Hollow Core Camber

Every time we provide a lunch & learn on hollow-core plank there are several questions pertaining to the amount of camber on hollow-core plank.  Camber is inherent in all prestressed precast products. It is the upward deflection created by the prestressed forces in the strands located below the center of gravity. Camber is required to resist design loads and in the hollow-core plank it compresses the bottom more than the top. Span length, plank thickness and design load requirements will determine the amount of prestressing force needed in the plank. The benefit of camber in prestressed precast concrete products is that it allows for longer spans, shallower depth sections and higher load carrying capabilities than conventional building materials. Large cambers resulting from long spans and/or heavy loads will affect the quantity of topping, assuming a level floor is required. Two inches of composite topping at mid span is minimal, and additional thickness at the ends of the plank may be required to maintain level floor elevations. Camber should not be specified as a design parameter.
Differential Camber
Differential camber refers to varying amounts of camber between adjacent hollow-core planks. Camber differences occur because no two planks have the same exact strength gain, creep and exposure to the elements in storage. Planks in the yard exposed to direct sunlight will experience more camber growth than plank in the shade. Adjacent planks with different span lengths will differ in camber as will those with different prestressing strand patterns.
Theoretical Camber
Theoretical camber is the upward deflection based on design parameters. It is time dependent due to the curing of concrete and can vary from actual camber on site. The ideal storage time for plank in the yard is 2 to 3 weeks. Schedule delays with longer storage time can lead to camber growth. 

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