Concrete Natural Draught Cooling Towers (NDCT) and Induced Draught Cooling Towers (IDCT) represent a significant investment in Power Plants. The cooling water is often saline/ corrosive. Most coastal power plant use Seawater for cooling which is highly corrosive. The corroding ions reach the reinforcing steel of the concrete where the high volume of corrosion products leads to cracking and spalling of the concrete in a vicious cycle. Purethane Coatings effectively stop permeation of the corrosion ions into the concrete surface.
In high quality concrete the transport of gases and ions is a slow process. However it is much faster through an interconnected network of micro cracks and capillary voids by capillary action. Such a network of micro cracks already exists in reinforced concrete and the widening of cracks in service (by thermal and humidity changes) precede the corrosion process. As soon as the Chloride ions reach the rebar, one has the powerful anodic-cathodic potential, and with free moisture abundantly available corrosion will proceed.
Permeability (not chemistry) of concrete is the most important factor for long term durability. To prevent the permeation of Chloride ions into the concrete an impermeable barrier in the form of a surface coating is required. The barrier must also be able to span existing and fresh micro cracks in the concrete.
Purethane Coatings provide an excellent barrier to permeation of ions.
- Chloride Ion transmission rate of Purethane coated concrete has been tested as 1.8% of uncoated concrete!
- Oxygen diffusion is tested as 0.0001 cm2/sec compared to 1,100 cm2/sec of uncoated concrete.
- There is ZERO liquid water permeation through the coating.
- Purethane spans cracks of up to 1.40 mm which may open under the coating.
- Purethane coatings have ‘unlimited build capability’ i.e. they can be applied at any thickness without sagging or dripping using multiple wet-on-wet passes. This unique feature allows pinhole (defect) free coating application on rough concrete. Coverage is even over peaks and valleys affording equal protection.
Brittle coatings like Epoxy and Glass Flake Polyester / Vinyl Ester are not suitable for concrete as they have ZERO CRACK SPANNING. Further they shrink with curing / ageing leading to stresses on the bonding surface and disbondment. Epoxy and Glass Flake Coatings have thickness limitations and cannot be applied more than 1.00mm. This leaves many defects in the coating on rough concrete through which corroding ions can enter the concrete.
Application capability is critical in a cooling tower. The fill support structure comprises thousands of narrow columns and beams at height up to 18 metres levels. The fast spray results in quick coating of accessible area and the operator needs to shift frequently to the next area. This means that scaffoldings are not suitable and high reach equipment like Scissor Lifts and Articulating Booms need to be deployed.