Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 8—Potential Defects Associated with Conduitsformation in the CMP are the chemicals dissolved in or transported by the naturalwater. All coatings and linings have some minor flaws (holidays). Corrosion tendsto concentrate at these flaws, since water can seep between the coating or lining andthe base metal moisture can become trapped, increasing the rate of corrosion. Thus,it may be possible for a coated CMP to become deteriorated in less time than anuncoated CMP in the same environment. Coatings applied to existing surfaces ofconduits are generally not very effective due to difficulties involved in obtaining agood bond with the conduit surface.For guidance on estimating the service life of CMP conduits, see the NationalCorrugated Steel Pipe Association’s CSP Durability Guide (2000) and FHWA’s,Durability of Special Coatings for Corrugated Steel Pipe (1991). Caution should beexercised in attempting to determine the service life of CMP used in conduitsthrough embankment dams. Many of these CMPs may have used no corrosionprotection, and many of the coatings and linings available today were not availablewhen the embankment dams were originally constructed. Prior to 1950, galvanizedsteel was the only metallic coating available for CMP. If a CMP has experienceddenting during installation, this could result in corrosion in areas where theprotective coating has been damaged. Figures 110 through 113 show the results ofcorrosion affecting CMP outlet works conduits.Corrosion of reinforcement in concrete conduits can also occur when it becomesexposed. Reinforcement can become exposed by cracking or spalling of theconcrete (figure 114), inadequate cover, or porous concrete. When reinforcement isexposed to corrosive elements, the iron oxides formed expand (requiring more spacewithin the concrete than the original reinforcement), resulting in tensile stresseswithin the surrounding concrete. These tensile stresses cause cracking anddelamination of the concrete. Rust stains on the conduit surface may be an indicatorof reinforcement corrosion.Cathodic protection attempts to retard electrochemical corrosion through theapplication of reverse direct current to the protected metal and to another metalwhich acts as a sacrificial anode. This sacrificial anode, typically consisting of eitherzinc, magnesium, graphite, or aluminum alloys, must be periodically replaced. Newconcrete installations in hostile corrosive environments should place special attentionon crack widths and concrete cover, as well as consider the use of protectivecoatings, before considering this often problematic and costly means of protectingagainst steel corrosion. Galvanic reaction between dissimilar metals can also result incorrosion. This can occur when a galvanic reaction forms between reinforcing steeland stainless steel outlet works components.Another form of corrosion is bacterial corrosion (Patenaude, 1984) caused byanaerobic sulfate-reducing bacteria. Bacterial corrosion typically has been found tooccur on galvanized steel pipe. This type of corrosion can exist in two environments185