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Alternate Materials <br />Concrete <br />Without a doubt, concrete represents the world's most <br />extensively used material of construction. However, it is <br />subject to direct corrosive attack as well as spalling, or <br />cavitation. Good examples of corrosive attack involve <br />acids, including even dilute acid associated with acid <br />rain. Sulfates are also especially aggressive to concrete, <br />which presents problems when used in the vicinity of <br />FGD applications. Protection of concrete floors with a <br />layer of FRP is common practice. Acid resistant grades <br />of concrete have been developed, as well as so-called <br />polymer concrete wherein resin is used to replace all, or <br />a portion, of the Portland cement used in the concrete <br />formulation. <br />Almost all concrete is reinforced with steel mesh or <br />rebar due to the low tensile strength of concrete. Upon <br />cracking and permeation by acids or salt solutions the <br />steel is attacked by galvanic corrosion. This then spalls <br />and weakens the structure due to high tensile stress in <br />the vicinity of the corroding steel. Dangerous situations <br />sometimes exist with concrete used in infrastructure <br />applications. Composite structures including composite <br />rebar offer novel approaches. <br />Another corrosion mechanism associated with concrete <br />is carbonation. It occurs when carbon dioxide from the <br />surrounding air reacts with calcium hydroxide contained <br />in the concrete, to produce calcium carbonate. <br />Because calcium carbonate is more acidic than the <br />parent material, it effectively depassivates the alkaline <br />environment of concrete. At pH levels below about 9.8, <br />the concrete mass can reduce the passive film which <br />serves to protect the steel reinforcement. This type of <br />attack is commonly observed with concrete hyperbolic <br />cooling towers, where elevated temperature and high <br />humidity promote the progression of a carbonation <br />front. The same conditions promote diffusion inside <br />of the hyperbolic tower. This can lead to corrosion <br />of steel, especially around cracks or in the vicinity of <br />joints associated with slip forms used in construction. <br />Due to water conservation as well as scarcity of fresh <br />water, greater use of evaporative cooling is leading to <br />new designs in cooling towers. As a result, more scale <br />formation along with higher salt concentrations favors <br />composities which can be used more extensively as an <br />alternative to concrete. <br />