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Concrete In Australia : March 2008
Rehabilitating concrete access chambers Concrete pipes and structures are an integral part of all sewerage systems. They are however susceptible to corrosion which, if not addressed, can ultimately lead to a failure of the system and costly replacement. Concrete access and inspection chambers are some of the assets where corrosion can occur. However, it has also been observed within concrete pipes, sewerage pumping stations and sewerage treatment plants. These assets represent considerable fi nancial investment by those in the water industry and need to be well maintained to obtain maximum operational life. Protection against destructive microbes requires specialised coatings and knowledge of how microbiologically induced corrosion occurs. Xypex, a corporate member of the Australian Concrete Repair Association (ACRA), has been involved in the repair and rehabilitation of numerous access holes (manholes) affected by hydrogen sulphides and sulphuric acid. It is well understood that sulphate attack on concrete results from a chemical reaction between sulphate ions and hydrated calcium hydroxide and/or the calcium aluminate components of hardened cement paste, in the presence of water. The products resulting from these reactions are calcium sulphate hydrate, known better as gypsum and calcium sulphoaluminate hydrate, commonly referred to as ettringite. These solids have a very much higher volume than the solid reactants and, as a consequence of their formation, stresses are produced within the concrete matrix that result in the cracking of the paste and ultimately, in the reduction of concrete’s integrity. Importantly, water is not only a necessary reactant in sulphate attack on concrete but, as it readily enters concrete through capillary action, it is also the medium for the transport of the sulphate ions into the matrix of the concrete, resulting in the formation of expansive by-products at depth. Concrete is a highly alkaline material and generally not resistant to the effects of strong acids or other compounds which may convert to acids. The most vulnerable component in hardened concrete is calcium hydroxide which is readily dissolved and neutralised when attacked by certain acids. The most common manifestation of this attack is the obvious surface erosion which under conditions of lengthy exposures to acidic conditions may continue unabated. There are many kinds of acid attack on construction materials. The attack can be from water containing free carbon dioxide or acid rain (which may contain dissolved gases such as carbon dioxide, sulphur dioxide and/or nitrous oxide), pure water from desalination plants or melting ice, sewage and food acids and those generated from organic wastes. There are also high levels of naturally occurring sulphates in soils in many regions throughout the world which give rise to concerns for the durability of concretes placed in these environments. In sewerage facilities, including tunnels, pits and access holes in particular, sulphate-producing anaerobic bacteria produce hydrogen sulphide (H2 atmosphere. The H2 S) which volatilises in the sewer S dissolves in the moisture condensing on the concrete surface and is further oxidised by bacteria forming sulphuric acid. The resulting high levels of sulphuric acid give rise to a quite aggressive attack on the cement paste, resulting in accelerated deterioration of the concrete. The process of remediation involves relining the cementitious substrate to reinstate the loss of cover with a high performance repair material. As the environment described earlier is extremely acidic, an internal coating to protect the alkaline concrete repair mortar will also be required. Severe sulphate attack is evident in this access chamber (top). The same access chamber post-repair is show below. Several high performance resins and epoxies have been used successfully in combination with the Xypex system for repair and rehabilitation of access holes, all of which are designed to protect concrete in chemically aggressive environments. The specialised nature of the repair process and the use of ACRA members, who have the trained technical and corporate resources, will successfully ensure the long-term repair of these important assets. This article was supplied by Xypex Australia. Concrete in Australia Vol 34 No 1 23