lightweight fill to level sloping structure

huangyhg

lightweight fill to level sloping structure
i am looking at an existing office building with a concrete pan joist floor over a basement. part of this floor extends beyond the occupied space and supports a driveway. the driveway consists of a 4" topping over a waterproof membrane over the pan joist structure. the plan is to expand the occupied space over the driveway, which means we need build up the driveway to the floor level. the amount of buildup will be as much as 2'-8". any input on how to do this would be appreciated. on another project, i needed to level off an existing sloping auditorium floor, and considered using geofoam. some of the negatives encountered were perceived difficulty in cutting the blocks on a taper, cost, and the possibility of odor from the geofoam under an occupied space. that particular floor was a slab-on-grade, so we ended up using earth fill instead.
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you can use a combination of things depending on the fill amount.  we have used rigid foamboard to create ribs of concrete (24" wide foam, 6" rib, 24" foam, 6" rib, etc) with a 2 1/2" slab over the foam.  or you can build up lines of cmu stub walls and top with a metal deck and lightweight concrete slab.
just be sure to correctly calculate the added load and check the existing structure - a driveway may have been designed for 50 psf (parking garage-type loading) to 250 psf (typically used in the past for sidewalks over basemens in downtown areas for firetruck loading), so you may have plenty of capacity - just check it.