manufacture of hollow slabs

huangyhg

manufacture of hollow slabs
how are hollow slabs manufactured ?
hollow slabs are manufactured by machines generally called extruders although some operate more like a slip form paver.  the manufactures place concrete on beds up to 700 feet long one day and saw into specified lengths the next day.  reinforcing strands are stretched the full length of the bed before the concrete is placed.  very low slump concrete is used.
the holes in the slab are formed by long tapered tubes called cores.  some machines simply force concrete between the cores while the machine moves down the bed.  the more fancy machines will oscillate the cores alternately in opposite directions while tamping the concrete into place.  this is all a very low tech operation.  in fact some of these machines are based on engineering concepts that are decades out of date.  
regards,
-mike
there are 3 ways that i am familiar with.
mrmikee described the extruded method (spancrete, dycore, etc.). there are even variations of this where some are cast with the top up and bottom weld plates are tied to the bed and top weld plates are dug into the wet surface after the machine passes by. some plank are cast with the top down, so the method of cast in plates is just the opposite. the beds are typically 600 or 700 feet in length.
fabcon extrudes plank with stone creating the cores until the concrete has hardened. i understand it is then removed. the core inside has a very rough surface.
the wet cast method (flexicore) is poured like normal site cast concrete in steel forms. i believe the hollow cores are done with infatible forms.
i am not sure how much these methods have changed over the years.
plank comes in many different widths, 48" being the most common.