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Question DetailsAsked on 1/25/2017

how close to foundation to jack hammer concrete floor

9200 sq. feet , concrete tiltup building; need to make 1"x1" trenches to install new floor electrical outlets ;

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1 Answer


Since a concrete tilt-up building, I am going to assume the slabs are structural, even if on-grade - and certainly any elevated floor slabs will be.

With a structural concrete building, get a structural engineer (preferably the one who designed the building, so he has the plans) to consult on this - because cut reinforcing and you could end up with a collapsing house of cards on your hands.

Normal method - run the power in the ceiling (in joist bays or in the plenum (the space between suspended ceiling and the floor above), then down from the ceiling to the floor in metal posts, with the outlets usually on the posts rather than in the floor itself - the ones you see at the corners of office cubicles, or in conduit hidden in decorative wood or acrylic/fiberglass columns.

Much less normal for this type work if it HAS to be in the flooring - use under-rug direct layment cable tray and cable, or more rarely come in from the suspended ceiling below the slab up through the slab with the wiring (avoiding reinforcing and any in-slab water/heating piping), or probably most rare get the rebar mapped out and a definitive route designed by the engineer (which is commonly on a diagonal to the reinforcing, not the shortest route), then use direct-embedment cable or cable in conduit laid in a trench sawn (not jackhammered) into the slab - pretty much whst you are talking about except I think your trench size is undersized unless low-voltage wire.

Note - need check on local building code for burial in floor slabs - generally only low voltage wiring can be direct-embedded in slabs, regular outlet wiring requires EMT (metal conduit), which has to generally have 3/4" concrete over it AND not be over 1/3 the slab thickness in diameter - so generally that means 1" (OD) conduit buried 3/4 to the top, or 1-3/4 to the bottom of it. To clear normal say #4-#6 rebar in the center of a normal slab, that usually means a slab at least 4-1/2 to 5 inches thick to avoid cutting into the rebar - not all floor slabs are that thick, with residential slabs commonly being 4 inches thick and high-rise buildings more like 5-8 inches thick. Gets nastier with pre/post-tensioned slabs where normally to cut in and put in conduit you are talking 5-7 inch minimum slab thickness to meet all code requirements which might exceed your actual slab thickness - and in those type floors commonly the cables "sag" down from near the top to nearer the bottom in mid-span, to your clearance dimension changes with location on the slab - not a problem to lay out before the slab is cast, running both above and below th reinforcing with the conduit, but generally not reasonably doable in an existing slab.

Depending on locale, you might get away with 1/2" conduit or even smaller rather than 3/4" I assumed (which is slightly oer 1" OD) - depends on area.

Another consideration - locating the in-slab outlets so they do not disrupt the structural integrity of the slab, because they are going to be nearly slab depth for the box, in all likelihood.

Another alternative commonly used - especially in partitioned buildings - run in walls or partitions or under approved raceway sill strips across walkways, and avoid open-floor area in-floor outlets entirely.

Answered 3 years ago by LCD

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