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<br />Acid water stems from two sources, mineral and organic. Mineral acidity comes <br />from sulfurous wells and springs, and drainage from coal mines. These sources <br />contain dissolved sulfur and iron sulfide which may form sulfurous and <br />sulfuric acids. Mineral acidity as strong as pH 2.3 has been encountered. <br />Organic acidity usually found in swampy land and barnyards rarely produces a <br />pH of less than 4.0. Alkalinity in water is caused by strong alkali-forming <br />minerals and from limed and fertilized fields. Acid water (low pH) is more <br />common to wet climates and alkaline water (high pH) is more common to dry <br />climates. As the pH of water in contact with culvert materials, either <br />internally or externally, deviates from neutral, 7.0, it generally becomes <br />more hostile. <br /> <br />b. Electrical Resistivity--This measurement depends largely on the nature <br />and amount of dissolved salts in the soil. The greater the resistance the <br />less the flow of electrical current associated with corrosion. High moisture <br />content and temperature lower the resistivity and increase the potential for <br />corrosion. Soil resistivity generally decreases as the depth increases. The <br />use of granular backfill around the entire pipe will increase electrical <br />resistivity and will reduce the potential for galvanic corrosion. <br /> <br />Several states rely on soil and water resistivity measurements as an important <br />index of corrosion potential. Some states and the FHWA have published <br />guidelines that use a combination of the pH and electrical resistivity of soil <br />and water to indicate the corrosion potential at proposed culvert sites. The <br />collection of pH and electrical resistivity data during culvert inspections <br />can provide valuable information for developing local guidelines. <br /> <br />c. Soil Characteristics--The chemical and physical characteristics of the <br />soil which will come into contact with a culvert can be analyzed to determine <br />the potential for corrosion. The presence of base-forming and acid-forming <br />chemicals is important. Chlorides and other dissolved salts increase <br />electrical conductivity and promote the flow of corrosion currents. Sulfate <br />soils and water can be erosive to metals and harmful to concrete. The <br />permeability of soil to water and to oxygen is another variable in the <br />corrosion process. <br /> <br />d. Abrasion Potentia1--Velocity of the water in the culvert can be <br />determined if the amount of flow, slope, size of pipe, material of pipe, and <br />inlet and outlet conditions are known. The higher the velocity, the more the <br />potential for abrasion. The soil characteristics of the streambed are also <br />indicators of abrasion potential. Erosive large-grained materials such as <br />sand, gravel, and stones will increase the potential for abrasion. <br /> <br />3-4.2 Methods to Increase Service Life. <br /> <br />There are various methods available to culvert designers and maintenance <br />forces to offset identified potential for corrosion and abrasion. Each <br />culvert site should be considered individually and the culvert material, <br />shape, and size selected that best fits the conditions of that site while <br />meeting the agency's objectives for hydraulic effectiveness, service life, <br /> <br />42 <br />