We produce precast exposed-aggregate sandwich panels using concrete made with Type III cement, a 0.38 water-cementitious materials ratio and enough superplasticizer to achieve a 7- to 8-inch slump. The forming bed is made of wood. Using a bucket that passes down the center of the bed, we pour enough concrete for a 3-inch-thick layer and rake the concrete to the edges. We consolidate that layer with an immersion vibrator, then screed it off before adding a 2-inch-thick insulation board and 3 more inches of concrete. When we sandblast the formed panel surface to expose the 1/2-inch limestone, the limestone particles appear in groups or swirls. What can we do to eliminate this problem?
Sid Freedman of the Precast/Prestressed Concrete Institute says such swirls are signs of segregation caused by the placing method. You should place concrete on top of concrete in successive bucket passes because blending separate puddles or passes of concrete creates areas that are heavy in mortar. Raking can cause a similar problem. Ed Boyle, plant manager at Lombard Architectural Precast, Alsip, Ill., suggests using a placing bucket with a wider-than-normal gate, then overlapping each successive pass to eliminate paste streaks. He also suggests using a vibrating screed that hangs off of the form side rails at the correct depth to consolidate your first layer of concrete. Immersion vibrators may be needed to ensure adequate consolidation for deep layers, but for 3-inch-thick layers a vibrating screed provides more uniform consolidation.