Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 8—Potential Defects Associated with Conduitsconcrete. Overconsolidation is usually defined as a situation in which theconsolidation effort causes all of the coarse aggregate to settle to thebottom while the paste rises to the surface. If this situation occurs, it isreasonable to conclude that there is a problem of a poorly proportionedconcrete rather than too much consolidation.5. Movement of formwork.—Movement of formwork during the period whilethe concrete is going from a fluid to a rigid material may induce crackingand separation within the concrete. Cracks open to the surface allowaccess of water to the interior of the concrete. An internal void may giverise to corrosion problems if the void becomes saturated.6. Settling of the subgrade.—Poor foundation support can impart tensilestresses, resulting in cracking of the concrete conduit. This often occursduring the period after the concrete begins to become rigid, but before itgains enough strength to support its own weight; cracking may also occur.7. Settling of the concrete.—During the period between placing and initial settingof the concrete, the heavier components of the concrete settle under theinfluence of gravity. This situation may be aggravated by the use of highlyfluid concretes. If any restraint tends to prevent this settling, cracking orseparations may result. These cracks or separations may also developproblems of corrosion, if saturated.8. Vibration of freshly placed concrete.— Most construction sites are subjected tovibration from various sources, such as blasting and from the operation ofconstruction equipment. Freshly placed concrete is vulnerable toweakening of its properties if subjected to forces that disrupt the concretematrix during setting.9. Premature removal of shores or reshores.—If shores or reshores are removed toosoon, the concrete affected may become overstressed and cracked. Inextreme cases, there may be major failures.10. Improper curing.—Curing is probably the most abused aspect of theconcrete construction process. Unless concrete is given adequate time tocure at a proper humidity and temperature, it will not develop thecharacteristics that are expected and that are necessary to providedurability. Symptoms of improperly cured concrete can include varioustypes of cracking and surface disintegration. In extreme cases, where poorcuring leads to failure to achieve anticipated concrete strengths, structuralcracking may occur.189