Compacting stone for basement floor/ not
My house is at the drywall phase of construction and the basement floor is about to be poured soon. The several inches (7-10+”) of stone there has been settling by gravity and rain and snow since December. 6mil poly(my request), 2″ foam(my request), 6″ steel mesh, Wirsbo IFR tubing and concrete will be installed on top of the stone. I would like the builder to compact the stone. He says it’s not necessary and a compactor only compacts the top 4″ or so of stone and he says he’s quite confident without it. It’s a finished walkout basement, and rebar dowels will support the edge of the slab where the footing doesn’t. I’m in southern Ontario and wonder what the norm is or what the concensus is on this procedure.
If the stone is compacted, settling will still occur anyway, but to a lesser degree. So if the stone settles 1/2″ with compaction, and settles 2″ without compaction, then the slab is unsupported in both scenarios and compaction is irrelevant. (unless, of course, the slab collapses enough to then rest on the stone). That’s my line of thinking now, but my builder is not sufficiently technical to be able to give me a good scientific explanation. (Although his reputation alone is really sufficient for me, I would like a more technical explanation anyway.)
The books I have don’t give much detail on this. So what do you guys do out there? What would you do if it were your own house you were building? Please and thankyou – Brian.
Replies
Compaction can do 8-12" so he is wrong there on his 4".
I always compact.
But he may be more or less right for results under your conditions, if he is very experienced at this. Her's why.
The work and weight of pouring the concrete will compact the stone to some degree as he places it. And you are only using it as a walkon slab for floor so it will not be subject to heavy loads other than it's own weight.
Now if this were a load bearing unit or for a garage with vehicular loads, I would definitely recommend that you continue to insist on compaction but you might do OK without mechanical packing for a floor.
BTW, with both SR finishing and pouring the crete, you are adding a tremendous load of moisture to the inside of the building envelope all at once. You might want to leave the windows cracked for a week or two.
Excellence is its own reward!
I've never found stone to compact very well in the first place, stone just seems to bounce. You may drive a few of them into the soil below but I don't think it will make much of a difference. As long as the soil below is a scraped surface and doesn't contain any fill you can get by without tamping. Personally I would have used sand in place of stone to get a smoother surface for the foam to rest on.
Turtleneck
The only difference between a rut and a grave is depth
I've had lots of experience in trenching, for sewer etc. Boy those engineers are sticklers for compaction. If the basement stone seen a lot of rain/water then it is all ready compacted, water is the best compactor.
If you've ever seen a project in construction you always see the voids or holes after it rains, water is the best.
Plus every stone has a natural state of compaction, for instance sand has the highest rate, so you really can't compact it that much more. Another factor to consider is if you used a round stone with little or no fines (sand), you need the sand to make it all lock together and the right moisture content (can't be dry).
Much depends on the stone (size, shape, & fines), but if you've seen lots of rain, mother nature can be your best friend sometimes, I agree with piffin on the 8-12" for being able to compact also, this is correct, when you don't compact 8-12 feet then you inundate your trench with ... WATER
Luck to your project
Mark,
A few thoughts;
1) All good advice so far. I've never met anyone who says "Gee, I sure wish I hadn't compacted that subgrade."
2) How any material behaves under compaction is, as has already been pointed out, the result of many factors. As I think Gabe and Johnnie have pointed out here before, almost any soil can be compacted.
The frequency of the compaction equipment (as in cycles per second) will have a lot to do with the results you get. Sand, stone, and similar materials need to be "rattled" into place, soils that tend toward the clay end of the spectrum need to be more "pounded".
3) I'm wondering about your rebar dowels. Might be worth detailing them so that the perimeter of the slab is not restrained as the slab cures. Not essential, but may help control cracking.
4) With the rigid foam and VB, again, watch your cure rate. Nothing at all wrong with the arrangement, especially indoors, but it is a different animal than a regular slab on grade. The VB and foam tends to hold a lot of moisture.
5) If your contractor is reputable and you have good reason to trust him, do so. This usually results in a better job.
Please let us know how it goes. This stuff is always interesting.
DRC
Thanks all - some good info here.
Dave - I'm not sure what, in total, you mean by "detailing" the dowels. Are you referring only to making sure they are positioned so as to not restrict slab shrinkage, or is there something else I should be looking for (like maybe "smoothe" dowels?) The dowels are a sloppy fit in the holes, so shrinkage in the direction of withdrawal of the dowels should be OK, and the sloppiness may or may not help in side shear depending on how they set at the time of pour. The dowels will support the slab for the entire length of one wall, and extend along the ajoining two walls for about I guess 8-10'. I would then say that shrinkage would mostly occurr in a direction tending to pull the slab off the footings of the back wall, except that there is a pier and point load support post just about in the middle of the slab, which would certainly restrict this movement. I say certainly because there was a 4'X4' footing (miss)placed next to it, a new one poured next to it, and the excess concrete from the truck was poured into the hole, which I estimate ended up being a mass of concrete about 4'X8'x5' thick! (Surveyor's error, don't ask.) The post was placed directly on this mass of concrete with no pier needed in between. Although my builder didn't specifically mention it, I think the presence of this support contributes to minimizing the probability of settlement problems.
As far as watching my cure rate, are you referring to making sure that they don't try and tool the slab too early before the surface water has evaporated? (a waiting time increase because of the VB) Or maybe that I should pay attention to keeping the surface moist for the first few days because the surface layer will dry faster than the underlayer because of the VB? Or both?
There seems to me to be a conflict in requirements for the stone fill. On one hand you want a uniform size to have air spaces between them so that it can drain freely (which is not the best for good, firm compaction) and on the other hand you want a good mix of sizes which facilitate compaction, and is not so good for free drainage.
It's too late now to go with sand, but I'm curious, is one more expensive than the other?
My contractor is reputable, however he does have to make a profit, and paying attention to ALL the details is just not economically possible (he would have to charge me even more : )), I acknowledge that, and I am willing to do a few of these extra details myself to make a better job (such as keeping the slab moist while it cures). However, it's a delicate balancing act for me because when I ask to do these extra little things it is at the same time implying that he is somehow not doing the job "right" by not doing them himself. So he usually tells me that he has found that these little things are not necessary. I guess I'm in good hands if that is the worst of my problems, eh? But, I digress, sorry.
I will keep you posted about what happens, and thankyou - Brian.
According to Walker's Building estimator Reference Book. The shrinkage factor for sand is 1.08 and Gravel is 1.12. In other words if you filled an area 100' by 50' feet with gravel 6" thick you would order 100' x 50' x 0' 6" x 1.12 = 2800 cubic feet or 104 cubic yards because it shrinks that much do to Tamping.
Wally O
You should compact to the degree you can. It's to late now but next time. Don't use Ca6 or road stone with the fines in it. Use 3/4" stone with no fines, it is self compacting. If you do use road stone with fines, apply it in "lifts" or layers. 6" to 8" thick. Compact each lift.
The rebar should not be a sloppy fit. If it is #4 rebar (1/2"), it should be pounded into a 1/2" drilled hole. Use a 3# hammer. A vibratory plate compactor would make a world of difference in compaction level in your space. An experienced operator could do your basement in a couple of hours, tops. Isn't it worth this, at least to help eliminate future cracks in the floor, especially one with radiant heat buried in it ?
carpenter in transition
Tim, you said rebar should not fit slopply into the hole.
I would disagree. Most dowel details, that he has described, are for support, not expansion and contraction. In those situations where a slab is moving along the horizontal plane a tight dowel will restrict the movement and cause stress. From my commercial days, I learned to use smooth dowels and grease one side of them. I have had this detailed speced on many many comercial slabs. If I am pouring agianst a previouse pour, I drill the hole to size and grease the end of the dowel I drive in the hole. If I am pouring and stub the dowels out for a future pour, I cast the smooth dowels in place and grease the exposed stub beore the next pour. In both cases I use smooth dowels. The rough surface of rebar, even if greased, allows the concrete bonding points that you want to avoid. The slab is going to shrink, as well as expand and contract. You can not retrict this with tight dowels. Don't fight it, control it by using smooth dowels and lubrication to reduce building stress and causing cracking.
Dave
I alway thought stone and gravel were self compacting. Walker's states that all fill materials will shrink under compaction- some more then others. That is why you order more then what it would take to achieve your finial depth.
They list cinder, crushed limestone, granulated slag, gravel and sand.
My question is when is it not gravel and it is stone? I know there is a size limit where it stops being one and becomes the other. What is it?
Also according to their opening statement all material will shrink under compaction making nothing self compacting. I too often heard stone is self compacting so is it or is it not?
To my knowledge the only self-compacting material used in construction is rounded stone,(like from a stream or river), if I recall correctly, about 1 1/2 to 2 " in diameter. When dumped from a truck this material will always naturally self repose into its most compact state. To visualize this in a scale model way, think glass marbles in a glass jar, they cannot be compacted. I guess this stone is not widely used because other characteristics of this material make it unsuitable for most applications, (and maybe price?) It would also tend to sink into any non-hard (compressing) substrate it was placed on.
I found out today that my builder has called for fibrated concrete, which eases my mind somewhat further. They pour tomorrow, but unfortunately I will be out of town and will miss it. I also found out that except for an area in the utility room within a few feet of the floor drain that the entire basement is being poured flat, with no slope towards the walkout side, which I originally wanted. Is this the normal way that basements are poured now? I'm leary about requesting a slope now because it means that the slab will now have to be thicker on one side, which I know is one recipe for increasing the likelyhood of cracking.
Thanks all - Brian.
The only welf compacting building material is concrete with an 8-inch slump....that's not a mistake, it's rustic
Only done one basement but after the concrete sub told me "2b don`t settle. We never compact", I went out and rented one. The deeper areas (2') came down about 2" with water. I`ll do it next time too.
stone will compact, A sled will not do it. one must use a jumping jack. most open grain stone and rocks will still compacted but will not lock together wiyhout fines. these are stones like 57 rock, These material are good for water retained footing to bridge material. all material under loads need to be compacted not matter what the material is. Contractor did not want to do it because it was extra work for him. Will the house settle, maybe not, a house really has not must weight and like said earler some compaction has accord due to placing of concrete(weight).
Me, I would compact the hell out of it, also we do not use stone under slabs here because the sharp points will tear the plastic. Everything here is slab on grade so we worry about moisture migration through slab and when placing concrete the sub soil sucking moisture out of the wet concrete creation cracks.
also if you compacted the rock and elevation goes down remember you are also compction sub soil under rock.
stone will compact, A sled will not do it. one must use a jumping jack
Not to start a debate but, I used a plate vibrator and a hose and got very good results. A jumping jack may have been better, don`t know.
Me, I would compact the hell out of it, also we do not use stone under slabs here because the sharp points will tear the plastic. Everything here is slab on grade so we worry about moisture migration through slab and when placing concrete the sub soil sucking moisture out of the wet concrete creation cracks.
Again, I don`t want to promote the idea that I am a pro at this. I am not. I have read that a layer of sand (2"?) is an exellent buffer for both puncture and water migration. Sounds like a lot of detail work that a lot of people probably don`t want to include because of time and cost. The guys I hired were not very picky or careful and can`t really picture them going to that amount of trouble. I also can`t picture hiring them again. :)
Wally - I wanted the slope slightly to the walkout because it faces a beach, and while I'm well above the 100 yr. high water mark, I would like to know that if the high limit of wave attack does ever reach the bottom of the door during a storm that the basement will self-drain as opposed to self-fill. Also I will be putting wet kayaks, canoes, windsurfers in the basement and would like the puddle to flow toward and out the doors as opposed to toward the stud wall and drywall. ( I should point out that one of the doors is a single garage door, we call it a " lake garage"). I will ask that the drywall be raised off the floor at the bottom of the wall, and will probably end up running a bead of silicone or something around the bottom of the baseboard to minimize wetting.
To All - Seems to be routine in my area to make the basement floor flat. The 30X40 basement of the house I'm in now all slopes slightly toward the floor drain in the laundry room. (It's been wet-tested once, in 45yrs.)
I did get to see some of the pour and while I didn't slump-test it, it was a very stiff mix, I'd guess 3-4" slump tops. I'm glad to see that it wasn't a soupy mix.
I guess if it does crack I should see it within the next few weeks?
Rgds - Brian
I never heard of a floor sloping to the walk out side. Your exterior soil or concrete patio if any should slope away from the house of course.
Any granular with fines is not self compacting. Under DOT spec's 3/4" crushed limestone stone can be used as trench backfill and considered self compacting. This is 3/4" stone with no fines. Call any quarry, they will know what you want.
Rounded or washed river stone is not necessarily a good choice, it rolls in place. Crushed stone is broken into faceted stones and interlocks into place.
Soaking stone with fines will also help with compacting.