A Treatise On The Principles And Practice Of Harbour Engineering

72 HARBOUR ENGINEERING. The coatings applied to ironwork under the head of paint comprise those which are composed of red lead and those which have oxide of iron for their base. The latter of these has been advocated on the ground that it removes the tendency to galvanic action produced by two diverse metal substances in contact with one another in the presence of moisture. Other coatings are mineral or vegetable tar, black varnish, siderosthen, and various bituminous solutions. It is obvious that only the surfaces of piles which lie above the water level can be treated with these applications after érection. For cast ironwork, and especially for cast iron pipes, no better preservative could be devised than the Angus-Smith treatment, which consists in dipping the pipes while hot into a liquid mixture of coal-tar, pitch, linseed oil, and resin. Iron and steel are galvanised by dipping them into a bath of molten zinc so that a veneer of the latter metal covers them completely. To effect this treatment properly, the surface of the metal treated must be absolutely clean and free from scale and grease. The process is effective against ordinary atmospheric influences, provided the zinc covering be maintained intact. If a crack or perforation occurs, corrosion sets in and proceeds rapidly. Against sea air and water, galvanising does not afford much protection. It is obviously no simple matter, therefore, to find a satisfactory and reliable method for insuring the permanence of iron and steelwork in mari- time situations, and particularly in the case of piling, where the work is so difficult of access. The desired result, however, has been achieved by the ingenious expedient of enveloping the metal in conerete, and this brings us to the system of combined steel and conerete which now generally goes by the name of reinforced conerete. Reinforced concrète consists essentially of a core or internal network of metal, completely embedded in conerete, so that no part of the metal is exposed to, or in contact with, any external atmospheric or aqueous influences. As applied to piling, the system has many and important ad- vantages. Reinforced conerete piles are not subject to oxidisation, dé- composition, or decay. Experience has demonstrated that steel bedded in Portland cement conerete does not rust even when iminersed in water, and that a rusty bar so treated manifests no inerease in corrosion. Moreover, reinforced conerete piles do not offer the least incentive or attraction to sea-worms or insects; they are fireproof as well as waterproof; their dura- bility is beyond question; they cost less than long greenheart piles, and little, if anything, more than creosoted pitchpine; they can be jointed, and lengthened or shortened at will; and, finally, their compressive strength and supporting power is very great. Reinforced conerete piles vary considerably in design, according to the individual ideas of numerous inventors. It will only be necessary, however, to refer to a few of the better known examples, which are distinetly applicable to harbour work. The circumstauces of foundation piles for