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. This is required to resist design loads and in the hollowcore 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, from which the engineer can calculate an estimated camber. 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. Camber should not be specified as a design parameter.
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. Minimize differential camber by one or more of these methods: jack up low pieces; shim shorter planks at bearing; flash patch; apply self-leveling course.
Theoretical camber is the calculated upward deflection based on relevant design parameters. It is time dependent due to the curing of concrete and can vary significantly from actual camber on site. The optimal storage time for plank in the yard is 2 to 3 weeks. Extensive job schedule delays and increased storage time can lead to camber growth. This is especially true in the early summer where warmer temperatures and low humidity accelerates the curing process.
Important Considerations about Camber
Specify realistic design loads to avoid overly conservative load requirements. Limit plank span to depth ratio to 45 and if necessary, increase plank thickness.
Increase 2-inch topping overlays if maximum plank camber exceeds 3/4 inch. Plan finished floor elevations accordingly.
Minimize differential plank camber prior to grouting keyways by one of more of the following methods: use temporary shoring to jack up low planks; use sandwich plates and bolts at mid span to draw planks vertically together; shim shorter and low planks at bearing points; flash patch (feather) joints between adjacent planks; apply self-leveling coatings (gypcrete) or use structural topping.
Advise subcontractors installing door frames to account for plank camber and topping if any. Contact Mid-States Concrete Industries today to learn more and be geared up for your next project.