Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 4—Structural Design of Conduitsprovided by dowels, reinforcement is never continuous across a contractionjoint. A minimum of 7 days should elapse between adjacent placements atcontraction joints. Waterstops should be placed across contraction joints.However, excessive movement at the joint may damage the waterstop.Therefore, contraction joints are not typically used in the construction ofconduits, since they may not ensure a watertight joint. Contraction joints areoften used in the construction of entrance and terminal structures.• Control joints.—Control joints are typically used in reinforced cast-in-placeconcrete conduit to provide for control of initial shrinkage stresses and cracksof monolithic units. Control joints are constructed as described for contractionjoints, except that reinforcement is always continuous across the joint. Thereinforcement prevents the longitudinal forces from opening the joints.Waterstops should be placed across control joints to provide a watertight seal.The surface of concrete first placed at joints should be coated with sealingcompound, so no bond develops between the ends of adjoining conduitsections. Figure 73 illustrates a typical control joint used in conduitconstruction. These joints are very effective in minimizing differentialmovement between conduit sections. A minimum of 7 days should elapsebetween adjacent placements at vertical control joints and 3 days betweenadjacent placements at horizontal joints.• Expansion joints.—Expansion joints are used in concrete to prevent damage dueto the compressional forces from movement caused by the expansion ofabutting concrete. Expansion joints separate adjoining structural elements.Dowels and keyways can be used across these joints to resist movement.Preformed joint filler is placed in all expansion joints. The joint filler shouldcover the entire surface of the concrete at each joint and be laid against theconcrete and held rigidly in place while concrete is placed on the other side ofeach joint. All joints in the filler should be tightly fitting butt joints. Expansionjoints are not typically used in the construction of conduits, since they do notprovide for a watertight joint. However, expansion joints are often used in theconstruction of entrance and terminal structures.• Construction joints.—Construction joints are typically used in reinforced cast-inplaceconcrete conduit to facilitate construction; to reduce initial shrinkagestresses and cracks; to allow time for installation of embedded metalwork; or toallow for subsequent placing of other concrete. For conduit construction, bondis required at these joints, and reinforcement is continuous across the joint.The surface of the joint should be cleaned to expose aggregate beforeplacement of the next concrete lift. A minimum of 7 days should elapsebetween adjacent placements at vertical joints and a minimum of 3 daysbetween adjacent placements at horizontal construction joints. As much as103