I've seen ads where companies come in and level concrete by injecting "stuff" under the concrete and raising it until it's level. Usually this is with sidewalks that aren't weight bearing. Would this work with a garage floor addition that's sloping ~.75" over 8'. The length is 20'. I'm sure it could probably be done but is it worth the cost? l assume that the slab and stem wall for the exterior wall& roof are separate.