing wires and engaged an engineering firm to assess the damage. The condition report noted that the engineers assessed approximately 43% of the external wall area (1310m²) and identified 7% as being delaminated (37m²). Extrapolating for the entire tank, that was a delaminated area of 86m².
Our scope of works was as follows
- Conduct a full Delamination Survey, identifying and marking out the area of remediation required
- Concrete repair to the identified area
- Saw cut extent of area to be patched
- Break out drummy concrete
- Clean Prestressing wires and paint with galvanic paint
- Install sacrificial anodes
- Apply repair mortar to repair
- Apply a primer or concrete barrier coating
However, our own surveys found the area of repair to be 223m² compared to the original report of 86m². The repair area was greater where the wall was more exposed to the sun.
When removing of the mortar layer it was discovered the a number of prestressing wires had failed and were broken. It was also found that wires had failed in the lower 1/3 of the tank wall raising serious concern about structural integrity at full capacity.
Additional prestressing strands (12.7mm) were introduced to compensate for the loss of prestressing wires. Each strand was mono stressed by over-lapping halfway around the reservoir. Sacrificial anodes were installed around each broken out area before the spraying application of repair mortar.
We sealed the top of the wall, then used paint to seal the sprayed concrete overlay. It was applied with an airless pump.
The problems we came across during the repair process highlighted the fact that what is on the drawings may not always be how something was built, therefore it is wise to use an experienced contractor that understands design, project specific issues and the technical aspects of the construction, especially when critical infrastructure is involved.