Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams• Alkali-aggregate reaction.—Alkali-aggregate reaction (AAR) occurs when certaintypes of sand and aggregate (e.g., opal, chert, flint, or volcanic material with ahigh silica content) are exposed to sodium and potassium hydroxide alkalies inportland cement. In a moist environment, a gel is formed around the reactiveaggregate, creating tension cracks around the aggregate and extensive expansionand fracturing of the concrete. This expansion, cracking, and loss of concretestrength can lead to pathways for seepage or localized collapse of the conduit.Concrete containing alkali-reactive aggregate may show immediate expansion anddeterioration, or it may remain undisturbed for many years. Concreteexperiencing AAR is characterized by pattern cracking on the surface. Figure 107shows a concrete wall that has experienced AAR. In new construction, aggregatesources containing negligible potentially alkali-reactive materials, low alkalicements, and pozzolan replacement of a portion of cement, should be used.When abundant potentially alkali-reactive materials are available, low alkaliportland cements and fly ash pozzolan have been found to eliminate or greatlyreduce the deterioration of reactive aggregates. There is no proven method foreliminating AAR in existing structures (Reclamation, 1997, p. 6).• Sulfate attack.—Sodium, magnesium, and calcium sulfates existing in soils andgroundwaters react chemically with the hydrated lime and hydrated aluminate inthe cement paste in concrete. The volume of the reaction byproducts is greaterthan the volume of the cement paste from which they are formed, resulting indisruption of the concrete from expansion. Concrete experiencing sulfateattack is characterized by a disintegrated appearance. In new construction, asulfate resistant portland cement or a combination of suitable cement andpozzolan should be specified, when it is recognized that concrete will beexposed to soil and groundwater with sulfates. The application of a thinpolymer concrete overlay or sealing compounds may be beneficial for existingstructures experiencing sulfate attack. Otherwise, removal and replacement ofconcrete with a sulfate resistant cement should be considered (Reclamation,1997, p. 23).Polyethylene plastic pipe, if exposed to ultraviolet (UV) radiation and oxygen, canexperience degradation affecting the physical and mechanical properties of the pipe.Ultraviolet light is present in sunlight. Typical applications using polyethylene pipeinvolve sliplining of existing conduits. In this type of application, exposure to UVlight is limited. Any exposed surfaces would require long term UV protection. Thisprotection is provided by compounding 2 to 3 percent carbon black into thematerial, which prevents UV penetration.8.1.3 CavitationCavitation is an erosional process and often causes deterioration in concrete, plastic,and metal conduits with high heads, where high velocity vortices are formed. The180