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Question DetailsAsked on 3/13/2016

Can I use stone dust as my base for a poured concrete patio?

I currently have a patio with blue stone and would like to replace it with poured concrete. The stone dust has been in place for about 15 years so it is compacted well.

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1 Answer


"Stone Dust" is not a technical materials specification - in some cases it is truly sand-like or actually dust, used to compact into the joints to hold the block tightly in place, same as masons sand would be used. In other cases it is rock spalls - the fines from blasting and processing the rock, and basically a coarse sand or rock chips with fines. The latter generally would be a good base material for pavers or concrete - the dusty/fine sand type is commonly used about 1/4-1/2" thick under pavers as a levelling course because the pavers can be pounded into it a fraction of an inch to level them out, but not a great base material in thick layer because it is too deformable and potentially frost susceptible. Not the best material for concrete base - generally OK for load bearing (for patio/slab) if compacted, but the risk is if it starts washing away with seepage under the slab or freezes seasonally.

So - bottom line - if basically a stone chip/coarse angular sand probably fine, if a dusty or coarse sand product not the best but if it is uneroded after 15 years probably will work fine for concrete slab too - just have to be careful about scuffing and loosening it up with feet while prepping for the concrete pour. If fine sand or basically just dust not so good.

WHY you are replacing pavers/stone with concrete might answer your question too - if because the stone keeps kicking up and getting out of level, that is because the base material is too soft/fine and is moving around when wet, or it is frost heaving - which would mean the material under it is not suitable for concrete base either - or that there was no true base (tuypically 3-4 inches thick for this use) and maybe they just put a thin levelling layer on native unsuitable soil or even topsoil, which should come out before construction.

Best practice would be to remove fine material (probably not over 1/2" thick or so) down to previously placed base (hopefully) or remove unsuitable material (topsoil uneven gradation or overly coarse material, silt and and mud) down 2-6 inches (depending on load capacity of what is under it) and then put in a proper concrete base material, which is commonly a 1/2 or 3/4" minus crushed stone product, compacted in damp (but no free water) condition with a plate compactor to a dense base that does not scuff up when worked on. If having this done commercially, you are probably paying $3-7/SF for the concrete patio ($ difference depending largely local concrete/labor cost and accessibility) - redoing the base is probably about $0.50 or maybe $1/SF of that cosst, so not a major difference if in doubt, and cheaper than replacing a damaged slab a few years down the road.

Answered 4 years ago by LCD

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