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sewai this type detail by Morris (1940) aud in a discussion of Morris's paper by WooUey (1940). The form of corrosion under Australian conditions is apparently similar, the chief eharaeteristies being as follows (1) it is assoeiated with the presence of hydrogc;ri sulphide in the sewer atmosphere; (' 2 )it oecurs ahove water level where the eonerete surface is iu contact with the gaseous H2S emitted from the sewage; (3; the ultimate result of corrosiou is the conversion ol: the concrete structure to a putty-like mass in which the particles of aggregate aie embedded; (4) test specimens placed in the sewer show a definite lag period in the development of eorrosiou. Once corrosion commences it proceeds rapidly.Concrete is a mixture of cement, fine aggregate (usually quartz sand), coarse aggregate (basalt is frequently used in Melbourne) and water. The chemically active constituents are the cement and water, the properties of concrete being dependent on the reaction which occurs between them. In mixing concrete, hydration of the cement constituents, tricalcium aluminatc, tricalcium silicate, dicalcium silicate and tetrn calcium aJuminoferrite, occurs. It is these hydration compounds which are responsible for the properties of concrete. The aggregate may be considered to be inert material bound together by the hydrated cement compounds. Dnring hydration, calcium hydroxide is liberated and the set concrete is highly alkaline (pH 11-12-5). On exposure to the atmosphere, carbonation occurs until the calcium hydroxide at the exposed surface is completely converted to calcium carbonate (saturated CaCOg solution has pH 10'2). In the presence of moisture some of the carbonate is dissolved as bicarbonate and the equilibrinm, solid calcium carbonate-saturated solution of calcium bicarbonate atmospheric CO2, is set up. The bicarbonate solution under sneh conditions has a pH of 8-4. Concrete exposed to the atmosphere becomes chemically stable at this stage.Concrete exjjosed to corrosive sewer atmospheres undergoes further changes producing a continued shift towards more acid conditions. In the early stages of the conosion process a powdery white deposit forms which becomes moist and develops into flakes which can easily be jemoved. These flakes are largely calcium sulphate and the pH of a water extract is between ft and 7. The surface becomes increasingly moist and finally assumes a putty-like consistency. Large masses ean then be broken away and the particJes of coarse aggregate seen embedded in the decomposed cement. At this stage free snlphuric acid is preseut and the pH is frequently less than 2'0. Small amounts of free sulphur can frequently be detected in the corrosion products.

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