Lava Rock Under Basement Slab
I’m considering using 1/8″ – 3/4″ lava rock, also known as “cinder” with No-Fines under the non-structural slab in our “to-be-finished” and conditioned basement.
I’ve read a lot about this, and I’m typically told you want to use “state-spec” gravel with fines, watered down and well compacted.
However, when I used to live in WI, we used regular crushed gravel all the time, with no fines, which was a huge benefit when it came to leaving air gaps within the substrate to allow for passive or active radon mitigation.
In our case, we also want to enable radon mitigation (thus no fines) and additionally we like the fact the cinder is local to our area (Central OR) and that it provides an R-value of around 1.25/inch thus serving double duty.
I’ve been advised to compact the cinder (which would destroy it) and told that it is not strong enough and will crush and degrade over time.
Thinking about this critically, it seems that
1) the cinder will not degrade over time as it will be stable and undisturbed underneath the slab insulation (barring any earth quakes)
2) it might benefit from mild “vibrating-plate” compaction (maybe setting the plate compactor on top of a piece of plywood to reduce the damage to the cinders) which might just help jiggle the cinders into place before the slab is poured, or maybe this is unnecessary if the cinders are walked on enough while prepping the slab for plumbing, insulation, etc.
3) the cinders (although not nearly as strong as crushed gravel in part because they are porous [where the R-value comes from] ) will be plenty strong to support the weigh of the slab, as well as anything that might be set on top of it including furniture, people, etc. because the weight will be widely distributed and somewhat buffered by the rockwool CB-110 rigid insulation boards we are placing between it and the concrete.
I will definitely be adding rebar (we’re using GatorBar) in both directions on 18″ centers to reinforce the slab, as well as adding fiber mesh particles to the concrete.
I’m wondering if any one has any knowledge or first hand experience with using cinder under a slab, and if so, how’d it work out, and do you have any advice.
Note, our engineer did not bat an eye when we asked if using cinder or pumice under the slab would be O.K… but he did say so long as it is not used under the mid-bearing footings that cut through the middle of the slab.
Any and all advice would be appreciated. Thanks!
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I'm doing this is Sacramento. We're pouring a 4" Adobe slab on top. We used about 12" of 3/4" lava. I don't have experience to share other than to say that the lava rock holds up to the plate compactor well without being noticeably crushed. It's quite a 'sharp' material so it locks together well. In 5 years I'll be able to tell you if it held up...for now I'm not worried.
Hi Kent, it’s been 2 years since this post… any updates on how the cinder has held up?
Hi Jeff, no news is good news. There is no noticeable settling of the lava that I can detect from the finish floor. It's adobe, so I'm sure it would crack if there was a problem underneath. I'd use it again.
I think of lava rock and cinders as very different stuff.
When I hear cinders I think of coal cinders from coal furnaces as a child I recall the track at school was a hard pack of coal cinders. Today the same material is hazardous waste but no one burns lumps of coal anymore except for a few black smiths.
I imagine when they burn pulverized coal as dust the unburned residue seems unlikely to resemble the stuff I recall.
My point is you want to be clear about what you are getting and where it came from and you do not want coal ash under your slab.
Walta
Thanks Walta, back in the Midwest everyone called it lava rock… out here in Bend, everyone calls it (lava rock) cinder.
"Lava Rock" would be the naturally occurring volcanic rock aka pumice. "Cinder" has generally meant coal cinder in my experience. I suppose "pumice" might be a specific mineral or mix, and other volcanic foamed rock might have other names. These names take on regional meanings, and you do want to ask what you're getting. I would second the notion that you don't want coal ash under your slab. Pumice would be fine. With the past volcanism in the northwest, it may be that there are local deposits of pumice that are available for construction.
Yeah, +1 for regional differences in names. Where I'm at we call the red or black stuff lava rock or scoria, and a lighter white stuff pumice. I'm not sure I would use pumice under a slab as it's less strong, at least what we get around here...it crushes pretty easily. Seems like there is room for some materials/building science folks to do some work here.