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Rob:
Your “flowable fill”, (I call lean concrete) makes excellent fill but IMO expensive. The greater problem in the posters situation is the 6′ wall. You would likely have a blowout pouring concrete against a 6′ foundation wall. I wouldn’t use gravel fill either, good clean dirt fill will suffice if , as already mentioned, the fill is brought up in 6″ lifts, properly moistened and compacted. I’d look into a small Ramax vibratory compactor .
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Replies
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I agree w/ Albert. I would add that essentially your walls are retaining walls and should have been designed accordingly prior to placing concrete....
A buttress would help unless the walls are designed as a pure cantilever retaining wall. Another method would be to bring the fill up part way and pour an interior bond beam or strongback (properly anchored to the existing wall), then continue w/ the fill.
*Putting rebar in the slab will have minimal value if the fill is not properly compacted. W/out compaction of suitable fill, the slab will crack.
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Bill Curry
you mentioned using re-bar in the slab. How much and what spacing would you suggest? I wish I had consulted this website prior to starting this project I am learning a lot and trying to make the best of my situation. Also I am planning to pour some footings on solid ground and use tubes with internal re-bar extending up into the slab so I can pour these columns along with the slab.for additional support. I want to use five of these eight inch tubes located along the center lines of the slab both ways. Do you have any suggestions as to spacing re-bars in conjunction with these columns? If anyone else has any ideas please reply.
*Edge bearing slabs will always crack due to settling of the fill underneath. Most commercial jobs spec a proctor of 92-98% compaction, verified by a soils engineer. We know these will settle over time. We can guess that without testing the average residential contractor will compact fill to 85-90% if he is trying. This results in even more settling and eventually more cracking. Floating slabs may crack - but only due to shrinkage, or an anomoly in the fill, but not due to designed-in defects.In my opinion - Rebar in a two car garage with an eight foot ceiling is total overkill. Especially because the slab is probably too thin to allow proper bar placement so to be considered a structural slab. Stick with the mesh to prevent shrinkage cracks.-Rob
*Sorry Rob if you want to give it the best chance rebar is the way to go. As far as slab thickness goes have you ever heard of grade beams? Wall keys? Expansion joints? Again most people in residental construction have little or no idea about concrete. Please don't tell me about how many years that someone was done this. Wire is just a poor choice in slabs, unless price is the only reason.
*Arrow just a few things that I would do if the soil is as unstable as you say. Your idea of pouring tubes down thur the fill is ok but how will you dig these? Is the ground that the fill is on stable enought to handle this? I rought idea of how to do this as follows 1 Figure a 5"slab at 4000psi(6000 if this is for heavy duty use)2 Deep grade beams roughtly 12 wide by 6" deep on 6' centers running the 36' wide3 Cover whole area with plastic4 Add 2" chairs to grade beam and #4 or #5 steel spaced at 4" centers5 Run #4 or #5 steel paralell to your grade beams on 12 centers. Use 2" chairs as needed. Remember to lap all steel 18" both ways if you need to splice.6 Bend all cornors7 Install expansion joint material around entire slab againest wall.What you are doing is building a floating slab. By isolating the slab from the wall you can help in settlement of the slab.Hope this helps and wthout seeing the place this is a rought idea. Email me if you need more info
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Arrowshooter, It sounds like the backfill has not been done yet? so you just have a hole with 4 walls? if so, then really the best thing to do would be to knock them down and start over with an engineered design. There is probably a very real risk of the back wall atleast tipping, if not falling over. It would be much cheaper to fix the walls now instead of after the rest of the structure is built.
Leaving aside the walls for now, Will this slab be poured inside the walls, or over the top of them? I'd guess that the former is more likely, but someone mentioned edge bearing slabs, so it seems there is some question. What is this floor going to be used for? you said "garage", if it really is just a garage, I'd think that a bit of settling and a few cracks are not the end of the world, but if it's actualy a workshop the floor requirements may be more critical.
If the slab is going inside the walls, and it is "just" a garage, I'd make a resonable attempt to compact the fill, then use expansion joint material at the edges, #4 rebar @ 1'o.c. both directions and just let the slab settle along with the fill.
If the slab goes over the walls, and is securly tied to the walls, then the slab will stop the top of the wall from rotating outward (but only after the slab is in place, won't help any while backfilling and won't stop the wall from breaking in the middle, is there any vertcal reinforcement?) but in this case the edges of the slab will be supported while the middle settles, causing more distortion and cracking than if the slab was floating, unless of course the slab was strong enough to span the distance between supports, but at 30x36 feet that is not likely, adding cloumns would reduce the slab span, but again, unless the slab is strong enough to span the distance between supports, all this will do is give you bumps in the floor when the fill settles, and worse cracking than if the slab was floating.
Jay
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Fred - this stuff is really not anything like concrete. It is black and sort of like hard and spongy. It is $25/yard but look at the time savings!
I'll try for an apples to apples comparison. Around here you'll end up paying about $8-$10 per ton (about $6 per yard?) for good fill. If you want to test it for organics that will cost you about $100. Then there is the labor for placing and compacting 6" lifts. You need at minimum a backhoe, plate compactor, and three laborers. To insure compaction you have to have a soils tester for about $350 per day.
In contrast, you back the concrete truck up, run it up to the governor, and pour it out as fast as the chute will let it, then you walk away. To resolve the pressure issue you pour a load a day until it's full, once the material below solidifies it will no longer transfer outward pressure unless it fractured. Even then, it would not be as much as liquid would.
A stone slinger is also an option, though I still like to see post-placement compaction.
-Rob
*Bill, I think you missed my point. I didn't say ALL slabs, I said residential garage slabs with an 8' ceiling. You can't get enough weight into the space to endanger the slab, thus requiring the rebar. The slab will also be too thin as to allow proper placement of the rebar to take the tensile stress on the underside of the slab.If the slab is not edge-bearing, then the tensile stress on the underside of the slab is automatically lower.What do grade beams, wall keys, and expansion joints have to do with slab thickness in this discussion?My old boss who was "Mister Overkill, P.E." never put rebar in residential garages, unless they were very large (4 or 5 car) clear spanned. This would allow the possibillity of weight concetration in the center of the slab. He did lots of heavy duty concrete design, including 12" thick two way slabs with designed variable spacing on the rebar. Looks impressive, but a total pain to place the bar.-Rob
*Okay Bill, now I see what you meant by grade beams in the other post. This system is a ton of work though! My impression was that Arrowshooter was a DIY doing this himself. Your answer will definetly work, but now requires not only all of the problems with fill compaction, but now you need a drawing for the rebar, and days and days of fill placement and rebar placement.This system is easy on material costs, but heavy on labor costs - which is the expensive part.I do agree with the floating slab and the expansion joit material at the edge.-Rob
*Compact gravel or sand using a compactor. Be careful near the 6'wall. If it's properly reinforced and keyed in a footing it should be ok. If your grade permits, "balance" the fill on the outside with fill up part of the way. For additional support of the slab, you could lay up about 6 block piers on footings spaced evenly and tie #4 rebar 12"o.c. both ways.
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Thanks to all for the information. Seems like there are a lot of good ideas being posted. Some of these were not an option especially tearing down the wall and starting over again with an engineer. I have been unable to connect with this web page for a while so here is whats happened.
I went ahead and backfilled using good old Georgia red dirt which packs pretty good under the weight of a bobcat I cant believe the slab will settle due to lack of compaction. I put in three 10 inch plastic drain pipes equally spaced along the 36 ft centerline These rest on undisturbed earth the longest of these is 54 inches. Each of these will have four vertical rebars. I intend to form up a 12" X 8" grade beam across the tops of these piers, the beam will have three 1/2" rebars along its length. I will use wire mesh over the whole slab. The slab will rest on top of the walls, the wall contractor placed ten foot high ^ shaped rebar all around the wall which will tie the walls to the slab. The slab,beam and piers will be poured at the same time. This is not a commercial type garrage and will probably be used for storage or For a shop. I have talked to several homebuilders in the area who say I wont have a problem. I will keep you posted as to how it all comes out.
*I AM BUILDING A 24 X 24 GARAGE WITH A 24 X24 DRIVE SHOULD I DIG FOOTINGS AND IF SO HOW DEEP 8FT WALLS 2 X4 STUDS
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I am building a two car garage on a sloping lot. The building is 36ft across the front and 30ft deep.I have poured walls that are three ft high at the front and about 6 ft high at the back. I am planing to backfill this space and pour a 4" slab which will be reinforced with 6x6 wire mesh. I've been told the dirt will settle and the slab is likely to crack. can someone give me some suggestions as to how this should be done? The construction will be two by four studs with thirty ft trusses over 9 ft ceiling ht.
*The best way to prevent slab settling and cracking is to compact the fill as you place it. Make sure that there is no original top soil left inside the foundation. Use good quality fill (clean sand or gravel) avoid any soils with clay as good compaction is difficult if not impossible. If the fill is very dry add water to help compaction. The fill should be compacted in approximately 6 inch lifts depending on the size of the compactor. You should be able to rent a small compactor from your local rental shop. Just tell them what you are doing and they should set you up with what you need.
*A friend did the same thing with his garage/site 32 x 26, only the contractor told him "You don't need to do all that." He then proceeded to backfill the foundation with tree stumps, sod clumps, asphalt, and tires. He compacted it by running the loader around on it. I was mortified when I showed up to help frame it. The guy who put the fill in told him edge bearing slabs don't need compacted fill!! I told my friend not to pay him a penny!In our area we can buy a flyash-portland cement-aggregate mix commonly called "flowable fill" from the concrete company. It's $25 per yard, you tailgate it off the truck, fill the space and walk away. It cures to a black pumice like consistency, you can dig thru it with picks, ripping hammers, etc. Costs more than gravel fill but saves tons of labor. Can pour concrete on it the next day!-RobP.S. - Edge bearing slabs are bad umm okay, don't do it okay (Impression from Southpark)
*If arrowshooter correctly compacts 6' of fill, mightent the foundation walls blow out?A buddy of mine once gave me some good advice: "only 2 things you need to know about concrete... It gets hard, and it's gonna crack!" A bit oversimplified, but I pretty much agree with the second part.Rob: Please expand on your statement "edge bearing slabs are bad". Not argueing, just asking for info.
*Yes there is a good chance the walls will fail regardless whether or not it is compacted. Especially with a thirty foot wall. That is why it is imperative that rebar is used in the walls. The rebar gives the concrete tensile strength. I should have mentioned it in my earlier post but it sounds like the walls are already poured. I've actually seen concrete walls fail in a similar situation although the walls were about 8 to 10 feet tall. The contractor even used rebar but failed to bend the rebar around the corners in the form so when the fill was being placed inside the garage foundation the walls stayed intact but came apart at the corners. For a thirty foot wall as described in this topic I would also recommend a buttress at the mid point of the wall. The buttress can be inside the garage so it wont be seen, just make sure it is tied into the main wall with rebar corners. When ever in doubt about structural situations such as this consult an engineer and do it right the first time.
*The walls are poured I dont have any buttreses etc the wall contractor put two rebars near the top and two near the bottom all around but he did not wrap the bars around the corners or tie them so Ive got what Ive got. A friend suggests putting some pipe columns on footings and extending up into the slab then backfilling around these to better support the slab this seems like it will take some pretty nimble bobcat work. Would putting rebars in the slab in addition to the wire mesh help?
*Arrowshooter In order to give your slab a chance you need the following;1 Compacted fill. This does not mean running the backhoe over the fill at the top. 4" to 12" lifts with a "jumpimg jack" should do the trick without blowing out your walls.2 Use of grade beams in the slab. Remember this important tip. friends don't let friends use wire.Rebar is the only way to go in your slab. Make sure you use "chairs" to keep the steel off the ground.
*Rob:Your "flowable fill", (I call lean concrete) makes excellent fill but IMO expensive. The greater problem in the posters situation is the 6' wall. You would likely have a blowout pouring concrete against a 6' foundation wall. I wouldn't use gravel fill either, good clean dirt fill will suffice if , as already mentioned, the fill is brought up in 6" lifts, properly moistened and compacted. I'd look into a small Ramax vibratory compactor .