Coatings on concrete serve as impermeable membranes, which prevent corrosive liquids like acids and acid producing gases such as H₂S ( Sewage ) , CO₂ ( Carbonation ) from attacking the concrete aggregate.

Reinforcing steel bar corrosion is caused by the permeation of corrosive ions like chlorides through the concrete cover and onto the reinforcing steel. Another major cause is the loss of alkalinity of concrete ( to below 12.5 pH ) by leaching or carbonation which destroys the passive and stable ferric oxide film on the reinforcing steel. The corrosion product, ferrous oxide is highly voluminous and the expansive pressure exerts significant tensile forces on concrete which causes it to crack and spall. Concrete coatings reduce / eliminate the permeation of corrosive gases and ions through the concrete onto the reinforcing steel.

As stated in NACE International Standard RP0187-96 “Design Considerations For Corrosion Control Of Reinforcing Steel In Concrete“, the basic approach to corrosion prevention is to isolate the reinforcing steel from the environment. Use of an durable, impermeable concrete coating which is compatible with the service conditions is recommended, apart from measures to be taken in the design and placement of dense concrete cover.

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 exist in reinforced concrete and the widening of cracks in service, by thermal and humidity changes precedes the corrosion process. Permeability (not chemistry) of concrete is the most important factor for long term durability (American Concrete Institute  ; ACI - SP109 “Concrete In Marine Environment”). It is therefore important for concrete coatings to be highly impermeable and also sufficiently elastic to span hairline shrinkage cracks.

• Alkali resistance (concrete has > 12.5 pH) and resistance to chemicals in the environment.

• Low permeability towards oxygen, moisture, chloride ions, gases like H₂S , CO₂ etc.

• High build characteristics in order to obtain a pinhole and defect free coating on the rough concrete surface.

• Elasticity to prevent cracking due to expansion / contraction cycles. Ability to span hairline shrinkage cracks.

• High adhesion.

• Good weathering characteristics.

Purethane^® 386/9000 has excellent chemical resistance to alkali and a wide range of industrial chemicals.  Permeation rates of of chlorides and oxygen to the reinforcing bar are many orders of magnitude lower than un-coated dense concrete , effectively starving the corrosion mechanism. Unlimited film build capability allows application of defect free coating even on rough concrete with a sufficient film thickness. It is elastic , will withstand cyclic thermal expansion / contraction and can span cracks in the concrete. Adhesion is greater than the tensile strength of concrete – in pull off tests, 90% or more of the break will take place within the concrete and not at the coating – concrete interface. It is highly resistant to degradation by U.V and weathering.

Tests by Taylor Woodrow Research Laboratories, U.K, highlight the chloride and oxygen diffusion rates, crack spanning capacity, water permeation, adhesion and weathering resistance. Other technical resources available include a comparative study of the efficacy of different approaches to combating corrosion of concrete in marine environments including concrete chemistry, re-bar coating, cathodic protection, use of Purethane^® 386/9000.  Ask for a copy (Adobe Acrobat) By E-Mail !

Click on the link to see newsletter describing application of 2.00 mm thick Purethane^® 386/9000 on a Seawater Cooling Tower for  a 2 X 500 MW Thermal Power Plant. One of the largest single installation of 100% Solids Polyurethane Coatings in the world !