Hydrophobic and Pore‐blocking Ingredient (HPI) for corrosion-resistant concrete. Wastewater treatment plants are among the harshest environments for concrete, where there are many causes that degrade concrete and once this process starts, its effects are usually rapid

In very brief and simple terms, the process of sewage treatment involves its transport from houses, offices, hospitals, factories etc to the treatment plant via subterranean tunnels usually made of concrete, like the Thames Tideway Tunnel.

Protecting against chemical and biological attack

In terms of chemical and biological attack, the problem is that concrete has a pH of about 12, so very alkaline and vulnerable to attack by acids. This level of alkalinity is finite, whereas the attacking acids have an infinite source.

Sewage contains sulphate-reducing bacteria and in the presence of oxygen and hydrogen, and a complicated series of chemical reactions the concrete ends up being attacked by sulphuric acid. What makes this worse is that due to the porosity of concrete, this acid gets sucked into the concrete so that the deterioration occurs ever deeper.

The steel reinforcement in the concrete is protected by the high alkalinity of the concrete. However, this protection will be lost if moisture, chloride ions and oxygen penetrate pores or cracks to reach the steel. Once this happens, local corrosion cells will be formed and the steel will start to oxidise.

This process increases the volume of the steel to create tensile forces in excess of the tensile strength of the concrete, which causes cracking in the concrete and once this happens, the extent of damage quickly increases. Stainless steel has been proposed to counter this issue; unfortunately, metals such as stainless steel (steel with added nickel and chromium) oxidise as well, just a bit more slowly.

In climates with cold winters, concrete will degrade when exposed to differential thermal conditions, where one face is much colder than the other, wetting and drying of the concrete on different sides and intermittent exposure to water in freezing temperatures. Such conditions will result in cracking and continuous spalling of the concrete.

Waterproof concrete

The Caltite waterproof concrete system is a time-proven, practical system which produces damp-proof, highly durable concrete.

Caltite and Cementaid have a successful track record of over 50 years helping designers, developers and owners protect concrete structures, even under the most aggressive conditions and was used in some 600,000m3 of concrete in two of the world’s most notable sewage treatment schemes, the Hong Kong Harbour Area Treatment Scheme and the Singapore Changi Water Reclamation Plant, which can treat 1.7 million and 2.4 million cubic metres of wastewater per day respectively.

If you are involved in a project where real concrete durability is important, please contact Cementaid to discover how Everdure CALTITE concrete can work for you and the huge cost savings that can be made by making structures last, info@cementaid.co.uk or telephone Greg Waring on 01293 653 900.

Alistair McDonald

Cementaid (UK) Limited

1 Baird Close, Crawley, RH10 9SY Tel: 01293 653 900

SOURCE

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